doi.org/10.26434/chemrxiv.11798568.v2

Solvent Dependence of Structural Dynamics and Spin-flip Processes in3,4,5-tri(9H-carbazole-9-yl)benzonitrile (ortho-3CzBN)Masaki Saigo, Kiyoshi Miyata, Hajime Nakanotani, Chihaya Adachi, Ken Onda

Submitted date: 13/04/2020 • Posted date: 15/04/2020Licence: CC BY-NC-ND 4.0Citation information: Saigo, Masaki; Miyata, Kiyoshi; Nakanotani, Hajime; Adachi, Chihaya; Onda, Ken(2020): Solvent Dependence of Structural Dynamics and Spin-flip Processes in3,4,5-tri(9H-carbazole-9-yl)benzonitrile (ortho-3CzBN). ChemRxiv. Preprint.https://doi.org/10.26434/chemrxiv.11798568.v2

We have investigated the solvent-dependence of structural changes along with intersystem crossing of athermally activated delayed fluorescence (TADF) molecule, 3,4,5-tri(9H-carbazole-9-yl)benzonitrile(o-3CzBN), in toluene, tetrahydrofuran, and acetonitrile solutions using time-resolved infrared (TR-IR)spectroscopy and DFT calculations. We found that the geometries of the S1 and T1 states are very similar inall solvents though the photophysical properties mostly depend on the solvent. In addition, the time-dependentDFT calculations based on these geometries suggested that the thermally activated delayed fluorescenceprocess of o-3CzBN is governed more by the higher-lying excited states than by the structural changes in theexcited states.

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Solvent Dependence of Structural Dynamics and Spin-flip Processes in 3,4,5-tri(9H-carbazole-9-yl)benzonitrile (ortho-3CzBN)

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 *Corresponding author. E-mail: konda@chem.kyushu-univ.jp Abstract We have investigated the solvent-dependence of structural changes along with intersystem crossing of a thermally activated delayed fluorescence (TADF) molecule, 3,4,5-tri(9H-carbazole-9-yl)benzonitrile (o-3CzBN), in toluene, tetrahydrofuran, and acetonitrile solutions using time-resolved infrared (TR-IR) spectroscopy and DFT calculations. We found that the geometries of the S1 and T1 states are very similar in all solvents though the photophysical properties mostly depend on the solvent. In addition, the time-dependent DFT calculations based on these geometries suggested that the thermally activated delayed fluorescence process of o-3CzBN is governed more by the higher-lying excited states than by the structural changes in the excited states. Keywords: Time-resolved infrared spectroscopy (TR-IR); Solvent dependence; Thermally activated delayed fluorescence (TADF)

1. Introduction

Thermally activated delayed fluorescence (TADF) molecules receive tremendous

attention owing to their unique ability to improve the efficiency of organic light-emitting

diodes (OLEDs). TADF molecules convert excitons at the lowest triplet state (T1) to the

lowest singlet excited state (S1) via reverse intersystem crossing (RISC) driven by thermal

excitation. To achieve an efficient RISC process, the energy gap between S1 and T1 (∆EST)

needs to be sufficiently small (<0.1 eV). The basic design strategy for obtaining small

∆EST is to separate the highest occupied molecular orbital (HOMO) from the lowest

unoccupied molecular orbital (LUMO), typically achieved by connecting electron donor

and electron acceptor chromophores1,2.

TADF molecules often have a charge transfer (CT) character in the excited states.

The CT character results in the strong solvent-dependence of photophysical properties

such as emission wavelength, photoluminescence quantum yield (PLQY), and lifetime.3–

5. Moreover, TADF activities also depend on the solvent. Although the mechanism of the

solvent-dependence is essential when designing high-performance devices, the

microscopic understanding of the mechanism is still elusive6–8. Recently, the contribution

of higher-lying excited states for TADF activity was suggested from the temperature

dependence of photoluminescence (PL) decay rate9, transient absorption spectroscopy6,

and theoretical calculations8,10. Hosokai et al. compared TADF activities of carbazole-

benzonitrile (Cz-BN) derivatives in a non-polar solvent (toluene, ε = 2.4) and a polar

solvent (acetonitrile, ε = 37). They found the solvent-dependent TADF in these

derivatives from the viewpoint of the difference in the shifting of energy levels and their

matching, including higher-lying excited states5. In addition to the T1 and S1 states, which

possess CT characters, Hosokai et al. proposed the existence of the higher-lying excited

state, T2, with a locally excited (LE) character to explain the solvent dependence. While

the S1 and T1 states are significantly stabilized in polar solvents, the energy level of a

locally excited T2 state is less affected; therefore, they determined the positive correlation

between TADF activity and energy matching of S1,CT and T2,LE.

TADF activity is also expected to be modulated by intra-molecular structural change

in the excited state. Nevertheless, discussions in many previous works were often based

on the geometry of the ground state. The structural change in the excited states has mainly

been discussed from theoretical calculations, but the lack of experimental justification of

the calculations has been problematic. Therefore, the correlation between the solvent

dependence of TADF activity and the structure in the excited state largely remains elusive.

We previously reported that the suppression of structural changes along with intersystem

crossing (ISC) assists the TADF process in Cz-BN derivatives with an ∆EST of about 0.2

eV11. Because structural changes in the excited states are highly sensitive to external

environments such as solvents, it is necessary to investigate the excited state structural

modulations with solvents to explore the solvent effects on TADF processes.

Here, we have studied the solvent dependence of structural change associated with

ISC using the combination method of time-resolved infrared spectroscopy (TR-IR) and

quantum chemical calculations. TR-IR is a powerful tool to investigate the structural

dynamics of functional molecules in excited states.11–15 We focused on 3,4,5-tri(9H-

carbazole-9-yl)benzonitrile (o-3CzBN, Figure 1a), which shows drastic structural

changes in the excited state. Although all Cz units of o-3CzBN in the ground state are in

the same plane, the location of one Cz unit shifts towards an out-of-plane direction in the

T1 state, as revealed by TR-IR and quantum chemical calculations.11 Intriguingly, o-

3CzBN shows no TADF activity in toluene solution although it becomes TADF active in

acetonitrile (MeCN) solution.5 Therefore, the o-3CzBN is an ideal system to explore

structural changes in higher-lying excited states by comparing the solvent dependence of

TR-IR in toluene and MeCN. We first carefully characterized the structural and electronic

properties of the lowest excited states, i.e., S1 and T1 states, in different solvents and

discuss the roles of the higher-lying excited states in these solvents based on these results.

2. Time-resolved spectroscopy and calculations

2.1 Time-resolved photoluminescence (TR-PL) measurements

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

synchronized with a nanosecond Nd:YAG laser (EKSPLA, NL220, central wavelength:

1064 nm, pulse duration: 6 ns). The samples were pumped by the third harmonic

generation (THG) of the output of the nanosecond Nd:YAG laser. 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.

2.2 Time-resolved infrared (TR-IR) measurements

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

been reported previously.12,13,16 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 a regenerative Ti:sapphire

amplifier (Spectra-Physics, Spitfire Ace, ~120 fs, 1 kHz, 4 mJ/pulse, 800 nm). For a pump

light, we employed one of the 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 fluences were 2.9

mJ/cm2 for toluene solution, 3.6 mJ/cm2 for THF solution, and 3.7 mJ/cm2 for MeCN

solution. We also confirmed that the measurements were in a linear regime (Figure S1),

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.

2.3. Calculations

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

performed using the Gaussian 16 package.17 Vibrational spectra were calculated after

geometry optimization of each state. We employed the 6-31G(d,p) basis set and the

B3LYP functionals following previous research.11 The solvent effect was examined using

the polarizable continuum model of toluene 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.

3. Results and discussion

First, we compared the photophysical properties of o-3CzBN in toluene,

tetrahydrofuran (THF, ε = 7.6), and MeCN. The absorption spectra showed no significant

difference except for a slight intensity decrease of the CT absorption band at 370 nm as

the polarity increased. In contrast, the photoluminescence spectra shifted to longer

wavelengths in more polar solvents (Figure 1b). These shifts indicate that o-3CzBN in

the excited states has more CT character than in the ground states. The TADF activity of

o-3CzBN also depends on the solvent. Although o-3CzBN showed no delayed

fluorescence in toluene, delayed emission was prominently observed in MeCN (Figure

1c), which is consistent with a previous report.5

For an in-depth study of structural alteration in the excited state, the transient IR

spectra of o-3CzBN were measured in toluene, THF, and MeCN solutions. We focused

on the region of 1100–1700 cm−1, where most sensitive cooperative vibrations of

carbazole units show significant stretching bands (Figure 2). The vibrational spectra

showed ground state bleaching and excited state absorption. The ground state bleach

(GSB) signals were well matched with the ground state vibrational spectra (Figure S2).

Figure 1. (a) 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 (blue), 445–465 nm in THF (green), and 460–480 nm in MeCN (red). PL

spectra and decay profiles were obtained by pumping at 355 nm.

The spectral shapes evolved with time, which is coincident with the time scale of ISC

(Figure S3). Therefore, we assigned the spectra at a delay time of 1–2 ns representing

vibrational spectra of the S1 state and the spectra at the delay time of t=100 ns representing

the T1 state.

Because the TR-IR spectra reflect molecular geometries in the excited states, we

compared the spectra in each solution. The peak position of o-3CzBN in the S1 and T1

states showed negligible difference regardless of solvent polarity (Figure 2). In all the

solutions, drastic spectral changes were observed in the region from 1270 to 1360 cm−1;

although the peak in the region of 1280−1310 cm−1 was dominant at the S1 state, the peaks

vanished in the spectra at the T1 state, and a new peak appeared at 1320−1350 cm−1

(Figures 3a, S4). The spectral changes are similar to the trend in the THF solution reported

in our previous report.11

Because TR-IR in the region is sensitive to intramolecular charge transfer and

distortion of molecular geometry11, this spectral change suggests a structural change

associated with the ISC from S1 to T1. Although infrared spectral peaks of a specific

functional group often change depending on solvents18,19, the IR stretching bands of o-

3CzBN in the 1100–1700 cm−1 region were independent of the solvent polarity, indicating

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).

nearly identical

structural changes

among the solutions.

Thus, the independence

of molecular

geometrical change in

the excited states from

TR-IR hardly explains

the solvent-dependent

TADF activities.

We next examined the contribution of molecular configuration and higher-lying

excited states in more detail to explore the solvent-dependent structural changes using

quantum chemical calculations. We employed DFT calculations and time-dependent DFT

(TD-DFT) calculations to investigate molecular geometries in the ground and excited

states. A comparison between the experimentally observed TR-IR spectra and simulated

spectra from quantum chemical calculations allowed us to confirm the reliability of the

quantum chemical calculations. The spectral simulation in the T1 state was extensively

Figure 4. Optimized geometries of o-3CzBN at the S0, S1, and T1 states in (a) toluene solution and

(b) MeCN solution. (1) Side view and (2) front view.

Figure 3. (a) Temporal evolutions of TR-IR spectra and (b) the

calculated vibrational spectra for S1 (blue) and T1 for o-3CzBN in

investigated in a previous report and correlated well to the experimentally observed

spectra.11 Here, we examined the S1 state spectra of o-3CzBN in toluene and MeCN

solvents, by conducting similar calculations to simulate vibrational spectra (Figure S5).

The simulated spectra successfully reproduced the trends of peak shifts at approximately

1300 cm−1 between the S1 and T1 states (Figures 3b, S6, S7). Based on the calculations,

we investigated the molecular geometries in the S1 structure in toluene, THF, and MeCN

solutions (Figures 4, S8). As deduced from the TR-IR spectra, similar trends of structural

changes in each solution were confirmed by quantum chemical calculations. Because the

dihedral angle represents a local structural distortion, we estimated the dihedral angles

between BN and Cz in o-3CzBN, clarifying a nearly similar tilted angle in their S0 states.

In contrast, the angles became perpendicular to each other in their S1 states. In the T1

states, the angle of Cz is distorted in the out-of-plane direction concerning BN. However,

the quantum chemical calculations also revealed that the degree of structural change was

slightly suppressed in polar solvents (Tables S1–S5). This slight suppression of structural

change is presumably responsible for the TADF activity of o-3CzBN in polar solvents.

We next discussed the orbital distribution of the highest occupied/lowest unoccupied

natural transition orbitals (HONTO/LUNTO). The calculations estimated a similar

optimized structure at each state in all solutions; the structures at S0 and T1 were similar

to those presented in previous reports11, and the locations of all Cz units were in the same

plane and were orthogonal to BN at S1. The HONTO is delocalized over the three Cz

units, and the LUNTO is localized to the BN unit in S0. However, the HONTO in T1

delocalized over the whole molecule. The distributions of the LUNTOs were similar

among the three states (Figures 5, S9–S11). These differences in MOs lead to charge

transfer (CT) behavior in excited states.

Those characteristics can be

clarified from the contribution

of exchange interactions

between the HOMO and

LUMO (JHL) to the energetics

of S1 and T1. Owing to the

repulsive nature of electrons, S1

prefers a small HOMO–LUMO

overlap, which is efficiently

achieved by a vertical dihedral

angle configuration between

the Cz units (donor) and the BN

unit (acceptor). On the

contrary, small dihedral angles

between Cz and BN in the T1

states result in considerable overlap between HOMO and LUMO. We obtained almost

identical results in the calculations using MeCN, implying that the above discussion can

be generalized independent of the surrounding dielectric functions (Figures S6–S7).

The effects of higher-lying excited states on the RISC process were estimated by

optimizing structural geometries in T1 and S1 energy levels. In Figure 6, we summarized

the energy deviations of excited states from the S1 state of o-3CzBN in toluene, THF, and

MeCN solutions. The energy difference between the S1 and T1 levels (∆ES1–T1) did not

correlate with the solvent polarity, but ∆ET2–S1 was significantly modified and became

considerably smaller in the highly polar medium. Moreover, T3 or S2 is closer to the S1

state in more polar solvents, which accelerates the spin conversion between S1 and T1.

We argue that the proximity of the energy of S1 and T2–T3 better explains the activation

of the TADF in MeCN. Although more accurate calculations, such as QM/MM

calculations, are necessary to gain quantitative mechanisms, the contribution from the

higher-lying state explained the solvent dependence of the TADF activity on the basis of

the determined S1 and T1 characters qualitatively.

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

We noticed that the NTOs of T2, T3, or other high-lying excited states were not

completely identical to the NTOs of T1 and S1 (Figures S9–11). The existence of other

states with different configurations is preferential to gain a large spin-orbit coupling based

on El-Sayed rules20,21. Although the RISC process is also governed by the electronic

couplings between S1 and T1, the interstate couplings are also increased by closer energy

between the S1 state and the T2 or higher-lying states. Because their environments control

the energy levels of the excited state for TADF materials22, we can further optimize the

RISC efficiencies by controlling the surrounding environment.

4. Conclusions

We studied the solvent-dependent excited state structural dynamics and photophysics

of a carbazole benzonitrile derivative, o-3CzBN, by TR-IR spectroscopy. To the best of

our knowledge, this is the first study investigating the solvent dependence of the

structures in the S1 and T1 states both experimentally and theoretically. We found that the

drastic structural changes were independent of the solvents but the photophysical

properties were heavily modulated by choice of solvent. However, our calculations based

on the TR-IR spectra suggested that the structural change was slightly suppressed in polar

solvents. In conjunction with the theoretical results, solvent-independent geometries in

the excited states implied dominant contributions of higher-lying excited states. To

achieve an efficient RISC process in o-3CzBN, along with the active ISC process

Figure 6. Relative energy levels of excited states with respect to the energy level 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.

associated with structural change suppression, the existence of higher-lying triplet states

near the S1 state is also important. The detailed understanding of the effect of the

environment is essential to control the optoelectronic properties of molecules from the

choice of the environment, such as host materials in practical optoelectronics devices.

Acknowledgment

This work was supported in part by JSPS KAKENHI Grant Numbers JP17H06375,

JP18H05170, JP18H02047, JP18H05981, JP19K15508, and the Qdai-jump Research

(QR) Program Wakaba Challenge. The computations were performed using the Research

Center for Computational Science (National Institute of Natural Sciences) and Research

Institute for Information Technology (Kyushu University). We thank Kyulux Inc. for

supplying samples and Dr. Raj Kumar Koninti for fruitful discussions.

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Solvent Dependence of Structural Dynamics and Spin-flip Processes in 3,4,5-tri(9H-carbazole-9-yl)benzonitrile (ortho-3CzBN) Masaki Saigo, 1 Kiyoshi Miyata, 1 Hajime Nakanotani, 2,3 Chihaya Adachi, 2,3 and Ken Onda 1*

1 Department of Chemistry, Kyushu University, 744 Motooka, Nishi, Fukuoka 829-0395 2 Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395 3 JST, ERATO, Adachi Molecular Exciton Engineering Project, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395

Table of Contents

Methods

I. Sample Preparation

II. Steady State Spectroscopy

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.

Table S5. The differences between at S1 and at S0 of diheadral angle, torsion angle, and bond length between BN and Cz.

Figures S1. TR-IR signal intensity for toluene and MeCN solution.

Figures S2. Comparison of the TR-IR and FT-IR spectra for o-3CzBN.

Figures S3. Comparison of the temporal decay of TR-IR signals and TR-PL intensity for o-3CzBN.

Figures S4. Temporal evolutions of TR-IR spectra for o-3CzBN in MeCN.

Figures S5. Comparison of the experimentally observed spectra and calculated vibrational spectra of o-3CzBN.

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

Figures S7. The comparison of calculated vibrational spectra of o-3CzBN for S1 and T1.

Figures S8. The optimized geometries of o-3CzBN at S0, S1, and T1 states in THF.

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

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

Figures S11. NTOs for some states from S0 to S2 in 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. Steady State Spectroscopy

1) UV-Vis absorption spectroscopy

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

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

2) Fourier transform infrared (FT-IR) spectroscopy

The IR spectra in the ground state were recorded with a FT-IR spectrophotometer

(Shimadzu, IRPrestige-21). The samples were measured in the KBr pellets, which were

prepared by mixing sample with KBr powder at a ratio of 1:100 and using a Hydraulic press.

KBr purchased from JASCO.

Table S1-S5

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

geometries were calculated in the same way as Figure S3.

S0 Toluene THF MeCN

Diheadral angle θ

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 φ

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 Length 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. Dihedral angle, torsion angle, and bond length between BN and Cz at S1. These

geometries were calculated in the same way as Figure S3.

S1 Toluene THF MeCN

Dihedral angle θ

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 φ

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 Length 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. Dihedral angle, torsion angle, and bond length between BN and Cz at T1. These

geometries were calculated in the same way as Figure S3.

T1 Toluene THF MeCN

Dihedral angle θ

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 φ

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 Length 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

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

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

∆(S1-S0) Toluene THF MeCN

Dihedral angle θ

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 φ

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 Length 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 dihedral 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

Dihedral angle θ

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 φ

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 Length 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 S1. 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.

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

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

were measured as KBr pellet.

Figure S3. 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 S4. 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 S5. 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 S6. 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 S7. The comparison of calculated vibrational spectra of o-3CzBN for (a) S1 and (b) T1.

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

Figure S8. 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 in the same way as Figure S3.

Figure S9. NTOs for selected states from S0 to S2 in toluene solution. The NTOs in the ground, triplet-excited, and singlet excited states 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 S10. NTOs for selected states from S0 to S2 in THF solution. The NTOs in the ground, triplet excited, and singlet excited states 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 S11. NTOs for some states from S0 to S2 in MeCN solution. The NTOs in the ground, triplet excited, and singlet excited states 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.

References

1. Hosokai, T. et al. Evidence and mechanism of efficient thermally activated delayed

fluorescence promoted by delocalized excited states. Science Advances 3, e1603282

(2017).

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