Distinct luminescent properties between thiophene-S-oxide and Thiophene-S, S-dioxides incorporated ladder-type molecules

Article abstract of DOI:10.1016/j.dyepig.2019.108147

Oxidized thiophene incorporated molecules show great potential as luminescent materials. However, most works focused on thiophene-S, S-dioxides (TDO) moiety, rarely on thiophene-S-oxide (TO) incorporated molecules. Herein, TO and TDO incorporated ladder-t

Full text of DOI:10.1016/j.dyepig.2019.108147

Journal Pre-proof
Distinct luminescent properties between thiophene-S-oxide and Thiophene-S, S-
dioxides incorporated ladder-type molecules
Jian Cao, Shiqi Li, Hua-Chun Wang, Si-Jie Bai, Zhangyan Wang, Xiancheng Ren,
Yun-Xiang Xu
PII:
S0143-7208(19)32768-8
DOI:
https://doi.org/10.1016/j.dyepig.2019.108147
DYPI 108147
Reference:
To appear in: Dyes and Pigments
Received Date: 27 November 2019
Revised Date: 14 December 2019
Accepted Date: 16 December 2019
Please cite this article as: Cao J, Li S, Wang H-C, Bai S-J, Wang Z, Ren X, Xu Y-X, Distinct luminescent
properties between thiophene-S-oxide and Thiophene-S, S-dioxides incorporated ladder-type molecules,
Dyes and Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2019.108147.
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Graphical abstract
Thiophene-S-oxide and thiophene-S,S-dioxides incorporated ladder-type molecules have been designed and their
luminescent features are systemically studied.
Optical Features for Oxidized Thiophene Incorporated Molecules
n=1, IDT2O: bathochromic emission
stronger fluorescence
anti-Kasha phosphorescence
n=2, IDT4O: normal photoluminescence
weaker fluorescence

Distinct Luminescent Properties Between Thiophene-S-oxide and Thiophene-S,
S-dioxides Incorporated Ladder-Type Molecules
Jian Cao‡, Shiqi Li‡, Hua-Chun Wang, Si-Jie Bai, Zhangyan Wang, Xiancheng Ren, Yun-Xiang
Xu*
College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials
Engineering, Sichuan University, Chengdu 610065, China.
* Correspondence author
E-mail: yxxu@scu.edu.cn (Y.-X. Xu)
‡ J. Cao and S. Li contribute equally to this article.
Abstract
Oxidized thiophene incorporated molecules show great potential as luminescent materials. However,
most works focused on thiophene-S, S-dioxides (TDO) moiety, rarely on thiophene-S-oxide (TO)
incorporated molecules. Herein, TO and TDO incorporated ladder-type molecules, namely
IDT2O-H and IDT4O-H, have been designed and synthesized to study their luminescent properties.
Compared to IDT4O-H, IDT2O-H not only shows stronger and bathochromic fluorescence
emission in solution and film states but also exhibits unusual anti-Kasha phosphorescence in film
state. The experimental spectra and theoretical calculations demonstrate that the distinct energy
levels and orbital features originated from their structures are crucial for the different luminescent
properties of IDT2O-H and IDT4O-H.

Keywords: Thiophene-S-oxide, Thiophene-S, S-dioxide, Ladder-type molecule, Anti-Kasha
phosphorescence, Luminescent property
1. Introduction
Oxidized thiophenes are common building blocks in the development of organic optics and
electronics materials, owing to the excellent optical and electronic properties.[1-7] Among the
oxidized thiophene moieties, thiophene-S, S-dioxides (TDO) is the most common one and a large
number of TDO-incorporated molecules have been published.[4, 8-18] In comparison,
TO-incorporated molecules may have some interesting luminescent and photophysical
properties.[7, 19-21] However, the synthesis of TO-incorporated molecules is still a challenge
because the TO will easily be further transformed into the more stable TDO or other dimeric
cycloaddition side-products.[21] Therefore, it is valuable to develop new stable materials based on
TO and investigate their luminescent and photophysical characteristics by comparing with TDO
counterparts.
Generally speaking, TO and TDO incorporated molecules exhibit some similar optical and
photophysical properties which caused by the oxidized of thiophene units. Typical two properties
are listed as following[1, 7, 14, 17, 22-24]: (1) electron affinity will be increased leading to deeper
lowest unoccupied molecular orbital (LUMO) energy levels; (2) the absorption and
photoluminescence spectra will be redshifted due to formation of intramolecular charge transfer
(ICT) state or the dearomatization of the thiophene ring. On the other hand, TO moiety not only
exhibited stronger aromaticity due to the existence of one unshared electron pair, but also possessed
weaker electron withdrawing properties compared with TDO moiety thus leading to distinct and

interesting optical and photophysical properties. For example, TO incorporated molecules exhibit
higher solubility, smaller energy gaps and oxidation potentials, bathochromic emission.[7, 19, 20,
25-27] In addition, other unknown optical and photophysical properties may be explored by
developing new materials.
Indacenodithiophene (IDT), one of the most important and common thiophene-incorporated
ladder-type conjugated molecule, was a popular donor unit for high-performance photovoltaic
materials. In our previous work, it is proved that ladder-type molecules with steric side chains could
generate photoluminescent emission in both solution and solid states after proper modification.[28]
The rigid, planar and fused backbone of ladder-type molecules rendered them some superior
properties such as facilitated charge transfer, strong absorption and so on.[29] In addition, the
fused-ring structure of IDT could be beneficial to the stability of TO moiety by preventing the
self-dimerization.[21] Therefore, IDT was transformed to stable indaceno[1,2-b:5,6-b’]dithiophene
1,6-dioxides (IDT2O) and indaceno[1,2-b:5,6-b’]dithiophene 1,1,6,6-tertaoxides (IDT4O) by
appropriate oxidation on thiophene rings in this work. Hexylbenzene or methylbenzene side chains
were adopted for IDT2O and IDT4O, named IDT2O-H/IDT2O-M and IDT4O-H/IDT4O-M,
respectively (Scheme 1). Both types of molecules consisted of highly planar conjugated backbones
and steric side chains. The out-of-plane side chains were supposed to prevent the strong
intermolecular π-π interactions thus leading to photoluminescence for both molecules. Indeed, both
of IDT2O and IDT4O molecules showed fluorescent emission in solution and film states, except
those of IDT2O are much stronger. Moreover, IDT2O films displayed unusual and interesting
anti-Kasha phosphorescent emission with higher energy than its fluorescence.

2. Experimental
All chemicals, unless otherwise noticed, were purchased from Adamas or other commercial
resources and used as received. The solvents were used as received. IDT-2Br-H and IDT-M were
synthesized according to previously reported procedure.[30, 31]
Scheme 1. Synthetic route of IDT2O and IDT4O.
Synthesis of IDT-2Br-M: 4,4,9,9-tetra-p-tolyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene
(IDT-M, 500 mg, 0.789 mmol) and N- Bromosuccinimid (NBS, 312 mg, 1.76 mmol) were
dissolved in 10 mL chloroform. The reaction mixture was stirred in dark for 6 h. Then the reaction
was quenched with water and extracted with CH2Cl2. The organic layer was dried over anhydrous
Na2SO4 and evaporated under vacuum. The crude product was washed with methanol three times to
1
give the product as a yellow solid (550 mg, 88 %). H NMR (400 MHz, CDCl3, δ) 7.32 (s, 2H),
7.12 – 7.04 (m, 16H), 6.96 (s, 2H), 2.31 (s, 12H). 13C NMR (100 MHz, CDCl3, δ) 154.70, 152.50,
141.36, 141.13, 136.71, 135.04, 129.16, 127.78, 125.92, 117.13, 113.86, 63.36, 21.00. HRMS (ESI)
m/z: [M+Na]+, calcd for C44H32Br2S2, 805.0204; found, 805.0208.
Synthesis of IDT2O-H and IDT4O-H: IDT-2Br-H (1.00 g, 0.939 mmol) and 1.20 g sodium
bicarbonate were dissolved in the mixture solvents of water (20 mL), acetone (16 mL) and
dichloromethane (18 mL). Then a portion of oxone (2KHSO5·KHSO4·K2SO4, 3.46 g, 5.63 mmol)

were added into the mixture carefully. The mixture was stirred at room temperature for 72 h.
Additional portion of oxone (1.73g, 2.81 mmol) was added twice to the mixture after 24 h and 48 h,
respectively. The mixture was poured into water and extracted with CH2Cl2. The organic layer was
dried over anhydrous Na2SO4. The crude compound was purified by silica gel chromatography
using a mixture of petroleum ether/CH2Cl2 (6:1) as the eluent to afford the products. Then, the
products were recrystallized from CH2Cl2/MeOH to give an orange solid of IDT2O-H (195 mg,
19%) and a yellow solid of IDT4O-H (185 mg, 18%), respectively.
IDT2O-H: 1H NMR (400 MHz, CDCl3, δ) 7.54 (s, 1H), 7.32 (s, 1H), 7.18 – 7.02 (m, 16H), 7.01 (s,
1H), 6.94 (s, 1H), 2.57 (dd, J = 16.0, 9.2 Hz, 8H), 1.58 (d, J = 6.4 Hz, 8H), 1.28 (t, J = 9.8 Hz,
24H), 0.88 (dd, J = 6.7, 5.6 Hz, 12H). 13C NMR (100 MHz, CDCl3, δ) 156.28, 153.58, 153.04,
143.06, 141.95, 140.54, 140.49, 138.02, 137.16, 136.44, 130.06, 129.12, 128.56, 127.71, 127.65,
126.30, 125.78, 123.97, 119.10, 117.04, 115.30, 64.45, 63.63, 35.55, 31.70, 31.68, 31.28, 29.12,
29.06, 22.57, 14.09. HRMS (ESI) m/z: [M+H]+, calcd for C64H72Br2O2S2, 1095.3413; found,
1095.3406.
1
IDT4O-H: H NMR (400 MHz, CDCl3, δ) 7.54 (s, 2H), 7.13 (d, J = 8.3 Hz, 8H), 7.02 (d, J = 8.3
Hz, 8H), 6.96 (s, 2H), 2.61 – 2.55 (m, 8H), 1.65 – 1.58 (m, 8H), 1.31-1.29 (m, 26H), 0.88 (t, J = 6.7
Hz, 12H). 13C NMR (100 MHz, CDCl3, δ) 155.66, 153.95, 143.34, 137.25, 135.64, 132.14, 129.32,
127.63, 125.64, 125.21, 118.92, 64.76, 35.55, 31.67, 31.23, 29.06, 22.56, 14.09.
Synthesis of IDT2O-M and IDT4O-M: The compounds were prepared according to the similar
procedure as IDT2O-H and IDT4O-H. Using IDT-2Br-M (500 mg, 0.639 mmol) as the starting

compound, IDT2O-M (70 mg, 14 %) as an orange solid and IDT4O-M (170 mg, 34 %) as a yellow
solid were afford.
IDT2O-M: 1H NMR (400 MHz, CDCl3, δ) 7.53 (s, 1H), 7.31 (s, 1H), 7.15 – 7.02 (m, 16H), 6.99 (s,
1H), 6.92 (s, 1H), 2.33 (d, J = 10.8 Hz, 12H). 13C NMR (100 MHz, CDCl3, δ) 156.22, 153.61,
153.52, 153.07, 140.51, 140.42, 138.07, 137.21, 136.96, 136.32, 130.12, 129.86, 129.31, 127.73,
127.66, 126.17, 125.61, 124.17, 119.05, 116.95, 115.49, 64.41, 63.59, 21.04, 20.99. HRMS (ESI)
m/z: [M+Na]+, calcd for C44H32Br2O2S2, 837.0103; found, 837.0142.
IDT4O-M：1H NMR (400 MHz, CDCl3, δ) 7.52 (s, 2H), 7.13 (d, J = 8.1 Hz, 8H), 7.01 (d, J = 8.2
Hz, 8H), 6.94 (s, 2H), 2.34 (s, 12H). HRMS (ESI) m/z: [M+Na]+, calcd for C44H32Br2O4S2,
869.0001; found, 869.0004.
3. Results and discussion
Compounds IDT2O and IDT4O were readily synthesized from IDT-2Br by oxidation reaction as
shown in Scheme 1. At first, many oxidants including m-chloroperbenzoic acid (m-CPBA), H2O2
and H2O2/CH3COOH had been tried (Table S1, Supporting Information), but no target product was
found. Then, an excess of dimethyldioxirane (DMD) was found to be effective for the oxidation
reaction, which was formed in situ by reacting acetone with oxone (2KHSO5·KHSO4·K2SO4) in the
presence of sodium hydrogen carbonate at room temperature. Moreover, both IDT2O and IDT4O
could be obtained in one pot with different ratios by adjusting reaction time. All molecules were
1
characterized by H-NMR, 13C-NMR and high-resolution mass spectrometry. Notably, IDT2O-H
and IDT2O-M were both mixture of diastereoisomers (syn- and anti-isomers in the viewpoint of
1
S=O group) according to the H and 13C -NMR (Figure S14, S15, S18 and S19, Supporting

Information). We tried to seperate the diastereoisomers, but it failed due to the poor
resolution. Therefore, IDT2O-H and IDT2O-M were used directly as diastereoisomers, respectively.
The solid-state structures of IDT2O-M and IDT4O-M were determined by single crystal X-ray
diffraction analysis and the parameters were summarized in Table S2. The crystals of IDT2O-M
and IDT4O-M were obtained by slow evaporation from tetrahydrofuran and
Figure 1. ORTEP diagrams of IDT2O-M (CCDC: 1950198) and IDT4O-M (CCDC: 1950199) with displacement
ellipsoids at 50% probability. Hydrogen atoms are omitted for clarity. In IDT2O-M, the oxygen atom is disordered
over the two sites with occupancy factor of 0.516 and 0.484, only one is shown.
dichloromethane solution, respectively. As shown in Figure 1, two TO units for IDT2O-M and two
TDO units for IDT4O-M were contained in their backbones. Both of IDT2O-M and IDT4O-M
possessed highly planar backbones (Figure S1, Supporting Information). In comparison, the TO
groups in IDT2O-M are still aromatic due to the existence of one unshared electron pair and the
C1-C2, C2-C3 and C3-C7 bond distances are 1.338 (5), 1.438 (4) and 1.345 (4) Å, respectively.
Conversely, the TDO groups in IDT4O-M lose its aromatic character and show a diene structure
with C1-C2, C2-C3 and C3-C7 bond distance of 1.319 (4), 1.465 (4) and 1.344 (4) Å, respectively.
To further evaluated the aromaticity of the TO and TDO groups for IDT2O-M and IDT4O-M, the

nucleus independent chemical shift (NICS)[32, 33] values at the points off the TO and TDO ring
centers (1.0 Å) were calculated. Generally speaking, positive NICS values indicated
anti-aromaticity and negative NICS values suggested aromaticity. The NICS(1)zz (the tensor
z-component of NICS(1)) values of TO and TDO rings in IDT2O-M and IDT4O-M were calculated
to be -2.11 and 5.04 ppm, respectively. The negative value of TO ring for IDT2O-M suggested
aromaticity and the positive value of TDO ring for IDT4O-M indicated anti-aromaticity. The
molecular packing modes and intermolecular interactions of IDT2O-M and IDT4O-M were
depicted in Figure S2 (Supporting Information). Both IDT2O-M and IDT4O-M crystallized in the
triclinic unit cells of P-1 space group with one molecule. The molecular packed in a typical
3.5x105
(a)
(b)
8
6
4
2
0
IDT2O-H
IDT4O-H
IDT2O-H
IDT4O-H
3.0x105
2.5x105
2.0x105
1.5x105
1.0x105
5.0x104
0.0
250 300 350 400 450 500 550
Wavelength(nm)
400 450W50a0ve5l5e0ng6t0h0(n65m0)700 750
J-aggregation but without significant π-π stacking for IDT2O-M and IDT4O-M, showing an
intermolecular distance of 4.000 and 4.430 Å, respectively. The intermolecular interactions were
mainly O-π and C-H···O interactions with a distance of 3.094 and 3.318 Å (distance between C and
O atoms) for IDT2O-M. For IDT4O-M, only C-H···O interaction existed between molecules with a
distance of 3.286 Å (distance between C and O atoms). These results revealed loose packing modes
for IDT2O-M and IDT4O-M.
Figure 2. (a) UV-vis absorption spectra and (b) photoluminescence spectra for IDT2O-H and IDT4O-H in
chloroform solution. c = 3×10-5 M. Excitation wavelength: 380 nm.

The photophysical properties of IDT2O-H and IDT4O-H in dilute solution were measured and
shown in Figure 2 and the parameters were summarized in Table S3 (Supporting Information).
IDT2O-H and IDT4O-H showed similar absorption profiles with two bands in the range of 230 -
550 nm (Figure 2a). The absorption bands in 230 - 350 nm represented π-π* transitions, while the
bands in 370 - 550 nm were assigned to intramolecular charge transfer (ICT). As shown in Figure
S3 (Supporting Information), the highest occupied molecular orbitals (HOMOs) are delocalized in
entire backbones, while the LUMOs predominantly resided on TO or TDO parts for IDT2O-M or
IDT4O-M (the substitution of methyl for hexyl for convenience), respectively. Such a distribution
of HOMO and LUMO contributes to the occurrence of ICT process. The ICT absorption peaks of
IDT2O-H and IDT4O-H were located at 451 nm and 445 nm with the molar absorption coefficient
of 3.00×104 and 5.28×104 L·mol-1cm-1, respectively. Compared to IDT4O-H, IDT2O-H showed a
narrower optical band gap because of the higher HOMO level, which was similar to previous
reports.[7, 26] IDT2O-H and IDT4O-H showed distinct photoluminescence (PL) properties. Under
UV light irradiation, IDT2O-H displayed bright orange color, while IDT4O-H showed yellow color.
As depicted in Figure 2(b), the PL emission band at 400 - 500 nm with fine vibrational structures
was slightly affected by the solvent polarities (Figure S4, Supporting Information), which was
ascribed to locally excited (LE) state for IDT2O-H solution. While the broad band at 500 - 700 nm
showed obvious bathochromic shifts with increased solvent polarities (Figure S4, Supporting
Information) which represented the ICT process. For IDT4O-H solution, an obvious LE state
emission at 400 - 450 nm was observed besides a very weak ICT emission peak at 500 nm.

In film states, their photophysical properties of IDT2O-H and IDT4O-H are significantly
different. As shown in Figure 3(a), there are two obvious absorption peaks at 311 nm and 443 nm
for IDT2O-H film similar to its solution, corresponding to π-π* transition and ICT process,
respectively. The photoluminescence spectra of IDT2O-H film excited at 253 nm and 340 nm were
measured and shown in Figure 3(b). When IDT2O-H film was excited at 253 nm, dual PL peaks at
approximately 400 and 570 nm were observed corresponding to the LE and ICT emission. By
increasing the
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(a)
IDT2O-H film (b)
IDT2O-H film (c)
IDT2O-H film
Phosphorescence
1.6
1.2
0.8
0.4
0.0
Excitation at
253 nm
Absorption
340 nm
350 400 450
550 600
Wavelength(nm)650
W4a0v0elength (nm)
350
450
500
250 300 350 400 450 500 550
Wavelength (nm)
1.5
1.2
0.9
0.6
0.3
0.0
IDT2O-H film
IDT4O-H film
(d)
(e) 1.2
(f)
IDT2O-H film @ 410nm
104
103
102
101
100
Emission at
403 nm
Abs
PL
Ph
1.0
589 nm
0.8
0.6
0.4
0.2
0.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4
300 400 500 600 700 800
250 300 350
450 500 550
Wavele4n0g0th (nm)
Wavelength(nm)
Time(ms)
Figure 3. (a) Normalized UV-vis absorption spectra of IDT2O-H film. (b) Normalized photoluminescence spectra
of IDT2O-H films excited at 253 nm (black line) and 340 nm (red line). (c) Phosphorescence spectra of IDT2O-H
film (excitation at 269 nm). (d) The lifetime decay curve of 410 nm emission excited at 280 nm for IDT2O-H
film. (e) Normalized excitation spectra of IDT2O-films recorded at 403 nm (black line) and 589 nm (red line). (f)
Normalized UV-vis (black line), photoluminescence (red line, excited at 257 nm) and phosphorescence (blue line,
excited at 276 nm) spectra of IDT4O-H films. These results are measured under ambient conditions.

excitation wavelength to 340 nm, the LE emission peak disappeared and only the ICT emission
peak at 570 nm was detected. Obviously, the emission peaks were dependent on the excitation
wavelength. Furthermore, the peak at 400 nm was proved to be phosphorescent emission with a
lifetime of 0.102 ms (Figure 3 (c), (d) and Table S4, Supporting Information). In the contrary, the
lifetime of peak at 570 nm was measured to be only 0.812 ns (Figure S5 and Table S4, Supporting
Information), indicating a typical fluorescent emission. Moreover, the photoluminescence quantum
yield (PLQY) of IDT2O-H film (emission band at 480 - 750 nm) was measured to be 30.39 %,
suggesting a strong fluorescence emission. These results revealed that IDT2O-H exhibited obvious
anti-Kasha phosphorescence emission property. Then, the excitation spectra of IDT2O-H films for
different emission peaks (403 nm and 589 nm) were collected to further understand the anti-Kasha
phenomenon. The excitation spectra for 589 nm covered the range of 230 - 550 nm, exhibiting
similar curves to its absorption spectra (Figure 3 (e)). While only a distinct sharp peak at 269 nm
existed in the excitation spectra for 403 nm emission peak (Figure 3 (e)). These results meant that
the phosphorescent peak at 403 nm could be excited at 230 - 300 nm with higher energy. Up to
now, many molecules with anti-Kasha fluorescence properties had been synthesized and
studied.[34-36]
However,
only
a
few
pure
organic
and
molecules
such
as
dibenzo[b,d]thiophen-2-yl(phenyl)methanone
(BDBT)
dibenzo[b,d]thiophen-2-yl
(4-chlorophenyl)methanone (ClBDBT) synthesized by Tang Groups[37] have been proved to
exhibit anti-Kasha phosphorescence emission. Therefore, IDT2O-H in this study may facilitate
further study of anti-Kasha phosphorescence properties. Compared to IDT2O-H film, IDT4O-H
film showed similar absorption profiles but obviously different photoluminescence characterization

(Figure 3(f)). Only a fluorescent emission peak at 430 nm was detected without excitation
dependent properties. Moreover, there was no significant phosphorescence signal in IDT4O-H film.
To further gain the phosphorescence properties and mechanism of IDT2O-H film, the
photoluminescence and phosphorescence spectra at 77 K were collected. As shown in Figure 4(a)
and
Figure 4(c), IDT2O-H and IDT2O-M showed similar three PL emission bands in dilute
2-methyltetrahydrofuran solution. After 1 ms decay, only two phosphorescence emission bands at
380 - 600 nm and 620 - 800 nm were observed. The disappearance of the emission band at 580 nm
in the
1.2
(a) 1.2
(b)
77K
PL
Ph
77K
IDT2O-H film
PL
Ph
IDT2O-H solution
1.0
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500
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800
Wavelength(nm)
40W0 avelengt6h0(0nm)700
(c)1.2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(d)
77K
77K
IDT2O-M film
PL
Ph
PL
Ph
IDT2O-M solution
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
400
700
800
Wavelength(nm)
W5a0v0elen6g0t0h(nm)
Figure 4. Steady-state photoluminescence (back line) and phosphorescence (red line) spectra after a decay time of
1 ms of (a) IDT2O-H in 2-methyltetrahydrofuran solution; (b) IDT2O-H film under 77 K; (c) IDT2O-M in
2-methyltetrahydrofuran solution and (d) IDT2O-M film under 77 K. Excitation wavelength: 270 nm.

phosphorescence spectra revealed its short-lived feature and is well correlated with the lifetimes
(8.09 ns for IDT2O-H and 8.23 ns for IDT2O-M) as shown in Table S5, Figure S6 and S7
(Supporting Information). Compared to the PL spectra, the phosphorescence emission band at 410
nm became broaden and redshifted after 1 ms decay for both IDT2O-H and IDT2O-M solution.
Moreover, it seemed that two peaks exist in the curves during 380 - 600 nm, especially for
IDT2O-M. However, the similar lifetime profiles for IDT2O-M as shown in Figure S7 (a) and (b)
revealed that the emission bands at 410 nm and 510 nm belong to the same excited states.[37] The
lifetimes of IDT2O-H solution at 410 nm and 695 nm were measured to be > 1 s and 5.98 ms,
respectively, revealing that the emission bands at 410 nm and 695 nm corresponded to the higher
and lower triplet energy levels. Next, the emission properties of IDT2O-H in aggregation state were
studied under 77 K. As illustrated in Figure 4(b), IDT2O-H film mainly exhibited the emission band
at 500 - 700 nm with a lifetime of 4.35 ns (Table S5 and Figure S8) in PL spectra. After 1 ms decay,
the phosphorescence emission band at 350 - 500 nm was detected with a lifetime of 0.63 s (Table
S5 and Figure S8). Notably, the emission band at 500 - 800 nm for IDT2O-H film still existed in the
decay spectra with a lifetime of 4.35 ns which may originated from the strong self-absorption effect
in aggregation state. Compared to the diluted solution, the phosphorescence signal at 695 nm was
not detected. This suggested that the lower triplet level of IDT2O-H was quenched in aggregation
state. IDT2O-M film showed the same properties with IDT2O-H film under 77K and the results
were displayed in Figure 4(d), S9 and Table S5.
Then, theoretical calculation was carried out to probe the mechanism of the anti-Kasha
phosphorescence emission of IDT2O-H film. For convenience, methyl was used to replace hexyl

for calculations. The energies of the excited states of IDT2O-M and IDT4O-M were calculated and
displayed in Figure S10. Generally speaking, the small energy gap between the singlet excited states
and triplet excited states facilitate the intersystem crossing (ISC) process.[38] All the energy gaps
between S1 and Tn for IDT2O-M exceed 0.3 eV, which lead to no effective ISC process. On the
contrary, the lower energy gaps between S2, S3 and Tn facilitate ISC process thus leading to the
generation of anti-Kasha phosphorescence. However, there are no obvious small energy gaps
between singlet and triplet for IDT4O-M, resulting in no phosphorescence generated. The natural
transition orbitals (NTOs) of S1 - S3 and T1-T5 for IDT2O-M were displayed in Figure S11 to
further study the characters of excited states. The characters of S1, S2 T1 and T2 are hybrid locally
charge transfer (HLCT) states and the others are locally excited (LE) states. The HLCT state could
facilitate the ISC process because the character combines the Frenkel and CT type.[39, 40] So, the
HLCT character
Sn
IC
ISC
S2
Tn
T1
S1
Abs
Fluo
Phos
S0
Figure 5. The proposed mechanisms for the anti-Kasha properties for IDT2O-H film. Abs, IC, ISC, Fluo and Phos
represent the absorption, internal conversion, intersystem crossing, fluorescence and phosphorescence.
of S2 state could be favorable for the ISC process in IDT2O-M. Furthermore, the hole and particle
of T1 and T2 are located at entire backbones with apparent (π, π*) transition feature, whereas the T3

and T4 mainly located at the thiophene-S-oxide moiety with (n, π*) transition character. According
to El-Sayed rule[41], T3 state with more (n, π*) transition character would result in large spin-orbit
coupling between T3 and S0 states thus leading to phosphorescence emission with higher
energy.[37, 42] Meanwhile, the internal conversion (IC) from T3 to T2 and T1 states seems very
weak because of the large energy gap. According to above results, a possible mechanism for the
anti-Kasha emission of IDT2O-H film was proposed and shown in Figure 5. Under light irradiation
in 230 - 500 nm, the excitons generated in upper excited states (Sn) could fast relax to lowest singlet
state (S1) by IC process and further emit fluorescence with the peak at 570 nm. However, in
addition to relaxation to S1, part of excitons in Sn could also transition to upper triplet state (Tn)
through the ISC process when the excitation energy is higher than 300 nm. And the excitons in Tn
could directly transition to the ground state (S0) with the generation of phosphorescence at 410 nm.
4. Conclusions
In summary, ladder-type molecules with thiophene-S-oxide (IDT2O) and thiophene-S, S-dioxides
(IDT4O) moieties have been designed and synthesized. Compared to IDT4O-H, IDT2O-H showed
bathochromic absorption and stronger photoluminescence emission in both solution and film states.
In addition, the unusual anti-Kasha phosphorescence emission at 410 nm with a lifetime of 0.102
ms for IDT2O-H film was observed. The optical properties and theoretical calculations revealed
that the generation of anti-Kasha phosphorescence relied on the modulated ISC process between
upper excited levels and the direct radiation decay from upper triplet to ground state. Notably,
IDT2O-H has potential to be used in large area flexible fluorescence/phosphorescence devices due
to the excellent film-forming properties and fluorescence /phosphorescence dual emission.

Meanwhile, it is possible utilizing the excitation wavelength dependent emission of IDT2O-H film
to detect deep-ultraviolet light and so on.
Acknowledgements
This study is supported by the National Natural Science Foundation of China (51403135) and the
Sichuan Science and Technology Program (Grant No. 2019YJ0123). The authors would like to
thank Prof. Peng Wu from Analytical & Testing Center of Sichuan University for help of
fluorescent and phosphorescent test.
Research data for this article.
CCDC 1950198 and1950199 contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.
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Highlights
•
•
•
Thiophene-S, S-dioxides (IDT4O) and thiophene-S-oxide (IDT2O)
incorporated ladder-type molecules were synthesized.
IDT2O showed stronger and bathochromic photoluminescence emission in
both solution and film states.
IDT2O film exhibited unusual anti-Kasha phosphorescence emission under
ambient conditions.

‡ J. Cao and S. Li contribute equally to this article.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: