Facile synthesis and photo-physical properties of cyano-substituted styryl derivatives based on carbazole/phenothiazine

Article abstract of DOI:10.1016/j.saa.2012.04.090

Four new cyano-substituted styryl derivatives based on carbazole and phenothiazine were facilely synthesized by conventional Knoevenagel condensation and well characterized. The UV-vis spectra of these dyes showed the lowest-energy absorption bands were mainly caused by the HOMO-LUMO one-electron promotion as conformed by TD-DFT calculations. The thermogravimetric analysis showed these dyes were thermally stable up to 350 °C. The UV-vis absorption and fluorescence emission spectra were also studied in solvents of different polarity, these dyes exhibited unusual large Stokes shift.

Full text of DOI:10.1016/j.saa.2012.04.090

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 128–134
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Facile synthesis and photo-physical properties of cyano-substituted styryl
derivatives based on carbazole/phenothiazine
b
b
b
Mingzhu Lu a,b, Yulan Zhu a,b, , Kuirong Ma , Li Cao , Kun Wang
⇑
a School of Petrochemical Engineering, Changzhou University, Changzhou 213164, PR China
b Jiangsu Key Laboratory for Chemistry of Low-dimensional Materials, School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huaian 223300, PR China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
Four cyano-substituted styryl
derivatives based on carbazole and
phenothiazine were synthesized.
The TGA results show these
derivatives are thermally stable.
These derivatives exhibited unusual
large Stokes shifts.
"
"
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new cyano-substituted styryl derivatives based on carbazole and phenothiazine were facilely syn-
thesized by conventional Knoevenagel condensation and well characterized. The UV–vis spectra of these
dyes showed the lowest-energy absorption bands were mainly caused by the HOMO–LUMO one-electron
promotion as conformed by TD-DFT calculations. The thermogravimetric analysis showed these dyes
were thermally stable up to 350 °C. The UV–vis absorption and fluorescence emission spectra were also
studied in solvents of different polarity, these dyes exhibited unusual large Stokes shift.
Ó 2012 Elsevier B.V. All rights reserved.
Received 4 January 2012
Received in revised form 1 March 2012
Accepted 19 April 2012
Available online 28 April 2012
Keywords:
Donor–acceptor
Photophysical properties
Stokes shift
DFT
Introduction
form D-
chemical properties by tuning different donor or acceptor units.
In most of the well-studied D- -A architectures, carbazole has
p-A molecules. It is easy to modify their physical and
During the past few decades, donor–acceptor (D–A) organic
materials have attracted considerable interests owing to their po-
tential applications in electro/photoactive materials, such as non-
linear optical (NLO) materials [1–3], organic light-emitting diodes
(OLEDs) [4], organic field-effect transistors (OFETs) [5,6], dye-sen-
sitized solar cells (DSSCs) [7–9]. In such D–A dyads, the D and A
p
been widely used as electron–donor in opto- and electroactive
materials, due to its good electron-donating and transporting capa-
bilities [10,11]. With electron-rich sulfur and nitrogen hetero-
atoms, phenothiazine is also a well-known heterocyclic compound
with high electron-donating ability. Its nonplanar butterfly confor-
mation in the ground state can inhibit molecular aggregation and
the formation of intermolecular excimers [12]. These advantages
make phenothiazine and its derivatives suitable for diverse appli-
cations in organic devices [13–15]. Functional materials with car-
bazole and phenothiazine as basic building blocks have desirable
photophysical and electron-transfer (ET) properties [16–18].
moieties are often connected through a
p-conjugated bridge to
⇑
Corresponding author at: Jiangsu Key Laboratory for Chemistry of Low-
dimensional Materials, School of Chemistry and Chemical Engineering, Huaiyin
Normal University, Huaian 223300, PR China. Tel.: +86 0517 83525091; fax: +86
0517 83525100.
E-mail address: yulanzhu2008@126.com (Y. Zhu).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.04.090

M. Lu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 128–134
129
N
N
POCl3/DMF
CN
CN
Ph
t-BuOK/MeOH
S
CHO
S
5a
7
H
N
N
S
CH3CH2Br
NaOH/DMSO
S
1
2
N
N
S
8
POCl3/DMF
POCl3/DMF
CN
CN
CN
Ph
t-BuOK/MeOH
Ph
OHC
S
CHO
5b
N
N
CN
CN
Ph
t-BuOK/MeOH
CHO
6a
9
H
N
N
CH3CH2Br
NaOH/DMSO
3
4
N
N
POCl3/DMF
CN
NC
Ph
CN
Ph
t-BuOK/MeOH
OHC
CHO
10
6b
Scheme 1. Synthetic route of compounds 7–10.
Fig. 2. Emission spectra of 7–10 in dichloromethane.
Fig. 1. UV–vis absorption of 7–10 in dichloromethane.
ously. Herein, we presented the facile synthesis and characteriza-
tion of cyano-substituted styryl derivatives based on carbazole
and phenothiazine. Their thermal stability, UV–vis absorption
and fluorescence emission characteristics in different solvents
were studied. DFT calculations were also presented, in order to
investigate the electronic structures and properties of these dyes.
On the other hand, the cyano group is well-known for its elec-
tron-withdrawing characteristic which is beneficial to enhance
electron affinity of the chromophores. Thus, cyano-substituted sty-
ryl derivatives also have increasing interests because of their more
and more applications in fluorescent organic nanoparticles (FONs)
[19,20], heterojunction solar cells [21,22], two-photon absorption
(TPA) materials [23,24] and aggregation-induced emission
enhancement (AIEE) materials [25–28]. However, there have been
always efforts to design and synthesize novel and well-defined cy-
ano-substituted styryl derivatives with prospective applications.
To the best of our knowledge, cyano-substituted styryl derivatives
based on carbazole/phenothiazine have not been reported previ-
Experimental
Materials and equipments
All chemicals and solvents were purchased from commercial
supplies and used without further purification. Flash column

130
M. Lu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 128–134
Fig. 3. (a) Absorption spectra of 7 in different solvents with the same concentration of 1 Â 10À5 M. (b) fluorescence spectra of 7 in different solvents with the same
concentration of 1 Â 10À6 M.
Fig. 4. (a) Absorption spectra of 8 in different solvents with the same concentration of 1 Â 10À5 M. (b) fluorescence spectra of 8 in different solvents with the same
concentration of 1 Â 10À6 M.
chromatography was performed employing 200–300 mesh silica
gel. IR spectra were obtained on an AVATA 360 infrared spectrom-
eter in the 400–4000 cmÀ1 region. The UV–visible spectra were re-
corded on an Australia GBC UV/Vis 916 spectrophotometer in the
range of 250–700 nm. The fluorescence spectra were obtained on
a Horiba Fluorolog 3. 1H and 13C NMR spectra were recorded on
a Bruker 400 NMR spectrometer operating at 400 and 100 MHz,
respectively, using CDCl3 as solvent and tetramethylsilane as inter-
nal reference. MS spectra were measured on LCQ Advantage instru-
ment and MALDI AXIMA-CFR(PLUS). The thermal gravimetric
analyses (TG-DSC) were carried out with a NETZSCH STA 449F3
thermal analyzer with a heating rate of 10 K minÀ1 under N2 atmo-
sphere. All reactions were performed under a nitrogen atmosphere.
Synthesis and characterization
Fig. 5. Mataga–Lippert plots observed for 7 (black) and 8 (red). (For interpretation
of the references to color in this figure legend, the reader is referred to the web
version of this paper.)
Synthesis of 10-ethyl-10H-phenotiazine 2, 9-ethyl-9H-carbazole 4, 10-
ethyl-10H-3-phenotiazine carbaldehyde 5a, 10-ethyl-10H-3,7-phenotia-
zine dicarbaldehyde 5b, 9-ethyl-9H-carbazole-3-carboxaldehyde 6a,
9-ethyl-9H-carbazole-3,6-dicarboxaldehyde 6b.
Compounds 2, 4, 5a–b, 6a–b were synthesized according to the
published literature [29,30].
butoxide (1.22 g, 10 mmol) were dissolved in methanol (50 mL).
The mixture was heated to reflux and stirred for 12 h under an
atmosphere of N2. After cooling to room temperature, the mixture
was poured into water (200 mL), neutralized to pH 7 with 1 M HCl
aqueous solution, and then extracted with dichloromethane. The
extract was washed with plenty of brine and dried over Na2SO4,
Synthesis of 3-[2-phenyl-2-cyanovinyl]-10-ethyl-10H-phenothiazine 7
10-Ethyl-10H-3-phenotiazine carbaldehyde (0.31 g, 1.2 mmol)
5a, 2-phenylacetonitrile (0.29 g, 2.5 mmol) and potassium tert-

M. Lu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 128–134
131
Fig. 6. HOMO and LUMO distributions obtained from DFT calculations of 7–10.
Table 1
Electronic spectral data of 7–10 calculated with TDDFT at the B3LYP/6-31G⁄ level.
Compounds
7
f
Composition
E (eV, nm)
Exp. (eV, nm)
0.4796
0.5418
0.8406
0.1897
0.9970
0.2351
H ? L (98%)
H-1 ? L (88%)
H ? L (99%)
H-1 ? L (97%)
H ? L (99%)
H-2 ? L (79%)
H ? L + 1 (13%)
H ? L (99%)
2.63 (474)
3.59 (346)
2.58 (481)
3.62 (343)
3.20 (388)
4.20 (296)
3.02 (413)
4.02 (309)
2.86 (435)
3.94 (315)
3.32 (374)
4.30 (289)
8
9
10
1.2929
0.2309
2.94 (422)
4.26 (292)
3.23 (384)
4.39 (283)
H-2 ? L + 1 (74%)
H-3 ? L (18%)
Synthesis of 3,7-bis[2-phenyl-2-cyanovinyl]-10-ethyl-10H-
phenothiazine 8
Fig. 7. Schematic representation of the frontier orbital energy levels of 7–10.
10-Ethyl-10H-3,7-phenotiazine
dicarbaldehyde
(0.28 g,
1 mmol) 5b, 2-phenylacetonitrile (0.35 g, 3 mmol) and potassium
tert-butoxide (1.22 g, 10 mmol) were dissolved in methanol
(50 mL). The mixture was heated to reflux and stirred for 12 h un-
der an atmosphere of N2. After cooling to room temperature, an or-
ange solid precipitate was filtered off, washed with cold methanol,
and purified by column chromatography on silica gel using ethyl
acetate/petroleum ether (1:6) as eluent. Yield: 0.32 g (67%) of or-
ange powder. 1H NMR (CDCl3, 400 MHz, ppm) d: 1.48 (t,
J = 6.8 Hz, 3H), 4.00 (q, J = 6.8 Hz, 2H), 6.92 (d, J = 8.8 Hz, 2H),
7.36–7.39 (m, 4H), 7.42–7.46 (m, 4H), 7.52 (s, 2H), 7.64–7.66 (d,
J = 7.2 Hz, 4H), 7.82–7.82 (d, J = 7.2 Hz, 2H); 13C NMR (CDCl3,
100 MHz) d: 145.29, 145.20, 144.84, 142.57, 140.41, 134.63,
132.89, 129.77, 129.35, 129.24, 129.05, 128.93, 128.81, 128.68,
128.66, 128.61, 128.42, 128.31, 128.16, 125.83, 123.40, 123.34,
123.04, 120.41, 118.28, 114.98, 114.92, 114.52, 112.41, 109.35,
the solvent was removed under reduced pressure. The residue was
purified by column chromatography on silica gel using ethyl ace-
tate/petroleum ether (1:4) as eluent. Yield: 0.38 g (89%) of or-
ange-red oil. 1H NMR (CDCl3, 400 MHz, ppm) d: 1.44 (t, J = 6.8 Hz,
3H), 3.95 (q, J = 6.8 Hz, 2H), 6.86–6.89 (m, 2H), 6.93 (t, J = 7.6 Hz,
1H), 7.09–7.16 (m, 2H), 7.34–7.44 (m, 4H), 7.54 (s, 1H), 7.63 (d,
J = 7.6 Hz, 2H), 7.80 (dd, J = 8.4, 1.6 Hz, 1H); 13C NMR (CDCl3,
100 MHz) d: 146.71, 143.58, 140.81, 134.79, 130.03, 129.16,
129.05, 128.80, 128.25, 128.06, 127.95, 127.55, 127.43, 125.79,
124.25, 123.08, 118.49, 117.94, 115.30, 114.76, 108.57, 42.18,
12.88; IR (NaCl): 3061, 3031, 2981, 2935, 2208, 1590, 1572,
1497, 1467, 1406, 1368, 1330, 1286, 1251, 1206, 1137, 1111; MS
(ESI): m/z 355.0 [M+H]+.

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M. Lu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 128–134
42.54, 12.72; IR (KBr): 3127, 2926, 2208, 1574, 1473, 1404, 1369,
(1 Â 10À5 M) at room temperature were shown in Fig. 1. The UV–
vis absorptions of these compounds have two absorption maxima
(kmax ꢀ 280–310 nm and kmax ꢀ 370–430 nm). The low energy
absorption band which can be assigned to an intramolecular
charge-transfer (ICT) transition has lower absorption coefficient
at kmax than the high energy absorption band for all of these four
compounds, and the absorptions of 7 and 9 which have only one
electron-withdraw group have higher absorption coefficient than
8 and 10, respectively, which have two electron-withdraw groups
(Table S1). On the other hand, 7 and 8 with phenothiazine as the
donor group have lower energy absorption band both of the two
absorption maxima than that of 9 and 10 with carbazole as the do-
nor group, respectively, which could be attributed to the lower
HOMO of phenothiazine than that of carbazole. The emission spec-
tra for 7–10 in dilute dichloromethane (1 Â 10À6 M) at room tem-
perature are displayed in Fig. 2. Overall, the emission spectrum for
7 displays a maximum at 579 nm without significant red-shift
comparing with that of 8 (kmax = 573 nm). This reinforces that both
3 and 7 positions-substituted phenothiazine by electron-withdraw
group imparts a negligible effect on the emission when compared
with only 3 position-subsitituded phenothiazine. However, the
fluorescence spectra for 9 and 10 show nearly identical fluores-
cence emission with a maximum at 436 nm and a shoulder at
411 nm (Table S1), indicating that substitution of carbazole with
withdraw group at 3 and 6 positions causes no significant red-shift.
Nevertheless, it is interesting to note that 7 and 9 which have only
one electron-withdraw group have higher absorption intensities
but lower fluorescence emission intensities than 8 and 10, respec-
tively (Fig. 2).
In order to investigate the solvent effect on photophysical
behaviors of these dyes, we also examined their photophysical
spectra of 7 and 8 in different solvents varying from toluene to
DMSO in sequence of dielectric constants. The UV–vis absorption
and fluorescence emission data of 7–8 were shown in Table S2.
Although the UV–vis absorption bands of 7 and 8 roughly re-
mained the same in all of these solvents, the emission spectra were
significantly red-shifted and weakened in intensity with increasing
solvent polarity (as shown in Figs. 3 and 4). For example, the emis-
sion maximum of 7 in DMSO (598 nm) is about 47 nm red-shifted
from that in toluene (551 nm) (Table S2). Similarly the emission
maxima of 8 gradually red-shifts from toluene (562 nm) to DMSO
(595 nm). Generally, a fluorophore has a larger dipole moment in
the excited state than that in the ground state. After excitation, sol-
vent dipoles can reorient or relax lowering the energy of the ex-
cited state. Therefore, this strong solvent effect can be attributed
to a decrease in the energy of the singlet excited states when the
polarity of the solvent increases. When compared the absorption
spectra with the emission spectra in different solvents, both 7
and 8 showed large Stokes shifts, which are significantly larger
than other simple styryl compounds [31]. For a given donor–accep-
tor compound, the Stokes shift increase was associated with the
conjugation length increase. Compound 7, however, showed a lar-
ger Stokes shift (from 139 to 181 nm) than that of 8 (from 122 to
154 nm) despite its shorter conjugation. To obtain further insight
into the observed positive solvent effect on fluorescence emission,
by plotting the Stokes shifts of 7 and 8 as a function of the solvent
orientation polarizability, the corresponding Mataga–Lippert plots
[32] of 7 and 8 based on different solvents were constructed and
shown in Fig. 5. These plots illustrated the typical behavior of these
compounds in the excited states with the variation of orientation
polarizability of the solvent. The linear relationship suggested that
only one excited state was present upon excitation. The slope of
the Mataga-Lippert plots of both 7 and 8 seen from the Mataga–
Lippert equation suggested that the excited-state dipole moment
was larger than the ground-state dipole moment. However, the
similar slope of the Lippert plots of 7 and 8 indicated that the
1253, 1205, 1176, 1133, 1113; MS (TOF): m/z 481.10 [M+].
Synthesis of 3-[2-phenyl-2-cyanovinyl]-9-ethyl-9H-carbazole 9
9-Ethyl-9H-carbazole-3-carboxaldehyde (0.34 g, 1.5 mmol) 6a,
2-phenylacetonitrile (0.23 g, 2 mmol) and potassium tert-butoxide
(1.22 g, 10 mmol) were dissolved in methanol (50 mL). The mix-
ture was heated to reflux and stirred for 12 h under an atmosphere
of N2. After cooling to room temperature, the mixture was poured
into water (200 mL), neutralized to pH 7 with 1 M HCl aqueous
solution, and then extracted with dichloromethane. The extract
was washed with plenty of brine and dried over Na2SO4, the sol-
vent was removed under reduced pressure. The residue was puri-
fied by column chromatography on silica gel using
dichloromethane/hexane (1:1) as eluent. Yield: 0.38 g (76%) of yel-
low oil. 1H NMR (CDCl3, 400 MHz, ppm) d: 1.42 (t, J = 7.2 Hz, 3H),
4.33 (q, J = 6.8 Hz, 2H), 7.27–7.52 (m, 7H), 7.68–7.70 (m, 3H),
8.09–8.14 (m, 2H), 8.61 (s, 1H); 13C NMR (CDCl3, 100 MHz) d:
143.51, 141.15, 140.56, 135.33, 129.22, 129.02, 128.48, 127.93,
127.19, 126.47, 125.75, 124.84, 123.35, 122.93, 122.54, 120.80,
119.85, 119.17, 108.97, 108.82, 107.36, 37.78, 13.85; IR (NaCl):
3057, 2976, 2932, 2206, 1627, 1586, 1496, 1471, 1448, 1387,
+
1346, 1234, 1159, 1132; MS (ESI): m/z 323.2 [M+H]
.
Synthesis of 3,6-bis[2-phenyl-2-cyanovinyl]-9-ethyl-9H-carbazole 10
9-Ethyl-9H-carbazole-3,6-dicarboxaldehyde (0.15 g, 0.6 mmol)
6b, 2-phenylacetonitrile (0.29 g, 2.5 mmol) and potassium tert-
butoxide (1.22 g, 10 mmol) were dissolved in methanol (50 mL).
The mixture was heated to reflux and stirred for 12 h under an
atmosphere of N2. After cooling to room temperature, yellow solid
precipitate was filtered off, washed with cold methanol, and puri-
fied by column chromatography on silica gel using dichlorometh-
ane/hexane (1:1) as eluent. Yield: 0.17 g (63%) of yellow powder.
1H NMR (CDCl3, 400 MHz, ppm) d: 1.51 (t, J = 7.2 Hz, 3H), 4.43 (q,
J = 7.2 Hz, 2H), 7.37–7.51 (m, 8H), 7.72–7.74 (d, 6H), 8.21–8.23
(d, J = 8.8 Hz, 2H), 8.65 (s, 2H); 13C NMR (CDCl3, 100 MHz) d:
142.99, 141.62, 135.07, 129.04, 128.67, 127.92, 127.52, 125.83,
125.80, 123.27, 122.95, 118.93, 109.37, 108.38, 38.14, 13.93; IR
(KBr): 3126, 2976, 2924, 2206, 1629, 1586, 1490, 1446, 1392,
1348, 1239, 1160, 1130; MS (ESI): m/z 472.2 [M+Na]+.
Results and discussion
Synthesis
The preparation of the target compound was synthesized in three
synthetic steps from commercially available phenotiazine and car-
bazole (Scheme 1). The first step was the alkylation of phenotiazine
and carbazole in the presence of sodium hydroxide followed by the
formylation of 10-ethyl-10H-phenotiazine, 2 and 9-ethyl-9H-car-
bazole 4 using the Vilsmeier-Haack condition (POCl3/DMF) to get
mono- and diformyl compounds 5a–b and 6a–b, respectively. In
the last stage of the synthesis, the four target compounds were pre-
pared by classical Knoevenagel condensation, aldehyde 5a–b, 6a–b
and 2-phenylacetonitrile were refluxed in absolute ethanol in the
presence of potassium tert-butoxide as the strong base under an
atmosphere of N2 to obtain the desired compounds in moderate
yields. All the newly synthesized compounds 7–10 were easily sep-
arated and purified by column chromatography and fully character-
ized and verified by FT-IR, 1H NMR, 13C NMR and Mass spectra.
Photophysical properties
The photophysical properties of these dyes were investigated in
dilute dichloromethane solution. The comparison of UV–vis
absorption spectra of 7–10 in dichloromethane solution

M. Lu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 128–134
133
intramolecular charge-transfer (ICT) features of 7 and 8 were
Conclusions
equally significant.
In conclusion, we have demonstrated an efficient and facile syn-
thesis of cyano-substituted styryl derivatives based on carbazole
and phenothiazine. They were soluble in common organic solvents,
showing broad absorption curves. The solvent effect on photophys-
ical behaviors of these dyes was also investigated, and the results
showed that the maximal emission displayed a large wavelength
shifted and the unusual large Stokes shift increased with the in-
crease of the solvent polarity. DFT calculation results confirmed
these dyes based on phenothiazine had narrower energy gaps than
that of carbazole, and the lowest-energy absorption bands were
mainly due to the HOMO–LUMO one-electron promotion. The
thermogravimetric analysis results showed that they were ther-
mally stable hence these dyes may have various application. Fur-
ther investigations of these dyes on various applications are
currently underway.
Thermal gravimetric analysis
The thermal stability is one of the key requirements for some
practical applications. In order to gain more insight into these dyes,
8 and 10 were subjected to the thermogravimetric analysis to
investigate their thermal stabilities. The thermal stability studies
were performed under nitrogen gas at a heating rate of 10 °C/
min, and the results were shown in Fig. S1. Above 350 °C the ther-
mogravimetric curves of 8 and 10 show a major loss in weight,
with decomposition temperatures at 352, 373 °C for 8 and 10,
respectively. These results confirm that both of them are thermally
stable.
Quantum chemistry calculations
Acknowledgements
To visualize geometries and electronic structures of these four
compounds, all these dyes have been subjected to quantum chem-
ical analysis by density functional theory (DFT). DFT calculations
were performed using the B3LYP method with 6-31G* basis set
implemented in Gaussian 03 program package [33,34]. We opti-
mized the molecular structures of these compounds without sym-
metry constraints. Fig. 6 shows the molecular geometries and
HOMO–LUMO surfaces of all these dyes synthesized in this study.
Generally, the HOMOs are expected to lie on the electron-rich
groups while the LUMOs are localized on the electron-poor groups.
As expected, DFT studies of these dyes clearly indicated that the
HOMOs of compounds 7 and 8 were primarily localized on center
phenothiazine core, and in contrast the LUMOs mostly existed on
the cyano moiety, while the HOMOs of compounds 9 and 10 were
This work was supported by National Natural Science Founda-
tion of China (No. 20671038) and Jiangsu Key Laboratory for the
Chemistry of Low-Dimensional Materials (No. JSKC11098).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.04.090.
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