conjugated polymers 
 have been studied extensively because of their 
 semiconducting and optical properties , which offer great potential 
 for a diverse range of applications such 
 as organic light - emitting diodes , field - effect 
 transistors , and polymer solar cells . recently , macrocycles have attracted increasing attention as quasi - infinite 
 -conjugated systems exhibiting unique electronic and optical 
 behavior . several studies have investigated 
 electron or energy transfer , nonlinear optical phenomena , and topological 
 effects in these structures . a promising approach to synthesis of nanosized fully -conjugated 
 macrocycles based on porphyrin units however , the difficulty of synthesizing 
 suitable templates has greatly limited the size of macrocycles accessible . 
 to this end , vernier effects were exploited to direct synthesis of 
 large nanorings using multiple smaller templates . vernier templating not only proves to be a powerful new 
 strategy to construct large nanorings , but also renders shape - persistent 
 structures with diverse geometries . exciton 
 delocalization and emission depolarization processes are expected 
 to be strongly affected by the change of geometry in the structures . conformational effects on photophysical 
 properties have been examined 
 extensively using both experimental and theoretical 
 methods on a variety of conjugated polymer 
 systems , such as ppv , p3ht , and dendrimers . in particular , effects 
 on polarization memory loss and energy transfer have attracted much attention . at the center of our study , 
 we address the question of how deliberate 
 distortion of a molecular nanoring influences the emissive properties 
 of a molecule following excitation . we examine to what extent out - of - plane 
 distortions of a particular nanoring topology affect the polarization 
 memory of the transition dipole moment and whether such effects can 
 be predicted by molecular mechanics simulations . in search for answers 
 to these questions , a range of conjugated zinc porphyrin nanorings 
 of similar size but different conformations are examined using ultrafast 
 time - resolved spectroscopy and the results are compared to molecular 
 dynamics simulations of ring structures . the synthesis and characterization of the 
 nanoring templates and complexes are described in detail elsewhere . figure 1 introduces the porphyrin nanorings under investigation , 
 which comprise porphyrin units joined by butadiyne bridges . through 
 insertion of templates binding to the porphyrin units , a range of 
 different molecular shapes are created with spatial extent on the 
 nanometer scale . untemplated nanorings c - p10 and c - p12 only differ slightly in 
 ring diameter and have almost identical conformation and absorption / emission 
 spectra ( further information in the supporting 
 information ( si ) ) . both samples are prepared in toluene/1% 
 pyridine solution to prevent aggregation . templating changes the geometry 
 considerably by rigidifying the nanoring into two smaller porphyrin 
  loops  and introducing significant out - of - plane distortions , 
 such as bending and twisting . molecular mechanics simulations ( vide 
 infra ) show that c - p10(t5)2 and c - p10(t6)2 have similar structures , 
 with the main difference being in the shape of templates as shown 
 in figure 1b , c . the angle between the mean 
 template planes in c - p10(t5)2 is calculated to be  
 = 72. the twist in c - p12(t6)2 is even more pronounced and 
 has the form of a figure - of - eight , with  = 28 , as shown 
 from crystal structure data in a previous study . on the other hand , these conformations have been deduced 
 from molecular mechanics and dynamics simulation using hyperchem , which will be described later in detail . samples with template 
 complexes were prepared in pure toluene solution , as the templates 
 would otherwise be displaced by competition with pyridine . chemical structures 
 of the budadiyne - linked zinc porphyrin nanoring 
 assemblies under investigation : ( a ) c - p10 , ( b ) c - p10(t5)2 , ( c ) c - p10(t6)2 , ( d ) c - p12 , ( e ) c - p12(t6)2 , and ( f ) c - p12(t8)2 . all compounds have octyloxy 
 side chains ( ar ) which are omitted in the graphic for clarity . the photoluminescence 
 ( pl ) upconversion technique was engaged to investigate pl dynamics 
 of sample solutions held in quartz cuvettes as described in detail 
 elsewhere . an excitation pulse was generated 
 by a mode - locked ti : sapphire laser with pulse duration of 100 fs and 
 a repetition rate of 80 mhz . pl is collected and optically gated in 
 a beta - barium - oxide ( bbo ) crystal by a vertically polarized time - delayed 
 gate beam . the upconverted signal , which consists of sum - frequency 
 photons from the gate pulse and the vertical component of the pl , 
 was collected , dispersed in a monochromator , and detected using a 
 nitrogen - cooled ccd . using a combination of a half - wave plate and 
 a glan - thompson polarizer , the polarization of the excitation pulse 
 was varied and pl intensity dynamics were recorded separately for 
 components polarized parallel ( i ) and perpendicular ( i ) to the 
 excitation pulse polarization . the pl anisotropy is defined using 
  = ( i  i ) /(i + 2i ) and calculated from the 
 measured components . the full - width - half - maximum ( fwhm ) of the instrumental 
 response function ( irf ) was 
 measured to be 270 fs , which gives the time - resolution limit 
 of the system ( further details in the si ) . by recording the irf with both polarizations , a temporal shift 
 of 15 fs between i and iwas found , and thus , the calculation 
 of (t ) was adjusted accordingly ( further information 
 in the si ) . to investigate pl decay dynamics at longer delay time after 
 excitation ( > 1 ns ) , electronic gating through time - correlated single - photon 
 counting ( tcspc ) technique was explored , using a becker & hickl 
 module . here , emission was detected with a silicon single - photon avalanche 
 diode , yielding a temporal resolution of around 40 ps . by fitting 
 the experimental data to a single exponential decay model ipl steady - state absorption and time - integrated pl spectra at room 
 temperature were recorded using a perkinelmer lambda 1050 uv / vis / nir 
 spectrometer and a horiba fluorolog fluorimeter , respectively . in order to gain basic 
 insight about the photophysical and electronic properties of the nanorings , 
 steady state absorption and emission spectroscopy was performed . the 
 resulting spectra for c - p12 and c - p12(t6)2 are compared in figure 2 ; spectra for 
 further structures are provided in the si . zinc porphyrin monomers exhibit a strong s0  
 s2 transition at 400 nm ( soret band ) and a weaker 
 s0  s1 transition at 550 nm 
 ( q - band ) . for the nanorings , -conjugation 
 extends through the butadiyne bridges , lifting the degeneracy in both 
 the lowest - energy q band and soret band of the porphyrin 
 monomers . transitions polarized parallel ( qx ) and perpendicular ( qy ) to the acetylenic backbone can therefore be observed 
 ( see figure 2 ) . when the nanorings bind to 
 the templates , qx is 
 observed to be red - shifted with respect to the soret band , while qy shows no considerable change . in addition , a significant sharpening of features in both emission 
 and absorption spectra can be observed for the templated complexes . templates bring rigidity to the system ; the molecules are less prone 
 to interporphyrin torsional motions and the molecular backbone may 
 be planarized , resulting in the observed 
 red shift of qx . the 
 four templated nanorings hence exhibit similar spectral features ( see 
 also further spectra in the si ) . normalized 
 steady - state absorption ( blue lines ) and time - integrated 
 photoluminescence ( green lines ) spectra at 295 k for ( a ) c - p12 in toluene/1% pyridine and ( b ) c - p12(t6)2 in toluene solution . the emission spectra were recorded after excitation 
 at 520 nm ( into the soret band ) . pl upconversion under ambient condition was performed to reveal 
 the extent of emission depolarization following photoexcitation with 
 a linearly polarized laser pulse . the templated nanorings were excited 
 at 820 nm and the untemplated rings at 760 nm , with emission being 
 monitored at the peak wavelength for each nanoring . to avoid pl quenching 
 via exciton figure 3 depicts the time - dependent pl 
 of c - p12 and c - p12(t6)2 over 
 the first 15 ps after excitation with a pulse polarized either parallel 
 ( i ) or perpendicular ( i ) to the polarization of the detected 
 emission . for c - p12 , the pl intensity remains constant over the experiment 
 time window after the onset of the signal at t = 
 0 , whereas c - p12(t6)2 shows a slight decaying trend , in agreement 
 with the shorter lifetime obtained using tcspc ( shown in table 1 ) . time - resolved pl dynamics of ( a ) c - p12 in toluene/1% pyridine ( concentration 0.2 mm ) and ( b ) c - p12(t6)2 in toluene solution ( concentration 0.4 mm ) . samples were excited 
 by excitation pulse polarized either parallel ( blue dots ) or perpendicular 
 ( red dots ) to the detection polarization , as illustrated in the inset . selective excitation near the qx band of a rigid butadiyne - bridged 
 porphyrin oligomer is expected 
 to result in an excited state with a transition dipole moment along 
 the molecular backbone . in the absence 
 of any depolarization mechanisms , emission will occur in the same 
 polarization direction as the absorption , and for a randomly oriented 
 distribution of such molecules in solution , an initial anisotropy 
 value of 0.4 is expected . however , for a complete 
 depolarization of the transition dipole moment within the 2d plane , 
 as may be the case for the untemplated rings , a pl anisotropy value 
 of 0.1 should be found . other effects , such 
 as out - of - plane distortion , could further lower the value toward zero . figure 4 shows 
 the temporal dependence of 
 the anisotropy (t ) for a number of the investigated 
 nanoring topologies . nanoring topologies 
 investigated , the anisotropy is found to be static over the first 
 20 ps after excitation , within the temporal resolution of 270 fs . it is possible that 
 excitation near the band edge ( 900 nm ) may lead to creation 
 of localized states that have higher anisotropy and/or slower pl depolarization ; however , we are unable to probe such effects 
 here . these transients therefore indicate that while an initial ultrafast 
 depolarization process may occur within the first hundred femtoseconds 
 after excitation , slower effects such as molecular reorientation are 
 absent over these time scales . the values 
 extracted in this way for the initial pl anisotropy  are listed 
 in table 1 for all molecules investigated . these values have been independently determined multiple times at 
 5 ps after excitation , and an overall accuracy of 0.02 was achieved . the untemplated rings c - p10 and c - p12 display anisotropy values around 0.1 , 
 as expected for a complete memory loss in the 2d plane and in the 
 absence of significant out - of - plane distortions of the rings . for all templated rings on the other hand , a 
 significant reduction of  below the value of 0.1 was observed . these results suggest that the excitation can access any segment 
 on the rings , both templated and untemplated , within the first few 
 hundred femtoseconds after excitation . such dynamics may be accounted 
 for by either of two different mechanisms . in the first possible scenario , 
 excitons are initially fully delocalized over the entire ring in the 
 absorbing state , but may subsequently self - localize 
 following geometric relaxation . in the second scenario , the absorbing state is only delocalized 
 over a subset of monomers , but excitons are subject to ultrafast energy 
 migration along the ring within hundreds of femtoseconds and thus 
 beyond the time - resolution of the upconversion system , similar to 
 observations reported in lh2 antenna complexes . it has recently been shown that as the size of untemplated 
 porphyrin nanorings increases , a shift from the former to the latter 
 scenario may gradually occur . we find that 
 the measured anisotropy values below 0.1 for the templated 
 nanorings can be explained by the presence of significant out - of - plane 
 distortions . molecular dynamics simulations discussed in detail below 
 show that the measured  is closer to zero for nanoring topologies 
 with larger distortions . as illustrated in figure 4 , the rigid planar c - p12(t8)2 has an appreciably higher anisotropy 
 (  = 0.07 ) than the other three templated nanorings which are 
 found to display significant out - of - plane distortions . pl polarization anisotropy 
 as a function of time after excitation 
 at 820 nm for c - p12 ( blue circles , detection 
 at 872 nm ) , c - p12(t8)2 ( red triangles , detection at 904 nm ) , and c - p12(t6)2 ( green squares , detection at 914 nm ) . the structures shown 
 on the right are energy minimized geometries calculated using a modified 
 form of mm+ force field in hyperchem . side chains 
 are not shown in the diagram but were included in the simulations . to quantitatively examine the 
 distortions present in the different 
 nanoring topologies , molecular dynamics simulations were conducted 
 for the templated nanorings with side chains using hyperchem . the starting configurations were energy - minimized geometries ( at 
 0 k ) calculated using a modified form of hyperchem s mm+ force field . additional bond stretch , angle bend , and 
 torsion terms were added to describe metalloporphyrins , alkynes , and butadiynes . molecular dynamics simulation were conducted 
 assuming ambient vacuum at two constant temperatures , 300 and 600 
 k. simulations ran in time steps of 1 fs for a time window of 500 
 ps , which gave a representative average . since the templates are fairly 
 rigid , it can be assumed that distortions mainly originate from motions 
 between two template planes where two fundamental motions can be distinguished : 
 twisting and bending , as illustrated in figure 5a , b for the case of 300 k. corresponding results for 600 k are shown 
 in the si . the corresponding angles  
 and  are introduced in the figure to quantitatively describe 
 the deviation of the structure from a planar conformation . all starting 
 configurations hold (0 )  0. the distortion angles 
 of the conformations in the 500 ps simulation window are plotted as 
 a histogram , and the results are summarized in figure 5c  f , with the root - mean - square ( rms ) values listed 
 in table 1 . due to the nature of the simulations 
 and the ambiguity in defining the distortion angles it is remarkable 
 that , compared to the bending angle  , the spread in the twisting 
 angle  is generally much narrower and the rms remains closer 
 to the energetically optimized values , which indicates that , in the 
 molecular dynamics , thermal energy has a greater influence on the 
 bending motion than the twisting motion . c - p10(t5)2 and c - p10(t6)2 show large out - of - plane distortions in twisting ( around 65 ) 
 as well as bending motion at 300 k , consistent with the experimentally 
 observed low pl anisotropy values of  = 0.05 . c - p12(t6)2 and c - p12(t8)2 both show similar distributions in  around 
 25 , much lower than that for the templated c - p10 topologies in comparison and in good agreement 
 with structures obtained from crytallographic investigations . these simulations explain the only slight decrease 
 in  from 0.1 to 0.07 experimentally observed for c - p12(t8)2 . however , c - p12(t6)2 has a much lower pl anisotropy , which can 
 be accounted for by its large bending angle similar to c - p10(t5)2 and c - p10(t6)2 . in contrast , c - p12(t8)2 remains rigid :  
 is narrowly distributed around the planar starting conformation . in summary , the molecular dynamics simulations reveal that the 
 templates provide an effective means to control the nanoring structure , 
 as can be seen in the narrow distribution in figure 5c  f as well as in the close resemblance between rms 
 values at 300 k and the energetically optimized values at 0 k for 
 all nanorings . furthermore , a good correspondence is evident between 
 simulated conformations and measured pl anisotropy : the more pronounced 
 the out - of - plane distortions in both twisting and bending motions , 
 the further the anisotropy is found to be lowered from 0.1 . using a simplified model based 
 on the results from hyperchem as described in the 
 main text , simulated anisotropy values s are given . to link the value experimentally 
 obtained for the pl anisotropy 
 with the calculated molecular structures , simple simulations were 
 performed using a distributed point - dipole model . the porphyrin units in each nanoring are 
 assumed to be arranged in two smaller circles around each template 
 as a result of the template s rigidity , with their transition 
 dipole moments aligned along the molecular backbone , as shown in the 
 schematic in the si . the angle between 
 the two template planes in the calculation is taken to be the rms 
 values simulated by hyperchem at 300 k. corresponding 
 values for 600 k are given in the si and 
 exhibit similar trends . for simplicity , the emitting and absorbing 
 states are assumed to extend over one monomer unit . this approximation 
 seems sensible as the pl depolarization dynamics are observed to be 
 sufficiently fast ( < 300 fs ) to suggest that all sites are visited 
 with similar probability . therefore , the calculated average based 
 on contributions from individual monomers will yield representative 
 results , independent of the extent of exciton wave function delocalization . the polarization anisotropy is then calculated using1with the only parameter being  , the 
 angle between absorbing and emitting dipole moments . excitation can 
 access any segment of a nanoring , and so each monomer can act as absorbing 
 or emitting site with equal probability . therefore , angles between 
 dipoles of any two porphyrin monomers on the rings are considered 
 and the average of  is calculated according to eq 1 . even though this model is rather simplified and primarily 
 focuses on the effect of bending and twisting distortions on the pl 
 anisotropy , the results agree well with the experimental findings . 
 the calculated anisotropy results s are listed in 
 table 1 and lie within the error range of the 
 experimental values . this correspondence confirms that the out - of - plane 
 distortion is in fact the major contribution causing the lowered anisotropy 
 value from 0.1 . the modeled values tend to be slightly lower than 
 the experimental values , but are surprisingly close , given the simplicity 
 of the model . deviations between experimental and simulated values 
 may arise because simulations were carried out assuming vacuum conditions 
 ( screening of intermolecular interactions arising from the solvent 
 was not taken into account ) and because of some variation with the 
 molecular mechanics potential chosen . in addition , a more complete 
 description of exciton depolarization dynamics ought to include effects 
 such as exciton localization dynamics , for example , to sites that are preferred through specific local 
 geometries . however , the good agreement between the experimental results 
 and the simple simulation performed here ultimately results from the 
 intimate link between the topology of the molecule to the polarization 
 directions its emitting state can explore . ( a ) the twisting angle 
  between two template planes is defined using c - p12(t6)2 as an example . four atoms on the porphyrin ring ( red ) are chosen 
 to form three vectors a , b , and c , with a and c lying approximately parallel to each 
 other when the molecule does not exhibit any twist . is the 
 torsional angle defined by the four atoms and is the angle between 
 the planes formed by a , b and b , c.  
 represents the twisting angle from a planar position . ( b ) the bending 
 angle  is illustrated using c - p12(t8)2 as an example . two angles  and  are 
 calculated , where  =  +  describes the deviation 
 from a planar position . simulations were carried out with octyloxy 
 side chains , which are omitted in the diagram for clarity . ( c  f ) 
 area normalized histogram of twisting angle  ( blue ) and bending 
 angle  ( red ) for ( c ) c - p10(t5)2 , ( d ) c - p10(t6)2 , ( e ) c - p12(t6)2 , ( f ) c - p12(t8)2 . molecular dynamics simulation carried 
 out using hyperchem at 300 k for 500 ps , as described 
 in detail in the text . -conjugated nanorings containing 
 10 or 12 porphyrin units 
 have been investigated to explore the influence of molecular topology 
 on exciton dynamics and polarization memory loss . it is found that 
 excitations can access any part of the templated nanorings , despite 
 their twisted conformations , thus showing similar exciton dynamics 
 to their more planar untemplated counterparts . time - dependent ultrafast 
 spectroscopy reveals that all nanorings exhibit static pl anisotropy 
 transients within the first 20 ps after excitation , but with an anisotropy 
 much lower than the value of 0.1 found for untemplated rings . molecular 
 dynamics simulations show that this phenomenon can be fully explained 
 and well described by the out - of - plane distortions of the rings from 
 a planar conformation . numerical simulations of the pl anisotropy 
 taking into account such distortions yield values that correlate well 
 with the experimentally obtained results . in summary , template - directed 
 synthesis not only plays a vital role in the synthesis of large porphyrin 
 rings , but it also rigidifies the nanorings and introduces a means 
 to control their geometric structures while maintaining excellent 
 -conjugation . these findings open exciting new possibilities 
 for fast and directional energy transfer in nanoscale molecular structures .