Synthesis and optical properties of two 2,2′: 6′,2″-Terpyridyl-based two-photon initiators

Article abstract of DOI:10.1016/j.molstruc.2006.10.044

Two donor-π-acceptor (D-π-A) type 2,2′: 6′,2″-terpyridyl-based organic heterocyclic molecules have been efficiently synthesized via solvent-free Wittig reactions in good yields. One crystal structure of them was determined by single crystal X-ray diffract

Full text of DOI:10.1016/j.molstruc.2006.10.044

Journal of Molecular Structure 839 (2007) 50–57
www.elsevier.com/locate/molstruc
0
0
00
Synthesis and optical properties of two 2,2 : 6 ,2 -Terpyridyl-based
two-photon initiators
a
a,b,
a,b,
a
*
*
Zhang-Jun Hu , Jia-Xiang Yang
, Yu-Peng Tian
, Hong-Ping Zhou ,
b
b
b
b
b
Xu-Tang Tao , Gui-Bao Xu , Wen-Tao Yu , Xiao-Qiang Yu , Min-Hua Jiang
a
Department of Chemistry, Anhui University, Hefei 230039, PR China
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
b
Received 4 August 2006; received in revised form 25 October 2006; accepted 27 October 2006
Available online 27 December 2006
Abstract
0
0
00
Two donor-p-acceptor (D-p-A) type 2,2 : 6 ,2 -terpyridyl-based organic heterocyclic molecules have been efficiently synthesized via
solvent-free Wittig reactions in good yields. One crystal structure of them was determined by single crystal X-ray diffraction determina-
tion. The two compounds exhibit sensitive single-photon-excited fluorescence (SPEF) emission with high fluorescence quantum yields
and long lifetimes. The two-photon-excited fluorescence (TPEF), two-photon absorption (TPA) cross-sections and two-photon initiation
polymerization (TPIP) microfabrication experiments have been investigated. Experimental results confirm that they are effective organic
two-photon photopolymerization initiators.
ꢀ
2006 Elsevier B.V. All rights reserved.
Keywords: Terpyridine; Two-photon photopolymerization initiator; Solvent-free Wittig reactions; Optical material
1
. Introduction
the focal point, providing a high resolution of the excita-
tion volume. Thus, photon-induced chemical reaction is
localized to this small volume. This characteristic of
TPA-based photopolymerization may lead to many poten-
tial applications, such as photonic devices, micromachines,
zero-threshold lasers and integrated optical waveguides
with losses [8,9].
Over the past decade, two-photon absorption (TPA) has
attracted considerable attention due to its various applica-
tions in the areas of optical limiting [1], two-photon photo-
dynamic therapy [2], two-photon pumped up-conversion
lasing [3], three-dimensional (3D) fluorescence imaging
[
4], optical data storage [5] and three-dimensional litho-
Two-photon initiated polymerization (TPIP) is the most
important approach of TPMF technique [9]. Ordinarily,
conventional UV–vis photoinitiators are employed, but
these initiators present very weak two-photon sensitivities
and often require high laser intensities with long exposure
time to realize initiation, which results in damage to the
microstructure. This limits the versatility of 3DLM and
its scope of applications. To overcome these shortcomings,
another approach has been advocated recently, which uti-
lizes high efficient initiators and excites them by laser pulses
in near IR [7]. Such systems allow lower power and shorter
exposure time without damage, which could lead to a
cheaper technology. Therefore, the two-photon initiators
with efficient TPA activity await further development.
graphic microfabrication (3DLM) [6,7]. As far as 3DLM
is concerned, the two-photon microfabrication (TPMF)
technique, for the fabrication of complex micrometer-size
3
D structures in one step with high spatial resolution, has
gained much attention. TPMF technique is based on the
fact that the two-photon absorption (TPA) probability
depends quadratically on the excitation intensity. This qua-
dratic dependence of TPA as a function of light intensity
limits the absorption process to the immediate vicinity of
*
Corresponding authors. Tel.: +86 551 5108151; fax: +86 551 5107342.
E-mail addresses: jxyang@ahu.edu.cn (J.-X. Yang), yptian@ahu.
edu.cn (Y.-P. Tian).
0
022-2860/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.10.044

Z.-J. Hu et al. / Journal of Molecular Structure 839 (2007) 50–57
51
The need for improved two-photon dyes in recent years
has led to a significant amount of research that has been
focused on the development of structure/properties rela-
tionships of TPA activity. Various design strategies have
successfully been used to synthesize organic molecules with
large TPA cross-sections (d) and high photochemical quan-
tum yield (U). As Albota et al. [10] and Reinhardt et al. [11]
reported, the conjugation length, electron donor (D) and
acceptor (A) strength, and planarity of the p-center are
important parameters for enhancing the TPA of molecules.
Under these considerations, many efficient D-p-D type ini-
tiators have been developed [6]. Our group also made
efforts to develop a series of two-photon initiators with
symmetric and asymmetric molecular structure of types
A-p-D-p-A, D-p-A and D-p-D. These initiators showed
efficient TPA activities and were applied in TPIP success-
fully [6c,7]. However, some complicated hetero-aromatic
unit cell parameters and data collections were performed
with Mo-Ka radiation (k = 0.71073 A). Unit cell dimen-
˚
sions were obtained with least-squares refinements, and all
structures were solved by direct methods using SHELXS-
97. The other non-hydrogen atoms were located in
successive difference Fourier syntheses. The final refinement
was performed by full-matrix least-squares methods with
anisotropic thermal parameters for non-hydrogen atoms
2
on F . The hydrogen atoms were added theoretically and
riding on the concerned atoms.
UV–vis absorption spectra were recorded on UV-3100
spectrophotometer. Fluorescence measurements were car-
ried out with an Edinburgh FLS920 fluorescence spectrom-
eter equipped with a 450 W Xe lamp and a time correlated
single-photon counting (TCSPC) card. All the fluorescence
spectra were collected. The SPEF quantum yields U were
measured by using a standard method under the same
experimental conditions for all compounds. Coumarin
307 dissolved in ethanol (U = 0.56) [16], at the same con-
centration as the other sample, was used as the standard.
Lifetime values were obtained by reconvolution fit analysis
of the decay profiles with the aid of F900 analysis software.
The fitting results were judged by their values of ‘‘reduced
chi-squared’’. TPEF spectra and TPIP microfabrication
were measured using a mira 900-D Ti: sapphire femtosec-
ond laser with a pulse width of 200 fs and a repetition rate
of 76 MHz. All measurements were carried out in air at
room temperature. TPA cross-sections were measured
using two-photon-induced fluorescence measurement tech-
nique [17].
0
compounds still relatively lack studies in TPIP [12]. 2,2 :
0
00
6
,2 -Terpyridine is a multiple nitrogen hetero-aromatic
compound with a rigid framework plane, and its deriva-
tives often show good photochemical and photophysical
properties [13]. Up to now, however, there are a few exam-
ples concerning their NLO properties [14,15], and only
recently a paper reported some studies about the TPA
response of D-p-A terpyridyl-based molecules [15]. These
systems can be depicted as traditional push–pull stilbene
chromophores, where the terpyridyl ring is the acceptor
part and a dialkylamino group the donor one. In this work,
two new compounds of this kind of dyes were utilized as
initiators to fabricate microstructure with TPIP technology
under laser source, demonstrating that they can be used as
efficient two-photon photopolymerization initiators. More-
over, one single crystal structure and the linear/non-linear
optical properties of two compounds were described, as
well as a brief discussion about the relationships of struc-
ture/properties.
2.2. Preparation
All chemicals were available commercially and the
solvents were purified as conventional methods
before use. 4-(N,N-Diphenylamino)benzaldehyde (a),
4
-(N-hydroxyethyl-N-methylamino)benzaldehyde (b) [18]
0
0
0
00
0
2
. Experimental
and 4 -(p-tolyl)-2,2 : 6 ,2 -terpyridine (1 ) [12] were
synthesized according to the reported methods. As shown
in Scheme 1, we synthesized the title compounds by the
following reactions.
2
.1. Apparatus and measurements
1
13
The H NMR and C NMR spectra recorded on at
0
0
0
00
2
5 ꢁC using Bruker Avance 600 spectrometer were reported
2.2.1. 4 -(Phenyl-p-bromomethyl)-2,2 : 6 ,2 -terpyridine
0
as parts per million (ppm) from TMS (d). Coupling
constants J were given in Hertz. Mass spectrum was
determined with a Micromass GCT–MS (EI source).
Electrospray ionization mass spectrum (ESI-MS) was
acquired by a model LCQ ion trap mass spectrometer
(2 )
0
1 (5.0 g, 15.5 mmol), NBS (2.9 g, 16.1 mmol) and a cat-
alytic amount of benzoyl peroxide (BPO) were dissolved in
tetrachloromethane (200 mL). The mixture was refluxed
for 8 h. After the mixture cooled to room temperature,
the solid residue was filtered. The filtrate was washed with
water and brine, and then dried over anhydrous magne-
sium sulfate, filtered and concentrated; the residual solid
was recrystallized from anhydrous ethanol to give 5.0 g
(
Finnigan) equipped with a Finnigan MAT electrospray
ion source. The melting points were measured on a
METTLER-TOLEDO DSC822 differential scanning
calorimeter at a heating rate of 20 ꢁC min
nitrogen atmosphere. IR spectra were recorded on NEXUS
ꢀ
1
under a
0
1
white crystals 2 with yield 80%. Mp: 176 ꢁC. H NMR
ꢀ1
8
70 (Nicolet) spectrophotometer in the 400–4000 cm
(CDCl , 600 MHz) ppm d: 4.58 (s, 2H), 7.38 (m, 2 H),
3
region using a powder sample on a KBr plate.
X-ray diffraction data of single crystals were collected on
Bruker P4 four-circle differactometer. The determination of
7.56 (d, J = 8.00 Hz, 2H), 7.90 (m, 4H), 8.69 (d,
J = 8.00 Hz, 2H), 8.45–8.77 (m, 4H). C NMR (CDCl3
400 MHz) ppm: 156.07, 155.95, 149.50, 149.09, 138.62,
1
3

52
Z.-J. Hu et al. / Journal of Molecular Structure 839 (2007) 50–57
CH
2
Br
CH PPh Br
2 3
i)
ii)
N
N
N
N
N
N
N
N
N
1
'
2'
3'
a
b
iii)
N
N
N
N
N
N
N
N
OH
1
2
4 3
Scheme 1. (i) NBS, BPO, CCl , reflux; (ii) PPh , toluene, reflux; (iii) a (4-(N,N-diphenylamino) benzaldehyde) and b (4-(N-hydroxyethyl-N-
methylamino)benz-aldehyde), t-BuOK, grind.
1
3
38.57, 136.94, 129.63, 127.76, 123.89, 121.38, 118.82, and
121.52, 120.32, 120.02, 118.72, 109.85. MS: m/z
(%) = 578.25 (100).
2.96. MS: m/z (%): 324.15 (100), 404.05, 401.05, 405.06.
ꢀ
1
FT-IR (KBr, cm ): 3779, 3697, 3635, 3049, 1807, 1719,
1
6
658, 1581, 1483, 1386, 1221, 1031, 893, 831, 785, 738,
72, 590, 492.
0
2.2.4. 4 -(4-{2-[4-(N-Hydroxyethyl-N-methylamino)
phenyl]ethenyl}phenyl)-2,2 : 6 ,2 -terpyridine (2)
0
0
00
0
It was also prepared similar to 1 via b and 3 to afford
solid product (yield 67%). Mp 216.5–220.1 ꢁC. H NMR
DMSO-d , 600 MHz) ppm d : 2.50 (s, 3H), 3.49 (s, 2H),
.55 (s, 2H), 6.72 (d, J = 7.8 Hz, 2H), 7.07 (d,
J = 16.2 Hz, 1H), 7.25 (d, J = 16.0 Hz, 1H), 7.47 (d, J =
.8 Hz, 2H), 7.53 (s, 2H), 7.74 (d, J = 7.2 Hz, 2H), 7.93
d, J = 7.2 Hz, 2H), 8.04 (t, 2H), 8.06–8.70 (m, 6H).
NMR (CDCl , 600 MHz): d = 156.47, 155.99, 149.87,
1
0
0
00
0
2
phosphonium bromide (3 )
.2.2. 4-(2,2 : 6 ,2 -Terpyridyl-4 )-benzyl triphenyl
0
(
6
3
0
A mixture of 2 (5.0 g, 12.4 mmol) and triphenylphos-
phine (TPP) (3.6 g, 13.6 mmol) in anhydrous toluene
7
(
(
80 mL) was refluxed for 3 h. The product was precipitated
1
3
C
from the resulting solution. The off-white precipitates
were filtered and dried under vacuum to give 3 (6.6 g,
yield 81%). The recrystal product from ethanol was
characterized by ESI-MS: m/z (%): 584.2 (100)
0
3
1
1
6
49.66, 149.12, 139.10, 136.82, 136.58, 129.62, 127.89,
26.59, 126.45, 124.14, 123.72, 121.41, 118.54, 112.97,
0.25, 55.26 and 38.92. MS: m/z (%): 484.23 (100). FT-IR
+
[
C H N P ].
4
0
31
3
ꢀ1
(
KBr, cm ) m: 2902, 1797, 1589, 1520, 1469, 1443, 1420,
1
8
386, 1351, 1262, 1181, 1123, 1075, 1043, 996, 895,
27,792, 739, 687, 662, 624, 536.
0
.2.3. 4 -(4-{2-[4-(N,N-Diphenylamino)phenyl]ethenyl}
2
phenyl)-2,2 : 6 ,2 -terpyridine (1)
0
0
00
t-BuOK (1.3 g, 12.3 mmol) was placed into a dry mortar
and milled to very small, then 3 (2.0 g, 3.0 mmol) and a
3. Results and discussion
0
(
0.8 g, 3.0 mmol) were added and mixed. The mixture
3.1. Structure features
was milled vigorously for about 20 min. The mixture
became sticky and 5 mL tetrahydrofuran (THF) was add-
ed, and then continuously milled for 10 min. After comple-
tion of the reaction (monitored by TLC), the mixture was
dispersed in 100 mL ethanol. The residual solid was filtered
and recrystallized from anhydrous dichloromethane/etha-
The crystal of 1, suitable for the X-ray analysis, was
obtained from slow evaporation CH Cl covered with iso-
2
2
propyl alcohol at room temperature several days later
(Table 1). The selected bond distances and angles are listed
in Table 2. As shown in Fig. 1, it is obvious that the C@C
exists in trans-isomer, making for planar construction of
molecular. In the terpyridyl acceptor moiety, the central
pyridinyl (labeled P0) is also arranged with a phenyl (P5)
and two pyridine planes (P6 and P7) being 14.1ꢁ (between
P0 and P5), 7.0ꢁ (P0 and P6) and 2.3ꢁ (P0 and P7). The
result indicates that the terpyridyl group is seldom copla-
1
nol, to give yellow crystals 1 (1.3 g, yield 75%). H NMR
(
7
DMSO-d , 600 MHz) ppm d: 6.90–7.34 (m, 14 H), 7.50–
6
.79 (m, 6H), 8.00 (d, 4H), 8.6–8.78 (m, 6H). C NMR
1
3
(
CDCl , 600 MHz) ppm d: 156.12, 155.83, 149.71, 148.98,
3
1
1
40.81, 137.97, 137.66, 137.10, 136.37, 128.88, 128.54,
27.73, 127.25, 127.18 125.98, 127.4, 123.92, 123.48,

Z.-J. Hu et al. / Journal of Molecular Structure 839 (2007) 50–57
53
Table 1
Unfortunately, the single crystal of 2 was not obtained,
but we could predict that it should share a similar structure
with described above, the conjugated p-bridges and terpyr-
idyl groups of which should be basically coplanar. From
Crystal data collection and structure refinement of 1
Molecule
1
Formula
C
578.69
41 30 4
H N
1
Formula weight
Crystal system
Space group
H NMR spectra of 2, it could be found that the double
Monoclinic
C2/c
293(2)
0.71073
0.074
28.921(4)
9.6831(14)
23.386(3)
90
110.811
90
bond protons are seen as a pair of double peaks, intensities
corresponding to 2H, centered at 7.25 and 7.07 ppm with
J = 16.0 and 16.2 Hz, respectively, a visible trans-isomer.
These structure information seem to confirm that solvent-
free Wittig reaction is an efficient and stereoselective way
to build C@C bond. Meanwhile, it provides further evi-
dences to support that this facile synthetic technique is an
efficient approach in forming C@C bond [19].
Temperature (K)
˚
a
k of Mo-K (A)
ꢀ1
Absorption coefficient (mm
)
˚
a (A)
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
˚
3)
V (A
6121.7(15)
8
7197
3.2. Linear absorption spectra and single-photon-excited
fluorescence (SPEF)
Z
Reflections collected
Reflections unique
6013
R
int
0.0232
The linear absorption spectra of these two dye mole-
Final R indices [I > 2r(I)]
R
1
= 0.0588, wR
2
= 0.1137
cules were measured in various dilute solutions (1 ·
ꢀ5
ꢀ1
10
mol L ). As shown in Fig. 2, the first absorption
bands of two compounds are located around 391 and
97 nm in DMF solution, respectively. And there is a slight
Table 2
Selected bond lengths (A) and angles (ꢁ) for of 1
3
˚
red-shift from 2 to 1. As shown in Scheme 1, structure
modification occurs only in donor terminal moiety, where
a diphenylamino was replaced by a methyl(hydroxy-
ethyl)amino. As well known, a strong electron donor could
help to stabilize the charge-separated excited state of the
molecule; this red-shift could be explained by the elec-
tron-donating strength of donor groups, indicating that 2
contains the stronger donor [20]. This donating strength
C(16)AC(19)
C(20)AC(21)
C(7)AN(1)
C(7)AN(1)AC(6)
C(13)AN(1)AC(6)
1.460(3)
1.464(3)
1.421(3)
119.2(2)
117.7(2)
C(19)AC(20)
C(13)AN(1)
C(6)AN(1)
C(13)AN(1)AC(7)
1.317(4)
1.421(3)
1.426(3)
121.4(2)
1
nar. At the diphenylamino donor end, the central nitrogen
and its three bended carbon atoms are basically coplanar,
forming a quasi-equilateral trigonal NC3 plane (defined
as the P1 plane), with the sum of the three CANAC angles
sequence has been discussed from H NMR chemical shift
values of the aldehyde protons in compounds a and b,
which is inversely related to electron density at the proton
[18]. This electron density could be interpreted by the delo-
calization of the lone electron pair of the N atom in the
donor units. Investigating the structure of 1, in the NC3
core, three CAN bond lengths are almost equal and dihe-
dral angles to the neighbor three planes are quite small (less
than 44ꢁ). Therefore, the lone electron pair may be delocal-
ized averagely over the three phenyl units. The similar
structural features also exist in the methyl(hydroxy-
ethyl)amino units in 2, but the delocalized tendency of
the electron pair is prior to the neighboring phenyl ring
than to alkyl group. So the electron-donating strength of
(
357.8ꢁ) being close to 360ꢁ. Around the central nitrogen,
three phenyl ring planes (P2, P3 and P4, respectively) are
arranged in a propeller-like fashion (P1 and P2: 43.2ꢁ, P1
and P3: 34.4ꢁ, P1 and P4: 43.7ꢁ). The linkage between
two phenyl rings (P2 and P5) is quite conjugated with bond
˚
˚
lengths of C(16)AC(19): 1.460 A, C(19)@C(20): 1.317 A
˚
and C(20)AC(21): 1.464 A. These structural features sug-
gest that all non-hydrogen atoms between donor and
acceptor are highly conjugated, leading to a p-bridge for
the charge transfer from amino to terpyridyl.
Fig. 1. Crystal structure of 1. Hydrogen atoms are omitted for clarity.

54
Z.-J. Hu et al. / Journal of Molecular Structure 839 (2007) 50–57
8
6
4
00
00
00
1
1
TPEF
Compound 1
Compound 2
SPEF
0.4
0.3
0.2
0.1
0.0
2
2
4
00 450 500 550 600 650 700
200
0
450
500
550
600
650
700
750
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
ꢀ5
ꢀ1
Fig. 3. SPEF (c = 1 · 10 mol L ) spectra of two compounds in DMF.
Fig. 2. Linear absorption spectra of two compounds in DMF
ꢀ3
ꢀ1
The inset shows that TPEF (c = 1 · 10 mol L ) spectra of two
ꢀ5
ꢀ1
(
c = 1.0 · 10 mol L ).
compounds in DMF.
units should be in the following sequence: methyl(hydroxy-
ethyl)-amino > diphenylamino, corresponding to the red-
shift sequence: 1 < 2. These relationships indicate that the
wavelength of the maximum TPA cross-section could be
tuned by modifying appropriate substituents, because the
wavelength of TPA almost corresponds to twice of that
of linear absorption. Such a modification could be expected
result in notable changes in the electron-donating strength
or p-conjugated length. Note that the absorption profiles
and the absorption maxima are mainly dominated by the
nature of the excited state p-electron system [21]. As shown
in Table 3, all the linear absorption spectra of two com-
pounds show quite slight solvatochromism, indicating that
the surrounding solvent molecules take weak effect on the
compounds in the excited state.
0
.5
acetone
chloroform
DMF
4
96
0.4
3
91
5
46
381
ethanol
386
528
0.3
542
0.2
0.1
0.0
3
00 350 400 450 500 550 600 650 700
The SPEF spectra of the two molecules at concentration
ꢀ
5
ꢀ1
Wavelength (nm)
of 1 · 10 mol L
in DMF solution are presented in
Fig. 3. A similar result was observed that the emission max-
Fig. 4. Linear absorption (left) spectra and SPEF (right) of 1 in various
solvents, with c = 1.0 · 10 mol L
SPEF
ꢀ5
ꢀ1
ima (kmax Þ shows a parallel increase with linear absorption
.
maxima (k ). Noted that the emission profiles and the
max
emission maxima mainly reflect the nature of the ground
state p-electron system [21], the results above imply that
more p-electrons and longer p-conjugated structure may
be involved in 2 than that in 1.
Table 3
Photophysical data of two molecules
As summarized in Table 3, the SPEF spectra of two
compounds show strong solvatochromism, kmax with
SPEF
a
max
SPEF
max
b
c
d
TPEF
max
e
Molecule
Solvent
k
k
U
s
k
remarkable bathochromic shifts. As shown in Fig. 4, for
instance, kmax of 1 is located at 496 nm in chloroform
1
Acetone
381
386
391
386
528
496
546
538
0.62
0.69
0.69
0.44
2.47
1.95
2.73
2.12
531
502
548
553
SPEF
CHCl
DMF
3
and red-shifted to 546 nm in DMF. In contrast, the shift
of kmax (abs) of the absorption spectra of 1 in chloroform,
relative to that in DMF, is very limited (less than 5 nm).
The facts indicated that dipole moment of two compounds
are dependent on solvent polarity. The redistribution of
electrons occurs within the molecule of each compound,
which proves that they both process a highly delocalized
conjugated system.
EtOH
2
Acetone
390
376
397
386
561
509
584
542
0.36
0.36
0.43
0.13
2.50
1.67
2.27
1.74
563
512
588
559
CHCl
DMF
3
EtOH
a
k
k
max of the one-photon absorption spectra in nm.
max of the one-photon fluorescence spectra in nm.
Fluorescence quantum yield.
The lifetime of one-photon fluorescence in ns.
max of the two-photon fluorescence spectra in nm.
b
c
d
e
The fluorescence lifetime measurement of compounds
was performed on the same Edinburgh FLS920 spectroflu-
k

Z.-J. Hu et al. / Journal of Molecular Structure 839 (2007) 50–57
55
orometer with a Hydrogen flash lamp (pulse duration
1 ns) as the excitation source. During the lifetime mea-
2
1
1
000
500
000
<
acetone
chloroform
DMF
surements, all solutions were excited the optimal excitation
wavelength in order to give the largest relative fluorescence
intensity in each solution. Emission peaks were observed
irrespective of the excitation wavelength. The time distribu-
tion of the light source pulse was measured immediately
after the measurement of the fluorescence decay. The
observed decay of fluorescence is given by convolution of
the lump pulse with the impulse response of the sample.
The decays were analyzed by the ‘‘least-squares’’ and iter-
ative reconvolution method. The quality of the exponential
ethanol
5
00
2
fits was evaluated by the goodness of fit (v ). For all the
2
0
decays in this experiment, v is bigger than 0.9 and less than
400
450
500
550
600
650
700
1
.1, which indicates a good fit. As shown in Table 3, all
Wavelength (nm)
compounds shows long fluorescence lifetime. The results
from the quantum yield measurement and lifetime calcula-
tion of the two compounds can clearly demonstrate the
sensitivity of their fluorescence behaviors to the solvents.
From Table 3, one can see that two compounds have rela-
tively high SPEF quantum yields and lifetime.
ꢀ3
ꢀ1
.
Fig. 6. TPEF of 1 in various solvents, with c = 1.0 · 10 mol L
Iout on Iin data can fit in well with the quadratic parab-
olas. This gives direct experimental evidence of a two-
photon excitation mechanism.
While the concentrated solutions (1.0 · 10 mol L
of the chromophores were excited by the wavelength
60 nm of the femtosecond laser beam, strong fluorescence
ꢀ
3
ꢀ1
)
3.3. Two-photon-excited fluorescence (TPEF) and
two-photon absorption (TPA)
7
can be observed and detected (see Table 3). As shown in
Fig. 6, the TPEF characteristic of the compound 1 is also
affected by its molecular environment. We could find that
there are similar vibronic structures and visible solvato-
chromism in the TPEF and SPEF spectra of 1 (see Figs.
As shown in Fig. 2, there is no linear absorption in
the range of 600–900 nm for two compounds, which sug-
gests that there are no molecular energy levels corre-
sponding to this spectral range. Therefore, upon
excitation from 600 to 900 nm, it is impossible to pro-
duce single-photon-excited up-converted fluorescence. If
frequency up-converted fluorescence that induced with
laser in this spectral range appears, it should be mainly
attributed to TPEF. Fig. 5 shows that the output inten-
sity of frequency up-converted fluorescence of 1. It was
found that the fluorescence intensity has a square depen-
dence on the input laser intensity, i.e., the experimental
6
and 4). This implies that they come from the same fluo-
rescent excited state. Similar to SPEF properties, 1 shows
more sensitive TPEF properties than 2 in the same solution
(see Fig. 3). From Table 3, we could observe slight red-shift
of the peak position of TPEF relative to their correspond-
ing SPEF. This is due to re-absorption from the fluores-
cence in the concentrated solution.
The TPA cross-sections have been measured using the
two-photon-induced fluorescence measurement technique
with the following equation: [17]
6
200000
150000
100000
5
4
3
2
1
0
Y = 1.9371X
2
U
ref
c
c
ref
n
ref
F
R
= 0.9863
r ¼ rref U
n F ref
Here, the subscripts ref stands for the reference molecule. r
is the TPA cross-section value, c is the concentration of
solution, n is the refractive index of the solution, F is the
integrated area of the detected two-photon-induced fluo-
rescence signal, and r is the fluorescence quantum yield.
The rref value of reference is taken from the literature [22].
By referencing the TPA cross-section of flouorescein to
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Lg I
in
50000
Experimental data
Fitted result
0
ꢀ
50
ꢀ4
ꢀ1
be 36 GM (1 GM = 10
cm s photon ), the TPA cal-
0
100
200
300
400
500
600
700
culated cross-sections of the two molecules are 153 and
96 GM, respectively. They are a few times larger than that
of the reference molecule. According to Albota et al. [10]
the terminal substitution with strong electron donors will
bring an increase of electron delocalization in the excited
state and result in a substantial increase in the transition
I
(mW)
in
Fig. 5. Dependence of output fluorescence intensity (Iout) of 1 on input
laser power (Iin). Excitation carried out at 760 nm, with c =
ꢀ
3
ꢀ1
1
.0 · 10 mol L in DMF. The inset shows the linear dependence of
lgIout on lgIin
.

56
Z.-J. Hu et al. / Journal of Molecular Structure 839 (2007) 50–57
terpyridyl-based molecules; and the two initiators were
successfully employed in photopolymerization reactions.
Experimental results confirm that this new kind of p-conju-
gated framework, consisting of terpyridyl acceptor and a
vinyl p-conjugated center is a useful conjugated p-electron
building block, which has potential applications in forming
novel and effective TPIP initiator molecules, especially
after proper structure modification.
Acknowledgements
This work was supported by a grant for the National
Natural Science Foundation of China (50532030,
50325311 and 50335050), Doctoral Program Foundation
of the Ministry of Education of China, Education Commit-
tee of Anhui Province (2006KJ032A, 2006KJ135B), Team
for Scientific Innovation Foundation of Anhui Province
Fig. 7. Optical micrograph of the lattice fabricated via two-photon
polymerization of an acrylic ester oligomer using 1 or 2 as an initiator.
dipole moment, which is the contributor to the enhanced
TPA cross-section. However, 1 exhibits the higher TPA
cross-section than 2 under the same experimental condi-
tions; it may be partially attributed to the better planar
characteristic and conjugation in p-electron system of 1
than that of 2.
(2006KJ007TD) and Person with Ability Foundation of
Anhui Province (2002Z021), Person with ability Founda-
tion of Anhui University.
Appendix A. Supplementary data
Crystallographic data for the structural analysis have
been deposited at the Cam-bridge Crystallographic Data
Center, CCDC-292526 for molecule 1. Copy of this infor-
mation may be obtained free of charge via www: http://
www.ccdc.cam.ac.uk or from The Director, CCDC, 12
Union Road, Cambridge CB221EZ, UK (fax: +44 1223/
3.4. Two-photon initiated polymerization (TPIP)
microfabrication
In our TPIP experiments, three-dimensional cross-
linked lattice periodic microstructures were fabricated
with negative resins system, which contained oligomer
336 033; email: deposite@ccdc.cam.ac.uk). Structural fac-
(
bisphenyl A epoxide dimethyl acrylate) and 0.5% 1 or 2
tors are available on request from the authors. Supporting
information can be found in the web version of this paper.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.
as initiator and a little 1,2-dichloroethane (increasing the
compatibility and controlling the viscosity). The same
mode-locked Ti: sapphire laser that was used in the TPF
measurements was utilized for TPMF at and 760 nm. The
laser was tightly focused via an objective lens (·50,
NA = 0.45), and the focal point was modulated in the gel
resin that was on a xyz-step monitored stage controlled
by computer. The pulse energy after being focused by the
objective lens was ꢁ20 mW. The polymerization results
were observed with a polarization microscope (Opton,
Germany) and the photographs of micostructures were
shown in Fig. 7. The distance of each two neighboring
streaks is 10 lm. The polymerization reaction does not take
place in the part out of the laser focus, which is easily
washed out by organic solvent.
2006.10.044.
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