Protecting Benzylic CH Bonds by Deuteration Doubles the Operational Lifetime of Deep‐Blue Ir‐Phenylimidazole Dopants in Phosphorescent OLEDs

Much effort has been dedicated to increase the operational lifetime of blue phosphorescent materials in organic light‐emitting diodes (OLEDs), but the reported device lifetimes are still too short for the industrial applications. An attractive method for increasing the lifetime of a given emitter without making any chemical change is exploiting the kinetic isotope effect, where key CH bonds are deuterated. A computer model identifies that the most vulnerable molecular site in an Ir‐phenylimidazole dopant is the benzylic CH bond and predicts that deuteration may hamper the deactivation pathway involving CH/D bond cleavage notably. Experiments show that the device lifetime until the initial luminance diminishes to 70% (LT70) of a prototype phosphorescent OLED device can be doubled to 355 hours with a maximum external quantum efficiency of 25.1% at 1000 cd m−2. This is one of the best operational performances of blue phosphorescent OLEDs observed to date in a single stacked cell.


Introduction
Phosphorescent transition metal complexes have attracted much attention as emitting materials in organic lightemitting diodes (OLEDs) due to their short excited state lifetime, and because their internal quantum efficiency can in principle reach 100%. [1][2][3][4][5] Cyclometalated Ir(III) complexes are particularly interesting, as their color can be tuned over the entire visible range from blue to red. [6][7][8][9][10][11][12][13][14][15][16] But finding blue phosphorescent materials that are robust enough for industrial applications proved challenging, which may be rationalized by the large band gap [17][18][19] compared to red or green emitters, leading to high-energy excited states that are consequently more reactive and are therefore expected to degrade more easily. Such degradation may give rise to short device lifetime and low efficiency. It is therefore necessary to develop blue phosphorescent materials that are chemically much more stable to engineer long device lifetime and high efficiency.
Despite being conceptually easy, it proved difficult to find rational ways of inhibiting the degradation to increase operational lifetime and efficiency. [20][21][22] A number of factors such as structural degradation, [23][24][25] triplet-polaron, [26][27] and triplet-triplet annihilation [28][29][30] have been considered as potential causes of these inefficiencies, but much remains unclear and a unified strategy for systematically increasing the performance of blue emitters has not emerged. One of the main difficulties in developing systematic strategies lies in the fact that little is known about what causes the loss in activity at the molecular level and a quantitative structure-reactivity relationship could not be established thus far. As a result, optimization efforts cannot build upon a precise mechanistic understanding and must rely on conjecture and empirical observations. For instance, bis(4′,6′-difluorophenylpyridinato)iridium(picolinate) (FIrpic) is one of the most extensively studied blue dopants in phosphorescent OLEDs. [31][32][33] The dissociation of the ancillary picolinate ligand and the cleavage of CF bonds [34][35][36][37][38] were proposed to be responsible for its deactivation. Consequently, fluorine-free homoleptic blue Ir(III) complexes [39][40][41][42] were prepared, and the Much effort has been dedicated to increase the operational lifetime of blue phosphorescent materials in organic light-emitting diodes (OLEDs), but the reported device lifetimes are still too short for the industrial applications. An attractive method for increasing the lifetime of a given emitter without making any chemical change is exploiting the kinetic isotope effect, where key CH bonds are deuterated. A computer model identifies that the most vulnerable molecular site in an Ir-phenylimidazole dopant is the benzylic CH bond and predicts that deuteration may hamper the deactivation pathway involving CH/D bond cleavage notably. Experiments show that the device lifetime until the initial luminance diminishes to 70% (LT 70 ) of a prototype phosphorescent OLED device can be doubled to 355 hours with a maximum external quantum efficiency of 25.1% at 1000 cd m −2 . This is one of the best operational performances of blue phosphorescent OLEDs observed to date in a single stacked cell.
Using the kinetic isotope effect (KIE) [48] to stabilize emitters is an unusual, but interesting strategy. Unlike conventional methods where the composition of the emitter must be changed to impact its chemical property, replacing an element within a molecule by its heavier isotope does not alter the chemical composition, but renders bond-cleavage reactions slower, which may potentially improve the performance of blue phosphorescent materials. Specifically, if the cleavage of a CH bond is involved in the rate-determining step of the deactivation reaction, its rate can be lowered by replacing the protium by deuterium. Because the zero point vibrational energy (ZPE) of a CH bond is typically ≈3000 cm −1 and is lowered to ≈2100 cm −1 in CD, the chemical reaction rate may decrease by a factor of up to ≈6 depending on how prominently the CH bond cleavage is featured in the transition state. [49] And indeed, incorporating deuterium into emitter molecules was reported to enhance the luminescence quantum yield, OLED efficiency and improve device lifetime, previously, [50][51][52][53] but whether KIE can be used to stabilize the notoriously instable triplet blue emitters was not examined.
To better understand the KIE on the device performance, we first wished to establish a precise relationship between deuteration and the CH/D activation. We constructed a computer model to identify the most vulnerable molecular sites and found that the benzylic CH bonds are the weakest and most vulnerable CH bonds. Thus, deuterating the weakest benzylic CH bond is expected to interrupt its bond cleavage and eventually the decomposition, giving rise to a more stable blue emitter. In order to verify this notion, we prepared various hydrogenated and deuterated homoleptic iridium complexes based on phenylimidazole ligands. The OLED devices were fabricated based on these eight iridium complexes. All complexes exhibit high external quantum efficiencies (EQEs) of 18.4-22.5% at 456-462 nm, and deuteration was found to significantly enhance the lifetime. In particular, the blue iridium complex Ir1D where the most vulnerable benzylic position was deuterated displayed an operational lifetime until the initial luminance diminishes to 70% (LT 70 ) that was nearly doubled when compared to the protium-analogue to 355 h at 1000 cd m −2 with high maximum EQE of 25.1% and good color purity (0.175, 0.285). To the best of our knowledge, this is the first case in which the instability of Ir-based blue emitters is overcome to such decisive extent by KIE, increasing the operational lifetime of blue OLEDs significantly. This finding also confirms the long-held suspicion that structural decomposition involving the activation of CH bonds plays an important role in the deactivation of the blue dopant. The operational performances of the blue OLED device doped with Ir1D are one of the best among reported blue OLED devices. The novel device structures with the gradient doping in emission layer (EML) have shown to engender long device lifetime, [26,66] but here we focus on a design of stable Ir-complexes embedded in a single stacked cell.

CH Bond Strengths
To construct high-performing blue phosphorescent emitters, we first sought to identify the key factor that could affect the degradation of the device. Considering the observations from previous studies mentioned above, it is clear that multiple factors contribute to the operational degradation of the emitter material with the most likely reactive components being the aromatic amines, carbazoles, hydrocarbons, and metal complexes. [23,25,57,58] Whereas it is currently impossible to precisely identify the most disabling decomposition pathway, we can assess in more general terms the vulnerabilities that develop as the emitter molecule undergoes excitation. The triplet excited state in the Ir(III)-dopants is typically dominated by the metalto-ligand-charge-transfer (MLCT) excitations, which can be formally envisioned as a reduction of the ligand. [54] Among the many chemical reactions that this process may accelerate, we envisioned that the CH activation and subsequent abstraction of hydrogen from the dopant are most likely. It is easy to imagine that the abstracted hydrogen atom can be transferred to an acceptor [A + ], as outlined in Equation (1), following an exciton-polaron annihilation mechanism, in which the hydrogen transfer from the excited triplet state of the dopant to a nearby receptor, e.g., the host material, takes place. The dehydrogenated Ir-dopant, which contains a carbon-centered radical, is likely to undergo further rearrangement and decomposition leading to unrecoverable loss of device performance.
To identify the CH bond that is most vulnerable to such a transformation, we chose a series of homoleptic Ir-phenylimidazole emitters Ir1-4 and calculated the adiabatic bond dissociation energies of all CH bonds in the excited triplet state using density functional theory (DFT) calculations, as summarized in Table 1. To account for the screening environmental effect in the host material, a polarizable continuum solvation model was employed using a dielectric constant of toluene (ε = 2.38). The computed CH bond dissociation energies range from ≈60 to as low as ≈29 kcal mol −1 , with the benzylic CH bonds showing the lowest bond dissociation energies. The benzylic CH bonds (3,9,10) are calculated to require 31-28 kcal mol −1 in all Ir-complexes considered. The pyrimidyl (1,2) and phenylic (4-7) CH bonds are also somewhat activated, but are unlikely to break at ≈60 and ≈50 kcal mol −1 , respectively. Given that the benzylic CH bonds are also the weakest in ground state molecules with tabulated bond energies being ≈90 kcal mol −1 compared to the phenylic bond being worth 113 kcal mol −1 , these calculated values are intuitively understandable. The calculated energy difference of the benzylic versus phenylic CH bonds of ≈30 kcal mol −1 is also consistent with standard bond energies and indicate that all CH bonds are activated by a similar amount in the MLCT state and the benzylic CH bond becomes vulnerable simply because it is the www.advopticalmat.de weakest among the CH bonds present in the phenyl-imidazole ligand.
Thus, the benzylic CH bond is most vulnerable toward hydrogen abstraction, which may trigger the chemical decomposition of the device. DFT calculations were employed to estimate the barrier of the hydrogen abstraction reaction, where Ir1 was chosen as a model. In these calculations, a cationic phenylcarbazole moiety was used as the hydrogen atom acceptor, representing one possible acceptor functionality of a host-localized hole, as illustrated in Figure 1a. Our calculations show that the barrier of the H-abstraction reaction is 23.7 kcal mol −1 , which is sufficiently low to take place even under mild conditions. The resulting benzyl radical and carbazolium species (3) were located at a relative energy of 21.5 kcal mol −1 , indicating the reaction is highly endergonic, and suggesting that the transition state should be late and resemble the electronic structure of the product state according to the Hammond postulate. [59] As shown in Figure 1b, the benzylic hydrogen in the transition state (TS) is located near the nitrogen with the length of CH and NH bonds being 1.52 and 1.19 Å, respectively.

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To predict the effect of replacing CH by CD, we estimated the difference of the zero-point vibrational energy variation (ΔΔZPE) between Ir1 and Ir1' where the benzylic hydrogen participating in the reaction was replaced by a deuterium. ΔZPE is the difference between the zero point vibrational energies at the transition state and the reactant. The calculated ΔΔZPE is 0.74 kcal mol −1 . Assuming Arrhenius-like dependence of the forward reaction rate on the barrier, the KIE can be estimated as where k and T are the Boltzmann constant and temperature, respectively. At room temperature the KIE can be estimated to be 3.5 for Ir1, suggesting that the deuterated dopant would undergo the H-abstraction reaction notably slower than in the original dopant. Details of ZPE calculations are given in Table S1 (Supporting Information). Taken together, these calculations highlight that the benzylic CH bonds are potentially vulnerable functionalities and deuterating them may notably reduce the reaction rate of the dehydrogenation reaction. As predicted in the computational model, we sought to prepare a series of Ir-complexes with various degrees of deuterium incorporation and study the effect of the deuterium incorporation experimentally.

Synthesis and Characterization
The synthetic procedure for accessing Ir1-4 and Ir1D-4D are shown in Figure 2. The ligands L1-4 and L1D-4D were prepared using a palladium catalyzed cross coupling procedure. [40,55] Deuterium incorporation can be achieved by using deuterium oxide (D 2 O) instead of water (H 2 O) during the synthesis of L1D-4D. Ir(III) complexes were prepared via a one-pot reaction of bis(1,5-cyclooctadiene)iridium(I) tetrafluoroborate, Ir(COD) 2 (BF 4 ) and the different ligands in an 1-methyl-2-pyrrolidinone (NMP). All Ir(III) complexes were purified by column chromatography and sublimation, and their structure were confirmed by 1 H and 13 C NMR, MALDI-TOF and elemental analysis. All complexes are identified as facial structures by straightforward NMR spectra due to C 3 symmetry. [56] The total deuterium content of each complex is confirmed by 1 H NMR and MALDI-TOF. Not surprisingly, the deuteration is not quantitative. Special attention was given to the degree of deuteration of the benzylic CH bonds. Deuterium/protium ratios of Ir1D-4D were assigned by 1 H NMR and quantified by comparing the integration of the corresponding NMR signals to an internal standard, the 5 or 6-position of the 3-cyanophenyl moiety that cannot be deuterated ( Figures S1-S8, Supporting Information).
Our experiments indicate that all methyl groups of Ir1D-3D are deuterated to ≈90%. The methyl and hydrogen of the isopropyl functionality in Ir4D show substitution levels of ≈48% and ≈85% and the hydrogen at the 4-position of the diisopropylphenyl of Ir4D is deuterated at ≈74%. The 2, 3-positions of the imidazole fragment in Ir1D-4D are confirmed to carry deuterium at ≈69% to 88%. MALDI-TOF measurements of Ir1D-4D also show consistently that the molecular weight has increased by deuteration ( Figure S9, Supporting Information), fully consistent with the conclusions drawn from the NMR analysis.

Photophysical Properties
With the Ir-complexes in hand, the photophysical properties were studied. As expected, Ir1-4 and Ir1D-4D have practically identical absorption and emission spectra, and display the same electrochemical and thermal properties, as detailed in Table 2. The UV-vis absorption and photoluminescence (PL) spectra are shown in Figure 3. The absorption spectra of all compounds feature an intense absorption band (λ abs ) at 260-330 nm that can be assigned to the spin-allowed p-p* (ligandcentered) transition of the phenylimidazole ligand. The broad low-energy absorption band in the region of 360-450 nm is an admixture of the 1 MLCT and 3 MLCT transitions. When excited for emission at 360 nm, the emission spectra of all complexes in dichloromethane (DCM) solution show blue emission with maximum peaks (λ em ) displayed at 456-462 nm. The UV-vis edge of Ir3 and Ir3D were blue-shifted by ≈0.03 eV and the PL were blue-shifted by 6 nm when compared to Ir2 and Ir2D that do not contain the methyl substituent on the 3-cyanophenyl fragment. The UV-vis and PL spectrum of Ir1-4 and deuterated Ir1D-4D iridium complexes are almost identical, confirming that the deuteration has a minimal impact on the photophysical properties.
The quantum yields (Φ) of all complexes measured in poly methyl methacrylate (PMMA) film employing the integrating sphere method were in the range of 0.94-0.99. Although it was reported previously that CH vibrational oscillators suppressed by CD replacement can enhance luminescent quantum efficiency and lifetime, [60,61] we were unable to observe such a differential effect, presumably because the heavy transition metal overrides and minimizes the impact of the deuterium on the reduced mass relevant for the vibrations. [53] The emisson lifetimes (τ) of 2.15-2.69 µs for all complexes measured in PMMA film confirm the expected phosphorescence emission. The methyl functionality introduced in Ir3 and Ir3D shift the excited state characteristics away from MLCT more toward the ligandcentered (LC) character, which results in reduced spin density on the iridium atom in the excited state and suppresses of the spinorbit coupling between singlet and triplet manifolds, resulting in the decrease of the radiative decay rate compared to Ir2 and Ir2D ( Figure S14 and Table S2, Supporting Information).
The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the iridium complexes were determined by cyclic voltammetry and optical band gap (Table 1 and Figure S10, Supporting Information). The HOMO energy levels were estimated from the onset of the oxidation potential and the LUMO energy levels were derived by adding the optical bandgaps to the HOMO energy levels. The HOMO for all complexes were in the range of −5.25 to −5.34 eV, and the adding the methyl groups to the 3-cyanophenyl moiety in Ir3 and Ir3D gave rise to a shift of the HOMO/LUMO levels due to their electron-donating inductive nature. As the methyl groups are closer to the LUMO geometrically, the destabilizing effect is greater for the LUMO than the metal-centered www.advopticalmat.de HOMO, resulting in a greater HOMO-LUMO gap (E g ) and a blue-shift of the emission. This finding is consistent with DFT calculations that showed a more pronounced blue-shift of Ir3 and Ir3D. The thermal properties were investigated by thermogravimetric analysis (TGA). The deposition temperatures at 5% weight loss (T d5 ) were found to be 415-475 °C ( Figure S11, Supporting Information), which is consistent with other phenylimidazole-based iridium complexes.

Device Performances
To evaluate the effect of deuteration, OLED device were fabricated as follows: Indium tin oxide (ITO, 150 nm)/p-doped  [62,63] The Ir complexes Ir1-4 and Ir1D-4D were studied in the same device structure and a detailed energy level diagram is shown in Figure 4a. A p-doped layer was used to achieve effective hole injection from ITO into hole transfer layer (HTL). For the enhancement of exciton confinement in the emission layer (EML), we employed the exciton and hole blocking layer (HBL) with high triplet energies (T 1 of H1: 3 eV, T 1 of HBL: 3.02 eV). The EQE versus luminance, normalized electroluminance (EL) spectra and lifetime curves (LT 70 @1000 cd m −2 ) are plotted in Figure 4b-d and chemical structures of H1, H2, and HBL are shown in Figure 4e. EML is composed of cohost system with H1 and H2 doped with the Ir complexes. H1 is the modified 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP) which has twisted structure by adopting ortho-like bridge and can achieve higher triplet energy (3 eV) and higher hole mobility. H2 has the CN modified carbazole unit and biphenyl contributing to deep HOMO/LUMO levels and affords enhanced electron acceptor characteristics. H1 and H2 have effective HOMO/LUMO levels to enhance the hole and electron carrier injection, respectively, which improve the device performance. In particular, this cohost combination can form the stable exciplex at ≈421 nm to efficiently support the triplet state of the Ir-dopant. [64] Adv. Optical Mater. 2021, 9,2100630  All OLED devices showed high efficiency and relatively long device lifetime at CIE (international commission on illumination) < 0.3, and all relevant EL characteristics are summarized in Table 3. The devices incorporating Ir1, Ir2, and Ir4 show an EL emission at 462 nm, whereas Ir3 displays slightly blueshifted emission at 457 nm associated with a deep-blue color characterized by CIE coordinates of (0.172, 0.258). Ir1, Ir2, and Ir4 show good device performance with EQE > 22%, LT 70 > 140 h. Ir1 has the highest EQE of 22.5% and a device lifetime LT 70 of 182 h, which may be attributed to the sterically bulky tert-butyl groups that inhibit intermolecular interaction that may lead to triplet-triplet annihilation. [65] The Ir3 device shows the lowest device performance among the dopants tested. Because the HOMO levels are relatively shallow, holes are easily trapped in the Ir3 devices and the operational voltage must be increased from ≈4 to 4.5 V. Moreover, since the Ir3 shows a blue-shifted emission peak, the energy transfer from cohost exciplex might not be sufficient. The Ir3 shows a relatively long decay time of ≈2.69 µs and a roll-off increase of ≈10%, which is slightly higher than what is seen in the other dopants Ir1 (7.9%), Ir2 (5.4%), and Ir4 (4.8%), resulting in the reduced EQE. Consequently, Ir3 has device lifetime LT 70 of ≈51 h at 1000 cd m −2 that is notably shorter than what is found with Ir2 that exhibited a lifetime of over 140 h, but the color of the emission was shifted to a deeper blue, indicated by CIE coordinates of (0.172, 0.258).

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Adv. Optical Mater. 2021, 9, 2100630 92 h, respectively, whereas the lifetime enhancement of Ir4D to 192 h is relatively low. Of particular interest is the observation that Ir1D displays highly efficient and stable device performance with a maximum EQE of 25.1% and the LT 70 being 355 h emitting a deep-blue color characterized by CIE coordinates of (0.175, 0.285) over high brightness of 1000 cd m −2 . These device characteristics are found to be almost the best of the reported performances of blue phosphorescent OLED devices.
To relate these KIE-based improvements of the lifetime to the calculated KIE mentioned above, we must account for the fact that deuterium incorporation was not quantitative, which will of course lower the experimentally observable KIE. To estimate the effective KIE eff the fractional deuterium incorporation can be accounted for by where r is typical deuteration ratio. The KIE eff becomes 3 ± 0.2 for a deuteration ratio in the range of 85-90% that was observed in Ir-complexes, which is in reasonably good agreement with the experimentally observed value ≈2.
Isothermal experiment was performed to confirm that the stability enhancement originates from increased stability of the deuterated dopants. The purity change of Ir1-4 and Ir1D-4D after the isothermal process applying a constant temperature ≈280 °C for 1 h is shown in Table 4. The impurities of each hydrogenated and deuterated dopant have similar patterns, but in the case of the deuterated dopants, the purity change after the isothermal process is notably smaller compared to the undeuterated analogues. For example, Ir3 has a purity change of 5.37% after being exposed to the isothermal conditions, but the deuterated analogue Ir3D shows a much improved behavior at 1.44%. The mass of major impurities was found to be oxidation products (see Figure S15, Supporting Information), which is worrisome, as device degradation can be affected by oxidized material during electro-and photochemical processes. [57,67] These isothermal experiments offer further support for the proposal that the durability of the Ir-dopants can be significantly enhanced by deuteration.

Conclusions
In this study, we demonstrate that KIE can significantly increase the operational lifetime of blue phosphorescent OLED devices. We examined four homoleptic iridium(III)-complex carrying phenylimidazole ligands and their deuterated analogues to investigate whether the KIE can be employed to protect CH bonds and extend the lifetime of deep-blue emitters in phosphorescent OLED devices. Computational studies were used to calculate the strengths of all CH bonds in the triplet excited state, which indicated a substantial weakening of all CH bonds due to repulsive nature of triplet state after its eventual localization on dissociating bond. The benzylic positions were identified as particularly vulnerable, and synthetic efforts were focused on deuterating these positions. The quantum yields of all complexes measured in PMMA film with integrating sphere were excellent ranging from 0.94 to 0.99. Prototype OLED devices were constructed and all eight dopants exhibited blue emissions in the range of 456-462 nm. Ir1D-4D maintained high efficiency and device lifetime could be doubled when compared to Ir1-4, demonstrating that deuterium incorporation targeting the most vulnerable CH bonds is a viable strategy to significantly increase the operational lifetime without introducing any change in chemical composition and altering the photophysical properties of the dopant. In particular, Ir1D showed one of the best device performances measured to date with a maximum EQE of 25.1% and a device lifetime of 355 h

Experimental Section
Device Fabrication and Measurements: All organic layers were deposited on patterned ITO glass using a thermal evaporation system with a vacuum pressure of <1.0 × 10 −7 torr after pre-cleaning of acetone, Isopropyl alcohol and deionized water sequentially. The common layers (HITL, EBL, HBL, ETL) and EML were deposited by 0.1 and 0.05 nm s −1 , respectively. The active area overlapped by the ITO anode and aluminum (Al) cathode was defined as 4 mm 2 . The EL device structure was fabricated as follows; ITO/p doped BCFA/BCFA/H1/H1:H2:Ir complexes/mCP-CN/ETL/Al. EL devices were encapsulated by the glass lid in high purity nitrogen glove box. Current density−voltage−luminance (J−V−L) characteristic including current, voltage was tested with a Keithley, 2400 Source-Meter and EL spectra and was collected by SR3 spectroradiometer. The operational lifetime of EL devices were measured in a constant current mode (designated current@1000 nit and 0.2 mA/0.1 mA, respectively). LT 70 is operation time where initial luminance was diminished to 70%.
DFT Simulations: All calculations were performed using density functional theory (DFT) implemented in the Jaguar 9.1 suite of programs. [68] Geometries of all ground state (S 0 ) and the lowest triplet excited state (T 1 ) were optimized using PBE0 [69] functional including Grimme's D3 dispersion [70,71] correction with the 6-31G** basis set [72] for main group elements and the Los Alamos double-zeta basis [73][74][75] containing effective core potentials for the transition metals. Single point calculations with Dunning's correlation-consistent triplet-ζ basis set, cc-pVTZ(-f) [76] were carried out to reevaluate the electronic energies. For iridium, LACV3P containing decontracted exponents to match the effective core potential with triple-ζ quality was used. The zero-point energy (ZPE) and entropy were obtained from the frequency calculations at the same level of theory used in the geometry optimization. For the evaluation of vibrational entropy, low frequency vibrational modes below 50 cm −1 except the imaginary frequency in TS were replaced with a value of 50 cm −1 . Solvation energies were computed by a self-consistent reaction field (SCRF) approach [77][78][79] with the dielectric constant ε = 2.379 (toluene) using the gas phase optimized structures. All free energies (kcal mol −1 ) in the energy profile in the manuscript were calculated at 298.15 K.
For the reactant state, the cationic acceptor and dopant (T 1 state) were treated as infinitely separated and since both components were immobilized in the device, the translational entropy was eliminated when calculating the free energy. Therefore, the entropy corrections only consider the contributions from the vibrational and rotational degrees of freedom.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.