Efficiently tuning the absorption and fluorescence spectroscopy of the novel branched p-nitro-stilbene derivatives with chemical strategy

Article abstract of DOI:10.1007/s10895-009-0564-x

Suitable chemical strategy is a useful approach on the tuning color and photoluminescence of organic dyes. This paper presented tuning novel branched p-nitro-stilbene derivatives efficiently with a new chemical strategy through variation of chemical bridg

Full text of DOI:10.1007/s10895-009-0564-x

J Fluoresc (2010) 20:353–364
DOI 10.1007/s10895-009-0564-x
ORIGINAL PAPER
Efficiently Tuning the Absorption and Fluorescence
Spectroscopy of the Novel Branched p-Nitro-stilbene
Derivatives with Chemical Strategy
Fang Gao & Liufeng Yang & Long Yang & Hongru Li &
Shengtao Zhang
Received: 15 September 2009 /Accepted: 3 November 2009 /Published online: 2 December 2009
#
Springer Science+Business Media, LLC 2009
Abstract Suitable chemical strategy is a useful approach
on the tuning color and photoluminescence of organic dyes.
This paper presented tuning novel branched p-nitro-stilbene
derivatives efficiently with a new chemical strategy through
variation of chemical bridged bond. Linking bonds played
significant effects on the absorption and fluorescence
spectroscopy of the branched p-nitro-stilbene derivatives.
A change from “D-π-A” to “A-π-A” chemical structural
characteristics occurred for the branched p-nitro-stilbene
derivatives as ester bond was attached. This led to not only
large hypsochromic shift of the maximal absorption
wavelength of the branched p-nitro-stilbene derivatives,
but considerable reduction of the fluorescence intensity.
While in contrast, the branched p-nitro-stilbene derivatives
with ether bond exhibited longer wavelength absorption
and much stronger fluorescence emission in modest polar
solvent. The cyclic voltammograms of these branched
p-nitro-stilbene derivatives were determined. Different
electrochemistry processes were observed for the branched
p-nitro-stilbene derivatives with various linking bonds. The
energies of frontier orbital of the branched p-nitro-stilbene
derivatives were estimated from their corresponding redox
potentials. Molecular geometry optimization of the
branched p-nitro-stilbene derivatives was performed, and
the electron density distribution of frontier orbital was
analyzed. Thermal stabilities of these branched nitro-
stilbene derivatives were investigated via the analysis of
the differential scanning calorimetry (DSC) and thermog-
raving (TGA) curves. This paper presented strong eviden-
ces that the absorption and fluorescence spectroscopy of the
branched stilbene derivatives could be mediated efficiently
by chemical strategy.
.
.
Keywords Stilbene derivatives Branched compound
.
.
Synthesis Spectroscopy Molecular geometry
.
.
optimization Electrochemistry Chemical strategy
Introduction
During the past years, organic dyes have been investigated
extensively due to their broad applications such as in
biomedical, photoelectronic material, and three-dimensional
micro-device fabrication, and they offer many advantages
such as easy fabrication and reducing production costs [1–
6]. In looking for high quality organic dyes with ideal
spectroscopic nature, the chemical structures of the dyes
have been shown to a strong relationship with their spectral
characteristics [7]. Hence, the investigation of the effects of
chemical structures on the absorption and emission spec-
troscopic properties of organic dyes becomes one of
significantly important research areas in photochemistry
and photophysics. Gratzel et al demonstrated that tuning the
frontier orbital of organic dyes by various substituent
groups was very useful strategy for the fabrication of high
efficient dye-sensitized solar cells [8, 9]. It is well accepted
that substituent groups not only can alter the extent of
intramolecular charge transfer, but the properties of excited
state properties, which is achieved by the variation of the
energy and the electron density distribution of frontier
orbital [10, 11].
:
:
:
:
F. Gao (*) L. Yang L. Yang H. Li S. Zhang
College of Chemistry and Chemical Engineering,
Chongqing University,
Stilbene-like dye is one of the most important classes of
dyes, which has received considerable attention as organic
materials for optical power limiting and electrooptical
Chongqing 400044, China
e-mail: fanggao1971@gmail.com

354
J Fluoresc (2010) 20:353–364
devices [12, 13]. The polarizablility and second hyper-
polarizabilities of these molecules tightly depends on
molecular structures of stilbene derivatives, thus enormous
efforts have been devoted on the development of stilbene
derivatives with various structural features such as donor-π-
acceptor and donor-π-donor [14–16]. Some studies have
been concentration on the enlarging absorption wavelength
and enhancement of fluorescence emission of stilbene
derivatives [17, 18]. The photophysical properties of
several selectively bridged stilbenes with donor-acceptor
substituent groups of various were investigated [19]. While
to date, few comparable investigations on development of
new stilbene derivatives with various spectral character-
istics of blue-shifted or red-shifted UV/visible absorption
spectroscopy and strong or weak fluorescence emission
have been reported. This paper presented some novel
branched nitro-stilbene derivatives, of which particular
interest is if we could tune successfully the absorption
wavelength and fluorescence emission via new chemical
strategy. Various linking chemical bonds were proposed to
tune color and photoluminescence of nitro-stilbene deriva-
tives. Molecular geometry optimization and electrochemis-
try of these dyes were carried out to further understand
interrelationship of the chemical structures and absorption
and fluorescence spectroscopy. The thermal stabilities of
the compounds were determined and analyzed.
Instruments
The UV/visible absorption spectra were recorded with a
Cintra spectrophotometer. The emission spectra were checked
with Shimadzu RF-531PC spectrofluorophotonmeter. Qui-
nine sulfate in 0.5 M H2SO4 (Φ=0.546, 1×10−6-×10−5mol/L
[21]) was used as a reference to determine the fluorescence
quantum yields of the compounds in this study. The melting
point was determined using an uncorrected Beijing Fukai
melting point apparatus. Nuclear Magnetic Resonance
(NMR) was carried out at room temperature with a Bruker
500 MHz apparatus with tetramethylsilane (TMS) as internal
standard and CDCl3 was used as solvent. Element analysis
was detected by CE440 elemental analysis meter from
Exeter Analytical Inc.
The fluorescence quantum yields of the compounds in
various solvents were determined based on the following
equation [22, 23]:
R
ꢀ
ꢁ
n20A If lf dlf
n2A I0f lf dlf
Φf ¼ Φ0f
ð1Þ
R
ꢀ
ꢁ
Wherein n0 and n are the refractive indices of the
solvents, A0 and A are the absorption at excited wave-
length, Φf and Φf0 are the quantum yields, and the integrals
denote the area of the fluorescence bands for the reference
and sample, respectively.
The time-resolved fluorescence curves were determined
on an Edinburgh FLS920 time-correlated single photon
counting unit. The lifetimes were calculated from decay
curves using the least-squares method.
Experimental
Materials
Molecular structure optimization calculations
The chemicals and reagents were purchased from Chongqing
Medical and Chemical Corporation unless otherwise speci-
fied. The organic solvents were dried using standard
laboratory techniques according to published methods [20].
The starting materials were further purified via reinstalla-
tion or recrystallization before use. C1, C2, C3 and C4
were synthesized in our laboratory and their chemical
structures were shown in Fig. 1.
Structure optimization was performed with the HyperChem
8.0 package [24] choosing the with AM1 semiempirical
quantum chemical method [25] to keep computations
tractable.
Electrochemistry
Electrochemical measurements were carried out using a
Shanghai Chenhua working station. Two Pt work electrodes
and an Ag/Ag+ reference electrode, i.e. three electrodes
system, were included in cell. Typically, a 0.05 mol/L
solution of tetra-n-butylammonium hexaflorophosphate in
CH2Cl2 containing of sample was bubbled with argon for
15 min. The scan rate was 0.1 V/s.
O2N
O2N
O
O
O
O
C1
C2
O
O
O2N
O2N
O
O
O
O
Thermal analysis
C4
C3
O
O
The differential thermal analysis (DTA) and thermogravi-
metric analysis (TGA) were conducted under nitrogen gas
Fig. 1 Chemical Structures of Nitro-Stilbene Derivatives

J Fluoresc (2010) 20:353–364
355
flow in Shimadzu DTG-60H equipment at heating rate
10°C min−1.
solution by filtration and THF was removed fully by
evaporation. The resulting mixture was dissolved in CHCl3
and washed by water for three times. The organic layer was
dried with anhydrous sodium sulfate and then concentrated.
The product was purified by column chromatography.
Further purification was carried out with twice recrystalli-
zation from benzene to yield 1.6 g (3.6 mmol) golden
Synthesis of nitro-stilbene derivatives
Synthesis strategies of C1 to C4 were shown in Scheme 1.
p-Nitro-3’,4’-dihydroxyl-stilbene was prepared according
to published method [26].
1
yellow of C2 (yield 46%). H-NMR ( δ: ppm): 8.19 (d, J=
C1 (3, 4-diethoxy-4’-nitrostilbene) C1 was prepared as
followings. (2.0 g, 7.8 mmol) p-Nitro-3’, 4’-dihydroxyl-
stilbene and bromoethane (2.5 g, 23.4 mmol) was dissolved
in 100 ml dried acetone, Anhydrous potassium carbonate
(4.3 g, 31.2 mmol) and a few 18-C-6 were added to the
solution. The reactant mixture was stirred at room temper-
ature under argon for 24 hr. The solid was got rid of
solution by filtration and THF was removed fully by
evaporation. The resulting mixture was dissolved in CHCl3
and washed by water for three times. The organic layer was
dried with anhydrous sodium sulfate and then concentrated.
The product was purified by column chromatography.
Further purification was carried out with twice recrystalli-
zation from benzene to yield 0.95 g (3.0 mmol) salmon
pink of C1 (yield 39 %).1H-NMR (δ: ppm): 8.19 (d, J=
9.0 Hz, 2H, CCHCH), 7.58 (d, J=8.5 Hz, 2H,CHCHC),
7.16 (d, J=16.0 Hz, 2H, CCHCH), 7.0 (d, J=7.5 Hz, 2H,
CCHCH), 6.84 (d, J=8.5 Hz, 1H, CHCHC), 4.16 (m, J=
7.0 Hz, 4H, OCH2CH3), 1.47 (t, J=7.0 Hz, 6H, CH2CH3).
13C-NMR(δ: ppm), 14.830, 64.626, 111.467, 112.032,
120.258, 124.138, 126.570, 129.771, 133.048, 144.207,
146.039, 146.458, 146.658. Melting point: 152–153.5°C.
Anal. calculated, C, 68.99, H, 6.11, N, 4.46, O, 20.42,
Found, C, 68.11, H, 5.92, N, 4.51.
9.0 Hz, 2H, CCHCH), 7.57 (d, J=9.0 Hz, 2H, CHCHC),
7.48 (dd, J=7.5 Hz, 4H, CCHCH), 7.39 (dd, J=7.5 Hz,
CHCHCH), 7.33 (t, J=2.8 Hz, 2H, CHCHCH), 7.15 (d, J=
17.5 Hz, 2H, CCHCH), 7.08 (d, J=9.5 Hz, 1H, CHCHC),
6.95(d, J=6.5 Hz, 2H, CCHCH), 5.22 (s, 4H, OCH2C).
13C-NMR(δ: ppm):71.119, 113.216, 114.718, 121.394,
124.128, 126.594, 127.234, 127.352, 127.954, 128.558,
129.856, 136.922, 144.083, 146.468, 149.135, 149.915.
Melting point:136.5–137°C. Anal. Calculated, C, 76.87, H.
5.30, N, 3.20, O, 14.63, Found, C, 76.10, H, 5.38, N, 3.31.
C3: (3, 4-diacetoxy-4’-nitrostilbene) C3 was prepared as
followings: p-Nitro-3’,4’-dihydroxyl-stilbene (1.5 g,
5.8 mmol) and triethylamine (TEA) (4.6 g, 4.64 mmol)
was dissolved in dry THF. Acetic anhydride (3.5 g,
34.8 mmol) was dropped slowly. The mixture was stirred
at room temperature under argon for 24 hr. The solid was got
rid of solution by filtration and the solvents were removed
fully by evaporation. The resulting mixture was dissolved in
CHCl3 and washed by water for three times. The organic
layer was dried with anhydrous sodium sulfate and then
concentrated. The product was purified by column chroma-
tography. Further purification was carried out with twice
recrystallization from benzene to yield 0.84 g (2.5 mmol)
deep yellow color of C3 (yield 42%).1H-NMR (CDCl3,
δppm): 8.19 (d, J=8.0 Hz, 2H, CCHCH), 7.58 (d, J=8.0 Hz,
2H, CHCHC), 7.38 (d, J=9.5 Hz, 2H, CCHCH), 7.18 (m, J=
9.5 Hz, 2H, CHCHC), 7.05(d, J=18.0 Hz, 1H, CCHCH),
2.31 (s, 6H, CCH3). 13C-NMR, δppm: 20.657, 121.540,
124.158, 127.460, 131.408, 135.167, 142.272, 142.437,
143.303, 146.964, 168.212. Melting point: 160–161.5°C.
Anal. Calculated, C, 63.34, H, 4.43, N, 4.10, O, 28.13,
Found, C, 63.90, H, 4.36, N, 3.86.
C2 (3, 4-dibenzyloxy-4’-nitrostilbene) C2 was prepared
as followings. (2.0 g, 7.8 mmol) p-Nitro-3’, 4’-dihydroxyl-
stilbene and benyl bromide (4.0 g, 23.4 mmol) were
dissolved in 150 ml dried acetone, Anhydrous potassium
carbonate (4.3 g, 31.2 mmol) and a few 18-C-6 were added
to the solution. The reactant mixture was stirred at room
temperature under argon for 24 hr. The solid was got rid of
C4: (3, 4-dibenzoyloxy-4’-nitrostilbene) The title com-
pound was prepared as followings: p-Nitro-3’, 4’-dihy-
droxyl-stilbene (1.5 g,5.8 mmol) and triethylamine (TEA)
(4.6 g, 46.4 mmol) was dissolved in dry THF. Benzoyl
chloride (4.9 g, 34.8 mmol) was dropped slowly. The
mixture was stirred at room temperature under argon for
24 hr. The solid was got rid of solution by filtration and the
solvents were removed fully by evaporation. The resulting
mixture was dissolved in CHCl3 and washed by water for
three times. The organic layer was dried with anhydrous
sodium sulfate and then concentrated. The product was
purified by column chromatography. Further purification was
carried out with twice recrystallization from benzene to yield
COOH
OHC
OH
OH
+
O2N
piperdine
Br acetone
/
Br acetone
/
Ph
O2N
C1
C2
OH
OH
K2CO3 /18-C-6
K2CO3 /18-C-6
Cl
O
O
O
TEA
O
C3
C4
Scheme 1 Synthesis route of branched nitro-stilbene derivatives

356
J Fluoresc (2010) 20:353–364
chloride was shown in Fig. 2. Clearly, the maximal
absorption wavelength of C3 exhibited considerable blue-
shift with respect to that of C1 in methylene chloride. The
data shown in Table 1 suggested clearly: (1) the maximal
absorption wavelength of C1 and C2 underwent evident
bathochromic shift in polar solvents (ca. 15 nm); while as
sharp contrast, C3 and C4 featured with tiny red-shift (ca.
4 nm). This is an indication that polar solvents and ether
bond could stabilize the intramolecular charge transfer
states of C1 and C2, while the intramolecular charge
transfer states of C3 and C4 could be destabilized by polar
solvents and ester bond [27, 28]. (2) Nitro-stilbene
derivatives with ester bridged bonds displayed ca. 30–
40 nm blue-shift on the maximal absorption wavelength
with respect to nitro-stilbene derivatives with ether bridged
in various solvents. This caused remarkably different color
of C1 and C2 from C3 and C4 as shown in Fig. 3. This
suggested that the intramolecular linking bonds had
significant effects on the absorption spectroscopy of the
nitro-stilbene derivatives. It was mostly due to the effects of
linking bond on the extent of intramolecular charge transfer.
The ultraviolet/visible absorption characteristics of C1 and
C2 indicated a highly allowed π-π* electron transition with
strong intramoclecular charge transfer feature [29, 30],
while the low optical density and the blue-shifted wav-
length implied that the absorption of C3 and C4 could be
ascribed to prohibited n-π* electron transition nature [31].
The extent of intramolecular charge transfer of C1 and C2
could be diminished by the electron deficiency effect of
ester bridged bond. While, ether linking bond could
increase the extent of intramolecular charge transfer due
to its electron-donating effect. The chemical structural
characteristics of C3 and C4 exhibited great change from
donor-π-acceptor to acceptor-π-acceptor as the introduction
of the ester bond, which could lead to the reduction in the
intramolecular charge transfer of C3 and C4. Typical
comparison of the sketch of molecular structures of C2
and C4 was shown in Fig. 4. Hence, the π-π* electron
transition of C1 and C2 was strengthened, and the n-π*
electron transition of C3 and C4 was further inhibited.
Fig. 2 Typical comparison of absorption spectroscopy of C1 and C3
in methylene chloride (1×10−5mol/L)
1.1 g (2.3 mmol) light yellow color of C4 (yield 40%).
1H-NMR δ(ppm): 8.19 (d, J=9.0 Hz, 2H, CCHCH), 7.57
(d, J=9.0 Hz, 2H, CHCHC), 7.48 (dd, J=7.5 Hz, 4H,
CCHCH), 7.39 (dd, J=7.5 Hz, CHCHCH),7.33 (t, J=2.8 Hz,
2H, CHCHCH), 7.15 (d, J=17.5 Hz, 2H, CCHCH), 7.08
(d, J=9.5 Hz, 1H, CHCHC), 6.95 (d, J=6.5 Hz, 2H,
CCHCH), 5.22 (s, 4H, OCH2C). 13C-NMR(δ: ppm):
121.637, 123.951, 124.187, 127.494, 128.562, 128.621,
130.188, 131.562, 133.816, 142.768, 142.962, 143.357,
146.971, 164.242. Melting point: 153–154.5°C. Anal.
Calculated, C, 72.25, H, 4.11, N, 3.01, O, 20.62, Found, C,
71.67, H, 4.16, N, 3.26.
Results and discussion
Spectroscopic properties
The UV-visible spectroscopy
C1 and C2 exhibited remarkably different absorption
characterization from C3 and C4. Typical ultraviolet/visible
absorption spectroscopy of C1 and C3 in methylene
Table 1 Experimental absorp-
tion spectral data of C1 to 4 in
Solvents
la,max (nm)
ε (1×105) L/mol·cm
various solvents
C1
C2
C3
C4
C1
C2
C3
C4
Cyclohexane
Benzene
Ethyl acetate
THF
357
379
375
383
381
379
360
375
374
381
381
379
333
337
334
338
339
338
335
341
337
337
341
342
0.134
0.118
0.215
0.336
0.344
0.251
0.121
0.114
0.189
0.200
0.273
0.358
0.105
0.108
0.146
0.111
0.189
0.102
0.0894
0.109
0.101
0.117
0.149
0.108
CH2Cl2
CH3CN

J Fluoresc (2010) 20:353–364
357
Fig. 3 Colors of C1 to C4 in
THF solvent
A
A
O
A-pi-A
O2N
O
O
intramolecular charge transfer (decreased)
D-pi-A
A
D
O2N
intramolecular charge transfer (increased)
These could result in blue-shift of the maximal absorption
wavelength of nitro-stilbene derivatives with ester linking
bonds. Thus, this is possible to tune the absorption
spectroscopy of nitro-stilbene derivatives by various chem-
ical linking bonds.
was assumed to be diminished. While in contrast, the
electron density distribution nitro-stilbene part of C1 and
C2 could be raised by ether bridged bond. These could cause
further decreasing (n, π*) electron transition of C3 and C4
and further increasing (π, π*) electron transition of C1 and
C2 [34]. The results showed the significant effects of various
linking bonds on the (π, π*) and (n, π*) electron transitions
of the compounds. As a result, the fluorescence emission of
C3 and C4 was considerably reduced. While the fluores-
cence emission of C1 and C2 was enhanced and it displayed
remarkable red-shift with the increasing of solvent polarity.
The difference on the fluorescence quantum yields of these
compounds was shown clearly shown in Table 2.
On the other hand, we observed that the fluorescence
quantum yields of C1 and C2 was much reduced in very
strong polar solvents such as acetonitrile. The variation of
the fluorescence quantum yields (Table 2) of C1 and C2 in
various solvents suggested that there existed two mecha-
nisms dominating the decay of the excited singlet state of
C1 and C2 [35, 36]. One was called as “negative
solvatokinetic effect”, which meant decreasing in the
fluorescence quantum yields with suitable enhancement of
intramolecular charge transfer. In other words, in non-polar
solvents, the radiative transition decay of the excited state
played significant role. The other mechanism was “positive
solvatokinetic effect”, which suggested that fluorescence
quantum yields were much reduced by the strong intramo-
lecular charge transfer for the larger polarity of solvents.
We supposed that the twisted intramolecular chare transfer
state of C1 and C2 could be formed in strong polar solvents
and the fluorescence emission was thus quenched.
The fluorescence spectroscopy
C1 and C2 featured with strong emission in modest polar
solvents. While in sharp contrast, C3 and C4 exhibited
exceptional weak fluorescence emission in various solvents.
A typical comparison of fluorescence spectroscopy of C1
and C3 in methylene chloride was shown in Fig. 5. The
results showed that linking bond played significant effect
on the fluorescence spectroscopy of nitro-stilbene deriva-
tives as well. This implied that the excited state properties
of these compounds were different.
The fluorescence emission of C1 and C2 could be
ascribed to π*→π excited state decay. In contrast, the
fluorescence emission of C3 and C4 could be from π*→n
excited state decay. It is well accepted that (n, π*) electron
transition is typical forbidden transition [32], while (π, π*)
electron transition is allowed transition [33]. As discussed,
the change from donor-π-acceptor to acceptor-π-acceptor
occurred as ester linking bond was introduced and the
electron density distribution of stilbene part of C3 and C4
The fluorescence lifetimes
Because the fluorescence emission of C3 and C4 was
extremely weak, only the fluorescence lifetimes of C1 and
C2 could be determined. Typical time-resolved fluores-
cence decay curves of C1 and C2 in methylene chloride
were presented in Fig. 6, which showed a little different
decay shape. This indicated that the methoxy and benzy-
loxy groups had influences on the fluorescence lifetimes of
nitro-stilbene derivatives, as shown in Table 3. The
Fig. 4 A sketch of chemical structural characteristics of C2 and C4

358
J Fluoresc (2010) 20:353–364
Table 2 Fluorescence quantum
yields of compounds 1 to 4 in
various solvents
Solvent
lf,max (nm)
Fluorescence quantum yields
C1
C2
C3
C4
C1
C2
C3
C4
Cyclohexane
Benzene
Ethyl acetate
THF
428
485
533
537
549
548
427
494
531
531
560
560
428
394
392
392
403
393
428
395
392
392
413
393
0.0054
0.024
0.18
0.0084
0.062
0.29
0.0078
0.0029
0.0068
0.034
0.0069
0.0070
0.024
0.13
0.39
0.053
CH2Cl2
0.13
0.15
0.0022
0.0036
0.0094
0.0052
the error within 10%, five times
measurements
CH3CN
0.0009
0.0047
radiative transition constants and non-radiative transition
constants of the compounds 1 to 2 were calculated
according to the following equations:
process of the excited singlet states of C1 and C2 rather
than only “cis-trans” isomerization due to their “D-π-A”
nature, and the fluorescence lifetimes of C1 and C2 were
thus enlarged.
ΦF
Kr ¼
ð2Þ
t
Electrochemistry
1 ꢀ ΦF
The cyclic voltammograms of the C1 to C4 were recorded in
methyl chloride from 50mVs-1 to 150mVs-1. The results were
presented in Fig. 7. Obviously, four compounds displayed
different redox curves, although all of the compounds were
characterized with irreversible redox processes. No pair
peaks were found for all compounds, and the redox processes
of all compounds were dominated by diffusion-controlled
electron transfer reactions resulting from linear increasing of
peak currents with the square root of scan rates.
C1 showed three reduction peaks under all sweeping
rates at the negative region at 100 mV/s scan rate:
−0.088 V, −0.640 V, −1.653 V. Three oxidation peaks:
−0.266, −1.355, 1.237 V and were recorded for C1. The
cyclic voltammetry of C2 exhibited one reduction and one
oxidation peaks at 100 mV/s sweeping rate. The reduction
peak was got at the negative region: −0.788 V, and the
oxidation peak was found at positive region: 1.087 V.
Figure 7(c) shows typical cyclic voltammograms of C3 at
Knr ¼
ð3Þ
t
wherein ΦF is fluorescence quantum yield, τ represents as
fluorescence lifetime, Kr is radiative transition constant, Knr
is non-radiative transition constant.
Seen from Table 3, the radiative transition constant of
C2 was larger, while non-radiative transition constant of C2
was lower. This interpreted why the fluorescence quantum
yields of C2 were bigger than C1, as shown in Table 2.
Stiffer structure and stronger electron-donating effects of
benzyloxy groups could make the fluorescence emission of
C2 enhanced. Interesting, the fluorescence lifetimes of C1
and C2 was longer than that of simple stilbene (τ, 0.075 ns)
[37]. It was well demonstrated that for a simple stilbene,
“cis-trans” isomerization was the only approach of deacti-
vation of the excited singlet states [38]. In the present study,
intramolecular charge transfer and twisted intramolecular
charge transfer states could dominate the deactivative
C1
C2
Fig. 5 Typical comparison of fluorescence spectroscopy of compounds 1 and 3 in methylene chloride (1×10−6mol/L), Ex: 350 nm, Slit window:
Ex: 5 nm. Em: 3 nm

J Fluoresc (2010) 20:353–364
359
Table 3 Fluorescence lifetimes, radiative transition constant, non-
radiative transition constant of C1 and C2 in various solvents
−1.273 V could be regarded as reductive potentials of nitro
group of C1, C2, C3 and C4 respectively. This showed that
as compared with C1 and C2, the reduction of nitro group
of C3 and C4 became more difficult. This could be ascribed
to the effect of electron-withdrawing or electron-donating
of various linking chemical bonds on the redox process of
nitro group. Likewise, ester bond in C3 and C4 as an
electron-withdrawing group could undergo reduction pro-
cess as well [39]. 0.609, 0.374 V could be from the
reduction process of ester group of C3 and C4. Further-
more, as discussed, the electron density distribution of C-C
double bond could be decreased by electron-withdrawing of
ester bond, and could be enhanced electron-donating effect
of ether bond. This could lead to easy or difficult
occurrence of redox processes on C-C double bond for
these compounds respectively [40, 41], and it could be the
main reason why different redox processes of these
compounds were detected. The different electrochemical
process reflected the difference on their HOMO and LUMO
energies of these compounds [42]. Hence, we further
estimated the energy of frontier orbital based on the
Solvents
C1
C2
τ
Kr
Knr
τ
Kr
Knr
THF
1.93
1.21
1.81
0.674
1.488
0.718
4.508
6.777
4.807
3.03
2.16
1.73
1.287
1.343
0.867
2.013
3.287
4.913
AcOEt
CH2Cl2
τ: ns, Kr, Knr: 108 /s
various scan rates. Three reduction peaks at −0.187,
−0.937, 0.609 V and one oxidation peak at −0.079 V were
observed at scan rate 100mVs−1. Typical cyclic voltammo-
gram of C4 at different scan rates was shown in Fig. 7(d).
Three reduction peaks were found as 0.374, −0.498, −1.273
V, and three oxidation peaks were observed −0.535 V,
−0.094 V and 0.878 V at scan rate 100 mV/s.
Nitro group is a powerful electron-withdrawing species
in C1 to C4 and the occurrence of its reduction reaction
could be easily observed [38]. −0.640, −0.788, −0.937,
Fig. 6 Fluorescence lifetime of
C1 and C2 in CH2Cl2
(b) C2
(a) C1
(d) C4
(c) C3

360
J Fluoresc (2010) 20:353–364
S0
HOMO
LUMO
corresponding redox potentials and optical parameters. The
results were listed in Table 4. It clearly showed that the
energy of HOMO and LUMO was enhanced by ester
linking bond.
Molecular geometry optimization
C1
The electron density distribution of HOMO and LUMO
orbitals of C1 to C4 at S0 and S1 states was investigated. As
shown in Fig. 8, the changes on the electron density
distribution of these compounds from HOMO to LUMO
transition exhibited some difference. The electron density
distribution moved to nitro group and its connecting
diphenyl ethylene parts in HOMO→LUMO transition at
S0 state. While as compared with C3 and C4, electron
density distribution of C1 and C2 was much closer to the
side of nitro group. Electron density distribution of C3 and
C4 exhibited more delocalization due to the deficiency
effect of ester chemical bond at LUMO orbital of S0 state.
At the S1 state, the change on the electron density
distribution of C1 and C2 from HOMO to LUMO
transition was quite different from that of C3 and C4. C1
and C2 showed a similar change as S0 state. However, C3
and C4 showed larger change on the electron density
distribution from HOMO to LUMO in S1 state. Consider-
able electron density distribution was found in the ester
chemical linking bonds in the LUMO orbitals of C3 and C4
in S1 state, especially for the ester chemical bond of C4.
This confirmed our assumption that the electron density
distribution of stilbene part of C1 and C2 was enhanced
greatly by ether chemical bond due to the electron-donating
effect. While as sharp contrast, the electron density
distribution of stilbene part of C3 and C4 was much
decreased by the electron-withdrawing effect of ester
chemical bond. We further calculated the dipole moment
change of these compounds between the excited state and
the ground state and listed in Table 5. Obviously, C1 and
C2 had larger dipole moment change than C3 and C4. In a
word, the molecular geometry optimization calculations
demonstrated ester linking bond played two significant
C2
C3
C4
S1
HOMO
LUMO
C1
C2
C3
Table 4 The estimated energy of HOMO and LUMO from Redox
potentials
Compounds
EOX
(V)
HOMO
(eV)
λonset
(nm)
Gap
( eV)
LUMO
(eV)
S0
HOMO
LUMO
C1
C2
C3
C4
1.237
1.087
−0.079
0.878
−5.58
−5.43
−4.26
−5.22
381
381
339
341
3.25
3.25
3.66
3.64
−2.33
−2.18
−0.60
−1.58
C4
HOMO ( eV )=− EOX − 4.34 [43, 44], Gap (eV)=1240/λonset,
LUMO ( eV )=HOMO+gap
Fig. 7 Cyclic voltammograms of compounds 1 to 4 in various scan rates

J Fluoresc (2010) 20:353–364
361
Table 5 The change of the dipole moment between the excited state
and ground state respectively obtained from experiments and
theoretical calculations
C2 respectively: C1: Y=11855x+5228.5, C2: Y=13383x+
5070.7. Similar slopes of C1 and C2 implied that excited
decay processes of C1 and C2 were similar [49]. However,
the linear correlationship between Stokes shift and orienta-
tional polarizability for C3 and C4 could not be obtained,
suggesting the excited state decay process of C3 and C4
was much different from that of C1 and C2. The changes of
dipole moments on the excited state and ground state could
be calculated from the slopes, and the data were listed in
Table 5. Although the experimental and theoretical results
had some difference, the data have confirmed simulta-
neously that C1 and C2 had larger charge transfer than C3
and C4 in the excited state. In a word, the results suggested
that the electron coupling effects induced by excitation
delocalization were greatly affected by various linking
bonds. Thus, the electron density distribution of chromo-
phore part of C1 and C2 was different from C3 and C4 in
the excited states, which eventually lead to different
spectral characteristics.
Methods
Dipole moments change Δμ (D)
C1
C2
C3
C4
From experiment
From theory
4.89
3.23
3.22
5.06
0.82
3.18
1 D=3.336×10-30 c.m
roles: (1) decreasing the extent of intramolecular charge
transfer of C3 and C4; (2) increasing the delocalization of
electron density distribution of overall molecule. In other
words, the electron density distribution of stilbene part was
decreased. The results confirmed that (n, π*) electron
transition C3 and C4 was considerably inhibited and (π,
π*) electron transition of C1 and C2 was further strength-
ened. As a consequence, the absorption of C3 and C4
exhibited obvious blue-shift and the fluorescence emission
was much reduced. The calculations could well interpret
why the absorption and fluorescence wavelength of
branched nitro-stilbene derivatives could be efficiently
tuned by the chemical linking bond.
Thermal analysis
The analysis of the thermal properties was conducted with
differential thermal analysis (DTA) and thermogravimetric
analysis (TGA) under a nitrogen atmosphere and the results
were listed in Fig. 10. The endothermic peaks determined
by DTA were found at 170.57, 143.17, 169.59 and 164.77°C
for C1, C2, C3 and C4 respectively as presented in Fig. 10.
Considering the heating rate was 10°C min−1, the DTA data
are very close to the melting points of C1 to C4
respectively determined with melting point apparatus. The
decomposition temperatures of compounds 1 to 4 deter-
mined by TGA were 334.76, 306.62, 359.02 and 376.37°C
respectively as shown in Fig. 10. The weight loss of C1
(MW=313.13) was 37.852% at 334.76°C, which was close
to the loss of diethoxy part (MW=87.89). Interestingly, the
We further estimated the changes on the dipole moments
between the excited state and ground state based on Lippert
equation [45, 46]:
ꢂ
ꢃ
2
2 me ꢀ mg
hcðvabs ꢀ vemÞ ¼
Δf þ const
ð4Þ
4p"0a3
ꢄ
ꢅ
" ꢀ 1
n2 ꢀ 1
Δf ¼
ꢀ
ð5Þ
2" þ 1 2n2 þ 1
Wherein h is Planck’s constant, c is the speed of light,
and Δƒ is called the orientation polarizability. υabs, υem are
the wavenumbers of the absorption and emission, n is the
refractive index, and ε is the relative low-frequency
dielectric constant of the solvents.
10000
9000
8000
7000
6000
5000
4000
Lippert equation, the chromophore group was regarded
as a dipole, which located in a cavity with a radius of a in a
continuous solvent-dipole environment. Moreover, this
equation reflected a solvent effect of the index of refraction
and relative dielicetric constant. Consequently, the linear
correlation between Stokes shifts and Δƒ could not show
some special interaction between the fluorophore and
solvent molecules such as hydrogen bonding. Plots of
Stokes shift as a function of the solvent orientation
polarizability (Δƒ) was shown in Fig. 9. The linear
correlation of C1 and C2 between Stokes shift and
orientational polarizability suggested the dipolar solvent
effects [47, 48]. Two equations were obtained for C1 and
3000
2000
1000
0
C1
C2
0
0.05
0.1
0.15
0.2
0.25
0.3
Orientation polarizability
Fig. 8 Electron density distribution of HOMO and LUMO at S0 and
S1 states

362
J Fluoresc (2010) 20:353–364
(a) C1
(b) C2
(c) C3
(d) C4
Fig. 9 Relationship between stokes’s shifts of C1 and C2 and orientation polarizability (Δƒ) of solvents
weight loss of C2 (MW=437.16) was 17.354%, which was
also close to the loss of benzyl group (MW=76.03). The
weight loss of C3 (MW=341.09) was 60.198%, which was
to close to the loss of nitro group and two acetoxy groups
(Total: MW=164.10). C4 (MW=465.12) displayed
27.932% weight loss, which was close to the loss of two
benzoyloxy groups (MW=120.02). The analysis showed
that: (1) these compounds had remarkable thermal stability
and the decomposition temperatures of the compounds 1 to
4 were over 300°C, and (2) intramolecular linking bond did
not show large effect on the thermal stabilities.
(n→π*) electron transition and the excited state decay
process of these compounds were influenced by various
chemical bonds. The estimated HOMO and LUMO
energies of these compounds revealed further the effects
of linking bonds on the electron density distribution of
frontier orbital. These made it much possible to tune
Conclusions
To summarize, a range of new branched nitro-stilbene
derivatives with various linking bonds was synthesized and
characterized. The linking bonds showed significant effects
on the electron density distribution of HOMO and LUMO,
and thus the extent of intramolecular charge transfer
exhibited considerably different. This resulted in the
different dipole moment changes between the excited state
and the ground state for these branched nitro-stilbene
derivatives. The results suggested that the (π→π*),
Fig. 10 DTA and TGA curves of the compounds 1 to 4

J Fluoresc (2010) 20:353–364
363
twisted intramolecular charge transfer fluorescence. J Am Chem
Soc 115:5035–5040
efficiently the spectroscopic properties of the branched
nitro-stilbene derivatives by linking bond. While, the
thermal stabilities of nitro-stilbene derivatives were not
afftected by the linking bonds. The results presented in this
paper would be great interest in the development various
branched substituted nitro-stilbene dyes with required
spectroscopic characteristics based on chemical strategy.
12. Chen L, Cui Y, Mei X, Qian G, Wang M (2007) Synthesis and
characterization of triphenylamino-substituted chromophores for
nonlinear optical applications. Dyes & Pigments 72:293–298
13. Jin Z, Xu Q, Li N, Lu J, Xia X, Yan F, Wang L (2008) Facile
di-color emission tuning of poly[1-(4-vinylstyryl)naphthalene]
with naphthalimide end group via ATRP. Eur Poly J 44:1752–
1757
14. Calaminici P, Jug K, Koster AM, Arbez-Gindre C, Screttas CG
(2002) Mechanism for large first hyperpolarizabilities of phos-
phonic acid stilbene derivatives. J Comput Chem 23:291–297
15. Gorman CB, Marder SR (1993) An investigation of the
interrelationships between linear and nonlinear polarizabilities
and bond-length alternation in conjugated organic molecules.
PNAS 90:11297–11301
16. Marder SR, Cheng L, Tiemann BG, Friedli AC (1994) Large first
hyperpolarizabilities in push-pull polyenes by tuning of the bond
length alternation and aromaticity. Science 5146:511–514
17. Fayed TA, Etaiw SEDH, Khatab HM (2005) Excited state
properties and acid-base equilibria of trans-2-styrylbenzoxazoles.
J Photochem Photobio A: Chem 170:97–103
Acknowledgements The authors appreciate financial support from the
National Natural Science Foundation of China (Nos. 20776165, 20702065,
20872184), Natural Science Foundation of Chongqing Science and
Technology Commission (Nos. CSTC 2009BB4216) and the Key
Foundation of (CSTC 2008BA4020). H. Li thanks “A Foundation for the
Author of National Excellent Doctoral Dissertation of PR China (200735)”
for financial support. This paper is part sponsored by the Scientific
Research Foundation for the Returned Overseas Chinese Scholars, State
Education Ministry as well (Nos. 20071108-1, 20071108-5). L. Yang
appreciates support from Chongqing University Postgraduates’ Science and
Innovation Fund (Nos. 200904A1A0010304).
18. Jha PC, Das M, Ramasesha S (2004) Two-photon absorption cross
sections of trans-stilbene, and 7, 8-disubstituted stilbenes in
different molecular conformations: A model exact study. J Phys
Chem 108:6279–6285
References
19. Lapouyade R, Czeschka K, Majenz W, Rettig W, Gilabert E,
Rulliere C (1992) Photophysics of donor-acceptor substituted
stilbenes. A time-resolved fluorescence study using selectively
bridged dimethylamino cyano model compounds. J Phys Chem
96:9643–9650
20. Perrin DD, Armarego WLF, Perrin DR (1966) Purification of
laboratory chemicals. Pergamon, New York
21. Eaton DF (1988) Reference materials for fluorescence measure-
ment. Pure & Appl Chem 60:1107–1114
22. Maus M, Retigg W, Bonafoux D, Lapouyade R (1999) Photoin-
duced intramolecular charge transfer in a series of differently
twisted donor-acceptor biphenyls as revealed by fluorescence. J
Phys Chem A 103:3388–3401
23. Lukeman M, Veal D, Wan P, Ranjit V, Munasinghe N, Corrie JET
(2004) Photogeneration of 1, 5-naphthoquinone methides via
excited-state (formal) intramolecular proton transfer (ESIPT) and
photodehydration of 1-naphthol derivatives in aqueous solution.
Can J Chem 82:240–253
1. Geddes CD, Parfenov A, Roll D, Uddin MJ, Lakowicz JR (2003)
Fluorescence spectral properties of indocyanine green on a
roughened patinum electrode metal-enhanced. J Fluor 13:453–457
2. Geddes CD, Apperson K, Birch DJS (2000) New fluorescent
quinolinium dyes-applications in nanometre particle sizing. Dyes
& Pigments 44:69–74
3. Wu X, Chowdhury MH, Geddes CD, Aslan K, Domszy R,
Lakowicz JR, Yang AJM (2008) Use of surface plasmon-
coupled emission for enhancing light transmission through top-
emitting organic light emitting diodes. Thin Solid Films
516:1977–1983
4. Dvornikov AS, Rentzepis PM (1999) Novel organic ROM
materials for optical 3D memory devices. Optics Commu.
136:1–6
5. Sun HB, Tanaka T, Takada K, Kawata S (2001) Two-photon
photopolymerization and diagnosis of three-dimensional micro-
structures containing fluorescent dyes. Appl Phys Lett 79:1411–
1413
24. Hyperchem 8.0 Package, Hyperchem Inc., Gainesville, FL
25. Dewar MJS, Zoebisch EG, Healy EF, Stewart JJP (1985) AM1: a
new general purpose quantum mechanical molecular model. J Am
Chem Soc 107:3902–3909
6. Sayama K, Tsukagoshi S, Hara K, Ohga Y, Shinpou A, Abe Y,
Suga S, Arakawa H (2002) Photoelectrochemical properties of J
aggregates of benzothiazole merocyanine dyes on a nanostruc-
tured TiO2 film. J Phys Chem B 106:1363–1371
26. Cavallini G, Massarani E, Process for the preparation of 4-
hrdoxystilbene and its derivatives, US Patent 2,878,291
27. Deperasińska I, Dresner J (1998) On the charge-transfer electronic
configurations of excited 1, 1′-dinaphthylamine, J. Mol. Stru.:
THEOCHEM. THEOCHEM 422:205–212
28. Yang J, He Q, Lin H, Bai F (2001) Probing twisted intramolecular
charge transfer state in hyper-branched conjugated polymers. Anal
Sci 17:a203–a206
7. Peng X, Song F, Lu E, Wang Y, Zhou W, Fan J, Gao Y (2005)
Heptamethine cyanine dyes with a large stokes shift and strong
fluorescence: A paradigm for excited-State intramolecular charge
transfer. J Am Chem Soc 127(12):4170–4171
8. Hagfeldt A, Gratzel M (2000) Molecular Photovoltaics. Acc
Chem Res 33:269–277
9. Hagberg DP, Yum JH, Lee HJ, Angelis FD, Marinado T, Karlsson
KM, Humphry-Baker R, Sun L, Hagfeldt A, Gratzel M,
Nazeeruddin MK (2008) Molecular engineering of organic
sensitizers for dye-sensitized solar cell applications. J Am Chem
Soc 130:6259–6266
29. Domagalska BW, Wilk KA, Wysocki S (2003) Experimental and
theoretical studies on solvent effects of amphiphilic conjugated
polyenals. Phys Chem Chem Phys 5:696–702
30. Wang S, Kin S (2009) Photophysical and electrochemical
properties of D-π-A type solvatifluochromic isophorone dye for
pH molecular switch. Curr Appl Phys 9:783–787
31. Mikroyannidis JA, Stylianakis MM, Cheung KY, Fung MK,
Djurisic AB (2009) Alternating phenylenevinylene and thienyle-
nevinylene copolymers with cyano groups: Synthesis, photo-
physics and photovoltaics. Syn Met 159:142–47
10. Maus M, Rettig W, Bonafoux D, Lapouyade R (1999) Photoin-
duced intramolecular charge transfer in a series of differently
twisted donor - acceptor biphenyls as revealed by fluorescence. J
Phys Chem A 103:3388–3401
11. Hamasaki K, Ikeda H, Nakamura A, Ueno A, Toda F, Suzuki I,
Osa T (1993) Fluorescent sensors of molecular recognition.
Modified cyclodextrins capable of exhibiting guest-responsive

364
J Fluoresc (2010) 20:353–364
32. Rocha AB, Bielschowsky CE (2001) Intensity of the n→π*
symmetry-forbidden electronic transition in acetone by direct
vibronic coupling mechanism. Chem Phys Lett 337:331–334
33. Roy BC, Gupta MD, Bhowmik L, Ray JK (2001) Synthesis and
characterization of poly (2, 5-dimethoxyaniline) and poly (aniline-
Co-2, 5- dimethoxyaniline): The processable conducting poly-
mers. Bull Mater Sci 24:389–396
34. Barlow S, Bunting HE, Ringham C, Green JC, Bublitz GU, Boxer
SG., Perry JW, Marder SR (1999) Studies of the electronic
structures of metallocene-based second-order nonlinear optical
dyes. J Am Chem Soc 121:715–3723
35. Li ZM, Wu SK (1996) Study on Photophysical behaviors of styryl
pyrazine aerivatives. Acta Phys-Chim Sin 12:418–422
36. Li ZM, Wu SK (1997) The effect of molecular structure on the
photophysical behavior of substituted styryl pyrazine derivatives.
J Fluor 7:237–242
37. Segura JL, Gomez R, Martin N, Guldi DM (2001) Synthesis of
photo- and electroactive stilbenoid dendrimers carrying dibutyla-
mino peripheral groups. Org Lett 3:2645–2648
38. Uda M, Mizutani T, Hayakawa J, Momotake A, Ikegami M,
Nagahata R, Arai T (2002) Photoisomerization of stilbene
dendrimers: The need for a volume-conserving isomerization
mechanism. Photochem & Photobio 76:596–605
39. Squella JA, Sturm JC, Weiss-Lopez B, Bonta M, Nunez-Vergara
LJJ (1999) Electrochemical study of β-nitrostyrene derivatives:
steric and electronic effects on their electroreduction. J Electroana
Chem 466:90–98
40. Ramesh P, Sampath S (2001) Electrochemical and spectroscopic
characterization of quinone functionalized exfoliated graphite.
Analyst 126:1872–1877
41. Boudon C, Gisselbrecht JP, Gross M, Anthony J, Boldi AM, Faust R,
Lange T, Philp D, Van Loon JD, Diederich F (1995) Electrochemical
properties of tetraethynylethenes, fully cross-conjugated Π-
chromophores, and tetraethynylethene-based carbon-rich molecular
rods and dehydroannulenes. J Electroanal Chem 394:187–197
42. Akiyama S, Tajima K, Nakatsuji S, Nakashima K, Abiru K,
Watanabe M (1995) Direct C≡C triple bond formation from the
C=C Double bond and direct hydroxylation into the o-position of
a nitro group on the benzene nucleus with potassium t-butoxide in
N, N-dimethylformamide in the air. Bull Chem Soc Japan
68:2043–2051
43. Liu J, Tu G, Zhou Q, Cheng Y, Geng Y, Wang L, Ma D, Jing X,
Wang F (2006) Highly efficient green light emitting polyfluorene
incorporated with 4-diphenylamino-1,8-naphthalimide as green
dopant. J Mat Chem 16:1431–1438
44. Bard AJ, Faulkner LA (1984) Eelctrochemical methods-
fundamentals and applications. Wiley, New York
45. Lippert E (1957) Spectroskopische bestimmung des dipolomo-
mentes aromatischer verbindugen im ersten angeregten singulett-
zustand. Z. Electrochem. 61:962–975
46. Lakowicz JR (1999) Principles of fluorescence spectroscopy.
Kluwer Academic, New York
47. Belfield KD, Bondar MV, Przhonska OV, Schafer KJ (2002)
Steady-state spectroscopic and fluorescence lifetime measure-
ments of new two-photon absorbing fluorene derivatives. J Fluor
12:449–454
48. Gaber M, El-Daly SA, Fayed TA (2008) El-Sayed YS Photo-
physical properties, laser activity and photoreactivity of a
heteroaryl chalcone: A model of solvatochromic fluorophore.
Optics & Laser Tech 40:528–537
49. Slyadnev MN, Inoue T, Harata A, Ogawa T (2000) A rhodamine
and a cyanine dye on the water surface as studied by laser induced
fluorescence microscopy, Colloids and Surfaces A: Physicochem.
Eng. Asp. Eng. Asp. 164:528–537