Quantitative Sequential Photoenergy Conversion Process from Singlet

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Quantitative Sequential Photoenergy Conversion Process from Singlet Fission to Intermolecular Two-Electron Transfers Utilizing Tetracene Dimer Shunta Nakamura, Hayato Sakai, Hiroki Nagashima, Yasuhiro Kobori, Nikolai V. Tkachenko, and Taku Hasobe ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01964 • Publication Date (Web): 20 Nov 2018 Downloaded from http://pubs.acs.org on November 21, 2018

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ACS Energy Letters

Quantitative Sequential Photoenergy Conversion Process from Singlet Fission to Intermolecular TwoElectron Transfers Utilizing Tetracene Dimer Shunta Nakamura,† Hayato Sakai, † Hiroki Nagashima,§ Yasuhiro Kobori,*,§ Nikolai V. Tkachenko*,‡ and Taku Hasobe*,† †

Department of Chemistry, Faculty of Science and Technology, Keio University, Yokohama, Kanagawa 223-8522, Japan

§

Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan ‡

Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, 33101 Tampere, Finland

ACS Paragon Plus Environment

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ABSTRACT. Singlet fission (SF) theoretically enables to perform the sequential photoenergy conversion process starting from the singlet state and leading to electron transfer (ET) with the radical ion pair quantum yield approaching 200%. Additionally, the long lifetime of the triplet state opens possibility for intermolecular ET process in diffusion-limited reaction. However, the quantitative two-electron transfer process through SF has yet to be reported. Herein we demonstrated the quantitative sequential process involving SF and leading to intermolecular twoelectron transfers using 2,2’-biphenyl-bridged tetracene dimer (Tet-BP-Tet: SF and electron donor) and chloranil (Ch: electron acceptor). The high-yield and long-lived individual triplet excited states of Tet-BP-Tet by SF (ΦT = 175 ± 5% and τT = 0.29 ms) resulted in the quantitative two-electron transfer process (ΦET = 173 ± 5%) with Ch in benzonitrile.

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Singlet fission (SF) generates two triplet excitons from one photon absorption.1-3 This photophysical process theoretically enables to perform the sequential photo-energy conversion process from SF to electron transfer (ET), yielding the radical ion pairs at ~200% quantum yield. Although the related and conceptual systems were recently published,4-6 the quantitative photoinduced process from SF to two-electron transfers has yet to be reported, so far. The typical examples for intramolecular SF (ISF) are covalently linked pentacene (Pc) dimers with highyield triplet quantum yields (ΦT).7-13 However, the high-yield intermolecular ET through ISF is generally difficult to achieve for a few reasons. Firstly, the faster recombination of triplet excited states relative to the conventional intersystem crossing (ISC) pathways preclude the intermolecular ET process considering the diffusion-limited reaction. This means that the longer lifetime of the triplet excited state through ISF is required for the high-yield intermolecular reaction. The other reason also includes the relatively low excitation energies of triplet excited states considering energy matching condition between a singlet and two triplet excited states [E(S1) ≥ 2E(T1)]. For example, the triplet excitation energy of Pc is ~0.8 eV.13 The oxidation potential of the triplet excited state of Pc: [E(Pc•+/3Pc*)] is ~+0.1 V vs. SCE considering the oxidation potential of Pc ground state: E(Pc•+/Pc) = ~0.9 V vs. SCE.10 It’s quite difficult to find appropriate electron acceptor molecules for ISF-mediated intermolecular ET. Therefore, the strong oxidation property of the triplet excited state with the long lifetime is definitely essential for the achievement of high-yield intermolecular ET through ISF.14 In contrast, tetracene (Tet) is a good candidate for ISF-mediated intermolecular ET considering the E(T1): ~1.3 eV and long-lived triplet excited state (0.63 ms).15 It should be noted that there is no example of covalently linked Tet dimers with the quantitative ISF yield and long-lived triplet state in homogeneous solution regardless of a large number of reports on SF.16-19 To attain the


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high-yield and long-lived triplet excited states, we employed a 2,2’-biphenyl unit as a linker of Tet dimer because of the appropriate control of the electronic coupling between two chromophores. We previously reported the high-yield and long-lived triplet excited states by ISF using 2,2’-biphenyl-bridged Pc dimer.9 On the other hand, tetrachloro-1,4-benzoquinone, i.e., chloranil (Ch) is a good electron acceptor according to the reported reduction potential (~0 V vs SCE).20 Herein, we demonstrated the quantitative sequential process from ISF to intermolecular two-electron transfers using 2,2’-biphenyl-bridged Tet dimer (Tet-BP-Tet in Scheme 1). The obtained high-yield and long-lived triplet excited states of Tet-BP-Tet by ISF (ΦT = 175 ± 5% and τT = 0.29 ms) resulted in the quantitative intermolecular two-electron transfer process with Ch in benzonitrile (PhCN) (ΦET = 173 ± 5%).

Scheme 1. (A) Chemical Structures of Tetracene Derivatives and Chloranil. (B) A Conceptual Scheme of Sequential Photoenergy Conversion Process from SF to intermolecular two-electron transfers in this study.

Tet-BP-Tet was synthesized by Suzuki-Miyaura coupling of TIPS-tetracene with pinacol borane and 2,2’-dibromobiphenyl.21 The synthetic details are shown in Scheme S1 in Supporting Information (SI). The structure of obtained Tet-BP-Tet was also confirmed by 1H and 13C NMR


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and MALDI-TOF MS (Figures S1-S3 in SI). To study the structure of Tet-BP-Tet, the density functional theory (DFT) calculations were performed with Gaussian 09 at B3LYP/6-31G** level of theory for optimization of the molecular structure (Figure S4 in SI). For example, the centerto-center distance between two Tet units was ca. 13.9 Å. Then, absorption and fluorescence spectra were measured in PhCN as shown in Figure S5 in SI. The absorption spectrum of TetBP-Tet became red-shifted as compared with Tet-ref (Figure S5A in SI). Noticeably, there is no absorption of Ch in the range from ca. 340 to 650 nm. This means that we can selectively excite the Tet unit. The normalized fluorescence spectrum of Tet-BP-Tet also shows the similar redshifted trend as compared with Tet-ref (Figure S5B in SI). Based on these results, the excitation energy of the singlet excited state in Tet-BP-Tet [E(S1)] was estimated to be 2.23 eV, which is approximately two times larger than the triplet energy in Tet unit [(E(T1) = 1.23 eV)].22 To evaluate the ultrafast excited-state dynamics (i.e., ISF) of Tet-BP-Tet, femtosecond transient absorption (fsTA) spectra were measured in PhCN (Figure 1A). The triplet-triplet (T– T) absorption band of the Tet unit in Tet-BP-Tet was separately assigned by using the energy transfer from anthracene to Tet-BP-Tet (Figure S6 in SI). The T-T spectra of Tet-BP-Tet were seen in the range from 450 to 550 nm. The singlet-singlet (S–S) absorption spectrum of Tet-ref (monomer) is separately shown in Figure S7 in SI for comparison. After fs pump pulse excitation at 350 nm and a subpucosecond thermalization the system is the lowest energy singlet excited state, and S-S absorption spectra of Tet-BP-Tet can be observed at 2.6 ps (Figure 1A). A triplettriplet (T-T) band at ca. 530 nm and ca. 605 nm gradually developed together with decay of S-S bands at ca. 410 nm and ca. 770 nm, whereas the competitive reaction between SF and triplettriplet annihilation (TTA) could be noticed since some S-S absorption bands remain in the region from 600-750 nm. The comparison of rise and decay in the time-profiles at 604 nm and 772 nm


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corresponding to the T-T and S-S absorption signals also suggests the occurrence of ISF (see the inset of Figure 1A). Here we will suppose that the reverse reaction, TTA, can be ignored relative to ISF. The apparent rate constant of ISF (kISF) was determined to be 7.1 × 1010 s–1 by the decay component analysis (see: Figure S8 in SI). This value is five orders of magnitude greater than that of the intersystem crossing (ISC) pathway (kISC: 8.5 × 105 s–1) (See: Figure S9, Table S1 and the calculation process in SI).

Figure 1. (A) fsTA spectra of Tet-BP-Tet in PhCN (excited at 350 nm) and the inset shows the time-profiles at 604 nm (blue) and 772 nm (red). (B) nsTA spectra of Tet-BP-Tet in PhCN (excited at 550 nm) and the inset shows the time-profile at 530 nm (black) and fitting curve at 530 nm (red). (C) µsTA spectra of Tet-BP-Tet in PhCN (excited at 532 nm) and the inset shows the time-profile at 500 nm (black) and fitting curve at 500 nm (red).


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To carefully examine the transient spectra of Tet-BP-Tet in the longer time scale, nanosecond transient absorption (nsTA) spectra were measured as shown in Figure 1B. With increasing the time, the spectral shapes became similar to the pristine T-T absorption spectrum confirmed by energy transfer process (Figure S6 in SI). This is in sharp contrast with the transient spectra observed by fsTA. The trace of Tet-BP-Tet at 530 nm was fully fitted by a biexponential function (at t >1 ps). The initial and faster component is attributable to the mixtures of singlet and correlated triplet-triplet (TT) states because of the competitive reaction between ISF and TTA. The other much slower one corresponds to the individual triplet states (T + T) according to the previous report.10 Thus, we clearly identified the T-T absorption of Tet-BP-Tet. The individual triplet quantum yield (ΦT) of Tet-BP-Tet was quantitatively calculated to be 175 ± 5% following the calculation process outlined in SI and using data presented in Figure S10 and Table S2-S3 in SI.10 Additionally, the rate constant of the dissociation of TT (kD) (i.e., the transition from TT to T + T) was determined to be 6.4 × 106 s–1 (see the calculation process in SI). Finally, microsecond transient absorption (µsTA) spectra demonstrated the long lifetime species of 3Tet*BP-3Tet*. The lifetime of 3Tet*-BP-3Tet* was also determined to be τT = 0.29 ms (i.e., kT = 3.4 × 103 s–1) by monoexponential fitting as shown in Figure 1C. Moreover, to confirm the direct evidence of ISF in Tet-BP-Tet, the quintet state of 5(TT) was observed using the time-resolved electron spin resonance measurements (TRESR), as shown in Figure S11 in SI.10, 23-24 To further discuss the intermolecular ET process from 3Tet*-BP-3Tet* (Electron Donor: D) to two Ch units (Electron Acceptor: A), first, the cyclic voltammograms of Tet-BP-Tet and Ch were measured (Figure S12 in SI). The first one-electron oxidation potential (Eox) of Tet-BP-Tet was determined to be +1.02 V vs SCE, while the first one-electron reduction peak of Ch (+0.04 V vs SCE) was also seen. Noted that the comparison of voltammograms between Tet-BP-Tet and Tet-


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ref indicates that the Tet unit in Tet-BP-Tet behaves like a monomer (see: Figure S12 in SI). This means the relatively weak interaction between two Tet units. Based on the Eox value of Tet unit and the reported energy of 3Tet* (1.23 eV),22 the first one-electron oxidation potential of 3Tet* was estimated to be −0.21 V vs SCE. Because the one-electron reduction potential of Ch (Ered = 0.04 V vs SCE: Figure S12C in SI) is higher than the one-electron oxidation potential of 3Tet*, the driving force of intermolecular ET from 3Tet* to Ch is negative (−ΔGET = 0.25 eV) and the ET process is energetically favorable.

Figure 2. (A) µsTA spectra of Tet-BP-Tet (50 µM) in the presence of 250 µM Ch in Arsaturated PhCN after excitation at 532 nm. (B) The corresponding time-profiles at 450 nm and 520 nm. The inset shows the second-order plots. (C) The time profiles of absorption at 520 nm in the presence of different concentrations of Ch. (a) 0 mM, (b) 0.25 mM, (c) 0.53 mM, (d) 0.63 mM, (e) 0.72 mM Insert: plots of the pseudo-first-order rate constant (kobs) vs the concentration of Ch.


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Additionally, absorption spectral changes from Tet to Tet•+ and from Ch to Ch•– were successfully observed by spectroelectrochemical methods (Figure S13 in SI). The molar absorbance coefficients (ε) of Tet radical cation (Tet•+) and Ch radical anion (Ch•–) were determined to be 1,110 M−1 cm−1 at 940 nm and 3,850 M−1 cm−1 at 450 nm, respectively (Figures S13 and S14 in SI). Based on the above results, intermolecular ET between 3Tet*-BP-3Tet* (D) to 2Ch (A) were studied in PhCN by µsTA measurements (Figure 2). The excitation wavelength was chosen to be 532 nm to excite selectively the only Tet unit (see: Figure S5 in SI). Figure 2A demonstrates the µsTA spectra in a mixed solution containing Tet-BP-Tet (50 µM) and Ch (0.25 mM). The T-T absorption of Tet-BP-Tet was immediately seen at 520 nm after laser pulse excitation. The decay of T-T absorption of Tet-BP-Tet coincides with the developments of Ch radical anion (Ch•–) at 450 nm and Tet radical cation (Tet•+) at 1000 nm (Figure 2A), indicating the occurrence of intermolecular ET from 3Tet*-BP-3Tet* to Ch. Noted that the red-shifted trend of Tet•+-BP-Tet•+ relative to Tet•+-ref may be due to the substituent effect in the dimeric form. The decay of the absorbance at 450 nm arises from the back electron-transfer (BET) reaction. The second-order plot (the inset of Figure 2B and Figure S15 in SI) for the decay of Ch•– was obtained from the absorbance at 450 nm and molar absorption coefficient of Ch•– determined by the electrochemical reduction of Ch (3,850 M–1 cm–1 at 450 nm). From the slope of the linear plot, the second-order rate constant of the back electron transfer (BET) from Ch•– to Tet•+ was calculated to be 5.0 × 109 M–1 s–1.25 This value is very close to the diffusion-limit in PhCN (5.6 × 109 M–1 s–1).26-28


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The decay rate constant of the T−T absorption at 520 nm increased with increasing the concentrations of Ch (0 - 0.72 mM) (Figure 2C). Figure 2C also shows a linear plot of the observed decay rate constant (kobs) at 520 nm based on the concentrations of Ch. From the slope of the linear plot, the second-order rate constant of intermolecular ET from 3Tet*-BP-3Tet* to Ch was determined to be 4.8 × 108 M–1 s–1.25 This value is slightly smaller than that of diffusion limited value in PhCN (5.6 × 109 M–1 s–1).26-28 The possible reason may be due to a small driving force of the intermolecular ET from 3Tet* to Ch (vide supra) or two-electron transfer processes in Tet-BP-Tet. Finally, the intermolecular ET yield (ΦET) was quantitatively estimated to be 167 ± 7% at 0.25 mM Ch utilizing the ε of Ch·–. The maximum ΦET value attains up to 173 ± 5% with increasing the concentrations of Ch (the maximum concentration of Ch: 0.72 mM) as shown in the calculation processes and Tables S4-S10 in SI. The ΦET value evaluated by the ε of Tet·+ was also similar to the above one (ΦET: ~1.7) (see: Table S11 and Figure S16 in SI). Noted that the ΦET values remain constant at higher concentration of Ch (> 1 mM).

Figure 3. Photoenergy conversion process in this study.


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Table 1. Summary of Rate Constants for Sequential Photoenergy Conversion Process. kISF [s–1]

k0 [s–1]

kT [s–1]

kET [M–1 s–1]

kBET [M–1 s–1]

7.1 × 1010

8.5 × 107

3.4 × 103

4.8 × 108

5.0 × 109

Based on the above results and electrochemical data, the intermolecular ET from Tet-BP-Tet to 2Ch was initiated by ISF of Tet-BP-Tet (kISF: 7.1 × 1010 s–1) as summarized in Figure 3 and Table 1. The kISF is much larger than the corresponding deactivation pathway of the singlet excited state (k0 = 8.5 × 107 s–1: Figure S17 in SI). Noted that the bimolecular ET process form the singlet excited state of Tet (1Tet*) to Ch is impossible considering the diffusion-limit in PhCN. The decay rate constant of 3Tet*-BP-3Tet* with 2Ch by intermolecular ET is estimated to be kET = 3.5 × 105 s–1 at the maximum concentration of Ch (0.72 mM) considering the above-mentioned bimolecular rate constant between 3Tet*-BP-3Tet* and Ch (4.8 × 108 M–1 s–1). The much larger kET (3.5 × 105 s–1) relative to kT (3.4 × 103 s–1) suggested the main occurrence of the intermolecular ET process. Finally, the BET reaction from Ch•– to Tet•+ proceeded with the diffusion-limited rate constant. In conclusion, we demonstrated the quantitative sequential photoenergy conversion process from ISF to intermolecular two-electron transfers using Tet-BP-Tet (SF and donor molecule) and chloranil (electron acceptor). The initial ISF process was confirmed by transient absorption and TRESR measurements. The high-yield and long-lived individual triplet pairs of Tet-BP-Tet by intramolecular SF (ΦT = 175 ± 5% and τT = 0.29 ms) resulted in the quantitative intermolecular two-electron transfer process (ΦET = 173 ± 5%) with chloranil in homogeneous solution (benzonitrile). Such a molecular system provides a new perspective for construction of future solar energy conversion systems (e.g., photocatalysis and photovoltaics).


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX. Synthetic details of Tet-BP-Tet and tetracene derivatives, optimized structures of Tet-BPTet estimated by density functional theory (DFT) calculations, steady-state absorption and fluorescence spectra, fsTA spectra, µsTA spectra, fluorescence lifetime profiles, and detailed calculation processes of quantum yields and rate constants of triplet excited states and electron transfer (pdf) AUTHOR INFORMATION *E-mail: [email protected] (T.H.).

*E-mail: [email protected] (Y.K). *E-

mail: [email protected] (N.V.T.). ORCID Taku Hasobe: 0000-0002-4728-9767

Yasuhiro Kobori: 0000-0001-8370-9362

Nikolai

V. Tkachenko: 0000-0002-8504-2335 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was partially supported by JSPS KAKENHI Grant Numbers Nos. JP18H01957, 18K19063, JP17H05270 and JP17H05162 to T.H. and Nos. JP17K14476 and JP17H05381 to


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H.S. We are grateful to Dr. Tatsuo Nakagawa and Mr. Hiroaki Hanada (Unisoku Co., Ltd.) for picosecond transient absorption measurements by Randomly Interleaved Pulse Train Method. This work was carried out by the joint research program of Molecular Photoscience Research Center, Kobe University. REFERENCES (1) Smith, M. B.; Michl, J., Singlet Fission. Chem. Rev. 2010, 110, 6891-6936. (2) Chan, W.-L.; Berkelbach, T. C.; Provorse, M. R.; Monahan, N. R.; Tritsch, J. R.; Hybertsen, M. S.; Reichman, D. R.; Gao, J.; Zhu, X. Y., The Quantum Coherent Mechanism for Singlet Fission: Experiment and Theory. Acc. Chem. Res. 2013, 46, 1321-1329. (3) Monahan, N.; Zhu, X. Y., Charge Transfer–Mediated Singlet Fission. Annu. Rev. Phys. Chem. 2015, 66, 601-618. (4) Kim, H.; Keller, B.; Ho-Wu, R.; Abeyasinghe, N.; Vázquez, R. J.; Goodson, T.; Zimmerman, P. M., Enacting Two-Electron Transfer from a Double-Triplet State of Intramolecular Singlet Fission. J. Am. Chem. Soc. 2018, 140, 7760-7763. (5) Rao, A.; Wilson, M. W. B.; Hodgkiss, J. M.; Albert-Seifried, S.; Bässler, H.; Friend, R. H., Exciton Fission and Charge Generation Via Triplet Excitons in Pentacene/C60 Bilayers. J. Am. Chem. Soc. 2010, 132, 12698-12703. (6) Trinh, M. T.; Pinkard, A.; Pun, A. B.; Sanders, S. N.; Kumarasamy, E.; Sfeir, M. Y.; Campos, L. M.; Roy, X.; Zhu, X. Y., Distinct Properties of the Triplet Pair State from Singlet Fission. Sci. Adv. 2017, 3, e1700241. (7) Lukman, S.; Chen, K.; Hodgkiss, J. M.; Turban, D. H. P.; Hine, N. D. M.; Dong, S.; Wu, J.; Greenham, N. C.; Musser, A. J., Tuning the Role of Charge-Transfer States in Intramolecular Singlet Exciton Fission through Side-Group Engineering. Nat. Commun. 2016, 7, 13622. (8) Basel, B. S.; Zirzlmeier, J.; Hetzer, C.; Phelan, B. T.; Krzyaniak, M. D.; Reddy, S. R.; Coto, P. B.; Horwitz, N. E.; Young, R. M.; White, F. J., et al., Unified Model for Singlet Fission within a Non-Conjugated Covalent Pentacene Dimer. Nat. Commun. 2017, 8, 15171. (9) Sakuma, T.; Sakai, H.; Araki, Y.; Mori, T.; Wada, T.; Tkachenko, N. V.; Hasobe, T., Long-Lived Triplet Excited States of Bent-Shaped Pentacene Dimers by Intramolecular Singlet Fission. J. Phys. Chem. A 2016, 120, 1867-1875. (10) Sakai, H.; Inaya, R.; Nagashima, H.; Nakamura, S.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T., Multiexciton Dynamics Depending on Intramolecular Orientations in Pentacene Dimers: Recombination and Dissociation of Correlated Triplet Pairs. J. Phys. Chem. Lett. 2018, 9, 3354-3360. (11) Kato, D.; Sakai, H.; Tkachenko, N. V.; Hasobe, T., High-Yield Excited Triplet States in Pentacene Self-Assembled Monolayers on Gold Nanoparticles through Singlet Exciton Fission. Angew. Chem. Int. Ed. 2016, 55, 5230-5234. (12) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Trinh, M. T.; Choi, B.; Xia, J.; Taffet, E. J.; Low, J. Z.; Miller, J. R.; Roy, X., et al., Quantitative Intramolecular Singlet Fission in Bipentacenes. J. Am. Chem. Soc. 2015, 137, 8965-8972.


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(13) Zirzlmeier, J.; Lehnherr, D.; Coto, P. B.; Chernick, E. T.; Casillas, R.; Basel, B. S.; Thoss, M.; Tykwinski, R. R.; Guldi, D. M., Singlet Fission in Pentacene Dimers. Proc. Natl. Acad. Sci. USA 2015, 112, 5325-5330. (14) Mirzayev, R.; Mustonen, K.; Monazam, M. R. A.; Mittelberger, A.; Pennycook, T. J.; Mangler, C.; Susi, T.; Kotakoski, J.; Meyer, J. C., Buckyball Sandwiches. Sci. Adv. 2017, 3, e1700176. (15) Burgdorff, C.; Ehrhardt, S.; Loehmannsroeben, H. G., Photophysical Properties of Tetracene Derivatives in Solution. 2. Halogenated Tetracene Derivatives. J. Phys. Chem. 1991, 95, 4246-4249. (16) Cook, J. D.; Carey, T. J.; Damrauer, N. H., Solution-Phase Singlet Fission in a Structurally Well-Defined Norbornyl-Bridged Tetracene Dimer. J. Phys. Chem. A 2016, 120, 4473-4481. (17) Liu, H.; Wang, Z.; Wang, X.; Shen, L.; Zhang, C.; Xiao, M.; Li, X., Singlet Exciton Fission in a Linear Tetracene Tetramer. J. Mater. Chem. C 2018, 6, 3245-3253. (18) Burdett, J. J.; Bardeen, C. J., The Dynamics of Singlet Fission in Crystalline Tetracene and Covalent Analogs. Acc. Chem. Res. 2013, 46, 1312-1320. (19) Korovina, N. V.; Das, S.; Nett, Z.; Feng, X.; Joy, J.; Haiges, R.; Krylov, A. I.; Bradforth, S. E.; Thompson, M. E., Singlet Fission in a Covalently Linked Cofacial Alkynyltetracene Dimer. J. Am. Chem. Soc. 2016, 138, 617-627. (20) D'Souza, F., Molecular Recognition Via Hydroquinone−Quinone Pairing: Electrochemical and Singlet Emission Behavior of [5,10,15-Triphenyl-20-(2,5-DihydroxyPhenyl)Porphyrinato]Zinc(II)−Quinone Complexes. J. Am. Chem. Soc. 1996, 118, 923-924. (21) Fuemmeler, E. G.; Sanders, S. N.; Pun, A. B.; Kumarasamy, E.; Zeng, T.; Miyata, K.; Steigerwald, M. L.; Zhu, X. Y.; Sfeir, M. Y.; Campos, L. M., et al., A Direct Mechanism of Ultrafast Intramolecular Singlet Fission in Pentacene Dimers. ACS Cent. Sci. 2016, 2, 316-324. (22) Stern, H. L.; Musser, A. J.; Gelinas, S.; Parkinson, P.; Herz, L. M.; Bruzek, M. J.; Anthony, J.; Friend, R. H.; Walker, B. J., Identification of a Triplet Pair Intermediate in Singlet Exciton Fission in Solution. Proc. Natl. Acad. Sci. USA 2015, 112, 7656-7661. (23) Weiss, L. R.; Bayliss, S. L.; Kraffert, F.; Thorley, K. J.; Anthony, J. E.; Bittl, R.; Friend, R. H.; Rao, A.; Greenham, N. C.; Behrends, J., Strongly Exchange-Coupled Triplet Pairs in an Organic Semiconductor. Nat. Phys. 2017, 13, 176-181. (24) Nagashima, H.; Kawaoka, S.; Akimoto, S.; Tachikawa, T.; Matsui, Y.; Ikeda, H.; Kobori, Y., Singlet-Fission-Born Quintet State: Sublevel Selections and Trapping by Multiexciton Thermodynamics. J. Phys. Chem. Lett. 2018, 9, 5855-5861. (25) Kawashima, Y.; Ohkubo, K.; Fukuzumi, S., Enhanced Photoinduced Electron-Transfer Reduction of Li+@C60 in Comparison with C60. J. Phys. Chem. A 2012, 116, 8942-8948. (26) Davis, C. M.; Kawashima, Y.; Ohkubo, K.; Lim, J. M.; Kim, D.; Fukuzumi, S.; Sessler, J. L., Photoinduced Electron Transfer from a Tetrathiafulvalene-Calix[4]Pyrrole to a Porphyrin Carboxylate within a Supramolecular Ensemble. J. Phys. Chem. C 2014, 118, 13503-13513. (27) D'Souza, F.; Ito, O., Photoinduced Electron Transfer in Supramolecular Systems of Fullerenes Functionalized with Ligands Capable of Binding to Zinc Porphyrins and Zinc Phthalocyanines. Coord. Chem. Rev. 2005, 249, 1410-1422. (28) Fukuzumi, S.; Ohkubo, K.; Imahori, H.; Guldi, D. M., Driving Force Dependence of Intermolecular Electron-Transfer Reactions of Fullerenes. Chem. Eur. J. 2003, 9, 1585-1593.


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