Synthesis and characterizations of benzothiadiazole-based fluorophores as potential wavelength-shifting materials

Article abstract of DOI:10.1016/j.jphotochem.2012.01.011

The synthesized benzothiadiazole-based series fluorophores as potential wavelength-shifting materials exhibit large Stokes shifts (>160 nm) with multiple broad absorbance bands from UV region to 600 nm and a strong fluorescence peak around 700 nm (in CHCl₃). Intramolecular charge transfer (ICT) characters of the synthesized compounds are examined using UV-vis and photoluminescence solvatochromic shift measurements. Among the synthesized compounds, the fluorophores with asymmetrical structures exhibit larger Stokes shifts than those with symmetrical structures due to large dipole moment changes upon excitation. The fluorophores with electron-donating methoxyl groups attached to the triphenylamine donors are found to have strong ICT properties. Photophysical experimental results are supported by theoretical calculations using Density Function Theory (DFT) and Time Dependent Density Function Theory (TD-DFT) methods. Calculated frontier molecular orbitals (MOs) of ground states on these fluorophores showed an increase in ICT character up to 50% from HOMO to LUMO. Geometric optimization calculations of the excited state reveal that these fluorophores show a more planar structure for the excited state than the ground state, which allows more π-π* overlap and leads to larger Stokes shifts and higher quantum yields.

Full text of DOI:10.1016/j.jphotochem.2012.01.011

Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 51–59
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and characterizations of benzothiadiazole-based fluorophores as
potential wavelength-shifting materials
Yilin Lia,b, Louis Scudieroc, Tianhui Rena, Wen-Ji Dongb,∗
a School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
b Voiland School of Chemical Engineering and Bioengineering and Department of Veterinary & Comparative Anatomy, Pharmacology & Physiology, College of Veterinary Medicine;
Washington State University, Pullman, WA 99164, USA
c Department of Chemistry and Materials Science and Engineering, Washington State University, Pullman, WA 99164, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesized benzothiadiazole-based series fluorophores as potential wavelength-shifting materials
exhibit large Stokes shifts (>160 nm) with multiple broad absorbance bands from UV region to 600 nm
and a strong fluorescence peak around 700 nm (in CHCl3). Intramolecular charge transfer (ICT) charac-
ters of the synthesized compounds are examined using UV–vis and photoluminescence solvatochromic
shift measurements. Among the synthesized compounds, the fluorophores with asymmetrical structures
exhibit larger Stokes shifts than those with symmetrical structures due to large dipole moment changes
upon excitation. The fluorophores with electron-donating methoxyl groups attached to the tripheny-
lamine donors are found to have strong ICT properties. Photophysical experimental results are supported
by theoretical calculations using Density Function Theory (DFT) and Time Dependent Density Function
Theory (TD-DFT) methods. Calculated frontier molecular orbitals (MOs) of ground states on these fluo-
rophores showed an increase in ICT character up to 50% from HOMO to LUMO. Geometric optimization
calculations of the excited state reveal that these fluorophores show a more planar structure for the
excited state than the ground state, which allows more ␲–␲* overlap and leads to larger Stokes shifts
and higher quantum yields.
Received 20 September 2011
Received in revised form 4 January 2012
Accepted 15 January 2012
Available online 25 January 2012
Keywords:
Benzothiadiazole
Wavelength-shifting materials
Large Stokes shift
ICT
DFT
Published by Elsevier B.V.
1. Introduction
short wavelength photons and re-emits photons at longer, more
favorable wavelengths before they reach the photovoltaic device.
Conjugated organic compounds have been extensively explored
in many fields of science [1]. Their special ability to transfer light
from lower wavelength to higher wavelength makes them unique
materials applied in organic light-emitting diodes [2], organic pho-
tovoltaics [3–5] and biochemical sensors [6]. Because of unique
spectroscopic properties associated with some of these com-
pounds, they can also be used as wavelength-shifting materials to
increase solar energy transfer efficiency of plant photosynthesis
and some man-made solar cells [7–9]. For example, many photo-
voltaic materials, particularly cadmium telluride (CdTe), exhibit a
poor spectral response to short wavelength light, which means that
an appreciable fraction of incident photon energy is lost during
energy conversion. One approach to improving the energy con-
version efficiency at short wavelength (<500 nm) is to modify the
spectrum that is incident on the surface of photovoltaic materi-
als by placing a layer of a wavelength-shifting material in front of
the photovoltaic device. The wavelength-shifting material absorbs
To achieve significant enhancement in energy conversion efficiency
of photovoltaic device using wavelength-shifting materials, it is
desirable to have materials with spectral properties of absorp-
tion at full UV range and emission at 680–700 nm. This means
that these materials should have large Stokes shifts (>150 nm)
with high quantum yield to avoid self-quenching and energy loss
[10]. The ␲-conjugated organic compounds are good candidates for
wavelength-shifting material development.
Recent functional and structural studies on ␲-conjugated com-
pounds have revealed the correlations between their spectroscopic
properties and structures [11–13]. Different methods have been
used to tune the spectral properties of ␲-conjugated compounds
such as increasing the molecular conjugation to make the absorp-
tion spectra red shift [14], tuning the intramolecular charge transfer
(ICT) character to shift the emission spectra [15] and building
energy-transfer system to obtain large Stokes shift [16].
In addition, recent studies have shown that small organic
molecules with benzothiadiazole structure exhibit large Stokes
shifts with high quantum yields and red light-emitting proper-
ties. These benzothiadiazole-based materials have been widely
employed in photovoltaic applications [17–20]. Furthermore,
∗
Corresponding author. Tel.: +1 509 335 5798.
E-mail address: wdong@vetmed.wsu.edu (W.-J. Dong).
1010-6030/$ – see front matter. Published by Elsevier B.V.
doi:10.1016/j.jphotochem.2012.01.011

52
Y. Li et al. / Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 51–59
recent report revealed that benzothiadiazole in these compounds
showed charge-acceptor properties [21]. Also, it is known that
triphenylamine-based derivatives are electron-rich molecules that
can be introduced as donor to increase the ICT character [15]. There-
fore, constructing a benzothiadiazole-triphenylamine based ICT
system can be a useful approach to tuning the photophysical prop-
erties of benzothiadiazole-based compounds to meet the specific
requirements of wavelength-shifting materials.
To develop these ␲-conjugated compounds with desired pho-
tophysical properties, we synthesized a series of fluorophores
containing benzothiadiazole core structure linked to tripheny-
lamines with different functional groups. In addition to their
specific intrinsic photophysical properties, the synthesized fluo-
rophores exhibit large Stokes shifts with multiple broad absorbance
bands from the UV region to 600 nm and a strong fluorescence
band around 700 nm (in CHCl3). Theoretical computational stud-
ies of these compounds not only revealed strong ICT during their
photo-excitation, but also provide information on the relationship
between their photophysical properties and structures.
analyzer. C, H & N percentage analyses were carried out on 1a–d to
prove that these compounds were pure enough for the fluorescence
measurements.
General procedure of palladium complex catalytic reaction: A mix-
ture of reactants, base and Pd-catalyst in toluene was refluxed
for 24 h. The resulting mixture was filtered, washed by water and
extracted by DCM three times. The combined organic layers were
collected and dried over MgSO4 anhydrous. After the solvent was
removed under reduced pressure, the residue was purified by flash
column chromatography on SiO2 with hexane and ethyl acetate as
eluant to obtain the desired product.
General procedure of NBS bromination: To a solution of reactant,
NBS was added in small portions, and the reaction solution was
stirred under room temperature for appropriate time. The resulting
solution was filtered, washed by water and extracted by DCM three
times. The combined organic layers were collected and dried over
MgSO4 anhydrous. After the solvent was removed under reduced
pressure, the residue was purified by flash column chromatogra-
phy on SiO2 with hexane and ethyl acetate as eluant to obtain the
desired product.
2. Experimental
2.3.1. 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (1)
Red solid (1.6 mg, yield 78.3%). Rf = 0.33 (Hex: EA = 15: 1). 1H
NMR (300 MHz, CDCl3, ␦): 8.13 (dd, 2H, J = 3.7 Hz), 7.88 (s, 2H), 7.47
(dd, 2H, J = 5.1 Hz), 7.23 (dd, 2H, J = 5.1 Hz).
2.1. Spectroscopic measurements
Absorption spectra were recorded with a Bechman Coulter
DU730 Life Science UV-Vis spectrophotometer. Absorption spec-
tra of the compounds above 250 nm were presented in this study.
The abrupt changes in absorption at 340 nm were caused by instru-
mental artifact primarily due to automatic lamp switch during
spectral measurements. Emission spectra were collected on an ISS
PC1 photon counting spectrofluorometer using the lowest absorp-
tion maximum as the excitation wavelength. Fluorescence lifetimes
were measured in the hexane solution on a HORIBA JOBIN YVON
fluorocube using a LED 459 nm for compound 1 or LED 495 nm for
1a–d as excitation light source. The relative fluorescence quantum
yields were determined in the hexane solution using perylene (0.94,
ꢀex = 435 nm) as standard for compound 1 and then using com-
pound 1 (0.59, ꢀex = 475 nm) as standard for compounds 1a–d. The
fluorescence quantum yield was calculated by [22]:
2.3.2. 4,7-bis(5-bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole
(3b)
Dark red solid (2.1 g, yield 86.1%). Rf = 0.47 (Hex: EA = 15: 1). 1
H
NMR (300 MHz, CDCl3, ␦): 7.82 (d, 2H, J = 4.0 Hz), 7.80 (s, 2H), 7.17
(d, 2H, J = 4.0 Hz).
2.3.3. Bis(4-methoxylphenyl)amine (5)
White solid (6.0 g, yield 98.0%). Rf = 0.30 (Hex: EA = 5: 1). 1H NMR
(300 MHz, CDCl3, ␦): 6.96 (d, 4H, J = 8.9 Hz), 6.84 (d, 4H, J = 8.9 Hz),
5.28 (s, 1H), 3.78 (s, 6H).
ꢀ
ꢁꢀ
ꢁ
2
Gradx
Gradst
nx
2.3.4. 4-bromo-N,N-bis(4-methoxylphenyl)aniline (6)
White solid (2.2 g, yield 65.6%). Rf = 0.26 (Hex: EA = 5: 1). 1H NMR
(300 MHz, CDCl3, ␦): 7.27 (d, 2H, J = 9.1 Hz), 7.08 (d, 4H, J = 9.1 Hz),
6.87–6.81 (m, 6H), 3.81 (s, 6H).
˚
x
= ˚st
nst
Where the subscripts st and x denote standard and sample respec-
tively, ˚ is the fluorescence quantum yield, Grad is the gradient
from the plot of integrated fluorescence intensity vs. absorbance,
and n is the refractive index of the solvent. The fluorescence lifetime
decays obtained from time-resolved measurements were fitted
with the one-exponential decay equation:
2.3.5. 4-methoxyl-N-(4-methoxylphenyl)-N-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline
(4b)
I(t) = Ae−t/ꢁ
White solid (1.1 g, yield 45.3%). Rf = 0.18 (Hex: EA = 5: 1). 1H NMR
(300 MHz, CDCl3, ␦): 7.61 (d, 2H, J = 8.7 Hz), 7.08 (d, 4H, J = 9.0 Hz),
6.88–6.81 (m, 6H), 3.80 (s, 6H), 1.32 (s, 12H). MS-MALDI: m/z calcd
f
+
2.2. Computational methods
for C26H30BNO4 431.2268, found 431.2214.
The S0 and S1 geometries of 1a–d were optimized using
B3LYP/6-31G(d). For the excited state, TD-DFT with B3LYP func-
tion was used on the optimized structures. All calculations were
performed with Gaussian 09 package [23].
2.3.6. N,N-diphenyl-4-(5-(7-(thiophen-2-
yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)aniline
(1a)
Red solid (145 mg, yield 50.6%). Rf = 0.29 (Hex: EA = 10: 1). 1H
NMR (300 MHz, CDCl3, ␦): 8.12 (d, 2H, J = 4.1 Hz), 7.88 (s, 2H),
7.58 (d, 2H, J = 8.7 Hz), 7.47 (dd, 1H, J = 5.1 Hz), 7.34 (dd, 4H,
J = 6.9 Hz), 7.23 (dd, 1H, J = 8.9 Hz), 7.16–7.03 (m, 9H). 13C NMR
(300 MHz, CDCl3, ␦): 123.5 123.7 124.9 125.3 126.1 126.8 127.0
2.3. Synthesis and characterization
Unless otherwise noted, all reagents were used as received and
without further purification. Chromatography was carried out with
60–200 mesh silica gel for flash columns. 1H NMR spectra and
13C NMR were collected on a 300 MHz spectrometer at room tem-
perature. Mass spectra were recorded on a 4800 MALDI TOF/TOF
+
128.2 129.0 129.6 147.6. MS-MALDI: m/z calcd for C32H21N3S3
543.0898, found 543.1697. Anal. (%): calcd (found) for C32H21N3S3:
C 70.69(68.02), H 3.89(3.62), N 7.73(7.23).

Y. Li et al. / Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 51–59
53
Fig. 1. Molecular structures of compounds 1a–d.
2.3.7. 4,4ꢀ-(5,5ꢀ-(benzo[c][1,2,5]thiadiazole-4,7-
3. Results and discussion
diyl)bis(thiophene-5,2-diyl))bis(N,N-diphenylaniline)
(1b)
3.1. Molecular structures and synthesis
Dark red solid (365 mg, yield 42.5%). Rf = 0.28 (Hex: EA = 5:
1). 1H NMR (300 MHz, CDCl3, ␦): 8.07 (d, 2H, J = 3.9 Hz), 7.79 (s,
2H), 7.56 (d, 4H, J = 11.1 Hz), 7.31 (t, 8H), 7.16–7.03 (m, 18H).
13C NMR (300 MHz, CDCl3, ␦): 123.5 123.7 124.9 125.5 126.8
128.3 128.9 129.6 147.6. MS-MALDI: m/z calcd for C50H34N4S3
786.1946, found 786.1304. Anal. (%): calcd (found) for C50H34N4S3:
C 76.30(79.45), H 4.35(4.59), N 7.12(6.11).
Compounds 1a–d (Fig. 1) are all benzothiadiazole-based fluo-
rophores with different triphenylamine (TPA) functionalities. They
can be classified by their asymmetrical (1a and 1c) or symmetrical
(1b and 1d) structures and the absence of methoxyl groups (1a and
1b) or the presence of methoxyl groups (1c and 1d) which serve as
electron-donating groups. Compound 1b has been reported before
[24] and is herein used as a comparison of other compounds.
+
The
synthesis
of
compounds
1a–d
from
4,7-
2.3.8. 4-methoxyl-N-(4-methoxylphenyl)-N-(4-(5-(7-(thiophen-
2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)phenyl)aniline
(1c)
dibromobenzo[c][1,2,5]thiadiazole (2) is depicted in Scheme 1.
A modified previously reported procedure is used to synthesize
compound 1 without further purification [25]. Compounds 3a
and 3b are obtained by the selective bromination [15,26] on the
thiophene group of compound 1. Compound 4b is synthesized
from two step Buchwald–Hartwig coupling [27,28] followed by the
reaction with B2(pin)2 [29,30]. The compounds 3a and 3b react-
ing with 4-(Diphenylamino)phenylboronic acid 4a or 4b under
palladium complex catalyst lead to products 1a–d. Compounds
1a and 1c are red solid and are both donor–acceptor (DA) type
fluorophores while compounds 1b and 1c are dark-red solid and
donor–acceptor–donor (D2A) type fluorophores.
Dark red solid (75 mg, yield 23.6%). Rf = 0.31 (Hex: EA = 5: 1).
1H NMR (300 MHz, CDCl3, ␦): 8.12 (d, 2H, J = 3.9 Hz), 7.87 (dd, 2H,
J = 1.9 Hz), 7.52 (d, 2H, J = 8.7 Hz), 7.46 (dd, 1H, J = 5.1 Hz), 7.29 (d, 1H,
J = 4.0 Hz), 7.23 (dd, 1H, J = 8.9 Hz), 7.11 (d, 4H, J = 8.9 Hz), 6.96 (d, 2H,
J = 10.7 Hz), 6.87 (d, 4H, J = 8.9 Hz), 3.81 (s, 6H). 13C NMR (300 MHz,
CDCl3, ␦): 55.7 115.0 120.4 122.9 125.2 125.6 126.1 126.3 126.7
126.9 127.0 127.5 128.2 129.1 137.4 139.7 140.7 146.2 148.8 152.7
+
152.9 156.4. MS-MALDI: m/z calcd for C34H25N3O2S3 603.1109,
found 603.1437. Anal. (%): calcd (found) for C34H25N3O2S3: C
67.63(68.08), H 4.17(4.03), N 6.96(6.85).
3.2. Characterization of photophysical properties
2.3.9.
4,4ꢀ-(5,5ꢀ-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(thiophene-5,2-
diyl))bis(N,N-bis(4-methoxylphenyl)aniline)
Photophysical properties of these compounds were investigated
by spectroscopic measurements. Fig. 2 shows the absorption and
emission spectra of compounds 1a–d in chloroform, and the per-
tinent photophysical parameters are summarized in Table 1. In
our measurements, tetra-N-phenylbenzidine (TPB) and compound
1 are used as references to investigate the ICT character of com-
pounds 1a–d.
Without conjugation to TPA, compound 1 in chloroform exhibits
a strong fluorescence at 565 nm and a multiple-band absorbance
with major maximal peaks at 300 nm and 445 nm, respectively. This
results in a 120 nm Stokes shift (Fig. 2 left). Upon conjugation to TPA
derivatives, high energy absorption bands of compounds 1a–d are
(1d)
Dark red solid (251 mg, yield 63.4%). Rf = 0.48 (Hex: EA = 2: 1).
1H NMR (300 MHz, CDCl3, ␦): 8.09 (d, 2H, J = 3.9 Hz), 7.83 (s, 2H),
7.51 (d, 4H, J = 8.7 Hz), 7.28 (d, 2H, J = 3.8 Hz), 7.11 (d, 8H, J = 8.9 Hz),
6.96 (d, 4H, J = 6.8 Hz), 6.87 (d, 8H, J = 8.9 Hz), 3.81 (s, 12H). 13C
NMR (300 MHz, CDCl3, ␦): 55.7 115.0 120.5 122.9 125.3 125.8 126.3
126.7 127.0 128.9 137.6 140.8 146.0 148.7 152.8 156.3. MS-MALDI:
m/z calcd for C54H42N4O4S3 906.2368, found 906.2276. Anal. (%):
calcd (found) for C54H42N4O4S3: C 71.50(70.16), H 4.67(4.42), N
6.18(6.09).
+

54
Y. Li et al. / Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 51–59
Scheme 1. Synthesis route of compounds 1a–d.
Table 1
Photophysical parameters of compounds 1 and 1a–d including absorption (ꢀabs), molar absorption coefficient (ε) and emission (ꢀem) maximum, Stokes shift (ꢀem − ꢀabs),
fluorescence quantum yield (˚f) and lifetime (ꢁf), radiative (kr) and nonradiative (knr) rate constants.
Compound
ꢀabs (nm)a
ε (M−1 cm−1
)
ꢀem (nm)b
Stokes shift (nm/cm−1
)
˚
f
ꢁf (ns)
kr (107 s−1
)
knr (107 s−1
)
1
445
495
530
510
545
8016
9007
15,185
15,082
18,804
565
670
690
705
710
120/4800
175/5300
160/4400
195/5400
165/4300
0.59
0.33
0.23
0.28
0.18
10.24
6.62
5.13
6.55
4.90
5.76
4.99
4.48
4.27
3.67
4.00
10.12
15.01
10.99
16.73
1a
1b
1c
1d
a
Lower energy absorption maximum.
Photoluminescence emission maximum, excited at ꢀabs in chloroform.
b

Y. Li et al. / Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 51–59
55
Fig. 2. Normalized UV–vis absorption (solid lines) and photoluminescence (dashed lines) spectra. Left: TPB (ꢀex = 350 nm, black), 1 (ꢀex = 445 nm, red) in chloroform. Right:
1a (ꢀex = 495 nm, green), 1b (ꢀex = 530 nm, blue), 1c (ꢀex = 510 nm, cyan) and 1d (ꢀex = 545 nm, magenta) in chloroform. Note: the sharp changes observed in UV absorption
at 340 nm are due to instrumental artifacts.
broadened and red-shifted with maximal peak at 375 nm. The low
energy absorption bands are red-shifted to 495–545 nm. The molar
absorption coefficients associated with the low energy absorption
bands are also increased, which allows compounds 1a–d to absorb
more light than compound 1. Emission spectra of these compounds
are also red-shifted to 670–710 nm (Fig. 2 right). Conjugation to TPA
increased Stokes shifts of the target compounds by 40–75 nm.
Each molecule of compounds 1a–d can be considered as a
donor–acceptor system, in which TPA derivatives serve as electron
donors while the benzothiadiazole core structure serves as an elec-
tron acceptor. Typically, TPA derivatives (such as TPB) have strong
absorbance from 250 nm to 400 nm (Fig. 2 left) and compound
1 also has a strong absorbance around 300 nm. Thus, the higher
energy absorbance (250–400 nm) of compounds 1a–d inherits UV
absorbance spectral characteristics of both donor and acceptor
and can be ascribed to the ␲–␲* transitions of the fluorophores
[31]. The lower energy absorbance of these compounds at 500 nm
is believed to be caused by electronic transition from donor to
acceptor, which can be used to reveal the ICT character of the
molecules. Furthermore, the red-shift observed in absorption and
emission spectra of compounds 1a–d suggests that molecules with
the electron-donating methoxyl group (1c) or with one more con-
jugated group of TPA derivatives (1b and 1d) will have better
molecular conjugation. The observed large Stokes shifts (>160 nm)
of these fluorophores may be the result of changes in molecular
structure from the less planar geometry in their ground state to
the more planar geometry at their excited state [32,33]. It is also
noted that compounds 1a and 1c with asymmetrical structures
show larger Stokes shift than compounds 1b and 1d with symmet-
rical structures. It is known that asymmetrical molecules display
larger changes in dipole moments going from ground to excited
states [34].
Without conjugation to TPA derivatives, compound 1 exhibits a
quantum yield of 0.59 and a fluorescence lifetime of 10.24 ns.
Radiative process (5.76 × 107 s−1) can effectively compete with
nonradiative process (4.00 × 107 s−1) in the course of the relax-
ation of the excited state of the compound. Upon conjugation with
TPA derivatives, the quantum yields and lifetimes decreased from
0.33 to 0.18 and from 6.62 ns to 4.90 ns, respectively. Calculated
nonradiative rates (Table 1) suggest that the observed decrease
in quantum yields and fluorescence lifetimes in the conjugated
compounds are largely caused by dominating roles of rapid nonra-
diative processes that are significantly increased upon conjugation.
The increase in the nonradiative process in these compounds is
likely due to more freedom of internal structural rotation, which is
consistent with the fact that compounds 1b and 1d have both sym-
metrical structure and more freedom of internal structural rotation.
Therefore, compounds 1b and 1d show lower quantum yields and
shorter fluorescence lifetimes than compounds 1a and 1c.
It is known that ICT plays an important role in tuning flu-
orescence spectral properties of ␲-conjugated compounds. Our
spectroscopic study has shown that ICT occurs upon excitation
of the fluorophores (Fig. 2). Because the ICT process is sensitive
to environmental polarity and dipole moment of the molecules,
the emission spectra of the fluorophores are expected to shift in
response to different solvents, which can be used to investigate ICT
properties of the fluorophores. In this study, we examine emission
spectral changes of each fluorophore with different solvents. Fig. 3
shows fluorescence photography (left) and normalized emission
spectral changes (right) of compound 1a in various solvents with
different polarities. With the increase in polarity of the solvents
from hexane, toluene, 1,4-dioxane, chloroform, dichloromethane,
acetone to acetonitrile, the fluorescent color of sample solution
containing 1a changes from yellow to deep red. Furthermore,
brightness or intensity of sample 1a decreases with increasing
solvent polarity. The decrease in fluorescence intensity is largely
caused by strong dipole–dipole interaction between fluorophore
and solvent molecules, which increases solvent relaxation time
upon fluorophore excitation and expedites nonradiative process.
Due to a strong dipole–dipole interaction between fluorophore and
solvent molecules, the observed spectral changes are the result of
changes in dipole moments between the delocalized ground state
and the highly localized excited state of the fluorophore. The highly
localized excited state is believed to be caused by ICT from the
donor component to the acceptor component of the molecule. Rela-
tionship between emission wavelength and the difference in the
Further characterization of compounds 1 and 1a–d with time-
resolved fluorescence measurement allows the determination of
the radiative and nonradiative rate constants using the following
equations [35] (results are given in Table 1).
˚
f
kr =
ꢁf
1 − ˚f
ꢁf
knr
=

56
Y. Li et al. / Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 51–59
Fig. 3. Equimolar fluorescence photography (left) and solvatochromatic shift (right) of compound 1a with different solvents (from left to right): hexane, toluene, 1,4-dioxane,
chloroform, dichloromethane, acetone and acetonitrile.
dipole moment of typical CT compounds can be expressed by the
Lippert–Mataga equation [36–38].
3.3. Quantum chemistry computation
To further understand the electronic properties of the ground
and excited states of the synthesized fluorophores, we performed
quantum chemistry computations on compounds 1a–d. Ground
state (S0) geometric optimizations on 1a–d were first performed
at the B3LYP/6-31G(d) level of DFT theory without symmetry con-
straints. Molecular orbital calculations based on the optimized
S0 geometries of 1a–d were then carried out. The computa-
tional results show that HOMOs are located on both donor and
acceptor groups while LUMOs are only located on the acceptors.
These results are consistent with observed photophysical data and
Lippert–Mataga plots of solvatochromic shift that show that ICT
occurred upon the excitation. The localized LUMOs suggest that the
benzothiadiazole core structure serves as acceptor upon ICT occur-
rence. The energy of HOMO and LUMO and orbital contributions of
donor and acceptor are given in Table 2.
Compounds 1a–d have electronically delocalized HOMO in the
ground states, to which electron contributions are 56–73% from
donor groups and 27–44% from acceptor groups. In contrast, the
highly localized LUMO is dominantly contributed by acceptor elec-
trons (>94%), which is likely caused by ICT from the ground state to
the excited state of the fluorophores. The calculated frontier orbitals
(HOMO and LUMO) are shown pictorially in Fig. 5, where the HOMO
and LUMO are considered to be largely ␲ and ␲* orbitals. Compared
to compounds 1a and 1b, compounds 1c and 1d show a decrease
in electron contribution in their HOMO from acceptor groups to
the molecular orbitals due to the presence of the methoxyl group,
which causes an enriched electron density in the donor group. It
is noted that electron contribution to the LUMO from donors of
symmetrical structures (1b and 1d) was doubled compared to the
asymmetrical ones (1a and 1c) because of the D2A type nature of
the fluorophores. Furthermore, the calculated energy gaps between
HOMO and LUMO (HLG) of compounds 1a–d (Table 2), which are
correlated to the lower excitation energy of the fluorophores, are
consistent with the experimental observations (i.e., the excitation
wavelengths of compounds 1a–d shifted to red with the decrease
of HLG) (Table 2).
ꢃ
ꢄ
ꢅꢆ
−→ −→
−→
n2 − 1
n2 + 1
2ꢃe(ꢃe − ꢃg)
∈ − 1
1
2
CT
flu
vac
ꢂ
= ꢂ
−
−
flu
hca30
2 ∈ + 1
ꢄ
ꢅ
∈ − 1
1
2
n2 − 1
ꢄf =
−
2n2 + 1
2 ∈ + 1
CT
flu
where ꢂ is the emission wavenumber excited at the ICT absorp-
−→ −→
in the gas phase; ꢃg and ꢃe is the dipole
vac
flu
tion peak and ꢂ
moment in the ground state and excited state, respectively; h is
the Planck’s constant and c is the speed of light in vacuum; a0 is the
radius of Onsagar’s cavity around a fluorophore; ∈ is the dielectric
constant; n is refractive index ∈ and n can be expressed by ꢅf, which
represents the effect of solvent. It should be noted that this equa-
tion contains many assumptions. For example, the fluorophore is
assumed to be spherical, and there is no consideration of specific
interactions with the solvent. Furthermore, this equation ignores
the polarizability of the fluorophore and assumes that the ground
and excited state dipole moments orient in the same direction.
Fig. 4 shows Lippert–Mataga plots to illustrate the relation-
ship of spectral shifts (given in wavenumbers) of the fluorophores
1a–d and solvent polarity (represented by ꢅf). The results show
that compounds 1a and 1b exhibit linear decrease of emission
wavenumber with ꢅf (Fig. 4), suggesting that the dipole moment
of the excited state of the fluorophores is larger than the dipole
moment of the ground state. Lippert–Mataga plots of 1c and 1d
(solid lines in Fig. 4) obtained from measurements in all solvents
show poor linear relationship. This can be explained by the para
position of the methoxyl group of TPA which alters the electronic
distribution of the molecular conjugation in the ground state and
forms hydrogen bond with higher polar solvent molecules. This
explanation is supported by measurements performed in the sol-
vents with low or medium polarities, in which the wavenumbers of
compounds 1c and 1d decreased linearly as ꢅf increased (dashed
lines in Fig. 4). The observed results from Lippert–Mataga plots of
1a–d definitely reveal the ICT occurrence during the excitation of
these fluorophores.
The ICT property can be characterized the difference in the elec-
tronic contribution of acceptor to LUMO and HOMO (shown in
Table 2). Our results suggest that the observed energy transitions
are ␲–␲* with more than 50% charge transfer characteristics (1a:

Y. Li et al. / Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 51–59
57
Fig. 4. Lippert–Mataga plots for compounds 1a–d. Solvent: 1 (hexane, ꢅf = 0.092), 2 (1,4-dioxane, ꢅf = 0.131), 3 (toluene, ꢅf = 0.126), 4 (chloroform, ꢅf = 0.254), 5
(dichloromethane, ꢅf = 0.320), 6 (acetone, ꢅf = 0.375), 7 (DMF, ꢅf = 0.378), 8 (ethanol, ꢅf = 0.379), 9 (acetonitrile, ꢅf = 0.393), 10 (methanol, ꢅf = 0.393). Solid line: linear fit
in the whole ꢅf range for 1a–d. Dashed line: linear fit in the low or medium ꢅf range for 1c–d.
Table 2
HOMO and LUMO energy and orbital contributions (%) of donor and acceptor in compounds 1a–d.
HOMO
Donor
LUMO
Donor
HLGa
eV
Ex. W. (exp)b
nm/eV
Acceptor
eV
Acceptor
eV
1a
1b
1c
1d
61%
56%
73%
64%
39%
44%
27%
36%
−4.480
−4.650
−4.612
−4.387
3%
6%
3%
6%
97%
94%
97%
94%
−2.560
−2.546
−2.480
−2.394
2.280
2.104
2.132
1.993
495/2.505
530/2.339
510/2.431
545/2.275
a
HLG: HOMO–LUMO gap = LUMO − HOMO.
b
Excitation wavelength, equals to absorption wavelength at lowest energy.
58%; 1b: 50%; 1c: 70%; 1d: 50%). In addition, the compound 1c
has more charge transfer character than the others due to more
electrons on donor group within the conjugated structure.
geometric difference between S0 and S1 states has a large Stokes
shift (e.g., 1a: 175 nm > 1b: 160 nm) due to longer relaxing time dur-
ing the excitation, which is supported by their lifetime results (e.g.,
1a: 6.62 ns > 1b: 5.13 ns). Computations on compounds 1c and 1d
were not performed due to extensive cost of computational time.
However, due to structural similarity, we predict similar behavior
for these compounds.
To further characterize the properties of spectral transitions of
the synthesized fluorophores, we utilized TD-DFT to examine the
molecular geometries of the ground states (S0) and the excited
states (S1) of compounds 1a–b. The results are shown in Fig. 6 and
Table 3. Although the calculated dihedral difference between S0
and S1 states of 1b is smaller than that of 1a, more energy loss in
the internal structural rotation during the excitation is expected
for compound 1b than that for compound 1a since 1b has one
more donor group than 1a. Furthermore, 1a shows more planar
structure in its S1 state, which allows the transition of the elec-
tron from donor to acceptor efficiently and leads to higher quantum
yield (e.g., 1a: 0.33 > 1b: 0.23). In addition, the molecule with larger
Table 3
Dihedral angles between donor (TPA) and acceptor (1) in optimized S0 and S1 geome-
tries of 1a.
Compound
S0 dihedral (◦)
S1 dihedral (◦)
S0 − S1 dihedral (◦)
1a
1b
23.2
24.0
5.6
10.0
17.6
14.0

58
Y. Li et al. / Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 51–59
Fig. 5. Frontier Orbitals for 1a–d.
Fig. 6. S1 and S2 geometrical structures of 1a and 1b.
4. Conclusion
structures exhibit larger Stokes shifts and higher quantum yields
due to the larger dipole moment change from the ground state to
the excited state and a more planar S1 state which reduces the
internal rotation. DFT and TD-DFT calculations support the pho-
tophysical experimental results by showing that lower HLG leads
to a red-shift of the emission wavelength and ICT occurs from
delocalized HOMO to localized LUMO. These specific photophys-
ical properties of compounds 1a–d make them good candidates
for potential wavelength-shifting materials. However, it is noted
that fluorescence quantum yields of the synthesized compounds
are range from 0.18 to 0.33, which may limit applications of these
compounds. The further efforts of our research will be made to
improve fluorescence quantum yields by fine tune molecular struc-
tures of the compounds. We will study the wavelength-shifting
properties of these compounds in their solid state, particularly in
solid film on a surface of man-made CdTe solar cells. It is expected
In conclusion, a series of benzothiadiazole-based fluorophores
including benzothiadiazole core structure and different TPA func-
tionalities have been synthesized. All of the fluorophores exhibit
large Stokes shifts (>160 nm) with multiple broad absorbance
bands from UV region to 600 nm and a strong fluorescence peak
around 700 nm (in CHCl3). Examination of the photophysical prop-
erties of these fluorophores suggest that TPA derivatives serve
as donor and benzothiadiazole as acceptor in these compounds.
The solvatochromic shifts of 1a–d on the basis of ICT mecha-
nism also were investigated. The emission spectra shift to red
due to the introduction of an electron-donor group to the donor
(1a vs. 1c or 1b vs. 1d) and the addition of one more conjugated
group of TPA derivatives (1a vs. 1b or 1c vs. 1d) to the molecules.
Our study also showed that the fluorophores with asymmetrical

Y. Li et al. / Journal of Photochemistry and Photobiology A: Chemistry 231 (2012) 51–59
59
that changes in medium environment of the compounds may sig-
nificantly changes in the fluorescence quantum yields and other
photophysical properties of the molecules.
[17] C.J. Brabec, C. Winder, N.S. Sariciftci, J.C. Hummelen, A. Dhanabalan, P.A. van Hal,
R.A. Janssen, A low-bandgap semiconducting polymer for photovoltaic devices
and infrared emitting diodes, Adv. Funct. Mater. 12 (2002) 709–712.
[18] F.L. Zhang, A. Gadisa, O. Inganas, M. Svensson, M.R. Andersson, Influence of
buffer layers on the performance of polymer solar cells, Appl. Phys. Lett. 84
(2004) 3906–3908.
[19] M. Velusamy, K.R.J. Thomas, J.T. Lin, Y.-C. Hsu, K.-C. Ho, Organic dyes incorpo-
rating low-band-gap chromophores for dye-sensitized solar cells, Org. Lett. 7
(2005) 1899–1902.
[20] D. Deng, Y. Yang, J. Zhang, C. He, M. Zhang, Z.-G. Zhang, Z. Zhang, Y. Li,
Triphenylamine-containing linear D-A-D molecules with benzothiadiazole as
acceptor unit for bulk-heterojunction organic solar cells, Organic Electronics
12 (2011) 614–622.
[21] J.-Z. Cheng, C.-C. Lin, P.-T. Chou, A. Chaskar, K.-T. Wong, Fluorene as the (-
spacer for new two-photon absorption chromophores, Tetrahedron 67 (2011)
734–739.
Acknowledgments
This work was supported by NSFC, China (No. 20872092). We
also thank the WSU Nuclear Magnetic Resonance Center for pro-
viding the NMR equipment to carry out the characterization of
compounds. The WSU NMR Center equipment was supported by
NIH grants RR0631401 and RR12948, NSF grants CHE-9115282 and
DBI-9604689 and the Murdock Charitable Trust.
[22] A Guide to Recording Fluorescence Quantum Yields, Jobin Yvon Ltd.
[23] Gaussian 09, Revision A.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuse-
ria, M. A. Robb, J. R. Cheeseman, G., Scalmani, V., Barone, B., Mennucci, G. A.
Petersson, H., Nakatsuji, M., Caricato, X., Li, H. P. Hratchian, A. F. Izmaylov, J.,
Bloino, G., Zheng, J. L. Sonnenberg, M., Hada, M., Ehara, K., Toyota, R., Fukuda, J.,
Hasegawa, M., Ishida, T., Nakajima, Y., Honda, O., Kitao, H., Nakai, T., Vreven, J.
A. Montgomery, Jr., J. E. Peralta, F., Ogliaro, M., Bearpark, J. J. Heyd, E., Brothers,
K. N. Kudin, V. N. Staroverov, R., Kobayashi, J., Normand, K., Raghavachari, A.,
Rendell, J. C. Burant, S. S. Iyengar, J., Tomasi, M., Cossi, N., Rega, J. M. Millam, M.,
Klene, J. E. Knox, J. B. Cross, V., Bakken, C., Adamo, J., Jaramillo, R., Gomperts,
R. E. Stratmann, O., Yazyev, A. J. Austin, R., Cammi, C., Pomelli, J. W. Ochter-
ski, R. L. Martin, K., Morokuma, V. G. Zakrzewski, G. A. Voth, P., Salvador, J. J.
Dannenberg, S., Dapprich, A. D. Daniels, O., Farkas, J. B. Foresman, J. V. Ortiz, J.
Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009.
[24] S.-I. Kato, T. Matsumoto, M. Shigeiwa, H. Gorohmaru, S. Maeda, T. Ishi-
i, S. Mataka, Novel 2,1,3-benzothiadiazole-based red-fluorescent dyes with
enhanced two-photon absorption cross-sections, Chem. Eur. J. 12 (2006)
2303–2307.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2012.01.011.
References
[1] S.R. Forrest, M.E. Thompson, Introduction: organic electronics and optoelec-
tronics, Chem. Rev. 107 (2007) 923–925.
[2] A.C. Grimsdale, K.L. Chan, R.E. Martin, P.G. Jokisz, A.B. Holmes, Synthesis of light-
emitting conjugated polymers for applications in electroluminescent devices,
Chem. Rev. 109 (2009) 897–1091.
[3] Y.J. Cheng, S.H. Yang, C.S. Hsu, Synthesis of conjugated polymers for organic
solar cell applications, Chem. Rev. 109 (2009) 5868–5923.
[4] P. Heremans, D. Cheyns, B.P. Rand, Strategies for increasing the efficiency of
heterojunction organic solar cells: material selection and device architecture,
Acc. Chem. Res. 42 (2009) 1740–1747.
[5] N. Martin, L. Sanchez, M.A. Herranz, B. Illescas, D.M. Guldi, Electronic communi-
cation in tetrathiafulvalene (TTF)/C60 systems: toward molecular solar energy
conversion materials? Acc. Chem. Res. 40 (2007) 1015–1024.
[6] S.W. Thomas III, G.D. Joly, T.M. Swager, Chemical sensors based on ampli-
fying fluorescent conjugated polymers, Chem. Rev. 107 (2007) 1339–
1386.
[7] E. Espi, A. Salmeron, A. Fontecha, Y. Garcia, A.I. Real, Plastic films for agricultural
applications, J. Plastic Film & Sheeting 22 (2006) 85–102.
[8] E. Klampaftis, D. Ross, K.R. Mclntosh, B.S. Richards, Enhancing the performance
of solar cells via luminescent down-shifting of the incident spectrum: a review,
Sol. Energy Mater. Sol. Cells 93 (2009) 1182–1194.
[25] Patent, WO01/49768.
[26] J.-J. Kim, H. Choi, J.-W. Lee, M.-S. Kang, K. Song, S.O. Kang, J. Ko, A polymer gel
electrolyte to achieve ≥6% power conversion efficiency with a novel organic
dye incorporating a low-band-gap chromophore, J. Mater. Chem. 18 (2008)
5223–5229.
[27] C.S. Li, Y.H. Tsai, W.C. Lee, W.J. Kuo, Synthesis and photophysical properties of
pyrrole/polycyclic aromatic units hybrid fluorophores, J. Org. Chem. 75 (2010)
4004–4013.
[28] K. Ohta, Y. Chiba, T. Ogawa, Y. Endo, Promising core structure for nuclear recep-
tor ligands: design and synthesis of novel estrogen receptor ligands based on
diphenylamine skeleton, Bioorg. Med. Chem. Lett. 18 (2008) 5050–5053.
[29] H. Konno, S. Aimoto, S.O. Smith, K. Nosaka, K. Akaji, Synthesis of [19,35, 36-
(13)C(3)]-labeled TAK779 as a molecular probe, Bioorg. Med. Chem. 17 (2009)
5769–5774.
[30] A. Pron, G. Zhou, H. Norouzi-Arasi, M. Baumgarten, K. Mullen, Controlling the
charge transfer in phenylene-bridged borylene-amine pi-conjugated systems,
Org. Lett. 11 (2009) 3550–3553.
[31] J. Svoboda, P. Stenclova, F. Uhlík, J. Zedník, J. Vohlídal, Synthesis and photophys-
ical properties of ␣, (-bis(terpyridine)oligothiophenes, Tetrahedron 67 (2011)
75–79.
[9] B.S. Richards, K.R. Mclntosh, Overcoming the poor short wavelength spectral
responses of CdS/CdTe photovoltaic modules via luminescence down-shifting:
ray-tracing simulations, Prog. Photovoltaics 15 (2007) 27–34.
[10] X. Zhang, Y. Xiao, X. Qian, A ratiometric fluorescent probe based on FRET
for imaging Hg2+ ions in living cells, Angew. Chem. Int. Ed. 47 (2008)
8025–8029.
[32] I.B. Berlman, Empirical correlation between nuclear conformation and certain
fluorescence and absorption characteristics of aromatic compounds, J. Phys.
Chem. 74 (1970) 3085–3093.
[11] R.E. Martin, F. Diederich, Linear monodisperse (-conjugated oligomers: model
compounds for polymers and more, Angew. Chem. Int. Ed. 38 (1999)
1350–1377.
[33] N.I. Nijegorodov, W.S. Downey, The influence of planarity and rigidity on the
absorption and fluorescence parameters and intersystem crossing rate con-
stant in aromatic molecules, J. Phys. Chem. 98 (1994) 5639–5643.
[34] H.L. Zhang, G. Zhao, Y. Ding, B. Wu, An efficient and enantioselective approach
to the azaspirocyclic core of alkaloids: formal synthesis of halichlorine and
pinnaic acid, J. Org. Chem. 70 (2005) 4954–4961.
[12] K.C. Moss, K.N. Bourdakos, V. Bhalla, K.T. Kamtekar, M.R. Bryce, M.A. Fox, H.L.
Vaughan, F.B. Dias, A.P. Monkman, Tuning the intramolecular charge transfer
emission from deep blue to green in ambipolar systems based on dibenzothio-
phene S,S-dioxide by manipulation of conjugation and strength of the electron
donor units, J. Org. Chem. 75 (2010) 6771–6781.
[13] J.L. Segura, N. Martin, Functionalized oligoarylenes as building blocks for new
organic materials, J. Mater. Chem. 10 (2000) 2403–2435.
[14] D.P. Hagberg, T. Marinado, K.M. Karlsson, K. Nonomura, P. Qin, G. Boschloo,
T. Brinck, A. Hagfeldt, L. Sun, Tuning the HOMO and LUMO energy levels of
organic chromophores for dye sensitized solar cells, J. Org. Chem. 72 (2007)
9550–9556.
[15] X. Tang, W. Liu, J. Wu, C.S. Lee, J. You, P. Wang, Synthesis, crystal structures, and
photophysical properties of triphenylamine-based multicyano derivatives, J.
Org. Chem. 75 (2010) 7273–7278.
[16] W. Lin, L. Yuan, Z. Cao, Y. Feng, J. Song, Through-bond energy transfer cas-
settes with minimal spectral overlap between the donor emission and acceptor
absorption: coumarin-rhodamine dyads with large pseudo-Stokes shifts and
emission shifts, Angew. Chem. Int. Ed. 49 (2010) 375–379.
[35] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd edition, Springer
Science+Business Media, NY, USA, 2006.
[36] F.B. Dias, S. Pollock, G. Hedley, L.O. Palsson, A. Monkman, I.I. Perepichka, I.F.
Perepichka, M. Tavasli, M.R. Bryce, Intramolecular charge transfer assisted by
conformational changes in the excited state of fluorene-dibenzothiophene-S,S-
dioxide co-oligomers, J. Phys. Chem. B 110 (2006) 19329–19339.
[37] Z.R. Grabowski, K. Rotkiewicz, W. Rettig, Structural changes accompanying
intramolecular electron transfer: focus on twisted intramolecular charge-
transfer states and structures, Chem. Rev. 103 (2003) 3899–4032.
[38] J. Herbich, A. Kapturkiewicz, Electronic structure and molecular conforma-
tion in the excited charge transfer singlet states of 9-acridyl and other aryl
derivatives of aromatic amines, J. Am. Chem. Soc. 120 (1998) 1014–1029.