Textile single-use products are dominantly used for hygiene and personal care, many of which are non-biodegradable and are frequently discarded into sewerage systems, thus causing blockages. Thus, there is a need to move towards water-soluble textiles. This research study focuses on transforming or repurposing biomass material and synthetic reusable waste plastic materials to improve waste. Chitosan (CS) nanofibers could be used in single-use nonwoven fabric or biodegradable tissues, as the water-soluble properties of chitosan nanofibers make them the perfect material for single-use applications. Furthermore, CS was blended with polyethylene terephthalate (PET) polymer and PET-based waste plastic (CS-WPET) to slow the CS nanofibers’ water degradability and strengthen the durability of the nanofiber which could be used as air filters. The CS-TFA and CS-TFA/DCM nanofiber diameters were 95.58 ± 39.28 nm or 907.94 ± 290.18 nm, respectively, as measured from the HRSEM images. The CS-PET and CS-WPET hybrid nanofibers had fiber diameters of 246.13 ± 96.36 or 58.99 ± 20.40 nm, respectively. The thermal durability of the nanofibers was tested by TGA, which showed that CS-TFA/DCM nanofibers had sufficient thermal stability up to 150 °C, making them suitable for filter or fabric use at moderate temperatures. The blended nanofibers (CS-PET and CS-WPET) were thermally stable up to 160 °C. In the aqueous medium stability test, CS-PET and CS-WPET hybrid nanofibers had a slower degradation rate and were easily dissolved, while the CS nanofibers were rapidly and completely dissolved in an aqueous medium. Blending waste PET with CS allows it to be recycled into a useful single-use, non-woven textile, with greater water solubility than unmodified PET nanofibers but more durability than CS nanofibers on their own. A large proportion of non-degradable plastic waste in the marine environment is the result of single-use synthetic plastics and discarded items . PlasticThe chitosan (CS) medium molecular weight, polyethylene terephthalate (PET), trifluoroacetic acid (TFA), and dichloromethane (DCM) were purchased from Sigma-Aldrich  and were used with no further treatment. The waste PET bottles were collected in the Cape Town municipality.The electrospinning method of chitosan was adapted from the previously conducted studies with minor modifications ,12. The The PET and WPET solutions were prepared following the same procedures described by Totito et al. (2021) . PET polThe CS-PET and CS-WPET hybrid nanofibers were produced via an electrospinning process. The volume mixing ratios of the two polymers were optimized in TFA solvent for the electrospinning process. The hybrid polymer solution ratios were prepared by mixing a certain volume of 7 wt% CS and 10 wt% PET or 10 wt% WPET solutions. Amongst the mixing volumes ratios, only a 1:1 ratio was electrospinnable. The volume ratio 1:1 was mixed at room temperature to form a homogeneous hybrid solution. Each polymer solution was electrospun following the same procedure as mentioned in 3. Then, 0.01 g of the PET and WPET nanofibers were weighed out. The nanofibers were immersed into the pH-adjusted deionized water and shaken at 190 rpm for 24 h. The nanofibers were then washed with deionized water and air-dried overnight at room temperature. The dried nanofibers were weighed thereafter to check the water stability of the nanofibers. The CS-TFA/DCM, CS-PET, and CS-WPET nanofibers were stabilized in various reagents to alter the diffusivity resistance towards aqueous mediums and decrease CS water absorption capacity. The CS-TFA/DCM, CS-PET, and CS-WPET nanofibers (0.01 g) were immersed in either 10 mL of 3 M of NaOH solution in methanol, 10 mL of saturated K2CO3 solution for 120 min, or neat ethanol solvent for 120 min; thereafter, the neutralized nanofibers were individually rapidly washed with deionized water, dried at room temperature, weighed and characterized by HRSEM.The PET and WPET nanofibers solubility test was carried out by adjusting the pH of deionized water to 1, 3, 5, 7, 9, or 11 with 0.1 M NaOH or 0.1 M HNO−1 to 650 cm−1 against transmittance with 2.0 resolutions at 64 scanning times. For every analysis, the baseline of the FTIR was corrected using the background spectrum of the blank scan. The FTIR diamond crystal and ATR holder were cleaned with ethanol before every analysis to prevent cross-contamination of the samples. The application of FTIR spectroscopy in analyzing the surface of the material involves the determination of the functional groups and chemical structures of various materials. Generally, infrared spectroscopy provides relevant information on short- and long-range coupling orders caused by other factors such as lattice bonding, electrostatics, and deterioration of the original material structure ..19]. TheThe 1:1 TFA solvent ratio CS-PET and CS-WPET hybrid nanofibers were characterized with the HRSEM to understand the morphology of the electrospun nanofibers. The HRSEM images of the 1:1 CS-PET and CS-WPET hybrid nanofiber blends show consistent, smooth, homogeneous nanofibers with an average diameter of 246.13 ± 96.36 nm for CS-PET hybrid nanofibers and 58.99 ± 20.40 nm diameter for CS-WPET hybrid nanofibers. The significant reduction in the CS-WPET nanofiber diameter size compared to the CS-PET and non-blended CS and WPET nanofibers see a,d is du−1. The stretching vibrations at 1671 cm−1 correspond to their carbonyl (C = O) functional group. The C = O represents the acetyl groups that were directly related to the backbone conformation of the chitosan [−1 band are ascribed to the combination of N-H bending, C-N, and C-C stretching vibrations [−1 corresponds to N-H deformation and C-N stretching vibration. The band at 1068 cm−1 confirms the C-O functional group [−1, 1651 cm−1, and 1417 cm−1 of the commercial chitosan used in the study reported by Kumirska et al. (2010); thus, the chitosan nanofibers were structurally similar to chitosan [In the ATR-FTIR results a, the dichitosan . The vibbrations . The waval group ,24. The chitosan ,26, not Thermogravimetric analysis of CS-TFA/DCM nanofibers is demonstrated in 2 and yielded similar outcomes as this research. El-Hefian et al. [The high thermal stability of the CS-TFA/DCM nanofibers agrees with the thermal degradation of chitosan experiments reported by de Britto et al. . CS-TFA/n et al.  also inv−1 that corresponds to the asymmetric stretching mode of the C-H bond [−1. The asymmetric C-C-O is present at 1260 cm−1 and the O-C-C stretching at the 1100 cm−1 band. The strong peak at 700 cm−1 represents the shifted aromatic functional group C-H wag, which has been affected by the presence of the carbonyl functional group [2 flow rate of 20 mL/min started with an 8.12% gradual increase in weight from 15 °C to ~380 °C. The 8.12% weight increase in the PET nanofibers is related to the expansion of the PET nanofibers, which is due to an endothermic reaction occurring. The successive thermal degradation of the PET nanofibers begins at ~380 °C and ends at ~450 °C, where a significant 70.40% weight of the PET nanofibers was lost. The depolymerization of the PET nanofibers was occurring at this second stage and formed a carbon black residue [The PET and WPET nanofibers were further characterized using the ATR-FTIR to understand the functional groups of the nanofibers as shown in C-H bond . The caral group . Thermog residue . The las residue .The first stage of the thermal decomposition for the WPET nanofibers started at about 290 °C to ~350 °C, which may be the result of the evaporation of volatile oligomers in the WPET nanofibers’ matrix. The WPET nanofibers lost about 4.55% of their weight in the first stage. Then, the weight of the WPET nanofibers was constant until degradation occurred from 400 °C to ~460 °C. At this stage, the WPET weight loss was about 72.96%. This significant weight loss was the contribution of WPET oligomers’ migration and the degradation of the ester group chain C-C-O together with the unsaturated chain of the carbonyl functional group . This waThe shape of the TGA profiles of PET and WPET nanofibers was found consistent with the increase in temperature. The first derivative plots corresponding with the TGA plots of the PET and WPET nanofibers pyrolysis at about 380 °C to ~460 °C are shown with the endothermic peak degradation in the first derivative plot at exactly 402 °C or 430.60 °C, respectively. Finally, the PET and WPET nanofibers’ thermal decomposition ended when the reaction ceased at a residue accumulation of ~9%. This corresponds with the thermal degradation investigation of waste polyethylene terephthalate (PET) under inert and oxidative environments studied by Das and Tiwari .−1 and 2964 cm−1 that corresponds to the C-H stretching and the combination of the carboxylic acid COOH and amino acids NH2 that indicates the presence of the PET or WPET in the nanofibers. The medium bands at 1792 cm−1 and 1718 cm−1 correspond to the C = O vibrations of the acetyl ester functional group also present in the PET and WPET structural formula. The medium band at 1020 cm−1 corresponds to the O bridge stretching in the chitosan structural formula. Asymmetrical C-H bending of the CH2 functional group is present at 1371 cm−1 and 1267 cm−1 bands, which also proves the presence of the CS molecules [The CS-PET and CS-WPET nanofibers were characterized using the ATR-FTIR technique to identify the functional groups of the 1:1 CS-PET and CS-WPET nanofibers e. The FTolecules .Thermogravimetric (TGA) analysis of 1:1 CS-PET and CS-WPET hybrid nanofibers is presented in PET, WPET, CS-TFA/DCM, CS-PET, and CS-WPET electrospun nanofibers were studied for pH stability and solubility behavior, and the recoverable materials were characterized using HRSEM and ATR-FTIR techniques. The electrospun nanofibers were tested at different pHs for the mass difference that may occur upon exposure to the solution. This will give an insight into each polymer nanofiber’s performance in acidic, basic, or neutral mediums. This stability test showed the pH-related degradation behavior of PET and WPET nanofibers, respectively .In The CS-TFA and CS-TFA/DCM nanofibers were completely soluble in all pH-adjusted aqueous solutions and thus would be immediately degraded in water when discarded after use. The CS-TFA/DCM nanofibers were stabilized to increase the diffusivity resistance in aqueous mediums and decrease its water solubility compared to CS-TFA nanofibers. The CS-TFA/DCM nanofibers were therefore stabilized by neutralization to check if the CS-TFA/DCM nanofibers could be stable in a wet wipes solution for single-use personal hygiene care. The CS-TFA/DCM nanofibers (0.01 g) were immersed in10 mL of 3 M of NaOH solution in methanol;2CO3 solution and ethanol solvent for 120 min.K2CO3 previously reported by Haider and Park [2O. Therefore, CS-TFA/DCM nanofibers stabilized with 3 M of NaOH solution in methanol, resulting in nanofibers with stable residues remaining. Further characterization was carried out on CS-TFA/DCM nanofibers stabilized with 3 M of NaOH solution in methanol.Thereafter, the neutralized CS-TFA/DCM nanofibers were individually rapidly washed with deionized water. The CS-TFA/DCM nanofibers that had been stabilized in Kand Park  completeand Park . Both ofThe HRSEM image shows that the CS-TFA/DCM nanofibers stabilized with 3 M NaOH in methanol lost their fibrous character after the stabilization protocol a. The CS−1 corresponds to the combination of the OH and NH functional groups, which were also present in the chitosan [−1 corresponds to the ammonium ions that may have remained after the stabilization. The weak band at 1650 cm−1 corresponds to the aromatics and amine functional groups of chitosan. The bending vibrations at 1410 cm−1 were assigned to the carboxyl functional group of chitosan. The band at 1025 cm−1 confirmed the presence of C-O functional groups of chitosan [The functional groups of the stabilized chitosan nanofibers are shown in the FTIR spectrum b above. chitosan ,42. The chitosan ,43. Thus−1 that corresponded to the C-H stretching and the combination of the carboxylic acid COOH and amino acids NH2 that represented the presence of the PET and WPET in the nanofibers. The medium bands at 1640 cm−1 and 1563 cm−1 corresponded to the C = O vibrations of the acetyl ester functional group also present in the PET and WPET structural formula. The medium band at 1023 cm−1 corresponded to the O bridge stretching in the chitosan structural formula, thus showing that traces of CS remained after treatment. Asymmetrical C-H bending of the CH2 functional group was present at the 1411 cm−1 band, which also proved the residual presence of the chitosan molecules [The HRSEM images a of CS-Polecules ,38. FromThe alarming amount of non-degradable single-use synthetic plastic waste found particularly in the marine environment has motivated scientists to implement new strategies for designing and synthesizing single-use biodegradable fabrics or textiles and reusing plastic waste. Chitosan, being the deacetylated polymer from waste shellfish, scales of fish, and insects, was electrospun into CS-TFA/DCM nanofibers using 0.1 mL/hr, 25 kV, and 14 cm conditions. The best solvent for electrospinning a chitosan polymer was found to be a mixture of trifluoroacetic acid and dichloromethane in 7:3 ratios with 7 wt% of CS-TFA/DCM. CS-TFA/DCM resulted in bead-free nanofiber morphology compared to CS-TFA. The thermal decomposition ranges of CS-TFA/DCM nanofibers started at lower temperatures than CS-PET and CS-WPET nanofibers, but CS-TFA/DCM nanofibers had sufficient stability up to 200 °C, which makes them sufficiently durable at moderate temperatures in dry environments. The blended nanofibers (CS-PET and CS-WPET) were electrospun using the TFA solvent, forming hybrid nanofibers that were thermally stable up to 160 °C, with a slower degradation rate and easily soluble in an aqueous medium. While the CS nanofibers completely dissolve in an aqueous medium, blending waste PET with CS allows it to be recycled into a useful non-woven textile with greater water solubility than unmodified PET nanofibers but more durability than CS nanofibers on their own. Stabilization with 3 M NaOH in methanol did not result in a beneficial outcome, as this procedure resulted in deformed nanofibers. CS nanofibers can be used as dry, single-use, nonwoven fabric for air filters, masks, or personal hygiene in medical applications, as the morphology of the nanofiber non-woven fabric presents fine interfiber pores suitable for trapping viruses and bacteria. If used for infection control, these contaminated fabrics could be thermally combusted through depolymerization in a furnace, forming carbon residue. PET and WPET were robust, and when blended with CS their biodegradability properties were enhanced, also affording possible application as a dry air filter or wipe, as they could be discarded after use without concern as the composite fabric would degrade much more rapidly than PET in the environment. The CS-TFA/DCM nanofibers are much more easily discarded, as these nanofibers will completely decompose and dissolve in water with no waste buildup in the environment. Therefore, CS-TFA/DCM nanofibers can be used for single-use personal hygiene care as a highly water soluble, dry nonwoven textile to replace synthetic polymer nanofibers. For water-durable fabric, the CS-WPET nanofibers made using chitosan and waste plastic bottles , offered a product that reuses waste and may have an extended utility before it degrades. The CS-WPET hybrid nanofibers were reported on for the first time in this research. For future work, it is recommended that upscaling of the chitosan and composite nanofibers is performed to replace personal hygiene single-use materials, minimize plastic pollution, and minimize marine and land pollution. This study also recommends that further studies be carried out on the intermediate stabilization of the nanofibers. It is recommended that before use, the CS-WPET nanofiber fabrics to be implemented for hygiene care undergo quality assurance and be tested according to ISO 9001, ISO 179, ISO 22716, and ISO/TC 217.


Molecular machines are at the frontier of biology and chemistry. The ability to control molecular motion and emulating the movement of biological systems are major steps towards the development of responsive and adaptive materials. Amazing progress has been seen for the design of molecular machines including light‐induced unidirectional rotation of overcrowded alkenes. However, the feasibility of inducing unidirectional rotation about a single bond as a result of chemical conversion has been a challenging task. In this Review, an overview of approaches towards the design, synthesis, and dynamic properties of different classes of atropisomers which can undergo controlled switching or rotation under the influence of a chemical stimulus is presented. They are categorized as molecular switches, rotors, motors, and autonomous motors according to their type of response. Furthermore, we provide a future perspective and challenges focusing on building sophisticated molecular machines. The possibility of generating unidirectional, controlled rotary motion by chemical conversion is a major challenge. This Review provides an overview of approaches to the design, synthesis, and dynamic properties of various classes of atropisomers that can perform controlled switching or directional rotational motion under the influence of a chemical stimulus. Future strategies and challenges for the development of autonomous molecular motors are also provided. In receIn this case, the rotation is considered to be “free” and nondirectional, and, in some cases the rotational speed can also be regulated by external chemical stimuli. When control of directionality is added to the motion of molecular switches and 360° clockwise or counter‐clockwise rotation can be achieved, we call such unidirectional switches molecular motors −C(sp3) bond at room temperature due to the low rotational barrier, restricted rotation can be observed around a crowded C(sp3)−C(sp3) as well as C(sp2)−C(sp3) bond  spectroscopy is the most widely used analytical technique for understanding the switching process, provided that the individual states of the switching process are diastereomeric. Diastereoisomers exhibit diagnostic differences in their NMR spectra so that the extent of the chemical switching can be readily determined. NMR techniques using HPLC (High‐performance liquid chromatography) with chiral stationary phases is known to be effective for the separation of enantiomers as well as diastereoisomers. HPLC can also be used to determine switching ratios of molecular machines, since different conformations of a motor or a switch can exhibit different retention times. If a chemical waste is generated during the switching process, it may interfere with the NMR spectrum but HPLC analysis may be utilized even in this case for tracing molecular switching. Chiral HPLC is also an excellent tool for analyzing isomers (like atropisomers) when a compound is chiral and cannot be analyzed by NMR spectroscopy.Single‐crystal X‐ray crystallography is the most powerful of all techniques as it allows the determination of three‐dimensional (3D) structures of molecules in their solid state. Despite the advantages of this method, its main drawback is that it requires a single crystal of a compound, which is typically difficult to obtain, in particular for compounds containing thermally labile chiral information. Nevertheless, if a suitable single crystal for a molecular machine is obtained for each conformational switching state, its rotation process can be visualized clearly.Another technique is electronic circular dichroism (CD) spectroscopy. CD spectroscopy is often used to determine the helicity of atropisomeric biaryls.2In the following section, various types of molecular switches will be discussed. They are categorized according to the following principles based on the parameters used to achieve conformational changes, i.e., molecular switching: 1) through modulation of hydrogen‐bonding; 2) solvent‐assisted conformational control; 3) pH‐responsive switching in biphenyl; and 4) through ion binding.2.1Hydrogen bonds provide a unique electrostatic noncovalent interaction between a hydrogen atom  and an electronegative atom bearing a pair of electrons .Rac‐1 a as shown in Scheme In 2009, the group of Shimizu reported an axially chiral diacid switch 2, by simply tuning the solvent polarity (Scheme M conformation in an apolar solvent (chloroform). On the other hand, in a polar solvent (THF), CH/O hydrogen bonds are proposed to become dominant to afford mainly the P conformation.A combination of noncovalent interactions such as hydrogen bonding and OH/π interaction has been used to control the axial conformation of cannabidiol derivative Hydrogen bonding is sometimes strongly dependent on the solvent–solute interaction.E/Z isomerization by controlling the hydrogen bonding. T0 T5 a/55 with a D−π−A interaction between R1 and R2 in the planar state results in an intense ICT band (356 nm for 5 a and 317 nm for 5 b) due to the π orbital overlap which facilitates the electron transport between the two aryl rings. However, upon hydrolysis with a base (tetrabutylammonium hydroxide), diminution of the ICT band is observed due to formation of a nonplanar dianionic carboxylate , recovering the original photoluminescence.Luminescence derived from intramolecular charge transfer (ICT)6 , the orientation of the ester group at the lower half gives two possible conformations for 6. Compound 6 a has the ester group H‐bonding to Hb and 6 b to Ha. The electron‐donating character of p‐NMe2 weakens the H‐donating capabilities of Ha, shifting the equilibrium towards 6 a with a 6 a : 6 b ratio of 1.3 : 1. This is due to the strong H‐bonding between the C=O of the ester group and Hb. In contrast, the electron‐donating ability of p‐NMe2 can be quenched via protonation upon addition of trifluoroacetic acid (TFA), which resulted in almost complete conversion towards +6 b‐H, i.e. the protonated form of 6 b. The authors further extended their design towards a series of benzamido‐diphenylacetylene (DPA) molecular switches where the relative hydrogen‐bonding capabilities of two amide groups determine the extent of conformation switching.Hamilton and co‐workers introduced a pH‐responsive biaryl‐type conformational switch based on a multifunctional diphenyl‐acetylene 7, in which an acid/base‐mediated uptake and release of halide (mainly chloride) anions occur with high selectivity, through a rotation around the C−N single bonds.7, owing to its close‐shaped cavity, displays a strong affinity towards halide anions. Upon deprotonation of the resorcinolic O−H group with an organic base , foldamer 7 unfolds into an open conformation (7‐2H) through the modulation of its intramolecular hydrogen‐bonding network. In the open conformation (7‐2H), due to the electrostatic repulsion between the halide anion and the resorcinolate oxyanions, the halide is released . In this section, illustrative examples of molecular machines showing rotatory motion around their single bonds by exploiting metal coordination are provided. Before going into details, distinctive features and advantages based on coordination complexes are noted:First, coordination bonds are often dynamic and reversible in nature; therefore formation and cleavage of coordination bonds can be achieved with simple chemical manipulations such as adding a metal ion source or additional ligand, treating with base/acid, or via redox control.2c, 2eMany coordination compounds are synthesized by hybridization of an inorganic metal ion and an organic ligand. Due to the abundant variety of these building blocks, a countless number of complexes have been studied, which enables one to select a desired motif suitable for the purpose of designing a molecular machine.Metal ions often possess multiple valence states that account for their different properties. Thus, fine‐tuning of the redox state of a metal center in a complex can directly alter the stability of its coordination bonds, reactivity, stereochemistry, and also physical properties. As a result, different geometries can be achieved due to different ligand‐binding modes by shuttling between two different metal redox states.Due to the above‐mentioned characteristics using coordination chemistry, the installation of a metal binding site in the design of a molecular machine can be highly beneficial to manipulate nanosystems.aryl−Caryl bond rotations.8 a and 8 b) as shown in Scheme 8 a and 8 b prefer the transoid conformations, which lead to helical structures stabilized by π–π interactions. Upon complexation with lead(II), the structure unfolds in order to adopt a tridentate binding mode, changing the conformation of the heterocyclic oligomer into a rigid and linear structure (8 a′ and 8 b′). In this case, the cisoid conformations are induced by tridentate metal ion coordination. This structural switch can be reversed by addition of a metal ion trap (cryptate), or acid and base, which extracts lead ions from the metal complex and the ligand folds in a helical structure.Lehn used metal coordination2e, 623+‐sensitive ligand 9 comprising imidazo‐quinazoline fluorophores was introduced as a conformational switch (Scheme 3 was used to induce single‐bond rotations to convert the switch to a different conformation (9′) which is stabilized by hydrogen‐bonding between an iminium NH and a chloride ion. The authors propose a possible mechanism of this structural switching where chloride anions generated from an aqueous solution of FeCl3 induce the H‐bonding interaction in 9′.Fe9′ is bathochromically shifted  unit connected to a rotatable rod. In this design, an axial zinc‐binding ligand (piperidine) and a copper‐chelating ligand (phenanthroline) are also present. With copper(I) ions, phenanthroline shields the py‐pym arm by forming a heteroleptic complex and inhibits axial coordination of pym to the zinc center. In this state, piperidine is bound to the Zn porphyrin core. When the copper ion is removed from the system by addition of a stronger ligand 10 b, the pyrimidine nitrogen works as a stronger axial ligand for the porphyrin system and binds to the zinc center, kicking out the piperidine ligand (10 a′).Schmittel et al. developed a multifunctional and reversible bimetallic nanoswitch and applied it to switchable catalysis, mimicking the behavior of calcium/calmodulin‐dependent protein kinase II Scheme .[65] MThe authors utilized this chemical response to regulate the Knoevenagel condensation reaction in which transformation the released piperidine acts as a base catalyst Scheme .11 was prepared using a phenanthroline with bulky groups and the ring inversion of the methylpyridylpyrimidine ligand was studied. Complex 11 exists in two forms, i.e., 11 a and 11 b, having different biaryl geometries , and by addition of decamethylferrocene as a reductant the original state was retrieved with reduction of CuII to CuI. Utilizing these rotamers, the electron‐transfer‐gating behavior to an electrode was successfully regulated by controlling the molecular rotation. In several follow‐up studies,The redox properties of copper‐pyridylpyrimidine complexes were explored by Nishihara and co‐workers to control the rotation of a pyrimidine ring of the organic bidentate ligand.anti‐to‐syn atropisomerization by the in situ formation of tetrahedral CuI complexes ethane (dppe) as secondary ligand triggered the formation of the syn‐conformer by a favorable C−C single‐bond rotation in the bpy unit. This is due to the stable complex formation between CuI and bidentate‐chelating bpy‐ligand. The combination of the favorably curved topology of corannulene and the syn‐conformation imposed by CuI leads to the generation of the stable 12 a‐Cu or 12 b‐Cu/fullerene complex (Switch‐ON) through a supramolecular host—guest interaction. One of the important aspects of this work is the reversibility of the switch ON/OFF process. Indeed, addition of 1 equivalent of dppe as a metal scavenger to the syn‐complexes 12 a‐Cu or 12 b‐Cu leads to the regeneration of the metal‐free anti‐bpy conformers 12 a or 12 b and releases the more stable [Cu(dppe)2]BF4 complex as a by‐product .trans‐13, photoirradiation in the presence of oxygen and a photosensitizer generated singlet oxygen and promoted photooxygenation at the anthracene core to give endoperoxide 13+2O. This compound exclusively exists as the cis‐isomer and delivered cis‐13 upon deoxygenation. Based on the proposed photooxygenation mechanism,cis‐isomer of 13 exclusively. Heating the cis‐isomer produces trans‐13 and consequently forces the C−C single bond to rotate, preferably forming the thermodynamically favored trans‐isomer with the trans/cis ratio of 90 : 10. Therefore, photooxygenation and thermal treatment resulted in a reversible molecular switch.Singlet oxygen can be used as “fuel” in order to perform switching around the C−C bond.Linker and co‐workers developed a bisarylanthracene that works as a molecular rotary switch  and 14 c (R=benzyl) depending on whether the phenol unit is protected or not. When phenol substituents are employed (14 b), a guest molecule can slow down the rotation speed of N,N′‐diarylnaphthalenediimides by two orders of magnitude.Likewise, a difference in rotational speed is observed in aryl−Naryl single bond in N‐substituted imide 15 a is restricted due to the lone‐pair–lone‐pair interaction between the imide carbonyl and the quinoline nitrogen. Based on this structure, proton‐responsive molecular rotor 15 was designed which rotates 107 times faster upon addition of methanesulfonic acid (MeSO3H) as a stimulus (Scheme +‐TS15 a‐H) is stabilized through an intramolecular H‐bond as shown in Scheme t1/2=24 min for the parent rotor (15 a) to t1/2=2×10−4 s for +15 a‐H.Rotation around a C16, which has two ortho‐substituents on opposite sides of the biaryl, can undergo free rotation around the C−C biaryl axle due to a low rotational barrier with ΔG≠=12.6 kcal mol−1 (at 298 K in acetonitrile). This free rotation can follow two different pathways as shown in Scheme TS1 through a facile n→π* interaction compared to the N,O repulsive interaction . Interestingly, the rotor rotation around the C−C single bond can be reversed by stabilizing the planar TS2 in Pathway 2, using a chemical stimulus. The authors introduced another additional competing intramolecular interaction in the form of H‐bonding by protonating the pyridine nitrogen. In the protonated form of the rotor  is stabilized over +TS1‐16‐H , thereby allowing rotation to take place following Pathway 2  shown in Scheme 18 in Scheme 18‐H has a high rotation speed (104 Hz at 298 K) at the original state.In 2018, the group of Wang demonstrated modulation of rotational speed of a related succinimide molecular rotor around its C−N bond in response to external metal cations  introduces an electrostatic bridge between the stator and the rotator and lowers the energy barrier for rotation. This acceleration is dependent on the radius of the metal cation . This results in multiple stages for the rotational speed, mimicking a macroscopic fan  to the five‐membered ring (h5) located close to the triptycene. This structural change slowed down the rotation speed of the triptycene by ca. 108 times.The dynamic nature of coordination bonds (formation and dissociation) can directly facilitate molecular switching. The first example of a rotor controlled by metal coordination is Kelly's molecular brake, reported back in 1994 Scheme h6 to the In the previous sections, we have shown several examples of chemically induced geometrical molecular switches and acceleration/deceleration of the rotation of C−C and C−N single bonds of molecular rotors. Some of these concepts were key to the development of chemically triggered unidirectional motors.However, these intriguing small molecular rotors and switches do not rotate unidirectionally because there is no directional constraint, and therefore they cannot be called molecular rotatory motors. In the following section, we will discuss various key principles required for the development of a new class of molecular motors and provide examples and the state of the art in the development of chemically driven biaryl‐based rotatory motors.4aryl−Caryl bond rotation in biaryls and the various approaches are illustrated. For a simple unsubstituted biphenyl, rotation around the axle is often considered to be “free”, i.e., the energy barrier is less than the thermal energy at room temperature.ortho positions of the phenyl ring are substituted with functional groups, the barrier of rotation becomes high enough to isolate individual conformers, which are called “atropisomers”.In this section, the design of molecular rotatory motors and their detailed mechanistic and stereochemical properties will be presented. The main discussion will focus on CStation I and III are non‐superimposable and display axial chirality  primarily depends on the size and the number of substituents at the ortho‐positions as mentioned earlier. In the case of biaryls with multiple substituents, rotation is restricted but atropisomerization can be achieved under certain conditions, i.e., “nondirectional” rotation of one of the aryl rings. One such process is thermal isomerization. Upon heating, biaryls can racemize via a sterically preferred nonplanar transition state where the ortho substituents can pass each other.XL by a C−H activationII catalyst.R,M)‐23 a]XL, according to the principle shown in Figure S,P)‐23 a, leading to a net clockwise 180° rotation. By switching the oxidation state of the Pd source from (II) to (0), the catalyst selectively undergoes addition to the C−Br bond and forms oxidative addition complex Pd[‐23 b]BrL. Next, another subsequent thermal helix inversion to the more stable diastereomer Pd[‐23 b]BrL proceeds. Reintroduction of bromide using N‐bromosuccinimide (NBS) “locks” the conformation to give the starting isomer ‐23 a, resulting in a directed net 360° rotation. It was also demonstrated that Pd0 can be directly regenerated from the remaining PdII reagent after the first half‐turn by using an excess amount of tricyclohexylphosphine.0/PdII redox cycle in combination with selective C−H and C−Br bond activation is an attractive approach towards the development of fully autonomous chemically powered molecular motor. In this study, the full 360° rotation cycle completes with an overall yield of 19 % over five steps. Clearly, there is still room for further improvement of the catalytic sequence, possibly by changing the directing group or the transformation methods of the catalyzed steps. This concept might be extended further towards an electric motor by the use of an electrochemical method to interconvert between the metal oxidation states.The next challenge in this field is to develop an autonomous molecular motor that does not require sequential addition of chemical fuels and that has the properties of a catalytic motor. Towards this goal, our group designed a molecular motor driven by switching of the oxidation states of a Pd catalyst Scheme .[96] I24 a can autonomously rotate about its C−N bond when fueled with carbodiimide ‐24′ and (+)‐24 to (+)‐24′; gray arrows Scheme 24 into the corresponding anhydride, 24′, using carbodiimide as fuel, producing urea as a waste (step 1). In the presence of chiral fuel ‐25, one atropisomer might react faster than the other ((−)‐24 reacts faster than (+)‐24 in a diastereoselective cyclization; Scheme 24′ would be configurationally labile in nature and should undergo a rapid atropisomerization ((−)‐24′⇌(+)‐24′; Scheme 24′ is then hydrolyzed using a chiral organocatalyst (R)‐26, giving rise to the diacid (+)‐24 through a dynamic kinetic resolution process, “locking” the conformation of the motor molecule. In the final step, thermal rotational isomerization of (+)‐24 can take place, completing the 360° rotation of the C−N bond and a motor cycle. Experimentally, the authors were not able to demonstrate that diacid 24 a is an autonomous directed motor, as claimed, because of the lack of experimental techniques available and free rotation around its C−N bond. However, they have elucidated the fundamental mechanism of each step during the rotational cycle with the analogous compounds 24 b and 24 c, as they exhibit high configurational stability at room temperature. For further investigation, they used (±)‐24 c as a model substrate for the continuous intramolecular anhydride formation and hydrolysis of the anhydride. In fact, anhydride formation using the chiral fuel ‐25 and its subsequent hydrolysis in the presence of the chiral organocatalyst (R)‐26 led to the enantioenriched compound 24 c with an e.r. of 71 : 29, meaning that there is a bias in the rotation direction. Finally, to test whether the carboxylic acid present in pyrrole rotator only passes through the substituent at the 6‐position of the aryl ring, the rotational barriers for 24 a–c were calculated. Gas‐phase DFT studies showed that the barrier for atropisomerization of carboxylic acids passing the X‐ substituents of the stator is much lower due to the steric hindrance between the two carboxylic acid groups. Moreover, the experimental fact that 24 c racemizes much more slowly than 24 b confirmed that racemization is caused by rotation of CO2H passing the ‐X substituent. This chemical cycle consumes more than 97 % of the fuel molecules, with directional bias up to 71 : 29 when chirality‐matched fuels and additives are used. During this autonomous catalytic cycle, the motor 24 acts as a catalyst for the hydrolysis of carbodiimide, which continues as long as the fuel molecules are present. The motor rotates approximately every three hours and makes a mistake in direction every three or four turns. Despite the fact that this system is still far from a perfect molecular ratchet motor, it addresses the important question of how to achieve autonomous and continuous rotary motion. The different approaches to chemically fueled unidirectional rotary motion have revealed the basic principles for further optimizations and novel designs that may result in genuine catalytic motors and stimulate future applications of the chemically driven molecular motors.As discussed earlier, molecular motors powered by chemical conversion could serve a variety of functions as molecular engines in nanotechnology, provided they operate autonomously, i.e., the rotor unit rotates 360° continuously as long as fuel is supplied. According to a recent study by Leigh and co‐workers, an arylpyrrole‐2,2′‐dicarboxylic acid derivative 6During three decades of combined efforts by researchers, several ingenious examples of chemically driven molecular switches and rotors based on atropisomers have been reported. They have all exploited chemical interactions such as hydrogen bonding, pH response, metal coordination, conformational change (rotation) due to bond formation and cleavage in dynamic systems. The introduction of responsive features allows molecular motion to be controlled by external chemical stimuli. Next, these fundamental studies laid the groundwork for the future development of molecular machines, and four examples of chemically driven functional rotatory molecular motors which demonstrate repetitive unidirectional 360° rotation have been successfully constructed to date. Although significant progress has been made in this field, and these proof‐of‐principle systems will spark novel designs, there are still major fundamental challenges to be addressed. Like biological motor proteins which are powered by ATP hydrolysis or a proton gradient, the ultimate goal of artificial chemically driven molecular machines is to develop a continuously and autonomously rotating motor as we discussed in the introduction section. Based on the strategies discussed in this Review, we provide two possible parallel routes for further investigation and future developments towards autonomous molecular motors.Unidirectional molecular rotors: One strategy is to modify the design of molecular rotors. Rotors undergo constant but random rotation due to the lack of directional bias in the movement of the components. Following the example of molecular motors, introduction of a chiral group in the close vicinity of the rotation axle of a rotor coupled to a chemical transformation to power the rotation is expected to provide full unidirectionality to molecular rotors.Continuous molecular rotatory motors: The other way is to make the motion of molecular motors continuous. Most of the current examples of molecular motors show unidirectionality in rotation, but multiple steps are necessary to achieve full rotation. By reducing the number of reaction steps as in Leigh's work or by using a catalytic  or redox approach  as shown in Figure b, autonomous motion might be accomplished.Specific key challenges are also listed in Figure Directionality: The major part of the work presented in this Review is dedicated to the acceleration of the rotor speed through a chemical modification. However, the rotational direction is not controlled. The design of alternative rotor structures where the nondirectional rotation of an existing rotor could be transformed into a unidirectional autonomous rotation, possibly using a chiral auxiliary or chiral catalyst  systems, is of key importance for future developments. Furthermore, the proof of principle used in the development of switches, rotors, and motors can provide a major source of inspiration for the development of novel designs of highly desired fully autonomously operating chemically driven rotatory molecular motors.The authors declare no conflict of interest.Anirban Mondal grew up in Kolkata, in the eastern part of India. He did his master's thesis at the Indian Institute of Technology Bombay, Mumbai, under the supervision of Prof. Debabrata Maiti and Rodney A. Fernandes. In 2016, he joined the group of Prof. Ben L. Feringa at the University of Groningen for his doctoral research. He recently completed his PhD focusing on various chirality transfer processes in the development of chemically driven molecular machines and asymmetric catalysis.Ryojun Toyoda obtained his PhD from the University of Tokyo (Japan) under the guidance of Prof. Hiroshi Nishihara in 2020. He then joined the group of Prof. Ben L. Feringa at University of Groningen as a postdoctoral researcher with support from a JSPS overseas fellowship. He worked on molecular machines and smart materials there. In 2022, he was appointed as an assistant professor at Tohoku University (Japan) where he is working on coordination chemistry, nanoscience, photofunctional molecules, molecular machines, and single‐molecule nanomaterials.Romain Costil studied Organic, Bio‐organic and Therapeutic Chemistry at the Ecole Nationale Supérieure de Chimie de Mulhouse (France), where he was awarded a Diplome d'ingénieur in 2014. He then joined the group of Prof. Jonathan Clayden at University of Bristol (UK) for a MSc project in foldamer chemistry, followed by PhD research on the synthesis and properties of a novel class of atropisomers. Following postdoctoral research in the Feringa lab working on novel molecular motors, he stayed as research manager in the same group.Ben L. Feringa obtained his PhD degree in 1978 at the University of Groningen in the Netherlands under the guidance of Prof. Hans Wynberg. After working as a research scientist at Shell he was appointed full professor at the University of Groningen in 1988 and named the distinguished Jacobus H. van't Hoff Professor of Molecular Sciences in 2004. He was elected foreign honorary member of the American Academy of Arts and Sciences and member of the Royal Netherlands Academy of Sciences. His research interests include stereochemistry, organic synthesis, asymmetric catalysis, molecular switches and motors, photopharmacology, self‐assembly and nanosystems.


These properties indicate that these molecular modeling structures as good candidates for utilization in organic DSSCs.In this manuscript, we report four series for 1,2,4-triazine derivatives as dye-sensitized solar cells (DSSCs). Density functional theory (DFT) methods via utilizing Becke's three-parameter functional and LeeeYangeParr functional (B3LYP) level with 6-31G  basis set to investigate their modeling molecular structures. Optimized molecular structures for studied molecular structures are obtained using the DFT/B3LYP/6-31G  method. In addition, the time-dependant density functional theory (TD-DFT) is used to study the optoelectronic properties and absorption spectra using DFT/CAM-B3LYP/ 6-31G +  +  level in the Gaussian 09 program. The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), energy gap (E Because of the great challenge used in new research on renewable energy sources, solar cells are considered one of the most important renewable energies recently –4. PhotoRecently, new organic compounds used in solar cells have been studied and developed. Among these materials are the dyes for sensitive solar cells (DSSCs), as they receive great interest among researchers due to their low cost and high efficiency in converting solar energy into electricity . MoreoveIn this manuscript, the results of quantum calculations for four organic compounds based on 1,2,4-triazine derivatives as dye-sensitized solar cells (DSSCs) are presented. In these structures, we used sulfur atom as electron donor unit (D) and the phenyl and triazine groups were used as electron acceptor groups (A) for all compounds.Time-Dependent DFT (TD-DFT) has been widely used to discuss the properties of organic compounds in their excited state because its high accuracy is reasonable with the ab-initio method and less computational time cost. In this paper, the obtained calculated results via utilizing TD-DFT/ CAM-B3LYP/ 6–31 +  + G  like the optoelectronic properties and photovoltaic properties (open-circuit voltage (Voc), oxidation potential energy, and electron injection force) of all molecular structures were investigated and presented.g) are obtained. The oscillator strengths (f) and the optical transitions have been illustrated using TD-DFT combined with CAM-B3LYP functional and 6- 31G +  +  basis set. Finally, the ultraviolet–visible (UV–Vis.) absorption spectra of the studied molecules were presented utilizing Gauss View software :1\d:1\d19]:1OC and JSC are the basis for producing photoelectric conversion efficiency. The maximum value for Voc is a significant photovoltaic parameter that can be obtained computationally via utilizing the difference between the HOMO of dye and the LUMO of the electron acceptor (conduction band of TiO2). The computational value of Voc has been calculated by utilizing the following (2) [From the formula, it can be seen that high Vwing (2) , 21:2\doWhilst, in DSSCs, Voc can be approximately obtained as the different energy among LUMO of the dye and conduction band (CB) of the semiconductor2). These values are positive; thus indicating that the electron transfer will be easy from all studied molecular structures  to TiO2. Furthermore, these values are sufficient to give the best efficient electron injection. Moreover, these triazine derivatives compounds can be utilized as sensitizers of the electron injection process from the excited dye to the conduction band of TiO2.The theoretically calculated value of Voc for all studied molecular structures is presented in Table Another parameter denoted (α) was calculated via the difference between the LUMO energy levels of the studied dyes and the LUMO energy level of PCBM (-3.2 eV) . The valf) [The light-harvesting efficiency (LHE) is obtained using the following equation (LHE = 1-10f) , where fAs presented in Table ex), oscillator strength (f), and the transition character of these dyes are listed in Table The optical properties and the electronic transition for the all studied dyes  are investigated using TD-DFT/ CAM-B3LYP / 6-31G +  +  method . The cal2-N4 π-bond to C1-N6 π-antibonding stabilized DDT molecular structure by 22.84 kcal/mol then from C3-N5 π-bond to C4-N38 π-antibonding via 32.31 kcal/mol. Also, the interaction between C20- C25 bond and C23-C24 π-antibonding is stabilized DDT molecule via 21.40 kcal/mol then from C20-C25 π-bond to C21-C22 π-antibonding that contributed 19.81 kcal/mol respectively as recorded in Table 7 to π*C3—N5 in DDT that assigned by 21.31 Kal/mol. The delocalization of MO between C1-C6 π-bond to C2-C3 and C4-C5 π-antibonding then from C4-C5 π-bond to C1-C6 π-antibonding is stabilized BDTTMB molecular structure via 21.01, 19.70 and 20.54 kcal/mol respectively. Also, the interactions among the following;  are stabilized BDTTB by 21.03, 19.46, 30.86, 31.96, 19.60 and 27.17 kcal/mol respectively. Finally, the stability of BDTTMP molecular structure due to the interaction between π-bond and π-antibonding; the values of interactions are listed in Table Since resonance is a fundamental phenomenon in the stability of the organic chemical structure, it was essential to study the stability of the studied molecular chemical structures  using Natural Bond Orbital (NBO) analysis. The hyperconjugation interactions in  molecular structures are examined via NBO analysis theory utilizing the B3LYP/6-31Glevel of theory . BesidesA molecular electrostatic potential map (MEPM) investigates reveals the electrophilic versus nucleophilic sites of all molecular structures under study, giving for the prediction of reaction areas . The MEP3H2O moiety: the titanium atom is an octahedral complex with two positions taken up by the oxygen atoms of the acrylic acid anchor [3H2O. The DTT/ Ti(OH)3H2O optimized structure, HOMO and LUMO MOs, and energy gap (Eg1) between HOMO and LUMO were obtained using DFT/B3LYP/LANL2DZ method and the calculated results are shown in Fig. 3H2O through a coordinated bond as shown in Fig. 3H2O complex. The fact that it is a charge-transfer state is again evident. Electrons are moving from the DTT benzene end to the other benzene end of the DTT molecule as shown in Fig. we continue our study of a prototypical dye molecule for DSSC devices from Fig. d anchor , three hIn this paper, the optimized molecular structure, and electronic and optical properties of four triazine derivative dyes DTT, BDTTB, BDTTMB, and BDTTMP were investigated via utilizing DFT/TD-DFT. According to the ground state geometry, we note that all stable conformations are not planar. The HOMO/LUMO energy gaps of DTT, BDTTB, BDTTMB and BDTTMP were calculated at DFT/B3LYP/6-31G  level are 4.168, 3.528, 3.965 and 3.921 eV respectively. So, the energy gaps differ slightly and decrease in the following order: BDTTB < BDTTMP < BDTTMB < DTT. Consequently, the calculated values of Voc/ TiO2 of our dyes are sufficient for possible efficient electron injection from the donor to the acceptor.


Anabaena sp. PCC 7120 and analyzed the seven candidates with predicted OprB-domain. Single recombinant mutants of the seven genes were created and their growth capacity under different conditions was analyzed. Most of the putative porins seem to be involved in the transport of salt and copper, as respective mutants were resistant to elevated concentrations of these substances. In turn, only the mutant of alr2231 was less sensitive to elevated zinc concentrations, while mutants of alr0834, alr4741 and all4499 were resistant to high manganese concentrations. Notably the mutant of alr4550 shows a high sensitivity against harmful compounds, which is indicative for a function related to the maintenance of outer membrane integrity. Moreover, the mutant of all5191 exhibited a phenotype which suggests either a higher nitrate demand or an inefficient nitrogen fixation. The dependency of porin membrane insertion on Omp85 proteins was tested exemplarily for Alr4550, and an enhanced aggregation of Alr4550 was observed in two omp85 mutants. The comparative analysis of porin mutants suggests that the proteins in parts perform distinct functions related to envelope integrity and solute uptake.Porins are essential for the viability of Gram-negative bacteria. They ensure the uptake of nutrients, can be involved in the maintenance of outer membrane integrity and define the antibiotic or drug resistance of organisms. The function and structure of porins in proteobacteria is well described, while their function in photoautotrophic cyanobacteria has not been systematically explored. We compared the domain architecture of nine putative porins in the filamentous cyanobacterium  Anabaena sp. strain PCC 7120 (Anabaena sp.) is approximately 14 nm thick, and the distance between PM and OM is about 45 nm , 7418, 7n source . Apparenild type . An alteAnabaena is rather dependent on functional TBDTs instead of porins [Synechocystis sp. are found in many freshwater and marine cyanobacteria species [Anabaena sp.With respect to the uptake of divalent ions, a relation to iron uptake could not be established in this study. Ferric iron ions are prevalently complexed to organic ligands like siderophores . Howeverf porins , 76. It f porins , 78. Mor species . This geAnabaena sp. could be made, as mutation of porins did not lead to a resistance against elevated concentrations (Anabaena sp. in form of cobalamin. This uptake is dependent on the BtuB proteins of the TBDT-family, which are predicted to exist in Anabaena sp. as well [Anabaena sp. [alr2231 shows a high resistance to elevated levels of zinc (2 to bicarbonate, which is important for the regulation of the pyruvate conversion to oxaloacetate and thus central for the cyanobacterial metabolism.Also for cobalt uptake, no relation to a porin in trations . Cobalt  as well . Similaraena sp. . This isaena sp. , 82. How of zinc , which malr0834, AFS-I-alr4741 and AFS-I-all4499 (alr0834 and alr4741 are upregulated in the absence of manganese, while all4499 belongs to the three most abundant transcripts among the porins (The uptake of other divalent metals like manganese and copper is discussed to depend on porin-like proteins in Gram-negative bacteria –88. The -all4499 . Interese porins . No signe porins , this sue.g. for respiration and photosynthesis. Consistent with reports for other bacteria [Anabaena sp. the TBDT IacT was identified which might be involved in copper uptake under highly limiting conditions [Copper is a globally important co-factor, bacteria , 88 mostnditions .tic22-mutant, but not the three omp85 mutants exhibits an elevated resistance towards high copper concentrations (alr4893 showed the highest accordance to the phenotypes of most porin mutants (except to that of AFS-I-alr4550, Interestingly, only the trations . Omp85 palr2269 and AFS-I-alr4893 the accumulation of the denatured form and a high abundance of aggregated Alr4550 were observed. Based on these results two interpretations are possible, which need to be explored in future. One could suggest that both Alr2269 and Alr4893 are involved in membrane insertion of porins, which would explain the rather moderate correlation between the phenotypes of porin mutants and omp85 mutants. Alternatively, the data would also be consistent with a functional diversification of the two Omp85 proteins. By this means Alr2269 could be specialized on the insertion of larger OM-proteins such as members of the Omp85 family, LptD or the TBDTs. On the other hand, Alr4893 could be specifically inserting porins into the OM. This could explain the observed diversity in the C-terminal amino acid composition between TBDTs and porins (alr4893, but not AFS-I-alr2269. The occurrence of the aggregated porin form in AFS-I-alr2269 would be the consequence of a reduced insertion of Alr4893. Both explanations suggest that Alr0075 only plays a minor role in porin insertion. Thus, Tic22 might globally act as periplasmic transfer protein including porins as substrate.Moreover, in AFS-I-d porins  and the alr4550 exhibited a unique phenotype by sowing hypersensitivity towards metals, salt and harmful compounds. Also the strong transcription of alr4550 suggests a crucial function of the protein, which apparently involves the maintenance of envelope integrity. First associations between single porin candidates and manganese, zinc or copper transport as well as diazotrophic growth capacity could be drawn, thus these connections need to be further examined. The density gradient centrifugation experiments showed that the three omp85 mutants in Anabaena sp. are affected in the migration of Alr2269 and Alr4550, but to different extents. The presence of aggregated intermediates of Alr4550 in the two omp85 mutants AFS-I-alr2269 and AFS-I-alr4893, but not in AFS-I-alr0075, suggests a functional specification on certain substrates for membrane insertion.Our results imply diverse functions of single porin-like proteins, reflected by the specific phenotypes of the mutants. Most prominently AFS-I-http://jmb.or.kr.Supplementary data for this paper are available on-line only at


Conspiracy theories have seen a rise in popularity in recent years. Spreading quickly through social media, their disruptive effect can lead to a biased public view on policy decisions and events. We present a novel approach for LDA-pre-processing called Iterative Filtering to study such phenomena based on Twitter data. In combination with Hashtag Pooling as an additional pre-processing step, we are able to achieve a coherent framing of the discussion and topics of interest, despite of the inherent noisiness and sparseness of Twitter data. Our novel approach enables researchers to gain detailed insights into discourses of interest on Twitter, allowing them to identify tweets iteratively that are related to an investigated topic of interest. As an application, we study the dynamics of conspiracy-related topics on US Twitter during the last four months of 2020, which were dominated by the US-Presidential Elections and Covid-19. We monitor the public discourse in the USA with geo-spatial Twitter data to identify conspiracy-related contents by estimating Latent Dirichlet Allocation (LDA) Topic Models. We find that in this period, usual conspiracy-related topics played a marginal role in comparison with dominating topics, such as the US-Presidential Elections or the general discussions about Covid-19. The main conspiracy theories in this period were the ones linked to “Election Fraud” and the “Covid-19-hoax.” Conspiracy-related keywords tended to appear together with Trump-related words and words related to his presidential campaign. The year 2020 was dominated by the Covid-19 pandemic. Facing the pandemic demands a competent administration and public support for its decisions. However, public decisions are taken under substantial uncertainty: knowledge about the virus and scientific recommendations to stop the spread of the Covid-19 disease are constantly being updated. Previous policy recommendations may become refuted by new insights and developments. This may lead to an erosion of confidence in scientific and political authorities. All these developments are a fertile ground for conspiracy theories to thrive and disrupt the public discourse . Public We investigate the public discourse on Twitter which has become a major tool of information acquisition in the recent years . DespiteOur Iterative Filtering framework allows us to descriptively explore which conspiracy theories exist on Twitter and how much attention they generate, either by their proponents, or their opponents. However, our framework cannot clearly identify misinformation attempts, only the public engagement with these phenomena.The paper is structured as follows. Sect. Analyzing topical content on Twitter has been the focus of numerous studies, many of which combine LDA and Sentiment Analysis to analyze Twitter Data and text data in general. Our proposed approach has the potential to improve such analyses in the future. Our work is especially closely related to the literature that uses different pre-processing approaches . Our coSeveral applications focus on analyzing Twitter data by LDA Topic Modeling to answer questions of socio-political significance , 17, 32.However, the topics produced by the models mentioned above seem to lack topical coherence and distinguishability in most cases, even though some models were fed with between two million  and overOur use case and technical analysis of Twitter data is closely related to the approach proposed by . Their sIn a further subset of the literature, filtering the dataset for Covid-19 related tweets becomes a crucial step. [1As mentioned before, the literature on the topics of misinformation detection on social media using topic modeling and sentiment analysis is very diverse. Further useful information can be found in  Before applying Hashtag Pooling on the text data, we implement conventional preprocessing steps. These include the removal of links, mentions, symbols and stop words. Additionally, all tokens were preprocessed as bigrams. These steps are well established and are standardized measures of data preparation . The fulqanon, wwg1wga, pizzagate, chemtrails, earthisflat, lizardpeople, newworldorder, covid19, virushoax, trumprussia, as well as fakenews. This choice aims to capture the “classical” conspiracy theories [Furthermore, we introduce a novel technique that we call Iterative Filtering. When analyzing Twitter data for a specific parent topic like “Conspiracy Theories,” we need an effective filter technique to remove unrelated tweets and at the same time include as many related tweets as possible. We start with defining a parent topic of interested, which is the discussion about conspiracy theories in the application of this paper. To do this, we search manually for relevant keywords in samples of Twitter data that are related to the parent topic. This results in a small dictionary of 11 filter terms: To increase our list of filter terms and thereby include more tweets into our analysis, we perform Iterative Filtering. For this we fit an LDA prematurely (after applying the pre-processing steps described above) to the dataset filtered with our initial few keywords. We explore the resulting topics for “new” words that serve as good filter terms and add them to the filter dictionary. The decision on which words to add is based on a subjective evaluation of the words’ importance and connection to the targeted theme . We filter our original tweet database again with the increased filtering dictionary and thereby obtain a larger dataset of filtered tweets, on which we again perform a premature LDA analysis to find new filter keywords. Note that the filtering is conducted on the unpooled  tweets. The pooling is performed as part of the pre-processing of the LDA estimation in each iteration.The three steps of filtering the database, fitting an LDA, and exploring and adding new filter terms are repeated until no new and relevant filter terms appear in the resulting estimated topics. Note that in every iteration round the same initial dataset is used to apply the filter based on the updated dictionary of keywords.nye, which stands for “new years eve.” This word occurred in some of the relevant topics in a few estimations, due to an increased usage around the end of 2020. We therefore put it on the blacklist due to its uninformative nature for our targeted topics. The blacklist is therefore essentially a means to clean the topics and to increase in the topics’ coherence. The interpretation of topics by humans is the subjective part in topic modeling. Similarly, the selection of keywords from the estimated topics as part of the updating step of the filter dictionary requires human interpretation and is therefore subjective. However, the benefit of this subjective element in our framework is that it allows the estimation of very specific topics. The algorithm is visualized on the left side of Figure  In a last step, we build a blacklist-dictionary to identify uninformative words in smaller topics of the LDA. For the final estimation round, we remove all tweets that contain uninformative words, defined in the blacklist dictionary. By this, we filter out those tweets that are uninformative even though they contain words from the Iterative Filtering steps, for instance, because they are using the keywords in a different context as the targeted themes or they are occurring temporally manner. An example are tweets that use the blacklist keyword In our overall pipeline  or to look up some tweets that use this specific term to get a feeling of the context it is used in. Rather than relying on a pre-specified threshold, we prefer to choose the words from the overall context of the topic of interest. For a tweet to become part of the data to analyze, a word that has been chosen in the Iterative Filtering process just has to occur once in a tweet. This tweet is then added to the dataset. This approach obviously does neglect tweets where typos in the keywords occur. Additionally, regarding the robustness of our approach, we want to point out the path-dependency of the framework: The subjective choice of which keywords are added to the filtering list determines the inclusion of the tweets in every following iteration. Therefore, it is important that the users of our framework critically reflect on keyword selection in every iteration step. Our approach produces a biased sample of the main dataset to identify highly specific and coherent topics of interest. These topics are not identifiable in the full dataset, because the LDA model is not able to discriminate between marginal topics when no filtering is applied.To show the capabilities and effectiveness of our Iterative Filtering framework, we included an example of the results of the LDA topic distributions, on which we only have used the initially selected conspiracy- and Covid-related filter keywords that were mentioned above. Figure   corpus” . By contfound in . Withoutentclass1pt{minimaFor Sentiment Analysis, we use the rule-based tool VADER , which wThe data collection and implementation of our analysis are visualized on the right side of Fig. The final dictionaries can be found in Figs. For each of the two datasets, a twenty-topic LDA model is estimated, covering the whole time period from September to December 2020. Additionally, grouping the tweets by month, we estimate four eight-topic LDA models for the larger, and four ten-topic LDA models for the smaller dataset. This results in a total of ten LDA Topic Models. The visualizations are implemented using the Python package PyLDAvis .Based on these ten models, we conduct an analysis on the spatial and sentiment level to find out more about the geographic distribution and sentiment content of conspiracy theory-related tweets in the USA. We hope to find indications that tweets with a high prevalence of conspiracy-related topics differ from non-conspiracy-related topics sentiment-wise.For the Sentiment Analysis, we consider the scores for the positive and negative sentiment categories and select tweets with sentiment scores that are larger than zero. By that we exclude tweets that receive a score value of one in the neutral category. The sentiment visualizations are based on the sentiment score. For example, if a tweet has a negative score of 0.7 and positive score of 0.3, both sentiment scores are visualized in the respective graphs for positive and negative sentiment.As a final note,  point ouThe Results Section is structured as follows. In Subsection traitortrump_trumptreason, obamagate) and, in general, are more critical toward Trump at the first sight. The tweets also directly target the president, accusing him of treason for example. The Covid-19-related topics dominate the results. Topic 5 (39.1%) is the main topic discussing the pandemic and is also the largest one in terms of the number of tokens for this LDA model. It contains almost exclusively the most important Covid-19-related keywords like pandemic, cases, healthcare and deaths. Topic 2 (17.4%) is another Covid-19-related topic, where people discuss social distancing, promote wearing masks, and advise on how to cope with the pandemic. The model also produces residual topics like topics number 3, 6 and 8 . Nevertheless, in comparison with the results of the LDA analysis in Fig. The plots in Fig.  Topic 2 .4% is anfakenews, msm, whereshunter) which is a Trump-supportive topic. Topic 3 is intersecting topic 2 and topic 1 by building a “bridge” between the Covid-19 pandemic and the election. It mainly covers the event of Donald Trump being infected with Covid-19. We find no conspiracy-related keywords among the top words (except for hoax in topic 3) for the topics of interest.The LDA model covering the tweets for October  and 10 (16.7%) are large conspiracy-related topics, discussing election fraud already well before the election, and also contain media critique in form of a “Fake News” discussion. Two other very interesting topics are topic 5 (5.3%) and 7 (7.9%). Topic 5 is a Q-Anon related topic, discussing “The Great Awakening,” while topic 7 perfectly represents the discussion about Covid-19 as planned or non-existing, also influenced by the Q-Anon group. As seen in the previous section, we find that the “Plandemic” conspiracy theory is rather marginalized in relation to all Covid-19 related discussions, but plays an important role in the context of all conspiracy-related topics. Finally, topic 8 (1.7%) shows that other well-known “classical” conspiracy theories like the Illuminati play a marginal role within the context of conspiracy theories in this time period.In September  that led to scenes like the ones seen at the Capitol a few weeks later. We find a small “New World Order” topic , another hint that “classical” conspiracy theories play a marginal role in this time.In November  and a much larger topic discussing resistance to election fraud . All accumulated, we still have over 80% election related conspiracy topics in December. Nevertheless, there are two more topics noteworthy, a topic about Covid-19 being a planned pandemic as we also have seen it in November  and a very small topic that describes the vaccine as part of the plan for a “New World Order” , building upon a “classical” conspiracy theory.Finally, in December . The associated time-series plot in Figure Notably, the LDA distinguishes between two different election fraud topics, namely topics 5 (6.5%) and topic 10 (9.2%). The main difference is that topic 10 includes calls for active resistance against the election results, whereas topic 5 appears more moderate. The time-series in Figure The biggest topic 12 (20.7%) is about “Fake News” and contains both sides of a controversial discussion. There are mainly tweets criticizing “Main Stream Media” (MSM) appearing in topic 12. Tweets with opposing opinions can also be found, but less frequently. These tweets criticize the inflationary use of the term “Fake News.” They also claim that “MSM”-critics are brainwashed by conspiracy theories. Figure Figure standwithsophie. The clearly conspiracy-related terms qanon and savethechildren hashtags are also found in the word distribution of topic 7. This might be a strategy to mobilize Twitter users via hard-to-oppose topics such as child protection and thus increase the reach of other conspiracy theories as well.Topic 7 (3.8%) is located in the PCA representation Fig.  somewhatThis Section evaluates if conspiracy-related topics differ sentiment-wise from other topics and how they are spatially distributed.Figure Figures In conclusion, our findings suggest that tweets with a high prevalence of conspiracy-related topics tend to be more negative in terms of their sentiment than tweets with a high prevalence of non-conspiracy-related topics. Nevertheless, this hypothesis could be further validated by additional research conducted on a larger dataset than ours.The results of the Iterative Filtering show a substantially improved coherence within the generated topics. The topic models perform well in terms of coherence and produce only some residual topics which can be clearly differentiated from the topics of interest. Most of the non-related tweets are grouped in “residual topics.” This is clearly different from the results produced by only using the initial filter keywords, in Figure In general, we saw that many conspiracy-related topics are marginal in comparison with other mainly discussed topics, like the US-Presidential Elections or general discussions about Covid-19. The main conspiracy theories present within the public discourse are related to the general focus of the discussion: they are dominated by “Election Fraud” in November and December and by the “Covid-19-hoax” theory in general. The conspiracy-related keywords are often grouped together with Trump-related words and words related to his presidential campaign. This implies that users who are utilizing conspiracy-related hashtags are often discussing Trump-related topics. Our framework can be utilized as a basis for further analyses of the topics of interest as we have shown with our brief geo-spatial Sentiment Analysis. We find that nation-wide conspiracy-related topics tend to be less positive and more negative sentiment-wise in comparison with topics focusing on Covid-19-related discussions in general.With our Iterative Filtering method, we are able to generate Twitter datasets that can be very well processed by LDA Topic Models. Thereby, we are able to gain insights into discussions of interest on Twitter and similar platforms: on the one hand, our method allows us to zoom into small sub-topics (like conspiracy-related topics) while also providing an overview of the broader context of the discussion (like election and Covid-19-related topics) over time. Hence, sub-topics, which are in general hard to identify due to small sample sizes in an overall larger dataset, are much more accessible within our framework. They also become much more coherent and therefore interpretable. Thereby we provide researchers a tool for further research on marginalized topics.