Article pubs.acs.org/JACS
Triplet Separation Drives Singlet Fission after Femtosecond Correlated Triplet Pair Production in Rubrene Ilana Breen,†,§ Roel Tempelaar,‡,§ Laurie A. Bizimana,† Benedikt Kloss,‡ David R. Reichman,‡ and Daniel B. Turner*,† †
Department of Chemistry, New York University, 100 Washington Square East, New York, New York 10003, United States Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, United States
‡
S Supporting Information *
ABSTRACT: Singlet fission, a multistep molecular process in which one photon generates two triplet excitons, holds great technological promise. Here, by applying a combination of transient transmittance and two-dimensional electronic spectroscopy with 5 fs laser pulses, we resolve the full set of fission steps before the onset of spin dephasing. In addition to its role as a viable singlet fission material, single-crystalline rubrene is selected because its energetics and transition dipole alignment uniquely allow for the unambiguous identification of the various fission steps through their contributions to distinct spectroscopic features. The measurements reveal that the neighboring correlated triplet pair achieves its maximum population within 20 fs. Subsequent growth of the triplet signal on picosecond time scales is attributable to spatial separation of the triplets, proceeding nonadiabatically through weakly coupled but near-resonant states. As such, we provide evidence in crystalline rubrene for a singlet fission step that, until now, has not been convincingly observed.
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INTRODUCTION Singlet fission is the spin-allowed conversion of one singlet exciton to two triplet excitons.1 This phenomenon has garnered recent attention for its potential utility in optoelectronic devices including next-generation solar cells.2−4 Proof-of-principle devices have demonstrated the prospects of singlet fission for use in photocatalysis, photovoltaics, and photodetectors.5−9 After photoexcitation, the key steps of the process are the conversion of one singlet into a neighboring pair of triplets correlated in an overall singlet state, 1(TT), a subsequent spatial separation of the triplets, 1(T...T), and finally spin relaxation resulting in uncorrelated triplet excitons, T + T. This process is often expressed as
glimpse into step (2), but these measurements involved disordered pentacene systems that produced strong background singlet signals and for which ES1 − E1(TT) > kBT, which complicates the interpretation. Incisive studies of step (2) would be enabled by measurements of a singlet-fission material in which ES1 ≲ E1(TT), thermodynamically ensuring that the spectroscopically measured dynamics primarily arise from an irreversible separation process, 1(TT) → 1(T...T), and for which it is possible to measure triplet dynamics without significant contamination by singlet signals in the form of ground-state bleach and stimulated emission. One singlet-fission material that appears to satisfy these demanding requirements is rubrene, a chemical derivative of tetracene having four pendant phenyl groups attached to the central rings. Rubrene is a singular material for organic electronics,14−17 displaying a high carrier mobility18,19 and a singlet-fission quantum efficiency of nearly 100%.20,21 Most studies indicate that the singlet and triplet pair energies are near resonance22−24 or that ES1 lies tens of meV below E1(TT).20,25−27 Here we report that indeed crystalline rubrene has the ideal properties for studying step (2) of the singlet-fission process. The time-resolved spectroscopic measurements reveal that the neighboring correlated triplet pair reaches its maximum population within 20 fs due to a near-isoenergetic alignment of the singlet and correlated triplet pair states, followed by
S0 ⎯→ ⎯ S1 ↔ 1(TT)⥂ 1(T ... T)⥂T + T ℏω
(1)
(2)
(3)
where steps (2) and (3) are denoted using uneven arrows to indicate the statistically preferred direction. Most modern timeresolved spectroscopic studies of singlet fission1,3,10 involved materials in which step (1) essentially finishes before the onset of triplet separation and spin relaxation, because ES1 − E1(TT) > kBT. As the different triplet pair species generated near-identical spectroscopic signals, the measured transients were therefore mostly representative of the conversion into 1(TT), while subsequent steps were unresolvable. Conventional magneticfield studies, on the other hand, have resolved the slower spin relaxation dynamics associated with step (3), which for the commonly studied polyacene materials occurs on the nanosecond time scale.11,12 One key pioneering study13 provided a © 2017 American Chemical Society
Received: March 15, 2017 Published: August 1, 2017 11745
DOI: 10.1021/jacs.7b02621 J. Am. Chem. Soc. 2017, 139, 11745−11751
Article
Journal of the American Chemical Society
noted this minor distinction,33 and we explore it further in the Supporting Information. These properties allow us to measure time-resolved spectra in which triplet signals dominate, generating ideal conditions for resolving the various steps of singlet fission. We prepared submicrometer thick rubrene single crystals suitable for femtosecond spectroscopy measurements and characterized their structural, vibrational, and optical properties. Photoluminescence spectra demonstrate that the samples are pristine; see Supporting Information for these details. The spectroscopic data presented in Figure 2 summarize the result of more than 100 complete transient-transmittance measurement scans of pristine single-crystal rubrene platelets performed using 5.2 fs laser pulses polarized along the slip-stack axis at room temperature. Each scan consisted of hundreds of pump− probe delay values, and at each value we averaged thousands of laser shots.34 The three normalized transient-transmittance spectra in panel (a) are negative in amplitude, which confirms that the positive-amplitude contributions from ground-state bleach and stimulated emission are negligible due to the unfavorable orientation of the S0 → S1 transition dipole. The suppression of such singlet signals in combination with low pulse energies minimizes the possible contribution of thermal effects.35,36 Furthermore, fluence-dependent data presented in the Supporting Information show no signs of thermal effects or exciton−exciton annihilation36−38 at the applied carrier densities. We can also discard the involvement of singlet photoinduced-absorption signals, S1 → Sn, because they appear outside the measured spectral window.20,39,40 Hence we can assign the signal as photoinduced absorption from the triplets. In panel (a) of Figure 2, the peak at 2.43 eV dominates, and, consistent with previous work,20,39,40 we assign this signal to the T1 → Tn transition. At early times, a strong shoulder appears at 2.29 eV. Its separation from the T1 → Tn transition is 0.14 eV, which matches the vibrational mode responsible for the vibronic progression in the linear absorption spectrum of rubrene.33 This correspondence suggests the presence of a vibrationally dressed triplet pair, 1(TT)′, that contributes through the T1′ → Tn transition and then undergoes vibrational relaxation to 1(TT). We substantiate this attribution below. Further growth of the signal from 10 to 800 ps does not change the peak shape. The extracted dynamics in panels (b) and (c) show that about 20% of the final triplet signal is present at the earliest possible time of 25 fs. Analysis presented in the Supporting Information shows that polaron pair formation, which has been observed at sub-100 fs time scales upon high-energy excitation in rubrene,20,39,41 has a negligible contribution in our measurements. No coherent oscillations are observable, in
spatial separation of the triplets occurring on picosecond time scales. These results from rubrene help in forming a complete understanding of the general sequence of photophysical steps constituting singlet fission.
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RESULTS AND DISCUSSION In crystalline form, rubrene molecules slip-stack along the crystallographic b axis and align in a face-to-edge herringbone motif along the c axis.28−30 Therefore, the linear absorption, Raman, and photoluminescence spectra of single-crystalline rubrene are anisotropic.31−33 To determine if this anisotropy could be exploited to suppress singlet signals, we performed ab initio calculations to obtain the S0 → S1 and T1 → Tn transition dipoles; see Supporting Information. The calculations confirm33 the short-axis polarization of the S0 → S1 transition. Most of the bright T1 → Tn transitions are polarized predominantly along the long molecular axis. Figure 1 shows
Figure 1. Structure of rubrene (left) and the bc plane of rubrene single crystals (right). The short molecular axis (z) of each rubrene molecule lies parallel to the crystallographic a axis. Calculations indicate that optical fields polarized in the bc plane will interact strongly with the T1 → Tn transition dipoles (purple arrows) and weakly with the S0 → S1 transition dipole (yellow arrows). This results in strongly suppressed singlet signals in spectroscopic measurements, which is ideal for resolving triplet signals and dynamics.
a representation of the singlet and triplet transition dipoles within the crystal. The short molecular axes are oriented parallel to the a-direction; hence for fields polarized along the crystallographic b and c axes, the contribution of S0 → S1 is negligible compared to T1 → Tn. Although the ab initio calculations predict the S0 →S1 transition to be rigorously a polarized, linear absorption spectra demonstrate that the singlet transition couples weakly to b- and c-polarized light. Prior work
Figure 2. Transient-transmittance spectra of rubrene single-crystal platelets. (a) Normalized spectra of the signal. Summation over the violet region in (a) reveals in (b) and (c) that the triplet signal forms almost instantaneously and then saturates by 200 ps. Signal amplitudes in panels (b) and (c) have the same normalization. 11746
DOI: 10.1021/jacs.7b02621 J. Am. Chem. Soc. 2017, 139, 11745−11751
Article
Journal of the American Chemical Society
Figure 3. 2D ES of rubrene single-crystal platelets. (a) Transient transmittance and 2D ES are two distinct time-resolved pump−probe methods. (b) 2D spectrum at a waiting time of 400 ps resolves a minor bleach signal at the vibronic shoulder. (c and d) Dynamics of three selected positions.
weakly absorb in this crystallographic plane, vibronic couplings, and couplings to 1(TT).49,50 Measuring the fastest dynamics required a very short laser pulse, which necessarily has a broad spectrum in the frequency domain. Thus, the transient-transmittance dynamics arise from excitation not only of the S0 → S1 transition but also of its first vibronic replica, S0 → S′1. This forms a plausible explanation of the vibrational relaxation observed in the triplet signal, since S′1 likely couples most strongly to the near-resonant 1(TT)′ state. To separate the contributions from these distinct excitation pathways, we performed two-dimensional electronic spectroscopy (2D ES), which has been employed for the investigation of singlet fission in only one previous study.47 2D ES is a coherent optical technique, highly analogous to 2D nuclear magnetic resonance,51 that provides maximum resolution of four-wave mixing signals.52,53 Existing reviews describe the signal-generation and interpretation methods.54−57 Essentially, as shown in panel (a) of Figure 3, the excitation and detection pulses of a transient-transmittance measurement become pulse pairs in 2D ES. Fourier transformation of the optical coherences during τexcite and τdetect yields excitation and detection energy axes, and the probed excitation−detection correlations are plotted as two-dimensional surfaces for each excitation−detection time (τ2). Panel (b) in Figure 3 displays the results from the 2D ES measurements polarized along the slip-stack axis. The salient feature is the dominant triplet signal in blue hues. The modest singlet peak in red hues is spectrally remote, and its positive sign indicates a ground-state bleach or stimulated emission origin associated with the photoexcited singlet state. Specifically, its coordinates indicate excitation near the main vibronic shoulder at 2.49 eV and detection at the band-edge exciton at 2.32 eV. The maximum of the triplet signal is located near the S0 → S1′ excitation frequency, albeit somewhat red-shifted due to the pulse spectrum and destructive interference with the positive-amplitude singlet signal at higher excitation frequencies. This demonstrates that most of the triplets detected in these measurements arise from photoexcitation into S1′ because it has a higher absorptivity than the S1 state, substantiating that vibrational relaxation among triplet excitons contributes to the dynamics observed in the 2D ES and transient transmittance data. Although photoexcition into S1′ is not the only important pathway through which singlet fission in rubrene can be
contrast to time-resolved spectroscopy studies of other singletfission materials.42−45 This reaffirms that singlet signals are negligible, through which vibrational quantum beats would be visible, and suggests that the conversion from singlet to triplets proceeds incoherently in crystalline rubrene at room temperature. The latter is consistent with the earlier notion that couplings in rubrene are weak24,46 and thermally activated by slow librational motion of the crystal. However, given the absence of significant coherence signatures, the early time component is unusual because other reports of femtosecond singlet-fission components involved strong couplings that induce coherent oscillations observable in spectroscopy.45,47 As we demonstrate below, the near resonance between S1 and 1 (TT) can drive the femtosecond dynamic even without strong couplings. Statistics based on multiexponential fits of measurements over carrier densities ranging from about 1.0 × 1017 cm−3 to 12.0 × 1017 cm−3 and a variety of pulse spectra reveal that, besides the ∼100 fs component, the dynamics have characteristic time scales of 2.1 ± 1.4 ps and 29 ± 2 ps for the slip-stack axis. Analogous fits to spectra polarized along the herringbone axis yield very similar time scales (∼100 fs, 4.0 ± 0.7 ps, and 45 ± 9 ps), as reported in the Supporting Information. These results clarify prior measurements of crystalline rubrene in which time scales varied from 2 to 100 ps.20,25 Unavoidable electronic nonresonant response48 of the sample and its substrate contaminates the first ∼25 fs, frustrating a rigorous quantification of the ∼100 fs dynamic. The 2 ps time scale is consistent with the dynamics of the T′1 → Tn transition in the transient-transmittance spectra, and we therefore attribute it to the presence of a vibrational relaxation process. The difference in time scales between the two axes is consistent with an anisotropy observed in polarized Raman spectra presented in the Supporting Information for the low-energy phonon modes that facilitate such relaxation. The longer time scale appears to be responsible for the irreversible depopulation of the photoexcited singlet into triplets beyond the early time equilibration between S1 and 1(TT). We therefore postulate that this time scale represents triplet separation, 1(TT) → 1 (T...T). The observed difference in time scales between the slip-stack and herringbone axes is likely caused by delocalization of S1, giving rise to anisotropy in the singlet dipoles that 11747
DOI: 10.1021/jacs.7b02621 J. Am. Chem. Soc. 2017, 139, 11745−11751
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Journal of the American Chemical Society initiated (rotating the crystal results in strong photoexcitation into S1, which does not involve vibrational relaxation), we proceed with an analysis that is consistent with the measurements in which S1′ is the initially excited state. This approach not only yields optimal insight into the detected photophysics but also provides valuable information pertaining to successful singlet-fission materials such as pentacene and hexacene, for which vibrational relaxation is thought to be an essential part of the fission process.47,50 Panels (c) and (d) of Figure 3 reveal the dynamics for selected coordinates in the 2D spectrum. The peak of greatest negative amplitude arises from excitation near S′1 and detection at the triplet transition. This peak, yellow marker and trace, is negative in value by 20 fs, which is the time resolution of the measurement. As in the transient-transmittance measurements, delay times before 20 fs are contaminated by electronic nonresonant response that makes quantitative “phasing” of the 2D spectra challenging. This instantaneous population accounts for about 20% of the total saturated signal. The red trace arises from excitation of S1 itself. Its amplitude is much smaller than that of the yellow trace, substantiating the assertion that the triplet population emanating from excitation of S1 is marginal. The weak signal prohibits a reliable quantification of the fast dynamics at this peak location given the level of noise in the measurements. It is notable that the longer-time dynamics lacks the few-picosecond component, which supports our attribution that the 2 ps time scale is indicative of vibrational relaxation. Finally, we plot in green the dynamics near the sole positiveamplitude signal, which corresponds to excitation at the vibronic shoulder and detection at the band-edge exciton. This signal remains marginally positive for the duration of the measurement. Consistent with the transient-transmittance measurements, no features in the 2D ES measurements revealed coherent oscillations. The time resolution of the transient-transmittance and 2D ES measurements, combined with the favorable energetics and transition dipole alignment of single-crystalline rubrene, allows us to resolve the scenario of multistep singlet fission summarized in Figure 4. The singlet states S1 and S′1 are quasi-resonant with the correlated triplet pair states 1(TT) and 1 (TT)′, respectively. Upon photoexcitation, weak couplings among them result in an ultrafast equilibration, yielding a nearinstantaneous 20% population of neighboring triplet pairs while leaving 80% of the population in the singlet state. Moreover, initial photoexcitation couples predominantly to S′1, after which vibrational relaxation induces a concomitant transfer of singlet and triplet populations to the band-edge states, S′1 → S1 and 1 (TT)′ → 1(TT). At longer times, separation of triplets to 1 (T...T) results in an irreversible depopulation of the singlet state, which is reflected in the picosecond growth of the triplet signal. The combination of vibrational relaxation and this triplet separation process gives rise to the characteristic change in peak shape observed in the transient-transmittance spectra. To substantiate our interpretation of the time-resolved spectroscopy measurements, we proceed with an analysis based on theoretical modeling. The quantum basis is composed of the vibronic states depicted in Figure 4. Specifically, a small coupling of 10 meV is imposed between S1 and 1(TT),26 while 1 (TT) is considered as a member of a linear chain of 1(T...T) states for which a nearest-neighbor (Dexter) coupling of 1 meV is used.58 Inclusion of a large number of 1(T...T) states ensures that the photoexcited singlet state becomes irreversibly
Figure 4. Schematic representation of singlet fission and the associated photophysics in crystalline rubrene. Shown is the energy level diagram including the ν = 0 (green) and ν = 1 (orange) vibrational manifolds. Photoexcitation (pe) to S1′ is followed by a fast equilibration between S1′ and 1(TT)′ (1). Subsequently, vibrational relaxation (vr) and an irreversible population of separated triplet pairs (2) take place at longer time scales and result in the characteristic trend in the inducedabsorption features shown at the bottom right.
depopulated, and this qualitatively represents the triplet separation process. The energy offset between S1 and 1(TT) is taken to be E1(TT) − ES1 = 41 meV, whereas the separated triplet pairs are taken to be quasi-resonant with 1(TT), E1(TT) − E1(T...T) = 10 meV. For each electronic state two vibrational sublevels separated by 150 meV are included. The quantum dynamics are evaluated through Markovian Redfield theory in the secular approximation. Full details are given in the Supporting Information. Shown in Figure 5 are the calculated transient-transmittance spectra at 200 fs and 10 ps as well as the calculated time transient of the integrated triplet signal. These data capture most of the salient features observed in the measurements. The integrated triplet signal shows a
