Relationship between Dynamic Planarization Processes and Exciton

Letter pubs.acs.org/JPCL

Relationship between Dynamic Planarization Processes and Exciton Delocalization in Cyclic Oligothiophenes Pyosang Kim,† Kyu Hyung Park,† Woojae Kim,† Tomoya Tamachi,‡ Masahiko Iyoda,*,‡ and Dongho Kim*,† †

Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 120-749, Korea ‡ Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan S Supporting Information *

ABSTRACT: In cyclic molecular structures, while the effect of conformational disorder on exciton delocalization is well understood, the impact of dynamic planarization processes remains unclear due to a lack of detailed investigation on the associated exciton dynamics. Thus, we have investigated the exciton delocalization of π-conjugated linear and cyclic oligothiophenes in the course of dynamic planarization processes by time-resolved fluorescence spectra measurements and theoretical calculations. Especially, through a comparative analysis of linear and cyclic oligothiophenes, we found that the evolution of 0−0 and 0−1 vibronic bands is strongly related to the conformations of cyclic molecular systems, reflecting the extent of exciton delocalization. Collectively, we believe that our findings are applicable to various π-conjugated organic materials and will provide new insights into the relationship between exciton delocalization and cyclic molecular conformation.

C

the relationship between the molecular conformation and exciton delocalization in the excited state of π-conjugated systems. In this sense, recently reported cyclic nanostructures have been considered as good candidates to investigate the effect of molecular conformations on the extent of exciton delocalization.15−31 In previous reports, the lack of chain end-effects, high symmetry, and the presence of ring strain were shown to cause remarkable changes in the linear and nonlinear optical properties as compared with linear molecular structures.15−28 However, Herz et al. have shown that in π-conjugated nanorings consisting of porphyrin units, an increase in ring size induces ring deformations and, hence, the localization of the exciton on the segment of the nanoring.32 According to the report from the Lupton group, recent single-molecule experiments on nanorings synthesized from phenylcarbazole units linked with phenylene-ethylene-butadiynylene groups have shown that structural relaxation following excitation induces exciton localization on the subunits of the π-system.33 Furthermore, the Spano group investigated theoretically the impact of chain bending on the steady-state emission spectra of conjugated polymers, which suggests that the conformational disorder in cyclic structures reduces the extent of exciton delocalization.34 These reports commonly demonstrate that the

onjugated molecular systems have attracted considerable attention in recent years due to their fundamental, technological, and practical advantages over existing inorganic semiconductors, particularly in the field of optoelectronics such as electroluminescent devices, solar cells, and transistors.1−6 The semiconducting and optical properties of π-conjugated systems are mainly dependent on the extent of exciton delocalization along the conjugated backbone.7 In this regard, tremendous experimental and computational efforts have been devoted to studying decisive factors to affect the exciton delocalization, which demonstrates that intrinsic conformational disorder makes them difficult to realize the desired semiconducting and optical properties.8 Moreover, timeresolved spectroscopic studies suggest that the electronic excited state of conjugated molecular systems undergoes significant geometrical relaxation of the underlying nuclear framework, which affects the delocalization of the excited-state wave function along the molecular backbone.9−14 Specifically, (1) the structural relaxation along the CC stretching coordinate is characterized by a change of CC bond alternation to create a domain of quinoidal bond sequences, which in turn leads to a rapid contraction of the excited-state wave function within the lattice and the minimization of bond length alternation in the excited-state. Likewise, (2) torsional reorganization on a picosecond time scale that planarizes and stabilizes the quinoidal structure of the excited state results in an extension of the excitonic wave function in contrast with CC stretching relaxation. Collectively, in order to obtain the desired performance of organic devices, it is crucial to elucidate © 2015 American Chemical Society

Received: November 11, 2014 Accepted: January 5, 2015 Published: January 5, 2015 451

DOI: 10.1021/jz502395z J. Phys. Chem. Lett. 2015, 6, 451−456

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The Journal of Physical Chemistry Letters conformational disorder localizes the exciton size on the segments of cyclic structure, which changes the specific photophysical properties of the cyclic system similar to that of the linear system. Despite the above reports, however, the influence of dynamic planarization processes on the exciton delocalization in cyclic π-conjugated systems remains unclear, in contrast to linear systems. Especially, because the torsional reorganization significantly affects the entire molecular conformation, the investigation of dynamic planarization processes in the cyclic structure will provide information on the correlation between molecular conformation and exciton delocalization. In this regard, we comparatively investigated the dynamic planarization processes of linear and cyclic oligothiophenes through time-resolved fluorescence spectra measured by a femtosecond fluorescence up-conversion technique. For this purpose, we prepared π-conjugated linear (10 repeating units) and cyclic (10, 15, 20, 25, 30 repeating units) oligothiophenes composed of thiophene, acetylene, and ethylene22 (see Scheme 1). Scheme 1. Molecular Structures of Linear and Cyclic Oligothiophenes

Figure 1. Steady-state absorption and fluorescence spectra (top) of linear and cyclic oligothiophenes showing the calculated oscillator strengths of singlet excited states (bottom) of L-10T and C-10T2V.

with L-10T. This feature can be understood qualitatively by DFT and TD-DFT calculations, as illustrated in Figure 1 and Figures S1 and S2 in the Supporting Information. First, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were delocalized over the ring of C-10T2V, in contrast with L-10T showing localized MO structures on the center of the backbone (Figure S1 in the Supporting Information). This feature resulted in a decrease of HOMO−LUMO gap and a red shift of the S1 state in C-10T2V as compared to L-10T. Second, the S0 → S1 transition of C10T2V at around 520 nm was mainly a symmetry-forbidden HOMO → LUMO transition, whereas the S0 → S1 transition of L-10T corresponded to an allowed transition near 450 nm (Figure S2 in the Supporting Information). Considering the completely forbidden transition of the S1 state in a complete planar geometry optimized without n-butyl substituents, a deviation from the planar ring shape may have led the forbidden S1 transition of C-10T2V to be slightly allowed, as shown in the absorption spectrum. Third, the S2 states (at around 450 nm) of C-10T2V exhibited a near degeneracy in which both HOMO−1 (HOMO−2) → LUMO and HOMO → LUMO+1 (LUMO+2) transitions were strongly allowed, while those of L-10T had an oscillator strength of nearly zero (Table S1 in the Supporting Information). Although the optimized molecular structure of L-10T does not represent all possible conformations in the solution, the calculated results suggest that the distinction between linear and cyclic molecular geometries leads to a difference in the steady-state absorption spectra between L-10T and C-10T2V. Furthermore, these results were supported by the steady-state fluorescence spectra and the radiative decay rates (kr). The fluorescence spectrum of C-10T2V was obviously red-shifted as compared to that of L-

Figure 1 shows the steady-state absorption and fluorescence spectra of linear (L-10T) and cyclic oligothiophenes (C10T2V−C-30T6V); the corresponding photophysical parameters are listed in Table 1. The absorption spectra of both linear and cyclic oligothiophenes were spectrally broad and structureless, whereas the fluorescence spectra were relatively narrow with clear vibronic features. The lack of mirror-image symmetry between the absorption and fluorescence spectra was due to coupling of the electronic transition with torsional motions of constituent units around the molecular backbone.35,36 Through photoexcitation, various conformations of the oligomer (reflecting the shallow ground-state torsional potential) moved to a steep excited-state potential. Then, relaxation along the potential energy surface in the excited state narrowed the distribution of torsional angles, leading to conformational changes from twisted to planar conformers and in turn resulting in the appearance of a narrow fluorescence spectrum. It is worth noting the difference in the absorption spectra between L-10T and C-10T2V, despite the same number of thiophene rings. C-10T2V exhibits a relatively sharp absorption spectrum as compared to L-10T, reflecting that the rigidity of the cyclic molecular structure reduces the conformational heterogeneities of the flexible linear chain. Interestingly, the shoulder at around 520 nm is observed in C-10T2V, in contrast 452

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The Journal of Physical Chemistry Letters Table 1. Photophysical Parameters for Linear and Cyclic Oligothiophenes in Toluene

a

sample

absorption (nm)

L-10T C-10T2V C-15T3V C-20T4V C-25T5V C-30T6V

448 446 469 480 486 488

fluorescence (nm) 522, 558, 562, 560, 560, 560,

553 602 606 602 602 602

Q.Y.

τr (ns)

kra (×108 s−1)

knra (×108 s−1)

0.29 0.17 0.28 0.33 0.32 0.30

0.38 0.95 0.67 0.61 0.58 0.57

7.6 1.8 4.1 5.4 5.5 5.3

19 8.7 10 11 12 12

Radiative and nonradiative decay rates calculated using fluorescence quantum yields and lifetimes.

10T, which indicated that the transition energy of the S1 state in C-10T2V was lower than that of L-10T. Morevoer, the kr values of C-10T2V were decreased as compared to that of L10T. As the kr value is proportional to the oscillator strength of the emissive state (vibrationally relaxed S1 state), this result also showed that the cyclic geometry of C-10T2V led to a reduced S0 → S1 transition, in contrast to L-10T. Together with above results, it is noteworthy that the intensity of the 0−0 band in C-10T2V is smaller than that of the 0−1 band, in contrast with L-10T. A recent theoretical report showed that as the chain conformation is bended up to the cyclic geometry, the intensity of the 0−0 band in the simulated emission spectrum becomes the value of zero, in contrast to that of the 0−1 band.34 Furthermore, temperature-dependent emission spectra of porphyrin nanorings show that Herzberg− Teller intensity borrowing increases the intensity of the 0−0 vibronic band as compared to the 0−1 band, resulting from the distortion away from the cyclic molecular structure.32 On the basis of these reports, the significant difference of the vibronic peak ratio between L-10T and C-10T2V stems from the structural distinction between linear and cyclic geometry. In the cases of larger cyclic oligothiophenes, C-15T3V−C30T6V, the peak positions of the maximum absorption bands were red-shifted as compared to that of C-10T2V, while the fluorescence spectra did not show a systematic shift. To obtain a qualitative analysis of contributions by the S1 and higher states based on the calculated results for C-10T2V, the absorption spectra of C-10T2V, C-15T3V, and C-30T6V were fit to three individual Gaussian functions, as shown in Figure S3 in the Supporting Information. The fit results revealed that the intensity of the S1 state increased as compared to higher states in going from C-10T2V to C-30T6V. Because there was no distinguishable shift in the transition energy of the S1 state, we suggest that the red-shifted main peak in the absorption spectra was induced by a change in the ratio between the intensities of S1 and higher states. Moreover, the kr values of cyclic oligothiophenes exhibited an increase from C-10T2V to C20T3V and saturating behavior in C-20T4V, C-25T5V and C30T6V, indicating that larger ring systems had allowed S1 transitions, which is consistent with the fit results of the absorption spectra. Furthermore, through the difference in the vibronic peak ratio in the steady fluorescence spectra between L-10T and C-10T2V, the larger intensity of the 0−0 band as compared to that of the 0−1 band suggests that the structural deformations away from cyclic geometry occur in the larger rings; hence, the S0 → S1 transition is allowed, in contrast with C-10T2V. Using time-resolved fluorescence spectra, we next investigated dynamic planarization (conformational change from twisted to planar form) in linear and cyclic oligothiophenes, as shown in Figure 2. According to previous reports, the dynamic planarization processes in π-conjugated poly- and oligomers

Figure 2. Reconstructed time-resolved emission spectra and fluorescence decay profiles (inset) of L-10T and C-10T2V after excitation at 400 nm.

play an important role in the evolution of exciton delocalization after exciton self-trapping through the CC stretching coordinate.11,37−40 In our experiments, the early emission dynamics of linear and cyclic oligothiophenes were nonexponential and depended strongly on the emission wavelength (Figure 2 and Figure S4 in the Supporting Information). The decay profiles were analyzed by global fitting with a summation of three exponential functions, where one of the three (τ3) was fixed as the lifetime of the S1 state. The fitted time constants of L-10T and C-10T2V were τ1 = 8.5 and 4.1 ps and τ2 = 46 and 35 ps, respectively, which were assigned to torsional relaxation as the molecular conformation changed from the ground-state geometry to a more planar excited-state geometry. The slightly shorter time constants for C-10T2V were accounted for by reduced conformational heterogeneities along the torsional coordinates as compared with L-10T, leading to a decreased overall time of dynamic planarization after photoexcitation. The early emission dynamics of the larger cyclic oligothiophenes also showed a similar dependence on the emission wavelength with time constants of τ1 = 4.6 ps and τ2 = 41 ps in C-15T3V and 2.3 and 24 ps in C-30T6V, respectively. Especially, the shorter time constants of C-30T6V were considered to represent excitation energy transfer to more planar subunits, reducing the overall time of conformational changes in the excited state.26 The femtosecond time-resolved fluorescence spectra of linear and cyclic oligothiophenes were reconstructed based on global fitting results, as shown in Figure 2 and Figure S4 in the Supporting Information. To reveal the spectral evolution 453

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The Journal of Physical Chemistry Letters

from a cyclic geometry, leading to an increase in the intensity of the S1 transition by dynamic planarization. To quantify the exciton size in linear and cyclic oligothiophenes, we obtained optimized structures without nbutyl substituents in the ground and S1 states of L-10T and C10T2V by using DFT calculations at the CAM-B3LYP level with the 6-31(d) basis set as implemented in the Gaussian 09 program. Figure 4 shows the bond length alternations of the

through dynamic planarization processes, the reconstructed time-resolved emission spectra of linear and cyclic oligothiophenes at various time delays after photoexcitation were fitted with the sum of three Gaussians, which have the transition energies of three vibronic peaks (0−0, 0−1, and 0−2) observed in the fluorescence spectra. The amplitude of each of the three Gaussians was allowed to fluctuate freely for the fit, while the widths were shared with each other. On the basis of the fit results, the full width at half-maximum (fwhm) of the vibronic peaks was calculated from the spectrally integrated emission intensity of the 0−0 and 0−1 vibronic peaks (Figure S5 in the Supporting Information). The fwhms of vibronic peaks of L10T and C-10T2V significantly decrease within 200 ps, supporting that the conformational heterogeneity of L-10T and C-10T2V in the ground state is obviously reduced by the dynamic planarization processes in the excited state. It is noteworthy that the relative intensity between the 0−0 and 0−1 bands of C-10T2V changed dramatically until reaching an inversion of intensity, which was in sharp contrast with the spectroscopic features of L-10T and larger rings. To perform a more detailed analysis of the relationship between the spectral evolution and dynamic planarization processes, the overall fluorescence intensity was extracted from the time-resolved fluorescence spectra through integration and is displayed in Figure 3. The increase in the total fluorescence intensity of L-

Figure 4. Bond length alternations for L-10T and C-10T2V in the ground and excited states. Numbers (1−10 presented in the plot represent the constituent thiophene rings.

constituted thiophene rings in the ground and S1 states for L10T and C-10T2V, respectively. Our calculations revealed that the bond length of double and single bonds in the S1 state increased and decreased, respectively, as compared to that in the ground state, indicating a conformational change from benzoid to quinoid structures. Therefore, together with recent theoretical reports exhibiting the extent of exciton delocalization in cycloparaphenylenes and porphyrin nanorings,42,43 a plot of bond length alternation can also be useful to predict the exciton delocalization length in conjugated cyclic chains.10,44 Despite the completely planar structure in the calculation results in both the ground and S1 state, the plot of L-10T and C-10T2V revealed a contrasting feature with respect to the exact localization length. In the case of L-10T, the bond length of single and double bonds in the S1 state was noticeably changed in the middle of the molecule but not in both sides as compared to the ground-state geometry. However, the S1 state geometry of C-10T2V exhibited an overall change in terms of bond length of single and double bonds as compared to ground-state geometry, reflecting a fully delocalized exciton over the entire ring in contrast to the localized exciton of L10T. Although the optimized structure of C-10T2V cannot completely represent the molecular structure in solution, combined with the inversion of intensity between the 0−0 and 0−1 bands observed in the time-resolved fluorescence spectrum of C-10T2V, the calculated results strongly support

Figure 3. Total emission intensity calculated by integration over the shown spectral range.

10T was considered to represent growth of the oscillator strength in the S1 state, supporting the increase in the degree of π-conjugation in the spectroscopic units through dynamic planarization processes.41 In contrast with L-10T, a decrease of total fluorescence intensity was observed in C-10T2V, indicating a reduced S1 transition by dynamic planarization processes. As we consider the TD-DFT calculation to show a forbidden S1 transition in a perfect cyclic geometry, the observation of C10T2V suggested two phenomena, namely, (1) the dynamic planarization along the cyclic backbone recovers the ring symmetry and (2) the spectroscopic unit in the ring has a nearly cyclic structure. In this respect, the decrease of the 0−0 band as compared to the 0−1 band observed in the timeresolved fluorescence spectrum of C-10T2V is directly related with the conformational change from distorted to planar cyclic molecular geometry by the dynamic planarization. Furthermore, on the basis of the contrasting results between L-10T and C-10T2V, the evolution of total emission intensity observed in C-15T3V and C-30T6V was taken as direct evidence of the shape of the spectroscopic unit in larger rings deforming away 454

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The Journal of Physical Chemistry Letters

(2) Sirringhaus, H.; Tessler, N.; Friend, R. H. Integrated Optoelectronic Devices Based on Conjugated Polymers. Science 1998, 280, 1741−1744. (3) Brédas, J.-L.; Beljonne, D.; Coropceanu, V.; Cornil, J. ChargeTransfer and Energy-Transfer Processes in π-Conjugated Oligomers and Polymers: A Molecular Picture. Chem. Rev. 2004, 104, 4971− 5004. (4) Barford, W. Electronic and Optical Properties of Conjugated Polymers; Oxford University Press: New York, U.K., 2005. (5) Heeger, A. J. Semiconducting Polymers: The Third Generation. Chem. Soc. Rev. 2010, 39, 2354−2371. (6) Facchetti, A. π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications. Chem. Mater. 2011, 23, 733−758. (7) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.; Durrant, J. R. Device Annealing Effect in Organic Solar Cells with Blends of Regioregular Poly(3-hexylthiophene) and Soluble Fullerene. Appl. Phys. Lett. 2005, 86, 063502. (8) Hwang, I.; Scholes, G. D. Electronic Energy Transfer and Quantum-Coherence in π-Conjugated Polymers. Chem. Mater. 2011, 23, 610−620. (9) Cornil, J.; Beljonne, D.; Heller, C. M.; Campbell, I. H.; Laurich, B. K.; Smith, D. L.; Bradley, D. D. C.; Mű llen, K.; Brédas, J. L. Photoluminescence Spectra of Oligo-Paraphenyllenevinylenes: A Joint Theoretical and Experimental Characterization. Chem. Phys. Lett. 1997, 278, 139−145. (10) Tretiak, S.; Saxena, A.; Martin, R. L.; Bishop, A. R. Conformational Dynamics of Photoexcited Conjugated Molecules. Phys. Rev. Lett. 2002, 89, 097402. (11) Banerji, N.; Cowan, S.; Vauthey, E.; Heeger, A. J. Ultrafast Relaxation of the Poly(3-hexylthiophene) Emission Spectrum. J. Phys. Chem. C 2011, 115, 9726−9739. (12) Busby, E.; Carroll, E. C.; Chinn, E. M.; Chang, L.; Moulé, A. J.; Larsen, D. S. Excited-State Self-Trapping and Ground-State Relaxation Dynamics in Poly(3-hexylthiophene) Resolved with Broadband Pump−Dump−Probe Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 2764−2769. (13) Westenhoff, S.; Beenken, W. J. D.; Yartsev, A.; Greenham, N. C. Conformational Disorder of Conjugated Polymers. J. Chem. Phys. 2006, 125, 154903. (14) Parkinson, P.; Müller, C.; Stingelin, N.; Johnston, M. B.; Herz, L. M. Role of Ultrafast Torsional Relaxation in the Emission from Polythiophene Aggregates. J. Phys. Chem. Lett. 2010, 1, 2788−2792. (15) Jasti, R.; Bhattacharjee, J.; Neaton, J. B.; Bertozzi, C. R. Synthesis, Characterization, and Theory of [9]-, [12]-, and [18]Cycloparaphenylene: Carbon Nanohoop Structures. J. Am. Chem. Soc. 2008, 130, 17646−17647. (16) Yamago, S.; Watanabe, Y.; Iwamoto, T. Synthesis of [8]Cycloparaphenylene from a Square-Shaped Tetranuclear Platinum Complex. Angew. Chem., Int. Ed. 2010, 49, 757−759. (17) Sisto, T. J.; Golder, M. R.; Hirst, E. S.; Jasti, R. Selective Synthesis of Strained [7]Cycloparaphenylene: An Orange-Emitting Fluorophore. J. Am. Chem. Soc. 2011, 133, 15800−15802. (18) Iwamoto, T.; Watanabe, Y.; Sakamoto, Y.; Suzuki, T.; Yamago, S. Selective and Random Syntheses of [n]Cycloparaphenylenes (n = 8−13) and Size Dependence of Their Electronic Properties. J. Am. Chem. Soc. 2011, 133, 8354−8361. (19) Krömer, J.; Rios-Carreras, I.; Fuhrmann, G.; Musch, C.; Wunderlin, M.; Debaerdemaeker, T.; Mena-Osteritz, E.; Bäuerle, P. Synthesis of the First Fully α-Conjugated Macrocyclic Oligothiophenes: Cyclo[n]thiophenes with Tunable Cavities in the Nanometer Regime. Angew. Chem., Int. Ed. 2000, 39, 3481−3486. (20) Zhang, F.; Götz, G.; Winkler, H. D. F.; Schalley, C. A.; Bäuerle, P. Giant Cyclo[n]thiophenes with Extended π Conjugation. Angew. Chem., Int. Ed. 2009, 48, 6632−6635. (21) Mayor, M.; Dischdies, C. A Giant Conjugated Molecular Ring. Angew. Chem., Int. Ed. 2003, 42, 3176−3179. (22) Nakao, K.; Nishimura, M.; Tamachi, T.; Kuwatami, Y.; Miyasaka, H.; Nishinaga, T.; Iyoda, M. Giant Macrocycles Composed

that the smaller 0−0 band relative to the 0−1 band in C-10T2V is due to the exciton delocalization along the cyclic molecular backbone. However, the increase in the ring size from C-10T2V induces the exciton localization on specific subunits of the entire rings, leading to the increase of the 0−0 band as compared to the 0−1 band. In summary, we have investigated the exciton delocalization of linear and cyclic oligothiophenes in the course of the dynamic planarization processes with the use of time-resolved fluorescence spectra and theoretical calculations. Especially, from the observation of the inversion between the 0−0 and 0− 1 bands in the time-resolved fluorescence spectra of C-10T2V, we propose that a cyclic geometry reduces conformational disorder of the chain, thereby leading to a rigid planar structure that effectively delocalizes the exciton over the molecular backbone. However, the increase in ring size leads to exciton localization on specific units in the rings, resulting from deformation away from a cyclic geometry. Collectively, our findings demonstrated that the evolution of the vibronic peak ratio between 0−0 and 0−1 bands provides a new insight into the degree of exciton delocalization in cyclic molecular systems. In this respect, we are currently using this approach to explore the correlation between the vibronic peaks and exciton delocalization in the excited states of these systems.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental methods, energetic MO levels and frontier MOs, time-resolved fluorescence spectra and fluorescence decay profiles for C-15T3V and C-30T6V, FWHMs of vibronic peaks in time-resolved fluorescence spectra, and calculated transition energies, oscillator strength and major contribution of transitions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.K.). *E-mail: [email protected] (M.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work at Yonsei University was supported by the Global Frontier R&D Program on Midcareer Researcher Program (2010-0029668) of the National Research Foundation (NRF) and Global Research Laboratory (GRL) Program funded by the Ministry of Science, ICT & Future, Korea (2013K1A1A2A02050183). The quantum calculations were performed using the supercomputing resource of the Korea Institute of Science and Technology Information (KISTI). The work at Tokyo Metropolitan University was partly supported by a Grant-in-Aid for Scientific Research from JSPS and by CREST of JST (Japan Science and Technology Corporation). We would like to thank Dr. Kazumi Nakao and Dr. Hideyuki Shimizu (Tokyo Metropolitan University) and Prof. Hiroshi Ikeda (Osaka Prefecture University) for helpful discussions.

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REFERENCES

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DOI: 10.1021/jz502395z J. Phys. Chem. Lett. 2015, 6, 451−456