Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX
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Singlet Fission in Fluorinated Diphenylhexatrienes Ryuzi Katoh,*,† Masaaki Hashimoto,† Akinori Takahashi,† Yoriko Sonoda,‡ Tomoaki Yago,§ and Masanobu Wakasa§ †
Department of Chemical Biology and Applied Chemistry, College of Engineering, Nihon University, Koriyama, Fukushima 963-8642, Japan ‡ Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan § Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-ohkubo, Sakura-ku, Saitama-shi, Saitama 338-8570, Japan ABSTRACT: Experimentally determined quantum yields, decay profiles, and magnetic field effects on fluorescence showed that fluorinated derivatives of diphenylhexatriene were singlet fission materials. The rate constant of singlet fission was estimated as a function of temperature from the initial rate of decay of the fluorescence profiles. The origin of the temperature dependence was discussed in relation to the molecular stacking structure of the crystals.
was found to be 1300 cm−1. The explanation for the quenching was that the SF process was assisted by thermal activation, that is, E(S1) was slightly smaller than E(2T1) in that case. Accordingly, the activation energy was equated to the energy difference between E(S1) and E(2T1). Although this model is intuitively reasonable, a detailed kinetic analysis is required to enhance understanding of the SF process. It should be noted that both prompt and delayed fluorescence pathways are included in the integrated fluorescence intensity obtained under stationary photoexcitation conditions. Recently, real time measurements using time-resolved fluorescence and transient absorption spectroscopy have therefore been widely used to study the details of the SF process. Time-resolved fluorescence measurements of deposited films of tetracene at 298, 77, and 4 K have revealed that the decay profiles are similar to each other.10 It has been argued that the initial decay rate of fluorescence corresponds to the rate of the SF process, namely, conversion of a S1 exciton into a T1 pair state [TT]. These experimental results for tetracene films have thus shown that the SF rate constant is temperature independent. Likewise, transient absorption measurements of the deposited films of tetracene in the temperature range 10− 270 K have shown no temperature dependence.11 The difference between these time-resolved studies and the previous integrated fluorescence intensity measurements5 has been
1. INTRODUCTION Singlet fission (SF) is a phenomenon in which one singlet exciton (S1) splits into two triplet excitons (2T1). The possibility of using SF to create a new kind of solar cell that generates two electrons from one photon was recently pointed out and was followed by extensive studies.1,2 The SF phenomenon was found for the first time in tetracene.3 After that initial discovery,1several SF materials, including pentacene4 and rubrene,5 were also discovered in the 20th century. The search for new SF materials has accelerated, and recently there have been reports of several new SF materials, including 6,13bis(triisopropyl-silyl-ethynyl)-pentacene (TIPS-pentacene),6 diphenylhexatriene (DPH),7 perylenediimide,8 and diketopyrrolopyrrole (DPP) derivatives.9 However, the number of SF materials is limited, mainly because of the severe energy matching required for SF: the energy of a singlet exciton, E(S1), must exceed twice the energy of a triplet exciton, E(2T1), i.e., E(S1) > E(2T1). This constraint severely limits the number of SF materials, and a systematic study of the SF process has therefore been difficult. Thus, the search for new SF materials is still an important subject, not only for future application to solar cells but also for detailed studies of the SF mechanism. The temperature dependence of fluorescence has been examined to provide a deeper understanding of the SF process. In a previous study of tetracene single crystals, fluorescence intensity was measured as a function of temperature under stationary photoexcitation conditions.3 Results showed that fluorescence quenching was effectively suppressed at low temperature. The activation energy of the quenching process © XXXX American Chemical Society
Received: July 13, 2017 Revised: October 25, 2017 Published: October 31, 2017 A
DOI: 10.1021/acs.jpcc.7b06905 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
2. EXPERIMENTAL SECTION Figure 1 shows the molecular structures of DPH and the four F-DPHs used in this study: MF, DF, TF, and PF. DPH was
attributed to the difference of crystal morphology, i.e., single crystals or vapor-deposited films. In this context, careful studies of the fluorescence lifetimes of tetracene single crystals have been conducted as a function of temperature in the range 250− 400 K. Thermally activated SF was found to have an activation energy of 450 cm−1,12 a result that has clearly shown that the SF process is sensitive to the morphology of crystals. It is worth noting that the activation energy of 450 cm−1 obtained from the time-resolved measurements is only 35% of the activation energy of 1300 cm−1 previously obtained from the integrated fluorescence intensity study. The temperature dependence of the rate of SF has also been investigated for rubrene crystals. Measurements of the timeresolved fluorescence of rubrene single crystals have revealed that the SF rate constant shows a thermal-activated type temperature dependence.13 For rubrene-deposited films, thermally activated SF was found in the temperature range 50−300 K, and the activation energy was estimated to be 110 cm−1.14 For powder samples, it has been observed that the fluorescence decay profiles do not change with temperature in the temperature range 566−856 K.15 This result clearly shows that a reliable fluorescence study with rubrene crystals will require that the quality of the crystals be carefully taken into account. In fact, fluorescence spectra of single crystals are known to be sensitive to the procedure used to grow the crystal.13,16 In addition, the magnetic field effect of the fluorescence has been reported to be very sensitive to the quality of the crystals.5 In transient absorption measurements with rubrene single crystals, a thermally active type of SF process with an activation energy of 290 cm−1 has been observed in the temperature range 35−180 K. The ultrafast SF process has also been observed to have no temperature dependence.17 The ultrafast process is attributed to coherent SF assisted by symmetry breaking followed by photoexcitation based on theoretical considerations.17,18 Although much information about SF processes can be inferred from their temperature dependence, the experimental results have not always been consistent with each other. These inconsistencies could reflect differences in the morphology of the molecules in the crystals, including structural defects. The implication is that local structure plays an essential role in the primary processes of SF. For a systematic study of the SF process with regard to molecular packing effects, derivatives of SF materials would be useful, because the S1 and T1 energy levels would be similar to those of the original molecule, and the crystal structure could be systematically controlled. In this context, we focused on fluorinated DPH derivatives (F-DPH). As previously noted, DPH is known to be a SF material, and there is a large magnetic field effect on its fluorescence.7 Accordingly, we have studied the magnetic field effect of a strong magnetic field (5 T)19 and have proposed a model of the magnetic field effect.20 As reported previously, the S1 energy level is not affected by F-substitution, and the molecular stacking geometry changes from the herringbone structure to the slipped parallel structure with increasing numbers of substituted fluorine atoms in DPH.21,22 In this study, we used fluorescence spectroscopy to confirm that F-DPHs were SF materials. The SF rate constants were systematically evaluated from the initial rate of decay of the fluorescence profiles as a function of temperature for DPH and F-DPHs. We discuss the origin of the temperature dependence in relation to the molecular stacking structure of the crystals.
Figure 1. Molecular structures of DPH and F-DPHs: (E,E,E)-1,6diphenyl-1,3,5-hexatriene (DPH), (E,E,E)-1,6-bis(4-fluorophenyl)1,3,5-hexatriene (MF), (E,E,E)-1,6-bis(2,4-difluorophenyl)-1,3,5-hexatriene (DF), (E,E,E)-1,6-bis(2,4,6-trifluorophenyl)-1,3,5-hexatriene (TF), and (E,E,E)-1,6-bis(perfluorophenyl)-1,3,5-hexatriene (PF).
used as purchased from Wako Pure Chemical Industries, Ltd. Monoclinic, and orthorhombic crystal polymorphisms are known for DPH. From XRD measurements, we confirmed that the DPH sample used in this study had an orthorhombic structure. We synthesized fluorinated DPH derivatives using the method previously reported.21,22 Fluorescence quantum yield (Φf) measurements were carried out with an absolute photoluminescence quantum yield spectrometer (Hamamatsu, model C11347). Fluorescence decay curves were obtained by time-correlated, single-photon counting with a time-correlated, single-photon-counting module (Becker & Hickl GmbH, model SPC-130). The sample was excited by the second harmonic (400 nm) of a Ti:sapphire laser (Spectra-Physics, Tsunami). The repetition rate of the oscillator (80 MHz) was reduced to 8 MHz with a pulse selector (Spectra-Physics, model 3980). Fluorescence was detected with a photomultiplier tube (Hamamatsu, model E3809U-50) after being dispersed with a monochromator (Jobin Yvon, model H20). The magnetic field was applied with an electromagnet (Takano Giken, MC 50 W, 100 A). Fluorescence intensity as a function of the magnetic field was measured with a UV-LED (350 nm) under steady-state excitation condition. Fluorescence intensity was detected with a CCD camera (PIXIS 256, Acton) after being dispersed with a monochromator (SP-150, Acton). B
DOI: 10.1021/acs.jpcc.7b06905 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C The temperature for the fluorescence decay measurements was controlled by placing the sample tube in a homemade electric heater for the high-temperature measurements and in a quartz dewar with dry ice−acetone as a cryogen for the lowtemperature measurements.
3. RESULTS AND DISCUSSION The absorption and fluorescence spectra of the F-DPHs in solution were similar to those of DPH, which is a result that is consistent with the previous report.21,22 The implication is that the electronic state is not significantly affected by F-substitution of DPH. The absorption and fluorescence spectra of the crystalline DPH and F-DPHs were shifted slightly (by less than 2500 cm−1) toward longer wavelengths compared with the spectra in solution.21,22 This result indicates that the intermolecular interactions of DPH and F-DPHs are weak. Specifically, the electronic states of the molecules are preserved in their crystals. Accordingly, these crystals probably have similar energy levels to their isolated molecules in excited states. Table 1 summarizes the fluorescence quantum yield (Φf) of DPH and F-DPH crystals, including the Φf in solution and the
Figure 2. Fluorescence modulation ΔF/F (= (IF(0) − IF(H))/IF(0)) of DPH and F-DPHs as a function of magnetic field.
conventional relaxation pathways, such as internal conversion and intersystem crossing, are involved. Nonexponential decays, similar to that of DPH, were observed for all F-DPHs (Figure 2). Initially, rapid decays with time constants of several hundred picoseconds were observed. The more slowly decaying components then remained. The lifetimes of the fluorescence in the solutions were 6−7 ns (Table 1), the indication being that dynamical quenching of S1 occurs efficiently in the crystals. These findings suggest that F-DPHs are SF materials. For further confirmation of the occurrence of SF, the effects of magnetic fields on the fluorescence intensities were examined. The fluorescence modulation ΔF/F by a magnetic field is defined as ΔF/F = (IF(0) − IF(H))/IF(0), where IF(0) and IF(H) are the fluorescence intensity with and without magnetic field H, respectively. Figure 2 shows ΔF/F as a function of H. It is clear that DPH and F-DPHs show magnetic field effect, indicating that F-DPHs are SF materials. The ΔF/F was smaller in DF, TF, and PF than in DPH and MF. At the present time, the reason for the difference is unclear, and further analysis of this phenomenon is underway in our group. Magnetic field effects on fluorescence decay have been reported in DPH, and a detailed model has been proposed based on their results.7,19,20 Accordingly, a SF process can also be recognized based on the change in the temporal behavior of the fluorescence under the influence of a magnetic field. According to the model of the magnetic field effect in the case of a SF process, the fast component of the fluorescence decay, which corresponds to the SF process from S1 to [TT], is not sensitive to the magnetic field. In contrast, a large magnetic field effect is expected to be observed in the slow component of the fluorescence decay, which is associated with the regeneration of S1 by the recombination of two T1. This magnetic field effect occurs because a change of spin multiplicity occurs in [TT], which is sensitive to the magnetic field. It should be noted that the decay profiles in nanosecond time range are expected to be insensitive to the magnetic field because only the slow component of the fluorescence is sensitive to the magnetic field. As shown in Figure 3 by the red solid line, the fluorescence decay profile of DPH was affected by the presence of the magnetic field (H = 0.5 T), a result that is consistent with previous reports.7,19 Similar magnetic field effects on fluorescence decay were observed with the F-DPHs. This result again indicates that F-DPHs are SF materials. We studied the temperature dependences of the fluorescence decays (Figure 4). In DPH, no effect of temperature was observed on either fast or slow processes. This result suggests that both the SF (fast) process and the recombination (slow)
Table 1. Fluorescence Quantum Yields (Φf) and Lifetimes (τf) of DPF and F-DPHs in Crystalline Phase and in Solution solutiona
crystal
a
compd
Φf
τf (ns)
Φf
DPH MF DF TF PF
0.013 0.012 0.011 0.010 0.008
7.1 7.4 6.8 6.2 8.0
0.38 0.40 0.47 0.57 0.43
References 21, 22.
fluorescence lifetime (τf) of DPH and F-DPH reported previously. In the case of the crystals, measurements of Φf were repeated using a reliable method based on an integrating sphere.23 The Φf values of the crystals were almost constant (within a range of ±10%) in the excitation wavelength range 300−400 nm. The Φf values of the crystals were significantly smaller than the Φf values of the solutions. This difference indicated that the fluorescence from the F-DPH crystals was effectively quenched by competition with the relaxation process, the suggestion being that F-DPHs are SF materials. To study the dynamics of the fluorescence quenching process, the decay profiles of the fluorescence were measured as shown by the solid blue lines (H = 0) in Figure 2. That nonexponential decays were obtained for DPH was similar to the results of a previous study.7 Accordingly, such nonexponential profiles can be explained as the result of SF and subsequent processes: S1 ↔ [TT] ↔ T1 + T1
(1)
The time series of the early period of the fluorescence decay corresponds to the conversion from S1 to [TT]. After that time, the regeneration of S1 by the recombination of [TT] and the dissociation of [TT] into two free T1 excitons occur competitively. Moreover, the regeneration of [TT] by the encounter of two T1 excitons after diffusive migration also occurs. Accordingly, the kinetics of the slower region of the fluorescence becomes complicated and shows nonexponential behavior, while single exponential decay is expected if only C
DOI: 10.1021/acs.jpcc.7b06905 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 3. Fluorescence decay curves of DPH and F-DPHs with and without magnetic field (0.5 T).
Figure 4. Fluorescence decay curves of DPH and F-DPHs recorded at different temperatures.
process involved no activation barrier. This absence of an activation barrier was probably due to the fact that the S1 and 2T1 energy levels are very similar, i.e., E(S1) ≈ E(2T1). Bardeen and co-workers have previously argued that the E(2T1) of the orthorhombic DPH is slightly (320 cm−1) larger than that of E(S1).7 Thermally activated SF would therefore be expected, which is in contrast to the implications of the results of the present study. Although the reason for the difference is unclear, it is probably due to differences in the experimental conditions of the various experiments used to estimate the energy position. For MF, there was almost no temperature dependence on the fast component, and the intensity of the slow component increased slightly at low temperatures. Such a small change with temperature suggests that the E(S1) and E(2T1) are very similar, which is similar to the case of DPH. The slight increase of the slow component was probably due to the increase of the rate of regeneration of the S1 exciton from two T1 excitons caused by the increase of the diffusion coefficient of T1. This similarity between DPH and MF is consistent with the expectation that the substitution of fluorine into the DPH chromophore would not significantly influence the energy
position. In contrast with DPH and MF, remarkable temperature effects were observed in DF, TF, and PF. In particular, the early decay rate decreased with decreasing temperature. This result indicates suppression of the SF process with decreasing temperature. The SF rate constants in the F-DPHs were obtained by fitting an exponential function to the data in the first part of the decay curves shown in Figure 4. The results are summarized in Figure 5 as Arrhenius plots. DPH and MF showed almost no temperature dependence. On the other hand, DF, TF, and PF showed thermal activation type temperature dependences. The activation energies for DF, TF, and PF were similar, about 600 cm−1. We then considered the origin of the temperature dependence of the SF rate constants of DPH and F-DPHs. In the case of DPH (Figure 4), the fluorescence decay showed no temperature dependence over a wide range of time. This lack of temperature dependence suggests that the energy levels of S1, [TT], and T1 + T1 are similar. It should be noted that no temperature dependence was observed, even for MF. This D
DOI: 10.1021/acs.jpcc.7b06905 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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rate and the molecular stacking structure because molecular structure and energy levels are not much different from each other. In recent SF studies of rubrene single crystals, which have a parallel stacking structure, there has been no electronic coupling for the SF process because of symmetry. It has been pointed out that thermal activation of the symmetry-breaking motion is needed for efficient SF, and theoretical analysis has successfully explained this phenomenon.18 For our present results, this concept can be applied, namely, the electronic coupling for SF would be weak in the slipped parallel structure of DF, TF, and PF. In fact, the electronic coupling for SF has been evaluated based on theoretical calculation for acene derivatives.25 Accordingly, relatively large values are obtained for the herringbone structure, whereas smaller values are obtained for the slipped parallel structure. It is thus very probable that the enhancement of the coupling by thermal excitation of the symmetry-breaking motion is necessary for efficient SF in DF, TF, and PF.
Figure 5. Arrhenius plot of the rate constant kSF of SF for DPH and FDPHs, as evaluated from the initial decay of the temporal profiles of fluorescence.
result again suggests that the energy levels of S1, [TT], and T1 are similar to each other. The implication is that no energy level of DPH is significantly affected by F-substitution. In previous studies, the temperature independence of SF has been explained in the context of the coherent type SF observed in rubrene single crystals where SF occurs within several tens of femtoseconds.17 For DPH and MF, on the contrary, the initial decay of the fluorescence, which corresponds to the SF rate, occurred over a time frame of several hundred picoseconds. It is thus unlikely that coherent SF occurs for DPH and MF. This conclusion is not surprising considering the weak intermolecular interaction of DPH compared with rubrene. For DPH, small Stokes shifts of the absorption and fluorescence spectra between isolated molecules and crystals have been observed, whereas a large Stokes shift has been observed for rubrene.13 In other words, DPH and MF have sufficient electronic coupling for SF, but it is not enough for coherent type SF. Next, we considered the fact that the initial fluorescence decay of DF, TF, and PF showed a large temperature dependence. One possible explanation for such a large temperature effect is that E(S1) is lower than E(2T1), which has been proposed previously for the SF in tetracene.3 However, because the positions of the energy levels are considered to be almost the same in MF and DPH, a large change of energy levels is not expected for DF, TF, and PF. Furthermore, it is unlikely a coincidence that DF, TF, and PF have the same activation energies, if these energy levels are affected by F-substitution. Accordingly, it is unlikely that the explanation for the activation energy is that the energy gap between E(S1) and E(2T1) is very different from the corresponding gap in DPH. Another possible explanation for the temperature dependence is originated in the difference of molecular stacking structure. Actually, the electronic coupling for SF has been argued to be sensitive to molecular stacking structure.18,24,25 As shown above, DPH and MF, which showed no temperature dependence, have the herringbone structure. In contrast, DF, TF, and PF, which showed thermal activation type dependence, have a slipped parallel structure.21,22 The good correlation between the presence and absence of temperature dependence and molecular stacking structure suggests that the molecular stacking structure could be a major factor determining the temperature dependence of the SF rate in DPH and F-DPHs. It is noted that DPH and F-DPH are a suitable system to discuss the correlation between the SF
4. SUMMARY We conclude that F-DPHs were SF materials based on the experimental findings, i.e., low fluorescence quantum yield, nonexponential fluorescence decay, and presence of magnetic field modulation of fluorescence. The SF rate constants were evaluated from fluorescence decay measurements. DPH and four derivatives thereof could be divided into two groups: one group (DPH and MF) showing no temperature dependence of the initial fluorescence decay and another group (DF, TF, and PF) showing thermal activation dependence. The activation energy was about 600 cm−1 in all three derivatives. We found that there is good correlation between the temperature dependence of the initial fluorescence decay and the molecular stacking structure of the crystal. The crystals of the group showing no temperature dependence had a herringbone packing structure; the crystals of the group showing a temperature dependence had a slipped parallel structure. The implication is that the molecular stacking structure of the crystal plays an important role in SF dynamics.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ryuzi Katoh: 0000-0002-1775-3127 Tomoaki Yago: 0000-0002-4507-245X Masanobu Wakasa: 0000-0001-9084-8459 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by a Grand-in-Aid for Scientific Research (B) (No. 16H04093) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan and SENTAN, JST, Japan.
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REFERENCES
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DOI: 10.1021/acs.jpcc.7b06905 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.7b06905 J. Phys. Chem. C XXXX, XXX, XXX−XXX
