Investigations and facile synthesis of a series of novel multi-functional two-photon absorption materials

Article abstract of DOI:10.1039/b703853d

Six centrosymmetric D-(π-A)₃ structural triphenylamine derivatives that can be used as two-photon photopolymerization and optical data storage chromophores, tris[4-(4-pyridylethenyl)phenyl]amine (1), tris[4-(2-pyridylethenyl)phenyl]amine (2), tris(4-cyanoethenylphenyl)amine (3), tris[4-butylacrylatephenyl]amine (4), tris[4-methylacrylatephenyl]amine (5) and tris[4-acrylicethenylphenyl]amine (6), have been successfully synthesized via a triple palladium-catalyzed Heck coupling reaction, and the novel chromophores were fully characterized by elemental analysis, IR, ¹H-NMR and ESIMS. The structure for 3 was determined by single crystal X-ray diffraction study. One- and two-photon absorption and fluorescence in various solvents were experimentally investigated. Two-photon initiated polymerization microfabrication and optical data recording experiments were carried out under 780 nm laser radiation, and the possible polymerization mechanism is discussed based on theoretical calculations. All the six chromophores have relatively large two-photon absorption cross-sections, and exhibit optical memory and highly efficient two-photon initiated polymerization abilities. The Royal Society of Chemistry.

Full text of DOI:10.1039/b703853d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Investigations and facile synthesis of a series of novel multi-functional
two-photon absorption materials{
Yu-Peng Tian,*ab Lin Li,a Ju-Zhou Zhang,a Jia-Xiang Yang,ab Hong-ping Zhou,a Jie-ying Wu,a
Ping-ping Sun,a Li-min Tao,c Ya-hui Guo,c Chuan-Kui Wang,cd Hui Xing,e Wen-hao Huang,e Xu-Tang Taob
and Min-Hua Jiangb
Received 14th March 2007, Accepted 15th June 2007
First published as an Advance Article on the web 2nd July 2007
DOI: 10.1039/b703853d
Six centrosymmetric D–(p–A)3 structural triphenylamine derivatives that can be used as two-photon
photopolymerization and optical data storage chromophores, tris[4-(4-pyridylethenyl)phenyl]amine
(1), tris[4-(2-pyridylethenyl)phenyl]amine (2), tris(4-cyanoethenylphenyl)amine (3), tris[4-butylacryl-
atephenyl]amine (4), tris[4-methylacrylatephenyl]amine (5) and tris[4-acrylicethenylphenyl]amine (6),
have been successfully synthesized via a triple palladium-catalyzed Heck coupling reaction, and the
novel chromophores were fully characterized by elemental analysis, IR, 1H-NMR and ESIMS. The
structure for 3 was determined by single crystal X-ray diffraction study. One- and two-photon
absorption and fluorescence in various solvents were experimentally investigated. Two-photon
initiated polymerization microfabrication and optical data recording experiments were carried out
under 780 nm laser radiation, and the possible polymerization mechanism is discussed based on
theoretical calculations. All the six chromophores have relatively large two-photon absorption cross-
sections, and exhibit optical memory and highly efficient two-photon initiated polymerization abilities.
configuration20,25–38 have gained a great deal of recent
Introduction
interest, which is largely due to their enhanced TPA with
Two-photon absorption (TPA) is a process in which two
respect to their monomer counterparts. Several experimental
photons are simultaneously absorbed to an excited state in a
and theoretical investigations have been carried out to gain a
medium via a virtual state under intense laser pulses when the
mechanistic understanding of structure–property relationships
sum of the energies of two absorbed photons matches well with
of TPA cross-sections. It has been shown from the measure-
the energy gap between an allowed excited state and the
ments that donor–acceptor strength, conjugation length, type
ground state. Since the theory was first proposed by Go¨ppert-
of p-bridge, geometry, and nature of ground and excited states
Mayer in 1931,1 TPA processes in conjugated organic
have significant influences on TPA cross-sections.39,40
molecules have aroused considerable attention due to their
Among several branching centres investigated, octupolar
potential applications in optical data storage,2,3 three-dimen-
systems were found to show better enhancement of TPA cross-
sional fluorescence imaging,4–6 photodynamic therapy,7–10
section with increased branching.20,25,39,41,42 By combining
two-photon upconversion lasing11,12 and three-dimensional
lithographic microfabrication.13–15 To realize these applica-
experiments with theory, Prasad et al.20 and Goodson et al.43
found that triphenylamine-cored branched molecules with
tions, several design approaches have been reported for
alkene p-bridges show exceptionally large TPA cross-sections.
Cho et al.21 have shown theoretically that, in branched mole-
synthesis of organic molecules with large TPA cross-sections.
Numerous new TPA chromophores including dipoles,16
cules, the TPA cross-section increases as the strength of the
quadrupoles,17–19 multibranched molecules,20–23 and dendri-
donor–acceptor interaction increases.
mers24 have been synthesized. Some of these materials have
To understand the effect of different acceptors on the
progressed to the level of real applications.6,16–21Among
TPA cross-section of multiply branched chromophores, we
several geometries investigated for better TPA properties,
have synthesized six new multi-functional chromophores 1–6
branched organic chromophores with the donor–p–acceptor
(Scheme 1), which have different acceptor (acrylonitrile,
acrylic acid, 4-vinylpyridine, 2-vinylpyridine, methyl acrylate,
aDepartment of Chemistry, Anhui University, Hefei, 230039,
butyl acrylate), and the same triphenylamine core and alkene
P. R. China. E-mail: yptian@ahu.edu.cn
p-bridges respectively as those used by Goodson et al.43
using the highly efficient Heck coupling reaction. These
chromophores have a common tri-dendritic centrosymmetric
D–(p–A)3 structural motif (where p is a p-conjugated bridge,
D is a donor, and A is an acceptor). The chromophores show
strong one- and two-photon fluorescence and relatively large
TPA cross-section. We also investigated the TPA initiated
photopolymerization properties of the chromophores by using
one of them as a photoinitiator and photosensitizer.
bState Key Laboratory of Crystal Materials, Shandong University,
Jinan, 250100, P. R. China
cDepartment of Physics, Shandong Normal University, Jinan, 250014,
P. R. China
dLaboratory of Theoretical Chemistry, Royal Institute of Technology,
SCFAB, S-10691, Stockholm, Sweden
eDepartment of Precision Machinery and Precision Instrumentation,
University of Science and Technology of China, Hefei, 230026,
P. R. China
{ Electronic supplementary information (ESI) available: chemicals and
measurements information. See DOI: 10.1039/b703853d
3646 | J. Mater. Chem., 2007, 17, 3646–3654
This journal is ß The Royal Society of Chemistry 2007

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Scheme 1 Synthesis routes to six initiators 1–6.
Up till now, however, effective TPA initiated photopoly-
merization materials are still few, especially with the D–(p–A)3
structure, the possible reasons being that the relationship of
the initiator structures and their properties for initiating two-
photon photopolymerization was not investigated in depth.
Common commercial initiators have small TPA cross-sections
7.63 (6H, d, J = 12 Hz). MS, m/z (%): 622.8 ([M]+, 80), 496.9
([M 2 I]+, 100), 242.0 ([M 2 3I]+, 30).
Tris[4-(4-pyridylethenyl)phenyl]amine 1. At room tempera-
ture, a suspension, in 24 mL DMF and 2.4 mL water, of 2.07 g
(15 mmol) potassium carbonate and 1.92 g (15 mmol) tetra-
n-butylammonium bromide was well stirred under nitrogen for
about 15 min. 0.156 g (0.6 mmol) triphenylphosphine, 3.60 g
(5.8 mmol) tris(4-iodophenyl)amine and 4.50 mL (40.5 mmol)
4-vinylpyridine were successively added, and the suspension
was stirred for another 15 min before addition of 0.066 g
(0.3 mmol) palladium(II) acetate. The mixture was maintained
at 70 uC for 2 days, then cooled to room temperature and
added to 500 mL water. A plentiful yellow solid was obtained
by filtration. This was dissolved in dichloromethane. The
solution was washed twice with water, dried over anhydrous
magnesium sulfate, then filtered and concentrated. The
resulting solution was purified by flash column chromatogra-
phy with ethanol as eluent, to give light yellow needle-shaped
crystals, 2.0 g. Yield: 66%. Anal. Calc. for C39H30N4: C, 84.45;
H, 5.45; N, 10.10. Found: C, 84.71; H, 5.77; N, 9.79%. IR
(KBr, cm21) selected bands: 3025 (w), 1631 (w), 1589 (s), 1503
around the order of 10250 cm4 s photon21 44
Recently, highly
.
photosensitive novel organic TPA photoinitiators have been
synthesized based on the idea of increasing their TPA cross-
sections45–47 by multi-step and complicated methods. So, it is
very significant to explore novel structures and facile synthesis
methods for efficient TPA photoinitiators and photosensitizers
with large cross-sections for practical applications. The results
of our experiment and theoretical calculations showed the
centrosymmetric D–(p–A)3 structural compounds exhibited
high efficiencies in both two-photon initiating polymerization
and data recording. The microstructures were fabricated by
the use of one of the series of compounds as initiator excited
with a 780 nm, 200 fs, 76 MHz Ti : sapphire laser source
and the possible mechanism is discussed in terms of the
charge-transfer process when the laser irradiation is applied.
Furthermore, we attempted to use the new chromophores as
optical data storage materials by the method of void creation
in polymer as in references 48–50. The optical system that we
used for data recording and reading with the new material was
similar to the instrumental setup used in the polymeriza-
tion.51,52 The novel chromophores possess potential applica-
tion prospects due to their simple preparation and efficient
initiating properties.
1
(s), 1321 (m), 1269 (m), 973 (m), 826 (m). H NMR (DMSO-
d6), d (ppm): 7.09, 7.11 (12H, d, J = 12 Hz), 7.52, 7.55 (6H, d,
J = 18 Hz), 7.63, 7.64 (6H, d, J = 6.0 Hz), 8.53 (6H, s). ESIMS,
m/z (%): 555.3 ([M + H]+, 100), 278.1 ([M + H]2+, 50), 226.4
([M 2 C5H4NCHCH2 + H]2+, 19).
Tris[4-(2-pyridylethenyl)phenyl]amine 2. Yellow micro-
crystals were prepared according to a similar procedure to 1
using 2-vinylpyridine instead of 4-vinylpyridine, and ethyl
acetate was used as eluent. Yield: 63%. Anal. Calc. for
C39H30N4: C, 84.45; H, 5.45; N, 10.10. Found: C, 84.81;
H, 5.79; N, 9.77%. IR (KBr, cm21) selected bands: 3031 (w),
1631 (w), 1581 (s), 1503 (s), 1323 (m), 1280 (m), 968 (m),
822 (m). 1H NMR (DMSO-d6), d (ppm): 7.08, 7.09 (6H, d,
J = 6.0 Hz), 7.20, 7.22, 7.23, 7.24 (6H, q, J = 7.8 Hz), 7.51,
7.53 (3H, d, J = 7.8 Hz), 7.62, 7.63, 7.64, 7.66 (9H, q, J =
11.4 Hz), 7.78, 7.77, 7.76 (3H, t, J = 7.2 Hz), 8.55(3H, s).
Experimental
Synthesis
The synthesis route towards the six compounds is described in
Scheme 1.
Tris(4-iodophenyl)amine. Tris-(4-iodophenyl)amine was
synthesized according to reported methods,30 yield 96%, mp:
169 uC. 1H NMR, d (ppm): 6.81, 6.83 (6H, d, J = 12 Hz), 7.61,
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ESIMS, m/z (%): 555.4 ([M + H]+, 100), 452.2 ([M 2
MoKa radiation with a v-scan mode [l = 0.71073A for 3]. The
structure was solved with direct methods using the SHELXTL
program53 and refined anisotropically with SHELXTL using
full-matrix least-squares procedures giving for 3 a final R1
value of 0.0576 for 280 parameters, and 1481 unique reflec-
tions with I ¢ 2s(I) and wR2 of 0.1389 for all 2627 reflections.
All the nonhydrogen atoms were located from the trial
structure and then refined anisotropically with SHELXTL
using full-matrix least-squares procedures. The hydrogen
atom positions were geometrically idealized and allowed to
ride on their parent atoms with fixed displacement parameters
(Table 1).
˚
C5H4NCHCH2 + H]+, 18).
Tris(4-cyanoethenylphenyl)amine 3. Yellow microcrystals
were prepared according to a similar procedure as 1 using
acrylonitrile instead of 4-vinylpyridine, and ethyl acetate–
petroleum ether (1 : 3) was used as eluent. Yield: 78%. A green-
yellow cubic single crystal was obtained by slow evaporation
of a solution in acetone. Anal. Calc. for C27H18N4: C, 81.39;
H, 4.55; N, 14.06. Found: C, 81.07; H, 4.69; N, 13.71%. IR
(KBr, cm21) selected bands: 3061 (w), 2212 (s), 1618 (m), 1591
1
(s), 1504 (s), 1326 (m), 1267 (s), 968 (m), 810 (m). H NMR
(DMSO-d6), d (ppm): 6.33, 6.36 (3H, d, J = 16.2 Hz), 7.07, 7.08
(3H, d, J = 8.4 Hz), 7.58 (6H, s), 7.61, 7.62 (6H, d, J = 7.8 Hz).
MS, m/z (%): 399.1([M+], 100).
CCDC reference number 648000. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b703853d
Results and discussion
Tris[4-butylacrylatephenyl]amine 4. Yellow microcrystals
were prepared according to a similar procedure to 1 using
butyl acrylate instead of 4-vinylpyridine, and ethyl acetate–
petroleum ether (10 : 1) was used as eluent. Yield: 81%. Anal.
Calc. for C39H45NO6: C, 75.09; H, 7.27; N, 2.25. Found: C,
75.41; H, 6.95; N, 2.58%. IR (KBr, cm21) selected bands: 3032
(w), 1710 (s), 1591 (s), 1505 (s), 1324 (s), 1267 (s), 1170 (m), 986
Synthesis and characterization
For the synthesis of the six chromophores, illustrated in
Scheme 1, an approach similar to that used by Goodson et al.43
was employed, using the palladium-catalyzed Heck coupling.43
All chromophores were synthesized through no more than two
steps, and purified either using column chromatography or
recrystallization. This synthesis was adapted from the synthesis
of other tribranched or multi-branched molecules, with the
only difference being the structure of the core, demonstrating
the versatility of this synthetic method. In the future by
utilizing a variety of cores with branches of different chemical
compositions, it will be possible to make a plethora of
molecules with varying HOMO and LUMO levels.
1
(w), 829 (m). H NMR (DMSO-d6), d (ppm): 0.89, 0.91, 0.92
(9H, t,), 1.35, 1.36, 1.37, 1.39 (6H, q), 1.60, 1.61, 1.62, 1.63
(6H, q), 4.12, 4.13 4.14 (6H, t), 6.51, 6.54 (3H, d, J = 16.2 Hz),
7.04, 7.06 (6H, d, J = 8.4 Hz), 7.58, 7.61 (3H, d, J = 17.6 Hz),
7.68, 7.69 (6H, d, J = 7.8 Hz). MS, m/z (%): 623.3 ([M]+, 100).
Tris[4-methylacrylatephenyl]amine 5. Yellow microcrystals
were prepared according to a similar procedure to 1 using
methyl acrylate instead of 4-vinylpyridine, and ethyl acetate–
petroleum ether (1 : 3) was used as eluent. Yield: 81%. Anal.
Calc. for C30H27NO6: C, 72.42; H, 5.47; N, 2.82. Found: C,
72.09; H, 5.28; N, 2.71%. IR (KBr, cm21) selected bands: 3032
(w), 1710 (s), 1591 (s), 1505 (s), 1324 (s), 1267 (s), 1170 (m), 986
(w), 829 (m). 1H NMR (DMSO-d6), d (ppm): 3.71 (9H, s), 6.52,
6.55 (3H, d, J = 16.2 Hz), 7.05, 7.07 (6H, d, J = 8.4 Hz), 7.60,
7.63 (3H, d, J = 15.6 Hz), 7.68, 7.69 (6H, d, J = 8.4 Hz). MS,
m/z (%): 497.2 ([M+], 100), 466.1 ([M 2 OCH3]+, 10).
The structures and purity of chromophores were determined
and confirmed with IR and 1H NMR. The infrared spectra are
very similar to those recorded for compounds with triphenyl-
amine cores47 with two groups of peaks centered around
1321 cm21 and 1267 cm21, except 1365 cm21 and 1175 cm21
for 6. The observed J-coupling constants for the vinyl protons
of chromophores were in the range of 12–18 Hz, which is
consistent with expected values for an all-trans configuration.
Mass spectrometry and elemental analyses were used to further
verify the structures of the chromophores. For the six mole-
cules, observed values agreed well with calculated values.
Tris[4-acrylicethenylphenyl]amine 6. At room temperature,
0.25 g (0.5 mmol) 5 and 0.40 g (10 mmol) sodium hydroxide
were dissolved in 50 mL ethanol and 15 mL water, respectively.
The two solutions were mixed together. The mixture was well
stirred for about 24 hours. After adding 200 mL water to the
mixture, the pH was adjusted to 2 by addition of diluted sulfuric
acid. Yellow solid (0.23 g) was obtained by filtration. Yield: 98%.
Anal. Calc. for C27H21NO6: C, 71.20; H, 4.65; N, 3.08. Found:
C, 71.58; H, 4.29; N, 2.96%. IR (KBr, cm21) selected bands: 3031
(s), 1683 (s), 1628 (s), 1591 (s), 1506 (s), 1265 (s), 1175 (s), 979 (s),
Table 1 Crystallographic data for 3
Formula
C27H18N4
398.45
Triclinic
Formula weight
Crystal system
Space group
¯
P1
˚
a, b, c/A
a, b, c/u
10.279(14), 10.283(13), 10.315(12)
95.59(3), 95.41(2), 95.42(2)
1074(2)
3
˚
V/A
Z
2
1.365
416
D(calc)/g cm23
F(000)
1
826 (s). H NMR (DMSO-d6), d (ppm): 6.41, 6.44 (3H, d, J =
Crystal size/mm
Temperature/K
˚
Radiation/A (MoKa)
0.41 6 0.32 6 0.21
273(2)
0.71073
15.6 Hz), 7.04, 7.06 (6H, d, J = 8.4 Hz), 7.45, 7.46 (3H, d, J =
8.4 Hz), 7.53–7.69 (6H, m). MS, m/z (%): 454.3 ([M]+, 50).
h
min-max/u
2.0–25.02
Data set
212: 12; 212: 4; 25: 11
1481
2627, 280
X-Ray crystallography
Observed data [I . 2.0 s(I)]
Nref, Npar
R1, wR2, S
Data collections [2.00 , h , 25.02u for 3] were performed
using a SMART CCD area detector diffractometer with
0.0576, 0.1642, 1.015
3648 | J. Mater. Chem., 2007, 17, 3646–3654
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where F, G, and H are coefficients dependent on the
polarization of the light. For linearly polarized light, F, G
and H are 2, 2 and 2, but for the circular case, they are 22, 3,
and 3, respectively. In the present work, we only consider the
results with the linearly polarized laser beam. The summation
goes over the molecular axes a, b = {x, y, z}. S is the two-
photon transition matrix element. In terms of sum-over-state
formulae, the two-photon matrix element for the two-photon
resonant absorption of identical energy is written as,54
"
#
X
S0jmajjTSjjmbj f T S0jmbjjTSjjmaj f T
ꢀ
ꢀ
Sab
~
z
vj{vf
2
vj{vf
2
j
where |0T and |fT denote the ground state and the final state,
respectively, |jT means all the intermediate states including the
ground state, vj is the excitation energy of the excited states,
and m is electronic dipole moment. In general, the total TPA
probability requires the information of all the excited states,
which is different to obtain clearly. On the other hand, a few-
state model often provide satisfying results for TPA cross-
section in the visible region.17,55–57
Fig. 1 Molecular structure of 3.
˚
Table 2 Selected bond lengths (A) and angles (u) for 3
N1–C1
N1–C19
N3–C18
C4–C7
C8–C9
C16–C17
C22–C25
C26–C27
C1–N1–C19
C10–N1–C19
C7–C8–C9
C13–C16–C17
C17–C18–N3
C25–C26–C27
1.422(4)
1.427(4)
1.143(5)
1.499(5)
1.459(6)
1.243(5)
1.490(5)
1.462(6)
119.0(3)
119.1(3)
123.2(5)
126.8(5)
176.6(6)
123.7(5)
N1–C10
N2–C9
N4–C27
C7–C8
C13–C16
C17–C18
C25–C26
1.428(4)
1.134(5)
1.139(5)
1.251(5)
1.499(6)
1.460(7)
1.246(5)
The TPA cross-section directly comparable with experi-
mental measurement is defined as17
4p2a50a v2gðvÞ
stpa
~
dtpa
15c0
Cf
C1–N1–C10
C4–C7–C8
C8–C9–N2
C16–C17–C18
C22–C25–C26
C26–C27–N4
119.0(3)
126.0(4)
176.3(5)
124.0(5)
126.5(5)
176.5(5)
Here a0 is the Bohr radius, c0 is the speed of light, a is the fine
structure constant, v is the photon energy of the incident
light, g(v) denotes the spectral line profile, here assumed to be
a d-function. Cf is the lifetime broadening of the final state,
which is assumed to be 0.1 eV.13
We utilized the GAUSSIAN 98 program package58 to
optimize the molecular geometrical structure with the hybrid
density functional theory (DFT/B3LYP) and a basis set 6-31G.
The few-state model is used to study the two-photon absorp-
tion cross-section of the molecules. The largest two-photon
absorption cross-sections of molecules are 8125.7 for 1, 8520.3
for 2, 3962.4 for 3, 4447.7 for 4, 4825.7 and 5061.7 GM
(1 GM = 10250 cm4 s photon21) for 5 and 6, respectively.
Structural studies
The crystal structure of 3 is shown in Fig. 1. Selected bond
lengths and angles are listed in Table 2. As shown in Fig. 1, the
central nitrogen (N1) and its three bonded carbon atoms are
in distorted pyramidal geometry and form a C3-symmetrical
species. The dihedral angles of the three benzene rings around
the central nitrogen are 65.2u, 65.2u and 64.9u respectively. In
the symmetrical tribranched compound, each branch com-
prised of a benzene ring and acrylonitrile unit is basally co-
planar with a dihedral angle of 14.5u. The linkage bond
lengths between benzene rings and acrylonitrile units are quite
conjugated, for example C4–C7 [1.499(5)], C7–C8 [1.251(5)]
and C8–C9 [1.459(6)]. These structural features suggest that all
nonhydrogen atoms of each branch are highly conjugated,
leading to a p-bridge for the charge transfer from donor to
acceptor. So, the nitrogen-centered symmetrical compounds
comprise three co-planar conjugated moieties, which is a
strategy of enhancing molecular TPA cross-section.
Linear absorption and single-photon exited fluorescence (SPEF)
Linear absorption spectra of 1 in toluene, acetone, THF,
ethanol and DMF with a solution concentration of C = 1.0 6
1025 mol L21 are shown in Fig. 2, from which one can see that
the linear absorption spectrum of 1 was slightly red shifted
with increasing solvent polarity and all the derivatives show
definite solvatochromic behavior. This positive solvatochro-
mism is indicative of a larger stabilization of the excited state
as compared to the ground state by a polar solvent, which
suggests that a significant charge redistribution takes place
upon excitation, consistent with a multidimensional intra-
molecular charge transfer (MDICT)55 occurring between the
core and the peripheral groups. There is no linear absorption
in the entire spectral range from 450 to 800 nm for the six
chromophores in THF (Fig. 3). The linear and nonlinear
optical properties for 1–6 in different polar solvents are listed
in Table 3. The absorption maxima of the six initiators
are located at 396, 387, 388, 399, 388 and 379 nm in ethanol,
Theoretical calculation
For molecules in gas phase and solutions, orientational
averaging over the two-photon absorption probability is
required, which gives
h
i
X
dtpa
~
F|SaaSbꢀbzG|SabSaꢀbzH|SabSbꢀa
ab
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Table 3 Single- and two-photon-related photophysical properties of
chromophores 1–6 in several different polar solvents
Solvents lmð1aaxÞ/nma lðm1afÞx/nmb Wc Dn/cm21 d t/ns lmð2afÞx/nme
1
2
3
4
5
6
a
toluene 379
THF 379
acetone 388
ethanol 396
452
479
500
511
517
448
467
482
493
498
439
464
486
492
495
441
471
493
515
509
443
473
495
510
505
472
470
496
502
503
0.15 4261
0.18 5508
0.07 5773
0.03 5683
0.06 5910
0.21 3518
0.16 4426
0.17 4986
0.09 5556
0.10 5759
0.57 2410
0.40 4422
0.14 5466
0.11 5447
0.20 5638
0.47 2200
0.65 4803
0.59 5423
0.10 5645
0.38 5479
0.39 3266
0.41 4900
0.14 5840
0.04 6165
0.16 5386
0.03 4653
0.30 5109
0.03 5813
0.02 6465
0.11 5892
492
502
520
525
DMF
toluene 387
THF 387
396
2.86 536
acetone 387
ethanol 387
DMF
toluene 397
THF 385
387
1.00 527
1.37 510
1.00 522
0.82 530
1.79 523
Fig. 2 Linear absorption spectra of
1 in several solvents with
acetone 384
ethanol 388
differing polarities.
DMF
toluene 402
THF 389
387
respectively, which correspond to the p–p* transition of the
main chain. From Table 3, one can observe that the absorption
maxima do not depend on the nature of the peripheral sub-
stituting groups due to their same principal part framework.
The one-photon fluorescence spectra of 1 are shown in Fig. 4
in five organic solvents. The concentration of the solution was
1.0 6 1026 mol L21. As shown in Fig. 4 and Table 3, the
solutions of the chromophores show emission maxima from
439 to 517 nm. It is clear that the emission peaks red shift
with increasing the solvent polarity indicating that the
acceptors with extended conjugation and/or stronger
electron-withdrawing ability stabilize the excited states more
than the Franck–Condon single state.59 The emission spectra
of the six chromophores were investigated in THF solutions
(Fig. 5) with the same concentration. The quantum yields (W)
of the six chromophores in different solvents were determined
by using coumarin as standard (Table 3). Seen from Table 3,
the quantum yields in ethanol solution are the least for all the
chromophores, which could be explained by the hydrogen
bonds of ethanol molecules consuming some energy of excited
state molecules. Comparing with Goodson’s chromophores,43
the relatively low quantum yield of 1 in THF could be
explained by the error of the measurements due to the sensi-
tivity of the fluorescence spectrophotometer and other
environmental conditions. Chromophore 4 shows the largest
quantum yields in the five solvents in comparison to the other
chromophores, especially in THF. Because of the hydrogen
acetone 389
ethanol 399
DMF
toluene 387
THF 384
398
acetone 384
ethanol 388
DMF
toluene 387
THF 379
397
acetone 385
ethanol 379
DMF
388
b
Peak position of the longest absorption band. Peak position of
c
SPEF, exited at the absorption maximum.
Quantum yields
.
determined by using coumarin as standard. Stokes’ shift in cm21
TPEF peak position in nm.
d
e
Fig. 4 One-photon fluorescence spectra of 1 in different solvents.
bonds between the acrylic acid moiety and solvent molecules,
the lowest numerical value of quantum yields belong to
chromophore 6 in the five solvents comparing to the others.
So, the solute–solvent interactions by which the excited state
could be rapidly quenched and due to decreased charge
transfer play an important role in decreasing the quantum
yield for the six chromophores.60
In order to further demonstrate the influence of solvent on
fluorescence, Table 3 lists the Stokes’ shift of the six chromo-
phores in solvents with different polarity. The Stokes’ shift
is defined as the loss of energy between absorption and
Fig. 3 Linear absorption spectra of 1–6 in THF.
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Fig. 5 One-photon fluorescence spectra of 1–6 in THF.
Fig. 7 Output fluorescence intensity (Iout) vs. the square of input
laser power (Iin)2 for 1 in DMF. Excitation carried out at 780 nm, with
C = 1.0 6 1024 mol L21 in DMF.
reemission of light, which is a result of several dynamic
processes. These processes include losses due to dissipation of
vibrational energy, redistribution of electrons in the surround-
ing solvent molecules induced by the altered dipole moment of
the excited chromophore, reorientation of the solvent mole-
cules around the excited state dipole, and specific interactions
between the fluorophore and the solvent or solutes. The
Lippert equation is the most widely used equation to describe
the effects of the physical properties of the solvent on the
emission spectra of fluorophores.61
at 780 nm. The linear dependence on the square of input laser
power suggests that it is a two-photon excitation mechanism.
Fig. 6 shows the experimental set up for TPEF measurement.
Detailed experiments reveal that from 700 to 900 nm the
peak position in the two-photon exited fluorescence (TPEF)
spectra of these chromophores is independent of the excita-
tion wavelengths, but the emission intensities of TPEF are
wavelength-dependent over that range. By tuning the pump
wavelengths incrementally from 700 to 900 nm while keeping
the input power fixed and then recording TPEF intensity, TPE
spectra are obtained; the TPE spectra of 1–6 are show in Fig. 8.
The spectra display one excitation peak, and this feature is
similar to that observed in the linear absorption spectra, expect
that the wavelengths are roughly doubled. The optimal
excitation wavelength for each of 1–6 is 780 nm. The TPE
peaks of 2, 3 and 6 are essentially twice the linear absorption
maxima (387, 387 and 388 nm), but TPE peaks for 1, 4 and 5
are at an energy slightly higher than twice the corresponding
linear absorption maximum (396, 398 and 397 nm). As show
in Fig. 8, the acceptor strength does not seem to play a major
role in determining the TPEF intensity, while the order of the
acceptor strength is acrylonitrile . acrylic acid . 4-vinyl-
pyridine . 2-vinylpyridine . methyl acrylate . butyl acrylate.
The result seems not to correspond with those reported in the
Dn = nabs 2 nem = (2/cha3)Df(me 2 mg)2 + const
In this equation, h is Planck’s constant, c is the speed of light, and
a is the radius of the cavity in which the fluorophore resides. The
wavenumbers of the absorption and emission are nabs and nem
(in cm21). In aprotic solvents, Stokes’ shifts are approximately
proportional to the orientational polarizability. But no obvious
shifts larger than expected are found in protic solvents, even
though hydrogen bonds may occur between the compounds and
the protic solvents.62,63
Two-photon excited luminescence spectral properties
As shown in Fig. 3, there is no linear absorption in the
wavelength range 450–800 nm for the six initiators, which
indicate that there are no energy levels corresponding to
an electron transition in this spectral range. If frequency
upconverted fluorescence appears upon excitation with a
tunable laser in this range, it should be safely attributed to
multi-photon absorption excited fluorescence. As shown in
Fig. 7, the emission intensity of compound 1 was investigated
Fig. 8 Two-photon (from a 200 fs, 76 MHz Ti : sapphire laser)
fluorescence intensity (peak) of 1–6 in DMF versus excitation
wavelengths of identical energy of 0.380 W.
Fig. 6 Experimental setup for two-photon photo fluorescence inten-
sity at different excitation wavelengths.
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Fig. 9 The two-photon fluorescence spectra of 1 in different solvents
(C = 1.0 6 1024 mol L21).
Fig. 10 Optical micrograph of microstructures fabricated via two-
photon polymerization of SR 349 using 4 as initiator.
literature,25,41–43,64 but it is in keeping with the tendency of the
theoretical calculation.
dichloromethane. The fabricated lattice was observed through
a polarization microscope (Olympus, BX-51).
As shown in Table 3, the peak positions of TPEF spectra of
1–6 are significantly shifted compared to those of the corres-
ponding SPEF in DMF solution. The TPA excited fluores-
cence of 1 is shown in Fig. 9 with a solution concentration of
C = 1.0 6 1024 mol L21 in different organic solvents at the
excitation wavelength of 780 nm. From the TPA excited
fluorescence spectra, one can see that 1 shows strong two-
photon fluorescence and the TPEF peak positions show a red
shift compared to those of SPEF (comparing Fig. 9 to Fig. 4).
This can be explained by the effect of reabsorption. For 1,
we can clearly find that the maxima of the TPA fluorescence
red-shift with increasing solvent polarity, which is also
similar to SPEF. Two-photon absorption cross-sections (s)
have been measured using TPEF in DMF. The largest s is
330.8 GM for 1, which is similar to Goodson’s value.43 In
addition, the values of s are 38.4 for 2, 57.9 for 3, 19.8 for
4, 66.1 and 19.4 GM (1 GM = 10250 cm4 s photon21) for 5
and 6, respectively. Due to the ideal molecules used in the
calculation, the experimental values are smaller than the
calculation results.
At present, the two-photon initiated photopolymerization
mechanism is still unclear. According to Marder et al.,13 strong
donor substituents in a molecular conjugated system would
make the conjugated system electron rich, and after single or
two-photon absorption and excitation, these chromophores
would transfer an electron even to relatively weak acceptors,
and this process could be able to activate some chemical
reactions, such as free radical polymerization. In order to
demonstrate the charge-transfer process, we attempted a
theoretical investigation. Our ab initio calculation at time-
dependent hybrid density functional theory B3LYP level coded
in the GAUSSIAN package for 1–6 shows that the first excited
state is the charge-transfer state. We have plotted the charge
density difference between the ground state and CT state for
1–6 in the gas phase (see Fig. 11), which is visualized by use of
the MOLEKEL program.65 The blue and grey parts represent
the electron gain and loss, respectively. It can be seen that
upon excitation, charge is mainly transferred from two
branches to the third branch. In the CT state, there are more
electrons at the third branch on average, indicating that the
TPA photopolymerization
The incident wavelength used for each of the six TPA initiators
for acrylate ester oligomer SR 349 polymerization was 780 nm.
The three dimensional lattices (Fig. 10) were fabricated with
negative resins by the two-photon polymerization of SR 349
using 4 as initiator. The resins consist of 0.5 wt.% photo-
initiator 4 in 99.5 wt.% monomer (SR 349 with a radical
inhibitor level of 100 ppm). Films with a thickness of about
100 mm were obtained by solvent evaporation from a 1,2-
dichloroethane solution. The film was exposed in the pattern
of a target structure at 780 nm laser at an average power of
500 mW and a linear scan speed of 50 mm s21. After irradiation,
the final three dimensional structures were obtained by
dissolving away the unexposed resin with dichloromethane.
Fig. 11 shows photongraphs of the polymerization region.
The distance between two neighboring polymerized streaks
is about 10.0 mm. Only in the region in the laser focus
does polymerization take place, and the remaining monomer
in the unpolymerized regions was easily washed out by
Fig. 11 Density difference between the charge-transfer and ground
states of 1–6 in the gas phase.
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molecule could be ready to give away its electron to its
surroundings. This picture was adopted to explain the
mechanism of two-photon induced polymerization.66 This
picture seems slightly different from Marder’s discussion.
However, whether the photoinduced electron-transfer reaction
can be energetically feasible needs to be further investigated
theoretically.
For the six chromophores, the central triphenylamine
moiety possesses relatively high electron density, and the
conjugated systems were further extended due to the introduc-
tion of 2- or 4-vinylpyridine, acrylonitrile, methyl acrylate or
other moieties by Heck coupling reaction. Then, the initiators
possessing high electron density and having relatively large
TPA cross-sections can efficiently initiate two-photon-induced
electron-transfer reaction, and microstructures can be created
by the free radical polymerization reaction.
TPA data storage
A polymer material has a low threshold of optical damage
and provides the possibility of doping with absorbing dyes,
which would allow the use of a long excitation wavelength to
reduce the scattering effect and the manipulation of the
refractive index. Poly(methyl methacrylate) (PMMA) is an
interesting material because of its high chemical resistance,
advantageous optical properties, and low cost. Recently,
PMMA and dye-doped PMMA were extensively studied with
respect to fs laser machining.48,49 A recent publication by
Yamasaki et al.48 demonstrates the formation of voids in an
undoped PMMA film by the use of single-shot ultrashort
pulses with a wavelength of 400 nm. It has been shown that
the voids in the PMMA can be read out under two-photon
excitation. Moreover, Day and Gu49 reported on the forma-
tion of submicrometer voids within dye-doped PMMA
under multiphoton absorption (MPA) excited by an infrared
laser beam.
Fig. 12 Example of one-layer data storage using 4 as initiator. The
distances between two adjacent bits in the layer is 4 mm.
Conclusions
In brief, a series of multi-branched chromophores with the
same core and varying acceptors have been synthesized by the
Heck coupling reaction. The chromophores show good one-
and two-photon excited fluorescence behaviour. The centro-
symmetric charge transfer D–(p–A)3 structural chromophores
exhibit relatively large TPA cross-section, and can be used for
two-photon polymerization and optical data storage when
initiated at 780 nm. A microstructure has been fabricated using
4 which has the largest quantum yield and the highest
solubility in organic solvents of the six chromophores. A
possible mechanism via the charge-transfer process when
laser irradiation is applied is discussed. A data recording
experiment using 4 proved the potential of these materials as
multifunctional materials. So, the results indicate that the six
compounds are multi-functional TPA chromophores and
possess potential applications due to simple preparation.
In the experiment, the storage medium used was PMMA
doped with a new photoinitiator, 4. To prepare the new
compound doped PMMA film for optical data storage, pure
compound (.99.5%) and PMMA were mixed together (with a
suitable weight ratio about 1 : 50) in a chloroform solution,
and the film was coated onto a glass slide with thickness of
about 150 mm in ambient air at room temperature. There is
almost no absorption above 500 nm by the chromophore;
therefore, it undergoes a typical two-photon excitation on
using a 780 nm femtosecond laser as the writing laser. Thus, as
described in the literature,50 bits were recorded successively by
a single pulse. A computer-controlled two-axis translation
stage was used to move the storage medium between pulses.
The recorded bits were retrieved through parallel reading with
a reflection-type confocal microscope, as shown in Fig. 12.
The shadow and blank can be detected and classified as
stored 1 and 0 data bits. The distance between adjacent dots in
the layer is about 4 mm. The laser irradiation duration was
50 ms for each bit.67 The differences in the bit size may be
caused by the inhomogeneity of the doped chromophore
in PMMA. The results of the experiment need improving,
however it is demonstrated that the chromophore 4 can be also
used as an optical data storage material.
Acknowledgements
The work was supported by a grant for the National Natural
Science Foundation of China (50532030), Doctoral Program
Foundation of the Ministry of Education of China, Education
Committee of Anhui Province (2006KJ032A, 2006KJ135B),
Team for Scientific Innovation Foundation of Anhui Province
(2006KJ007TD), Key Laboratory of Opto-Electronic
Information Acquisition and Manipulation, (Anhui Univer-
sity). We thank Dr Neil Cumpstey for helpful discussions.
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