Synthesis, photophysical and electrochemical properties of two novel carbazole-based dye molecules

Article abstract of DOI:10.1016/j.saa.2014.06.159

Two carbazole-based dye molecules: 3-(6-benzothiazol-2-yl-9H- hexylcarbazole-3-yl)-2-cyano-acylic acid (D3) and 3-[5-(6-benzothiazol-2-yl-9H- hexylcarbazole-3-yl)-thiophen-2-yl]-2-cyan-acylic acid (D4) were synthesized by an approach from carbazole derivate using Vilsmeier-Haack, Suzuki cross-coupling and Knoevenagel reactions. Their physical and electrochemical properties were investigated. D3 and D4 exhibit different optical properties, such as UV absorption, photoluminescence, fluorescence quantum yield and fluorescence lifetime in different solvents. Compared with D3 without a thiophene unit, the maximum absorption wavelength of D4 red-shift obviously and its fluorescence intensity is also enhanced. A shift of the E_HOMO and E_LUMO is observed for D3 (E_HOMO = 2.06 V, E_LUMO = -1.39 V vs. NHE) and D4 (E_HOMO = 1.73 V, E_LUMO = -1.33 V vs. NHE). D3 and D4 can be used as dyes for dye-sensitized solar cells (DSSCs) with TiO ₂ nanomaterial because their E_HOMO are lower than the conduction band edge of TiO₂ [-0.5 V (vs. NHE)] and their E _LUMO are higher than the I^3-/I^- redox potential [0.42 V (vs. NHE)].

Full text of DOI:10.1016/j.saa.2014.06.159

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 379–385
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, photophysical and electrochemical properties of two novel
carbazole-based dye molecules
⇑
Qing Zhang, Weiju Zhu, Min Fang, Fangfang Yin, Cun Li
School of Chemistry and Chemical Engineering & Anhui Province Key Laboratory of Environment-friendly Polymer Materials, Anhui University, Hefei 230601, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
We designed and synthesized two
novel carbazole-based dye molecules.
The dye D4 containing thiophene
group showed better photoelectronic
and photovoltaic properties than D3.
The two dyes could be applied as
sensitizers in nano-TiO2 DSSCs.
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
Two carbazole-based dye molecules: 3-(6-benzothiazol-2-yl-9H-hexylcarbazole-3-yl)-2-cyano-acylic
acid (D3) and 3-[5-(6-benzothiazol-2-yl-9H-hexylcarbazole-3-yl)-thiophen-2-yl]-2-cyan-acylic acid
(D4) were synthesized by an approach from carbazole derivate using Vilsmeier-Haack, Suzuki cross-
coupling and Knoevenagel reactions. Their physical and electrochemical properties were investigated.
D3 and D4 exhibit different optical properties, such as UV absorption, photoluminescence, fluorescence
quantum yield and fluorescence lifetime in different solvents. Compared with D3 without a thiophene
unit, the maximum absorption wavelength of D4 red-shift obviously and its fluorescence intensity is also
enhanced. A shift of the EHOMO and ELUMO is observed for D3 (EHOMO = 2.06 V, ELUMO = ꢁ1.39 V vs. NHE) and
D4 (EHOMO = 1.73 V, ELUMO = ꢁ1.33 V vs. NHE). D3 and D4 can be used as dyes for dye-sensitized solar cells
(DSSCs) with TiO2 nanomaterial because their EHOMO are lower than the conduction band edge of TiO2
[ꢁ0.5 V (vs. NHE)] and their ELUMO are higher than the I3ꢁ/Iꢁ redox potential [0.42 V (vs. NHE)].
Ó 2014 Elsevier B.V. All rights reserved.
Received 7 March 2014
Received in revised form 19 June 2014
Accepted 27 June 2014
Available online 17 July 2014
Keywords:
Carbazole derivate
Cyclic voltammetry
DFT calculation
DSSCs
Introduction
organic coumarin [5–6], indoline [7–8], oligoene [9–11], merocya-
nine [12], and hemicyanine [13] dyes. Carbazole is relatively easy
Dye-sensitized solar cells (DSSCs) have attracted great attention
over the last 15 years owing to their prospect of high energy con-
version efficiency and low production cost [1–4]. Recently, several
groups have developed metal free organic sensitizers to overcome
the prohibitive cost of ruthenium metal complexes, and the
impressive photovoltaic performance has been obtained with some
to functionalize the carbazolyl moiety at the 3-, 6-, or 9-positions
for tuning its properties. Carbazole derivative is a kind of promis-
ing candidates for photoluminescence and electroluminescence
materials as the hole-transporting materials (HTMs) for organic
light-emitting diodes (OLEDs) due to the high charge mobility
[14–15]. The design and synthesis of functional dyes have become
a focus of current research in view of their potential applications as
sensitizers in DSSCs technologies [16–17]. Organic molecules with
a wide range of absorption in the visible region and containing an
⇑
Corresponding author. Tel./fax: +86 551 63861279.
E-mail address: cun_li@126.com (C. Li).
http://dx.doi.org/10.1016/j.saa.2014.06.159
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

380
Q. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 379–385
anchoring group such as cyanoacylic acids are ideal candidates as
sensitizers [18]. The DSSCs process requires that these dyes absorb
sunlight and excite electron from HOMO to LUMO state. This
excited electron is then injected into the conduction band of TiO2
in a femtosecond life time by the anchoring group and in this pro-
cess the dye gets oxidized. Then the oxidized dye is neutralized to
ground state by Iꢁ3 /Iꢁ redox system [19].
In this paper, we report the synthesis of two novel
carbazole-based molecules, 3-(6-benzothiazol-2-yl-9H-hexylcar-
bazole-3-yl)-2-cyano-acylic acid (D3) and 3-[5-(6-benzothiazol-
2-yl-9H-hexylcarbazole-3-yl)-thiophen-2-yl]-2-cyano-acylic acid
(D4), which were both substituted with benzoth-iazole at 3-position
of carbazole, and decorated by cyanoacetic acid at 6-position. The
benzothiazole moiety is well-known as an excellent acceptor [20–
3-formyl-9H-hexylcarbazole (3)
To a stirring solution of compound 2 (2.51 g, 10 mmol) in anhy-
drous DMF at ice-water bath, phosphorus oxychloride (POCl3,
0.2 mol) was added, and the mixture stirred for another 30 min.
The mixture was warmed to 70 °C for 2 h and then stirred for over-
night at room temperature. It was poured rapidly into ice water
and neutralized with potassium bicarbonate. The solution was
extracted with ethyl acetate (3 ꢂ 50 mL) and dried over anhydrous
magnesium sulfate. The crude product was purified by column
chromatography (neutral alumina; ethyl acetate/petroleum ether,
1/10, v/v) to give a light yellow solid 2 1.60 g. Yield: 57.3%. 1H
NMR (Fig. S3, 400 Hz, CDCl3) d (pm): 10.10(s, 1H), 8.61(s, 1H),
8.16(d, J = 7.60 Hz, 1H), 8.01(d, J = 8.40 Hz, 1H), 7.45–7.55(m, 3H),
7.32(t, 1H), 4.33(t, 2H), 1.85–1.93(m, 2H), 1.26–1.42(m, 6H),
0.86(t, 3H). FT-IR (Fig. S4, KBr, cmꢁ1): 1692(AC@O), 2953–
2862(ACH3, ACH2A), 1375(ACH3), 1472(ACH2A), 1621–
1594(structure of carbazole).
23]. It gives rise to a donor-p-acceptor type compound [21–23]
due to its potential role in the modulation of the HOMO–LUMO
gap. We introduced a thiophene unit between the carbazole and
cyanoacetic acid on the principle of extension of the
can make electron delocalize to a plurality of atoms, reduce the
p-bridge which
p–
p*
3-benzothiazol-9H-hexylcarbazole (4)
energy, increase the molar extinction coefficient value and promote
the maximum absorption wavelength red shift.
A mixture of 3 (3.40 g, 13 mmol), 2-aminothiophenol (1.75 g,
14 mmol) in DMSO was kept at 170 °C for 12 h. The reaction mix-
ture was cooled down to room temperature and poured into brine.
After the solvents were removed in rotary evaporation, the crude
products were recrystallized from ethanol to obtain a yellow crys-
tal 2.74 g. Yield: 62.2%. 1H NMR (Fig. S5, 400 MHz, CDCl3) d (pm):
8.85(s, H), 8.21(t, 2H), 8.09(d, J = 8.40 Hz, H), 7.91(d, J = 8.00 Hz,
H), 7.52–7.41(m, 4H), 7.37(t, H), 7.31(t, H), 4.32(t, 2H), 1.88(m,
2H), 1.39–1.24(m, 6H), 0.86(t, H). FT-IR (Fig. S6, KBr, cmꢁ1):
2952–285(ACH3, ACH2A), 1627–1591(structure of carbazole),
3131, 1477–1423, 748–725(benzene), 1129–946(ASAC).
Experimental
Materials and instruments
Reagents were purchased as analytical grade from Sinopharm
Chemical Reagent Co., Ltd. (Shanghai, China) and used without fur-
ther purification unless otherwise state. Dimethylformamide
(DMF) was freshly distilled from molecular sieve.
1H NMR spectra were obtained on an AV-400 spectrometer
(Bruker, Switzerland). Chemical shifts (d) were given in ppm rela-
tive to CDCl3 7.26 (1H) or DMSO-d6 2.54 (1H). Elemental analysis
were determined on a Vario ELIII spectrometer (Elementar, Ger-
many). UV–Vis absorption spectra were recorded on a UV-3600
spectrophotometer (Shimadzu, Japan). Fluorescence emission
spectra were recorded on a LS-55 FL spectrophotometer (PerkinEl-
mer, USA). The fluorescence quantum yields were calculated using
quinine sulfate in 0.01 M H2SO4 solution as a standard at room
temperature based on the literature [24]. The fluorescence decay
behaviors were recorded on a FluoroMax-4P full functional tran-
sient and steady-state fluorescence spectrometer (HORIBA Jobin
Yvon, France). Cyclic voltammetry (CV) analysis was recorded on
a CHI660d electrochemical workstation (Chenhua, China), the
measurements were carried out with a conventional three-elec-
trode configuration consisting of a glassy carbon working elec-
6-benzothiazol-2-yl-9H-hexylcarbazole-3-carbaldehyde (5)
The product was synthesized by the similar procedure for syn-
thesis of 3, giving a faint yellow powder of the product in 57.2%
yield. 1H NMR (Fig. S7, 400 MHz, CDCl3) d (pm): 10.13(s, H),
8.88(s, H), 8.70(s, H), 8.28(d, J = 10.00 Hz, H), 8.08(m, 2H), 7.92
(d, J = 8.00 Hz, H), 7.52(t, 3H), 7.39(t, H), 4.37(t, 2H), 1.91(t, 2H),
1.37(m, 6H), 0.87(t, 3H). FT-IR (Fig. S8, KBr, cmꢁ1): 2952–
2854(ACH3, ACH2A), 1627–1596(structure of carbazole),
1682(C@O), 3062, 1479–1426, 759–725(benzene), 1164–
1128(ASAC).
3-(6-benzothiazol-2-yl-9H-hexylcarbazole -3-yl)-2-cyano-acylic acid
(D3)
To a solution of 5 (0.73 g, 1.9 mmol) in 10 mL acetonitrile was
added cyanoacetic acid (0.3 g, 3.5 mmol), dichloromethane and
piperidine (0.1 mL). The solution was refluxed for 18 h. After
cooled down to room temperature, the solvent was removed by
rotary evaporation. The product was obtained by silica gel chroma-
tography (CH2Cl2:MeOH = 10:1 as eluent) as a green solid 0.62 g.
Yield: 68.5%. 1H NMR (Fig. S9, 400 MHz, DMSO-d6) d (pm):
13.76(s, H), 9.03(s, H), 8.90(s, H), 8.48(s, H), 8.36(d, J = 4.40 Hz,
H), 8.24(d, J = 8.40 Hz, H), 8.15(d, J = 8.00 Hz, H), 8.07(d,
J = 4.80 Hz, H), 7.88(m, 2H), 7.55(t, H), 7.45(t, H), 4.50(t, 2H),
1.81(t, 2H), 1.24(m, 6H), 0.80(t, 3H). FT-IR (Fig. S10, KBr, cmꢁ1):
2925–2854(ACH2, ACH3), 1710–1631(structure of carbazole),
2212(AC„N), 1710(AC@O), 1430(AOH), 3066, 582–1430, 800–
708 (structure of benzene), 1023(C@N), 1203–1159(ASAC). Ele-
mental analysis: Anal. Calcd for C29H25N3O2S: C 72.63, H 5.25, N
8.76, S 6.69, Found; C 72.65, H 5.26, N 8.77, S 6.70.
trode,
a platinum-disk auxiliary electrode and a saturated
calomel reference electrode.
Synthesis
9H-hexylcarbazole (2)
1.0 g NaH was added in batches to the stirring solution of carba-
zole (4.0 g, 24 mmol) in anhydrous DMF. Then 4 mL (28 mmol) 1-
bromohexane in 15 mL DMF was added dropwise under stirring
at 80 °C for 3 h. When the mixture was cooled down to room tem-
perature, it was poured into water and adjusted the pH = 7 with
1:1(HCl/H2O, v/v) concentrated hydrochloric acid. The product
was obtained by filtering and recrystallization from ethanol as an
acicular crystal 6.35 g. The yield was 76.5%. 1H NMR (Fig. S1,
400 Hz, CDCl3) d (pm): 8.09 (d, J = 7.60 Hz, 2H), 7.38–7.48(m, 4H),
7.21(d, J = 8.00 Hz, 2H), 4.28(t, 2H), 1.86(m, 2H), 1.33(m, 6H),
0.86(t, 3H). FT-IR (Fig. S2, KBr, cmꢁ1): 3049(@CAH), 2856–
2953(ACH2A, ACH3), 1459(ACH3), 1323(ACH2A), 1621–1594
(structure of carbazole).
6-iodo-9H-hexylcarbazole-3-carbaldehyde (6)
Amount of 3 (1.40 g, 5 mmol) was added in the flask with 25 mL
glacial acetic acid and stirred vigorously till dissolved completely.
KI (1.66 g) and KIO3 (3.21 g) were added in the solution. The
mixture was stirred at 70 °C for 3 h. After cooled down to room

Q. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 379–385
381
temperature and it was washed with mixed solution of 100 mL
CH2Cl2 and 50 mL H2O. The extract was washed with NaHSO3
aqueous solution and adjusted to pH = 8 with ammonia. Then the
yellow organic solvents in lower was removed in rotary evapora-
tion, the crude products were recrystallized from ethanol to obtain
a yellow power (1.58 g). Yield: 78.2%. 1H NMR (Fig. S11, 400 MHz,
CDCl3) d (pm): 10.09(s, H), 8.54(s, H), 8.46(d, J = 8.40 Hz, H), 8.01(d,
J = 8.00 Hz, H), 7.75(d, J = 4.00 Hz, H), 7.45(d, J = 8.40 Hz, H), 7.25(d,
J = 8.40 Hz, 2H), 4.28(t, 2H), 1.84(t, 2H), 1.27(m, 6H), 0.96(t, 3H).
FT-IR (Fig. S12, KBr, cmꢁ1):1686(AC@O), 3079–2924(ACH3, ACH2-
A), 569(-I), 1623–1588(structure of carbazole).
brine and dried over MgSO4. After the solvents were evaporated in
rotary evaporation, the residue was purified by chromatography on
silica gel (petroleum:ethyl acetate = 5:1 as eluent) to give the prod-
uct as yellow solid (0.40 g), Yield: 45.4%. 1H NMR (Fig. S15,
400 MHz, CDCl3) d (pm): 9.90(s, H), 8.87(s, H), 8.49(d, J = 1.60 Hz,
H), 8.21(d, J = 10.00 Hz, 2H), 8.09(d, J = 8.00 Hz, 2H), 7.92(d,
J = 8.00 Hz, H), 7.78(m, 2H), 7.48(m, 4H), 7.38(t, H), 4.33(t, 2H),
1.90(t, 2H), 1.31(m, 6H), 0.96(t, 3H). FT-IR (Fig. S16, KBr, cmꢁ1):
2952–2852(ACH3, ACH2A), 1598(structure of carbazole),
1655(C@O), 3091, 1470–1433, 758(benzene), 797(structure of
thiophene).
3-benzothiazole-6-iodo-9H-hexylcarbazole (7)
3-[5-(6-Benzothiazol-2-yl-9H-hexylcarbazole-3-yl)-thiophen-2-yl]-2-
cyano-acylic acid (D4)
The product was synthesized according to the procedure for
synthesis of 4, giving a grey powder in 70.8% yield. 1H NMR
(Fig. S13, 400 MHz,CDCl3) d (pm): 8.80(s, H), 8.52(s, H), 8.28(d,
J = 8.40 Hz, H), 8.12(d, J = 8.00 Hz, H), 7.91(d, J = 4.00 Hz, H),
7.76(d, J = 8.40 Hz, H), 7.51(m, 2H), 7.40(t, H), 7.22(d, J = 8.40 Hz,
H), 4.30(t, 2H), 1.66(t, 2H), 1.32(m, 6H), 0.96(t, 3H). FT-IR
(Fig. S14, KBr, cmꢁ1): 2925–2853(ACH3, ACH2A), 1623–
1589(structure of carbazole), 3059, 1470–1435, 797–728(ben-
zene), 1288–1153(ASACA).
The product was synthesized similar as the procedure for syn-
thesis of D3, giving a red powder in 65.8% yield. 1H NMR
(Fig. S17, 400 MHz, DMSO-d6)
d (pm): 9.94(s, H), 9.11(d,
J = 8.00 Hz, H), 8.95(d, J = 12.40 Hz, H), 8.50(d, J = 4.40 Hz, H),
8.26(d, J = 8.40 Hz, H), 8.17(d, J = 8.00 Hz, H), 8.07(d, J = 4.80 Hz,
2H), 7.92(t, 2H), 7.84–7.78(m, 2H), 7.57(t, H), 7.47(t, H), 4.50(s,
2H), 1.83–1.80(m, 2H), 1.30–1.22(m, 6H), 0.83(t, 3H). FT-IR
(Fig. S18, KBr, cmꢁ1): 2924–2853 (ACH2, ACH3), 1579(structure
of carbazole), 2213(AC„N), 1707(AC@O), 1430(-OH), 1628, 756–
725(@CAH), 1189(ASAC), 797(structure of thiophene). Elemental
analysis: Anal. Calcd for C33H27N3O2S2: C 70.56, H 4.84, N 5.70, S
11.42, Found: C 70.59, H 4.82, N 7.51, S 11.41.
3-(2-carbaidenhyde-thiophene)-6-benzothiazol-9H-hexylcarbazole
(8)
To a solution of 7 (0.88 g, 1.8 mmol) and 5-formyl-2-thiophene
boric acid (0.32 g, 2 mmol) in the mixture of toluene and ethanol
(toluene:ethanol = 3:1, v/v), 2 M aqueous Na2CO3 solution (2 mL),
and Pd(PPh3)4 (10 mg ꢂ 4) was added in batches, the reaction mix-
ture was refluxed for 48 h under nitrogen atmosphere. After cooled
down to room temperature, the mixture was poured into a satu-
rated solution of ammonium chloride then extracted with ethyl
acetate (20 mL ꢂ 3). The combined organic phase was washed with
Preparation of dye-sensitized electrodes
The working TiO2 electrode for DSSCs was prepared as follows.
F-doped tin oxide (FTO) glass plates were immersed into 40 mM
TiCl4 at 70 °C for 30 min and rinsed by water and ethanol. To pre-
pare TiO2 paste commercial titania power was ground in a mortar
Scheme 1. The synthetic route of D3 and D4.

382
Q. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 379–385
Results and discussion
Synthesis and characterization
As shown in Scheme 1, D3 and D4 are 3-benzothiazole carba-
zole-based derivative molecules with cyanoacetic acid and thio-
phene cyanoacetic acid, respectively. The carbazole was easily
converted to 2 by N-alkylation and 3 was prepared by a Vilsmeier
reaction of N-hexylcarbazole with POCl3 in DMF [25–26]. Synthesis
of 4 from 3 (or 7 from 6) is Phillips Method to transform 2-amino-
thiophenol to benzothiazole. Treatment of the 5-formyl-2-thio-
phene boric acid with 7 through Suzuki coupling reaction gave
an important intermediate aldehyde 8. In this reaction, we added
Pd(PPh3)4 in batches to raises the yield obviously compared to
the literature [27]. The reason may be that Pd(PPh3)4 is sensitive
with the air, light, heat and moisture. The final reaction for D3
and D4 was the condensation of the respective aldehyde with
cyanoacetic acid by the Knoevenagel reaction with piperidine as
catalyst.
UV–Vis absorption spectral studies
The UV–Vis absorption spectra of the D3 and D4 in several sol-
vents with different polarity are illustrated in Fig. 1. The spectral
data are summarized in Table 1. D3 exhibits two absorption bands,
a high-energy absorption band at 308–319 nm and a low-energy
absorption band at 359–375 nm (except 391 nm in dichlorometh-
ane). D4 exhibits three absorption bands, a high-energy absorption
band at 319–327 nm,
a
low-energy absorption band at
396–406 nm (except 431 nm in dichloromethane) and an uncon-
spicuous band 345–349 nm. The high-energy absorption band is
attributed to the p–p
* electron transition of the conjugated system
between benzothiazole and cabazole moiety and the low-energy
absorption band is attributed to the intramolecular charge transfer
(ICT) from the carbazole unit to the cyanoacylic acid units. In addi-
tion, D3 and D4 exhibit different maximum absorption wave-
lengths (Fig. S19). The low-energy absorption band of D4
(400 nm) obviously red-shifted compared with D3 (359 nm),
which is ascribed to the incorporating thiophene unit between car-
Fig. 1. UV–Vis absorption spectra of D3 and D4 in different solvents (10.0 lM).
with a small amount of water TiO2 was evenly coated on the glass
surface, then the electrodes coated with TiO2 pastes were heated
under airflow at 450 °C for 30 min. The TiO2 films were immersed
into the dye solution (0.3 M in ethanol) for 24 h. After absorption of
the dyes, the electrodes were rinsed with chloroform and acetoni-
trile. The working and Pt-counter electrodes were assembled into a
sealed sandwich solar cell.
bazole moiety and cyanoacylic acid to extend the
p-conjugation
system while the high-energy absorption band is also red-shifted
[28–29]. As the solvent polarity increases the maximum absorp-
tion wavelength of D3 (391 to 359 nm) and D4 (431 to 396 nm)
blue shifted steming from a polar interaction and/or deprotonation
[30].
Table 1
UV–Vis spectra and fluorescence spectra data of D3 and D4 in different solvents.
Compounds Solvents
UV wavelength
Emission wavelength Excited wavelength
Stokes shift
Fluorescence quantum
Fluorescence
lifetime (s, ns)
b
kU (nm)a
Ex (nm)a
Em (nm)a
(cmꢁ1
)
yield (U)c
D3
D4
Dichloromethane 391,319
391
375
371
359
364
431
415
422
420
418
2374
2570
3257
4047
4579
0.057
0.070
0.112
0.091
0.095
0.39
0.59
0.28
1.05
1.06
Ethyl acetate
Ethanol
Acetonitrile
Dimethyl
375,312
371,308
359,313
364,312
formamide
Dichloromethane 431,347,319
431
406
404
400
396
484
480
496
492
491
2541
3798
4591
4675
4886
0.271
0.318
0.239
0.358
0.242
2.03
1.03
0.60
1.39
1.65
Ethyl acetate
Ethanol
Acetonitrile
Dimethyl
406,349,322
404,346,322
400,345,321
396,348,327
formamide
a
b
c
The concentration of solutions is 10.0 lM.
Stokes shift was calculated from 1/Ex ꢁ 1/Em.
2
FsAr nr
Us
¼
Ur
ðn
Þ
where F is the integration of the emission intensities, n is the index of refraction of the solution, U is the fluorescence quantum yield and A is the
Fr As
s
absorbance at the excitation wavelength, the subscripts ‘‘r’’ and ‘‘s’’ denote the reference and unknown samples respectively.

Q. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 379–385
383
The fluorescence spectral studies
the solvent polarity (fluorescence solvatochromism) which sup-
ports the excited state charge transfer character.
The fluorescence spectra of the two compounds were measured
in different polar solvents and the spectral data are also collected
in Table 1. The emission spectra of D3 and D4 consist of one broad
band. As shown in Fig. 2. and Table 1, D3 and D4 display mainly a
The fluorescence decay behaviors of compound D3 and D4 were
also studied in several solvents Fig. S19. The fluorescence lifetimes
(s, ns) of D3 and D4 are collected in Table 1. The fluorescence
lifetime of D4 is longer than that of D3. On the other hand, the life-
times of D3 and D4 in polar solvents are longer than in non-polar
solvents. With the increase of solvent polarity, the dipole–dipole
interaction of molecular in excited state is strengthened, leading
to the polarity increase of the excited state and the reduction of
the energy of molecular excited state. We can also find that the
lifetimes of D3 and D4 in ethanol are clearly shorter than in other
solvents because of lots of hydrogen bonds in it. Considering from
the application in DSSCs, the lifetimes of D3 and D4 are short. This
means that D3 (D4) can facilitate the charge recombination. The
facilitation can be interpreted as being that D3 (D4) attracts I3ꢁ
by dispersive force and there are spaces for Iꢁ3 to be around the
dyes [33].
broad emission band in several solvents owe to overlap of
p–
p*
transition of their skeletons and
p–
p* transition of the ICT from
the carbazole unit to the cyanoacylic acid unit. The polarity of
the solvent has a slight impact on fluorescence emission wave-
length. The fluorescence intensity in dichloromethane, ethyl ace-
tate and ethanol show a decreasing tendency. This may be
caused by the different interaction between solvent and dye mol-
ecules [31].
In order to further investigate the fluorescence emission prop-
erties of D3 and D4, their Stokes shifts and fluorescence quantum
yields are studied and summarized in Table 1. Stokes shifts of D3
and D4 become larger with the increase of solvent polarity. It is lar-
ger in the case of the polar solvents when the hydrogen bonds or
dipol–dipol interactions are most possible [32]. It could be seen
that the introduction of thiophene moiety raises the fluorescence
quantum yield that might resulted from the elongation of the con-
jugated bridge length of D4, so the molar absorption coefficient is
The electrochemical properties
In order to investigate the properties of the D3, D4, the highest
occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) energy levels, cyclic voltammetry (CV)
analysis were performed. The measurements were carried out in
CH3CN solution containing 0.1 M n-Bu4NClO4 as supporting elec-
trolyte [34]. The results are shown in Fig. 3 and summarized in
Table 2. The CV curves of two molecular remain unchanged under
successive oxidation, indicating their excellent stability against
electrochemical oxidation. The oxidation potential (Eox) corre-
sponds to HOMO while the reduction potential (Ered) to LUMO. A
positive shift of the EHOMO and ELUMO was observed for D3
(EHOMO = ꢁ5.73 V, ELUMO = ꢁ2.28 V) vs. D4 (EHOMO = ꢁ5.92 V,
ELUMO = ꢁ2.86 V), which resulted from introduction of the thio-
phene moiety.
increased and
p electron is more easily excited. The fluorescence
spectra have been recorded in different solvents and the fluores-
cence maximum of these compounds red shift upon increasing
Frontier molecular orbitals
The time-dependent hybrid density functional theory (TDDFT)
calculations were performed using B3LYP/6-31G*(d, p). B3LYP is a
hybrid function modified from the three-parameter exchange–cor-
relation functional of Becke [37]. Table 3 and Fig. 4 present the cal-
culation results of D3 and D4. The results from theoretical
calculation revealed that the HOMO of D3 and D4 are localized
on main conjugated framework between the carbazole and cyano-
acetic acid, and the LUMO are mainly located on the thiophene
Fig. 2. The normalized fluorescence spectra of D3 and D4 in different solvents
(10.0
lM).
Fig. 3. Cyclic voltamograms of D3 and D4 in acetonitrile.

384
Q. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 379–385
Table 2
Electrochemical properties of D3 and D4.
Compounds
kabs (nm)a
Eg (V)b
Eonset(ox) (V)c
EHOMO (V)d
ELUMO (V)d
Eox (V vs. NHE)
Eox ꢁ Eg(V vs. NHE)
D3
D4
359
405
3.45
3.06
1.09
1.28
ꢁ5.73
ꢁ5.92
ꢁ2.28
ꢁ2.86
2.06
1.73
ꢁ1.39
ꢁ1.33
a
b
c
Absorption spectra were measured in acetonitrile.
Eg values were calculated from intersect of the normalized absorption and the emission spectra (kabs): Eg = 1240/kabs).
The oxidation potentials (vs. NHE) of compounds were measured on 0.1 M tetrabutylammonium perchlorate (TBAP) in acetonitrile using a glass carbon working
electrode, a Pt counter electrode and a saturated calomel reference electrode.
d
EHOMO = ꢁ[Eonset(ox) + 4.4 eV + 0.24 eV] [35–36], ELUMO = EHOMO + Eg.(vs. SCE).
Table 3
Calculated single-photon-related photophysical properties of D3 and D4.
Compound
D3
Excited state
E (eV)
k (nm)
f
Composition
Character
ICT
1
2
3.3770
3.8789
359.14
313.64
0.2932
0.4276
HOMOꢁ1 ? LUMO
HOMO ? LUMO+2
p
?
p*
D4
1
2
3
2.9884
3.5244
3.9687
400.89
345.79
321.40
0.3368
0.2876
0.5030
HOMOꢁ1 ? LUMO
HOMO ? LUMO+1
HOMOꢁ1 ? LUMO+1
ICT
ICT
p
?
p*
While the transitions at ca. 400 nm (f = 0.3368), 345 nm
(f = 0.2806) 321 nm (f = 0.5030) originating from HOMO-
ꢁ1 ? LUMO, HOMO ? LUMO+1 and HOMOꢁ1 ? LUMO+1 transi-
tions of D4, showed that the distributions of HOMO and LUMO
levels are separated in D4, indicating that the HOMO-
ꢁ1 ? LUMO+1 transition can be ascribed to
a p–
p* mixed
charge-transfer transition.
There is a very important application of D3 and D4 as organic
sensitizers of Dye-sensitized solar cells (DSSCs), which emerged
as a new generation photovoltaic device and had received consid-
erable attention in recent years because of their high efficiency and
low-cost [38–40]. Fig. 5 shows the schematic energy diagram of
these dye-sensitized TiO2 electrodes. To get an efficient charge sep-
aration, the LUMO of the dye from where the electron injection
occurs has to be sufficiently more negative than the conduction
band edge of the TiO2 (Ecb) [41], and the HOMO has to be more
positive than the redox potential of Iꢁ3 /Iꢁ [42]. The HOMO levels
of D3 and D4 dyes are 2.06, and 1.73 V (vs. NHE), respectively.
We considered that the potential levels of the Eox ꢁ Eg corre-
sponded to the LUMO levels of the dyes. The HOMO levels of the
dyes are sufficiently more positive than Iꢁ3 /Iꢁ redox potential value,
indicating that the oxidized dyes formed after electron injection to
TiO2 could accept electrons from Iꢁ ions thermodynamically [41].
From these values, we can find the introduction of thiophene unit
into the molecules shift the HOMO levels negatively, which thus
decrease the gap between the HOMO level and the redox potential
of Iꢁ3 /Iꢁ. This might increase the efficiency of regeneration of the
oxidized dye by Iꢁ and the overall solar-to-electrical energy con-
version efficiency at the same time. These LUMO are sufficiently
more negative than the Ecb of TiO2 (ꢁ0.5V vs. NHE). The relatively
large energy gaps between the LUMO and Ecb allow for the addition
of the 4-tert-butylpyridine (TBP) to the electrolyte [43], which shift
the Ecb of the TiO2 negatively and, consequently, improve the volt-
age and the total efficiency.
Fig. 4. The frontier molecular orbitals of the HOMO and LUMO.
Fig. 5. Schematic energy level diagram for a DSSC based on dyes.
Solar cell performance
cyanoacetic acid units, respectively. Additionally, the results also
show that the D4 possess two charge-transition states compared
to the D3 because the insertion of thiophene group can enhance
All DSSC were fabricated from a TiO2 film, which consists of a
layer of TiO2 nanoparticle film (TiO2 P25 21diameter).We compared
the J–V curves of D3 and D4 based divices in Fig. 6 and the results are
summarized in Table 4. The devices fabricated from D3 achieved a
power conversion efficiency of 0.45%. Theꢁd2evice fabricated from
D3 showed Voc = 0.66 V, Jsc = 1.33 mA cm and FF = 0.51, while
D4 showed Voc = 0.53 V, Jsc = 2.10 mA cmꢁ2 and FF = 0.44. It is
the ICT and effective
p-electron delocalization. Two transitions of
D3 resemble those observed in the experimental linear absorption
spectra. The transitions at ca. 359 nm (f = 0.2932) and 313 nm
(f = 0.4276) originating from HOMOꢁ1 ? LUMO and HOMO ? -
LUMO+2 transitions are assigned as the ICT transitions of D3.

Q. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 379–385
385
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Acknowledgments
This research was financially supported by the Key Natural Sci-
ence Research Project of Anhui Provincial Universities
(KJ2013A013) and the Natural Science Fund of Anhui Province
(No. 11040606M33), the Dr. Research Project Fund of Anhui Uni-
versity (No. 33190105, 33190213), Youth Scientific Research
Found of Anhui University and the Excellent Youth Fund in Univer-
sity of Anhui Province (No. 2012SQRL025), the Graduate Academic
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Appendix A. Supplementary data
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