Absence of Intramolecular Singlet Fission in Pentacene

Hasharoni , K.; Levanon , H.; Greenfield , S. R.; Gosztola , D. J.; Svec , W. A.; Wasielewski , M. R. Mimicry of the Radical Pair and Triplet-States i...
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Letter Cite This: J. Phys. Chem. Lett. 2017, 8, 5609-5615

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Absence of Intramolecular Singlet Fission in Pentacene− Perylenediimide Heterodimers: The Role of Charge Transfer State Long Wang,†,‡ Yishi Wu,*,†,‡ Jianwei Chen,†,‡ Lanfen Wang,†,‡ Yanping Liu,†,‡ Zhenyi Yu,†,‡,∥ Jiannian Yao,†,‡,§ and Hongbing Fu*,†,‡,§,∥ †

Beijing National Laboratory for Molecules Science (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species & Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ Department of Chemistry, University of Chinese Academy of Sciences, Beijing 100049, China § Beijing Key Laboratory for Optical Materials and Photonic Devices, Department of Chemistry, Capital Normal University, Beijing 100048, China ∥ Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Department of Chemistry, Tianjin University, Tianjin 300072, China S Supporting Information *

ABSTRACT: A new class of donor−acceptor heterodimers based on two singlet fission (SF)-active chromophores, i.e., pentacene (Pc) and perylenediimide (PDI), was developed to investigate the role of charge transfer (CT) state on the excitonic dynamics. The CT state is efficiently generated upon photoexcitation. However, the resulting CT state decays to different energy states depending on the energy levels of the CT state. It undergoes extremely rapid deactivation to the ground state in polar CH2Cl2, whereas it undergoes transformation to a Pc triplet in nonpolar toluene. The efficient triplet generation in toluene is not due to SF but CT-mediated intersystem crossing. In light of the energy landscape, it is suggested that the deep energy level of the CT state relative to that of the triplet pair state makes the CT state actually serve as a trap state that cannot undergoes an intramolecular singlet fission process. These results provide guidance for the design of SF materials and highlight the requisite for more widely applicable design principles. inglet fission (SF), the ultrafast splitting of a singlet exciton (S1) into two triplets (2T1) with spin conservation, may increase the solar energy conversion efficiency beyond the Shockley−Queisser limit.1−3 On the basis of the experimental and theoretical studies, a generalized dimer model mechanism has been postulated for SF, possibly involving a charge transfer (CT) state, as in eq 1

S

3 3 [1(S1S0) ↔ CT ↔ 1(TT)] ⇝ 1(TT) 1 1 1 1 ⇝ T1 + T1

polarity could serve to make SF competitive with CT state formation in a terrylenediimide dimer.19 The Musser group reported that efficient intramolecular singlet fission (iSF) was observed in pentacene (Pc) dimers via either a virtual or distinct CT state, which was governed by the energy gap between singlet and CT states with different side groups.20 In these dimers, SF is an intrinsic property of the molecule referred to as iSF; therefore, local order and intermolecular coupling are no longer design constraints. These results also provide some preliminary design criteria for new iSF-capable materials and suggest that the CT state as a virtual form or distinct intermediate plays a significant role in ultrafast SF processes.19−22 Although significant progress has been obtained, the pool of SF materials remains relatively small, with most focus on the same or similar chromophores, such as polyacenes,12−16,18,23−28 polyenes,7,29−31 and their derivatives.32,33 Campos, Wasielewski, and Guldi et al. have independently reported that some acene derivatives and terrylenediimide can be used as the active

(1)

where S0 is the ground state and the intermediate 1(T1T1) is the correlated triplet pair with both singlet spin and doubleexcitation characters.4−6 To observe SF, the minimum requirement is the presence of a pair of adjacent chromophores with the correct energetics condition [2E(T1) ≈ E(S1)] and moderate interchromophore electronic coupling. It is evident that these processes require a delicate balance of chemical design parameters, such as film morphology and crystal packing in solid films and aggregates.7−11 Recently, covalently linked dimers have been successfully developed and provided uniquely detailed insight into the SF mechanism, particularly with regard to the influence of CT states.12−18 For example, the Wasielewski group presented that adjusting the CT state energy via chromophore slip-stacking and decreasing solvent © XXXX American Chemical Society

Received: October 1, 2017 Accepted: October 31, 2017 Published: October 31, 2017 5609

DOI: 10.1021/acs.jpclett.7b02597 J. Phys. Chem. Lett. 2017, 8, 5609−5615

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The Journal of Physical Chemistry Letters unit for the construction of new iSF-active materials.12,16−19 It has also been demonstrated that the donor−acceptor (DA) linked copolymers, which consist of a strong donor and strong acceptor, can promote highly efficient CT-mediated iSF.22 Inspired by these works, we presented our audacious attempts to develop potential SF materials by combining a chromophore module combination strategy with SF mechanistic studies in the reported homodimers and polymers, i.e., DA heterodimer (Chart 1). We choose the prototype compound of Pc as a donor due to its relatively low triplet energy and exoergic character for SF. The modularity of the DA scheme allows us to select perylenediimide (PDI) as a good acceptor unit to tune the absorption relative to the triplet energy. Therefore, the heterodimer system composed of a Pc and PDI chromophore unit not only fulfills the energetic condition for SF (ES1(Pc) ≈ ET1(Pc) + ET1(PDI)) but exhibits CT character because of DA electronic interaction, which might facilitate fast SF either via an intermediate CT state or as a virtual state accessible through superexchange (Chart 1).19,20 Recently, Yong et al. have demonstrated that the intermediate 1TT state is bound with respect to free triplets with an energy of about dozens of meV. We suggest that the involvement of the intermediate state, either the 1(TT) or CT state, may be helpful for facilitating endothermic SF, which is reasonable considering that the energy barrier of each subreaction is easier to overcome by thermal activation.34,35 In present work, a new family of DA heterodimers (PP and PPP, Chart 1) based on Pc and PDI units has been successfully synthesized and characterized. Combining steady-state measurements and transient absorption (TA) spectroscopy, the role of CT state on the exciton dynamics was investigated as a function of solvent polarity and excitation wavelength. The CT state, which generates rapidly upon excitation of PP and PPP,

demonstrated that this high triplet yield in toluene is actually due to a CT-mediated intersystem crossing (ISC) process. The occurrence of ISC and absence of iSF in the studied systems have been discussed in correlation with the role of the CT state in the energy diagram. The deep energy level of the CT state relative to that of the 1(TT) state is suggested to prohibit the iSF process from CT to the 1(TT) state. Substitution at the bay positions (1,6,7,12-positions) of PDI introduces varying degrees of PDI core distortion away from planarity, while at the headland positions (2,5,8,11-positions), it does not lead to these distortions.36 Considering that the planarity geometrical arrangement has a vital effect on the molecular electronic interaction, the Pc unit is covalently linked to the PDI headland position in the studied system. In addition, a phenyl spacer is also used to separate Pc from the PDI unit in dimer PPP, which is also adopted in the reported dimers to adjust the interchromophore couling reaction and relative CT energy.15,19 The synthetic route of heterodimers of PP and PPP is outlined in Scheme 1. The desired products were gained via a six step method including Ru-mediated headland C−H activation37,38 with overall yields of 3.2 and 2.4%, respectively. These new Pc−PDI heterodimers were then characterized by 1 H, 13C NMR, and HR-MS (for details, see the Supporting Information). Steady-state UV−vis absorption spectra of PP and PPP, compared with monomer units, were measured in different solvents at ambient temperature (Figure 1). Results show that absorption spectra exhibit weak solvent polarity dependence and major vibrational bands at around 460, 490, 527, 590, and 650 nm, which roughly matches the sum of the spectra of model compounds of Pc and PDI. It should be noted that the oscillation strength of Pc absorption in PP decreases by over 30% in comparison with that in PPP, indicative of efficient electronic coupling in directly linked PDI−Pc dimer. The absorption spectra from 450 to 550 nm results largely from the PDI subunit, while that in the 600−700 nm range results exclusively from Pc. In sharp contrast to the parent PDI and Pc, the heterodimers show very low photoluminescence efficiency (ΦF < 0.01).39,40 This implies that there are other efficient nonradiative pathways for excited-state quenching. To evaluate the excited-state photophysics, time-resolved absorption spectra in different solvents were measured by femtosecond transient absorption (fs-TA) spectroscopy with an excitation wavelength at 650 nm, where the excitation light is absorbed by nearly 100% Pc unit. Figure 2 illustrates the fs-TA spectra of PPP in CH2Cl2. Immediately after excitation, the spectrum is characterized by ground-state bleaching (GSB) of Pc at 600−650 nm and the intense broad excited-state absorption (ESA) band of 1Pc* in the 430−550 nm range (Figure S2). The 1Pc* absorption band is then rapidly replaced by a strong ESA band featured over 700 nm and another negative structured signal shown in the 470−550 nm range (3.4 ps, Figure 2a). The negative absorption band matches well the ground-state absorption of PDI and can be assigned to the GSB of PDI. Because the energy transfer process is prohibited as a result of the lower energy level of 1Pc* than that of 1PDI*, the emerging absorption band over 700 nm is safely identified as the PDI radical anion (PDI−), which is consistent with the characteristic absorption spectrum of PDI− measured by a spectroelectrochemistry apparatus (Figure S3). This indicates that a CT state is generated. The absorption of the Pc radical cation is at 443 nm, which is partially overlapped with 1Pc* absorption (peaked at 451 nm, Figures S2 and S3). The PDI−

Chart 1. Design Strategy of Heterodimers PP and PPP for SF

recombines directly to the ground state in CH2Cl2. However, population transfer from CT to 3Pc* has been observed in toluene, resulting in triplet yields (ΦT) exceeding 70%. It is 5610

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The Journal of Physical Chemistry Letters Scheme 1. Synthetic Route for PP and PPPa

a) BPO, NBS; b) K2CO3, KI, DMF; c) iPrMgCl in THF(2M), THF; d) SnCl2·2H2O; e) Pd(dppf)2Cl2−CH2Cl2, bis(pinacolato)diboron or 1,4benzenedilboronic acid bis(pinacol) ester, KOAc, 1,4-dioxane; f) RuH2CO(PPh3)2, pinacolone, mesitylene.

a

Scheme 2. Excited-State Dynamic Pathways of PPP in CH2Cl2

Figure 1. Steady-state UV−vis absorption spectra of PP (gray and black lines) and PPP (green and blue lines) in CH2Cl2 and toluene (1 × 10−5 M), along with Pc and PDI monomers.

CH2Cl2. Similarly, photoinduced CT and CR were also observed for PP in CH2Cl2 with τCT = 0.7 ± 0.01 ps and τCR = 5.2 ± 0.8 ps (Figure S4, Table 1). The observed CR rate in PP and PPP is more than 1 order of magnitude larger than that in the reported homodimers published elsewhere.19 It should be noted that there are no other long-lived transient species observed in these dimers in polar solvent, excluding the possibility of CT-mediated iSF. On the basis of the measured steady-state spectra and electrochemical redox potentials as well as the Weller equation, we can calculate the energy levels of the

absorption decays at roughly the same rate as GSB of PDI and Pc, confirming that charge recombination (CR) to the ground state is the dominant deactivation channel of the CT state (Scheme 2). Analysis of the 720 nm kinetic curve results in one rise time constant of 1.3 ± 0.1 ps and one decay time constant of 8.2 ± 0.6 ps (Figure 2b, Table 1). That means ultrafast CT state formation and subsequent recombination for PPP in

Figure 2. (a) fs-TA spectra and (b) kinetic traces of PPP in CH2Cl2 excited at 650 nm. 5611

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Table 1. Summary of Exponential Kinetic Fits for the Excited-State Features of PP and PPP in CH2Cl2 and Toluene Excited at 650 and 480 nm CH2Cl2 λex/nm PP PPP a

650 480 650 480

τCT/ps 0.7 0.3 1.3 0.3

± ± ± ±

0.01 0.03 0.1 0.1

toluene τCR/ps 5.2 7.3 8.2 8.5

± ± ± ±

0.8 0.8 0.6 0.8

τCT1 /ps a

0.5 0.2 1.1 0.6

± ± ± ±

0.2 0.2 0.3 0.2c

τCT2b/ps 8.9 11.0 17.0 15.0

± ± ± ±

0.1 2.0 2.0 1.0

τCR/ns

τT/μs

ΦT/deg

± ± ± ±

38.6

87±16

37.2

74±15

0.37 0.58 2.2 2.9

0.06 0.03 0.5 0.2

CT from the “hot” 1Pc* state. bCT from the “relaxed” 1Pc* state. cEnergy transfer process.

Figure 3. PPP in toluene solution: (a) fs-TA spectra and (b) kinetic traces excited at 650 nm. c) ns-TA spectra and kinetic traces (inset) excited at 532 nm. (d) Comparison of fs-TA spectra at a delay time of 3.1 ns (red line) and ns-TA spectra at a delay time of 0.6 us (black line) and an inverted steady-state absorption curve (blue line).

driving force for CT in PPP is 0.56 eV. The large driving force for CT promotes a rapid CT process. Meanwhile, it can be seen that the CT state energy is far below the energy of the triplet pair state (∼2.1 eV), which makes the iSF process from the CT state highly endoergic. It is known that decreasing the solvent polarity can increase the CT energy and reduce the CT driving force as well.19 Consequently, it can be anticipated that a highlying CT state might open the CT-mediated iSF channel. Meanwhile, the long-lived CT state in nonpolar solution compared with that in polar solution is also beneficial for population transfer from the CT state. Therefore, fs-TA measurements were also conducted on PP and PPP in nonpolar toluene. Actually, the excited-state dynamics of PPP change drastically in toluene (650 nm excitation). As shown in Figure 3, the 1Pc*-associated spectral feature at ∼470 nm was also clearly recognized. Accompanied with the decay of this feature, the positive band corresponding to PDI− appeared in the 700− 750 nm range. Analysis of the 720 nm kinetics results in two rising time constants of 1.1 ± 0.3 and 17 ± 2 ps, indicating CT from “hot” 1Pc* and “relaxed” 1Pc* states.20 As the CT decays,

Scheme 3. Excited-State Dynamic Pathways of PPP in Toluene

locally excited states, the triplet pair state, and CT states (Scheme 2, Table S1; for more details, see the SI). It should be noted that the energy of the triplet pair state is calculated by approximately summing up the energies of 3Pc* and 3PDI*. The energy of the CT state is 1.34 eV in CH2Cl2, and the 5612

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Figure 4. (a) fs-TA spectra and (b) kinetic traces of PPP in toluene excited at 480 nm.

the population of 1Pc* and a subsequent CT process. At this wavelength, the direct iSF process is endothermic by 0.2 eV (Table S1). When the PDI unit is selectively excited, the process becomes exothermic by 0.24 eV and possibly feasible to be observable. As an example of PPP, selective 480 nm photoexcitation of the PDI unit in toluene results in a positive 1 PDI*-based band at 700−750 nm (Figure 4 and S2). The 1 PDI* undergoes an initial decay and then an additional increase during its evolution. Meanwhile, the 1Pc*-centered band appears in concert with the fast decay of 1PDI*, indicating a rapid energy transfer (EnT) process from PDI to Pc. Global analysis of 470 and 720 nm kinetic curves results in two time constants of 0.6 ± 0.2 and 15 ± 0.1 ps, with the former due to EnT and the latter ascribed to CT. Therefore, EnT is perhaps the dominant process after direct light excitation. The formed 1 Pc* via EnT then decays rapidly as the aforementioned pathway to the CT state and finally transforms to a long-lived Pc triplet state. Again, no signal of 3PDI* can be observed, and iSF has been ruled out. On the basis of the above-mentioned results, the CT yield could be roughly estimated from the kinetic traces of PDI GSB at 528 nm (Figure 4b). Compared to the GSB intensities at 0.1−100 ps, a yield value of 80% can be extracted out. Due to the absence of any competitive process and taking the spectra overlapping (at 100 ps) into account, a quasi-quantitive CT yield can be obtained readily. The triplet yield via ISC was also calculated from ns-TA spectra following the procedure reported previously (for details, see the SI).37 87 ± 16 and 74 ± 15% were then determined for PP and PPP, respectively. We suggest that a spin−orbit coupling ISC (SO-ISC) mechanism, namely, 1CT → T1, plays a dominant role in triplet generation in the heterodimers.43,46−49 Our chemical strategy for a long-lived triplet state in a DAbased covalent dimer appears to be successful. However, the triplet state does not originate from the desired SF. These results showed that formation of a CT state whose energy is below those of the singlet state and an estimated triplet pair state results in the CT state serving as a trap state that competes effectively with other processes, including SF. The excited-state dynamics of the Pc−PDI heterodimers are summarized. The major features involve competition between singlet energy transfer and ultrafast CT to form a geminate CT state followed by competition between recombination to a long-lived 3Pc* state and formation of the singlet ground state. A new family of DA heterodimers has been successfully synthesized and characterized. By using steady-state measurements and fs- and ns-TA spectroscopy, the excited-state

another new broad absorption band appears in the 470−550 nm region, overlapping with the PDI GSB signal, and the spectral pattern persists beyond the fs-TA detection range (3.3 ns). Complementary nanosecond flash photolysis (ns-TA) has been performed to make a proper assignment of this long-lived spectral species. After 532 nm light excitation of PPP in toluene, a broad absorption band from 400 to 570 nm along with a distinct negative signal at around 650 nm appears immediately within the instrument response function and features a decay lifetime of τ = 37.2 μs (Figure 3c). In ns-TA spectra, the distinct negative signal at around 650 nm is attributed to the GSB of the Pc unit (peaked at 600 and 650 nm), but there is no accompanying signature of the GSB of the PDI unit (peaked at 490 and 525 nm). Considering the extinction coefficient magnitude of Pc and PDI units in the absorption spectrum of the heterodimer, the slow component can be unambiguously attributed to 3Pc*, which is also consistent with the reported spectra in Pc dimers published elsewhere.15−17 In Figure 3d, we also compare the spectra of fsTA (red solid line, 3.3 ns) and ns photolysis (black solid line, 600 ns) as well as an inverted steady-state absorption spectrum. The residual PDI bleaching is due to the undecayed radical anionic form of PDI. Therefore, the transient species on the μs time scale is exclusively the 3Pc*. This assignment can be also supported by the fact that no appreciative change is observed in the spectral dynamics of ns-TA spectra in aerated or deaerated solution (Figure S5) because of the suppressed triplet energy transfer from 3Pc* to the dissolved oxygen as a result of lower 3 Pc* energy (0.8 eV) compared to that of 1O2 (0.98 eV). PP also shows very similar photodynamics, including CT generation with time constants of 0.5 ± 0.2 and 8.9 ± 0.1 ps and CR (0.37 ns) to yield long-lived 3Pc* (Figure S6). The above-mentioned results of spectra and kinetics analysis clearly proved the ultrafast CT state formation and subsequent CR to yield 3Pc*. No spectral evidence correlated with 3PDI* can be observed, excluding the possibility of iSF from the CT state. We suggest that a Pc triplet generates in these heterodimers via efficient ISC.41−45 The sum of the energy of two independent triplet states is about 2.1 eV (Pc ≈ 0.8 eV, PDI ≈ 1.3 eV), while the energy of the corresponding CT state is calculated to be 1.74 eV in toluene. Triplet pair formation via iSF from the intermediate CT state is energetically uphilled by at least 0.35 eV (Table S1, Scheme 3), which makes the overall energetics highly unfavorable for iSF in these heterodimers. Finally, we have inspected the possibility of a direct iSF pathway from the singlet excited state of PDI (480 nm excitation). For PPP in toluene, excitation of 650 nm results in 5613

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from the Quantum Mechanical Superposition. J. Am. Chem. Soc. 2012, 134, 18295−18302. (6) 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. (7) Dillon, R. J.; Piland, G. B.; Bardeen, C. J. Different Rates of Singlet Fission in Monoclinic Versus Orthorhombic Crystal Forms of Diphenylhexatriene. J. Am. Chem. Soc. 2013, 135, 17278−17281. (8) Johnson, J. C.; Nozik, A. J.; Michl, J. The Role of Chromophore Coupling in Singlet Fission. Acc. Chem. Res. 2013, 46, 1290−1299. (9) Renaud, N.; Sherratt, P. A.; Ratner, M. A. Mapping the Relation between Stacking Geometries and Singlet Fission Yield in a Class of Organic Crystals. J. Phys. Chem. Lett. 2013, 4, 1065−1069. (10) Vallett, P. J.; Snyder, J. L.; Damrauer, N. H. Tunable Electronic Coupling and Driving Force in Structurally Well-Defined Tetracene Dimers for Molecular Singlet Fission: a Computational Exploration Using Density Functional Theory. J. Phys. Chem. A 2013, 117, 10824− 10838. (11) Wang, L.; Olivier, Y.; Prezhdo, O. V.; Beljonne, D. Maximizing Singlet Fission by Intermolecular Packing. J. Phys. Chem. Lett. 2014, 5, 3345−3353. (12) 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. (13) Müller, A. M.; Avlasevich, Y. S.; Schoeller, W. W.; Müllen, K.; Bardeen, C. J. Exciton Fission and Fusion in Bis(Tetracene) Molecules with Different Covalent Linker Structures. J. Am. Chem. Soc. 2007, 129, 14240−14250. (14) 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. (15) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Appavoo, K.; Steigerwald, M. L.; Campos, L. M.; Sfeir, M. Y. Exciton Correlations in Intramolecular Singlet Fission. J. Am. Chem. Soc. 2016, 138, 7289− 7297. (16) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Steigerwald, M. L.; Sfeir, M. Y.; Campos, L. M. Intramolecular Singlet Fission in Oligoacene Heterodimers. Angew. Chem., Int. Ed. 2016, 55, 3373− 3377. (17) 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. (18) 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. U. S. A. 2015, 112, 5325−5330. (19) Margulies, E. A.; Miller, C. E.; Wu, Y.; Ma, L.; Schatz, G. C.; Young, R. M.; Wasielewski, M. R. Enabling Singlet Fission by Controlling Intramolecular Charge Transfer in π-Stacked Covalent Terrylenediimide Dimers. Nat. Chem. 2016, 8, 1120−1125. (20) Lukman, S.; Chen, K.; Hodgkiss, J. M.; Turban, D. H. P.; Hine, N. D. M.; Dong, S. Q.; Wu, J. S.; 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. (21) Margulies, E. A.; Logsdon, J. L.; Miller, C. E.; Ma, L.; Simonoff, E.; Young, R. M.; Schatz, G. C.; Wasielewski, M. R. Direct Observation of a Charge-Transfer State Preceding High-Yield Singlet Fission in Terrylenediimide Thin Films. J. Am. Chem. Soc. 2017, 139, 663−671. (22) Busby, E.; Xia, J.; Wu, Q.; Low, J. Z.; Song, R.; Miller, J. R.; Zhu, X. Y.; Campos, L. M.; Sfeir, M. Y. A Design Strategy for Intramolecular Singlet Fission Mediated by Charge-Transfer States In DonorAcceptor Organic Materials. Nat. Mater. 2015, 14, 426−433.

photophysical processes are studied systematically for the possibility for SF. Results show that the CT state appears on ultrafast time scales and then undergoes extremely rapid deactivation to the ground state in polar CH2Cl2, whereas it undergoes transformation to long-lived species in nonpolar toluene. Although these long-lived species are confirmed to be triplet states, there is no evidence for the desired SF process. An efficient ISC is responsible instead. Our work reveals that formation of a CT state whose energy is below those of singlet state and triplet pairs results in the CT state serving as a trap state that competes effectively with SF. Because the CT energy is mostly influenced by the redox potentials of the electron donor and electron acceptor, on the basis of the calculated energy level diagram, we suggest that a heterodimer strategy might work when a less strong donor unit and moderate acceptor unit are covalently linked in a proper manner. Our results set important lessons for new SF material design and highlight the requisite for more widely applicable design principles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02597. Materials and methods, synthetic procedures, 1H NMR, 13 C NMR, full transient absorption data, and fitting (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.W). *E-mail: [email protected] (H.F.). ORCID

Hongbing Fu: 0000-0003-4528-189X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973) 2017YFA0204503, the National Natural Science Foundation of China (Grant Nos. 21190034, 91222203, 21273251, 21221002, 91333111, 21503139, and 21673144), the Beijing Natural Science Foundation of China (Grant No. 2162011), Project of State Key Laboratory on Integrated Optoelectronics of Jilin University (IOSKL2014KF16), and the Youth Innovative Research Team of Capital Normal University.

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REFERENCES

(1) Smith, M. B.; Michl, J. Singlet Fission. Chem. Rev. 2010, 110, 6891−6936. (2) Smith, M. B.; Michl, J. Recent Advances in Singlet Fission. Annu. Rev. Phys. Chem. 2013, 64, 361−386. (3) Congreve, D. N.; Lee, J.; Thompson, N. J.; Hontz, E.; Yost, S. R.; Reusswig, P. D.; Bahlke, M. E.; Reineke, S.; Van Voorhis, T.; Baldo, M. A. External Quantum Efficiency above 100% in a Singlet-ExcitonFission-Based Organic Photovoltaic Cell. Science 2013, 340, 334−337. (4) Chan, W.-L.; Ligges, M.; Jailaubekov, A.; Kaake, L.; Miaja-Avila, L.; Zhu, X. Y. Observing the Multiexciton State in Singlet Fission and Ensuing Ultrafast Multielectron Transfer. Science 2011, 334, 1541− 1545. (5) Chan, W.-L.; Tritsch, J. R.; Zhu, X. Y. Harvesting Singlet Fission for Solar Energy Conversion: One- Versus Two-Electron Transfer 5614

DOI: 10.1021/acs.jpclett.7b02597 J. Phys. Chem. Lett. 2017, 8, 5609−5615

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The Journal of Physical Chemistry Letters (23) Wilson, M. W. B.; Rao, A.; Clark, J.; Kumar, R. S. S.; Brida, D.; Cerullo, G.; Friend, R. H. Ultrafast Dynamics of Exciton Fission in Polycrystalline Pentacene. J. Am. Chem. Soc. 2011, 133, 11830−11833. (24) Roberts, S. T.; McAnally, R. E.; Mastron, J. N.; Webber, D. H.; Whited, M. T.; Brutchey, R. L.; Thompson, M. E.; Bradforth, S. E. Efficient Singlet Fission Discovered in a Disordered Acene Film. J. Am. Chem. Soc. 2012, 134, 6388−6400. (25) Lee, J.; Bruzek, M. J.; Thompson, N. J.; Sfeir, M. Y.; Anthony, J. E.; Baldo, M. A. Singlet Exciton Fission in a Hexacene Derivative. Adv. Mater. 2013, 25, 1445−1448. (26) Wu, Y.; Liu, K.; Liu, H.; Zhang, Y.; Zhang, H.; Yao, J.; Fu, H. Impact of Intermolecular Distance on Singlet Fission in a Series of Tips Pentacene Compounds. J. Phys. Chem. Lett. 2014, 5, 3451−3455. (27) Margulies, E. A.; Wu, Y.-L.; Gawel, P.; Miller, S. A.; Shoer, L. E.; Schaller, R. D.; Diederich, F.; Wasielewski, M. R. Sub-Picosecond Singlet Exciton Fission in Cyano-Substituted Diaryltetracenes. Angew. Chem., Int. Ed. 2015, 54, 8679−8683. (28) Zhang, Y.-D.; Wu, Y.; Xu, Y.; Wang, Q.; Liu, K.; Chen, J.-W.; Cao, J.-J.; Zhang, C.; Fu, H.; Zhang, H.-L. Excessive Exoergicity Reduces Singlet Exciton Fission Efficiency of Heteroacenes in Solutions. J. Am. Chem. Soc. 2016, 138, 6739−6745. (29) Gradinaru, C. C.; Kennis, J. T. M.; Papagiannakis, E.; van Stokkum, I. H. M.; Cogdell, R. J.; Fleming, G. R.; Niederman, R. A.; van Grondelle, R. An Unusual Pathway of Excitation Energy Deactivation in Carotenoids: Singlet-to-Triplet Conversion on an Ultrafast Timescale in a Photosynthetic Antenna. Proc. Natl. Acad. Sci. U. S. A. 2001, 98, 2364−2369. (30) Wang, C.; Tauber, M. J. High-Yield Singlet Fission in a Zeaxanthin Aggregate Observed by Picosecond Resonance Raman Spectroscopy. J. Am. Chem. Soc. 2010, 132, 13988−13991. (31) Musser, A. J.; Maiuri, M.; Brida, D.; Cerullo, G.; Friend, R. H.; Clark, J. The Nature of Singlet Exciton Fission in Carotenoid Aggregates. J. Am. Chem. Soc. 2015, 137, 5130−5139. (32) Trinh, M. T.; Zhong, Y.; Chen, Q.; Schiros, T.; Jockusch, S.; Sfeir, M. Y.; Steigerwald, M.; Nuckolls, C.; Zhu, X. Intra- to Intermolecular Singlet Fission. J. Phys. Chem. C 2015, 119, 1312− 1319. (33) Wu, Y. S.; Wang, Y. C.; Chen, J. W.; Zhang, G. X.; Yao, J. N.; Zhang, D. Q.; Fu, H. B. Intramolecular Singlet Fission in an Antiaromatic Polycyclic Hydrocarbon. Angew. Chem., Int. Ed. 2017, 56, 9400−9404. (34) Stern, H. L.; Cheminal, A.; Yost, S. R.; Broch, K.; Bayliss, S. L.; Chen, K.; Tabachnyk, M.; Thorley, K.; Greenham, N.; Hodgkiss, J. M.; et al. Vibronically Coherent Ultrafast Triplet-Pair Formation and Subsequent Thermally Activated Dissociation Control Efficient Endothermic Singlet Fission. Nat. Chem. 2017, DOI: 10.1038/ nchem.2856. (35) Yong, C. K.; Musser, A. J.; Bayliss, S. L.; Lukman, S.; Tamura, H.; Bubnova, O.; Hallani, R. K.; Meneau, A.; Resel, R.; Maruyama, M.; et al. The Entangled Triplet Pair State in Acene and Heteroacene Materials. Nat. Commun. 2017, 8, 15953. (36) Shoer, L. E.; Eaton, S. W.; Margulies, E. A.; Wasielewski, M. R. Photoinduced Electron Transfer in 2,5,8,11-Tetrakis-Donor-Substituted Perylene-3,4:9,10-Bis(Dicarboximides). J. Phys. Chem. B 2015, 119, 7635−7643. (37) Yu, Z. Y.; Wu, Y. S.; Peng, Q.; Sun, C. L.; Chen, J. W.; Yao, J. N.; Fu, H. B. Accessing the Triplet State in Heavy-Atom-Free Perylene Diimides. Chem. - Eur. J. 2016, 22, 4717−4722. (38) Yu, Z. Y.; Wu, Y. S.; Chen, J. W.; Sun, C. L.; Fu, H. B. OrthoHeterofluorene Perylenediimides: Synthesis, Photophysical, and Exciton Dynamic Properties. Phys. Chem. Chem. Phys. 2016, 18, 32678−32681. (39) Benjamin, W. E.; Veit, D. R.; Perkins, M. J.; Bain, E.; Scharnhorst, K.; McDowall, S.; Patrick, D. L.; Gilbertson, J. D. Sterically Engineered Perylene Dyes for High Efficiency Oriented Fluorophore Luminescent Solar Concentrators. Chem. Mater. 2014, 26, 1291−1293. (40) Pensack, R. D.; Tilley, A. J.; Parkin, S. R.; Lee, T. S.; Payne, M. M.; Gao, D.; Jahnke, A. A.; Oblinsky, D. G.; Li, P. F.; Anthony, J. E.;

et al. Exciton Delocalization Drives Rapid Singlet Fission in Nanoparticles of Acene Derivatives. J. Am. Chem. Soc. 2015, 137, 6790−6803. (41) Wiederrecht, G. P.; Svec, W. A.; Wasielewski, M. R.; Galili, T.; Levanon, H. Novel Mechanism for Triplet State Formation in Short Distance Covalently Linked Radical Ion Pairs. J. Am. Chem. Soc. 2000, 122, 9715−9722. (42) Wu, Y. S.; Zhen, Y. G.; Wang, Z. H.; Fu, H. B. Donor-Linked Di(Perylene Bisimide)s: Arrays Exhibiting Fast Electron Transfer for Photosynthesis Mimics. J. Phys. Chem. A 2013, 117, 1712−1720. (43) Okada, T.; Karaki, I.; Matsuzawa, E.; Mataga, N.; Sakata, Y.; Misumi, S. Ultrafast Intersystem Crossing in Some Intramolecular Heteroexcimers. J. Phys. Chem. 1981, 85, 3957−3960. (44) Carbonera, D.; Di Valentin, M.; Corvaja, C.; Agostini, G.; Giacometti, G.; Liddell, P. A.; Kuciauskas, D.; Moore, A. L.; Moore, T. A.; Gust, D. EPR Investigation of Photoinduced Radical Pair Formation and Decay to a Triplet State in a Carotene-PorphyrinFullerene Triad. J. Am. Chem. Soc. 1998, 120, 4398−4405. (45) Hasharoni, K.; Levanon, H.; Greenfield, S. R.; Gosztola, D. J.; Svec, W. A.; Wasielewski, M. R. Mimicry of the Radical Pair and Triplet-States in Photosynthetic Reaction Centers with a Synthetic Model. J. Am. Chem. Soc. 1995, 117, 8055−8056. (46) Benniston, A. C.; Harriman, A.; Li, P. Y.; Rostron, J. P.; van Ramesdonk, H. J.; Groeneveld, M. M.; Zhang, H.; Verhoeven, J. W. Charge Shift and Triplet State Formation in the 9-Mesityl-10Methylacridinium Cation. J. Am. Chem. Soc. 2005, 127, 16054−16064. (47) Lor, M.; Thielemans, J.; Viaene, L.; Cotlet, M.; Hofkens, J.; Weil, T.; Hampel, C.; Mullen, K.; Verhoeven, J. W.; Van der Auweraer, M.; et al. Photoinduced Electron Transfer in a Rigid First Generation Triphenylamine Core Dendrimer Substituted with a Peryleneimide Acceptor. J. Am. Chem. Soc. 2002, 124, 9918−9925. (48) Veldman, D.; Chopin, S. M. A.; Meskers, S. C. J.; Janssen, R. A. J. Enhanced Intersystem Crossing via a High Energy Charge Transfer State in a Perylenediimide-Perylenemonoimide Dyad. J. Phys. Chem. A 2008, 112, 8617−8632. (49) Benniston, A. C.; Harriman, A.; Li, P. Y.; Rostron, J. P.; Verhoeven, J. W. Illumination of the 9-Mesityl-10-Methylacridinium Ion Does Not Give a Long-Lived Photoredox State. Chem. Commun. 2005, 2701−2703.

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DOI: 10.1021/acs.jpclett.7b02597 J. Phys. Chem. Lett. 2017, 8, 5609−5615