A comparative study of symmetrical and unsymmetrical trimethine cyanine dyes bearing benzoxazolyl and benzothiazolyl groups

Article abstract of DOI:10.1016/j.dyepig.2011.11.001

Six symmetrical and unsymmetrical trimethine cyanine dyes bearing benzothiazolyl and benzoxazolyl groups were synthesized. Their linear optical properties were studied. It was found that the variation of hetero atoms play an important role in the spectral properties of the dyes. Sequential replacement of the oxygen atom by the sulfur atom resulted in a change in shade from yellow to purple. In order to understand the relationship between the molecular structures and the spectral properties of these dyes, theoretical calculations were made using the Gaussian programme. Key parameters related to absorption and emission spectra were reported and discussed.

Full text of DOI:10.1016/j.dyepig.2011.11.001

Dyes and Pigments 93 (2012) 1506e1511
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A comparative study of symmetrical and unsymmetrical trimethine cyanine dyes
bearing benzoxazolyl and benzothiazolyl groups
,
b
b
a,b,
Quan-Quan Shi a, Ru Sun a, Jian-Feng Ge a , Qing-Feng Xu , Na-Jun Li , Jian-Mei Lu
*
*
a Key Laboratory of Organic Synthesis of Jiangsu Province, Soochow University, Suzhou 215123, China
b Key Laboratory of Functional Materials for Environment Protection of Suzhou city, Soochow University, Suzhou 215123, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Six symmetrical and unsymmetrical trimethine cyanine dyes bearing benzothiazolyl and benzoxazolyl
groups were synthesized. Their linear optical properties were studied. It was found that the variation of
hetero atoms play an important role in the spectral properties of the dyes. Sequential replacement of the
oxygen atom by the sulfur atom resulted in a change in shade from yellow to purple. In order to
understand the relationship between the molecular structures and the spectral properties of these dyes,
theoretical calculations were made using the Gaussian programme. Key parameters related to absorption
and emission spectra were reported and discussed.
Received 29 July 2011
Received in revised form
7 November 2011
Accepted 7 November 2011
Available online 13 November 2011
Keywords:
Cyanine dye
Ó 2011 Elsevier Ltd. All rights reserved.
Trimethine cyanine
Spectral property
DFT calculation
Benzoxazolyl
Benzothiazolyl
1. Introduction
dyes [15]. In this study, the synthesis and comparative study of
symmetrical and unsymmetrical trimethine cyanine dyes bearing
Cyanine dyes have attracted interest as functional dyes in many
fields. The extensively conjugated structures generate changeable
absorption, high extinction coefficients and considerable quantum
yields [1]. The applications of cyanine dyes and derivatives were
mainly focused on the material and biological applications. In
materials field, they may serve as non-linear optical (NLO) mate-
rials [2], data storage materials [3] and organic semiconductor
materials [4]. In the biological arena, cyanine dyes have been used
as probes for the detection of metal ions [5], DNA [6e8] and
proteins [9]. In addition, their derivatives were also explored for
anti-MRSA [10] and anti-protozoal [11] activity.
Much research on the spectral properties of cyanine dyes
explored the influence of pH values [12], substituent groups [13],
isomerism phenomenon [14] and the number of the methines. It is
noted that benzothiazolyl and indolin-2-yl groups were the most
common heterocyclic end groups in the studies of cyanine dyes.
However, little research has explored the benzoxazolyl cyanine
benzoxazolyl and benzothiazolyl groups are discussed.
2. Experimental
2.1. General information
All the starting materials were purchased from either TCI or
Sinopharm Chemical Reagent Co, Ltd. All analytic grade solvents
(A.R.) were obtained from commercial suppliers and used directly.
Melting points were determined on X-4 microscope electron
thermal apparatus (Taike, China) without correction. Absorption
spectra were taken on U-3900 UVeVis spectrophotometer at room
temperature. Fluorescence emission spectra were measured on
Shimadzu RF-5300PC Spectroscope at room temperature. Quantum
yields were determined using fluorescein in 0.1 M NaOH or cresyl
violet in methanol as the reference. 1H NMR spectra were recorded
on either a Varian-300 or 400 NMR spectrometer, and TMS was
used as an internal standard. Mass spectra were recorded on Fin-
nigan MAT95 mass spectrometer (ESIþ). Elementary analysis was
conducted on a Carlo Erba-MOD1106 elementary analysis appa-
ratus. Differential scanning calorimetry (DSC) and thermogravi-
metric analysis (TGA) were conducted on a Universal V3.7A TA
instrument in flowing N2 with a heating rate of 10 ꢀC/min.
* Corresponding authors. 199 Ren’ai Road, Suzhou Industrial Park, College of
Chemistry, Chemical Engineering and Material Science, Soochow University,
Suzhou 215123, China. Tel.: þ86 (0) 512 6588 4717; fax: þ86 (0) 512 6588 0367.
E-mail addresses: ge_jianfeng@hotmail.com (J.-F. Ge), lujm@suda.edu.cn
(J.-M. Lu).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.11.001

Q.-Q. Shi et al. / Dyes and Pigments 93 (2012) 1506e1511
1507
2.2. Synthesis
C19H17ClN2O2$2.5H2O: C, 59.14; H, 5.75; N, 7.26. found: C, 58.87; H,
5.55; N, 7.20.
2.2.1. 2,3-Dimethylbenzoxazol-3-ium iodide (1a)
A solution of 2-methylbenzoxazole (2.66 g, 20.0 mmol) and
iodomethane (11.28 g, 80.0 mmol) in DMF (10.0 mL) was heated
at 60 ꢀC for 24 h. The solution was cooled to room temperature,
and then ethyl acetate (100.0 mL) was added slowly. The
precipitated solid was collected by filtration and washed with
ethyl acetate. The crude product was collected and heated
under reflux with ethyl acetate (100.0 mL) for 15 min. The
suspension was cooled to room temperature. The product was
collected by filtration and washed with ethyl acetate to afford
1a as bright yellow solid. Yield: 80.5%. 1H NMR (400 MHz,
DMSO-d6, TMS) dH: 8.16e8.11 (m, 2H), 7.79e7.77 (m, 2H), 4.08
(s, 3H), 3.07 (s, 3H).
2.2.7. 3-Methyl-2-(3-(3-methylbenzothiazol-2(3H)-ylidene)prop-
1-en-1-yl)benzoxazol-3-ium chloride (3b)
To a mixture of 2a (0.10 g, 0.25 mmol) and 1c (0.072 g,
0.25 mmol) in ethanol (6 mL), Et3N (0.1 mL) was added in portions,
and then the mixture was heated under reflux for 1 h. The reaction
was cooled to room temperature; ethyl ether (10 mL) was added
slowly during 0.5 h. The solid was filtrated, washed with ethyl ether
(3 ꢁ 10.0 mL) and water (3 ꢁ 10.0 mL) to afford the iodide salt. Then
the iodide salt was dissolved in methanol. The methanol solution of
the iodide salt was passed through the anion-exchange resin
(Amberlite IRA-400, chloride form), and the resin was washed with
methanol. After the concentration of the eluent chloride salt, the
product was collected to give 3b as dark red solid. Yield: 59.1%.
2.2.2. 2,3,5-Trimethylbenzoxazol-3-ium iodide (1b)
Compound 1b was synthesized by the similar procedure of 1a.
Yield: 97.7%. 1H NMR (400 MHz, DMSO-d6, TMS) dH: 8.01 (d,
J ¼ 8.6 Hz, 1H), 7.93 (s, 1H), 7.59 (d, J ¼ 8.6 Hz, 1H), 4.03 (s, 3H), 3.04
(s, 3H), 2.54 (s, 3H).
mp.210.0e210.9 ꢀC 1H NMR (300 MHz, DMSO-d6, TMS) dH
:
8.05e7.93 (m, 2H), 7.81e7.65 (m, 3H), 7.60e7.35 (m, 4H), 6.50 (d,
J ¼ 12.4 Hz, 1H), 6.08 (d, J ¼ 12.8 Hz, 1H), 3.82 (s, 3H), 3.74 (s, 3H).
ESIþ m/z: calcd for C19H17N2OSþ: 321.1056, found: 321.1061. Anal.
Calcd. for C19H17ClN2OS$4H2O: C, 53.20; H, 5.87; N, 6.53. found: C,
53.37; H, 5.45; N, 6.54.
2.2.3. 2,3-Dimethylbenzothiazol-3-ium iodide (1c)
Compound 1c was synthesized by the similar procedure of 1a.
Yield: 98.4%. 1H NMR (400 MHz, DMSO-d6, TMS) dH: 8.46 (d,
J ¼ 8.1 Hz, 1H), 8.30 (d, J ¼ 8.5 Hz, 1H), 7.91e7.87 (m, 1H), 7.82e7.77
(m, 1H), 4.21 (s, 3H), 3.19 (s, 3H).
2.2.8. 3-Methyl-2-(3-(3-methylbenzothiazol-2(3H)-ylidene)prop-
1-en-1-yl)benzothiazol-3-ium chloride(3c)
Dye 3c was synthesized by condensing 1c and triethoxy-
methane by a similar procedure described for dye 3a. Yield: 41.7%.
mp. 217.0e218.0 ꢀC 1H NMR (300 MHz, DMSO-d6, TMS) dH: 7.99 (d,
J ¼ 7.7 Hz, 2H), 7.78e7.66 (m, 3H), 7.56 (t, J ¼ 7.5 Hz, 2H), 7.39 (t,
J ¼ 7.4 Hz, 2H), 6.54 (d, J ¼ 12.5 Hz, 2H), 3.82 (s, 6H). ESIþ m/z: calcd
for C19H17N2Sþ2 : 337.0828, found: 337.0821. Anal. Calcd. for
C19H17ClN2S2$4H2O: C, 51.28; H, 5.66; N, 6.30. found: C, 50.94; H,
5.20; N, 6.25.
2.2.4. 2,3,5-Trimethylbenzothiazol-3-ium iodide (1d)
Compound 1d was synthesized by the similar procedure of 1a.
Yield: 83.6%. 1H NMR (400 MHz, DMSO-d6, TMS) dH: 8.30 (d,
J ¼ 8.4 Hz, 1H), 8.14 (s, 1H), 7.64 (d, J ¼ 8.4 Hz, 1H), 4.16 (s, 3H), 3.15
(s, 3H), 2.57 (s, 3H).
2.2.5. 3-Methyl-2-(2-(N-phenylacetamido)vinyl)benzoxazol-3-ium
iodide (2a)
2.2.9. 2-(3-(3,5-Dimethylbenzoxazol-2(3H)-ylidene)prop-1-en-1-
yl)-3-methylbenzoxazol-3-ium chloride (3d)
A suspension of 1a (1.58 g, 5.7 mmol) and N, N0-diphe-
nylformamidine (1.36 g, 6.9 mmol) in acetic anhydride (10.0 mL)
was heated under reflux for 2 h. The reaction was cooled to room
temperature; ethyl ether (30.0 mL) was added slowly over more
than 0.5 h. The product was collected by filtration, then washed
with ethyl ether (3 ꢁ 15.0 mL) and water (3 ꢁ 5.0 mL) to afford 2a as
a yellow solid. Yield: 46.1%. mp. 173e174 ꢀC 1H NMR (400 MHz,
DMSO-d6, TMS) dH: 9.06 (d, J ¼ 13.8 Hz, 1H), 8.06 (d, J ¼ 7.4 Hz, 1H),
7.94 (d, J ¼ 7.4 Hz, 1H), 7.69e7.65 (m, 5H), 7.53 (d, J ¼ 7.5 Hz, 2H),
5.46 (d, J ¼ 13.7 Hz, 1H), 3.80 (s, 3H), 2.06 (s, 3H). ESIþ m/z: calcd for
C18H17N2Oþ2 : 293.1285, found: 293.1276.
Dye 3d was synthesized by condensing 2a and 1b by a similar
procedure described for dye 3b. Yield: 38.1%. mp. 213.9e214.1 ꢀC 1H
NMR (300 MHz, DMSO-d6, TMS) dH: 8.25 (t, J ¼ 13.1 Hz, 1H), 7.74 (d,
J ¼ 6.9 Hz, 1H), 7.65e7.61 (m, 2H), 7.54e7.35 (m, 3H), 7.20 (d,
J ¼ 7.2 Hz,1H), 6.10e5.92 (m, 2H), 3.71 (s, 6H), 2.43 (s, 3H). ESIþ m/z:
calcd for C20H19N2Oþ2 : 319.1447, found: 319.1436. Anal. Calcd. for
C20H19ClN2O2$2.5H2O: C, 60.07; H, 6.05; N, 7.01. found: C, 60.24; H,
5.56; N, 6.98.
2.2.10. 2-(3-(3,5-Dimethylbenzothiazol-2(3H)-ylidene)prop-1-en-
1-yl)-3-methylbenzoxazol-3-ium chloride (3e)
Dye 3e was synthesized by condensing 2a and 1d by a similar
procedure described for dye 3b. Yield: 91.7%. mp. 203.7e204.1 ꢀC
1H NMR (300 MHz, DMSO-d6, TMS) dH: 8.05e7.89 (m, 1H), 7.83 (d,
J ¼ 7.5 Hz, 1H), 7.76e7.72 (m, 1H), 7.67e7.65 (m, 1H), 7.58 (s, 1H),
7.52e7.33 (m, 2H), 7.22e7.20 (m, 1H), 6.49 (d, J ¼ 12.7 Hz, 1H), 6.03
(d, J ¼ 13.0 Hz, 1H), 3.80 (s, 3H), 3.72 (s, 3H), 2.43 (s, 3H). ESIþ m/z:
calcd for C20H19N2OSþ: 335.1218, found: 335.1212. Anal. Calcd. for
C20H19ClN2OS$4H2O: C, 54.23; H, 6.14; N, 6.32. found: C, 53.79; H,
5.68; N, 6.08.
2.2.6. 3-Methyl-2-(3-(3-methylbenzoxazol-2(3H)-ylidene)prop-1-
en-1-yl)benzoxazol-3-ium chloride (3a)
To a solution of 1a (3.56 g, 13.0 mmol) in acetic anhydride
(10.0 mL) under reflux, triethoxymethane (1.92 g, 13.0 mmol) was
added dropwise during 45 min under nitrogen. The reaction was
heated under reflux for 1 h. After it was cooled to room tempera-
ture, the residue was collected by filtration, washed by ethyl acetate
(3 ꢁ 15.0 mL) and water (3 ꢁ 15.0 mL). The iodide salt was obtained
by recrystallization in ethanol. Then the iodide salt was dissolved in
methanol. The methanol solution was passed through the anion-
exchange resin (Amberlite IRA-400, chloride form), and the resin
was washed with methanol. After the concentration of the eluent
chloride salt, the product was collected to give 3a as red solid. Yield:
2.2.11. 2-(3-(3,5-Dimethylbenzothiazol-2(3H)-ylidene)prop-1-en-
1-yl)-3-methylbenzothiazol-3-ium chloride (3f)
Dye 3f was synthesized by condensing 2c and 1d by a similar
procedure described for dye 3b. Yield: 72.3%. mp. 210.7e211.2 ꢀC 1H
NMR (300 MHz, DMSO-d6, TMS) dH: 7.97 (d, J ¼ 7.6 Hz, 1H), 7.85 (d,
J ¼ 7.8 Hz, 1H), 7.75e7.67 (m, 2H), 7.58e7.53 (m, 2H), 7.39 (t,
J ¼ 7.4 Hz, 1H), 7.22 (d, J ¼ 7.9 Hz, 1H), 6.51 (t, J ¼ 11.0 Hz, 2H), 3.80
(s, 6H), 2.43 (s, 3H). ESIþ m/z: calcd for C20H19N2Sþ2 : 351.0990,
8.7%. mp. 214.6e215.0 ꢀC 1H NMR (300 MHz, DMSO-d6, TMS) dH
8.29 (t, J ¼ 13.7 Hz, 1H), 7.76 (d, J ¼ 7.5 Hz, 2H), 7.67 (d, J ¼ 7.3 Hz,
2H), 7.53e7.33(m, 4H), 6.03 (d, J ¼ 13.2 Hz, 2H), 3.74 (s, 6H). ESIþ m/
z: calcd for C19H17N2Oþ2 : 305.1285, found: 305.1285. Anal. Calcd. for
:

1508
Q.-Q. Shi et al. / Dyes and Pigments 93 (2012) 1506e1511
found: 351.0977. Anal. Calcd. for C20H19ClN2S2$2.5H2O: C, 54.47; H,
5.71; N, 6.35. found: C, 54.99; H, 5.42; N, 6.23.
3. Results and discussion
3.1. Synthesis
The synthetic strategy used to obtain the dyes 3aef was shown
in Scheme 1. Compounds 1aed were prepared by the reaction of
the requisite starting material and iodomethane in DMF at 60 ꢀC
[16e18]. The intermediates 2a and 2c were synthesized in the
similar way described in references [19,20] with some modifica-
tions. The symmetrical cyanine dyes (3a, 3c) were obtained by the
condensation of 1a and 1c with triethoxymethane in acetic anhy-
dride, respectively. The unsymmetrical cyanines were obtained by
the reaction of compounds 1 and 2, which followed the last step in
the preparation of rhodacyanine [11]. To improve the solubility of
the final products, the chloride salts were obtained from the cor-
responding iodide salts using ion-exchange technology. The TGA
analyses were shown in Fig. S1 (Supplementary data). The mole-
cules of water of crystallization associated with the dyes were lost
during heating between 50 and 150 ꢀC and the dyes were decom-
posed around 250 ꢀC.
Fig. 1. UV/visible absorption spectra of dyes 3aef at the concentration of
1.0 ꢁ 10ꢂ5 mol Lꢂ1 In DMSO solution.
Stokes’ Shifts of these dyes are 20e22 nm. Dyes 3a and 3d possess
moderate quantum yields (0.35, 0.34), comparing with dyes 3c (0.15)
and 3f (0.17).
3.2. Spectral properties of dyes 3aef
UVevis and fluorescence spectra of dyes 3aef in DMSO solution
are presented in Figs. 1 and 2, respectively. The spectral charac-
teristics including absorption and emission maxima (lmax), excita-
3.3. Theoretical calculations
3.3.1. Computational details
tion wavelength (lex), extinction coefficients (
3
)
All the calculations in this work were performed with the
Gaussian98 program package [21]. The geometric and electronic
structures of molecules were investigated with the DFT method. In
each optimization, the vibrational frequencies were calculated and
the results showed that all optimized structures were stable
geometric structures. The lowest singlet excited states were
computed with ab initio CIS/6-31G(d) on the basis of the optimized
geometries obtained from HF/6-31G(d) calculations. The transition
energies were calculated at the ground-state and excited-state
geometries using TD-DFT calculations. The maximum absorption
and emission wavelengths of these compounds were studied by
employing TD-DFT methods. The transition energies were
and fluorescence quantum yields ( ) are given in Table 1.
F
As shown in Fig. 1, the absorption spectra of dyes 3aef show
typical absorption bands of cyanine dyes: an intense dominant
absorption band with one shoulder band. Comparing 3a, 3b with 3c,
the maximum absorption bands gradually increase (3a < 3b < 3c).
The same situation is also found in 3d, 3e and 3f (3d < 3e < 3f). The
results indicate that the hetero atoms play an important role in the
absorption spectra. There is a 20 nm red-shift with the variation of
the hetero atoms from oxygen to sulfur. The methyl group on the
benzene ring induces a slight red-shift (about 3 nm). The fluores-
cence spectra of dyes 3a-f display the same phenomenon. The
Scheme 1. The synthetic route to dyes 3aef.

Q.-Q. Shi et al. / Dyes and Pigments 93 (2012) 1506e1511
1509
Table 2
Selected bond lengths (R, Å), bond angles (D, ꢀ) and dihedral angles (Di, ꢀ) of
3aDefD
.
3aD
3bDa
3cD
3dD
1.382
3eD
1.388
1.823
1.822
1.399
1.366
1.415
1.385
1.417
1.367
3fD
1.391
R(C1eC2)
R(C2eX1)
R(C4eX1)
R(C4eC5)
R(C4eN1)
R(C3eN1)
R(C7eX2)
R(C9eX2)
R(C7eN2)
1.381
1.417
1.384
1.391
1.367
1.414
e
1.390
1.825
1.818
1.397
1.366
1.416
1.384
1.416
1.367
e
1.390
1.824
1.818
1.397
1.366
1.416
e
1.417
1.383
1.392
1.367
1.415
1.385
1.416
1.367
1.512
107.36
1.823
1.817
1.398
1.365
1.417
1.818
1.824
1.366
1.511
89.35
115.88
89.38
115.91
179.69
179.04
e
e
e
e
Fig. 2. Fluorescence emission spectra of dyes 3aef at the concentration of
1.0 ꢁ 10ꢂ5 mol Lꢂ1 in DMSO solution.
R(R1eC10
)
e
e
1.513
89.58
D(C2eX1eC4)
D(C3eN1eC4)
D(C7eX2eC9)
D(C7eN2eC8)
107.40 89.38
109.16 115.90 115.89 109.15
89.39
115.93
107.38
109.17
179.88
179.19
e
107.43
109.16
e
107.40
109.17
calculated at the TD-DFT/B3LYP level of approximation by using the
ground (excited) state HF/6-31G(d) (CIS/6-31G(d)) geometries,
which are considered to precisely predict the absorption and
fluorescence spectra of these systems [22e25]. The polarizable
continuum model (PCM) was used in calculation of the absorption
and emission spectra in DMSO solution. The singletetriplet gaps
were calculated at the optimized geometry of the first excited
singlet. All DFT calculations were performed using the B3LYP
functional and the 6-31G (d) basis set.
e
e
Di(C1eC2eX1eC4) 179.96 179.68 179.75 179.91
Di(C5eC4eX1eC2) 179.70 179.02 179.17 179.35
a
For 3bD, X1 is sulfur atom.
the range from 0.411 to 0.414, while the NBO charges of the oxygen
atoms are from ꢂ0.461 to ꢂ0.416. The charges of carbon atoms
connected to sulfur atoms are from ꢂ0.193 to 0.126. However, the
charges are from 0.283 to 0.612 when they connected to oxygen
atoms. Compared with the oxygen atom the sulfur atom has
a stronger electronic donation ability.
3.3.2. Geometry optimization and NBO charge distribution
Some selected bond lengths, bond angles and dihedral angles
are shown in Table 2. The optimized structures of the cation parts
for dyes 3aef at the ground state are represented in Fig. S2
(Supplementary data). As shown in Table 2, the bond angles of
CeSeC are in the range from 89.35ꢀ to 89.58ꢀ, however, the bond
angles of CeOeC are from 107.36ꢀ to 107.43ꢀ. The bond lengths of
CeS are in the range from 1.817 Å to 1.825 Å, while the bond lengths
of CeO are from 1.383 Å to 1.417 Å. The calculated bond lengths and
bond angles suggest that the hetero atoms have a great influence on
the molecular structures of dyes 3a-f. From Table 2 and the opti-
mized structures of dyes 3aDefD (supplementary data), it can be
3.3.3. Absorption spectra
The key parameters related to absorption spectra are listed in
Table 4. As shown inTable 4, the excitation energies from dyes 3aD to
3cD and from dyes 3dD to 3fD decrease, meanwhile, the maximum
absorption bands increase. The dominant transitions of dyes 3aDefD
are HOMO to LUMO transitions (Table S1, supplementary data). The
frontier molecular orbitals of 3aDefD are shown in Fig. S3
(supplementary data), it can be seen that the HOMO of dyes
3aDefD are mainly located on the hetero rings and the carbon atoms
connected to the hetero rings. Compared with an oxygen atom, the
sulfur atom has a noticeable contribution to its HOMO component.
Meanwhile the LUMO are mainly located on the methine chain and
the sulfur atom also has a discernible contribution to its LUMO
seen that dyes 3aDefD are
p-conjugated planar compounds. The
dihedral angles of C1eC2eOeC4, C1eC2eSeC4, C5eC4eOeC2 and
C5eC4eSeC2 are in the range from 179.02ꢀ to 179.96ꢀ, which
indicate that the hetero atoms do not distort the planarity.
The NBO charge distributions are shown in Table 3, from these
data, it can be seen that the NBO charges of the sulfur atoms are in
Table 3
NBO charge distribution of 3aDefD
.
Table 1
3aD
3bDa
3cD
3dD
3eD
3fD
Absorption and emission maxima (lmax), excitation wavelength (lex), extinction
coefficients (
3
), Stokes’ shift (Dnst) and quantum yields (
F
) of dyes 3aef.
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
N1
N2
X1
X2
ꢂ0.256
0.291
0.115
0.611
ꢂ0.404
ꢂ0.102
e
ꢂ0.227
ꢂ0.187
0.143
ꢂ0.227
ꢂ0.186
0.143
0.125
ꢂ0.368
ꢂ0.112
e
ꢂ0.252
0.283
0.121
ꢂ0.221
ꢂ0.193
0.150
ꢂ0.221
ꢂ0.193
ꢂ0.150
0.122
ꢂ0.367
ꢂ0.114
0.122
Dyes
lmax(Abs)
lmax(Em)a
(nm)
lg
3
Dnst
F
(nm)
(L molꢂ1 cmꢂ1
)
(nm)
0.126
0.612
0.125
3a
3b
3c
3d
3e
3f
490
526
563
493
530
566
512
546
585
515
551
588
5.06
4.84
4.95
5.20
5.02
5.00
22
20
22
22
21
22
0.35b
0.26b
0.15c
0.34b
0.20b
0.17c
ꢂ0.375
ꢂ0.107
0.609
0.115
0.290
ꢂ0.404
ꢂ0.104
0.610
0.115
0.289
ꢂ0.008
ꢂ0.382
ꢂ0.384
ꢂ0.458
ꢂ0.460
ꢂ0.374
ꢂ0.109
0.609
0.115
0.290
ꢂ0.002
ꢂ0.363
ꢂ0.383
0.414
e
e
0.144
e
e
ꢂ0.186
ꢂ0.001
ꢂ0.362
ꢂ0.364
0.414
e
e
e
ꢂ0.390
e
ꢂ0.364
ꢂ0.381
0.412
ꢂ0.459
ꢂ0.362
e
a
The results were obtained by the excitation on the absorption maximum.
The fluorescence quantum yields (F) were measured in DMSO solution using
b
ꢂ0.461
e
0.411
e
fluorescein in 0.1 mol Lꢂ1NaOH as the reference.
ꢂ0.416
0.412
c
The fluorescence quantum yields (F) were measured in DMSO solution using
a
cresyl violet in methanol as the reference.
For 3bD, X1 is sulfur atom.

1510
Q.-Q. Shi et al. / Dyes and Pigments 93 (2012) 1506e1511
Table 4
Table 6
Absorption spectra were obtained using TD-DFT methods for dyes 3aDefD at the
Singletetriplet gaps of 3aDefD
(DEST).
B3LYP/6-31G(d) Optimized Geometries.
3aD
3bD
3cD
3dD
3eD
3fD
abs
electronic
transitions
expa
(nm)
l
f
excitation
energies (eV)
D
EST (cmꢂ1
)
Gas phase
DMSO
9347
7907
9239
7817
9142
7687
9177
7822
9056
7728
8994
7615
max
(nm)
3aD
3bD
3cD
3dD
3eD
3fD
gas phase
DMSO
gas phase
DMSO
gas phase
DMSO
gas phase
DMSO
gas phase
DMSO
gas phase
DMSO
S0 / S1
S0 / S1
S0 / S1
S0 / S1
S0 / S1
S0 / S1
S0 / S1
S0 / S1
S0 / S1
S0 / S1
S0 / S1
S0 / S1
409
434
431
458
455
485
409
435
435
461
460
488
1.57
1.75
1.47
1.66
1.39
1.58
1.58
1.79
1.48
1.64
1.40
1.59
3.02
2.85
2.87
2.70
2.72
2.55
3.02
2.84
2.84
2.69
2.69
2.53
490
526
563
493
530
566
intersystem crossing (ISC) processes and increase the rate of the
overall non-radiative processes, which are in agreement with the
experimental results.
4. Conclusion
Six symmetrical and unsymmetrical trimethine cyanine dyes
3aef were synthesized. Their linear optical properties were
investigated experimentally and theoretically. The experimental
results showed that their absorption bands and fluorescence bands
were shifted by about 20 nm when one oxygen atom was replaced
by one sulfur atom. The NBO charge distribution indicated that the
sulfur atoms possess stronger electronic donation ability. Theoret-
ical results showed that their excited energies decrease in the order
3a, 3b to 3c and 3d, 3e to 3f. The reason for the lower excited
energies is the presence of lower lying unoccupied states in the case
of the sulfur atoms, in comparison to oxygen atoms. All the domi-
nant transitions are HOMO to LUMO transitions, and the maximum
absorption bands shift both in the gas phase and in DMSO solution.
The simulated spectra results correlate closely with the experi-
mental results.
a
Measured in DMSO solution.
component. The simulated absorption spectrum in the gas phase and
in DMSO solution is shown in Fig. S4 (supplementary data). The
simulated results correlate closely with the experimental results.
3.3.4. Emission spectra
The key parameters related to fluorescence emission spectra are
listed in Table 5. From Table 5, it can be seen that the trend of the
calculated emission spectra data fits that of the experimental
results, although there are discrepancies between theoretical
values and experimental values. There are several reasons for the
discrepancies. Firstly, the currently available calculated methods
are approximate methods; the improved methods are still under
development. Secondly, the treatment of the solvent is also a limi-
tation and calculations of the specific interactions with solvent
molecules are not taken into account. As many researchers have
reported [26,27], solvent has a great influence on the fluorescence
emission spectra.
Acknowledgments
We thank the financial support from National Natural Science
Foundation of China (20876101, 21071105, 20902065), Natural
Science Fund (BK2009113) in Jiangsu Province.
The results show that the fluorescence quantum yields decrease
from dyes 3a to 3c and from 3d to 3f (Table 1). The possible
explanation is a low singletetriplet gap, which increases the
probability of the non-radiative processes and a strong spineorbit
Appendix. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.dyepig.2011.11.001.
coupling, duing to the presence of the sulfur atom in the
p-elec-
tronic system. According to the literature [28], the sulfur atom in an
aromatic system such as dibenzothiophene increases the ratio of
phosphorescence/fluorescence quantum yields, due to spineorbit
coupling and electronic effects. The calculated values for the sin-
gletetriplet splitting are listed in Table 6. A decrease of single-
tetriplet gap is found from 3aD to 3cD and 3dD to 3fD. The
calculated results in Table 6 predict a higher probability of the
References
[1] Peng X, Song F, Lu E, Wang Y, Zhou W, Fan J, et al. Heptamethine cyanine dyes
with a large stokes shift and strong fluorescence: a paradigm for excited-state
intramolecular charge transfer. J Am Chem Soc 2005;127:4170e1.
[2] Hales JM, Matichak J, Barlow S, Ohira S, Yesudas K, Brédas J-L, et al. Design of
polymethine dyes with large third-order optical nonlinearities and loss figures
of merit. Science 2010;327:1485e8.
[3] Mustroph H, Stollenwerk M, Bressau V. Current developments in optical data
storage with organic dyes. Angew Chem Int Edn 2006;45:2016e35.
[4] Spitler MT, Parkinson BA. Dye sensitization of single crystal semiconductor
electrodes. Acc Chem Res 2009;42:2017e29.
[5] Nolan EM, Lippard SJ. Tools and tactics for the optical detection of mercuric
ion. Chem Rev 2008;108:3443e80.
[6] Levitus M, Ranjit S. Cyanine dyes in biophysical research: the photophysics of
polymethine fluorescent dyes in biomolecular environments. Q Rev Biophys
2011;44:123e51.
[7] Flors C. DNA and chromatin imaging with super-resolution fluorescence
microscopy based on single-molecule localization. Biopolymers 2011;95:
290e7.
[8] Hannah KC, Armitage BA. DNA-templated assembly of helical cyanine dye
aggregates: A supramolecular chain polymerization. Acc Chem Res 2004;37:
845e53.
[9] Volkova KD, Kovalska VB, Balanda AO, Vermeij RJ, Subramaniam V,
Slominskii YL, et al. Cyanine dye-protein interactions: looking for fluorescent
probes for amyloid structures. J Biochem Bioph Meth 2007;70:727e33.
[10] Wainwright M, Kristiansen JE. Quinoline and cyanine dyes e putative anti-
MRSA drugs. Int J Antimicrob Ag 2003;22:479e86.
[11] Yang M, Arai C, Bakar Md A, Lu J, Ge J-F, Pudhom K, et al. Fluorinated rho-
dacyanine (SLJ-01) possessing high efficacy for visceral leishmaniasis (VL).
J Med Chem 2010;53:368e73.
Table 5
Calculated Emission Data in the gas phase and in DMSO solution for dyes 3aDefD
.
em
electronic
transitions
expa
(nm)
l
f
excitation
energies (eV)
max
(nm)
3aD
3bD
3cD
3dD
3eD
3fD
gas phase
DMSO
gas phase
DMSO
gas phase
DMSO
gas phase
DMSO
gas phase
DMSO
gas phase
DMSO
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
411
436
436
466
461
493
416
439
443
468
467
497
1.58
1.75
1.46
1.64
1.40
1.57
1.60
1.77
1.43
1.63
1.40
1.58
3.01
2.84
2.84
2.66
2.68
2.51
2.97
2.82
2.79
2.64
2.65
2.49
512
546
585
515
551
588
a
Measured in DMSO solution.

Q.-Q. Shi et al. / Dyes and Pigments 93 (2012) 1506e1511
1511
[12] Mazières MR, Duprat C, Wolf JG, Roshal AD. Ph dependent spectral properties
and electronic structure of benzothiazol containing cyanine dyes. Dyes Pigm
2009;80:355e60.
[13] Mitekura H, No T, Suzuki K, Satake K, Kimura M. Spectroscopic properties of
meso-substituted cyanine dyes: evidences for intramolecular charge transfer
from a julolidine moiety as a meso-substituent to the cyanine chromophore.
Dyes Pigm 2002;54:113e20.
[21] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al.
Gaussian 98 revision A.5. Pittsburgh, PA: Gaussian, Inc.; 2003.
[22] Zhang X-H, Wang L-Y, Zhai G-H, Wen Z-Y, Zhang Z-X. The absorption, emis-
sion spectra as well as ground and excited states calculations of some
dimethine cyanine dyes. J Mol Struc THEOCHEM 2009;906:50e5.
[23] Meguellati K, Ladame S, Spichty M.
A conceptually improved TD-DFT
approach for predicting the maximum absorption wavelength of cyanine
dyes. Dyes Pigm 2011;90:114e8.
[14] West W, Pearce S, Grum F. Stereoisomerism in cyanine dyes
e meso-
substituted thiacarbocyanines. J Phys Chem 1967;71:1316e26.
[15] Mishra A, Behera R, Behera P, Mishra B, Behera G. Cyanines during the 1990s:
a review. Chem Rev 2000;100:1973e2012.
[16] Deligeorgiev T, Kaloyanova S, Vasilev A. A novel general method for prepa-
ration of neutral monomethine cyanine dyes. Dyes Pigm 2011;90:170e6.
[17] Kabatc J, Paczkowski J. The photophysical and photochemical properties of the
oxacarbocyanine and thiacarbocyanine dyes. Dyes Pigm 2004;61:1e16.
[18] Peng Z-H, Zhou X-F, Carroll S, Geise HJ, Peng B-x, Dommisse R, et al. Structure
of rhodanine cyanine dyes, spectroscopy and performance in photographic
emulsions. J Mater Chem 1996;6:1325e33.
[24] Champagne B, Guillaume M, Zutterman F. TDDFT investigation of the optical
properties of cyanine dyes. Chem Phys Lett 2006;425:105e9.
[25] Guillaume M, Liégeois V, Champagne B, Zutterman F. Time-dependent density
functional theory investigation of the absorption and emission spectra of
a cyanine dye. Chem Phys Lett 2007;446:165e9.
[26] Tomasi J, Mennucci B, Cammi R. Quantum mechanical continuum solvation
models. Chem Rev 2005;105:2999e3094.
[27] Fdez Galván I, Elena Martín M, Muñoz-Losa A, Aguilar MA. Solvatochromic
shifts on absorption and fluorescence bands of N, N-dimethylaniline. J Chem
Theory Comput 2009;5:341e9.
[19] Carreon JR, Stewart KM, Mahon Jr KP, Shin S, Kelley SO. Cyanine dye conju-
gates as probes for live cell imaging. Bioorg Med Chem Lett 2007;17:5182e5.
[20] Jung ME, Kim W-J. Practical syntheses of dyes for difference gel electropho-
resis. Bioorgan Med Chem 2006;14:92e7.
[28] Ionescu S, Popovici D, Balaban AT, Hillebrand M. 2-phenoxathiinyl-5-
phenyloxazole and 5-phenoxathiinyl-2-phenyloxazole derivatives: experi-
mental and theoretical study of emission properties. Spectrochim Acta A
2007;66:1165e70.