Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2017, 8, 5700-5706
Harnessing Molecular Vibrations to Probe Triplet Dynamics During Singlet Fission Christopher Grieco,† Eric R. Kennehan,† Adam Rimshaw,† Marcia M. Payne,‡ John E. Anthony,‡ and John B. Asbury*,† †
Department of Chemistry, The Pennsylvania State University, State College, Pennsylvania 16801, United States Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States
‡
S Supporting Information *
ABSTRACT: Ultrafast vibrational spectroscopy in the mid-infrared spectral range provides the opportunity to probe the dynamics of electronic states involved in all stages of the singlet fission reaction through their unique vibrational frequencies. This capability is demonstrated using a model singlet fission chromophore, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn). The alkyne groups of the TIPS side chains are coupled to the conjugated framework of the pentacene cores, enabling direct examination of the dynamics of triplet excitons that have successfully separated from correlated triplet pair states in crystalline films of TIPS-Pn. Relaxation processes during the separation of triplet excitons and triplet−triplet annihilation after their separation result in the formation of hot ground state molecules that also exhibit unique vibrational frequencies. Because all organic molecules possess native vibrational modes, ultrafast vibrational spectroscopy offers a new approach to examine the dynamics of electronic intermediates that may inform ongoing efforts to utilize singlet fission to overcome thermalization losses in photovoltaic applications. inglet fission is an exciton multiplication process whereby photogenerated excited singlet states in conjugated organic molecules form two triplet excitons.1,2 Because this exciton multiplication process has been shown to approach 100% quantum efficiency in a number organic molecules,3,4 singlet fission represents an appealing strategy to enhance the power conversion efficiency of solar cells by potentially overcoming thermalization losses, particularly on the higher energy side of the solar spectrum.5−8 Although singlet fission is efficient in crystals of conjugated molecules such as pentacene, tetracene, rubrene, and others,9−20 the reported enhancements of the overall external quantum efficiency of solar cells that utilize such singlet fission sensitizers have been limited to date. Understanding the complete mechanism of singlet fission from the dynamics of the parent singlet excited states S1S0 through formation of fully separated triplet excitons T1 + T1 may inform efforts to more efficiently utilize singlet fission to enhance the power conversion efficiency of photovoltaic devices. While the precise mechanism of how singlet fission occurs is still a topic of debate,9,21−23 there is consensus that the photophysical pathways involve an intermediate state known as the correlated triplet pair (CTP) that must dissociate to form independent triplet excitons24 according to the reaction
S
S1S0 → 1(TT ) ← T1 + T1
We hypothesized that experimental techniques that could directly and uniquely probe the formation of separated triplet excitons and their relaxation processes might offer new opportunities to investigate the mechanism of the singlet fission reaction. Motivated by prior work using vibrational dynamics of organic photovoltaic materials to examine electronic processes,37,38 we undertook an ultrafast mid-infrared (mid-IR) spectroscopy study of singlet fission in crystalline films of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn). Herein, we show that probing native vibrational modes of singlet fission chromophores in conjunction with visible transient absorption methods indeed offers a new approach to selectively examine the spatial separation of triplet excitons as they dissociate from CTP states. Furthermore, we identify unique vibrational signatures of hot ground state molecules that form due to relaxation processes that occur during multiplied triplet exciton formation. Given that all organic singlet fission sensitizers1,2 possess native vibrational modes, our findings suggest this may be a general approach to examine the mechanism of singlet fission throughout all stages of the process from creation of parent singlet excited states through formation of fully independent triplet excitons. Figure 1a depicts the molecular structure of TIPS-Pn along with a Fourier transform infrared (FTIR) spectrum of a crystalline film of TIPS-Pn measured in the region of the symmetric νs and antisymmetric νas alkyne stretch modes. Crystalline TIPS-Pn films were prepared using spin coating and
(1)
Although ultrafast electronic spectroscopy measurements in the visible spectral range have been challenged to unambiguously distinguish CTPs from separated triplet excitons because they both have triplet character and overlapping absorption spectra,25,26 recent measurements in the near-infrared have revealed distinct absorptions of these intermediates.26−36 © XXXX American Chemical Society
Received: September 13, 2017 Accepted: November 7, 2017 Published: November 7, 2017 5700
DOI: 10.1021/acs.jpclett.7b02434 J. Phys. Chem. Lett. 2017, 8, 5700−5706
Letter
The Journal of Physical Chemistry Letters
when excitation intensities in the 5−300 μJ/cm2 range are used, as reported here.39 The excitation intensity dependent decay kinetics of the T1 → Tn triplet absorption feature represented in Figure S2b demonstrate that the decay rates increase with higher excitation intensity, as expected for a bimolecular triplet−triplet annihilation process. We examined the dynamics of singlet fission in the crystalline TIPS-Pn films using ultrafast vibrational spectroscopy in the mid-IR spectral range. Figure 1b depicts a two-dimensional (frequency-time) plot of ultrafast mid-IR transient absorption spectra following pulsed excitation at 655 nm with 150 μJ/cm2. The transient absorption spectra consist of a broad photoinduced absorption (PIA) superimposed with a narrow vibrational feature centered at 2132 cm−1. We recently assigned the broad PIA feature to the absorption of singlet excited states and CTP intermediates.43 The vibrational feature corresponds to the ground state bleach (GSB) peak of the alkyne stretch that results from depletion of the ground state population by excitation of the sample. Figure 1c represents spectral slices taken from the two-dimenisonal data that highlight the time evolution of these spectral components in greater detail. As a starting point to describe the transient absorption features in the spectra, we adopted a simple model in which the broad mid-IR PIA was described by a polynomial function, and the GSB peak was described by the FTIR spectrum of the crystalline TIPS-Pn film (Figure 1a). The FTIR spectrum should describe the GSB peak if no other vibrational features overlap it. The best fit spectra (solid curves) overlaid on the 1, 10, and 50 ps spectra in Figure 1c were computed by a nonlinear least-squares fitting procedure using these two basis functions. The 1 ps spectrum is well described using the simple model. However, the line shape of the vibrational feature changes at 10 and 50 ps time delays and cannot be adequately described by the model (see gray shaded region in Figure 1c). This result indicates that a new positive-going transient vibrational feature grows in on the lower frequency side of the GSB on the 10 to 50 ps time scale. To capture the dynamics of this new vibrational feature, we subtracted the polynomial functions used to fit the broad midIR PIA from the mid-IR transient absorption spectra. Figure S5 represents a two-dimensional (frequency-time) plot of the resulting transient vibrational spectra. The 1, 10, and 50 ps vibrational spectra plotted in Figure 1d are overlaid with the FTIR spectrum of the crystalline film that describes the GSB feature. The comparison highlights the increasing amplitude of the new vibrational feature that grows in around 2116 cm−1 (see gray shaded region in Figure 1d). The data reveal that the GSB feature increases in amplitude synchronously with the growth of the new vibrational feature between 1 and 10 ps. Notice that the center frequency of the GSB peak does not shift during the growth of this positive feature. Finally, Figure 1e depicts the time evolution of the transient vibrational spectra over the full range of time scales measured in the experiment. Unlike the spectra in Figure 1d, the data measured on longer time scales reveal the onset of time-dependent frequency shifts of both the GSB and the new positive-going vibrational feature on the 100 ps time scale that continue into the nanosecond time regime. We used time-resolved infrared (TRIR) transient absorption spectroscopy and temperature dependent FTIR spectroscopy to independently establish the assignments of the spectral features that evolve in the data represented in Figure 1d,e. Figure 2a depicts TRIR spectra of TIPS-Pn molecules dissolved
Figure 1. (a) Chemical structure and the FTIR spectrum of the alkyne stretch modes of a solvent-annealed film of TIPS-Pn. (b) 2dimensional frequency-time plot of the ultrafast mid-IR transient absorption spectra following excitation at 655 nm to the first singlet excited state. (c) Spectral slices taken from the 2D data in (b). Overlaid on the 1, 10, and 50 ps transient spectra are fits describing only the ground state bleach (GSB) and the broad photoinduced absorption (PIA) resulting from correlated triplet pairs (see ref 43). (d) Mid-IR transient absorption spectra with the PIA subtracted and measured at 1, 10, and 50 ps. The dashed vertical lines mark the frequencies of transient absorption features of triplet excitons (2116 cm−1) and hot ground state molecules (2124 cm−1). (e) Mid-IR transient absorption spectra measured on a broader range of time scales that capture the vibrational dynamics during singlet fission and heat effects due to relaxation processes and subsequent triplet−triplet annihilation.
solvent annealing procedures that were reported previously.39 We verified that TIPS-Pn molecules adopted the form-I brickwork packing motif in the films using visible absorption spectroscopy and X-ray diffractometry (see Supporting Information).40 We will refer to these as crystalline TIPS-Pn films here and throughout the following discussion. To compare the singlet fission dynamics measured through the molecular vibrations reported here with the work of others, we used conventional ultrafast transient absorption spectroscopy to measure the dynamics of the T1 → Tn triplet absorption in the visible spectral range.2,3,12,41 Figure S2a depicts the T1 → Tn transient absorption spectrum measured in a crystalline TIPS-Pn film at ∼10 ns time delay following excitation at 645 nm. Transient absorption kinetics measured at the peak of the T1 → Tn transition (520 nm) were fit with a biexponential growth model followed by a decay component in accordance with previous analysis methods described in the literature.14,26,39 The data confirm the growth of the triplet absorption feature on the ∼100 fs time scale followed by a few picosecond rise component. The subsequent decay of the triplet absorption feature results from triplet−triplet annihilation,7,42 which is known to occur on the nanosecond time scale 5701
DOI: 10.1021/acs.jpclett.7b02434 J. Phys. Chem. Lett. 2017, 8, 5700−5706
Letter
The Journal of Physical Chemistry Letters
Although the ability to control the concentration of TIPS-Pn in solution was essential to make the vibrational assignment of the alkyne stretch mode of triplet excited states, the vibrational frequency in solution is not the same as that in the crystalline film due to the difference in solvent environment. Therefore, we extended the TRIR measurements to examine the crystalline TIPS-Pn film (Figure 2b, top). A distinct vibrational spectrum is observed in the spectrum recorded at the earliest time delay that represents a time-average over the ∼20 ns instrument response time of the TRIR spectrometer.44 This spectrum captures the time range when triplet excitons are present in the TIPS-Pn film (Figure S2). Spectra recorded at later time-delays (25 ns and longer) exhibit changes in amplitude without significant spectral evolution. An infrared absorption spectrum of the crystalline film collected at the same frequency resolution as the TRIR experiment is plotted together with the transient spectra for comparison. We performed global target analysis of the TRIR spectra in Figure 2b using the Glotaran software package and a simplistic sequential unimolecular kinetic model in which triplets annihilate to form hot ground state species. We extracted two species-associated spectra (SAS) and verified that the spectrum representing the longer time delays (with time constant of 2.3 μs) quantitatively matched the steady-state temperature difference spectrum of the crystalline TIPS-Pn film measured with the same spectral resolution. This temperature difference spectrum is labeled as the 320−295 K spectrum in Figure 2b (middle). The measurements of the temperature difference infrared spectra of the crystalline TIPS-Pn film are described in detail in the Supporting Information (Figure S7). We note that thermal effects have been shown to contaminate transient absorption spectra in the perylenediimide system for example, which can complicate measurements of exciton dynamics during singlet fission.45 However, the species associated spectrum representing the earliest TRIR spectrum (with 34 ns time constant) does not match the temperature difference spectrum. Therefore, we assigned the positive-going vibrational feature in this spectrum (SAS1) to the alkyne stretch of triplet excitons in the crystalline TIPS-Pn film. To make this assignment, we recognized that triplet excitons persist on the nanosecond time scale under these experimental conditions (Figure S2)39 and that the alkyne stretch of triplet excited states appears on the lower frequency side of the GSB in the solution studies (Figure 2a). Subtraction of the infrared absorption spectrum of the film from the SAS1 spectrum allowed us to isolate the spectrum of the alkyne stretch of triplet excitons, which appears in Figure 2b (bottom). The center frequency of this peak (2115 cm−1) closely matches the new vibrational feature that grows in on 10−50 ps time scale in Figure 1d. We conclude that this new vibrational feature in Figure 1d arises from triplet excitons that form on an early picosecond time scale. The higher frequency feature that grows in on the subnanosecond time scale in Figure 1e arises from heat deposited during the singlet fission and triplet− triplet annihilation processes. Motivated by the vibrational assignments of triplet excitons and thermal effects in the ultrafast mid-IR transient absorption spectra, we developed a quantitative spectral modeling procedure to analyze the population dynamics of the electronic states of TIPS-Pn during singlet fission through the time evolution of their distinct vibrational features. Figure 3a depicts three basis spectra that were used to fit the transient vibrational spectra in Figure 1 at each time delay ranging from 1 ps to 3 ns.
Figure 2. (a) Nano-to-microsecond time-resolved vibrational spectra of TIPS-Pn in CCl4 solution at 1 mM (top) and 25 mM (bottom) concentrations. At low concentration, few triplet excited states form by intersystem crossing within the excited state lifetime. At high concentration, singlet fission occurs among TIPS-Pn molecules, leading to faster growth of triplet excited states that have unique alkyne stretch modes at lower frequencies than the ground state bleach of TIPS-Pn. (b) Nano-to-microsecond time-resolved vibrational spectra of a crystalline TIPS-Pn film (top). Results of a convolutionfitted global analysis performed using a sequential kinetic model (middle). Overlaid is a 320−295 K temperature difference spectrum for reference. The positive component of SAS1 (bottom) is assigned to the alkyne stretch mode of triplet states on the basis of the solution studies in panel a.
in CCl4 solutions at 1 mM and 25 mM concentrations in the top and bottom panels, respectively. It has been shown that TIPS-Pn molecules do not undergo singlet fission at the lower concentration.3 As a consequence, few triplet excited states form by intersystem crossing within the excited state lifetime of TIPS-Pn. However, at the higher concentration, diffusioncontrolled singlet fission has been reported, which results in the formation of triplet excited states on the nanosecond time scale. The onset of singlet fission at this concentration leads to faster growth of the triplet excited state population. The marked increase in amplitude of the transient absorption peak on lower frequency side of the GSB in the 25 mM solution allows us to assign this peak to the alkyne stretch of triplet excited states of TIPS-Pn. 5702
DOI: 10.1021/acs.jpclett.7b02434 J. Phys. Chem. Lett. 2017, 8, 5700−5706
Letter
The Journal of Physical Chemistry Letters
Figure 3. (a) Basis spectra used to fit the ultrafast vibrational transient absorption spectra measured in crystalline TIPS-Pn film. The GSB and S0* spectra were measured using FTIR spectroscopy. (b) Four transient absorption spectra at select time points highlight the characteristic time evolution of the transient populations and indicate the fidelity of the fitting procedure. (c) State-specific population kinetic traces highlight the dynamics of the ground state bleach, the triplet exciton population, and the growth of the hot ground state following triplet−triplet annihilation.
GSB feature from the PFID signal is challenging because both are superimposed on the broad mid-IR PIA signal (Figure 1c). Consequently, we analyzed the time and frequency evolution of the vibrational features beginning at 1 ps time delay after the pulse overlap region was complete (see Figure S4). The analysis reveals that the time-dependent amplitude of the GSB feature exhibits a small growth in amplitude on the few picosecond time scale. Following these dynamics, the GSB decays in amplitude on the time scale of triplet−triplet annihilation (see Figure S2) because this process restores TIPS-Pn molecules to their ground electronic state.46,47 Triplet−triplet annihilation in conjunction with other relaxation processes during singlet fission give rise to excess thermal energy that is deposited into the crystalline lattice of TIPS-Pn molecules in the film. The associated temperaturedependent perturbation of the vibrational features of TIPS-Pn are captured by the growth of the hot ground electronic state feature S0* on the mid- to late-picoseond time scale (Figure 3c, bottom). However, before the hot ground electronic state grows in, the transient absorption feature of triplet excitons T appears on the lower frequency side of the GSB (around 2116 cm−1) and increases in amplitude on the few picosecond time scale. After this initial growth, the triplet exction feature T decays on a similar time scale as the loss of the GSB, consistent with triplet excitons being lost as a result of triplet−triplet annihilation. It is interesting to note that singlet fission to form CTP intermediates is known to occur on the 100 fs time scale in crystalline TIPS-Pn films,12,28,48 yet the transient vibrational spectrum measured at 1 ps time delay contains little evidence of the alkyne stretch mode of triplet excitons T (Figure 3b). Rather, the triplet exciton feature is prominent in the spectra measured at 10 ps and longer time scales. The growth of this feature on the 10 to 50 ps time scale is evident in the transient absorption spectra represented in Figure 1c,d, even without the spectral modeling procedure (see gray shaded regions). Recent ultrafast near-IR measurements of pentacene derivatives revealed that the mechanism of singlet fission involves two CTP states known as interacting 1(TT) and noninteracting 1 (T···T) CTP states.26 These studies indicated that interacting CTP states form on the 100 fs time scale but spatially separate on the few picosecond time scale,42,43 consistent with our
The FTIR spectrum measured at 295 K was used to represent the GSB of TIPS-Pn because the transient vibrational spectrum at 1 ps time delay is well described by this FTIR spectrum (Figure 1d). We used the temperature difference spectrum of a crystalline TIPS-Pn film measured at 320−295 K to represent the basis spectrum describing the vibrational features of the hot ground state population that dominates on the nanosecond time scale (Figure 2b). We introduced a third basis spectrum represented in Figure 3a (middle) to describe the transient absorption feature of triplet excitons appearing around 2116 cm−1 (Figure 1d). A skewed Lorentzian line shape was adopted for this basis spectrum because the same line shape provided the best fit of the triplet exciton alkyne stretch mode measured in the crystalline film (Figure 2b). We used a nonlinear leastsquares method to simultaneously obtain the best fit of the broad mid-IR PIA (see Figure 1c), the amplitudes of the basis spectra describing the GSB and thermal effects, as well as the center frequency, skew, width and amplitude of the triplet exciton absorption feature for each time delay. Spectral slices at 1, 10, 100, and 1000 ps are represented in Figure 3b with the best fit spectra (thick black lines) overlaid on the data (circles). The corresponding basis spectra scaled according to their best fit amplitudes are represented in each panel for comparison. Note that the GSB spectrum cannot be distinguished from the data in the 1 ps spectrum because the triplet T and hot ground state features have not grown in on this time scale. The arrows highlight the growth of these populations on longer time scales. The data and fits demonstrate the fidelity of the spectral modeling procedure, which is described in greater detail in the Supporting Information. The population dynamics in Figure 3c were obtained from the spectral modeling procedure and indicate the time evolution of the electronic states of TIPS-Pn during singlet fission. The GSB population dynamics exhibit a pulse limited rise on the subpicosecond time scale that is convolved with the perturbed free-induction decay (PFID) of the alkyne stretch of ground state molecules. This signal appears at negative time delays and around the time-origin of the experiment. At these time delays, the mid-IR probe pulse creates a vibrational coherence between the ground and first excited vibrational levels of the alkyne stretch before the sample is excited by the visible pump pulse. Disentangling the pulse-limited rise of the 5703
DOI: 10.1021/acs.jpclett.7b02434 J. Phys. Chem. Lett. 2017, 8, 5700−5706
Letter
The Journal of Physical Chemistry Letters
The curves overlaid on the vibrational kinetics traces in Figure 3c (reproduced in Figure 4c) represent best fits obtained from a sequential kinetic model described in Section S6 that was used to quantitatively and simultaneously fit with a common set of time constants the evolution of the GSB, the triplet absorption T, and the hot ground state S0*. The time constant for CTP separation obtained from this modeling procedure is 3.5 ± 0.6 ps, as indicated in Figure 4c and Table S1. This time constant quantitatively matched within experimental precision the time constant for CTP separation of 3.2 ± 0.9 ps measured independently in ref 43 from the picosecond rise component of the transient absorption kinetics of the T1 → Tn transition under the same conditions. We note that the origin of the shift of the triplet vibrational feature to lower frequency is currently unclear. However, electronic excitation of TIPS-Pn does reduce the ground state absorption of the alkyne group, indicating that it is coupled to the lowest energy excited electronic states of the molecule. Therefore, it is reasonable to expect that the change in electronic structure associated with the transition from a singlet to a triplet spin state might also affect the vibrational frequency of this mode. It is useful to recall that the triplet exciton energy is half the singlet energy, indicating a significant difference in the electronic states. However, the triplet vibrational feature decays with the same rate as the total triplet population on the nanosecond time scale due to triplet−triplet annihilation. Furthermore, the comparison of the broad mid-IR PIA dynamics with the vibrational dynamics confirms that triplet excitons probed directly through their unique vibrational frequencies do not appear in the crystalline TIPS-Pn film until the few picosecond time scale, in contrast to measurements of the T1 → Tn transition in the visible spectral range. Because both CTPs 1(TT) and separated triplet excitons T1 + T1 have overlapping transitions and triplet excitonic character,25,26 the appearance of the T1 → Tn transition does not necessarily reflect the true triplet formation dynamics. This was one of our original motivations for developing the mid-IR approach for examining intermediates in the singlet fission reaction. It is curious that we have not observed the alkyne stretch mode of interacting CTP intermediates 1(TT) in the transient mid-IR spectra. While the reason for this absence has not been fully established, the time dependence of the GSB feature suggests that the alkyne stretch of interacting CTPs 1(TT) may overlap that of the ground state. For example, the growth of the (negative-going) GSB vibrational feature occurs on the same time scale as the growth of the triplet exciton absorption feature (Figure 1d and Figure 3c). This suggests that there may be a positive-going transient absorption feature overlapping the GSB peak that decays on the few picosecond time scale. The decay of this positive-going feature would then allow the full amplitude of the GSB peak to be observed. We speculate that this positive-going feature is the alkyne stretch mode of interacting CTP intermediates. We also note that the population of singlet excitons decays within less than 1 ps (see Figure S4),43 which prevents us from clearly measuring the alkyne stretch of this state due to complications from the PFID signal near the time-origin of the experiment. In summary, we demonstrated that ultrafast vibrational spectroscopy in the mid-IR provides a new approach to selectively examine the dynamics that lead to separation of triplet excitons from CTP intermediate states following singlet fission as well as the dynamics with which heat is dissipated
measurements of these dynamics through the triplet vibrational feature. The data in Figure 4 compare the key observations from the ultrafast mid-IR and visible transient absorption measurements
Figure 4. (a) Comparison of the fast decay of the broad mid-IR PIA signal on the subps time scale with the rise of the triplet absorption feature (T1 → Tn) at 520 nm. The decay of the mid-IR absorption coincides with the slow rise of the visible absorption, which results from CTP separation. (b) A kinetic model was used to simultaneously fit the visible kinetics and mid-IR CTP signals to extract the underlying population dynamics. These are compared to the population dynamics in (c) from analysis of the vibrational features. The comparison demonstrates that the rise of the triplet vibrational feature T coincides with the separation of CTPs detected in both the visible and broad mid-IR spectral regions.
described here. Figure 4a depicts the transient absorption kinetics trace of the T1 → Tn transition measured at 520 nm overlaid with the broad mid-IR PIA signal that arises from the absorption of singlet excited states and CTP intermediates in the crystalline TIPS-Pn film.43 The synchronous decay of the broad mid-IR PIA signal with the formation of the T1 → Tn transition indicates the initial formation of CTP intermediates 1 (TT) on the subpicosecond time scale. Figure 4b focuses on the evolution of the broad mid-IR PIA signal on time 1 ps and longer time regime that highlights the separation of CTP intermediates on the few picosecond time scale.43 In Figure 4c, we reproduce the visible transient absorption kinetics trace for comparison to the dynamics of the triplet vibrational feature T and the hot ground state feature S0* on the same logarithmic time axis. The comparison reveals that the triplet exciton vibrational mode at 2116 cm−1 appears synchronously with the few picosecond rise of the T1 → Tn transition and the decay of the broad mid-IR PIA signal. Because both the growth of the T1 → Tn transition on the picosecond time scale14,26,42 and the decay of the CTP signal 43 arise from separation of CTP intermediates, the data indicate that we do not observe the triplet exciton vibrational feature until after separation of the CTPs. We therefore define a CTP separation time from these data on the basis of the rise of the triplet exciton vibrational mode T in the vibrational dynamics, as indicated in Figure 4b. 5704
DOI: 10.1021/acs.jpclett.7b02434 J. Phys. Chem. Lett. 2017, 8, 5700−5706
Letter
The Journal of Physical Chemistry Letters during singlet fission and subsequent triplet−triplet annihilation. In this work, we focused on the alkyne stretch mode of TIPS-Pn to probe the dynamics of the electronic states during singlet fission. However, because all organic singlet fission sensitizers possess native vibrational modes, this technique offers a broadly applicable approach to examine electronic states involved in singlet fission throughout all stages of the reaction in a variety of molecular systems. For example, we recently demonstrated that the vibrational frequencies of conjugated C−C stretch modes of perylenediimides provide direct probes of the interactions of the molecules participating in delocalized excitonic states.49 We used these vibraional modes, which we called intermolecular coordinate coupled vibrational modes, to compare the extent of delocalization of excitons among the perylenediimide molecules in their molecular crystals. We believe this capability may be used to provide new insight about both the dynamics and delocalization of electronic states that lead to the formation of multiplied triplet excitons in new singlet fission sensitizers being developed for practical applications.
■
via Singlet Fission Exciton Sensitizer. Appl. Phys. Lett. 2012, 101, 113304. (7) Poletayev, A. D.; Clark, J.; Wilson, M. W.; Rao, A.; Makino, Y.; Hotta, S.; Friend, R. H. Triplet dynamics in pentacene crystals: applications to fission-sensitized photovoltaics. Adv. Mater. 2014, 26, 919−24. (8) Yang, L.; Tabachnyk, M.; Bayliss, S. L.; Bohm, M. L.; Broch, K.; Greenham, N. C.; Friend, R. H.; Ehrler, B. Solution-processable singlet fission photovoltaic devices. Nano Lett. 2015, 15, 354−8. (9) Zimmerman, P. M.; Zhang, Z.; Musgrave, C. B. Singlet fission in pentacene through multi-exciton quantum states. Nat. Chem. 2010, 2, 648−52. (10) Arias, D. H.; Ryerson, J. L.; Cook, J. D.; Damrauer, N. H.; Johnson, J. C. Polymorphism Influences Singlet Fission Rates in Tetracene Thin Films. Chem. Sci. 2016, 7, 1185−1191. (11) Sharifzadeh, S.; Wong, C. Y.; Wu, H.; Cotts, B. L.; Kronik, L.; Ginsberg, N. S.; Neaton, J. B. Relating the Physical Structure and Optoelectronic Function of Crystalline TIPS-Pentacene. Adv. Funct. Mater. 2015, 25, 2038−2046. (12) Yost, S. R.; Lee, J.; Wilson, M. W.; 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, 492−7. (13) 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, 17278−81. (14) 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, 6790−803. (15) 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, 8679−83. (16) 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, 6388−400. (17) 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, 7289−97. (18) Irkhin, P.; Biaggio, I. Direct imaging of anisotropic exciton diffusion and triplet diffusion length in rubrene single crystals. Phys. Rev. Lett. 2011, 107, 017402. (19) Jia, W.; Chen, Q.; Chen, L.; Yuan, D.; Xiang, J.; Chen, Y.; Xiong, Z. Molecular Spacing Modulated Conversion of Singlet Fission to Triplet Fusion in Rubrene-Based Organic Light-Emitting Diodes at Ambient Temperature. J. Phys. Chem. C 2016, 120, 8380−8386. (20) Hartnett, P. E.; Margulies, E. A.; Mauck, C. M.; Miller, S. A.; Wu, Y.; Wu, Y.-L.; Marks, T. J.; Wasielewski, M. R. Effects of Crystal Morphology on Singlet Exciton Fission in Diketopyrrolopyrole Thin Films. J. Phys. Chem. B 2016, 120, 1357−1366. (21) Zimmerman, P. M.; Bell, F.; Casanova, D.; Head-Gordon, M. Mechanism for singlet fission in pentacene and tetracene: from single exciton to two triplets. J. Am. Chem. Soc. 2011, 133, 19944−52. (22) Lukman, S.; Musser, A. J.; Chen, K.; Athanasopoulos, S.; Yong, C. K.; Zeng, Z.; Ye, Q.; Chi, C.; Hodgkiss, J. M.; Wu, J.; et al. Tuneable Singlet Exciton Fission and Triplet-Triplet Annihilation in an Orthogonal Pentacene Dimer. Adv. Funct. Mater. 2015, 25, 5452− 5461. (23) 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, 316−24. (24) Mirkovic, T.; Ostroumov, E. E.; Anna, J. M.; van Grondelle, R.; Govindjee; Scholes, G. D. Light Absorption and Energy Transfer in the Antenna Complexes of Photosynthetic Organisms. Chem. Rev. 2017, 117, 249−293.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02434. Detailed experimental methods, visible and mid-IR transient absorption spectra and kinetics, temperature dependence, and spectral modeling procedures (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
John E. Anthony: 0000-0002-8972-1888 John B. Asbury: 0000-0002-3641-7276 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS C.G., E.R.K., A.R., 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 research. J.E.A. and M.M.P. thank the National Science Foundation (CMMI-1255494) for support of organic semiconductor synthesis.
■
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, 361−86. (3) Walker, B. J.; Musser, A. J.; Beljonne, D.; Friend, R. H. Singlet exciton fission in solution. Nat. Chem. 2013, 5, 1019−24. (4) 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, 8965−72. (5) Tabachnyk, M.; Ehrler, B.; Bayliss, S.; Friend, R. H.; Greenham, N. C. Triplet Diffusion in Singlet Exciton Fission Sensitized Pentacene Solar Cells. Appl. Phys. Lett. 2013, 103, 153302. (6) Reusswig, P. D.; Congreve, D. N.; Thompson, N. J.; Baldo, M. A. Enhanced External Quatum Efficiency in an Organic Photovoltaic Cell 5705
DOI: 10.1021/acs.jpclett.7b02434 J. Phys. Chem. Lett. 2017, 8, 5700−5706
Letter
The Journal of Physical Chemistry Letters
lene-3,4:9,10-bis(dicarboximide)s. J. Am. Chem. Soc. 2012, 134, 386− 97. (42) Scholes, G. D. Correlated Pair States Formed by Singlet Fission and Exciton-Exciton Annihilation. J. Phys. Chem. A 2015, 119, 12699− 12705. (43) Grieco, C.; Kennehan, E. R.; Kim, H.; Pensack, R. D.; Brigeman, A. N.; Rimshaw, A.; Payne, M. M.; Anthony, J. E.; Giebink, N. C.; Scholes, G. D.; et al. Direct Observation of Correlated Triplet Pair Dynamics During Singlet Fission Using Ultrafast Mid-IR Spectroscopy. J. Phys. Chem. C 2017, submitted for publication. (44) Rimshaw, A.; Grieco, C.; Asbury, J. B. High Sensitivity Nanosecond Mid-Infrared Transient Absorption Spectrometer Enabling Low Excitation Density Measurements of Electronic Materials. Appl. Spectrosc. 2016, 70, 1726−1732. (45) Grieco, C.; Aplan, M. P.; Rimshaw, A.; Lee, Y.; Le, T. P.; Zhang, W.; Wang, Q.; Milner, S. T.; Gomez, E. D.; Asbury, J. B. Molecular Rectification in Conjugated Block Copolymer Photovoltaics. J. Phys. Chem. C 2016, 120, 6978−6988. (46) Lin, J. D. A.; Mikhnenko, O. V.; Chen, J.; Masri, Z.; Ruseckas, A.; Mikhailovsky, A.; Raab, R. P.; Liu, J.; Blom, P. W. M.; Loi, M. A.; et al. Systematic study of exciton diffusion length in organic semiconductors by six experimental methods. Mater. Horiz. 2014, 1, 280−285. (47) Zhang, Y.; Forrest, S. R. Triplet diffusion leads to triplet−triplet annihilation in organic phosphorescent emitters. Chem. Phys. Lett. 2013, 590, 106−110. (48) Herz, J.; Buckup, T.; Paulus, F.; Engelhart, J.; Bunz, U. H.; Motzkus, M. Acceleration of Singlet Fission in an Aza-Derivative of TIPS-Pentacene. J. Phys. Chem. Lett. 2014, 5, 2425−30. (49) Kennehan, E. R.; Grieco, C.; Brigeman, A. N.; Doucette, G. S.; Rimshaw, A.; Bisgaier, K.; Giebink, N. C.; Asbury, J. B. Using Molecular Vibrations to Probe Exciton Delocalization in Films of Perylene Diimides with Ultrafast Mid-IR Spectroscopy. Phys. Chem. Chem. Phys. 2017, 19, 24829−24839.
(25) 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. (26) Pensack, R. D.; Ostroumov, E. E.; Tilley, A. J.; Mazza, S.; Grieco, C.; Thorley, K. J.; Asbury, J. B.; Seferos, D. S.; Anthony, J. E.; Scholes, G. D. Observation of Two Triplet-Pair Intermediates in Singlet Exciton Fission. J. Phys. Chem. Lett. 2016, 7, 2370−2375. (27) Herz, J.; Buckup, T.; Paulus, F.; Engelhart, J.; Bunz, U. H.; Motzkus, M. Acceleration of Singlet Fission in an Aza-Derivative of TIPS-Pentacene. J. Phys. Chem. Lett. 2014, 5, 2425−30. (28) Herz, J.; Buckup, T.; Paulus, F.; Engelhart, J. U.; Bunz, U. H.; Motzkus, M. Unveiling Singlet Fission Mediating States in TIPSpentacene and its Aza Derivatives. J. Phys. Chem. A 2015, 119, 6602− 10. (29) Stern, H. L.; Musser, A. J.; Gelinas, S.; Parkinson, P.; Herz, L. M.; Bruzek, M. J.; Anthony, J. E.; 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, 7656−7661. (30) Stern, H. L.; Cheminal, A.; Yost, S. R.; Broch, K.; Bayliss, S.; Chen, K.; Tabachnyk, M.; Thorley, K.; Greenham, N. C.; 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. (31) 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. (32) Margulies, E. A.; Wu, Y.-L.; Gawel, P.; Miller, S. A.; Shoer, L. E.; Schaller, R. D.; Diederich, F.; Wasielewski. Sub-Picosecond Singlet Exciton Fission in Cyano-Substituted Diaryltetracenes. Angew. Chem., Int. Ed. 2015, 54, 8679−8683. (33) 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. (34) Yong, C. K.; Musser, A. J.; Bayliss, S.; 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. (35) Breen, I.; Tempelaar, R.; Bizimana, L. A.; Kloss, B.; Reichman, D. R.; Turner, D. B. Triplet Separation Drives Singlet Fission after Femtosecond Correlated Triplet Pair Production in Rubrene. J. Am. Chem. Soc. 2017, 139, 11745−11751. (36) Bakulin, A. A.; Morgan, S. E.; Kehoe, T. B.; Wilson, M. W. B.; Chin, A. W.; Zigmantas, D.; Egorova, D.; Rao, A. Real-Time Observation of Multiexcitonic States in Ultrafast Singlet Fission Using Coherent 2D Electronic Spectroscopy. Nat. Chem. 2016, 8, 16− 23. (37) Jeong, K. S.; Pensack, R. D.; Asbury, J. B. Vibrational Spectroscopy of Electronic Processes in Emerging Photovoltaic Materials. Acc. Chem. Res. 2013, 46, 1538−1547. (38) Barbour, L. W.; Hegadorn, M.; Asbury, J. B. Watching Electrons Move in Real Time: Ultrafast Infrared Spectroscopy of a Polymer Blend Photovoltaic Material. J. Am. Chem. Soc. 2007, 129, 15884− 15894. (39) Grieco, C.; Doucette, G. S.; Pensack, R. D.; Payne, M. M.; Rimshaw, A.; Scholes, G. D.; Anthony, J. E.; Asbury, J. B. Dynamic Exchange During Triplet Transport in Nanocrystalline Pentacene Films. J. Am. Chem. Soc. 2016, 138, 16069−16080. (40) Diao, Y.; Lenn, K. M.; Lee, W. Y.; Blood-Forsythe, M. A.; Xu, J.; Mao, Y.; Kim, Y.; Reinspach, J. A.; Park, S.; Aspuru-Guzik, A.; et al. Understanding polymorphism in organic semiconductor thin films through nanoconfinement. J. Am. Chem. Soc. 2014, 136, 17046−57. (41) Ramanan, C.; Smeigh, A. L.; Anthony, J. E.; Marks, T. J.; Wasielewski, M. R. Competition between singlet fission and charge separation in solution-processed blend films of 6,13-bis(triisopropylsilylethynyl)pentacene with sterically-encumbered pery5706
DOI: 10.1021/acs.jpclett.7b02434 J. Phys. Chem. Lett. 2017, 8, 5700−5706
