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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NMAT4154

NATURE MATERIALS | www.nature.com/naturematerials 1

Supplementary Information for

Highly efficient blue electroluminescence

based on thermally activated delayed fluorescence

By Shuzo Hirata1, Yumi Sakai1,2, Kensuke Masui1,3, Hiroyuki Tanaka1, Sae Youn Lee1,4, Hiroko Nomura1, Nozomi Nakamura1, Mao Yasumatsu1,

Hajime Nakanotani1,4,5,Qisheng Zhang1,4, Katsuyuki Shizu1,4,

Hiroshi Miyazaki1,6, Chihaya Adachi1,4,7*

1. Center for Organic Photonics and Electronics Research (OPERA), Kyushu

University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan 2. Research & Development Department, Dyden Corporation, 1-1 Hyakunenkouen,

Kurume, Fukuoka 839-0864, Japan 3. Advanced Core Technology Laboratories, Fujifilm Corporation, 577 Ushijima,

Kaisei, Ashigarakami, Kanagawa 258-8577, Japan 4. JST, ERATO, Adachi Molecular Exciton Engineering Project, 744 Motooka, Nishi,

Fukuoka 819-0395, Japan 5. Innovative Organic Device Laboratory, Institute of Systems, Information

Technologies and Nanotechnologies (ISIT), 744 Motooka, Nishi, Fukuoka 819-0395, Japan

6. Nippon Steel & Sumikin Chemical Co., Ltd., 46-80, Nakabaru Sakinohama, Tobata, Kitakyushu 804-8503, Japan

7. International Institute for Carbon Neutral Energy Research, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan

*Corresponding author. E-mail: [email protected]

© 2014 Macmillan Publishers Limited. All rights reserved.

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2 NATURE MATERIALS | www.nature.com/naturematerials

SUPPLEMENTARY INFORMATION DOI: 10.1038/NMAT4154

This PDF file includes:

1. Synthesis and Characterization

2. Details of Time-Dependent DFT Calculations (Figure S1)

3. Photophysical Properties (Figure S2-S10)

3.1 UV-vis absorption characteristics in toluene solution (Figure S2)

3.2 Emission spectral characteristics in toluene solution (Figure S3)

3.3. Emission lifetime in toluene solution (Figure S4)

3.4. Emission spectral characteristics in DPEPO films (Figure S5)

3.5. Emission lifetimes in DPEPO films (Figure S6 to S10)

4. Investigation of the Difference between ΔEST and ΔEaTADF (Figure S11-S19)

5. Table S1

6. References





1. Synthesis and Characterization

3a:

A solution of 3,6-dibromocarbazole (40.0 mmol), potassium hydroxide (KOH) (48.0

mmol) in dry acetone (200 mL) was stirred under nitrogen for 30 min. Then,

p-toluenesulfonyl chloride (44.0 mmol) in acetone was added to the solution. The

solution was heated under reflux at about 58 °C under a nitrogen atmosphere for 5 h.

After cooling, the solution was evaporated to remove acetone, and 1,2-dichloromethane

(50 mL) was added. The organic phase was washed with ice H2O (50 mL×3), dried over

sodium sulfate (Na2SO4), and then filtered. Evaporation of the filtrate gave the crude

product, which was purified by column chromatography (silica gel; eluent = 20%

toluene/hexane) to give 3a as white crystals (17.3 g, 36.1 mmol, 90.2%). 1H NMR

(500MHz, CDCl3) :(ppm) 8.20 (d, J=9.0 Hz, 2H), 7.98 (d, J=2.0 Hz, 2H), 7.64 (d,

J=8.5 Hz, 2H), 7.61 (dd, J=2.0 Hz, J=2.0 Hz, 2H), 7.13 (d, J=8.5 Hz, 2H), 2.29 (s, 3H);

13C NMR (125 MHz, CDCl3): δ (ppm); 145.4, 137.5, 130.9, 129.9, 127.0, 126.5, 123.1,

117.5, 116.7; HRMS-FAB (m/z): [M]+ calcd. for C19H13Br2NO2S, 478.90; found,

478.901; Anal. Calcd. for C18H11Br2NO2S: C, 47.62; H,2.73; N, 2.92. Found C, 47.73;

H, 2.68; N, 2.89.

3b:





A solution of 3,6-dibromocarbazole (40.0 mmol), phenylboronic acid (96.0 mmol),

sodium carbonate (Na2CO3) (207 mmol) in toluene (200 mL), ethanol (100 mL), and

H2O (53.0 mL) was stirred under nitrogen for 30 min. After addition of

tetrakis(triphenylphosphine)palladium (Pd(PPh3)4) (3.99 mmol) to the solution, it was

stirred at 80 C under nitrogen for 12 h. After cooling, the solution was evaporated to

remove acetone, and 1,2-dichloromethane (50 mL) was added. The organic phase was

washed with ice H2O (50 mL×3), dried over Na2SO4, and then filtered. Evaporation of

the filtrate gave the crude product, which was purified by column chromatography

(silica gel; eluent = 23% 1,2-dichloromethane/hexane) to give 3b as a white yellow

powder (4.33 g, 13.6 mmol, 33.9 %). 1H NMR (500 MHz, CDCl3): δ (ppm) 8.34 (d,

J=1.5 Hz, 2H), 8.11 (s, 1H), 7.72 (dd, J=1.0 Hz, J=1.0 Hz, 4H), 7.69(dd, J=1.5 Hz,

J=2.0 Hz, 2H), 7.51-7.46 (m, 6H), 7.36-7.33 (m, 2H); 13C NMR (125 MHz,

DMSO-d6): δ (ppm) 141.2, 139.7, 130.9, 128.7, 126.5, 126.3, 124.7, 123.3, 118.5,

111.3; HRMS-FAB (m/z): [M]+ calcd. for C24H17N, 319.14; found, 319.136; Anal. Calcd.

for C24H17N: C, 90.25; H, 5.36; N, 4.39. Found C, 90.31; H, 5.34; N, 4.35.

3c:

A solution of 3b (4.00 mmol), 4a (8.08 mmol), and copper(II) oxide (Cu2O) (9.70

mmol) in dodecylbenzene (3.5 mL) was stirred at 220 C under nitrogen for 12 h. After

cooling, chloroform was added to the solution, which was then filtered through Celite®.

The filtrate was evaporated by a rotary evaporator to yield the crude material, which





was purified by column chromatography (silica gel; eluent = 20% toluene/hexane) to

give 3c as a white powder (1.00 g, 1.05 mmol, 26.2%). 1H NMR (500 MHz, CDCl3): δ

(ppm) 8.66 (d, J=9.0 Hz, 2H), 8.40 (d, J=2.0 Hz, 4H), 8.18 (d, J=2.0 Hz, 2H), 7.97 (d,

J=8.5 Hz, 2H), 7.82 (dd, J=2.0 Hz, J=2.0 Hz, 2H), 7.72 (d, J=7.0 Hz, 8H), 7.67 (dd,

J=2.0 Hz, J=2.0 Hz, 4H), 7.48-7.46 (m, 14H), 7.36-7.33 (m, 6H), 2.42 (s, 3H); 13C

NMR (125 MHz, CDCl3): δ (ppm) 141.9, 141.1, 137.8, 133.9, 130.2, 128.8, 127.3,

127.1, 126.9, 126.7, 125.8, 124.1, 119.0, 118.7, 116.5, 110.0; HRMS-FAB (m/z): [M]+

calcd. for C67H45N3O2S, 955.32; found, 955.323; Anal. Calcd. for C67H45N3: C, 84.16; H,

4.74; N, 4.39; Found C, 84.28; H, 4.74; N, 4.31.

3d:

A solution of 3c (0.25 mmol) and KOH (2.5 mmol) in tetrahydrofuran (THF) (2.3 mL),

dimethylsulfoxide (DMSO) (1.4 mL), and H2O (0.3 mL) was stirred at 70 C under

nitrogen for 4 h. After cooling, the solution was neutralized with sulfuric acid (H2SO4)

and then toluene (30 mL) was added. The organic phase was washed with ice H2O (30

mL×3), dried over MgSO4, and then filtered. Evaporation of the filtrate gave 3d as a

white powder (0.16 g, 0.20 mmol, 80.0%). 1H NMR(500 MHz, CDCl3): δ (ppm) 8.52

(s,1H), 8.42 (d, J=2.0 Hz, 4H), 8.30 (d, J=2.0 Hz, 2H), 7.77 (d, J=8.5 Hz, 2H), 7.73 (d,

J=8.0 Hz, 8H), 7.70 (d, J=2.0 Hz, 1H), 7.67 (dd, J=2.0 and 1.5 Hz, 5H), 7.49-7.46 (m,

12H), 7.35-7.33 (m, 4H); 13C NMR (125 MHz, CDCl3): δ (ppm) 142.1, 141.8, 139.4,

133.5, 130.0, 128.8, 127.3, 126.6, 126.2, 125.7, 124.2, 123.9, 119.8, 118.9, 112.2, 110.1;





HRMS-FAB (m/z): [M]+ calcd. for C60H39N3, 801.31; found, 801.315; Anal. Calcd. for

C60H39N3: C, 89.86; H, 4.90; N, 5.24. Found C, 89.66; H, 4.78; N, 5.27.

2a:

2-Bromo-4,6-diphenyl-1,3,5-triazine (BDT) was prepared using the method reported in

Ref. 19. A solution of 3d (1.20 mmol), BDT (1.21 mmol), copper(I) iodide (CuI) (0.02

mmol), 18-crown-6-ether (0.13 mmol), and potassium carbonate (K2CO3) (1.43 mmol)

in dodecylbenzene (1.20 mL) and N-methylpyrrolidone (NMP) (0.50 mL) was stirred at

220 C under nitrogen for 48 h. After cooling, the solution was evaporated to remove

acetone, and 1,2-dichloromethane (50 mL) was added. The organic phase was washed

with ice H2O (50 mL×3), dried over Na2SO4, and then filtered. Evaporation of the

filtrate gave the crude product, which was purified by column chromatography (silica

gel; eluent = 23% 1,2-dichloromethane/hexane) to give 2a as a white yellow powder

(0.203 g, 0.183 mmol, 15.1%). 1H NMR (500 MHz, DMSO-d6): δ (ppm) 9.16 (d, J=9.0

Hz, 2H), 8.87-8.83 (m, 6H), 8.77 (d, J=8.5 Hz, 4H), 8.23 (d, J=8.5 Hz, 2H), 7.96 (d,

J=8.5 Hz, 2H), 7.86-7.80 (m, 16H), 7.76-7.71 (m, 4H), 7.55 (d, J=8.5 Hz, 4H),

7.52-7.49 (m, 8H), 7.37-7.34 (m, 4H); 13C NMR (125 MHz, CDCl3) δ (ppm) 142.0,

141.7, 133.6, 132.8, 131.1, 130.8, 128.8, 127.4, 125.8, 119.0, 110.1; HRMS-FAB (m/z):

[M]+ calcd. for C81H52N6, 1108.43; found, 1108.425; Anal. Calcd. for C81H52N6: C,

87.70; H, 4.72; N, 7.58; Found C, 87.63; H, 4.64; N, 7.53.





4a:

A solution of 3a (5.0 mmol), carbazole (12.1 mmol), Cu2O (24.6 mmol) in

dodecylbenzene (10 mL) was stirred at 200 C under nitrogen for 35 h. After cooling,

chloroform was added to the solution, which was then filtered through Celite®. The

filtrate was evaporated by a rotary evaporator to yield the crude material, which was

purified by column chromatography (silica gel; eluent = 20% toluene/hexane) to give 4a

as white crystals (1.00 g, 1.53 mmol, 30.7%). 1H NMR (500MHz, DMSO-d6): (ppm)

8.65 (d, J=14.0 Hz, 2H), 8.58 (d, J=8.5 Hz, 2H), 8.25 (d, J=7.5 Hz, 4H), 8.03 (d, J=8.0

Hz, 2H), 7.88-7.86 (m, 2H), 7.47 (d, J=8.0 Hz, 2H), 7.42-7.41 (m, 8H), 7.30-7.27 (m,

4H), 2.36 (s, 3H); 13C NMR (125 MHz, DMSO-d6): 146.1,010.4, 136.7, 133.8, 133.2,

130.4, 127.0, 126.7, 126.5, 1222.6, 120.4, 120.1, 120.0, 116.9, 109.7; HRMS-FAB

(m/z): [M]+ calcd. for C43H29N3O2S, 651.20; found, 651.198; Anal. Calcd. for

C43H29N3O2S: C, 79.24; H, 4.48; N, 6.45. Found C, 79.24; H, 4.34; N,6.44.

4b:

A solution of 4a (3.07 mmol) and KOH (30.0 mmol) in THF (35.0 mL), DMSO (17.0

mL), and H2O (3.5 mL) was stirred at 70 C under nitrogen for 5 h. After cooling, the

solution was neutralized with H2SO4 and then toluene (50 mL) was added. The organic





phase was washed with ice H2O (50 mL×3), dried over magnesium sulfate (MgSO4),

and then filtered. Evaporation of the filtrate gave 4b as a white powder (1.1 g, 2.21

mmol, 75.0%). 1H NMR (500 MHz, DMSO-d6): (ppm) 11.90 (s, 1H), 8.51 (d, J=2.0

Hz, 2H), 8.25 (d, J=7.5 Hz, 4H), 7.84 (d, J=5.5 Hz, 2H), 7.62-7.60 (m, 2H), 7.43-7.40

(m, 4H), 7.34 (d, J=8.5 Hz, 4H), 7.28-7.25 (m, 4H); 13C NMR (125 MHz, DMSO-d6):

141.1, 139.6, 128.1, 126.0, 125.3, 123.2, 122.3, 120.3, 120.0, 119.5, 112.5, 109.6;

HRMS-FAB (m/z): [M]+ calcd. for C36H23N3, 497.19; found, 497.189; Anal. Calcd. for

C36H23N3: C, 86.90; H, 4.66; N, 8.44. Found C, 86.94; H, 4.59; N, 8.43.

2b:

A solution of 4b (0.60 mmol), BDT (0.60 mmol), CuI (0.02 mmol), 18-crown-6-ether

(0.09 mmol), and K2CO3 (0.63 mmol) in dodecylbenzene (0.60 mL) was stirred at 220

C under nitrogen for 48 h. After cooling, the solution was evaporated to remove

acetone, and 1,2-dichloromethane (50 mL) was added. The organic phase was washed

with ice H2O (50 mL×3), dried over Na2SO4, and then filtered. Evaporation of the

filtrate gave the crude product, which was purified by column chromatography (silica

gel; eluent = 23% 1,2-dichloromethane/hexane) to give 2b as a white yellow powder

(0.17 g, 0.21 mmol, 35.2%). 1H NMR (500 MHz, CDCl3): δ (ppm) 9.13 (d, J=8.5 Hz,

2H), 8.85 (d, J=9.5 Hz, 4H), 8.31 (d, J=2.0 Hz, 2H), 8.17 (d, J=7.5 Hz, 4H), 7.98 (d,

J=8.5 Hz, 2H), 7.81 (d, J=9.0 Hz, 2H), 7.68-7.63 (m, 8H), 7.42 (d, J=3.0 Hz, 8H),

7.31-7.28 (m, 4H); 13C NMR (125 MHz, CDCl3): δ(ppm) 171.7, 170.8, 141.8, 140.9,





140.3, 136.1, 135.9, 132.8, 131.0, 129.1, 128.8, 126.9, 126.5, 124.4, 120.3, 119.9, 119.8,

111.5, 109.7; HRMS-FAB (m/z): [M]+ calcd. for C57H36N6, 804.30; found, 804.299;

Anal. Calcd. for C57H36N6: C, 85.05; H, 4.51; N, 10.44. Found C, 85.09; H, 4.45; N,

10.44.

2c:

3-Carbazolylcarbazole was prepared using the method shown in Ref. 16. A solution of

3-carbazolylcarbazole (0.74 mmol), BDT (0.80 mmol), CuI (0.11 mmol),

18-crown-6-ether (0.11 mmol), and K2CO3 (4.48 mmol) in dodecylbenzene (2.0 mL)

was stirred at 220 C under nitrogen for 48 h. After cooling, the solution was evaporated

to remove acetone, and 1,2-dichloromethane (50 mL) was added. The organic phase was

washed with ice H2O (50 mL×3), dried over Na2SO4, and then filtered. Evaporation of

the filtrate gave the crude product, which was purified by column chromatography

(silica gel; eluent = 20% 1,2-dichloromethane/hexane) to give 2c as a white yellow

powder (0.25 g, 3.95 mmol, 53.0%). 1H NMR (500 MHz, CDCl3): (ppm) 9.08 (d,

J=9.0 Hz, 2H), 8.84 (d, J=6.5 Hz, 4H), 8.32 (d, J=2.0 Hz, 1H), 8.20 (d, J=8.0 Hz, 2H),

8.15 (d, J=7.5 Hz, 1H), 7.91 (d, J=8.5 Hz, 2H), 7.75 (d, J=9.0 Hz, 1H), 7.67-7.59 (m,

8H), 7.55-7.51 (m, 1H), 7.45-7.41 (m, 4H), 7.38-7.35 (m, 1H), 7.33-7.29 (m, 2H); 13C

NMR (125 MHz, CDCl3): δ (ppm) 171.9, 170.9, 141.9, 141.3, 141.2, 139.6, 136.1,

135.5, 132.7, 130.8, 130.4, 129.1, 128.8, 126.9, 125.9, 124.8, 123.4, 123.2, 120.3, 119.7,

11.0, 109.8; HRMS-FAB (m/z): [M]+ calcd. for C45H29N5, 639.24; found, 639.242; Anal.

Calcd. for C45H29N5: C, 84.48; H, 4.57; N, 10.95. Found C, 84.58; H, 4.48; N, 10.94.





1a:

1a was prepared by method described in Ref. 14.





2. Details of Time-Dependent TDF Calculations (Figure S1)

Fig. S1. Distribution of HOMO and LUMO in guest compounds calculated by the TD-DFT

method. (i) Distribution of HOMO in the structure optimized at the ground state (S0). (ii)

Distribution of LUMO in the structure optimized at S0. (iii) Distribution of HOMO in the structure

optimized at the lowest singlet excited state (S1). (iv) Distribution of LUMO in the structure

optimized at the S1 state. This result indicates that distribution of HOMO and LUMO in the

structures optimized at S0 is similar to those at S1.





3. Photophysical Properties (Figure S2-S10)

3.1. UV-vis absorption characteristics in toluene solution (Figure S2)

Fig. S2. Absorption characteristics of guests in toluene solution. A, 1a. B, 2a. C, 2b. D, 2c. Lines

represent molar absorption coefficient (ε(νa)) vs. absorption energy (νa) characteristics. Red, green,

and blue dotted lines represent fitting curves when the fine spectra are expressed by a summation of

each curve based on the Gaussian model: εn(ν) = Anexp(-(ν-Bn)2/2Cn2) (n=1, 2, 3…), where An is

amplitude, Bn is average wavenumber, and Cn is distribution parameter. The absorption with the

lowest absorption wavenumber (red dotted lines) corresponds to CT absorption in each guest. The

values of ∫ε(νa)dνa in the CT absorption as shown in *3 of Table 1 were used to determine dipole

moment of fluorescence (Q) using Eq. 6.

A B

C D





3.2. Emission spectral characteristics in toluene solution (Figure S3)

Fig. S3. Fluorescence and phosphorescence spectra of guests in toluene. A, 1a. B, 2a. C, 2b. D,

2c. Blue and red lines represent fluorescence spectra at RT and phosphorescence spectra at 77 K,

respectively. Orange and green dotted lines represent supporting lines to determine S1 and the lowest

excited triplet state (T1) energy, respectively. For phosphorescence of 1a, 2b, and 2c, the emission

peak with the highest energy was used as the T1 energy because they have sharp emission peaks,

which means the T1 state contains ππ* characteristics. For the others, the onset energy of each

emission was used to determine the S1 or T1 energy because of broad emission spectra related to CT

characteristics.

A

B

C D





3.3. Emission lifetimes in toluene solution (Figure S4)

Fig. S4. Emission decay characteristics of guests in toluene.

A

B

C D





3.4. Emission spectral characteristics in DPEPO films (Figure S5)

Fig. S5. Fluorescence and phosphorescence spectra of 6 wt% guest:DPEPO films. A, 1a. B, 2a.

C, 2b. D, 2c. Blue and red lines represent fluorescence spectra at RT and phosphorescence spectra at

5K, respectively. Orange and green dotted lines represent supporting lines to determine S1 and T1

energy, respectively. In 1a, the phosphorescence peak with the highest energy was used as the T1

energy because they contain sharp emission peaks, which means it possesses ππ* characteristics. In

other cases, the onset energy of each emission was used to determine the S1 or T1 energy because of

broad emission spectra related to CT characteristics.

A

C D

B





3.5. Temperature dependence of emission lifetime in DPEPO films (Fig. S6 to S10)

Fig. S6. Photoluminescence decay characteristics from 0 to 100 ns of 6 wt% guest:DPEPO

films at various temperatures. A, 2a. B, 2b. C, 2c. Emission intensity at all wavelengths was

integrated to determine emission decay characteristics. Emission decays (I(t)) can be expressed by

summation of two exponential factors, A1exp(-t/τ1) and A2exp(-t/τ2). Values of A1exp(-t/τ1) at each

temperature were compared to see the change in intensity of prompt fluorescence at each

temperature. No significant temperature dependence was observed for 2ac.

A

B

C





Fig. S7. Delayed emission characteristics of a 6 wt% 2a:DPEPO film at various temperatures.

A, Photoluminescence decay characteristics from 0 to 8 ms. Integration of emission intensity at all

wavelengths was used to determine emission decay characteristics. B, Emission spectra at each delay

time. Delayed emission (green line) at 300 K is ascribed only to TADF because the shape of the

delayed emission spectra is different from that of phosphorescence (red line). With decreasing

temperature, the TADF signal gradually decreases in intensity but did not completely disappear (i).

Furthermore, phosphorescence gradually emerged (ii) because the spectral shape of the emission at

lower temperature corresponds to that of phosphorescence, as shown in Fig. S5B. Therefore, the

delayed emission contains not only TADF but also phosphorescence at low temperature, which

makes it impossible to separate TADF and phosphorescence intensity. The separation of TADF and

phosphorescence intensity was achieved by the method shown in Fig. S10.

A

B





Fig. S8. Delayed emission characteristics of a 6 wt% 2b:DPEPO film at various temperatures.





temperature, the TADF signal gradually decreases in intensity but did not completely disappear (i).


lower temperature corresponds to that of phosphorescence, as shown in Fig. S5C. Therefore, the

delayed emission contains both TADF and phosphorescence at low temperature, which makes it

impossible to separate TADF and phosphorescence intensity. The separation of TADF and

phosphorescence intensity was achieved by the method described in Fig. S10.

A

B





Fig. S9. Delayed emission characteristics of a 6 wt% 2c:DPEPO film at various temperatures.





temperature, TADF signal gradually decreases in intensity but did not completely disappear (i).


low temperature corresponds to that of phosphorescence, as shown in Fig. S5D. Therefore, the

delayed emission contains both TADF and phosphorescence at lower temperature, which makes it

impossible to separate TADF and phosphorescence intensity. The separation of TADF and

phosphorescence intensity was achieved by the method shown in Fig. S10.

A B





Fig. S10. Photoluminescence decay characteristics in the range from 390 to 428 nm of 6 wt%

guest:DPEPO films at various temperatures. A, 2a. B, 2b. C, 2c. The observation of PL decay

from 390 to 428 nm enables the observation of temperature changes of only the TADF signal

because phosphorescence of 2ac is not observed at energies higher than 2.9 eV, whereas

fluorescence and delayed fluorescence can be observed in this energy range, as shown in Fig.

S7B-S9B, respectively. These temperature changes of the TADF signal were used to determine the

inset plots of Fig. 3D. Some data at low temperature cannot be used because signals were weak.

A

B

C





4. Investigation of the Difference between ΔEST and ΔEaTADF (Figure S11-S19)

It has been observed that the energy difference between the prompt components of

fluorescence and phosphorescence (ΔEST) derived from the onset of photoluminescence

spectra is larger than the energy obtained from the plots of ln(ΦTADF/ΦT) versus 1/T

(ΔEaTADF). Monkman et al. suggested that this can be ascribed to the presence of a 3n-π*

state in their compounds35. In this section, we suggest another possibility, which is that

molecular conformation changes during the rather long transient lifetime of triplet

excitons cause ΔEST>ΔEaTADF.

First, we summarize our explanation using the illustration depicted in Fig. S11.

Figure S11 represents an example of the relationship between the total energy of a

molecule of 2c at the ground state (S0), the lowest singlet excited state (S1), and the

lowest triplet excited state (T1), and the angle between a donor unit and acceptor unit (θ),

as illustrated in the inset of Fig. S11A. Fig. S11A shows that the molecular

conformation of the TADF molecules at S0 is stable when the angle between the donor

and acceptor units (θ) is θG. After the photoabsorption process at θ=θG ((1)), θ remains

constant until generation of prompt fluorescence at wavelength (λ=λ1) within ns because

large rotational motion between the donor and acceptor units cannot occur in a rigid

solid matrix within the time rangeS1. Therefore, fluorescence is generated ((2)) and

intersystem crossing (ISC) from S1 to T1 occurs at θ≈θG ((3)). After generation of a

triplet exciton through ISC, corresponding to t=t1, θ can gradually change over time

because the rotational energy is comparable with the thermal energy at room

temperature (RT) and the lifetime of the triplet exciton is long enough to allow the

motion. Therefore, θ of the triplet excitons forms a distribution over time, as shown in

the red filled circles in Fig. S11B. After ISC (t=t1), many triplet excitons have θ1 at the





lowest total energy ((4)), and they hardly contribute to reverse intersystem crossing

(RISC) from T1 to S1 because they have large ΔEST ((5)) caused by the large overlap

between HOMO and LUMO ((6)). After RISC, delayed fluorescence at λ=λ1 occurs

((7)). Then, at t=t2, some triplet excitons with θ=θ2 exist ((8)). These can contribute to

the RISC efficiently ((9)) because they have small ΔEST due to separation of HOMO and

LUMO ((10)), and delayed fluorescence at λ=λ2 is generated ((11)). Although some

triplet excitons with θ=θ0, which are present in the same number as that with θ=θ2, exist

((12)), they hardly contribute to RISC because of the large ΔEST ((13)) caused by a large

overlap between HOMO and LUMO ((14)). Subsequently (at t=t3), although there are

few triplet excitons with θ=θ3 at RT ((15)), they can still contribute to the RISC

efficiently ((16)) because they have very small ΔEST due to considerable separation of

HOMO and LUMO ((17)), and they generate delayed fluorescence at λ=λ3 ((18)).

Although few triplet excitons with θ=θ-1 ((19)), which are present in the same number

as that with θ=θ3, exist, they cannot contribute to RISC because of very large ΔEST

((20)).

In this process, (22) in Fig. S11C should be phosphorescence energy at 5 K

because all triplet excitons are concentrated at state (4) at 5 K. Because (2) of Fig. S11A

is fluorescence energy, (23) of Fig. S11D can be defined as the difference of onset

energy between fluorescence and phosphorescence (ΔEST). However, not only the

process of (4)-(5) but also those of (8)-(9) and (15)-(16) contribute to the actual process

of RISC at RT, as shown in Fig. S11B, and ΔEST at states (8) and (15) is smaller than

that of state (4). In this case, the actual ΔEST at RT (ΔEaTADF) should be expressed as

shown in (24) of Fig. S11D. Therefore, ΔEST>ΔEaTADF occurs in TADF materials. In this

mechanism, we note that a gradual spectral shift of delayed fluorescence with an





increase of distribution of θ with t should occur, as indicated in the changes from λ1 to λ3

shown in Fig. S11B. These estimations were derived by considering the following

results of DFT calculations and experimentally obtained spectral data.

To explain the above phenomena using 2a, we defined N(<ΔEST>, t=0) and

ΔEaTADF. N(<ΔEST>, t=0) is the energy difference between the spectral onset of the

prompt component of fluorescence and that of phosphorescence, where t is the time

after the generation of triplet excitons. In the main text, N(<ΔEST>, t=0) is defined as

ΔEST. ΔEaTADF is the slope of an Arrhenius plot of the intensity of delayed fluorescence.

In the following paragraphs, we demonstrate that molecular conformational changes

during the rather long triplet excited lifetime are one reason for ΔEST>ΔEaTADF in 2a.

Investigation of 2a by DFT calculation indicated that N(<ΔEST>, t=0) of 2a is

0.20 eV. Figure S12A shows the relationships between total energy at S0, S1, and T1 and

the angle between bis-3,6-diphenyl-3carbazolylcarbazole and triphenyltriazine units (θ)

for 2a, which were calculated by TD-DFT (Gaussian 09, B3LYP, 6-31G*). In the

calculation, after the molecular conformation was optimized at T1, only θ was changed

in the optimized structure. Then, S0, S1, and T1 energies were calculated for the

structures with each θ. The S0 state of 2a has an optimized structure at θ=±54.2°.

Because θ=±54.2° hardly changes before generation of fluorescence when 2a is doped

in a rigid hostS1, a fluorescence energy corresponds to the S1 energy at θ=±54.2°; this

energy is 2.39 eV, as shown in Fig. S12. The S1 state of 2a has CT characteristics over

the whole range of θ as illustrated in (i)-(iii) of Fig. S13, which is observed

experimentally as a broad fluorescence spectrum without clear vibrational structure

(blue line of Fig. S7B). In contrast, the T1 state of 2a has an optimized structure at

θ=±44.3° and phosphorescence energy of 2.19 eV, as shown in Fig. S12. In the T1 state,





91% of the phosphorescence transition has CT character, as illustrated in (v) of Fig. S13.

Therefore, the T1 state is almost entirely CT, which is observed experimentally as a

broad phosphorescence spectrum without clear vibrational structure (red line of Fig.

S7B). Thus, N(<ΔEST>, t=0) is the energy difference between the S1 energy at θ=±54.2°

and the T1 energy at θ=±44.3°, and it is 0.20 eV, as shown in Fig. S12.

Conversely, DFT calculation revealed that N(<ΔEST>, t=∞) decreased markedly

to 0.020 eV, where t=∞ is after sufficient time to change θ has passed. After generation

of triplet excitons in 2a, θ of the triplet excitons forms a distribution with increasing t.

The θ of 2a can change at RT because many vibrational modes smaller than 400 cm-1,

which is comparable with the thermal energy at RT, contribute to the change of θ, as

shown in Fig. S14. The number density of triplet excitons of 2a as a function of θ at t=

∞ (N(θ, t=∞)) can be expressed as in Fig. S15A, which was calculated using the

Arrhenius model from the potential curve of T1 in Fig. S12A. The distribution of θ in 2a

ranged from θ=±44.3° to θ=±90° because the difference of T1 energy between θ=±44.3°

and θ=±90°, 0.062 eV, is not much larger than the thermal energy at RT, 0.026 eV, as

shown in Fig. S12. Furthermore, because the overlap integral between the HOMO in the

donor unit (Ψd) and LUMO in the acceptor one (Ψa) decreases with increasing θ, ΔEST

decreases with increasing θ, as shown in Fig. S15B. From Fig. S15A and B, a

distribution of ΔEST at t=∞ (P(ΔEST, t=Δ)) in 2a was calculated; this is depicted in Fig.

S16A. When RISC from T1 to S1 is approximated by the Arrhenius model, the number

density contributing to the RISC process at t=∞ as a function of ΔEST (N(ΔEST, t=∞))

can be expressed as follows:

))(exp(,(),( TkEtEPtEN

BST

STST . (Eq. S2)





Figure S16B shows the relationship between N(ΔEST, t=∞) and ΔEST. The actual ΔEST

at t=∞ (N(<ΔEST>, t=∞)) can be expressed as the average of N(ΔEST, t=∞) as

follows:

STSTST EdtENtEN ),(),( . (Eq. S3)

The dashed line in Fig. S16B indicates that the value of N(<ΔEST>, t=∞) in 2a is 0.020

eV. ΔEaTADF can be expressed as a time average of N(<ΔEST>, t) as follows:

tdt

dttENE

STTADFa

),(. (Eq. S4)

Because N(<ΔEST>, t=0) and N(<ΔEST>, t=∞) of 2a are 0.20 and 0.020 eV, respectively,

ΔEaTADF of 2a should be in the range from 0.020 to 0.20 eV. Therefore, DFT calculations

indicate that N(<ΔEST>, t=0)>ΔEaTADF in 2a.

Although the above consideration is based on the calculation of molecular orbitals,

the increase of N(<ΔEST>, t) with increasing t can be experimentally observed as a

gradual shift of delayed fluorescence spectra. Figure S17 shows that TADF spectra of

2a are shifted from higher to lower energy over time after photoexcitationS2. This can be

explained by the decrease of θ with increasing t. Figure S18 is an example of the

molecular orbital diagram when θ is increased from θ1 to θ2. When θ=θ1, the energy

difference between Ψd and the HOMO of 2a (ΨH) or the energy difference between Ψa

and the LUMO of 2a (ΨL) is E1. Likewise, the energy difference between Ψd and ΨH or





the energy difference between Ψa and ΨL is E2 when θ=θ2. Typically, E1 is larger than E2

because the overlap between Ψd and Ψa at θ=θ1 is larger than that at θ=θ2. Thus,

fluorescence energy at θ=θ1 (λ1) is larger than that at θ=θ2 (λ2), as shown in Fig. S18.

Consequently, delayed fluorescence spectra are red shifted, which can be explained by

the increase of θ. A gradual shift of the TADF spectra occurs from ns to ms when 2a is

doped into bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) host. We note that

this is reasonable because the rotational motion between two large units such as the

donor and acceptor units of 2a needs a large space (a few nm), and ranges from ns to ms

in a rigid host matrixS1. Because the DPEPO host has a glass transition temperature of

280 °C34, it is difficult for the large dynamic motion of guest molecules to occur

quicklyS3.

In previous reports and the main manuscript, ΔEST has been determined from the

energy difference between the onset of a prompt fluorescent spectrum and that of a

phosphorescence spectrum. However, the actual RISC process involves up-conversion

from the phosphorescence energy to the energy of a delayed fluorescence, which is

smaller than the energy of prompt fluorescence. Therefore, the actual ΔEST at RT should

be discussed as the energy difference between the onset of an integrated spectrum of the

delayed fluorescence spectra and that of the phosphorescence spectrum. Figure S17

shows the spectra of prompt fluorescence (purple), integration of delayed fluorescence

(black), and phosphorescence (red) in 2a. The time average of N(<ΔEST>, t) is

0.04±0.02 eV, which is comparable with ΔEaTADF of 0.055 eV.

Overall, both DFT calculations and spectral analyses suggest that the changes in

molecular conformation over time after the generation of triplet excitons is one reason

why ΔEST>ΔEaTADF.





Fig. S11. Explanation of ΔEST>ΔEaTADF using energy diagrams. A, Processes before generation

of triplet excitons. B, Processes after generation of triplet excitons. Ψd(r) and Ψa(r) represent the

HOMO of a donor and LUMO of an acceptor, respectively. C, Process of phosphorescence at 5 K. D,

Determination of ΔEST and ΔEaTADF in the case of the mechanism of reverse intersystem crossing

(RISC) as shown in A-C.

A B

C

D





Fig. S12. Relationship between total energy at S0, S1, and T1 states and θ in 2a. Data

were calculated by TD-DFT (Gaussian 09, B3LYP, 6-31G*). After the molecular

conformation was optimized at T1, only θ was changed in the optimized structure. Then,

S0, S1, and T1 energies were calculated for the structures with each θ.





Fig. S13. Distribution of HOMO and LUMO in S1 and T1 at each θ when 2a is at

RT. Data were calculated by TD-DFT (Gaussian 09, B3LYP, 6-31G*).





Fig. S14 Visualization of the vibrational energy of 2a calculated by TD-DFT

(Gaussian 09, B3LYP, 6-31G*). The frequency of each vibrational mode was

calculated using the optimized structure at T1. A, Infrared absorption spectra from 0 to

3500 cm-1. B, Vibrational modes ascribed to (i)-(iv) in A. C, Infrared absorption spectra

from 0 to 200 cm-1. D, Vibration modes from 0 to 200 cm-1. In B and D, blue arrows

represent the motions of each atom. Peaks from 0 to 200 cm-1 are ascribed to the

rotational mode between donor and acceptor units.

A B

C D





Fig. S15. Relationship between (A) N(θ, t=∞) and θ, and (B) ΔEST and θ in 2a.

A

B





Fig. S16. Relationship between (A) P(ΔEST, t=∞) and ΔEST, and (B) N(ΔEST, t=∞)

and ΔEST in 2a.

A

B





Fig. S17. Spectral shift of delayed fluorescence over time after generation of triplet

excitons in 2a. Samples were thin films composed of 6wt% guest (2a):DPEPO films.

Delayed fluorescence spectra were measured at RT under inert conditions.

Phosphorescence spectra were measured at 5 K under inert conditions. Black dotted

lines were used to determine the onset energy from the integrated delayed fluorescence

spectra and phosphorescence spectra.





Fig. S18. Change of wavelength of delayed fluorescence with θ. A, An example of a

molecular orbital diagram when θ=θ0. B, An example of a molecular orbital diagram

when θ=θ1. Ψd(r) and Ψa(r) represent the HOMO of a donor and LUMO of an acceptor,

respectively, and θ indicates the angle between them. λ1 and λ2 are the wavelengths of

delayed fluorescence when θ is θ1 and θ2, respectively. θ2 is larger than θ1. The

wavelength of delayed fluorescence increases with θ because the overlap between Ψd(r)

and Ψa(r) decreases.

A B





Fig. S19. Spectral shift of delayed fluorescence over time after generation of triplet

excitons when a molecule with large ΔEST was used as a guest in a rigid host.

Samples were thin films composed of 6 wt% guest:DPEPO films. Delayed fluorescence

spectra were measured at RT under inert conditions. Phosphorescence spectra were

measured at 5 K under inert conditions. Blue, orange, and black dotted lines were used

to determine the onset energy of the prompt component of fluorescence spectra,

integrated delayed fluorescence spectra, and phosphorescence spectra, respectively.





5. Table S1

Table S1. Other photophysical properties of guests

Calculation*1 Toluene solution DPEPO film

Compound λa*2

S1*3 T1

*3 S1

*4 T1*4

nm eV eV eV eV 1a 432 2.86/CT 2.74~2.77/MIX 2.85/CT 2.73-2.80/MIX 2a 466 3.02/CT 2.93/CT 3.01/CT 2.89/CT 2b 450 3.02/CT 2.85/MIX 3.05/CT 2.85/MIX 2c 441 3.05/CT 2.83/MIX 3.10/CT 2.81/MIX

*1: Results for structure optimized at S0 by TD-DFT (Gaussian 09/B3LYP/6-31G(d))

*2: Maximum wavelength of UV-vis absorption

*3: Results determined from measurements presented in Fig. S3. MIX indicates that T1 exhibits both ππ* and CT

characteristics, and CT means that T1 exhibits CT characteristics. The T1 energy of 1a is obscure because its

photoluminescence spectrum is broad, so the T1 energy cannot be clearly determined

*4: Data determined from measurements presented Fig. S5. MIX indicates that T1 exhibits both ππ* and CT

characteristics and CT means that T1 shows CT characteristics. Because the T1 energy of 1a is obscured by its

broad photoluminescence spectrum, the T1 energy cannot be clearly determined





6. References

S1. J. N. Turro, Modern Molecular Photochemistry p. 7 (University Science Books,

Sausalito, California, 1991) states that large rotational motions range from ns to ms

and depend on the rigidity and viscosity of the matrix.

S2. Although it has been reported that spectral shift of fluorescence with time after

excitation is sometimes caused by reorientation of the matrix, this is not applicable

to TADF molecules. This is because the degree of the spectral shift was increased

when a TADF molecule with large ΔEST was used as a guest in a rigid host, as

shown in Fig. S19.

S3. A red shift of delayed fluorescence spectra is not observed when a guest is doped

into a solvent. This is because the low viscosity of the solvent enables quick change

of θ in the guest molecules, so the distribution of θ was saturated within 1 ns after

finishing excitation. This is also described on p. 7 of reference S1.



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