Synthesis, photophysical properties and TD-DFT calculation of four two-photon absorbing triphenylamine derivatives

Article abstract of DOI:10.1007/s11426-012-4794-4

In this study, linear absorption, single-photon excited fluorescence, fluorescence quantum yields, fluorescence lifetime and two-photon excited fluorescence of a series of triphenylamine derivatives (L1, L2, L3 and L4) have been measured. L1 and L3 are D-π-A type dyes, while L2 and L4 are D-π-D-π-A type dyes (D = donor, A = acceptor). The investigated compounds consist of triphenylamine-bearing donor-substituted and/or systematically extended π-conjugated length, which are designed to gain insight into the effect of the ethoxyl unit and π-linkage length on the linear and nonlinear optical properties. The influence of solvent polarity on the photophysical properties was investigated. Employing time-dependent density functional theory (TD-DFT) calculations, the structure-property relationships are discussed.

Full text of DOI:10.1007/s11426-012-4794-4

SCIENCE CHINA
Chemistry
January 2013 Vol.56 No.1: 106–116
doi: 10.1007/s11426-012-4794-4
• ARTICLES •
Synthesis, photophysical properties and TD-DFT calculation of
four two-photon absorbing triphenylamine derivatives
KONG Lin1, YANG JiaXiang1, 2, ZHOU HongPing1, LI ShengLi1, HAO FuYing1,
ZHANG Qiong1, TU YuLong1, WU JieYing1, XUE ZhaoMing1 & TIAN YuPeng1, 2, 3*
1Department of Chemistry, Key Laboratory of Functional Inorganic Materials of Anhui Province, Anhui University, Hefei 230039, China
2State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
3State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, China
Received March 29, 2012; accepted August 10, 2012; published online December 21, 2012
In this study, linear absorption, single-photon excited fluorescence, fluorescence quantum yields, fluorescence lifetime and
two-photon excited fluorescence of a series of triphenylamine derivatives (L1, L2, L3 and L4) have been measured. L1 and
L3 are D--A type dyes, while L2 and L4 are D--D--A type dyes (D = donor, A = acceptor). The investigated compounds
consist of triphenylamine-bearing donor-substituted and/or systematically extended -conjugated length, which are designed to
gain insight into the effect of the ethoxyl unit and -linkage length on the linear and nonlinear optical properties. The influ-
ence of solvent polarity on the photophysical properties was investigated. Employing time-dependent density functional theory
(TD-DFT) calculations, the structure-property relationships are discussed.
triphenylamine derivatives, optical properties, structure-property relationship, TD-DFT calculation
ested in triphenylamine-derivatives. Triphenylamine has
1 Introduction
been known to exhibit rigid plane, long conjugation length
and good hole transporting properties [4]. Thus, triphenyl-
amine and its derivatives have displayed promising proper-
ties in the development of light emitting [5], organic
field-effect transistors (OFET) [6], photovoltaic devices
[79] and dye-sensitized solar cells [10]. In addition, elec-
tron transfer or electron separation between the tri-
phenylamine group and the accepting group provides this
class of compounds highly anisotropic structures and
results in interesting photophysical properties. Recently,
our group has synthesized a number of triphenylamine de-
rivatives with linear and/or nonlinear optical properties [11].
Considering all the aspects above, in this work, we se-
lected a series of ICT structural triphenylamine derivatives
L1L4 (Scheme 1), including 2-(4-(diphenylamino) ben-
zylidene)-malononitrile (abbreviated as L1), 2-(4-(4-
(diphenylamino)-styryl)-benzylidene)malononitrile (abbre-
Nowadays, there have been increasing interests in organic
optical materials, among which, intramolecular charge-
transfer (ICT) compounds are regarded as ideal candidates
due to their possible applications in optoelectronic and/or
photonic fields [1, 2]. ICT molecule is characterized by an
electron-donating (D) and an electron-accepting (A) group
linked through -conjugation. The optical properties of
ICT molecules depend mainly on the polarizability of the
electrons localized in front line molecular orbitals, which
lies on the chemical structure of D and/or A group, the
property of the donor and acceptor groups, the -conju-
gated spacer length, and the polarizability of the solvent
used for measurements [3].
When choosing electron-donating group, we are inter-
*Corresponding author (email: yptian@ahu.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

Kong L, et al. Sci China Chem January (2013) Vol.56 No.1
107
viated as L2), 2-(4-(bis(4-ethoxyphenyl)amino)-benzylidene)
malononitrile (abbreviated as L3) and 2-(4-(4-(bis-(4-eth-
oxyphenyl)-amino)styryl)benzylidene)malononitrile (abbre-
viated as L4). In molecule L1, the triphenylamine group is
employed as an electron-donating unit, the dicyano group
as an electron-accepting unit, and they are linked by a
vinyl bond. Based on the structure of L1, the -electron
conjugated system is enlarged by forming –C=C– to get L2,
which is chosen to study the influence of the -conjugated
spacer length on the optical properties. Furthermore, eth-
oxyl units are introduced to the structure of L1 and L2 to
form L3 and L4, respectively. The ethoxyl unit is expected
to balance the charge recombination of the whole structure
[12]. L1 and L2 are typical ICT compounds and have
caused many interests. In 2008, Chen [13] reported the
synthesis of L1. In 2009, Li [14] reported the crystal struc-
ture of L1. In 2012, Lin [15] used L2 as a colorimetric and
turn-on fluorescence chemosensor for cyanide anion. How-
ever, the optical properties (especially two-photon absorp-
tion and two-photon excited fluorescence) of these com-
pounds have not been thoroughly studied, not to say the
relationship between the structure and the photophysical
properties. Thus, in this study, linear and nonlinear optical
properties of L1–L4 are measured. Based on TD-DFT cal-
culations, the relationships between the structure and the
optical properties are discussed.
methane (CH2Cl2), ethyl acetate (EtAc), tetrahydrofuran
(THF) and N, N-dimethylformamide (DMF), were spec-
troscopic grade. The compounds were characterized by
NMR, FT-IR, and elemental analysis.
Elemental analysis was performed with a Vario ELⅢ
1
elemental analyzer. The H NMR spectra were recorded on
a Bruker Avance 400 spectrometer. FT-IR spectra were
recorded on a NEXUS 870 (Nicolet) spectrophotometer in
the 4004000 cm1 region using a powder sample on a KBr
plate. UV-Vis absorption spectra were recorded on a
UV-3100 spectrophotometer. The fluorescence spectra were
recorded on a Hitachi F-7000 fluorescence spectrophotom-
eter. The fluorescence quantum yields () were determined
against the fluorescein standard ( = 0.90) [9]. Fluores-
cence lifetime measurements were carried out using an
HORIBA FluoroMax-4P fluorescence spectrometer equip-
ped with a time-correlated single-photon counting (TCSPC)
card. The two-photon excited fluorescence (TPEF) spectra
were measured using a mira 900-D Ti: sapphire femto-
second laser with a pulse width of 140 fs and a repetition
rate of 80 MHz. Thermal analyses were performed with the
Netzsch 449F3 thermal analyzer under N2 atmosphere at a
heating rate of 10 °C min1.
2.2 Preparation of compound L3L4
The detailed synthesis procedures of L3 and L4 are listed
below. The synthesis procedures of other intermediates are
provided in Supporting Information.
2 Experimental
2.1 Materials and physical measurements
2.2.1 Preparation of L3
All chemicals were commercially available and used with-
out further purification. The solvents, benzene, dichloro-
4-(Bis(4-ethoxyphenyl)amino)benzaldehyde (0.71 g,
2
mmol), malononitrile (0.16 g, 2.4 mmol) and K2CO3 (0.34 g,
2.5 mmol) were dissolved in ethanol (30 mL), refluxed
overnight and monitored by TLC. Upon completion of the
reaction, the mixture was cooled to room temperature,
poured into CH2Cl2 (500 mL), washed with water (100 mL)
and dried over MgSO4. The solvent was evaporated, and the
residue was purified by column chromatography of silica
:
gel with ethyl acetate/petroleum ether mixture (1 50 v/v) as
1
the eluent. L3 was obtained as a red solid in 57% yield. H
NMR (400 MHz, CDCl3) δ (ppm): 8.13 (s, 1H); 7.78(d, 2H,
J = 8.8 Hz); 7.22 (d, 4H, J = 8.0 Hz); 6.99 (m, 5H); 6.67 (d,
2H, J = 8.8 Hz); 4.00 (m, 4H); 1.33 (t, 6H, J = 6.8 Hz).
FT-IR (KBr cm1): 2978, 2922 (m), 2216 (CN, m), 1712
(m), 1591 (s), 1510 (s), 1268 (s), 1238, 1047 (s). Anal. Calc.
for C26H23N3O2: C 76.26, H 5.66, N 10.26%; Found: C
76.03, H 5.14, N 11.47 %.
2.2.2 Preparation of L4
4-(4-(Bis(4-ethoxyphenyl)amino)styryl)benzaldehyde (0.46
g, 1 mmol), malononitrile (0.073 g, 1.1 mmol) and drops of
piperidine were dissolved in ethanol (30 mL), refluxed
overnight and monitored by TLC to ensure complete reac-
Scheme 1 Molecular structures of the investigated triphenylamine deriv-
atives L1–L4.

108
Kong L, et al. Sci China Chem January (2013) Vol.56 No.1
tion. Then, the mixture was cooled to room temperature,
poured into CH2Cl2 (300 mL), washed with water (50 mL 
4) and dried over MgSO4. The solvent was evaporated, and
the residue was purified by column chromatography of sili-
3 Results and discussion
3.1 Synthesis
Schemes 12 show the chemical structures of L1L4 and
the respective preparation processes in this study. The or-
ganic compounds are obtained through the Knoevenagel
condensation of malononitrile and the corresponding alde-
hydes. The foregoing spectroscopic data are in line with the
designed structures.
:
ca gel with ethyl acetate/petroleum ether mixture (1 50 v/v)
as the eluent. L4 was obtained as a yellow solid in 60%
yield. 1H NMR (400 MHz, d6-DMSO) δ (ppm): 7.95 (s, 1H),
7.55 (d, 2H, J = 8.0 Hz), 7.40(d, 2H, J = 8.8 Hz), 7.34(d, 2H,
J = 7.2 Hz), 7.28(s, 2H), 7.24 (d, 2H, J = 8.4 Hz), 7.17 (d,
2H, J = 17.2 Hz), 7.12 (d, 1H, J = 16.0 Hz), 6.97 (1H), 6.74
(d, 2H, J = 7.6 Hz), 6.57 (d, 1H, J = 8.0 Hz), 6.48 (1H),
3.97(m, 4H), 1.34(t, 6H). FT-IR (KBr cm1): 2982, 2922,
2861 (m), 2216 (CN, m), 1621 (m), 1561 (s), 1500 (s),
1248 (s), 1188 (s). Anal. Calc. for C34H29N3O2: C 79.82, H
5.71, N 8.21%; Found: C 79.50, H 5.43, N 7.94%.
3.2 Thermogravimetric analysis of L1L4
Thermal stability is an important property of a designed
organic compound. The results of the TG measurements
are listed in Figure 1. L1L4 exhibit high thermal stability
and the quickest decomposition temperature (Tdecomp.) of
L1L4 ranges from 318 °C to 410 °C.
In order to improve the thermal stability of D--A type
compounds, there are many routes to adjust the structure. In
this work, it is observed that the difference of an electron-
donating unit and the systematically extended -conju-
gated length affects the thermal stability of D--A type
compounds. L1 losses its weight from 274 °C to 326 °C
Scheme 2 The synthetic routes for L1L4. L1 (Ar = 4-(diphenylamino)
benzaldehyde), L2 (Ar = -(4-(diphenylamino)-styryl)benzaldehyde), L3
(Ar = 4-(bis(4-ethoxyphenyl)amino)benzaldehyde ), L4 (Ar = 4-(4-(bis
(4-ethoxyphenyl)-amino)styryl)benzaldehyde).
Figure 1 TG curves of L1L4. The inserts are DTG curves.

Kong L, et al. Sci China Chem January (2013) Vol.56 No.1
109
(Tdecomp. is 318 °C). Compared to the thermal stability of L1,
incorporation of either extended -conjugated length or
ethoxyl unit notably increases the thermal stability of
L2L4. L2 losses its weight from 318 °C to 405 °C (Tdecomp.
= 381 °C). L3 losses its weight from 278 °C to 355 °C
(Tdecomp. = 347 °C), and L4 losses its weight from 362°C to
496 °C (Tdecomp. = 410 °C).
one peak, which centers at about 420440 nm in the select-
ed solvents for L1 and 430440 nm for L3, which corre-
sponds to the -* transition of the main chain. The absorp-
tion spectra of L2 and L4 show two absorption maxima
(abs-max) between 250 nm and 450 nm, which are located at
294312 nm, 394407 nm for L2, and 283294 nm,
390400 nm for L4 in the five selected solvents, respec-
tively. The longer wavelength at about 400 nm of L2 and
L4 corresponds to the -* transition of the whole structure,
while the shorter wavelength at about 300 nm is mainly
associated with the -* transition of the triphenylamine
moiety [16, 17]. The linear absorption properties of L1L4
are further studied by TD-DFT calculation, which results in
similar conclusion.
The abs-max of L1–L4 is bathochromically shifted with
the increasing polarity of the solvent. abs-max of L1 is shift-
ed by 12 nm from 428 nm in benzene to 440 nm in DMF,
while that of L2 is shifted by 13 nm from 394 nm in ben-
zene to 407 nm in DMF, that of L3 is shifted by 10 nm (431
nm in benzene and 441 nm in DMF), and that of L4 is
shifted by 10 nm (390 nm in benzene and 400 nm in DMF).
This bathochromic shift reflects electron donating ability.
The results revealed that changing the -conjugated
length and/or introducing the ethoxyl unit to the electron-
donating group can influence the electron-donating ability
3.3 Linear absorption and single-photon excited fluo-
rescence (SPEF)
In this study, an important goal beyond synthesis and the
thermal property is to investigate the effect of the -conju-
gated spacer length and the ethoxyl unit on the optical
properties. The photophysical properties of L1L4 in five
different solvents (benzene, CH2Cl2, EtAc, THF, and DMF)
at the concentration of 1.0  106 mol/L are collected in
Table 1, including single-photon absorption and fluores-
cence, the corresponding fluorescence decay lifetime and
quantum yields.
3.3.1 Linear absorption
Linear absorption spectra of L1–L4 in five different sol-
vents are shown in Figure 2. As can be seen from Table 1
and Figure 2, the absorption spectra of L1 and L3 exhibit
Table 1 Photophysical properties of L1L4 in several of different polar solvents
Compounds
Solvents
Benzene
CH2Cl2
EtAc
THF
DMF

abs-max (nm)
428
0.98
514
0.06
3909
2.39
1.23
431
3.33
545
0.009
4853
3.38
0.98
433
3.08
572
0.01
5612
2.03
0.86
439
2.80
577
0.01
5448
3.70
1.20
440
2.10
585
0.02
5633
2.19
0.97
max104 a)

em-max (nm)
b)
L1
 (cm1) c)
τ (ns) d)
 e)

abs-max (nm)
294 394
3.29
295 395
5.32
303 399
3.47
308 406
4.18
312 407
3.96
max104 a)

em-max (nm)
b)
488
0.19
4953
2.10
1.06
524
0.10
6232
2.69
1.09
531
0.12
6230
2.32
1.32
550
560
0.04
6713
2.43
1.08
L2
L3
L4
0.078
6449
2.35
 (cm1) c)
τ (ns) d)
 e)
0.94

abs-max (nm)
431
1.40
509
0.14
3609
2.86
1.21
432
1.61
520
0.10
3917
1.42
0.97
434
1.57
519
0.09
3774
2.43
1.02
437
2.53
523
0.11
3763
2.41
0.92
441
1.06
533
0.11
3914
0.234
1.05
max104 a)

em-max (nm)
b)
 (cm1) c)
τ (ns) d)
 e)

abs-max (nm)
283 390
0.53
509
0.23
5995
3.63
289 393
0.67
527
0.11
6470
2.50
294 397
0.54
531
0.05
6356
1.63
1.09
288 399
0.63
552
0.026
6947
1.58
290 400
0.63
559
0.023
7174
3.07
max104 a)

em-max (nm)
b)
 (cm1) c)
τ (ns) d)
 e)
1.18
1.13
1.12
1.02
a) One-photon absorption coefficients ( is in unit of dm3 mol1 cm1); b) quantum yields determined by using fluorescein as standard; c) Stokes’ shift; d)
FL lifetime; e) goodness of fit.

110
Kong L, et al. Sci China Chem January (2013) Vol.56 No.1
Figure 2 Linear absorption spectra of L1L4 in five different solvents.
of the triphrnylamine group. Furthermore, the different
electron-donating ability may influence the bandgap of the
intramolecular charge-transfer material, which will result in
different linear absorption properties. Similar phenomenon
is expected to appear in single-photon excited fluorescence.
DMF with I = 666 a.u.). Large values of the Stokes’ shift
are observed for L1L4 in the selected solvents due to
strong solvent-solute dipole-dipole interactions, which ex-
hibit large orientational polarizability and dipole moment.
As L1L4 contain electron-donating group (triphenyla-
mine) and electron-withdrawing group (dicyano), charge
separation in the fluorophore is their most significant char-
acter, which results in dipole moment in the ground state.
Meanwhile, in the excited state, the degree of charge sepa-
ration increases, and a larger dipole moment appears than
that in the ground state [18]. The charge separation will be
even larger with the increasing of solvent polarity. Thus, the
emission spectra of the four compounds exhibit remarkable
solvent polarity sensitivity.
Comparing the bathochromically shifted emission wave-
length between L1 and L2 (69 nm for L1 and 72 nm for L2,
respectively), L3 and L4 (34 nm for L3 and 60 nm for L4,
respectively), one can see that extending -linkage length
results in increasing positive solvatochromism. The result is
consistent with the observations of UV-Vis absorption ex-
periments.
3.3.2 Single-photon excited fluorescence (SPEF)
The SPEF spectra (Figure 3) and the corresponding FL life-
time (Figure 4) have been measured under the same condi-
tions as the linear absorption spectra. As shown in Figure 3
and Table 1, the SPEF spectra of L1L4 display positive
solvatochromism (red-shift with increasing solvent polarity)
in their emission spectra. Moreover, the fluorescence inten-
sity decreases with the increasing solvent polarity. For ex-
ample, the maximum emission wavelength of L1 centers at
about 514 nm in benzene (I = 1262 a.u.), while in a higher
polar solvent DMF, the peak locates at 583 nm (I = 852
a.u.), which is red-shifted by 69 nm and one third decrease
of intensity due to different molecular environment. The
SPEF spectra of L2, L3 and L4 all exhibit the same trend.
The peak of L2 red shifts by 72 nm from 495 nm in benzene
(I = 10035 a.u.) to 567 nm in DMF (I = 1981 a.u.). The peak
of L3 red shifts by 34 nm (509 nm in benzene with I = 5313
a.u. and 533 nm in DMF with I = 2315 a.u.) and that of L4
is 60 nm (509 nm in benzene with I = 93082 a.u., 559 nm in
Comparing to the SPEF spectra of L1, L2L4 showed
blue-shifted emission wavelength no matter when extending
the -linkage unit or introducing the ethoxyl unit to the
triphenylamine group. This may be resulted from the easy-

Kong L, et al. Sci China Chem January (2013) Vol.56 No.1
111
Figure 3 The single-photon excited fluorescence spectra of L1L4.
larger than those of L1, e.g. 0.19 for L2 and 0.06 for L1 in
C6H6 solution. The same trend was observed for L3 and L4,
e.g. 0.23 for L4 and 0.14 for L3 in C6H6 solution, which
reveals that extending the -linkage unit has advantages to
improve the fluorescence quantum yields. Comparing the
data between L3 and L1, L4 and L2, one can see that in-
troducing the ethoxyl unit to the electron donating group
can result in higher fluorescence quantum yields.
assembly character of L1 [14] (the SPEF spectra of L1L4
at the concentration of 1.0  103 mol L1 is listed in Sup-
porting Information Figure S1). Either increasing the
-linkage length or introducing the ethoxyl unit will de-
crease the assembly of the molecules, thus blue-shifted
emission wavelength was observed.
As can be seen in Table 1, the fluorescence quantum
yields of L1L4 change a lot with different solvents. The
emission quantum yields show a trend of reduction along
with the red shift of SPEF. For example, the fluorescence
quantum yield of L1 in benzene is 0.06, while it is reduced
to 0.01 in EtAc and THF. The result indicates that in polar
solvents, the intermolecular interactions between the dye
molecules and the solvent molecules in the excited state are
stronger than that in the non-polar solvents, which induces a
further increase of the effect of energy- and/or charge-
transfer processes between the excited and the ground state
of the dye molecules. Thus, a decrease of energy in the ex-
cited state is expected, accompanied by a red shift of SPEF
and decrease of fluorescence quantum yield.
3.3.3 Time-resolved measurements
The time-resolved PL decays of L1L4 are measured using
a time-correlated single-photon counting (TCSPC) tech-
nique. SiO2 nanocrystals in H2O sample is used as an IRF
reference for the life-times reconvolution. During the meas-
urements, compounds are excited with the optimal excita-
tion wavelength (ex-max) in order to obtain the largest rela-
tive fluorescence intensity. The data shown in the study is
examined at the em-max of L1L4 respectively. The fluo-
rescence decay profiles of L1L4 in five different solvents
are shown in Figure 4. The results in THF solutions will be
discussed in detail as an example. The fluorescence decay
lifetimes and its amplitude in THF are summarized in Table
2. The fluorescence of L1, L3 in THF is found to decay
with two components, while L2, L4 in THF is found to de-
In this study, higher  values are obtained in non-polar
solvents with the highest  values being observed in C6H6.
Therefore, the results in C6H6 are chosen to be discussed as
an example. The fluorescence quantum yields of L2 are

112
Kong L, et al. Sci China Chem January (2013) Vol.56 No.1
Figure 4 Time-domain fluorescence intensity decay of L1L4.
Table 2 Fluorescence decay lifetime () and amplitude (A) of L1L4 in THF solution
 (ns)
2.03
2.26
2.41
2.50
χ
Compounds
A1 (%)
24.7
6
A2 (%)
75.3
91
A3 (%)

em (nm)
577
1 (ns)
0.80
2 (ns)
2.43
3 (ns)

4.20
L1
L2
L3
L4
0.86
0.98
0.92
1.13

3
550
1.21
2.26
523
1.48
10.5
64
2.52
89.5
28

12.50

7
552
0.89
3.76
cay with three components. There are two kinds of mecha-
nisms to explain the multi-exponential of FL lifetime. One
is that there exist more than one component in the excited
state and each component has its inherent FL lifetime. The
other is that the fluorophore exists as a dimer and/or trimer
in the solution. As for the latter mechanism, there will be a
longer wavelength emission in the SPEF [2022]. However,
the experimental result shows only one peak for L1L4,
which rules out the second mechanism. The result reveals
that there are more than one component in the excited state
of L1L4. As for L1 and L3, there exist two kinds of ex-
cited state, which is in accordance with the two excitation
bands (see Supporting Information Figure S2). On the other
hand, there exist three kinds of excited state of L2 and L4.
For detail, in the THF solution of L1 at 577 nm emission, a
short component has a decay lifetime of 0.80 ns (the corre-
sponding amplitude is 24.7%), and the following compo-
nent has 2.43 ns (75.3%). With extending -conjugated
length to L2, the fluorescence decay lifetime and the corre-
sponding amplitude of a short component are 1.21 ns and
6%, respectively. The decay lifetime of the following com-
ponent is 2.26 ns with amplitude being 91%. It is worth
noting that there exists the third component with the fluo-
rescence decay lifetime and the corresponding amplitude
being 4.20 ns and 3%, respectively. The similar trend is also
observed between L3 and L4. The results reveal that the
extended -conjugated length leads to additional excited
state. However, the emission band of L1L4 shows only

Kong L, et al. Sci China Chem January (2013) Vol.56 No.1
113
one peak. In the case of this point, the intense of other bands
should overlap the existing band or they appear as shoulder
on the spectrum.
The average fluorescence decay lifetime of L3 at 523 nm
emission is 2.41 ns, which is longer than that of L1 (2.03 ns
at 577 nm emission band). The same trend also appears be-
tween L2 and L4 ( = 2.26 ns for L2 at 550 nm emission
band and 2.50 ns for L4 at 552 nm), i.e, introducing the
ethoxyl unit to the electron-donating group seems to provide
a more stable environment for the excited state.
pounds. In the HOMO, the electrons are primarily concen-
trated on the nitrogen atom of the triphenylamine and its
adjacent benzene ring. In the LUMO, the electrons are
mainly concentrated on cyano group and its adjacent ben-
zene ring. Obviously, the electron transition from the
HOMO to the LUMO is accompanied by intramolecular
charge transfer from the central triphenylamine moiety to
the peripheric cyano unit. The calculated HOMO-LUMO
gap is 3.11 eV for L1, 2.40 eV for L2, 2.80 eV for L3 and
2.21 eV for L4.
Introduction of the ethoxyl unit to the triphrnylamine
group leads to little energy changes of the LUMO orbits
(2.50 eV for L1 and 2.48 eV for L3, for example), and
significant energy changes of the HOMO orbits (5.61 eV
for L1 and 5.28 eV for L3). Thus, the HOMO-LUMO gap
decreases from L1 (3.11 eV) to L3 (2.80 eV). The same
trend is also observed for L2 and L4. These results show
that the strong electron-donating unit (ethoxyl unit) may
raise the HOMO level and lead to smaller gap [12]. The
extended -conjugated length can both raise the HOMO
3.4 Theoretical calculations
As is well known that electronic excitation from the HOMO
to the LUMO produces the first singlet excited state. Thus,
molecular orbital calculations of TD-DFT at the B3LYP
level basis set (Gaussian 03 [23]) are performed on com-
pounds L1L4 to study their electronic structures. The or-
bital features presented in Table 3 might provide insights to
understand the corresponding optical properties of the com-
Table 3 Frontier orbital and the corresponding energy of L1L4 in the gas phase
Compounds
HOMO-1
HOMO
LUMO
LUMO+1
L1
energy
6.94 (eV)
5.61
2.50
0.95
Energy gap from HOMO to LUMO for L1 is 3.11 eV
L2
energy
6.23(ev)
5.20
2.80
0.051
Energy gap from HOMO to LUMO for L2 is 2.40 eV
L3
energy
0.033
6.78(eV)
5.28
2.48
Energy gap from HOMO to LUMO for L3 is 2.80 eV
L4
energy
5.96(eV)
4.93
2.72
0.046
Energy gap from HOMO to LUMO for L4 is 2.21 eV

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Kong L, et al. Sci China Chem January (2013) Vol.56 No.1
level and lower the LUMO level, which will lead to an even
smaller HOMO-LUMO gap (comparing L1 to L2, L3 to
L4).
3.5 Two-photon absorption (TPA) and two-photon
excited fluorescence (TPEF)
As shown in linear absorption spectra, there is no linear
absorption in the wavelength range of 550–900 nm for
L1L4. It is inferred that there are no energy levels corre-
sponding to one electron transition in this spectral range.
Therefore, the two-photon excited fluorescence (TPEF)
pathway is expected to appear if frequency up-converted
fluorescence induced with a laser in this range appears [24].
The insets of Figure 5 show log-log plot of the excited flu-
orescence signal vs. excited light power. The linear de-
pendence of the square of input laser power on the output
fluorescence intensity (with a slope of 2.04, 2.03, 1.98 and
2.02 for L1L4 respectively) provides direct evidence for
two-photon excited process.
The TPA cross-sections have been measured using the
two-photon-induced fluorescence measurement technique
[25]. The TPA cross sections (δ) are determined by com-
paring their two-photon excited fluorescence to that of flu-
orescein in the NaOH solution (aq, 0.1 mol/L) (pH = 11)
[26, 27] according to the following equation [28]
The theoretical spectral characteristics, detailed infor-
mation is shown in the Supporting Information Table S1,
show one main transition for L1 (λabs = 421 nm) and L3 (λabs
= 437 nm), which is just from the HOMO orbit to the
LUMO. When extending the -conjugated length of L2 and
L4, the theoretical spectra show two main optical transitions.
One transition corresponds to the transition from the con-
figuration HOMO-1 to LUMO with λabs = 386 nm (oscilla-
tor strength f(HOMO-1)-LUMO being 0.9789) for L2 and λabs
=
405 nm (f(HOMO-1)-LUMO being 0.9885) for L4, respectively.
The other optical transition for L2 corresponds to the con-
figuration HOMO to LUMO+3 with λabs = 308 nm and
fHOMO-(LUMO+3) being 0.1336. While the other transition for
L4 corresponds to the configuration HOMO to LUMO+4
with λabs = 300 nm and fHOMO-(LUMO+4) being 0.1733. The
calculated results fit with the experimental data and the
corresponding absorption bands are observable on the ab-
sorption spectra of L1L4.
Figure 5 TPF spectra of L1L4 in DMF solution pumped by femtosecond laser pulses under different pumped powers at their maximum excitation wave-
length, insert is the logarithmic plots of the output fluorescence (Iout) vs. the square of input laser power (Iin).

Kong L, et al. Sci China Chem January (2013) Vol.56 No.1
115
neglected. As mentioned above, the molecules of L1 tend to
self-assembly. Thus the aggregate effect may be the main
influencing factor for L1, which can explain the phenomena
that the TPEF peak position of L1 is very close to that of
the SPEF. However, the re-absorption effect may be more
important for L2L4.
According to Eq. (1), the largest δTPA is 87 GM (1 GM =
1050 cm4 s photon1) for L1 at 720 nm excitation, 3733 GM
for L2 at 800 nm, 18 GM for L3 at 780 nm, and 144 GM
for L4 at 800 nm, respectively. It is worthwhile noting that
δ is strongly enhanced with the increasing -linkage length.
Comparing L2 to L1, the value offers a > 40-fold increase.
Meanwhile, the ethoxyl unit also influences the optical
property. Comparing L1 to L3, the value offers a ~ 5-fold
increase, which confirms the effect of the ethoxyl unit on
the charge recombination in the whole structure.
ref Cref nref
F
  ref
(1)

C
n F
ref
In this equation, the subscripts ref refers to the reference
molecule, which is fluorescein.  is the TPA cross-section
value, c is the concentration of the solution, n is the refrac-
tive index of the solution, F is the TPEF integral intensities
of the solution emitted at the exciting wavelength, and  is
the fluorescence quantum yield. The δref value of reference
equals to 38, which is from the literature [26, 27, 29].
Detailed experiments reveal that the peak position in the
TPEF spectra of L1L4 are independent of the excitation
wavelengths from 680 to 900 nm, while the TPA cross sec-
tions change over this range. Thus, by tuning the pump
wavelengths incrementally from 680 to 900 nm while
keeping the input power fixed and then recording the TPEF
intensity, the two-photon absorption spectra are obtained as
shown in Figure 6. The best TPA wavelength is 720 nm for
L1, 800 nm for L2, 780 nm for L3, and 800 nm for L4. The
best excitation wavelength of L4 is essentially twice that of
the linear absorption maximum (400 nm). For L1L3, The
best excitation wavelength is slightly shorter than twice that
of the corresponding linear absorption maximum (440, 407
and 441 nm for L1, L2 and L3, respectively).
As shown in Figure 5, the emission maximum locates at
588 nm for L1, 586 nm for L2, 550 nm for L3 and 575 nm
for L4, respectively. The TPEF peak position of L1 is very
close to the peak fluorescence wavelength using SPEF,
while the TPEF peak position of L2L4 shows a red-shift
when compared to that of SPEF (Figure 3 and Table 1).
This can be explained by the effect of re-absorption. For
single-photon induced emission measurements, dilute solu-
tions (1 × 106 mol L1) were used, thus the re-absorption of
SPEF within the samples is negligible. In the case of TPEF,
concentrated solutions (1 × 103 mol L1) were used. The
emission is from the surface layer as well as inside the solu-
tion sample. Thus, re-absorption of the shorter wavelength
fluorescence by the concentrated sample can no longer be
4 Conclusions
Systematic study on preparation, thermal stability, linear
absorption, single- and two-photon excited fluorescence,
and theoretical calculation on a series of ICT type tri-
phenylamine derivatives allows us to derive significant
structure-property relationships. The chromophores show
high thermal stability, good single- and/or two-photon ex-
cited fluorescence behavior, and have potential applications
as lighter-emitting materials. The extended -conjugated
length leads to additional fluorescent excited state. It can
improve the fluorescence quantum yields and enhance
TPEF spectra as well as the corresponding TPA cross sec-
tions. Introducing the ethoxyl unit to the triphenylamine
group can improve the fluorescence quantum yields, pro-
vide a more stable environment for the excited state and
balance the charge recombination in the whole structure.
This study can provide some insights and strategies in de-
signing and preparing organic optical materials with poten-
tial values.
Figure 6 Two-photon absorption cross sections of L1L4 in DMF vs. excitation wavelengths of identical energy of 0.50 W.

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Kong L, et al. Sci China Chem January (2013) Vol.56 No.1
This work was supported by the National Natural Science Foundation of
China (NSFC, 21071001, 51142011, 21101001), Education Department of
Anhui Province (KJ2010A030) and the “211” Project of Anhui University.
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