Thermally activated delayed fluorescence materials based on 3,6-di-tert-butyl-9-((phenylsulfonyl)phenyl)-9H-carbazoles

Article abstract of DOI:10.1016/j.dyepig.2014.06.008

A series of bipolar materials for thermally activated delayed fluorescence based on 3,6-di-tert-butyl-9-((phenylsulfonyl)phenyl)-9H-carbazoles, is synthesized by Ulmann coupling reactions. In these materials, the 3,6-di-tert-butylcarbazole group is linked at the 3-, 4- position or 3′-, 4′- position of diphenyl sulfone. The effects of the conjugation connectivity on the electronic, photophysical and electrochemical properties of these materials, are studied by extensive UV-vis, fluorescence spectroscopic measurements, cyclic voltammetry and theoretical calculations as well as X-ray crystallographic analysis. The energy gap between singlet and triplet in these materials is tuned from 0.39 eV to 0.22 eV by manipulation of conjugation of the electron donor units.

Full text of DOI:10.1016/j.dyepig.2014.06.008

Accepted Manuscript
Thermally Activated Delayed Fluorescence Materials based on 3,6-Di-tert-butyl-9-
((phenylsulfonyl)phenyl)-9H-carbazoles
Bin Huang, Qi Qi, Wei Jiang, Jinan Tang, Yuanyuan Liu, Wenjuan Fan, Zhihui Yin,
Fachen Shi, Xinxin Ban, Huange Xu, Yueming Sun
PII:
S0143-7208(14)00232-0
10.1016/j.dyepig.2014.06.008
DYPI 4416
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 15 April 2014
Revised Date: 3 June 2014
Accepted Date: 6 June 2014
Please cite this article as: Huang B, Qi Q, Jiang W, Tang J, Liu Y, Fan W, Yin Z, Shi F, Ban X,
Xu H, Sun Y, Thermally Activated Delayed Fluorescence Materials based on 3,6-Di-tert-butyl-9-
((phenylsulfonyl)phenyl)-9H-carbazoles, Dyes and Pigments (2014), doi: 10.1016/j.dyepig.2014.06.008.
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ACCEPTED MANUSCRIPT
Graphic abstract

ACCEPTED MANUSCRIPT
Thermally Activated Delayed Fluorescence Materials based on
3,6-Di-tert-butyl-9-((phenylsulfonyl)phenyl)-9H-carbazoles
Bin Huang a,b, Qi Qi a, Wei Jianga,*, Jinan Tang a, Yuanyuan Liu b, Wenjuan Fan b, Zhihui Yin b,
Fachen Shi b, Xinxin Ban a, Huange Xu a, Yueming Sun a,*
a
School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu, P. R.
China 211189
b
Department of Chemical and Pharmaceutical Engineering of Southeast University Chengxian
College, Nanjing, Jiangsu, P. R. China 210088
* Corresponding author: Tel/fax: +86 25 52090621
E-mail address:101011462@seu.edu.cn
Abstract:
A series of bipolar materials for thermally activated delayed fluorescence based on
3,6-di-tert-butyl-9-((phenylsulfonyl)phenyl)-9H-carbazoles, is synthesized by Ulmann coupling
reactions. In these materials, the 3,6-di-tert-butylcarbazole group is linked at the 3-, 4- position or
3'-, 4'- position of diphenyl sulfone. The effects of the conjugation connectivity on the electronic,
photophysical and electrochemical properties of these materials, are studied by extensive UV-vis,
fluorescence spectroscopic measurements, cyclic voltammetry and theoretical calculations as well
as X-ray crystallographic analysis. The energy gap between singlet and triplet in these materials is
tuned from 0.39 eV to 0.22 eV by manipulation of conjugation of the electron donor units.
△
Keywords: carbazole, sulfone, phosphorescence, bipolar, EST, TADF
1. Introduction
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High-efficiency organic light emitting diodes (OLEDs) have attracted major interest due to their
potential for solid-state lighting and flat panel display applications [1-5]. In recent years, great
advances have been made in the development of materials for OLEDs [6-13]. These materials have
the potential to lead to cost-effective, light-weight and large area OLEDs. Among them, the energy
△
gap between singlet and triplet ( EST) of the materials plays an important role in realization of high
performance OLEDs. Xu et al reported a series of ambipolar ternary hosts with high triplet excited
△
energy and small EST less than 0.4 eV for low-voltage-driving high-efficiency blue PhOLEDs [7].
△
Duan et al reported bipolar host with EST of only 0.11 eV for low-voltage-driving green OLEDs
[11]. To enhance the performance of fluorescence-phosphoresce (F-P) hybrid white organic light
emitting diode (WOLED), Zhang et al reported two efficient blue fluorophors with sufficiently
△
small EST to sensitize green and red phosphors [14]. Due to high cost, pollution, low solubility and
poor long-term stability of traditional phosphors based on rare metals such as Ir(Ⅲ), Pt(Ⅲ), or Os(Ⅲ),
the development of low cost substitutes has attracted more and more attention [15-18]. In principle,
△
when EST is small, reverse intersystem crossing (ISC) can take place [19]. On the basis of this
concept, a series of efficient thermally activated delayed fluorescence (TADF) materials as
substitutes of phosphors based on rare metals for high-efficiency OLEDs were designed and
synthesized [20-29]. In particular, the PhOLEDs based on dicyanobenzene derivatives with a small
△
EST of 0.083 eV show a high external electroluminescent efficiency of 19% [3].
Generally, a small spatial overlap between the highest occupied molecular orbital (HOMO) and
△
lowest unoccupied molecular orbital (LUMO) is important to achieve a small EST. This task is
mostly solved by a design of compounds based on intramolecular charge transfer (ICT). In an ICT
based compound, the HOMO and LUMO would be localized at the donor (D) and acceptor (A)
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△
moieties, respectively. Thus, the HOMO and LUMO are well separated, leading to a small EST. In
△
the last few years, various materials with a small EST are designed and applied to OLEDs. Very
△
recently, Adachi et al reported an ICT based molecule with an EST of 0.003 eV, in which the
△
HOMO and LUMO are ideally separated which gave a EST of almost zero [30]. The PhOLEDs
based on these molecules show a high external electroluminescent efficiency of 14%. Although
△
various materials for TADF with small EST have been reported as summarized above, there are
few systematic studies focusing on the effects of structure on fundamental properties, such as the
△
optical and electrochemical properties particularly the EST.
Recently, a sulfone-containing material attracted major interest for high-efficiency OLEDs
because a sulfone moiety possesses a strong electron-accepting nature. Some sulfone based
materials have been applied to realize high-performance OLEDs as a host, an electron transporter,
or an emitter [31-39]. Kido et al developed a novel high triplet energy host material with a sulfone
and m-terphenyl moieties for phosphorescent OLED
using such a system as a host material are the highest levels in the scientic literature [33]. Zhang et
al reported two novel blue-violet emitting materials based on a D- -A structural strategy
incorporating carbazole as a mild electron-donor and sulfone as an electron-acceptor with a
-conjugation-breaking feature. The two compounds exhibit high-effciency uorescent emissions
. The performances of blue and green OLEDs
π
π
with impressively high quantum yields in both solution and film states. The nondoped devices based
on these compounds achieved excellent current/power effciencies up to 1.89 cd A-1/1.58 lm W-1 and
CIE below 0.06 [36]. Adachi et al developed
a
diphenyl sulfone derivative,
bis(4-(3,6-di-tert-butylcarbazole)phenyl)sulfone (3a) (Scheme 1) as an emitter for blue OLEDs. 3a
△
possesses a EST of 0.32 eV and the corresponding blue OLED achieved a high external
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electroluminescent efficiency of 10% [27].
In this context, we became interested in the carbazole/sulfone derivatives, in which the number
and position of a 3,6-di-tert-butylcarbazole unit are systematically changed while fixing the central
diphenyl sulfone backbone. Here, we report the synthesis, structural features of
3,6-di(tert-butyl)-9-((phenylsulfonyl)phenyl)-9H-carbazoles (1a-5a, Scheme 1). The thermal,
electronic, photophysical properties of 1a-5a were explored by extensive thermogravimetric
analysis, UV−vis, fluorescence spectroscopic measurements, cyclic voltammetry (CV) and
theoretical calculations as well as X-ray crystallographic analysis. The energy difference between
△
singlet and triplet ( EST) in TC-DPSs is tuned from 0.39 eV to 0.22 eV by manipulation the number
and position of 3,6-di-tert-butylcarbazole unit.
2. Experimental
2.1 General
All reactants and solvents were purchased from commercial sources and used without further
purification. 1H NMR and 13C NMR spectra were recorded on a Bruker ARX300 NMR spectrometer
with Si(CH3)4 as the internal standard. Elemental analysis was performed on an Elementar Vario EL
CHN elemental analyzer. Mass spectra were obtained using a Thermo Electron Corporation
Finnigan LTQ mass spectrometer. UV-vis absorption spectra were recorded with
a
spectrophotometer (Agilent 8453) and PL spectra were recorded with a fluorospectrophotometer
(Jobin Yvon, FluoroMax-3). The transient photoluminescence decay characteristics of 1a-5a in
toluene were recorded using a spectrofluorometer (Floromax-4, Horiba). The fast decay component
was recorded with a 350 nm LED excitation source, while the slow decay component was recorded
with a flash lamp source. Thermogravimetric analysis (TGA) was performed using a Netzsch
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simultaneous thermal analyzer (STA) system (STA 409PC) under a dry nitrogen atmosphere at a
o
heating rate of 10 C min-1. Cyclic voltammetry measurements were performed on a Princeton
Applied Research potentiostat/galvanostat model 283 voltammetric analyzer in CH2Cl2 solutions
(10-3 M) at a scan rate of 100 mV s-1 with a platinum plate as the working electrode, a silver wire as
the pseudo-reference electrode, and a platinum wire as the counter electrode. The supporting
electrolyte was tetrabutylammonium hexafluorophosphate (0.1M) and ferrocene was selected as the
internal standard. The solutions were bubbled with a constant argon flow for 10 min before
measurements.
X-ray crystallography: X-ray crystallographic analysis of 1a were performed on a Nonius
CAD-4 single-crystal diffractometer by using MoKα radiation (λ = 0.71073 Å) with an ω/2θ scan
mode at 293 K. The structure was solved by direct methods and refined by full-matrix least-squares
procedures on F2 using SHELXL-97 program. All non-hydrogen atoms were refined anisotropically,
and the hydrogen atoms were introduced at calculated positions. The isotropic temp. factors were
fixed to 1.2 times (1.5 times for CH3 groups) the equivalent isotropic displacement parameters of
the C-atom the H-atom is attached to. Drawings were produced using Diamond 3.0 and Mercury
1.4.1 software. CCDC 973776 (1a) contains supplementary crystallographic data. These data can be
obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc. cam. ac.
uk/data request/cif.
DFT calculations of 1a-5a were performed using the Gaussian 03 program package. The
calculations were optimized at the B3LYP/6-31G(d) level of theory. The molecular orbitals were
visualized using Gaussview [40].
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2.2 Synthesis of 1a-e
Synthesis of 4-bromo-diphenylsulfone (1b)
To a solution of bromobenzene (3.14 g, 20.0 mmol) and benzenesulfonyl chloride (3.53g, 20.0
mmol) in CHCl3 (40 mL) was added aluminum chloride (2.67g, 20.0mmol). The reaction mixture
was heated under reflux for 24h, the resulting mixture was cooled to room temperature and then
poured into 40ml water and extracted with CH2Cl2 (2×40 mL). The combined organic phase was
then washed with saturated aqueous NaCl solution (2×20 mL) and dried with anhydrous Na2SO4.
After removal of the solvent by rotary evaporation, the residue was purified by silica gel column
o
1
chromatography to afford 1b as a white crystal. Yield: 69.54%. M.p.:109-110 C. H NMR(300
MHz, CDCl3): δ(ppm) 7.94-7.91 (t, J=7.2Hz, 7.2Hz, 2H), 7.81-7.79 (d, J=5.7Hz, 2H), 7.65-7.48 (m,
5H).
Synthesis of 3-bromo-diphenylsulfone (2b)
To a solution of dipenylsulfone (10.9 g, 50.0 mmol) in 50 mL of H2SO4 was added
o
N-bromosuccnimide (8.9 g, 50.0 mmol) in fractions. The reaction mixture was stirred at 70 C for 2
h, the resulting mixture was cooled to room temperature and then poured into ice-water (100 mL).
The white precipitate was filtered and dried. The crude product was recrystallized from ethanol to
o
1
produce 2b as a white solid. Yield: 86.59%. M.p.:90-91 C. H NMR(300 MHz, CDCl3): δ (ppm)
8.08 (s, 1H), 7.96-7.94 (d, J=7.5Hz, 3H), 7.61-7.50 (d, 5H).
Synthesis of di(3-bromophenyl)sulfone (4b)
To a solution of dipenylsulfone (10.9 g, 50.0 mmol) in H2SO4 (50 mL) was added
o
N-bromosuccnimide (17.8 g, 100 mmol) in fractions. The reaction mixture was stirred at 100 C for
2 h, the resulting mixture was cooled to room temperature and then poured into ice-water (100 mL).
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The white precipitate was filtered and dried. The crude product was recrystallized from ethanol to
o
1
produce 4b as a white crystal. Yield: 0.60 g (58.08%). M.p.:98-100 C. H NMR(300 MHz, CDCl3):
δ (ppm) 8.07(s, 2H), 7.88-7.86 (d, J=7.8Hz, 2H), 7.73-7.71 (d, J=7.8Hz, 2H), 7.40-7.38 (t, J=7.8Hz,
8.1Hz, 2H).
Synthesis of di(4-bromophenyl)sulfide (3c)
To a solution of 4-bromoiodobenzene (5.64 g, 20.0 mmol), 4-bromothiophenol (3.78 g, 20 mmol)
1,10-phenanthroline monohydrate (0.8 g, 4 mmol) in DMSO (40 mL) was added K2CO3 (5.52 g, 40
mmol).The reaction mixture was then purged with nitrogen for ten minutes before adding Cu2O
(0.144 g, 1 mmol). After heated under reflux for 24h under nitrogen, the resulting mixture was
cooled to room temperature and then poured into water. The pale precipitate was filtered and dried.
The crude product was recrystallized from hexane to produce 3c as a white crystal. Yield: 66.57%.
o
1
M.p.:111-112 C. H NMR(300 MHz, CDCl3): δ (ppm) 7.45-7.41 (m, 4H), 7.22-7.16 (m, 3H),
7.65-7.48 (m, 5H) [36].
Synthesis of di(4-bromophenyl)sulfone (3b)
To a solution of 30 mL H2O2 (30%, g/g) and 30 mL of HOAc was added 3c (3.44 g, 10 mmol) in
fractions. After heated under reflux for 24h, the resulting mixture was cooled to room temperature
and then poured into water. The white precipitate was filtered and dried. The crude product was
o
1
recrystallized from ethanol to produce 3b as a white solid. Yield: 84.66%. M.p.:173-174 C. H
NMR (300 MHz, CDCl3): δ(ppm) 7.80-7.77 (d, J=8.7Hz, 4H), 7.67-7.64 (d, J=8.7Hz, 4H) [36].
Synthesis of 3, 4'-dibromo-diphenylsulfide (5c)
A procedure similar to that used for 3c was followed but with 3-bromoiodobenzene instead of
4-bromoiodobenzene. After heated under reflux for 24 h under nitrogen, the resulting mixture was
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cooled to room temperature and then poured into water. The mixture was extracted with
dichloromethane (3×20 mL). The combined organic phase was then washed with saturated aqueous
NaCl solution (2×10 mL) and dried with anhydrous Na2SO4. After removal of the solvent by rotary
evaporation, the residue was purified by silica gel column chromatography to afford 5c as a white
o
1
solid. Yield: 58.08%. M.p.:64-65 C. H NMR(300 MHz, CDCl3): δ(ppm) 7.49-7.43 (m, 3H),
7.39-7.36 (d, J=7.5Hz, 1H), 7.27-7.14 (m, 4H).
Synthesis of 3, 4'-dibromo-diphenylsulfone (5b)
A procedure similar to that used for 3b was followed but with 5c instead of 3c.Yield: 78.08%.
o
1
M.p.:140-141 C. H NMR(300 MHz, CDCl3): δ(ppm) 8.05 (s, 1H), 7.86-7.78 (t, J=7.8Hz, 8.1Hz,
8.4Hz, 3H), 7.71-7.64 (d, J=10.5Hz, 8.7Hz, 3H), 7.41-7.36 (d, J=7.8Hz, 8.1Hz, 1H).
Synthesis of 3,6-di-tert-butyl-9-(4-(phenylsulfonyl)phenyl)-9H-carbazoleꢀ1aꢁ
To a solution of 1b (0.594 g, 2.0 mmol), 18-crown-6 (0.027g, 0.1 mmol) and
3,6-di-tert-butylcarbazole (0.68g, 2.4 mmol) in tetrahydro-1,3-dimethyl-2(1H)pyrimidine (DMPU)
(5 mL) was added K2CO3 (0.55g, 4.0 mmol). The reaction mixture was then purged with nitrogen
for ten minutes before adding CuI (0.020 g, 0.1 mmol). The mixture was stirred at 170oC for 24h
under nitrogen, the resulting mixture was cooled to room temperature and then poured into water
and extracted with CH2Cl2 (3×20 mL). The combined organic phase was then washed with saturated
aqueous NaCl solution (2×10 mL) and dried with anhydrous Na2SO4. After removal of the solvent
by rotary evaporation, the residue was purified by silica gel column chromatography to afford 1a as
o
1
a white solid. Yield: 65.01%. M.p.:276-279 C. H NMR(300 MHz, CDCl3): δ(ppm) 8.16-8.11 (m,
4H), 8.06-8.04(d, J=6.6Hz, 2H), 7.75-7.72(d, J=8.4Hz, 2H), 7.63-7.58 (m, 3H), 7.47-7.44 (t,
J=8.7Hz, 1.8Hz, 2H), 7.40-7.37 (d, J=8.7Hz, 2H), 1.45 (s, 18H). 13C NMR (75 MHz, CDCl3): δ(ppm)
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143.7, 142.6, 141.1, 138.6, 137.9, 133.0, 129.1, 129.1, 127.4, 126.1, 123.7, 123.6, 116.1, 108.8,
34.4, 31.6. MS (MALDI-TOF) [m/z]: calcd for C32H33NO2S, 495.22; found, 495.3912. Anal. Calcd.
for C32H33NO2S (%): C, 77.54; H, 6.71; N 2.83; S, 6.46. Found: C, 77.40; H, 6.78; N 2.88; S, 6.47.
Synthesis of 3,6-di-tert-butyl-9-(3-(phenylsulfonyl)phenyl)-9H-carbazole (2a)
A procedure similar to that used for 1a was followed but with 2b (0.594 g, 2.0 mmol) instead of
o
1
1b. Yield: 55.48%. M.p.:189-191 C. H NMR(300 MHz, CDCl3): δ(ppm) 8.16-8.12 (d, J=8.7Hz,
1.5Hz, 3H), 8.01-7.99 (d, J=6.9Hz, 3H), 7.81-7.72 (m, 3H), 7.70-7.55 (m, 3H), 7.48-7.44 (t,
J=4.8Hz, 6.9Hz, 2H), 7.30-7.28 (d, J=8.4Hz, 2H), 1.47 (s, 18H). 13C NMR (75 MHz, CDCl3): δ(ppm)
143.4, 143.2, 140.9, 139.1, 138.1, 133.1, 130.4, 130.4, 129.1, 127.5, 125.0, 124.9, 123.6, 116.1,
108.4, 34.4, 31.6. MS (MALDI-TOF) [m/z]: calcd for C32H33NO2S, 495.22; found, 495.4332. Anal.
Calcd. for C32H33NO2S (%): C, 77.54; H, 6.71; N 2.83; S, 6.46. Found: C, 77.48; H, 6.79; N 2.87; S,
6.48.
Synthesis of bis(4-(3,6-di-tert-butylcarbazole)phenyl)sulfone (3a)
To a solution of 3b (0.376 g, 1.0 mmol), 18-crown-6 (0.027g, 0.1mmol) and
3,6-di(tert-butyl)carbazole (0.68g, 2.4 mmol) in DMPU (5 mL) was added K2CO3 (0.55g, 4.0 mmol).
The reaction mixture was then purged with nitrogen for ten minutes before adding CuI (0.020 g, 0.1
o
mmol). The following procedure was similar to that used for 1a. Yield: 58.21%. M.p.:367-370 C.
1H NMR (300 MHz, CDCl3): δ (ppm) 8.26-8.23 (d, J=8.4Hz, 4H), 8.13 (s, 4H), 7.83-7.80 (d,
13
J=8.4Hz, 2H), 7.50-7.43 (m, 8H), 1.46 (s, 36H). C NMR (75 MHz, CDCl3): δ (ppm) 143.7, 142.8,
138.4, 137.9, 129.2, 126.2, 123.7, 123.6, 116.1, 108.8, 34.4, 31.6. MS (MALDI-TOF) [m/z]: calcd
for C52H56N2O2S, 772.41; found, 772.5745. Anal. Calcd. for C52H56N2O2S (%): C, 80.79; H, 7.30; N
3.62; S, 4.15.found:C, 80.62; H, 7.38; N 3.67; S, 4.17.
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Synthesis of bis(3-(3,6-di-tert-butylcarbazole)phenyl)sulfone (4a)
A procedure similar to that used for 3a was followed but with 4b (0.376 g, 1.0 mmol) instead of
o
1
3b.Yield: 67.26%. M.p.:304-305 C. H NMR (300 MHz, CDCl3): δ (ppm) 8.22 (s, 2H), 8.11-8.10
(d, J=2.1 Hz, 4H), 8.03-8.00 (d, J=7.8Hz, 4H), 7.85-7.82 (d, J=8.4 Hz, 2H), 7.78-7.73 (t, J=8.4Hz,
8.4 Hz, 2H), 7.39-7.36 (t, J=1.8Hz, 6.9Hz, 1.8Hz, 4H), 7.37-7.28 (d, J=8.7Hz, 4H), 1.43 (s, 36H).
13C NMR (75 MHz, CDCl3): δ (ppm)143.4, 142.9, 139.3, 138.0, 130.7, 125.1, 125.0, 123.6, 116.1,
108.4, 34.4, 31.6. MS (MALDI-TOF) [m/z]: calcd for C52H56N2O2S, 772.41; found, 772.6925. Anal.
Calcd. for C52H56N2O2S (%): C, 80.79; H, 7.30; N 3.62; S, 4.15.found:C, 80.56; H, 7.42; N 3.68; S,
4.18.
Synthesis of 3,6-di-tert-butyl-9-(3-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenylsulfonyl)phenyl)
-9H-carbazole (5a)
A procedure similar to that used for 3a was followed but with 5b (0.376 g, 1.0 mmol) instead of
o
1
3b.Yield: 68.56%. M.p.:195-196 C. H NMR (300 MHz, CDCl3): δ(ppm) 8.24-8.08 (m, 8H),
13
8.87-8.78 (m, 4H), 7.50-7.41 (m, 6H), 7.36-7.33 (d, J=8.7Hz, 2H), 1.47-1.46 (d, J=2.4Hz, 36H). C
NMR (75 MHz, CDCl3): δ (ppm)143.8, 143.5, 143.1, 143.0, 139.3, 138.1, 138.0, 137.9, 130.6,
130.6, 129.3, 126.2, 125.0, 123.8, 123.6, 123.5, 116.2, 108.8, 108.5, 34.4, 31.6, 31.5, 29.4, 26.6. MS
(MALDI-TOF) [m/z]: calcd for C52H56N2O2S, 772.41; found, 772.8536. Anal. calcd. for
C52H56N2O2S (%): C, 80.79; H, 7.30; N 3.62; S, 4.15.found:C, 80.87; H, 7.25; N 3.63; S, 4.16.
3. Results and discussion
3.1 Synthesis and characterization
Scheme 1 and scheme 2 show the structures and synthetic routes of the synthesized 1a-5a. Firstly,
3,6-di-tert-butylcarbazole was synthesized according to the literature procedure [41]. Subsequently,
10

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bromobenzene reacted with benzenesulfonyl chloride under the catalyst of aluminum chloride to
provide the intermediate 1b. The intermediates 2b, 4b were synthesized via brominating by
N-Bromosuccinimide in sulfonic acid with diphenyl sulfone as the starting material. The
intermediates 3b, 5b were similarly synthesized through two steps. The diphenylsulfides were
prepared with 4-bromo-thiophenol, 4-bromo-iodobenzene and 3-bromo-iodobenzene as materials
via C-S coupling according to the literature procedure [42]. The resulting 3c, 5c went through
oxidization by H2O2 to provide 3b, 5b. Finally, Ulmann coupling reaction of bromo-substituted
diphenyl sulfone with 3,6-di-tert-butylcarbazole led to 1a-5a with a yield of 55.48% - 68.56%. The
products were purified by the silica column method and recrystallized from n-hexane/CH2Cl2,
1
yielding the very pure white powders. H NMR, 13C NMR, matrix-assisted laser desorption
ionization time-of-flight (MALDI-TOF) mass spectrometry and elemental analysis were employed
to confirm the chemical structure of 1a-5a in the experimental section.
3.2 Structure Properties
The molecular structure of 1a was further determined by single-crystal X-ray crystallographic
analysis. The single crystal of 1a was obtained from slow evaporation of toluene solution at room
temperature, and the deposition number was CCDC 973776, and the crystal data were summarized
in Table 1. As shown in Fig. 1, 1a reveals a twisted molecular structure with a dihedral angle of 49o
between carbazole unit and the adjacent phenyl ring, and a dihedral angle of 106.1o between two
phenyl rings. The fact that the angles for O2-S-O1 and C6-S-C7 are 120.0o and 106.1o may attribute
to the hydrogen bonds between O2, O1 and C12, C5. From the data mentioned above, the
non-planarity and rigidity of the molecule is established, features which are essential in preventing
molecule from close packing and hence severe concentration quenching. The sulfone unit in the
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structure is beneficial for the reduction of
3.3 Thermal properties of 1a-5a
π -conjugation, possibly leading to high triplet energy.
The thermal properties of 1a-5a were investigated by thermal gravimetric analyses (TGA) under
nitrogen atmosphere at a heating rate of 10 oC min-1. The thermal gravimetric curves of 1a-5a are
presented in Fig. 2. The comparison of the results is shown in Table 2. The results revealed that
1a-2a present two main decomposition processes and 3a-5a present only one main decomposition
o
o
process. The initial decomposition temperature of 1a-5a range from 172 C to 370 C and the final
o
o
decomposition temperature of 1a-5a range from 443 C to 574 C. The number of the donor affects
the Td of 1a-5a significantly, 1a-2a have similar lower Td values than 3a-5a. These may attribute to
more bulky structure of 3a-5a than that of 1a-2a [14].
3.4 The UV-vis absorption spectra
Fig. 3 depicts the UV-vis absorption spectra of 1a-5a in toluene. Three major absorption bands
locate at the range of 295-298 nm, 329-333 nm and 340-345 nm in the absorption spectra,
corresponding to the
π -π * transitions of phenyl in 3,6-di-tert-butylcarbazole, diphenyl sulfone
and the ICT transition. From the absorption edge of the UV-vis absorption, the optical bandgap (Eg)
of 1a-5a can be estimated to be 3.21 - 3.46 eV. The results show that the position of the donor
affects the Eg value significantly. 1a exhibits a lower Eg value than 2a, 3a and 5a exhibit lower Eg
values than 4a, these may be attributed to the longer conjugation of 1a, 3a and 5a than that of 2a, 4a.
The number of the donor has a slight effect on the value of the band gap. As shown in Fig. 3, the
magnitude of the band gap increased with the decrease of the number of the donor. These
observations suggest that a donor-acceptor (D-A)-type structure exhibits a higher Eg value than a
donor-acceptor-donor (D-A-D)-type structure. This may be attributed to the decrease of ICT leading
12

ACCEPTED MANUSCRIPT
to reduced
π - conjugation and wide bandgap.
3.5 Photoluminescence
Fig. 4 depicts the photoluminescence (PL) spectra of 1a-5a in toluene. Their photoluminescence
properties are presented in Table 3. 1a-5a exhibit broad and structureless emission bands with a
maximum at 404-422 nm in toluene, which can be ascribed to the ICT. As is the case with the λmax
values of 1a-5a, their emission maxima (λfl) are generally dependent on the position of the donor.
Introduction of 3,6-di-tert-butylcarbazole at the 3-position of the diphenyl sulfone induces a shift of
the emission maximum to longer wavelengths. The results are similar to dibenzothiophene S,
S-dioxide(DBTO)-based flourophores [37]. Among them, changing the position of N-carbazole
group from 3,6-substitutions to 2,7-substitutions induces a shift of the emission maximum from
424nm to 442nm. Conversely, the presence of fewer donors reduces the electron-donating ability
and produces a shift of the emission maximum to shorter wavelengths. We measured the
fluorescence spectra in various solvents with different polarities. In the polar solvent methanol, the
emission maximum of 1a-5a red-shift to 474-479 nm. Conversely, in the nonpolar solvent hexane,
the emission maximum of 1a-5a blue-shift to 369-393 nm. The Stokes shifts for 1a-5a are very large
in polar solvents as compared to non-polar solvents indicating a considerable energetic stabilization
of the excited state in polar solvents.
The phosphorescence spectra of 1a-5a measured from a frozen 2-methyltetrahydrofuran matrix at
77K is shown in Fig. 5. The triplet energy of 1a-5a was determined to the values of 2.75-3.13 eV by
the energy 0-0 phosphorescent emission. 1a has the highest triplet energy of 3.13 eV, which is in
agreement with the anticipation in single-crystal X-ray crystallographic analysis.
3.6 Lippert-Mataga calculation
13

ACCEPTED MANUSCRIPT
The solvent dependence of the fluorescence indicates that the excited state is stabilized in more
polar solvents, as expected for an ICT. To obtain more information about the change in the dipole
moment upon excitation, we use the Lippert-Mataga equation, Eq. (1), which express the Stokes
shift as a function of the solvent polarity parameter ∆f
ε
ꢂn [43-44].
–
(
)
∆ν
=
ν a
−
ν f = 2∆
µ
2 / hca3 ∆f
ε
ꢂn + A
(1)
(
)
(
)
Where ∆f
ε
ꢂn is calculated from Eq. (2):
(
)
2
 − n −1 / 2n2 +
1
(2)


∆f
ε
ꢂn = 
ε
−
1 / 2
ε
+
1
)
(
)
(
) (
(
) (
)




Here,
∆
ν
=
ν a
−
ν f corresponds to the Stokes shift (in cm-1) between the maxima of absorption and
corresponds to the Plank’s constant (6.6
10-34 J s),
108 m s-1) and
is the solute cavity radius (m),
are the refractive index and the static dielectric constant of the
fluorescence emission. The other terms are:
h
×
c
is the velocity of light in the vacuum (3.0
×
a
respectively. In Eq. (2), n and
solvent, respectively.
ε
Table 3 lists the
∆
ν
values for 1a-5a in different solvents. For owing the similar structure with
we assumed the cavity radius of 1a-5a
the reported compound based on carbazole and sulfone [36]
,
to be ca. 1 nm. Fig. 6 shows the linear Lippert-Mataga plots with the slope values of 9272 cm-1 -
17130 cm-1 for 1a-5a. From the slope of this plot, the difference of the dipole moment between the
excited state and the ground state ( ∆µ ) were estimated to be 33 - 45 D for 1a-5a. This large change
in dipole moment upon excitation is typical for photoinduced ICT processes, reflecting the effective
electronic communication between the donor 3,6-di-tert-butylcarbazole group and the acceptor
diphenyl sulfone [36].
3.7 Delayed fluorescence
In addition, we determined the absolute fluorescence quantum yields (Φf) of 1a-5a in toluene
14

ACCEPTED MANUSCRIPT
solution in air and in oxygen-free (by bubbling nitrogen through the solutions for 10 minutes to
exclude oxygen) by an integrating sphere system. The excitation wavelength used was 350 nm. As
shown in Table 3, the Φf values of 1a-5a are 0.221-0.548 in toluene solution in air. The positions of
the donor affect the Φf values significantly. Introduction of 3,6-di-tert-butylcarbazole at the
4-position of the diphenyl sulfone increases the Φf values as found in 1a, 3a and 5a compared to
those in 2a, 4a, this may be attributed to the longer conjugation of 1a, 3a and 5a than that of 2a, 4a.
The long conjugation leads to more overlap between the HOMO and LUMO levels, which will
increase the radiative decay rate of the materials according to the golden rule [45]. Meanwhile, 3a,
4a and 5a exhibit higher Φf values than 1a and 2a. The observation suggests that a
donor-acceptor-donor (D-A-D)-type structure exhibits a higher Φf value than a donor-acceptor
(D-A)-type structure. For all the five compounds, the Φf values increased after nitrogen bubbling,
suggesting that nitrogen bubbling suppressed the quenching of triplet states by oxygen in air and led
to increased delayed fluorescence and Φf values. The larger increase in Φf values of 3a, 4a than
those of 1a, 2a in the absence of oxygen show that (D-A-D)-type structures can emit more delayed
fluorescence than the corresponding (D-A)-type structures. These findings are in line with the
emitters reported by Lee [29]. The largest increase in Φf value indicates that 4a may be the best
TADF emitter among 1a-5a.
Table 3 lists the prompt and delayed fluorescence lifetime of 1a-5a in toluene. In air, these
solutions have a lifetime of 4-6 ns attributing to the prompt fluorescence. After bubbling nitrogen
through the solutions for 10 minutes to exclude oxygen, as shown in Fig. 7 and S1 (In the
supporting information), the transient photoluminescence decay characteristics these solutions show
two-component decays attributing to the prompt and delayed fluorescence, which are consistent
15

ACCEPTED MANUSCRIPT
with the results reported by Zhang et al [27]. The delayed fluorescence of 1a-5a in toluene, which
has a lifetime of 234-285 ꢀs, is more than four orders of magnitude longer than the prompt
fluorescence, which has a lifetime of 6-30 ns. Observation that 4a shows the largest increase in the
lifetime of delayed fluorescence comparing to prompt fluorescence reveals that 4a may be the most
potential TADF emitters among 1a-5a.
3.8 Electrochemical analysis
The electrochemical properties of 1a-5a were studied in solution through cyclic voltammetry (CV)
using tetrabutylammonium hexafluorophosphate as the supporting electrolyte and ferrocene as the
internal standard. The highest occupied molecular orbital (HOMO) energy levels of 1a-5a were
characterized (Fig. 8). During the anodic scan in CH2Cl2 solutions, 1a-5a exhibited similar
reversible oxidation process, which can be assigned to the oxidation of electron-donating
3,6-di-tert-butylcarbazole moiety. No reduction waves were detected. On the basis of the onset
potentials for oxidation, the HOMO energy levels of 1a-5a were estimated to be -5.35 - -5.42 eV.
The effect of the number of the donor on the HOMO energy level is not evident. The LUMO energy
level of 1a-5a estimated from the HOMO energy level and Eg were -2.21 - -1.93 eV.
3.9 DFT calculations
DFT calculations were performed to understand the physical properties of 1a-5a at the molecular
level. The results are listed in Table 4, and the HOMOs and LUMOs of 1a-5a are shown in Fig.9.
All the HOMOs of 1a-5a are evenly delocalized over the outer 3,6-di-tert-butylcarbazole. The
LUMOs of 1a-5a are mainly delocalized over the central diphenyl sulfone. The theoretically
calculated HOMO−LUMO gaps (ꢁEHOMO−LUMO) are higher than those obtained in the UV−vis
spectroscopic measurements (ꢁEg) by ca. 0.5 eV, probably reflecting that the calculations are
16

ACCEPTED MANUSCRIPT
employed under the gas-phase conditions. With respect to the effects of the conjugation in 1a-5a on
their HOMO and LUMO levels, the theoretical calculations are generally consistent with the results
obtained by the absorption spectra and CV. The five compounds exhibit small variations of HOMO
levels ranging from -5.37 to -5.46 eV, whereas their LUMO levels vary in a larger range from -1.48
to -1.61eV, depending on the coupling modes of electron-donating moiety carbazole and the
electron-accepting moiety diphenyl sulfone.
Time dependent density functional theory (TD-DFT) calculations can also predict the ꢁEST
changes of these five compounds, based on their ground state geometries when optimized by the
DFT (B3LYP/6-31G*). Using the most popular B3LYP density functional, the triplet energy of
1a-5a were calculated to be 2.98-3.13 eV, which are in good agreement with the results based on the
phosphorescence spectra of 1a-5a measured from a frozen 2-methyltetrahydrofuran matrix at 77K
except 2a. The singlet energy of 1a-5a was calculated to be 3.26-3.45 eV. The ꢁEST values of 1a-5a
were calculated to be 0.22-0.39 eV. In comparison with 1a and 3a, 2a and 4a show greater
separation of the HOMO and LUMO orbitals. The lower overlap between the HOMO and the
LUMO of 2a and 4a indicate smaller energy difference between their singlet and triplet CT states.
The ꢁEST of 1a, 3a and 5a are larger than that of 2a, 4a also confirm that the larger overlap between
the HOMO and LUMO of materials with 3,6-di-tert-butylcarbazole at the 4-position. The calculated
results are in good agreement with the observations in the Φf and lifetime. As shown in Fig. 9, the
energy difference between singlet and triplet ( EST) in 1a-5a is tuned from 0.39 eV to 0.22 eV by
△
manipulation of the position and the number of the electron donor units.
4. Conclusions
In
summary,
we
have
designed
and
synthesized
a
series
of
17

ACCEPTED MANUSCRIPT
3,6-di(tert-butyl)-9-((phenylsulfonyl)phenyl)-9H-carbazoles. Their thermal and photo-physical
properties have been investigated in detail by TGA, UV-vis, fluorescence spectroscopic
measurements, CV and DFT calculations. The energy gap between singlet and triplet ( EST) in these
△
compounds is tuned from 0.39 eV to 0.22 eV by manipulation of the position and the number of the
electron donor units. Based on this study, we present a novel strategy for tuning the energy gap
between singlet and triplet in ambipolar systems for TADF by manipulation of conjugation of the
electron donor units.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant
No.51103023 and 21173042), the National Basic Research Program of China (Grant
No.2013CB932900), the Research Fund for Graduate Innovation Project of Jiangsu Province
(NO.CXZZ13_0090), the science and technology support program (industry) project of Jiangsu
province (BE 2013118) and Jiangsu Provincial Natural Science Foundation for YouthsꢀNo.
BK20130749ꢁ.
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Figure Captions
Scheme 1. Structure of 1a-5a.
Scheme 2. Synthetic route of 1a-5a.
Table 1. Crystal data and structure refinement for 1a.
Table 2. Values of the initial (Tdi), final decomposition temperatures (Tdf), Td (corresponding to a
weight-loss of 5%) of 1a-5a from the TGA.
Table 3. Photoluminescence properties of 1a-5a.
Table 4. Cyclic voltammetry, photoluminescence properties and theoretically calculated energy
levels of 1a-5a.
Fig. 1. ORTEP diagram of 1a.
Fig. 2. TGA traces of the 1a-5a.
Fig. 3. Normalized UV-vis absorption spectra of 1a-5a.
Fig. 4. Normalized photoluminescence spectra of 1a-5a.
Fig. 5. The phosphorescence spectra of 1a-5a measured from a frozen 2-methyltetrahydrofuran
matrix at 77K.
Fig. 6. Lippert-Mataga plots of 1a-5a.
Fig. 7. Emission decay of 1a-5a in the absence of oxygen (by bubbling nitrogen through the
solution for 10 minutes to exclude oxygen) in toluene at 300 K.
Fig. 8. Oxidation part of the CV curves of 1a-5a in CH2Cl2 solutions (10-3M).
Fig. 9. Optimized geometries and calculated HOMO and LUMO density maps for 1a-5a according
to DFT calculations at B3LYP/6-31* level.
S1. Emission decay curves and fitting curves of 1a-5a in the absence of oxygen (by bubbling
25

ACCEPTED MANUSCRIPT
nitrogen through the solution for 10 minutes to exclude oxygen) in toluene at 300 K. The value
of Adj. R-Square (>0.967) is used to ensure the quality of the fit.
26

ACCEPTED MANUSCRIPT
Table 1 Crystal data and structure refinement for 1a.
Empirical formula
C32H33NO2S
CCDC No.
973776
Formula weight
Temperature
495.65
293(2)
Wavelength
0.71073 Å
Crystal system, space group
Unit cell dimensions
monoclinic
a = 9.945(2) Å
b = 11.848(2) Å
c = 11.506(2) Å
1355.7(5) Å3
2, 1.214 Mg/m3
0.148 mm-1
528
α = 90.00º
β =90.27(3)º
γ = 90.00º
Volume
Z, Calculated density
Absorption coefficient
F(000)
Crystal size
0.10 × 0.20 × 0.30 mm
1.77 to 25.39 deg.
Theta range for data collection
Limiting indices
0<=h<=12, -14<=k<=14, -13<=l<=13
4971 / 3667 [Rint = 0.0631]
0.9853 and 0.9569
Reflections collected / unique
Max. and min. transmission
Refinement method
Data / restraints / parameters
Final R indices [I>2sigma(I)]
Largest diff. peak and hole
Full-matrix least-squares on F2
4971 / 1 / 325
R1 = 0.0588, wR2 = 0.1435
0.251 and -0.177 e·Å-3
1

ACCEPTED MANUSCRIPT
Table 2 Values of the initial (Tdi), final decomposition temperatures (Tdf), Td (corresponding to a
weight-loss of 5%) of 1a-5a from the TGA
Compd.
Tdi (oC)
Tdf (oC)
Td ( oC)
1a
220
170
452
443
297
332
2a
3a
370
369
549
560
427
421
4a
5a
342
574
371
Table 3 Photoluminescence properties of 1a-5a
1a
PL
2a
PL
3a
4a
5a
PL
∆f ε，n
(
)
UV
UV
UV
PL
UV
PL
UV
∆
ν
∆
ν
∆
ν
∆
ν
∆ν
hexane
toluene
DCM
0
343
344
344
344
345
377
406
445
453
474
17130
45
2629
4439
6598
6995
7888
341
342
342
343
344
389
414
451
462
479
3619
5085
7067
7509
8193
344
345
345
345
346
369
404
449
460
475
11076
36
1969
4355
6714
7246
7849
341
342
342
342
343
393
422
458
470
484
9272
33
3880
343 379 2769
344 410 4679
344 452 6946
345 462 7340
345 477 8021
10963
0.013
0.217
0.274
0.307
5543
7406
7963
8493
DMF
MeOH
14899
42
Slope
36
∆
µ
（D）
Phos (nm)a
Triplet(eV)
Eg(nm)
396
3.13
375
451
2.75
358
403
3.08
386
405
3.06
364
416
2.98
379
Eg(eV)
3.31
0.405
0.587
0.182
5.57
3.46
0.221
0.507
0.286
4.68
3.21
0.536
0.690
0.154
5.31
3.41
0.362
0.656
0.294
5.59
3.27
0.548
0.777
0.229
5.20
b
Φf
c
Φf
∆
Φf
τp (ns) b
c
τp (ns)
6.38
16.6
6.34
22.35
8.36
2