Effect of thiophene substitution on the intersystem crossing of arene photosensitizers

Article abstract of DOI:10.1039/c8pp00230d

The effect of thienyl substitution on the intersystem crossing (ISC) of a few arenes was studied using steady state and time-resolved transient absorption and emission spectroscopies, as well as DFT/TDDFT computations. We found that the phenyl and thienyl substituents generally induce red-shifted absorptions for the chromophores, and the DFT/TDDFT computations show that the red-shifted absorption and emission are due to the increased HOMO and the reduced LUMO energy levels. Nanosecond transient absorption spectra indicate the formation of a triplet state, the triplet state lifetime is up to 282 μs, and the singlet oxygen quantum yields (Φ_Δ) are up to 60%. DFT/TDDFT computations indicate that introducing the thienyl substituent alters the relative singlet/triplet excited state energy levels, and the energy level-matched S₁/T₂ states are responsible for the enhanced ISC of the thienyl compounds. This information is useful for the design of heavy atom-free triplet photosensitizers and for the study of the fundamental photochemistry of organic compounds.

Full text of DOI:10.1039/c8pp00230d

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PAPER
Effect of thiophene substitution on the intersystem crossing of
arene photosensitizers
Received 00th xxxxxx 2018,
Accepted 00th xxxxxx 2018
Farhan Sadiq, Jianzhang Zhao,* Mushraf Hussain and Zhijia Wang
DOI: 10.1039/x0xx00000x
The effect of the thienyl substitution on the intersystem crossing (ISC) of a few arenes was studied with steady state and
timeresolved transient absorption and emission spectroscopies, as well as DFT/TDDFT computations. We found that the
phenyl and thienyl substituents generally induce red-shifted absorptions for the chromophores, and the DFT/TDDFT
computations show that the red-shifted absorption and emission is due to the increased HOMO and the reduced LUMO
energy levels. Nanosecond transient absorption spectra indicate the formation of triplet state, the triplet state lifetime is
www.rsc.org/
up to 282 μs, and the singlet oxygen quantum yields (
introducing the thienyl substituent alters the relative singlet/triplet excited state energy levels, and the energy
levelmatched S /T states are responsible for the enhanced ISC of the thienyl compounds. These information are useful

) are up to 60 %. DFT/TDDFT computation indicate that
1
2
for design of heavy atomfree triplet photosensitizers and for study of the fundamental photochemistry of organic
compounds.
atom effect and simple molecular structure profile are desired.
Previously, ISC was observed for thiophene oligomers and also
Introduction
32 38

for thienyl-containing chromophores.
Thienyl groups can
Intersystem crossing (ISC) and triplet photosensitizers play
important roles in photochemistry and photobiology.16 For
instance, triplet photosensitizers are important compounds
which have potential applications in the field of
be easily introduced to these molecules by cross coupling
reaction. Thiophenes are suitable for biological applications,
especially as thiophene derivatives are used as food flavouring
39,40
additives.
Moreover, attaching thienyl unit to
a
7 9

photocatalysis,
photodynamic therapy (PDT, which is a
chromophore may red

shift the absorption wavelength, which
10 12

promising approach for treatment of tumor),
are beneficial for preparation of new triplet photosensitizers.
Pyridine attached with thienyl moiety was reported and
efficient ISC was observed, but the absorption is in UV range.41
Previously thiophene moiety was introduced to copolymer
Bodipy backbone, ISC was proposed to be responsible for the
reduced fluorescence emission of the copolymer, but the
13
photovoltaics, and triplet

triplet annihilation upconversion
14 19

(TTAUC).
A good triplet photosensitizer show efficient
intersystem crossing (ISC) and long lived triplet excited state,
in addition to strong absorption of excitation light.
Conventional triplet photosensitizers usually contain heavy
atoms like iodo, bromo or metal atoms, Pt, Ir, Ru etc from d
42
triplet state was not studied. Thiophene fused Bodipy
20,21
and f block of periodic table.
The heavy atom effect is
4
3,44
derivatives were also reported to show ISC,
but it is clear
beneficial for the flip or rephrase of electron spin due to spin
orbit coupling (SOC) process. But toxicity and cost-efficiency
are major drawbacks of these photosensitizers. In order to
avoid these problems, triplet photosensitizers without heavy
atom are highly desired. So far some method, i.e., thermally
that more molecular structure diversity needs to be explored.
In order to study the effect of thienyl substitution on the
ISC of organic chromophores, herein we selected a few
representative arenes for preparation of thienyl compounds
(
(
Scheme 1). Perylene (Per), naphthalimide (NI) and anthracene
An) are among the most investigated chromophores, but the
22 24

activated reverse intersystem crossing,
singlet fission
(SOCT),26
spin
doublet excited state,29,30 and exciton
2
5
(
SF),
spin

orbit
charge
transfer
effect of thienyl substituents on the ISC of these
chromophores was not studied. Such compounds (Per NI and
An) are relevant for biological investigations as they don’t
2
2,27,28
convertor,
,
3
1
coupling to augment ISC of heavy atom free compounds have
been developed. But for most of these methods, the molecular
structures are synthetically demanding.
4
5-47
show dark toxicity.
Steady state and time resolved
transient absorption spectroscopies were used for study of the
photophysical properties. The properties of the thiophene-
appended arene fluorophores are compared with those of
phenyl counterparts. We found that the ISC can be enhanced
by introducing thienyl unit to the chromophores.
Thus new methods to achieve efficient ISC without heavy
a)State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology, E-208 West Campus, 2 Ling Gong Rd., Dalian 116024,
P. R. China. Email: zhaojzh@dlut.edu.cn
†
Electronic Supplementary Information (ESI) available. See
DOI: XXXXX/x0xx00000x
Experimental section
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General methods
porphyrin (TPP) was used as standard (
 = 62% in DCM) for

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DOI: 10.1039/C8PP00230D
measurement of singlet oxygen (
anthracene was used as standard (
measurement of of DT-An


) of MT-Per in DCM and
All chemicals used in the synthesis were of analytical grade
and were used as received. Solvents were dried and distilled
before use for synthesis. Detail of synthesis is given in supporting
information (see experimental section).


= 70 % in Methanol) for


,
MT-NI
,
MP-NI in methanol.
Nanosecond transient absorption spectroscopy
Results and discussion
LP920 laser flash photolysis spectrometer (Edinburg
Instruments, UK) equipped with Tektronix TDS 3012B
oscilloscope was used for studying nanosecond timeresolved
transient absorption spectroscopy. The samples were
deaerated with argon for 15 minutes before measurement and
flow of gas was kept constant during the measurements. The
data was processed by LP920 software.
Design and synthesis of the compounds
Previously, ISC properties of bodipy derivatives with fused
4
3,44
thiophene were reported.
Thiophene attached bodipy
4
2
copolymer were also reported. It was proposed that the
decrease in fluorescence emission is due to ISC process. To
explore the effect of thiophene substitution on ISC and triplet
state properties, we designed and synthesized different arene
compounds attached with thienyl units (Scheme 1).
Singlet oxygen sensitization
Perylene is known for its high fluorescence quantum yield.
Thus it is feasible to study the effect of thienyl substitution on
the ISC of this chromophore. We expect thienyl attachment to
the core of perylene will reduce the fluorescence quantum
Singlet oxygen of compounds were measured with
diphenylisobenzofuran (DPBF) as singlet oxygen scavenger. By
1
following the absorption (λmax = 414 nm) of DPBF, the O
2
production was monitored. For calculation of singlet oxygen
F
yield (Φ ) of MT-Per and increase the ISC ability of compound.
quantum yield (Φ

), the following equation (1) was used:
MP-Per was prepared as a reference compound. Thienyl unit
2
was introduced by Pd (0)
reaction (scheme 1).
catalyzed Suzuki cross coupling

Astd



110
 m sam



sam
sam  std
Asam 

(1)
1
10
mstd std
NI is a well known chromophore, which is electron
deficient. Introducing a thienyl unit results electron push pull
character in MT-NI, thus it is expected the that absorption
wavelength will be red shifted. Compounds related to MT-NI



In above equation, “std” and “sam” stands for standard and
sample. η, A, Φ and m stands for refractive index of the
solvent, absorbance at excitation wavelength, singlet oxygen
quantum yield and slope of the absorbance of DPBF changing
with time, respectively. For measurement, optically matched
solutions of sample and standard were used. Tetraphenyl

were previously reported but triplet state properties were not
48
investigated. It should be pointed out that the pristine NI
S
S
S
O
N
O
Br
Per
An
S
i
O O O
MT-NI
vii
Br
ii
viii
DT-An
MT-Per
iv
Br
Br
v
Br
ix
iii
Br-Per
vi
DBr-An
O
N
O
O
N
O
MP-Per
DP-An
Br-NI
MP-NI
○
Scheme 1 Reagents and conditions: (i) 1.2 equivalent of N-bromosuccinimide (NBS), DMF, 25 C, 24 h; (ii) Thiophene-2-ylboronic acid, K
2
CO
O (4/2/1, v/v), refluxed at 90 C for 8 h, under Ar; (iv) 2-
, CHCl , stirred at room temperature for 5
3 3 4
, Pd(PPh ) ,
○
○
PhCH
3
/Et

OH/H
2
O (4/2/1, v/v), 90 C, 8 h, Ar; (iii) phenylboronic acid, K
2
CO
3
, Pd(PPh
3
)
4
, PhCH
3
/ Et

OH/H
2
○
ethylhexylamine, Et-OH, refluxed at 90 C for 8 h, under Ar; (v) and (viii) the same to (ii); (vi) and (ix) the same to (iii); (vii) Br
2
3
h under Ar.
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(without any substituents at 4position) possesses an efficient compounds with thienyl substitution show more redshifted
ISC, but the absorption is in UV range. With introducing phenyl absorption than phenyl substituted compounds. This may be
group at 4-position (MP-NI), the fluorescence was reported to due to the increase in conjugation by attaching thienyl
49
be 78 %, i.e., the ISC decreased.
An is with efficient ISC (
= 70 %).50 It was proposed that
the T and T states are involved in the ISC of An, T
similar energy level to that of S state, and S (T
proposed. Substitution on the An may change the relative absorption is further red shifted (446 nm) due to increase in
energy levels of S /T states, thus the ISC may be inhibited to interaction between thiophene and Per core. It is noted that
large extent given the T state is with higher energy level than upon thienyl attachment the vibronic progression of
state. The relative energy level of S /T state of An also compounds become less significant. The effect of thienyl
moiety to the core of chromophore.

T
For instance, Per shows absorption band at 434 nm, with
is with the attachment of phenyl group, MP-Per shows red shift in
)T was absorption of about 8 nm while in case of MT-Per the
3 1
2
3
2
1
1

T
2
1
2
2
S
1
1
2
dependent on the matrix micro environment, for instance, the introduction to the core of chromophore is more prominent in
solvent polarity. We introduced two thienyl moieties on the An case of NI where the bathochromic shift in absorption for MT-
core (DT-An) to study the effect on ISC ability. The NI is about 27 nm as compared to reference compound (Br
NI)
photophysical properties of DT-An were reported previously (Fig 1b). However, MP-NI shows less red shifted absorption of

and triplet lifetime is investigated by indirect method,51 9 nm. Also the broad, structure less peak was observed by
however, triplet properties were not investigated in detail. We substitution of either phenyl or thiophene with core of NI. This
did not attempt to induce oligomeric thienyl moiety on to the may be due to electron donating ability of thiophene or phenyl
chromophores, previously it was reported that high oligomeric to electron deficient NI moiety.
52 55

thiophene may show reduced ISC ability.
The attachment of two thiophenes to the core of
anthracene also shows more red shifted absorption of about
20 nm as compared to the compound with phenyl substitution
16 nm) (Fig.1c). However the effect on vibronic progression
with substitution of either phenyl or thiophene is very less.

UVVisible absorption spectra
(
The UVVis absorption spectra of compounds were studied
and compared with reference compounds (Fig. 1). Both the
thienyl and phenyl substituted compounds show bathochromic
shift (red shift) as compared to reference compounds. The
0
.20
0
0
0
0
0
0
.5
.4
.3
.2
.1
.0
MP-Per
MT-Per
Per
MP-NI
MT-NI
Br-NI
DP-An
DT-An
An
a
b
0.15
c
0.15
0
.10
.05
0
.10
.05
0
0
0.00
0.00
3
00 360 420 480 540 600
300
360
420
480
300
360
420
480
Wavelength / nm
Wavelength / nm
Wavelength / nm
Fig. 1 UVVis absorption spectra of the compounds, c = 1.0 × 10 M in toluene, 20 C.
5
○
60
45
30
15
0
Toluene
DCM
120
80
Toluene
DCM
2.8
Toluene
DCM
a
c
b
CH CN
CH CN
2.1
.4
0.7
.0
3
3
CH CN
3
CH OH
3
CH OH
3
CH OH
3
1
40
0
0
4
20
480
540
600
420
490
560
630
400
500
600
700
Wavelength / nm
Wavelength / nm
Wavelength / nm
Fig. 2 Fluorescence emission spectra. (a) MT-Per (λex = 397 nm); (b) MT-NI (λex = 317 nm); and (c) DT-An (λex = 373 nm). Optically matched solutions were used (A = 0.16 for MT-Per,
A = 0.09 for MT-NI, A = 0.04 for DT-An). 20 C.
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Interestingly, the absorptivity of the thionated compound, DT- or triplet). Similar results were observed for MP-NI (see ESI Fig.
An, was enhanced as compared to that of An. For instance,

=
S23b†). Interestingly, the emission wavelength of other
compounds is not dependent on solvent polarity.
4

1

1
1
.2 × 10 M cm at 376 nm was observed for DT-An, but the
2

1

1
corresponding

of anthracene is 6.0 × 10 M cm at the
The emission spectra of compounds were studied and
same wavelength. The DT-An solution (c = 1.0 × 10 M in compare with the reference compounds (Fig. 3). MT-Per show
5
○
toluene, 20 C) was irradiated at λex = 377 nm (maximum decreased fluorescence intensity with emission band at 477
absorption) with Xe lamp for different time intervals (from 1 nm. This band is redshifted as compared to emission of Per
minute to 45 minutes). We found the decrease in UV-Visible moiety at 445 nm. In case of MP-Per, emission band was
absorption (see ESI† Fig. S39) with prolonged irradiation, observed at 461 nm. The red shift in emission band was also
which shows that the DT-An is not very stable if exposed to observed for MP-Per but fluorescence quenching is not
light for continuous long time.
significant (Φ
reference compound Per (Φ
F
= 94.0 %, Table 1) when compared with
= 98.0 % in n hexane, Table 1).
F

Fluorescence emission spectra
The effect of thiophene and phenyl substitution on the core
of anthracene and naphthalene imide is different from
The emission of all compounds in different solvents was
studied (Fig. 2). MT-Per show hypsochromic shift (blue shift) (
perylene core. By introducing phenyl group to the core of An
an increased fluorescence quantum yield ( = 97 %, Table 1)
was observed with emission band at 425 nm (Fig. 3c). In case
of DT-An, however, the fluorescence quantum yield ( = 2 %,
,


F
8
nm) in emission with increased solvent polarity. The
fluorescence intensity is also solvent dependent (slightly
intensified in polar solvent). In case of MT-NI the emission is
red shifted (λmax = 454 nm in toluene to λmax = 496 nm in
methanol) and the intensity of emission increases with
increasing polarity of solvent. Bathochromic shift (red shift)
was also observed in DT-An with increased solvent polarity.
MT-NI is interesting, the emission is intensified in polar

F
Table 1) was greatly reduced with red shifting of emission
band to 436 nm. The decrease in emission of DT-An may be
due to some radiation less decay and ISC (See later section for
detail).
For MP-NI, a broad structure less emission band was
observed at 414 nm (Fig. 3b). The emission of the compound
1 n
solvents (Fig. 2b). This may be due to the change of the S /T
energy gap in different solvents, for instance the n-π* state
(MP-NI) significantly increased with phenyl substitution on NI
may change its energy level in different solvents (either singlet
500
400
300
200
100
0
8
0
0
1
60
20
a
Br-NI
MT-NI
MP-NI
c
An
DT-An
DP-An
b
6
Per
1
MT-Per
MP-Per
40
20
0
8
0
0
0
4
4
40 480 520 560 600 640
350 400 450 500 550 600
Wavelength / nm
360
420
480
540
Wavelength / nm
Wavelength / nm
Fig. 3 Fluorescence emission spectra of (a) Per, MT-Per and MP-Per (λex = 406 nm), (b) Br-NI, MT-NI and MP-NI (λex = 340 nm), (c) An, DT-An and DP-An (λex = 344 nm) in toluene,
0 C.
2
4
3
2
1
Per
An
10
10
10
10
a
c

= 3.80 ns (100 %)
MP-NI
 = 3.4 ns
(100 %)
b
 = 3.3 ns(100 %)
4
3
2
3
2
1
MT-Per
10
DT-An
 = 0.4 ns + 2.9 ns + 13.4
1
0

= 0.58 ns+ 4.0 ns
(98 %)
(2 %)
(81 %) (17 %) (2 %)
MT-NI
DP-An
5.4 ns
(100 %)
MP-Per

1
0
0
10
= 3.3 ns
(100 %)
 = 3.2 ns
(100 %)
1
10
0
20
40
Time / ns
60
80 100
0
20
40
Time / ns
60
80 100
0
20
40
Time / ns
60
80 100
Fig. 4 Decay traces of the fluorescence of (a) MT-Per (λem = 475 nm), MP-Per (λem = 460 nm) and Per (λem = 445 nm) (b) MT-NI (λem = 475 nm) and MP-NI (λem = 445 nm) (c) DT-An
(λem = 445 nm), DP-An (λem = 430 nm) and An (λem = 450 nm). MT-Per, MP-Per and Per were excited with a picosecond pulsed laser (407 nm). MT-NI, MP-NI, DT-An, DP-An, and An
were excited with a picosecond pulsed laser (340 nm). c = 1.0 × 105 M in acetonitrile, 20 C.
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The photophysical parameters of the compounds were
core. In case of MT- NI, the emission band was observed at
4
1
55 nm (Fig. 3b). The fluorescence quantum yield of MT-NI is summarized in Table 1. The singlet oxygen quantum yield (Φ
4 % (Table 1) as compared to MP-NI compound ( = 78 %, of the compounds were determined. The results show that the
Br-NI is is up to 60 % (MT-Per). Moderate singlet oxygen quantum
fluorescent. Enhance in fluorescence is due to charge yield was observed for MT-NI = 22 %). For DT-An, the
transfer ability of aryl group to the electron deficient NI 13 %, which is much lower than that of An = 70 %).
The fluorescence lifetimes of the compounds were studied
Fig. 4). For Per, fluorescence lifetime of 4.3 ns was Nanosecond transient absorption spectroscopy: the triplet excited

)

(
F
Table 1), while the reference compound
non
)
Φ


(

 =

.
(


(
determined in acetonitrile. Similar fluorescence lifetime (
.6 ns) was observed for MP-Per. For MT-Per, however, a
short lived luminescence component was observed as 0.58 ns
98 %) / 4.0 ns (2 %). This result indicates that there is an
efficient quenching channel for the S state of MT-Per. This
quenching channel can be attributed to ISC (see later section).

F
=
states property of the compounds
3
In order to confirm the formation of triplet state with thienyl
attached chromophores, the nanosecond transient absorption
spectra of the compounds were studied (Fig. 5). For MT-Per
(
,
1
an excited state absorption (ESA) band at 548 nm was
0.06
0.04
0.02
0.00
0
0
0
.03
.02
.01
0.03
0.02
0.01
0.00
a
b
c
0
s
0
.0 s
3.62 s
..
5.34 s
13.3 s
...
186.2 s
0 s
.
2
6
6.6 s
.
. .
1665 s
0.00
-0.02
-0.01
360
480
600
720
400
500
600
700
300
400
500
600
Wavelength / nm
Wavelength / nm
Wavelength / nm
0.05
0.04
0.03
0.02
0.01
0.00
0.05
0.08
0.06
0.04
0.02
0.00
e
f
d
0.04
0.03
0.02
0.01
 T = 56.7 s
 = 5.9 s

= 282 s
0.00
100
200
Time / s
300
400
0
20
40
Time / s
60
80 100
0
20
40
Time / s
60
80 100
Fig. 5 Nanosecond time-resolved transient absorption spectra of (a) MT-Per (b) MT-NI (c) DT-An (d) Decay trace of MT-Per at 560 nm, λex = 355 nm (e) Decay trace of MT-NI at 560
nm, λex = 355 nm (f) Decay trace of DT-An at 397 nm, λex = 355 nm (pulsed nanosecond laser), c = 1.0×10 M for MT-Per, and c = 5.0 × 10−5 M for both MT-NI and DT-An. In
5
○
deaerated toluene, 20 C.
Table 1 Photophysical parameters of the compounds
a
 b
c
d
e
 f
g
h
Δ

abs / nm
λ
em / nm
Φ
F
/ %
λ
ex / nm
F
/ ns

T
/ μs
Φ / %
a
n
n
n
MT- Per
MP-Per
MT-NI
MP-NI
DT-An
DP-An
Per
421, 446
416, 442
367
2.81, 3.32
1.25, 1.56
1.55
477
15
94
14
78
2
97 i,48
98 i,49
 j
406
406
340
340
344
344
406
340
344
0.6 , 0.6
281.8
 j
60
 j
a
461, 489
455
3.3 ,3.6
a
3.2 , 3.4
56.7
70
22
a
n
349
1.33
414
3.4 , 3.7
21
a
n
n
n
377, 397
373, 393
407, 434
340, 355
357, 377
1.20, 1.16
0.96, 0.90
3.47, 4.50
1.74, 1.55
0.61, 0.58
447
1.0 , 1.4
5.7
 k
 k
13
a
 j
 j
 j
70 i
430, 409
445, 471
 k
5.4 , 6.0
a
3.8 , 4.3
Br-NI
 j
28.4
An
453, 426
27 i
3.3 , 3.8
a
n
64.5
a
In toluene (1.0 × 10 M), b Molar extinction coefficient at absorption maxima,

5
: 104
M
-1
-1
c
d
e

cm , Emission wavelength. Fluorescence quantum yield in toluene,
= 98% in n-hexane). For MT-NI MP-NI DT-
Excitation wavelength for measurement of the fluorescence, For MT-Per and MP-Per, perylene was used as standard (

F
,
,
f
h
An and DP-An, anthracene was used as standard (
DCM, TPP was used as standard ( = 62% in DCM), for MT-NI
Literature value. Not Observed. k Not applicable. Not determined. In acetonitrile.
 = 30% in ethanol), Fluorescence lifetimes in toluene. Singlet oxygen quantum yield, for MT-Per and MP-Per in
F
i



, MP-NI, DT-An and DP-An in methanol, anthracene was used as standard ( = 70% in methanol).
n
j
m
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Photochem. Photobio. Sci.
observed (Fig. 5a). For MT-Per, no significant ground state
bleaching (GSB) band was observed. We suppose this is due to
the overlap of the GSB and ESA bands. It is noticeable that the
Energy
eV]
Per
MT-Per
View Article Online
DOI: 10.1039/C8PP00230D
MP-Per
[
ESA band of MT-Per (548nm) is red

shifted as compared to
that of per (450 nm). It is reasonable because the steady
state UV Vis absorption of MT-Per is red shifted as compared
to that of perylene.
58



0.47 eV

0.51 eV
LUMO+1
The triplet state lifetime of MT-Per was determined as 282
μs, which is close to the previous report of 272 μs, accessed
with the intramolecular triplet photosensitizing method.58 It
should be pointed out that the triplet state lifetime of MT-Per
is much longer than Br-Per (62.2 μs), we attribute this to the
lack of significant heavy atom effect in MT-Per. Heavy atom

0.83 eV
1.98 eV
5.06 eV

2.02 eV
LUMO
HOMO
1 0
effect may enhance the T S transition, as such the triplet

2.13 eV
state life may be reduced. It should be pointed out that the
transient observed is not charge separated state (anion per
should give absorption at 585 nm).59 The enhanced ISC in the
thienyl compounds may be attributed to the alternation of the
excited state energy levels, and the establishment of the
5.01 eV

5.03 eV
55,60,61
1 n
energy level matched S /T states.
For MT-NI, ESA band at 561 nm was observed upon pulsed
laser excitation, a minor GSB band was observed at 360 nm.
The ESA band is similar to the previously reported ESA band of
NI moiety (450650 nm). The triplet state lifetime was
determined as 56.7 μs. Similar results were observed for DT-
An (Fig. 5c). The ESA band at 426 nm is close to the pristine An
HOMO1
62

6.31 eV

6.56 eV

6.71 eV
(see ESI†, Fig. S29). It is interesting that the triplet state
Fig. 6 Selected frontier molecular orbitals of Per, MP-Per and MT-Per calculated with
lifetime was determined as 5.9 μs, much shorter than the
triplet state lifetime of An (64.5 μs) (see ESI Fig. S29†). The
reason may be due to the intramolecular rotation of the
thienyl substituents, or the vibration induced enhanced
DFT theory at B3LYP/6-31G (d) with Gaussian 09.
1 0
coupling between T and S state.
LUMO
LUMO
405 nm
f = 0.0000
3
53 nm
f = 0.0012
DFT calculation
The molecular geometries of the compounds were optimized
HOMO-1
HOMO
LUMO
53 nm
S
4
S
4
3
LUMO
464 nm
f = 0.0000
C
3
4
4.2 
C
C
3
4
2.6 
f = 0.0001
3
3
.61 eVS
.51 eVS
3
C3
1
C
2
C
1
C
2
C2
2
S4
T
3 3.06 eV
C
1
ISC
2
.57 eVS
1
T
2
2
.67 eV
8
9.8 
HOMO-1
LUMO
HOMO-6
T
1 1.44 eV
MT-Per
MT-NI
DT-An
C
4
C
4
90.5 
LUMO
4
81 nm
862 nm
5
2.8  C
C
3
51.9 
C
3
f = 0.7304
C1
C2
f = 0.0000
C4
1
C
2
C
1
C
2
C3
HOMO
HOMO
S
0
MP-Per
MP-NI
DP-An
Fig. 7 proposed energy levels and frontier molecular orbital diagrams of MT-Per. DFT
calculation was performed at the B3LYP/6-31G (d) level using Gaussian 09W. Alkyl
chains are simplified as methyl groups to reduce the computation time.
Scheme 2 Optimized ground state conformations and dihedral angles of the
selected atoms of arenes, calculated by at B3LYP/6-31G level with Gaussian 09.
Photochem. Photobio. Sci.
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Paper
View Article Online
DOI: 10.1039/C8PP00230D
Table 2 Properties of the low

lying electronic excited states of MP-Per and MT-Per. Calculated by TDDFTB3LYP6-31G (d), based on the DFTB3LYP6-31G(d)
optimized ground state geometries.
States
Electronic
transition
Energy / eV/nm a
f b
Composition c
CI d
MT-Per
Singlet
0
S 
0
S 
0
S 
0
S 
0
S 
0
S 
0
S 
S
S
S
S
T
T
T
1
2
3
4
2.57
3.51
3.61
3.68
1.43
2.67
3.06







481
352
343
336
862
464
405
0.7304
0.0001
0.0012
0.0417
0.0000
0.0000
0.0000
H
H
H
H
H
H
H

L
0.7061
0.4934
0.5653
0.5004
0.6951
0.1087
0.4327

1L



6L

L+2
L+1
L
Triplet e
1
2
3
L+3
a

lying excited states are presented. b Oscillator strengths. c Only the main configurations are presented. d The CI coefficients are in absolute
Only the selected low
values.
(
a)
(b)
(
3.15 eV)
S
3
(3.08 eV)
T3
(3.13 eV)
S
3
T3 (3.06eV)
T2 (2.67 eV)
(
2.94 eV) S
2
(
2.97 eV) S
2
No ISC
ISC
T2
S
1
S
1
(
2.95 eV)
(
2.80 eV)
(2.78 eV)
T1
(1.48 eV)
4
77 nm
446 nm
T1 (1.43 eV)
4
61 nm
442 nm
S0
S0
(
c)
(
d)
(
3.50 eV)
(
3.46 eV)
(
3.45 eV)
T3
S
3
(3.21 eV)
T3
S
3
(
3.33 eV) S
2
(
3.29 eV) S
2
ISC
T2
T2
No ISC
S
1
S
1
(
3.24 eV)
(
3.19 eV)
(
3.15 eV)
(3.15eV)
3
94 nm
T1 (1.65 eV)
3
93 nm
4
61 nm
T1 (1.71 eV)
447 nm
S0
S0
Scheme 3. Simplified Jablonski diagram illustrating the photophysical processes in (a) MP-Per (b) MT-Per (c) DP-An and (d) DT-An. The excited state energy levels of
compounds are derived from spectroscopic data (UV Vis and fluorescence emission). The triplet excited state energy values were calculated by TDDFTB3LYP 6-31G
d). In toluene.


(
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View Article Online
DOI: 10.1039/C8PP00230D
(Scheme 2). For MT-Per, the dihedral angle between the The photophysical properties of the compounds were studied
thienyl plane and per plane is 44.2 Interestingly the dihedral with steady-state and time-resolved transient absorption and
angle for MP-Per is 52.8 These results indicate that the emission spectroscopies, as well as DFT computations. The


conjugation in MT-Per is slightly more effective than that in UV
MP-Per. Similar results were observed for MT-NI and MP-NI red
For DT-An and DP-An, interestingly, similar dihedral angles that ISC generally enhanced with attaching thienyl moiety on
were observed (89.8 and 90.5 respectively). The frontier the chromophores, and the singlet oxygen quantum yield is up
molecular orbitals of the compounds were studied base on the to 60 %. The triplet state lifetimes are as long as 282 μs.
optimized ground state geometry (Fig. 6). DFT/TDDFT computations on the excited states show that the

Vis absorption of the chromophores is generally
.
shifted upon introducing the thienyl moieties. We found

The general effect of attaching phenyl and thienyl relative energy levels of the excited state were altered by
substituents to the chromophores is the increased energy introducing thienyl substituents, and the energy level matched
levels for the occupied orbitals, and the decreased energy
levels of the frontier empty orbitals. This trend is responsible to the enhanced ISC ability. All these features are beneficial for
for the red shifted absorption bands of the compounds as the compounds to be used as triplet photosensitizers. These
1 1
S /T states of the thienyl substituted compounds contribute

compared to the reference compounds. We used DFT/TDDFT works propose new insights in the design of triplet
to calculate the excited state of the compounds to rationalize photosensitizers to be used in photophysics and photobiology,
the ISC abilities (Fig. 7). The excited states energy levels and and especially in photodynamic therapy.
the electronic transitions of MT-Per are presented. The
calculated S
1
state is with a vertical transition energy of 2.57 Acknowledgements
eV (481 nm), which is very close to the experimental results of
the absorption band at 446 nm (Fig. 1a). This agreement
rationalized out theoretical computation. The computation
also infers that there is no other major absorption band down
to 350 nm, which is verified by experimental observation.
We thank the NSFC (21473020, 21673031, 21761142005,
1603021, 21421005, and 21273028), the State Key
Laboratory of Fine Chemicals (ZYTS201801), and the
Fundamental Research Funds for the Central Universities
2
(Grants DUT16TD25, DUT15ZD224, and DUT2016TB12) for
The triplet excited states were also computed (Fig. 7). T
state (1.44 eV) is well below the S state (2.57 eV), however, T
state (2.67 eV) is with similar energy to S
electronic configuration of S state is with significant 
feature, for T state, however, the  feature is
predominant. These two characters make the S /T states
strongly coupled, and we suppose the S ISC is enhanced.
Ultrafast internal conversion T populates the T state
finally. We examined the excited state energy levels of per (see
ESI Fig. S36†), no T state sharing similar energy with S state
was observed. Thus the enhanced ISC in MT-Per can be
rationalized by the close lying S /T states. Previously
1
financial Support.
1
2
1
state. Moreover, the
Notes and references
1
2
1
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3
. For MP-Per, the T
2
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state
(
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1
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
Thus we propose the electronic coupling S
ISC should be poor.
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/T is weak and the
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8
9
Conclusions

In summary, in order to prepare heavy atom free triplet
photosensitizers, thienyl moiety was introduced to the
chromophores of perylene, naphthaleneimide and anthracene.
4
,

Photochem. Photobio. Sci.
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Photochemical & Photobiological Sciences
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DOI: 10.1039/C8PP00230D
Table of Contents Entry:
Effect of thiophene substitution on the intersystem crossing of arene
photosensitizers
a
a
a
a
Farhan Sadiq, Jianzhang Zhao, * Mushraf Hussai, and Zhijia Wang
Effect of Thiophene Substitution on ISC
Φ
Φ
∆
= 0 %
Φ = 60 %
∆
F
= 94%
S
Φ = 15%
F
No ISC ability
ISC ability
1
T
2
0.03
1
[F]
MT-Per
a
[F]
T
2
0
0
0
.02
.01
.00
0
ꢀ
s
6
6.6
. .
ꢀ
s
.
ISC
1665
ꢀ
s
No ISC
T
1
T
1
S
0
S
0
3
60 480 600 720
Wavelength / nm
Thiophene substitution gives energy level-matched S /T states and the ISC was enhanced,
1
2
which was not observed with phenyl substitution.