Letter pubs.acs.org/JPCL
Observation of Two Triplet-Pair Intermediates in Singlet Exciton Fission Ryan D. Pensack,† Evgeny E. Ostroumov,† Andrew J. Tilley,‡ Samuel Mazza,§ Christopher Grieco,∥ Karl J. Thorley,§ John B. Asbury,∥ Dwight S. Seferos,*,‡ John E. Anthony,*,§ and Gregory D. Scholes*,† †
Department Department § Department ∥ Department ‡
of of of of
Chemistry, Chemistry, Chemistry, Chemistry,
Princeton University, Princeton, New Jersey 08544, United States University of Toronto, Toronto, Ontario M5S 3H6, Canada University of Kentucky, Lexington, Kentucky 40506, United States The Pennsylvania State University, University Park, Pennsylvania 16802, United States
S Supporting Information *
ABSTRACT: Singlet fission is an excitation multiplication process in molecular systems that can circumvent energy losses and significantly boost solar cell efficiencies; however, the nature of a critical intermediate that enables singlet fission and details of its evolution into multiple product excitations remain obscure. We resolve the initial sequence of events comprising the fission of a singlet exciton in solids of pentacene derivatives using femtosecond transient absorption spectroscopy. We propose a three-step model of singlet fission that includes two triplet-pair intermediates and show how transient spectroscopy can distinguish initially interacting triplet pairs from those that are spatially separated and noninteracting. We find that the interconversion of these two triplet-pair intermediates is limited by the rate of triplet transfer. These results clearly highlight the classical kinetic model of singlet fission and expose subtle details that promise to aid in resolving problems associated with triplet extraction.
inglet fission is an excitation multiplication phenomenon specific to molecular systems where one spin-zero singlet excitation is converted into two spin-one triplet excitations.1−4 High singlet-to-triplet conversion efficiencies5−7 and long triplet-state lifetimes8 make singlet fission particularly attractive for practical applications.4 To realize efficiency gains in a device,9,10 carriers must be extracted from each triplet excitation generated through singlet fission. To date there are limited reports where singlet fission chromophores have been successfully incorporated into a solar cell,11 and a detailed understanding of the singlet fission mechanism remains obscure. Singlet fission was originally proposed to explain delayed fluorescence observed in crystals of polyacenes and was subsequently interrogated further via its magnetic field dependence.1,3 These pioneering studies leveraged a combination of experiment and numerical simulations to arrive at the following kinetic model
S
S1 + S0 ⇄ 1(TT) ⇄ T1 + T1
detailed interpretation of their excited-state dynamics has long been controversial (see ref 3 and refs therein). Considerable confusion has persisted into the contemporary literature, especially regarding how transient signatures relate to populations in the singlet fission process. On the basis of time-resolved two-photon photoelectron emission measurements, it was originally suggested that the singlet exciton and triplet pair are photogenerated via a quantum-mechanical superposition state.13 This model was considered to conflict; however, with high-time-resolution transient absorption measurements indicating the gradual evolution of singlet to triplet optical signatures.14 Several groups have further asserted that free triplet excitons are observed on a sub-100 fs time scale in transient absorption measurements15,16 and that there is no distinct optical signature for the initially formed triplet pair in the solid state.15,17 Very limited information has been gathered about the properties and dynamics of the correlated triplet pair,12 in no small part due to its elusiveness to detection.18,19 The correlated triplet pair is a critical intermediate in singlet fission; it is an overall singlet state, making the process spin allowed. As a result, fission can be rapid and the triplet pair can form with high efficiency. What is not clear from studies to date is how the triplet pair formed through fission evolves over time. On
(1)
This generally accepted model3 describes the interconversion of a singlet exciton with an intermediate state, colloquially known as the correlated triplet pair,12 and the subsequent interchange of this intermediate state with two independent triplet excitons. Recent technological advances in ultrafast spectroscopy now make it possible to directly observe individual elementary steps of photophysical processes. Pentacene and its derivatives have served as exemplar singlet fission chromophores, although a © XXXX American Chemical Society
Received: May 2, 2016 Accepted: June 9, 2016
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Table 1. Pentacene Derivatives Investigated in This Work Arranged According to the Magnitude of the Red Shift of the Lowest Energy Singlet Transition Observed in Their Aggregated Form (i.e., ΔEexc)
considerably to cover both visible and near-infrared spectral regions (Figure S2). We investigated singlet fission dynamics in colloidal aggregates of these compounds to minimize optical artifacts that are often apparent in crystalline films and that can obscure particular transient absorption features.25 Rapid precipitation of the compounds in water generally yields ca. 100 nm diameter aggregates.20 Absorption spectra of the aggregate suspensions indicate that the constituent chromophores are relatively strongly coupled, as is evidenced by the red shift of the lowest energy singlet transition (Figure S3). We first consider a relatively unexplored region in the nearinfrared. Figure 1a displays the near-infrared transient absorption of aggregates of one of these compounds, Cl4TIPS-Pn (see the Supporting Information for full chemical nomenclature). Near the time origin of the experiment we observe a population of excitons exhibiting transient spectra similar to that expected for photogenerated singlet excitations (Figure S4), whereas at longer time delays the transient spectra are assigned to triplet excitons.16 While this particular triplet transition is formally forbidden in isolated pentacene molecules,26 it has been observed in concentrated solutions27 and solids14,16 of pentacene and its derivatives. Aggregates of each pentacene derivative exhibit similar overall qualitative features and trends (Figure S5). A most remarkable feature of these data is the retention of signal amplitude in the vicinity of the singlet-induced absorption on the picosecond time scale, well exceeding the time scale for complete decay of the photogenerated singlet exciton population (Figure 1b). There are several possibilities to explain this observation (see the Supporting Information for detailed discussion). The most likely explanation is that the induced absorption arises from an intermediate state. This observation is therefore inconsistent with recent reports, suggesting that free triplet excitons form on time scales as short as a few hundred femtoseconds in solids of singlet fission chromophores.15,16,28 Figure 1b and Figure S6 show that the decay and growth of the singlet and triplet transient spectral features follow nonexponential kinetics. We previously proposed that, in addition to triplet-pair formation, triplet-pair separation is necessary to explain these nonexponential kinetics.20 Here we further hypothesize that triplet-pair separation requires the inclusion of an additional intermediate in the kinetic model of singlet fission. To test this hypothesis, we performed a global analysis of the transient data using a three-component scheme (Figure 1b, inset). The three-component scheme resulted in smaller root-mean-square values and better residuals (Figure
what time scale do the triplets spatially separate, and what are their properties before and after? These questions impact mechanisms of extraction, that is, how the triplets are converted into charge pairs. In this letter, we show how transient spectroscopy can detect interacting and noninteracting triplet pairs as well as determine the time scale for their separation in solids of a series of pentacene derivatives. The latter result provides compelling experimental evidence in support of a triplet energy transfer mediating their interconversion. In the transient absorption experiment, a small fraction of the sample is excited by a “pump” pulse that is resonant with the lowest-energy singlet transition (i.e., ca. 660−710 nm). A “probe” pulse then interrogates the sample at variable time intervals relative to the pump pulse, allowing one to directly monitor the formation and decay of both “bright” and “dark” excited-state (transient) species. The probe continua in these experiments covers an extensive spectral range spanning the visible to the nearinfrared (Figure S1). We demonstrate that singlet exciton fission in solids of pentacene derivatives is described by the following three-step kinetic scheme S1 + S0 → [1(TT)⥂1(T...T)]⥂T1 + T1
(2)
where we use the terms interacting and separated triplet pair to denote the two intermediates 1(TT) and 1(T···T), respectively. Although their existence was originally proposed nearly 40 years ago,18 surprisingly no direct experimental evidence verifying the presence of two triplet-pair intermediates has since been reported. A global analysis of our transient absorption measurements enables us to identify the spectroscopic features associated with these different triplet-pair intermediates when triplets reside on neighboring sites and when they are spatially separated. We chose to study solids of pentacene derivatives, whose chemical structures are shown in Table 1, because their molecular packing arrangementthat determines exciton energies, coupling strengths, and singlet fission dynamics20−22can be tailored through chemical functionalization.23 In contrast with tetracene where singlet fission is slightly endoergic24 and triplet-pair formation is expected to be ratedetermining, singlet fission in solids of pentacene derivatives is generally exoergic, and this enables us to resolve individual elementary steps of the process. We previously reported, for example, that triplet pairs form within subpicosecond time scales in aggregates of several pentacene derivatives.20 In the present work, the spectral window has been expanded 2371
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independent triplet excitons by the microsecond time scale, a transient absorption spectrum obtained on the microsecond time scale reveals that the photoinduced absorption bands of these overall singlet, correlated triplet-pair states are similar to those of independent triplet excitons (Figure S8). These results indicate that (i) the two correlated triplet-pair states are wellapproximated as a product function of two molecule-localized triplet states,32 and (ii) the latter correlated triplet-pair state makes a “spectrally silent” transition31 into independent triplet excitons. The two triplet-pair intermediates, labeled CTP1 and CTP2, are indicated to have slightly different photoinduced spectra in the global analysis. CTP1 has an induced absorption to the red of the onset of the near-infrared triplet transition that is featureless and extends to the longest wavelengths measurable (Figure 1c and Figure S9). This observation is significant and indicates that CTP1 does not bear considerable character of the parent singlet exciton. In addition, the vibronic progression of the triplet transition is different for the two triplet-pair intermediates. Variations in excitonic coupling are known to modify vibronic progressions in molecular solids.33 We attribute variations in the vibronic progressions in the triplet transition of the two triplet-pair intermediates to differences between the triplet and triplet-pair exciton states. The decay of the induced absorption in the 1200−1400 nm spectral range and concomitant change in vibronic structure indicate the change of excitonic interactions involving the triplets comprising the correlated triplet pair. We therefore assign CTP1 and CTP2 components of the global analysis to interacting and noninteracting triplet-pair intermediates, respectively. The visible spectral region is known to comprise strong transient absorption features characteristic of pentacene singlet and triplet populations. Figure 2a displays the transient absorption of aggregates of Cl4-TIPS-Pn in the visible spectral region. Again, we find that a three-component scheme successfully describes these data and that the first component is readily ascribable to photogenerated singlet excitons (Figures 2b and Figure S4). The transient spectra of the interacting and noninteracting triplet-pair intermediates exhibit two distinguishing differences in the visible spectral range. The transient spectrum of the interacting triplet pair (CTP1) exhibits a slightly broader photoinduced absorption band at ∼540 nm; that is, the absorption tail extends more to longer wavelengths and the primary photoinduced absorption band is blue-shifted compared with CTP2. These observations are consistent with the optical properties reported for a triplet-pair intermediate recently observed in concentrated solutions of a tetracene derivative,17 confirming our assignment of CTP1 to 1(TT). We emphasize that the corresponding near-infrared transient spectra clearly reveal negligible parent singlet exciton character in this state. As described above, loss of excitonic interactions accompanying the transition from CTP1 to CTP2 is indicated by a loss of the long wavelength-induced absorption along with a red shift of the triplet transition energy. While the presence of the long wavelength-induced absorption appears general to this series of pentacene derivatives, only in aggregates of the most strongly coupled chromophores is the red shift of the tripletinduced absorption clearly resolved (Figure 2b and Figures S10 and S11). The magnitude of the red shift in aggregates of these compounds is on the order of ca. 100 cm−1 (inset of Figure 2b). Such minor perturbations of the triplet electronic structure are
Figure 1. (a) Transient absorption of Cl4-TIPS-pentacene aggregates in the near-infrared spectral range. The measurements were performed with the pump wavelength at 710 nm and an incident pump fluence of ca. 300 μJ/cm2. The transient absorption data have been corrected for chirp in the probe continuum. The dashed line indicates a time slice at 1260 nm. The scale bar is indicated. (b) Transient absorption kinetics at 1260 nm. Overlaying the data is a fit resulting from a global analysis of the data. (c) Species-associated spectra obtained from a threecomponent global analysis of the transient absorption data. The first component represents the initially photogenerated singlet exciton, and the second and third components represent two different correlated triplet-pair intermediates.
S7), which are well-known criteria used to determine the number of distinguishable components contributing to the dynamics.29−31 Therefore, three states are necessary to correctly describe these dynamics. The resulting near-infrared species-associated spectra are shown in Figure 1c. Full details of the global analysis, including results for all compounds, are included in the Supporting Information. The first component is clearly ascribable to photogenerated singlet excitons, and the global analysis returns a lifetime of ca. 0.2 ps for this state in the case of the Cl2-TIPS-Pn aggregates. The second and third components exhibit considerably longer lifetimes of ca. 2 and ≫200 ps, respectively, and the transient spectra of these two latter components are qualitatively comparable to that expected for independent triplet excitons.16 Given that the correlated triplet pair is expected to persist well into the nanosecond time scale,18,19 we attribute the second and third components to two distinct correlated triplet-pair intermediates. On the basis of the assumption that the correlated triplet pair has transitioned to 2372
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exciton localization or triplet transfer could mediate spatial separation of the triplet pair. Kandada et al. proposed that triplet transfer likely mediates triplet-pair separation in crystalline pentacene34 on the picosecond time scale, consistent with the rate of triplet transfer generally expected in crystalline acene solids.2 Thus, the picosecond time constant measured here for triplet-pair separation is consistent with a triplet energy transfer mediating the interconversion between interacting and noninteracting triplet pairs. The mechanism for how energytransfer mediates separation of triplet pairs was recently elucidated.32 We arrive at the following three-step model for singlet exciton fission that includes two correlated triplet-pair intermediates (eq 2). Figure 3 displays a pictorial representa-
Figure 3. Schematic representation of the different singlet fission intermediates and singlet exciton fission energy level diagram. S0 + S0 represents the ground state, S1 + S0 represents a singlet exciton, 1(TT) represents an interacting triplet-pair intermediate, 1(T...T) represents a spatially separated, noninteracting triplet-pair intermediate, and T1 + T1 represents two independent triplet excitons. The general time scales for the elementary steps identified in this work are indicated; steps that we were unable to resolve are indicated by gray arrows.
Figure 2. (a) Transient absorption of aggregates of Cl4-TIPSpentacene in the visible spectral range. The measurements were performed with the pump wavelength at 710 nm and an incident pump fluence of ca. 120 μJ/cm2. The scale bar is indicated. (b) Species-associated spectra obtained from a three-component global analysis of the transient absorption data. The first component represents the initially photogenerated singlet exciton and the second and third components represent interacting and (noninteracting) spatially separated triplet-pair intermediates, respectively. The magnitude of the blue shift of the triplet-induced absorption in the visible spectral range for the interacting and separated triplet pairs is shown in the inset. (c) Time constants obtained for the formation (orange circles) and separation (black squares) of the triplet pair plotted as a function of singlet exciton red shift.
tion of the states involved along with a schematic representation of the kinetic model in the form of an energy level diagram. In this model, the singlet exciton undergoes a spin-allowed internal conversion to an overall singlet, correlated triplet-pair state over the course of a few hundred femtoseconds. The excitonic interactions evident in this triplet pair are lost as the triplets spatially separate. The time scale of this latter step is fundamentally limited by the rate of triplet transfer. Although not spectrally resolved in the present work, it is physically reasonable to conclude that spin decoherence subsequently transitions the correlated triplet pair into two independent triplet excitons.32 We note that the complete absence of photoluminescence in aggregates of these compounds20 suggests the absence of an emissive excimer intermediate from the kinetic model. The presence of two triplet-pair intermediates has important implications for the successful implementation of singlet fission for carrier multiplication. For example, a key finding of the present work is the presence of triplet exciton interactions exclusively in the interacting triplet pair, 1(TT). Chemical systems in which triplet exciton interactions are strong and the triplet pair cannot spatially separate might yield only the interacting triplet pair and, as a consequence, exhibit exceed-
consistent with weak coupling between the triplets comprising the triplet pair and explain the subtle differences in the CTP1 and CTP2 transient spectra. Having demonstrated that an additional triplet-pair intermediate is required in the kinetic model of singlet fission and having clarified the nature of the different triplet-pair intermediates, we now discuss fundamental aspects that mediate their interconversion. Figure 2c shows that the time constant associated with formation of the triplet pair is generally of the order of several hundred femtoseconds or less and exhibits a trend with singlet exciton red shift that likely reflects a driving-force dependence of this step.20−22 In contrast, the time constant associated with separation of the triplet pair is generally on the order of a few picoseconds and does not exhibit a clear trend with the singlet exciton red shift. As identified above, excitonic interactions change as the triplets comprising the triplet pair spatially separate. Either triplet 2373
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Superradiance and Exciton Fission. J. Chem. Phys. 2010, 133 (14), 144506. (8) Poletayev, A. D.; Clark, J.; Wilson, M. W. B.; Rao, A.; Makino, Y.; Hotta, S.; Friend, R. H. Triplet Dynamics in Pentacene Crystals: Applications to Fission-Sensitized Photovoltaics. Adv. Mater. 2014, 26 (6), 919−924. (9) Green, M. A. Third Generation Photovoltaics: Advanced Solar Energy Conversion; Springer Series in Photonics 12; Springer: New York, 2006. (10) Hanna, M. C.; Nozik, A. J. Solar Conversion Efficiency of Photovoltaic and Photoelectrolysis Cells with Carrier Multiplication Absorbers. J. Appl. Phys. 2006, 100 (7), 074510. (11) 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 (6130), 334−337. (12) Smith, M. B.; Michl, J. Recent Advances in Singlet Fission. Annu. Rev. Phys. Chem. 2013, 64 (1), 361−386. (13) 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 (6062), 1541−1545. (14) 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 (31), 11830− 11833. (15) Musser, A. J.; Liebel, M.; Schnedermann, C.; Wende, T.; Kehoe, T. B.; Rao, A.; Kukura, P. Evidence for Conical Intersection Dynamics Mediating Ultrafast Singlet Exciton Fission. Nat. Phys. 2015, 11 (4), 352−357. (16) Herz, J.; Buckup, T.; Paulus, F.; Engelhart, J. U.; Bunz, U. H. F.; Motzkus, M. Unveiling Singlet Fission Mediating States in TIPSPentacene and Its Aza Derivatives. J. Phys. Chem. A 2015, 119 (25), 6602−6610. (17) 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. U. S. A. 2015, 112 (25), 7656−7661. (18) Frankevich, E. L.; Lesin, V. I.; Pristupa, A. I. Rate Constants of Singlet Exciton Fission in a Tetracene Crystal Determined from the RYDMR Spectral Linewidth. Chem. Phys. Lett. 1978, 58 (1), 127−131. (19) Burdett, J. J.; Bardeen, C. J. Quantum Beats in Crystalline Tetracene Delayed Fluorescence due to Triplet Pair Coherences Produced by Direct Singlet Fission. J. Am. Chem. Soc. 2012, 134 (20), 8597−8607. (20) 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 (21), 6790−6803. (21) Yost, S. R.; Lee, J.; Wilson, M. W. B.; Wu, T.; McMahon, D. P.; Parkhurst, R. R.; Thompson, N. J.; Congreve, D. N.; Rao, A.; Johnson, K.; et al. A Transferable Model for Singlet-Fission Kinetics. Nat. Chem. 2014, 6 (6), 492−497. (22) Busby, E.; Berkelbach, T. C.; Kumar, B.; Chernikov, A.; Zhong, Y.; Hlaing, H.; Zhu, X.-Y.; Heinz, T. F.; Hybertsen, M. S.; Sfeir, M. Y.; et al. Multiphonon Relaxation Slows Singlet Fission in Crystalline Hexacene. J. Am. Chem. Soc. 2014, 136 (30), 10654−10660. (23) Anthony, J. E. Functionalized Acenes and Heteroacenes for Organic Electronics. Chem. Rev. 2006, 106 (12), 5028−5048. (24) Piland, G. B.; Bardeen, C. J. How Morphology Affects Singlet Fission in Crystalline Tetracene. J. Phys. Chem. Lett. 2015, 6 (10), 1841−1846. (25) Marciniak, H.; Pugliesi, I.; Nickel, B.; Lochbrunner, S. Ultrafast Singlet and Triplet Dynamics in Microcrystalline Pentacene Films. Phys. Rev. B 2009, 79 (23), 235318.
ingly short triplet-pair lifetimes. This is clearly apparent in the short triplet-pair lifetimes reported in conjugated polymers and covalently tethered molecular pairs.35−38 Polymer films or aggregates of molecular pairs might help to effect triplet-pair separation in these systems, although strong excitonic interactions can still facilitate rapid radiationless transitions and return to the ground state.39,40 Such losses are obviously prohibitive to the practical implementation of singlet fission, and a detailed understanding of the different triplet-pair intermediates identified in this work may greatly facilitate inhibiting these loss pathways.
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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.6b00947. The experimental methods section, acronyms and full chemical nomenclature, steady-state absorption data, and additional femtosecond and nanosecond transient absorption data are included in the Supporting Information. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*D.S.S. E-mail:
[email protected]. *J.E.A.: E-mail:
[email protected]. *G.D.S.: E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS G.D.S. thanks the United States Air Force Office of Scientific Research (FA9550-13-1-0005) for supporting the femtosecond transient absorption studies. J.E.A., K.J.T., and S.M. thank the U.S. National Science Foundation (CMMI-1255494) for supporting the synthesis of singlet fission chromophores. D.S.S. thanks the Natural Sciences and Engineering Research Council of Canada, DuPont, and the Alfred P. Sloan Foundation. C.G. and J.B.A. thank the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DESC0008120 for support of this work. A.J.T. thanks the Connaught Global Challenge Award for a postdoctoral fellowship. R.D.P. thanks J. Dean for general discussion.
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REFERENCES
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