A comprehensive investigation on the cooperative branch effect on the optical properties of novel conjugated compounds

Article abstract of DOI:10.1007/s10895-010-0741-y

This paper presents a variety of conjugated derivatives with different number of arms (4-styryltriphenylamine: C1, 4, 4'-di-styryltriphenylamine: C2, 4, 4', 4.-tri-styryltriphenylamine: C3). The linear absorption and fluorescence maxima and the molar extinction coefficients are in the order of C1

Full text of DOI:10.1007/s10895-010-0741-y

J Fluoresc (2011) 21:545–554
DOI 10.1007/s10895-010-0741-y
ORIGINAL PAPER
A Comprehensive Investigation on the Cooperative Branch
Effect on the Optical Properties of Novel
Conjugated Compounds
Long Yang & Fang Gao & Jian Liu & Xiaolin Zhong &
Hongru Li & Shengtao Zhang
Received: 11 April 2010 /Accepted: 29 September 2010 /Published online: 6 October 2010
#
Springer Science+Business Media, LLC 2010
Abstract This paper presents a variety of conjugated
derivatives with different number of arms (4-styryl-
triphenylamine: C1, 4, 4′-di-styryltriphenylamine: C2, 4,
4′, 4″-tri-styryltriphenylamine: C3). The linear absorption
and fluorescence maxima and the molar extinction
coefficients are in the order of C1<C2<C3 in various
solvents. Two-photon absorption (TPA) up-converted
emission of the derivatives were determined with Ti:
sapphire femtosecond laser. The maximal TPA emission
wavelength and the two-photon absorption cross section of
the derivatives are also in the order of C1<C2<C3 in various
solvents. The dipole moment changes of the derivatives
between the excited state and the ground state were
estimated from experiment, and they are in the order of
C1<C2<C3, which is confirmed further by the molecular
geometry optimization of the derivatives. The electron density
distribution and the energy levels of the frontier orbital of the
derivatives were analyzed. The cyclic voltammograms of the
derivatives were performed and discussed.
applications such as two-photon fluorescence sensors [1,
2], two-photon biomarkers [3, 4], two-photon imaging
reagents [5, 6], two-photon photodynamic therapy [7, 8]
and non-linear optical materials [9, 10]. An ideal dye
should have excellent spectral characteristics such as high
fluorescence quantum yield, large molar extinction coeffi-
cient and remarkable TPA cross section in the red-NIR range
(700–1200 nm), hence this could guarantee the use of lower
concentrations of the sample in applications, which stimulates
the development of new two-photon compounds with large
TPA cross sections [11–18], which normally have typical
phenylvinyl chemical structures with donor-π-donor, donor-
π-acceptor and donor-acceptor-donor characteristics.
Many branched compounds containing various electron-
donating or accepting groups in the conjugated arms were
reported, and the optical characteristics were found to be
related to the molecular structures [19, 20]. While to date,
the branched conjugated two-photon dyes without push-
pull groups have not been explored well. It is surprise
because such molecules could show clearly branch effect
on the optical properties of the armed derivatives. Such
investigation is very necessary because the results could
help us on the development of novel optical materials.
Consequently, we develop new fluorescent derivatives
containing only core and phenyl ring, which means that
no electron-donating or accepting groups are located in the
arms of the derivatives, thus the substituent effect could be
eliminated. This paper presents the synthesis of 4-styryl-
triphenylamine (C1), 4, 4′-di-styryl–triphenylamine (C2)
and 4, 4′, 4″-tri-styryl–triphenylamine (C3) (Fig. 1). Of its
particular interest is trying to investigate pure branch effect
on the optical properties (one- and two- photon). The
molecular geometry optimization and the electrochemistry
of the derivatives were studied to reveal further the deep
reasons for the different optical properties.
.
.
Keywords Synthesis Optical properties Molecular
.
.
geometry optimization Branch effect Conjugation
Introduction
In recent years, one of the central themes in organic and
material chemistry is to develop highly fluorescent and
two-photon absorption (TPA) dyes due to their wide
:
:
:
:
:
L. Yang F. Gao (*) J. Liu X. Zhong H. Li (*) S. Zhang
College of Chemistry and Chemical Engineering,
Chongqing University,
Chongqing 400044, China
e-mail: fanggao1971@gmail.com
e-mail: hongruli1972@gmail.com

546
J Fluoresc (2011) 21:545–554
Fig. 1 Chemical structures of
the derivatives studied in this
paper
N
N
N
N
C1
C4
C2
C3
Experimental
Φf and Φf0 are the quantum yields, and the integrals denote
the area of the fluorescence bands for the reference and
sample, respectively.
Reagents and Materials
Two-photon excited fluorescence spectra, pumped by Ti:
sapphire femto-second laser (Spectra-Physics Ltd., Tsunami
mode-locked, 80 MHz, <130 f s, average power ≤700 mW)
tuned by step of 20 nm in the range of 700~880 nm, were
recorded on Ocean Optics USB2000 CCD camera with
detecting range of 180~880 nm. TPA cross-section (σ) was
determined by up-conversion fluorescence method using
5×10−4 mol/L fluorescein in 0.1 mol/L solution of NaOH
as reference sample [26]. The sample was bubbled with
nitrogen for 15 min to eliminate oxygen before the
detection. TPA cross sections of the compounds were
determined by the following equations [27]:
Organic solvents were obtained from Chongqing Medical
and Chemical Corporation. Other chemicals and reagents
were purchased from Aldrich unless otherwise specified.
The organic solvents were dried using standard laboratory
techniques according to the published methods [21]. The
starting materials were further purified with redistillation or
recrystallization before use. The derivatives C1 to C3
(Fig. 1) were synthesized in our laboratory, and C2 and C3
were firstly reported.
Instruments
sTPE
The UV/visible absorption spectra (1×10−5 mol/L) were
recorded with a Cintra spectrophotometer. The emission
spectra (1×10−5 mol/L) were checked with Shimadzu RF-
531PC spectrofluorophotonmeter. Rodamin 6G in ethanol
(Φ=0.94, 1×10−6–1×10−5 mol/L) was used as reference to
determine the fluorescence quantum yields of the com-
pounds herein [22, 23]. To avoid self-quenching of
fluorescence emission, the low concentration of the com-
pounds (1×10−6 mol/L) was prepared for the survey of
fluorescence quantum yields. The melting point was
determined using a Beijing Fukai melting point apparatus.
Nuclear magnetic resonance (NMR) spectroscopy was
conducted at room temperature on a Bruker 500 MHz
apparatus with tetramethylsilane (TMS) as an internal
standard and CDCl3 as solvent. Elemental analysis was
performed by a CE440 elemental analysis meter from
Exeter Analytical Inc.
s ¼
ð2Þ
ΦF
TPE ccal ncal
S
sTPE ¼ scal
ð3Þ
c
n
Scal
Wherein σ is two-photon absorption section, σTPE is
two-photon excited crossing section, c is the concentration
of reference and sample molecules, n is refractive index of
the solvent, and S is two-photon up-conversion fluores-
cence intensity, cal represents as reference.
Molecular Geometry Optimization
Molecular geometry optimization was conducted with the
HyperChem 8.0 package [28] via AM1 semi-empirical
quantum chemical method to keep the computations tractable
[29].
The fluorescence quantum yields of the compounds in
solvents with different polarities were measured based on
the following equation [24, 25]:
R
ꢀ
ꢁ
Cyclic Voltammograms
n20A0 If lf dlf
n2A If0 lf dlf
Φf ¼ Φ0f
ð1Þ
R
ꢀ
ꢁ
Cyclic voltammograms was carried out using a Shanghai
Chenhua electrochemical working station. Two Pt work electro-
des and an Ag/Ag+ reference electrode, namely three electrodes
system, were included in cell. Typically, a 0.05 mol/L solution
wherein n0 and n are the refractive indices of the solvents,
A0 and A are the optical densities at excitation wavelength,

J Fluoresc (2011) 21:545–554
547
of tetra-n-butylammonium hexaflorophosphate in CH2Cl2
containing of dyes was bubbled with argon for 15 min.
phosphite was removed in vacuum, the crude benzyl-
phosphonate was obtained, which directly went to the next
step without further purification.
Synthesis
(1) C1: 4-styryl-triphenylamine
The synthesis of the derivatives was depicted in
Scheme 1. 4-Formyl-triphenylamine and 4′,4″-diformyl-
triphenylamine, 4′,4″,4‴—triformyl-triphenylamine were
prepared according to well-known procedures with mod-
ified procedure respectively [30]. Benzyl bromide
(17.5 mmol, 3.00 g) reacted with triethyl phosphite
(10 ml) under 130–160 °C for 6 h. After excess triethyl
Benzylphosphonate (8.77 mmol, 2.00 g) reacted with 4-
formyl-triphenylamine (7.31 mmol, 2.00 g) in 50 ml dry
tetrahydronfuran using sodium methoxide (17.5 mmol,
0.98 g) as base at room temperature overnight. After the
filtration of solid materials and the evaporation of solvent in
vacuum, the reactant mixture was dissolved in chloroform
and washed by water. The organic layer was dried with
Scheme 1 Synthesis routes of
C1, C2 and C3
O
P
NBS/CCl4
P(OEt)3
(OEt)2
130-1600C
reflux
Br
O
P
(OEt)2
N
POCl3/DMF
N
C5
N
C1
C4
,
NaOCH3
THF
CHO
O
P
OHC
(OEt)2
N
N
POCl3/DMF
N
C4
,
NaOCH3
THF
C6
C2
CHO
O
P
OHC
CHO
(OEt)2
N
POCl3/DMF
N
N
C4
C7
,
NaOCH3
THF
C8
CHO
CHO
O
P
(OEt)2
THF, NaOCH3
N
C3

548
J Fluoresc (2011) 21:545–554
a
δ(ppm): 7.532–7.548(d, 2H, J=8.0 Hz, Ar-H), 7.428–7.448
(d, 2H, J=10.0 Hz, Ar-H), 7.377–7.412 (t, 2H, J=7.75 Hz,
Ar-H), 7.283–7.325 (m, 4H, Ar-H), 7.269 (s, 1H, Ar-H),
7.156–7.172 (d, 2H, J=8.0 Hz, Ar-CH = CH), 7.129 (s, 2H,
Ar-H), 7.029–7.118(m, 6H, Ar-H). 13C-NMR (CDCl3,
125 MHz): 123.077, 123.658, 124.534, 126.368, 127.335,
127.417, 128.230, 128.710, 129.340, 137.679, 147.411,
147.610. Anal. Calcd (Found) for C26H21N (%): C, 89.81
(89.89), H, 6.15 (6.09), N, 4.06 (4.01).
1.0
0.8
0.6
0.4
0.2
0.0
Red Shift
C1
C2
C3
C4
(2) C2: 4,4′-di- styryl -triphenylamine
Benzylphosphonate (8.77 mmol, 2.00 g) reacted with
4,4′-diformyl-triphenylamine (3.65 mmol, 1.10 g,) in 60 ml
dry tetrahydronfuran using sodium methoxide (17.5 mmol,
0.98 g) as base at room temperature overnight. After the
filtration of solid materials and the evaporation of solvents
in vacuum, the reactant mixture was dissolved in chloro-
form and washed by water. The organic layer was dried
with anhydrous magnesium sulphate. After evaporated in
vacuum, C2 was purified with column chromatography
using benzene as eluent, which was further purified with
recrystallization in cyclohexane. C2: color: yellow, yield:
40%, m.p.: 152–153 °C. 1H-NMR(CDCl3, 500 MHz)
δ(ppm): 7.487–7.504 (d, 4H, J=8.5 Hz, Ar-H), 7.390–
7.415 (d, 4H, J=12.5 Hz, Ar-H), 7.330–7.361 (t, 4H, J=
7.7 Hz, Ar-H), 7.223–7.293 (m, 5H, Ar-H), 7.127–7.147 (d,
2H, J=10.0 Hz, Ar-H), 7.072–7.094 (d, 4H, J=11.0 Hz,
Ar-H), 7.025–7.076 (t, 4H, J=12.7 Hz, Ar-H), 13C-NMR
(CDCl3, 125 MHz): 123.363, 123.975, 124.763, 126.353,
127.273, 127.352, 127.430, 128.129, 128.686, 129.379,
131.901, 137.605, 147.039, 147.039. Anal. Calcd (Found)
for C34H27N (%): C, 90.90 (90.87), H, 6.01 (6.13), N, 3.13
(3.09).
280
320
360
400
440
Wavelength (nm)
b
Red Shift
C1
C2
C3
C4
1.0
0.8
0.6
0.4
0.2
0.0
300
350
400
450
Wavelength (nm)
Fig. 2 Typical UV/visible absorption spectroscopy of C1 to C4 in
benzene (c: 1×10−5 mol/L) a Real spectroscopy, b Normalized
Spectroscopy
(3) C3: 4,4′,4″-tri- styryl -triphenylamine
Benzylphosphonate (8.77 mmol, 2.00 g) reacted with
4,4′,4″-triformyl-triphenylamine (2.92 mmol, 0.96 g,) in
60 ml dry tetrahydronfuran using sodium methoxide
(17.5 mmol, 0.98 g) as base at room temperature overnight.
After the filtration of solid materials and the evaporation of
solvents in vacuum, the reactant mixture was dissolved in
chloroform and washed by water. The organic layer was
anhydrous magnesium sulphate. After evaporated in vacu-
um, C1 was purified with column chromatography using
benzene as eluent, which was further purified with
recrystallization in cyclohexane. C1: color: yellow, yield:
60%, m.p.: 141–142 °C, 1H-NMR (CDCl3, 500 MHz)
Table 1 Maximal absorption
Compounds
Solvents
wavelength (λa,max, nm) and the
molar extinction coefficients (ε)
of the derivatives
Benzene
THF
EtOAc
CH2Cl2
CH3CN
C1
C2
C3
10−5ε
λa,max
10−5ε
λa,max
10−5ε
λa,max
0.279
363
0.339
364
0.435
382
0.970
388
0.320
358
0.364
366
0.322
358
0.430
384
0.437
379
0.439
385
0.451
379
0.760
390
0.689
386
0.750
390
0.715
387
ε: mol−1 ·lcm−1

J Fluoresc (2011) 21:545–554
549
Table 2 Maximal linear emission wavelength (λf,max, nm) and the
quantum yields of C1 to C3
dried with anhydrous magnesium sulphate. After evaporat-
ed in vacuum, the crude 4-formyl-4′,4″-di-styryl–triphenyl-
amine (C8) was purified with column chromatography
using benzene as eluent, which was further purified with
recrystallization in cyclohexane. C8: color: yellow, yield:
52%, m.p., 94–95 °C. 1H-NMR (CDCl3,500 MHz) δ(ppm):
7.696–7.723(d, 2H, J=13.5 Hz, Ar-CHO), 7.494–7.515(d,
4H, J=10.5 Hz, Ar-H), 7.466–7.488(d, 4H, J=11.0 Hz, Ar-
H), 7.342–7.373(t, 4H, J=7.75 Hz, Ar-H), 7.245–7.274(t,
2H, J=7.25 Hz ,Ar-H), 7.139–7.165(d, 4H, J=13.0 Hz, Ar-
CH = CH), 7.043–7.116(t, 6H, J=18.3 Hz, Ar-H).
Compounds
Solvents
Benzene
THF
EtOAc
CH2Cl2
CH3CN
C1
C2
C3
φ
0.75
421
0.96
436
0.80
446
0.57
467
0.89
434
0.97
443
0.88
456
λf,max
φ
0.86
432
0.76
442
0.69
456
0.39
473
λf,max
φ
0.67
441
0.60
462
0.55
473
0.51
483
λf,max
Benzylphosphonate (8.77 mmol, 2.00 g) reacted with
4-formyl-4′,4″-di-styryl –triphenylamine (C8) (7.31 mmol,
3.49 g) in 60 ml dry tetrahydronfuran using sodium
methoxide (17.5 mmol, 0.98 g) as base at room temper-
ature overnight. The reactant mixture was processed in the
same procedure as C2 to obtain C3. C3: color: yellow,
double absorption peaks from 275 to 550 nm. The first
absorption peaks of the derivatives are similar to the
absorption spectroscopy of triphenylamine, indicating that
it could be from the local electron transition of triphenyl-
1
yield: 35%, m.p. >300 °C. H-NMR (CDCl3,500 MHz)
δ(ppm): 7.507–7.522(d, 6H, J=7.5 Hz, Ar-H), 7.426–443
(d, 6H, J=8.5 Hz, Ar-H), 7.348–7.379(t, 6H, J=7.8 Hz,
Ar-H), 7.242–7.271(t, 3H, J=7.3 Hz, Ar-H), 7.109–7.131
(d, 6H, Ar-CH = CH), 7.020–7.077(t, 6H, J=14.3 Hz, Ar-
H). 13C-NMR (CDCl3, 125 MHz): 124.9, 126.9, 127.4,
127.9, 128.5, 128.6, 129.7,137.5, 145.1. Anal. Calcd
(Found) for C30H27NO4 (%), C, 91.13 (91.24), H, 6.13
(6.01), N, 2.74 (2.67).
a
1.0
C1
C2
0.8
C3
0.6
0.4
0.2
0.0
Results and Discussion
Linear Optical Properties
A typical UV/visible absorption spectroscopy of C1 to C3
in benzene is presented in Fig. 2. The derivatives exhibit
350
400
450
500
550
600
Wavelength (nm)
b
Red Shift
1.0
1.0
C1
C2
C3
Benzene
THF
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
350
400
450
500
550
600
380
400
420
440
460
480
500
520
540
Wavelength (nm)
Wavelength (nm)
Fig. 4 Two-photon induced up-conversion fluorescence emission of
the derivatives in solvents excited by 700 nm Ti: sapphire laser a C1,
C2, C3 in benzene; b C1 in benzene and THF
Fig. 3 Normalized fluorescence spectroscopy of C1, C2 and C3 in
benzene

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J Fluoresc (2011) 21:545–554
Table 3 Two-photon optical data of C1 to C3 in 700 nm laser frequency
Solvents
C1
C2
C3
λmax (TPA)
σTPA/GM
λmax (TPA)
σTPA/GM
λmax (TPA)
σTPA/GM
Benzene
THF
418
441
46.6
57.6
436
462
218.6
290.7
443
479
254.0
288.0
λmax (TPA): nm, the maximal up-conversion fluorescence wavelength, σ: two-photon crossing section (GM, 1GM=10−50 cm·s·photo−1
)
amine core. The second absorption peaks of the derivatives,
namely the maximal absorption peak, could be ascribed to
the (π, π*) transition [31]. The absorption spectral data of
C1 to C3 in various solvents were listed in Table 1. The
data show that the maximal absorption wavelength displays
gradual red-shift in the order of C1<C2<C3. This suggests
that the extent of internal charge transfer is related to the
number of branches. The maximal absorption wavelength
of the derivatives exhibits some bathochromic shift with the
increasing of solvent polarity, which is due to internal
charge transfer nature of (π, π*) electron transition of the
derivatives [32]. Interesting, the molar extinction coeffi-
cients of the derivatives increase with the number of arms.
Table 1 shows that the ratio of the molar extinction
coefficients of C1, C2 and C3 is close to the ratio of the
number of arms, namely 1:2:3. It could be easily under-
stood from their chemical features. C1, C2 and C3 are
characterized with D-π, D-(π)2, D-(π)3 respectively, which
results in a cooperative effect of the molar extinction
coefficients of the derivatives. Figure 3 shows a gradual
red-shift for the maximal fluorescence wavelength as
C1<C2<C3. This indicates that the order of the extent of
internal charge transfer in the excited state is C1<C2<C3.
The fluorescence quantum yields and the maximal linear
emission wavelength of the derivatives are presented in
Table 2. The data show that C1, C2 and C3 exhibit strong
fluorescence in various solvents. The emission maxima of
the derivatives shift to long wavelength in polar solvents.
This means that the fluorescence spectroscopy displays
more remarkable solvent effect than the absorption spec-
troscopy, which implies that a large internal charge transfer
occurs in the excited state.
Two-Photon Optical Properties
We observed that TPA fluorescence of the derivatives was
quenched at high concentration solution, thus 5×10−4 mol/
L sample solution was used for the TPA determination. The
derivatives display remarkable TPA emission in 700 nm Ti:
sapphire femto-second laser excitation. Figure 4(a) presents
TPA emission of these compounds under 700 nm laser in
benzene. Clearly, the maximal TPA emission exhibit
gradual red-shift in the order of C1→C2→C3. Figure 4
(b) shows the TPA fluorescence of C1 in benzene and
tetrahydrofuran (THF) at 700 nm. Obviously, the maximal
TPA emission of C1 exhibits red-shift with the increasing
445
440
435
250
C1
C2
C3
200
150
100
50
430
425
420
415
C1
C2
C3
0
690
720
750
780
810
840
870
680
720
760
800
840
880
Two-photon excited wavelength (nm)
Wavelength (nm)
Fig. 5 Maximal TPA emission wavelengths of the derivatives in
benzene excited by different laser frequencies
Fig. 6 TPA cross-sections (σTPA) of C1, C2 and C3 in benzene in
various laser frequencies from 700 nm to 880 nm (c: 5×10−4 mol/L)

J Fluoresc (2011) 21:545–554
551
7000
6000
5000
4000
3000
2000
polarity of the solvents. Table 3 presents the maximal TPA
emission and TPA cross sections of the derivatives. The
results show that the orders of the maximal TPA emission
wavelength and TPA cross sections of the derivatives are
the same (C3>C2>C1), reflecting a cooperative branch
effect on two-photon optical properties. TPA cross sections
of the derivatives become larger with the increasing polarity
of the solvents, which could be ascribed to larger dipole
moment changes between the excited state and the ground
state caused by stronger intramolecular charge transfer in
polar solvents. It is well accepted that the larger dipole
moment changes between the ground state and the excited
state lead to not only the red-shift of the maximal TPA
emission, but the larger TPA cross section [33–35].
C1
C2
C3
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Orientation polarizability (Δf)
We also utilized different excited wavelength (700~880 nm)
to detect TPA fluorescence of the derivatives. The maximal
Fig. 8 Relationship between Stokes’ shifts of the derivatives and the
orientation polarizability (Δƒ) of solvents respectively
TPA emission wavelengths of the derivatives excited by
different laser frequencies are presented in Fig. 5. It is
interesting to observe that the maximal TPA emission of C1
is independence on the excited wavelength and it is almost
identical to one-photon maximal fluorescence wavelength,
which implies that one-photon and two-photon excited
fluorescence spectroscopy could be from the same or similar
excited state. Figure 6 shows the TPA cross sections of the
derivatives in various laser frequencies. Clearly, the TPA
cross sections are in the order C3>C2>C1 in most laser
frequencies, which reveals further the branch cooperative
effects on the two-photon optical properties. We shall point
out that the TPA cross section of C1 is very close to reported
data [36]. Herein, the relationship of TPA intensities of the he
derivatives and the pumped powers were determined further,
which follows well the square law in the 800 nm laser
excitation (Fig. 7), and the slopes of 1.916 and 1.973 for C2
and C3 respectively demonstrate that the derivatives have
excellent two-photon absorption nature [26].
a
5.2
C2
5.0
4.8
4.6
4.4
4.2
4.0
Slope=1.916
1.6
1.7
1.8
1.9
2.0
2.1
2.2
Log I
0
b 5.2
C3
5.0
4.8
4.6
4.4
4.2
4.0
Dipole Moment Changes
Slope=1.973
It is well accepted that the dipole moment changes are
related with the optical properties of organic compounds.
Table 4 Changes of dipole moment between the excited state and
ground state respectively obtained from experiments and theoretical
calculations
1.6
1.7
1.8
1.9
2.0
2.1
2.2
Dipole moments change Δμ (Derby)
C1
C2
C3
Log I0
From experiment
From theory*
2.59
2.76
3.92
2.87
4.01
3.05
Fig. 7 Square relationships between TPF (TPA fluorescence)
intensity of the derivatives and the excited laser powers at
800 nm a for C2, b for C3
1 D=3.336×10−30 c.m, * in vacuum, for single molecule

552
J Fluoresc (2011) 21:545–554
HOMO
LUMO
Table 6 The redox potentials of compounds C1 to C3 determined in
methylene chloride at 0.1 V·s−1scan rate
Compounds
Oxidation potentials (V)
Reduction potentials (V)
C1
C2
C3
C4
1.11/0.90
1.14/0.72/0.34
1.16
0.53/−0.13
0.74/0.49/−0.17
0.71/−0.14/−0.80
0.42/−0.26/−0.57
C1
C2
1.03
effect of the index of refraction and relative dielicetric
constant. As a consequence, the linear correlation between
Stokes shifts and Δƒ does not reflect 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) are shown in Fig. 8. The linear
correlations between Stokes shift and orientational polariz-
ability reflects the dipolar solvent effects. The equations are
obtained as: C1, Y=7402.6X+3454.2, C2, Y=7760.3X+
2538.1, C3, Y=8069.3X+2630.3. Thus, the dipole moment
changes between the excited state and the ground state are
calculated from the slopes and listed in Table 4. The order of
the dipole moment change are C3>C2>C1, which demon-
strates that the extent of internal charge transfer in the
excited state is greatly influenced by the number of arms.
The dipole moment changes between the excited state and
ground state were further calculated theoretically on the basis
of molecular geometry optimization. Table 4 shows that the
theoretical results are good agreement with the experimental
results.
C3
Fig. 9 Electron density distributions of frontier orbital of C1, C2 and C3
We further estimated the dipole moment changes between
the excited state and the ground state based on Lippert
equation [37, 38]:
ꢂ
ꢃ
2
2 me ꢀ mg
hcðnabs ꢀ nemÞ ¼
Δf þ const
ð4Þ
4p"0a3
As shown in Fig. 9, the derivatives have typical (π, π*)
character, while they exhibit different electron distribution
in frontier orbitals. Seen from Fig. 9, for the HOMO
orbitals of the derivatives, the electron density is mainly
distributed around N-core. While for the LUMO orbital of
the derivatives, the majority of the electron density
distribution is located at diphenylehylene part. Thus, it is
obvious that the majority of the electron density distribution
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, em are
υ
the wavenumbers of the absorption and emission respec-
tively, n is the refractive index, and ε is the relative low-
frequency dielectric constant of the solvents. The chromo-
phore group is considered as a dipole, which locates in a
cavity with a radius of a in a continuous solvent-dipole
environment, and Lippert equation describes a solvent
Table 7 Estimated HOMO and LUMO energies of compounds 1 to 3
from cyclic voltammograms
Compounds λonset (nm) Eg (eV) ELUMO (eV) EHOMO (eV)
Table 5 Energy gaps of HOMO and LUMO of the derivatives
obtained from theoretical calculations
C1
C2
C3
408.0
424.8
431.2
3.04
2.92
2.88
−5.45
−5.48
−5.50
−8.49
−8.40
−8.38
Derivatives
EHOMO (eV)
ELUMO (eV)
Energy gap (eV)
C1
C2
C3
−7.861
−7.793
−7.744
−0.4531
−0.5585
−0.5782
7.4079
7.2345
7.1658
Eg=1240/λ, LUMO(eV) = −EOX −4.34 [41, 42], HOMO(eV) = LUMO–
Eg , λonset:the longest absorption wavelength at ten percent of the maximal
UV peak

J Fluoresc (2011) 21:545–554
553
for C1 is in one arm in LUMO, and two arms have the most
of electron density distribution in LUMO of C2, and the
electron density is distributed in three arms in LUMO of
C3. As a consequence, the charge delocalization of frontier
orbitals of the derivatives are in the order of C3>C2>C1.
The theoretical HOMO-LUMO energy of the derivatives
were further calculated, and the results are listed in Table 5,
which shows that the order of HOMO-LUMO gap is
C1>C2>C3. The results show that the not only the electron
density distribution and the dipole moment changes of the
excited state and the ground state could be tuned by the
numbers of arms, but the energy gaps of the frontier orbital
could be regulated. The results explain the gradual red-shift
of absorption and fluorescence spectroscopy of C1, C2 and
C3. This also interprets why the maximal TPA fluorescence
wavelength and TPA cross section of the derivatives are
related to the number of arms.
Cyclic voltammograms of the derivatives were acquired at
various scan rates from 50 to 200 mV·s−1. No corresponding
potentials demonstrate that redox processes of the derivatives
are characterized with irreversible nature under all sweeping
rates. The linear increasing of peak currents with the square
root of scan rates indicates that the electron transfer reactions
are controlled by the solvent diffusion [39]. Table 6 presents
the redox potentials of C1 to C4 at 0.1·V·s−1scan rate.
Obviously, the first oxidation-reduction potentials of C1 to
C3 could be assigned to N-core. We further estimated
HOMO-LUMO gap based on the optical band gap (Eg),
which was calculated from the onset of the longest
absorption wavelength at ten percent of the maximal UV
peak [40], and the results are presented in Table 7. It clearly
shows the HOMO energy of the derivatives is in the order of
C3>C2>C1, while LUMO energy is the order of
C1>C2>C3, HOMO-LUMO gap is tuned efficiently by the
number of branches. It interpreters well why one and two-
photon optical properties of the derivatives have a close
relationship with the number of arms.
the development of new branched dyes with required one-
and two- optical properties.
Acknowledgements The authors appreciate financial support from
National Natural Science Foundation of China (Nos. 20776165,
20702065, 20872184). We would thank “the Foundation of Chongq-
ing Science and Technology Commission” (CSTC2008BA4020,
CSTC2009BB4216). H. Li thanks “A Foundation for the Author of
National Excellent Doctoral Dissertation of PR China (200735)”, and
thanks supports from the Key Laboratory of Functional Crystals and
Laser Technology, TIPC, Chinese Academy of Sciences. We thank Dr.
X. Chen for helpful discussion on molecular geometry optimization.
We also thank “Innovative Talent Training Project, the Third State of
‘211 Project, S-09103’, Chongqing University.
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