solvent dependence of structural dynamics and thermally

doi.org/10.26434/chemrxiv.11798568.v1

Solvent Dependence of Structural Dynamics and Thermally ActivatedDelayed Fluorescence in 3,4,5-tri(9H-Carbazole-9-Yl)benzonitrile(Ortho-3CzBN)Masaki Saigo, Kiyoshi Miyata, Hajime Nakanotani, Chihaya Adachi, Ken Onda

Submitted date: 04/02/2020 • Posted date: 06/02/2020Licence: CC BY-NC-ND 4.0Citation information: Saigo, Masaki; Miyata, Kiyoshi; Nakanotani, Hajime; Adachi, Chihaya; Onda, Ken(2020): Solvent Dependence of Structural Dynamics and Thermally Activated Delayed Fluorescence in3,4,5-tri(9H-Carbazole-9-Yl)benzonitrile (Ortho-3CzBN). ChemRxiv. Preprint.https://doi.org/10.26434/chemrxiv.11798568.v1

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1

Masaki Saigo, 1 Kiyoshi Miyata, 1 Hajime Nakanotani, 2,3 Chihaya Adachi, 2,3 and Ken Onda*1

1Department of Chemistry, Kyushu University, 744 Motooka, Nishi, Fukuoka 829-0395 2Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395

3JST, ERATO, Adachi Molecular Exciton Engineering Project, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395

E-mail: [email protected]

Thermally activated delayed fluorescence (TADF) 1 processes depend on the ambient environment such as 2 solvents. Here, we investigated solvent-based structural 3 changes of a TADF molecule, 3,4,5-tri(9H-carbazole-9-4 yl)benzonitrile (o-3CzBN), as a model system. Our findings 5 suggest that the photophysical properties of o-3CzBN heavily 6 modulated by solvents, but their drastic structural changes in 7 excited states are almost independent of the polarities. The 8 results highlight the contribution of higher-lying excited 9 states in the TADF process. 10

Keywords: Time-resolved infrared spectroscopy (TR-11 IR) | Solvent dependence | Thermally 12 activated delayed fluorescence (TADF) 13

Thermally activated delayed fluorescence (TADF) 14

molecules received tremendous attention due to their unique 15 ability to improve the efficiency of organic light-emitting 16 diodes (OLEDs). TADF molecules can convert excitons at 17 the lowest triplet state (T1) to the lowest singlet excited state 18 (S1) via reverse intersystem crossing (RISC) driven by 19 thermal excitation. To achieve an efficient RISC process, the 20 energy gap between S1 and T1 (EST) needs to be sufficiently 21 small (<0.1 eV). The basic design strategy for obtaining small 22 EST is to separate the highest occupied molecular orbital 23 (HOMO) from the lowest unoccupied molecular orbital 24 (LUMO), typically achieved by connecting electron donor 25 and electron acceptor chromophores1,2. 26

TADF molecules often have charge-transfer (CT) 27 character in the excited states. The CT character results that 28 the photophysical properties such as emission wavelength, 29 photoluminescence quantum yield (PLQY) and lifetime 30 strongly depend on solvents.3–5. TADF activities also depend 31 on the solvent selection. Although the mechanisms of the 32

dependence are essential to design high performance devices, 33 the microscopic understanding of the mechanism is still 34 elusive6–8. Recently, the contribution of higher-lying excited 35 states for TADF activity is suggested from the temperature 36 dependence of PL decay rate9, transient absorption 37 spectroscopy6, and theoretical calculations8,10. Hosokai et al. 38 compared TADF activities of carbazole-benzonitrile (Cz-39 BN) derivatives in a non-polar solvent (toluene, = 2.2) and 40 a polar solvent (acetonitrile, = 37), and clarified the solvent 41 dependent TADF in these derivatives from the viewpoint of 42 difference in shifting of energy levels and their matching 43 including higher-lying excited states5. In addition to the T1 44 and S1 states which possess CT characters, they proposed the 45 existence of the higher-lying excited state, T2, with locally-46 excited (LE) character to explain the solvent dependence. 47 While S1 and T1 are significantly stabilized in polar solvents, 48 the energy level of locally-excited T2 is less affected; 49 therefore, they claimed the positive correlation between 50 TADF activity and energy matching of S1,CT and T2,LE. 51

Intra-molecular structural change in the excited states 52 has also significant impacts on TADF activity. We previously 53 reported that the suppression of structural changes along with 54 intersystem crossing (ISC) assists the TADF process in Cz-55 BN derivatives with EST of about 0.2 eV11. Since structural 56 changes in the excited states are highly sensitive to external 57 environments such as solvents, it is necessary to investigate 58 the excited state structural modulations with solvents to 59 explore the solvent effects on TADF processes. 60

Here, we studied the solvent dependence of structural 61 change associated with ISC using time-resolved infrared 62 spectroscopy (TR-IR). TR-IR is a powerful tool to investigate 63

Solvent Dependence of Structural Dynamics and Thermally Activated Delayed Fluorescence in 3,4,5-tri(9H-carbazole-9-yl)benzonitrile (ortho-3CzBN)

Figure 1. (a) The molecular structure of o-3CzBN. (b) UV-Vis absorption spectra (dot) and photoluminescence (PL) spectra (line) of o-3CzBN, (c) PL decay profiles of o-3CzBN probed at 430-450 nm in toluene, 445-465 nm in THF, and 460-480 nm in MeCN. Red, green, and blue represent toluene, THF, and MeCN, respectively. PL spectra and decay profiles were obtained by pumping with 355 nm.

2

the structural dynamics of functional molecules in excited 1 states.11–15 We focused on 3,4,5-tri(9H-carbazole-9-2 yl)benzonitrile (o-3CzBN, Figure 1a) which shows drastic 3 structural changes in the excited state. While all Cz units of 4 o-3CzBN in the ground state are in the same plane, the 5 location of one of the Cz unit shift towards an out-of-plane 6 direction in the T1 state, revealed by TR-IR and quantum 7 chemical calculations.11 Interestingly, o-3CzBN shows no 8 TADF activity (PLQY of delayed fluorescence, DF = 0%) in 9 toluene solution (= 2.4), while it becomes TADF active 10 (DF = 21%) in acetonitrile (MeCN) solution.5 We believe 11 that the solvent dependent TADF activity of o-3CzBN may 12 be attributed to structural modulations in higher-lying excited 13 states in different polarities. The o-3CzBN is an ideal system 14 to explore structural changes in higher-lying excited states by 15 comparing the solvent dependence of TR-IR in toluene and 16 MeCN. 17

First, we compared the photophysical properties of o-18 3CzBN in toluene, tetrahydrofuran (THF), and MeCN. The 19 absorption spectra showed no significant 20 difference except a slight decrease in the CT 21 absorption band around 370 nm as the polarity 22 increases, whereas the photoluminescence 23 spectra shifted to longer wavelengths in more 24 polar solvents (Figure 1b). This implies that o-25 3CzBN in excited states has more CT character 26 than the ground states. 27

TADF activity of o-3CzBN also depends on 28 the solvent. While o-3CzBN showed no delayed 29 fluorescence in toluene, prominently the delayed 30 emission was observed in MeCN (Figure 1c), 31 which is consistent with the previous report.5 32

For an in-depth study of structural alteration in the 33 excited state, the transient IR spectra of o-3CzBN were 34 measured in toluene, THF and MeCN solutions. We focused 35 on the region of 1100 to 1700 cm−1 where most sensitive 36 cooperative vibrations of carbazole units show significant 37 stretching bands (Figure 2). The vibrational spectra showed 38 ground-state bleaching and excited state absorption. The 39 ground-state bleach (GSB) signals were well matched with 40 the ground state vibrational spectra (Figure S1). The spectral 41 shapes evolved with time which is coincident with the time 42 scale of ISC (Figure S2). Therefore, we assigned the spectra 43 at the delay time of 1-2 ns representing vibrational spectra of 44 the S1 state and the spectra at the delay time of t=100 ns 45 representing the T1 state. 46

Since the TR-IR spectra reflect molecular geometries in 47 the excited states, we compared the spectra in each solution. 48 The peak position of o-3CzBN in S1 and T1 states showed 49 negligible difference regardless of solvent polarity (Figure 2). 50 In all the solutions, drastic spectral changes were observed 51

Figure 2. TR-IR spectra (a) at 1 ns or 2 ns and (b) at 100 ns after 355-nm photoexcitation in toluene (upper, red), THF (middle, green) and MeCN (lower, red).

Figure 3. (a) Temporal evolutions of TR-IR spectra and (b) the calculated vibrational spectra for S1 (blue) and T1 for o-3CzBN in toluene.

3

in the region from 1270 to 1360 cm−1; while the peak in the 1 region of 1280−1310 cm−1 was dominant at the S1 state, the 2 peaks vanished in the spectra at the T1 state and a new peak 3 appeared at 1320−1350 cm−1 (Figure 3a, S3). The spectral 4 changes are similar to the trend in the THF solution reported 5 in our previous report.11 6

Since TR-IR in the region is sensitive to intramolecular 7 charge transfer and distortion of molecular geometry11, this 8 spectral change suggests a structural change associated with 9 the ISC from S1 to T1. While infrared spectral peaks of a 10 specific functional group often change depending on 11 solvents16,17, the IR stretching bands of o-3CzBN in 1100-12 1700 cm-1 region were independent of the solvent polarity, 13 which indicates common structural changes among the 14 solutions. Thus, the independence of molecular geometrical 15 change in the excited states from TR-IR cannot explain the 16 solvent-dependent TADF activities. 17

We examined the contribution of molecular 18 configuration and higher-lying excited states in more detail 19 to explore the solvent-dependent structural changes using 20 quantum chemical calculations. We employed density 21 functional theory (DFT) calculations and time-dependent 22 DFT (TD-DFT) calculations to investigate molecular 23 geometries in the ground and excited states. A comparison 24 between the experimentally observed TR-IR spectra and 25 simulated spectra from quantum chemical calculations allows 26 us to confirm the reliability of the quantum chemical 27 calculations. The spectral simulation in the T1 state was 28 extensively investigated in the previous report and correlated 29 well to the experimentally observed spectra.11 Here, we 30 attempted to examine the S1 state spectra of o-3CzBN in 31 toluene and MeCN solvents, by conducting similar 32 calculations to simulate vibrational spectra (Figure S4). The 33 simulation spectra successfully reproduced the trends of peak 34 shifts around 1300 cm-1 between S1 and T1 states (Figure 3b, 35 S5, S6). Based on the calculations, we investigated the 36 molecular geometries in the S1 structure in toluene and 37

MeCN solutions (Figure 4, S7). As 38 deduced from the TR-IR spectra, 39 similar trends of structural changes in 40 each solution were confirmed by 41 quantum chemical calculations. Since 42 the dihedral angle represents a local 43 structural distortion, we estimated the 44 dihedral angles between BN and Cz in 45 o-3CzBN, clarifying almost a similar 46 tilted angle in their S0 states, whereas 47 it became perpendicular to each other 48 in their S1 states. In the T1 states, one 49 of Cz is distorted in the out-of-plane 50 direction with respect to BN. 51 However, the quantum chemical 52 calculations also revealed that the 53 degree of structural change was 54 slightly suppressed in polar solvents 55 (Table S1-S5). This slight 56 suppression of structural change can 57 be responsible for TADF activity of 58 o-3CzBN in polar solvent. 59

We also discussed the orbital distribution of the highest 60 occupied/lowest unoccupied natural transition orbitals 61 (HONTO / LUNTO). The calculations estimated a similar 62 optimized structure at each state in all solutions; the 63 structures at S0 and T1 were similar to previous reports11, and 64 the locations of all Cz units are in the same plane and are 65 orthogonal to BN at S1. The HONTO is delocalized over the 66 3Cz units, and the LUNTO is localized to the BN unit in S0. 67 However, the HONTO in T1 delocalized over the whole 68 molecule. The distributions of the LUNTOs are similar 69 among the three states (Figure 5, S8-S10). Such a difference 70 in MOs leads to charge transfer (CT) behavior in excited 71 states. 72

Those characteristics can be clarified from the 73

contribution of exchange interactions between the HOMO 74 and LUMO (JHL) to the energetics of S1 and T1. Due to the 75

Figure 5. HONTO and LUNTO for S0, S1, and T1 in toluene.

Figure 4. The optimized geometries of o-3CzBN at S0, S1, and T1 states in (a) toluene solution and (b) MeCN solution. (1) Side view and (2) front view.

4

repulsive nature of the electrons, S1 prefers small HOMO-1 LUMO overlap, which is efficiently achieved by a vertical 2 dihedral angle configuration between the Cz units (donor) 3 and the BN unit (acceptor). Contrary, small dihedral angles 4 between Cz and BN in the T1 states result in considerable 5 overlap between HOMO and LUMO. We obtained the almost 6 the same results in the calculations in MeCN, implying that 7 the above discussion can be generalized independent of 8 surrounding dielectric functions (Figure S5-S6). 9

The effects of higher-lying excited states on the RISC 10 process are estimated by optimizing structural geometries in 11 T1 and S1 energy levels. In Table 1, we summarized the 12 energy deviations of excited states from the S1 state of o-13 3CzBN in toluene, THF, and MeCN solution. The energy 14 difference between the S1 and T1 levels (ES1-T1) did not 15 correlate with the solvent polarity, but ET2-S1 was 16 significantly modified and becomes much smaller in the high 17 polar medium. Moreover, T3 or S2 is closer to S1 state in more 18 polar solvents. We noticed that the NTOs of T2, T3, or other 19 high-lying excited states were not completely identical with 20 the NTOs of T1 and S1 (Figure S8-10). The existence of other 21 states with different configurations is preferential to gain 22 large spin-orbit coupling based on El-Sayed rules18,19. 23 Although the RISC process is also governed by the electronic 24 couplings between S1 and T1, the interstate couplings are also 25 increased by closer energy between the S1 state and the T2 or 26 higher-lying states. Since such energy levels of the excited 27 state can be controlled by their environments for TADF 28 materials20, we can further optimize the RISC efficiencies by 29 controlling the surrounding environment. 30

In conclusion, we studied the solvent dependent excited 31

state structural dynamics and photophysics of a carbazole 32 benzonitrile derivative, o-3CzBN, by TR-IR spectroscopy. 33 We found that the drastic structural changes are independent 34 of the solvents while the photophysical properties were 35 heavily modulated by the choice of solvent. However, our 36 calculations suggested that the structural change was slightly 37 suppressed in polar solvents. To achieve an efficient RISC 38 process in o-3CzBN, along with the active ISC process 39 associated with the structural change suppression, the 40 existence of higher-lying triplet states near S1 state also 41 equally important. The intrinsic characters of molecules can 42 be controlled by choosing the environment such as host 43 materials in practical devices. 44 45

46 Acknowledgment 47

This work was supported in part by JSPS KAKENHI 48 Grant Number JP17H06375, JP18H05170, JP18H02047, 49 JP18H05981, JP19K15508 and Qdai-jump Research (QR) 50 Program Wakaba Challenge. The computations were 51 performed using the Research Center for Computational 52 Science (National Institute of Natural Sciences) and Research 53 Institute for Information Technology (Kyushu University). 54 We thank Kyulux Inc. for supplying samples and Dr. Raj 55 Kumar Koninti for fruitful discussions. 56 57 Supporting Information is available on 58 http://dx.doi.org/10.1246/cl.******. 59

References and Notes 60 1 H. Tanaka, K. Shizu, H. Miyazaki, C. Adachi, Chemical 61

Communications 2012, 48, 11392. 62 2 H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, 63

Nature 2012, 492, 234. 64 3 R. Ishimatsu, S. Matsunami, K. Shizu, C. Adachi, K. Nakano, 65

T. Imato, Journal of Physical Chemistry A 2013, 117, 5607. 66 4 X.-K. Chen, Y. Tsuchiya, Y. Ishikawa, C. Zhong, C. Adachi, 67

J.-L. Brédas, Advanced Materials 2017, 1702767, 1702767. 68 5 T. Hosokai, H. Noda, H. Nakanotani, T. Nawata, Y. 69

Nakayama, H. Matsuzaki, C. Adachi, Journal of Photonics 70 for Energy 2018, 8, 1. 71

6 T. Hosokai, H. Matsuzaki, H. Nakanotani, K. Tokumaru, T. 72 Tsutsui, A. Furube, K. Nasu, H. Nomura, M. Yahiro, C. 73 Adachi, Science Advances 2017, 3, e1603282. 74

7 H. Noda, H. Nakanotani, C. Adachi, 2018, 1. 75 8 H. Noda, X.-K. Chen, H. Nakanotani, T. Hosokai, M. 76

Miyajima, N. Notsuka, Y. Kashima, J.-L. Brédas, C. Adachi, 77 Nature Materials 2019. 78

9 T. Kobayashi, A. Niwa, K. Takaki, S. Haseyama, T. Nagase, 79 K. Goushi, C. Adachi, H. Naito, Physical Review Applied 80 2017, 7, 1. 81

10 X. K. Chen, D. Kim, J. L. Brédas, Accounts of Chemical 82 Research 2018, 51, 2215. 83

11 M. Saigo, K. Miyata, S. Tanaka, H. Nakanotani, C. Adachi, 84 K. Onda, Journal of Physical Chemistry Letters 2019, 10, 85 2475. 86

12 N. Fukazawa, T. Tanaka, T. Ishikawa, Y. Okimoto, S. 87 Koshihara, T. Yamamoto, M. Tamura, R. Kato, K. Onda, The 88 Journal of Physical Chemistry C 2013, 117, 13187. 89

13 T. Mukuta, S. Tanaka, A. Inagaki, S. Koshihara, K. Onda, 90 Chemistry Select 2016, 1, 2802. 91

14 T. Mukuta, P. V. Simpson, J. G. Vaughan, B. W. Skelton, S. 92 Stagni, M. Massi, K. Koike, O. Ishitani, K. Onda, Inorganic 93 Chemistry 2017, 56, 3404. 94

15 C. Grieco, E. R. Kennehan, H. Kim, R. D. Pensack, A. N. 95 Brigeman, A. Rimshaw, M. M. Payne, J. E. Anthony, N. C. 96 Giebink, G. D. Scholes, J. B. Asbury, Journal of Physical 97 Chemistry C 2018, 122, 2012. 98

16 A. Allerhand, P. R. Von Schleyer, Journal of the American 99 Chemical Society 1963, 85, 371. 100

17 A. R. H. Cole, L. H. Little, A. J. Michell, Spectrochimica 101 Acta 1965, 21, 1169. 102

18 M. A. El-Sayed, The Journal of Chemical Physics 1963, 38, 103 2834. 104

19 H. Yersin, L. Mataranga-Popa, R. Czerwieniec, Y. Dovbii, 105 Chemistry of Materials 2019, 31, 6110. 106

20 P. L. Santos, J. S. Ward, P. Data, A. S. Batsanov, M. R. 107 Bryce, F. B. Dias, A. P. Monkman, Journal of Materials 108 Chemistry C 2016, 4, 3815. 109

110

Table 1. The energy difference with respect to that of S1. The triplet energy levels were calculated based on the optimized geometry of T1, and the singlet energy levels were calculated based on the optimized geometry of S1.

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Electronic Supporting Information for Chemistry Letters © 2019 The Chemical Society of Japan

Solvent Dependence of Structural Dynamics and Thermally Activated Delayed Fluorescence

in 3,4,5-tri(9H-carbazole-9-yl)benzonitrile (ortho-3CzBN) Masaki Saigo, Kiyoshi Miyata, Hajime Nakanotani, Chihaya Adachi, and Ken Onda*


Masaki Saigo, 1 Kiyoshi Miyata, 1 Hajime Nakanotani,2,3 Chihaya Adachi2,3, and Ken Onda1*

1Department of Chemistry, Kyushu University, 744 Motooka, Nishi, Fukuoka 829-0395

2Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka

819-0395

3JST, ERATO, Adachi Molecular Exciton Engineering Project, Kyushu University, 744 Motooka, Nishi, Fukuoka

819-0395

Solvent Dependence of Structural Dynamics and Thermally Activated Delayed Fluorescence in 3,4,5-tri(9H-carbazole-9-yl)benzonitrile (ortho-3CzBN)


Table of Contents

Methods

I. Sample Preparation

II. Spectroscopy

III. Calculations

Table S1. Diheadral angle, torsion angle, and bond length between BN and Cz at S0.

Table S2. Diheadral angle, torsion angle, and bond length between BN and Cz at S1.

Table S3. Diheadral angle, torsion angle, and bond length between BN and Cz at T1.

Table S4. The differences between at S1 and at S0 of diheadral angle, torsion angle, and bond

length between BN and Cz.


length between BN and Cz.

Figures S1. Comparison of the TR-IR and FT-IR spectra for o-3CzBN.

Figures S2. Comparison of the temporal decay of TR-IR signals and TR-PL intensity for o-

3CzBN.

Figures S3. Temporal evolutions of TR-IR spectra for o-3CzBN in MeCN.

Figures S4. Comparison of the experimentally observed spectra and calculated vibrational

spectra of o-3CzBN.

Figures S5. The calculated vibrational spectra of o-3CzBN for S1 and T1.

Figures S6. The comparison of calculated vibrational spectra of o-3CzBN for S1 and T1.

Figures S7. The optimized geometries of o-3CzBN at S0, S1, and T1 states in THF.

Figures S8. NTOs for some states from S0 to S2 in toluene solution.

Figures S9. NTOs for some states from S0 to S2 in THF solution.

Figures S10. NTOs for some states from S0 to S2 in MeCN solution.

Figures S11. TR-IR signal intensity for toluene and MeCN solution.


Methods

I. Sample Preparation

We synthesized o-3CzBN according to previous works.1 We prepared solutions of the

purified molecules in toluene, tetrahydrofuran (THF) and acetonitrile (MeCN) purchased from

Kanto Kagaku.

II. Spectroscopy

1) UV-Vis absorption spectroscopy

The UV-Vis absorption spectra was measured with a UV-Vis spectrophotometer

(JASCO, V-630). The concentration of the solutions was prepared to be 0.1 mM.

2) Time-resolved photoluminescence (TR-PL) measurements

TR-PL measurements were performed using a streak camera (Hamamatsu, C4334)

coupled to a regenerative Ti:sapphire amplifier (Spectra-Physics, Spitfire Ace, ~120 fs, 1 kHz, 4

mJ/pulse, 800 nm). The samples were pumped by the third harmonic generation (THG) from a

nanosecond Nd:YAG laser (EKSPLA, NL220, central wavelength: 1064 nm, pulse duration: 6

ns). The polarization angles of the light for pumping/detection were set to the magic angle. The

concentration of the solutions was prepared to be 1 mM. The solutions were continuously

circulated through a quartz cell to avoid potential damage from optical pumping.

3) Fourier transform infrared (FT-IR) spectroscopy

The IR spectrum at the ground state was recorded with a FT-IR spectrophotometer

(Shimadzu, IRPrestige-21). The sample was used the KBr pellets, prepared by mixing sample

with KBr powder at a ratio of 1:100 and using a Hydraulic press. KBr purchased from JASCO.

4) Time-resolved infrared (TR-IR) measurements

The experimental setup for the pump-probe femtosecond TR-IR measurements has been

reported previously.2–4 Briefly, a broadband mid-IR pulse for a probe light (pulse duration: 120

fs, bandwidth: 150 cm-1, tunable range: 1000-4000 cm-1) was generated by difference frequency


generation (DFG) of signal and idler lights from an optical parametric amplifier (OPA) coupled

to the output from the Ti:sapphire regenerative amplifier. For a pump light, we employed one of

three lasers below accordingly to make S1 excitation: the third harmonic generation (THG) from

a nanosecond Nd:YAG laser (EKSPLA NL220, central wavelength: 1064 nm, pulse duration: 6

ns). The polarization angles of the light for the pump and probe were set to the magic angle. The

pump pulse fluence were 2.87 mJ/cm2 for toluene solution, 3.56 mJ/cm2 for THF solution, and

3.67 mJ/cm2 for MeCN solution. We also confirmed that the measurements were in a linear

regime (Figure S11), meaning such spectral change is not due to high-order effects but rather

intrinsic change of the molecule. The sample solutions were continuously circulated through a

home-built optical cell equipped with BaF2 windows with an optical path length of 0.1 mm. A

probe pulse passed through the optical cell and was dispersed by a 19 cm polychromator followed

by detection using a 64-channel mercury cadmium telluride (MCT) infrared detector array. The

concentrations of the solutions were prepared to be 1 mM for toluene and MeCN solutions and 3

mM for THF solution. All measurements were conducted after 1-hour bubbling using N2 gas.

III. Calculations.

Quantum chemical calculations based on the density functional theory (DFT) were

performed using the Gaussian 16 package.5 Vibrational spectra were calculated after geometry

optimization of each state. We employed the 6-31G(d,p) basis set snd the B3LYP functionals

following previous research.6 The solvent effect was examined by using the polarizable

continuum model of PhMe solution (dielectric constant: 2.379) and MeCN solution (dielectric

constant: 36.64). The frequencies of the simulated spectra were appropriately scaled to take into

account frequency shifts caused by anharmonicity. The scaling factor of 0.97 was adopted.


Table S1-S5

Table S1. Diheadral angle, torsion angle, and bond length between BN and Cz at S0. These

geometries were calculated with the same way as Figure S3.

S0 Toluene THF MeCN

Diheadral angle

BN-Cz1 55.863 56.767 57.642

BN-Cz2 60.901 61.617 62.307

BN-Cz3 55.863 56.767 57.642

Torsion angle

BN-Cz1 177.222 177.337 177.450

BN-Cz2 180.000 180.000 180.000

BN-Cz3 177.222 177.337 177.450

Bond L

ength l

BN-Cz1 1.4170 1.4178 1.4184

BN-Cz2 1.4117 1.4129 1.4139

BN-Cz3 1.4170 1.4178 1.4184


Table S2. Diheadral angle, torsion angle, and bond length between BN and Cz at S1. These


S1 Toluene THF MeCN

Diheadral angle

BN-Cz1 88.444 88.208 88.600

BN-Cz2 89.106 88.973 89.325

BN-Cz3 88.424 88.171 88.592

Torsion angle

BN-Cz1 179.033 179.113 179.197

BN-Cz2 179.981 179.961 179.983

BN-Cz3 179.033 179.113 179.195

Bond L

ength l

BN-Cz1 1.4380 1.4377 1.4377

BN-Cz2 1.4401 1.4363 1.4341

BN-Cz3 1.4380 1.4377 1.4377


Table S3. Diheadral angle, torsion angle, and bond length between BN and Cz at T1. These


T1 Toluene THF MeCN

Diheadral angle

BN-Cz1 49.142 51.067 52.196

BN-Cz2 48.719 50.759 52.020

BN-Cz3 78.513 76.281 75.600

Torsion angle

BN-Cz1 175.571 175.780 175.912

BN-Cz2 153.742 156.796 158.412

BN-Cz3 177.495 178.086 178.422

Bond L

ength l

BN-Cz1 1.4136 1.4151 1.4160

BN-Cz2 1.4163 1.4170 1.4174

BN-Cz3 1.4267 1.4276 1.4281



length between BN and Cz. These geometries were calculated with the same way as Figure S3.

(S1-S0) Toluene THF MeCN

Diheadral angle

BN-Cz1 32.581 31.442 30.958

BN-Cz2 28.205 27.356 27.018

BN-Cz3 32.561 31.404 30.950

Torsion angle

BN-Cz1 1.811 1.776 1.746

BN-Cz2 -0.019 -0.039 -0.017

BN-Cz3 1.811 1.776 1.745

Bond L

ength l

BN-Cz1 0.0211 0.0199 0.0193

BN-Cz2 0.0284 0.0235 0.0202

BN-Cz3 0.0211 0.0199 0.0193


Table S5. The differences between at T1 and at S0 of diheadral angle, torsion angle, and bond

length between BN and Cz. These geometries were calculated with the same way as Figure S3.

(T1-S0) Toluene THF MeCN

Diheadral angle

BN-Cz1 -6.721 -5.700 -5.446

BN-Cz2 -12.182 -10.857 -10.287

BN-Cz3 22.650 19.515 17.958

Torsion angle

BN-Cz1 -1.651 -1.557 -1.539

BN-Cz2 -26.258 -23.204 -21.588

BN-Cz3 0.273 0.749 0.972

Bond L

ength l

BN-Cz1 -0.0034 -0.0027 -0.0023

BN-Cz2 0.0045 0.0042 0.0035

BN-Cz3 0.0097 0.0098 0.0098


Figure S1-S11

Figure S2. Comparison of the TR-IR and FT-IR spectra for o-3CzBN. The TR-IR spectra was

measured at 2 ns after 355-nm photoexcitation in THF solution. The FT-IR spectra of the sample

was measured as KBr pellet.

Figure S2. Comparison of the temporal decay of TR-IR signals (red) and TR-PL intensity (blue)

for o-3CzBN in (a) toluene solution and (b) MeCN solution. TR-IR signals are 1299 cm-1 in

toluene and 1303 cm-1 in MeCN.


Figure S3. Temporal evolutions of TR-IR spectra for o-3CzBN in MeCN solution ranging from

1260 to 1380 cm−1 after optical excitation to S1. Similar spectral changes were observed in both

solutions; from the peak at 1280−1310 cm−1 to the peak at 1320−1350 cm−1.


Figure S4. Comparison of the experimentally observed spectra (upper row) and calculated (lower

row) vibrational spectra of o-3CzBN for (a,c,e) S1 and (b,d,f) T1 in (a,b) toluene, (c,d) THF, and

(e,f) MeCN. Simulated spectra were calculated using TD-DFT with B3LYP/6-31G(d,p) for S1,

and DFT with B3LYP/6-31G(d,p) for T1. Solvent effects were taken into account using PCM for

each solvent.


Figure S4. The calculated vibrational spectra of o-3CzBN for S1 (blue) and T1 (red) in (a) THF

and (b) MeCN. Simulated spectra were calculated with the same way as Figure S3.

Figure S6. The comparison of calculated vibrational spectra of o-3CzBN for (a) S1 and (b) T1.

Simulated spectra were calculated with the same way as Figure S3.


Figure S7. The optimized geometries of o-3CzBN at S0, S1, and T1 states in THF solution. (1)

Side view and (2) front view. These geometries were calculated with the same way as Figure S3.


Figure S8. NTOs for some states from S0 to S2 in toluene solution. The NTOs at ground state,

triplet excites state, and singlet excites state were calculated with the optimized geometries at S0,

T1, and S1 of o-3CzBN respectively. The NTOs were calculated using TD-DFT with B3LYP/6-

31G(d,p). Solvent effects were taken into account using PCM for toluene.


Figure S9. NTOs for some states from S0 to S2 in THF solution. The NTOs at ground state, triplet

excites state, and singlet excites state were calculated with the optimized geometries at S0, T1, and

S1 of o-3CzBN respectively. The NTOs were calculated using TD-DFT with B3LYP/6-31G(d,p).

Solvent effects were taken into account using PCM for THF.


Figure S10. NTOs for some states from S0 to S2 in MeCN solution. The NTOs at ground state,

triplet excites state, and singlet excites state were calculated with the optimized geometries at S0,

T1, and S1 of o-3CzBN respectively. The NTOs were calculated using TD-DFT with B3LYP/6-

31G(d,p). Solvent effects were taken into account using PCM for MeCN.


Figure S11. TR-IR signal intensity for (a) toluene solution and (b) MeCN solution as a function

of pump pulse fluence. The linearity in the pump intensity was confirmed.


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