Structures and nonlinear optical properties of new symmetrical two-photon photopolymerization initiators

Article abstract of DOI:10.1039/b415354e

Four new symmetrical, two-photon photopolymerization initiators, 9-(4-{(1E,11E)-4-[(E)-4-(9H-carbazol-9-yl)styryl]-2,5-dimethoxystyryl}phenyl) -9H-carbazole (1), N-(4-{(1E,8E)-4-[(E)-4-(diphenylamino)styryl]-2,5- dimethoxystyryl}phenyl)-N-phenylbenzeneami

Full text of DOI:10.1039/b415354e

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P A P E R
Structures and nonlinear optical properties of new symmetrical
two-photon photopolymerization initiatorsw
Yun-Xing Yan,*a Xu-Tang Tao,a Yuan-Hong Sun,b Chuan-Kui Wang,b Gui-Bao Xu,a
Wen-Tao Yu,a Hua-Ping Zhao,a Jia-Xiang Yang,a Xiao-Qiang Yu,a Yong-Zhong Wu,a
Xian Zhaoa and Min-Hua Jianga
a State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, P. R. China.
E-mail: yxyan@icm.sdu.edu.cn
b Department of Physics, Shandong Normal University, Jinan, 250014, P. R. China
Received (in St. Louis, MO, USA) 3rd October 2004, Accepted 29th November 2004
First published as an Advance Article on the web 2nd February 2005
Four new symmetrical, two-photon photopolymerization initiators, 9-(4-{(1E,11E)-4-[(E)-4-(9H-carbazol-
9-yl)styryl]-2,5-dimethoxystyryl}phenyl)-9H-carbazole (1), N-(4-{(1E,8E)-4-[(E)-4-(diphenylamino)styryl]-
2,5-dimethoxystyryl}phenyl)-N-phenylbenzeneamine (2), 1,4-bis{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}-
2,5-bisdimethoxybenzene (3) and 1,4-bis{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}-2,5-bisdodecyloxybenzene (4),
have been synthesized and characterized. One-photon fluorescence, one-photon fluorescence quantum yields,
one-photon fluorescence lifetimes, and two-photon fluorescence have been investigated. The results show that
they are all good two-photon absorbing chromophores and effective two-photon photopolymerization
initiators. The two-photon absorption cross-sections of these molecules have been evaluated by theoretical
calculations. Microfabrication via two-photon-initiated polymerization has been studied and the possible
photopolymerization mechanism is discussed.
middle of the conjugated framework, provides the potential for
1. Introduction
symmetric charge displacement upon excitation and enhanced
TPA. However, effective initiators are still rare as the relation
between the structures and properties required for initiating
two-photon photopolymerization remains unclear. In this con-
tribution, we present the synthesis, characterization and calcu-
lated two-photon cross sections of a series of symmetrical
chromophores, namely 9-(4-{(1E,11E)-4-[(E)-4-(9H-carba-
zol-9-yl)styryl]-2,5-dimethoxystyryl}phenyl)-9H-carbazole (1),
N-(4-{(1E,8E)-4-[(E)-4-(diphenylamino)styryl]-2,5-dimethoxy-
styryl}phenyl)-N-phenylbenzeneamine (2), 1,4-bis{2-[4-(2-
pyridin-4-ylvinyl)phenyl]vinyl}-2,5-bisdimethoxybenzene (3) and
1,4-bis{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}-2,5-bisdodecyl-
oxybenzene (4). Compared to the known two-photon photo-
polymerization initiators,2 the acceptor groups are attached to
the ends of 3 and 4. Our results demonstrate that they are all
good two-photon absorbing chromophores and effective two-
photon photopolymerization initiators.
Organic materials possessing large two-photon absorption
(TPA) cross sections can be used for a variety of technical
applications, such as in three-dimensional (3D) optical data
storage and microfabrication,1–8 optical power limiting,9,10
localized photodynamic therapy,11 and two-photon laser scan-
ning fluorescence imaging.12–14 In TPA, the absorption transi-
tion probability is proportional to the square of the incident
intensity. This quadratic dependence of TPA on light intensity
limits the absorption process to the immediate vicinity of the
focal point, providing a large penetration depth and a high
resolution of the excitation volume. Thus, TPA-initiated
photopolymerization appears to be an interesting method for
the fabrication of complex micrometer-sized 3D structures
with high resolution in one step. When a chromophore, acting
as a polymerization photoinitiator, is excited by simultaneous
absorption of two photons, it can lead to the formation of solid
structures in the presence of a monomer. To be efficient
initiators of the chain reaction with acrylates, the selected
compounds must have a large TPA cross section and must
be able to produce radicals upon excitation.
It should be mentioned that, although 1 and some related
compounds have been synthesized previously,22–25 its crystal
structure, one-photon fluorescence lifetime and calculated two-
photon cross section have not been previously investigated.
Moreover, the use of 1–4 as initiators had not been studied.
Much effort has been expended to develop efficient two-
photon chromophores by use of either a symmetrically or
asymmetrically substituted p-conjugated unit.15–18 Recent re-
search encompassing synthesis, structures and theory has
revealed the importance of certain basic structural motifs for
TPA-active organic materials.19–21 In particular, a framework
for mobile p electrons with electron donor and acceptor groups
on the terminal sites, with or without donors/acceptors in the
2. Experimental
2.1. Chemicals and instruments
4-Carbazol-9-ylbenzaldehyde was synthesized according to
the reported methods.26 Triphosphonium chloride (10) was
synthesized by our laboratory. 4-Bromobenzyl(triphenyl)pho-
sphonium bromide (20) and 2,5-bisdodecyloxybenzene-
1,4-dicarbaldehyde (30) were synthesized according to the
literature methods.27,28 The 600 MHz H1 NMR and 13C
NMR spectra were obtained on a Bruker av600 spectrometer.
Elemental analyses were performed using a PE2400 elemental
analyzer. UV-Vis-near-IR spectra were measured on a Hitachi
w Electronic supplementary information (ESI) available: NMR spectra
of 1 and 2. Packing diagram of 1. The linear absorption and the one-
and two-photon fluorescence spectra of 1–4. See http://www.rsc.org/
suppdata/nj/b4/b415354e/
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
N e w J . C h e m . , 2 0 0 5 , 2 9 , 4 7 9 – 4 8 4
479

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Scheme 1 Synthesis of 1 and 2.
U-3500 recording spectrophotometer. Melting points were
measured on DSC822e Mettler–Toledo instruments.
The linear absorption spectra were measured in solvents of
9-(4-{(1E,11E)-4-[(E)-4-(9H-Carbazol-9-yl)styryl]-2,5-dimet-
hoxystyryl}phenyl)-9H-carbazole (1). Under argon atmosphere,
10 (1 mmol) and 4-carbazol-9-ylbenzaldehyde (2 mmol) were
dissolved in dry THF (100 ml) at 0 1C. Potassium tert-butoxide
(3 mmol) was then added portion-wise and the reaction
mixture was allowed to stand for 2 h. This mixture was then
ground and poured into distilled water (200 ml), neutralized
with dilute hydrochloric acid, and extracted with dichloro-
methane. The organic layer was then dried over anhydrous
magnesium sulfate and evaporated to yield a green powder,
which was further purified by column chromatography on
silica gel using ethyl acetate–petroleum benzine (1 : 10) as
eluent and crystallized using ethyl acetate with a yield of
different polarity at a concentration of c = 1 ꢀ 10ꢁ5 mol lꢁ1
,
and the solvent influence eliminated. The steady-state fluores-
cence spectra measurements were performed with an Edin-
burgh FLS 920 spectrofluorimeter. A 450 W Xe arc lamp
provided the B400 nm excitation radiation. The one-photon
fluorescence spectra were measured with the same concentra-
tions as those of the linear absorption spectra. The maximum
one-photon fluorescence excitation wavelengths for 1, 2, 3 and
4 are 400, 420, 420 and 430 nm, respectively. The two-photon
fluorescence spectra were observed with a laser beam from a
mode-locked Ti:sapphire laser (Coherent Mira 900F) as the
pump source with a pulse duration of 200 fs, a repetition rate
of 76 MHz, and a single-scan streak camera (Hamamatsu
35% and a mp 204.8 1C. The enthalpy is ꢁ71.76 mJ mgꢁ1
.
Anal. calcd for C48H36N2O2: C, 85.71; H, 5.36; N, 4.17; found:
1
C, 85.98; H, 5.44; N, 4.23%. H NMR (600 MHz, CDCl3) d:
Model C5680-01), together with a monochromator as
the recorder. The excitation wavelengths of 1, 2, 3 and 4 were
8.17 (d, 4H, J = 7.6 Hz), 7.79 (d, 2H, J = 7.8 Hz), 7.58 (q, 6H,
J = 7.3 Hz), 7.43 (m, 10H), 7.30 (q, 4H, J = 7.1 Hz), 7.23 (d,
1H, J = 12.5 Hz), 6.94 (d, 1H, J = 12.4 Hz), 6.89 (d, 1H, J =
12.1 Hz), 6.80 (d, 1H, J = 12.2 Hz), 3.97 (s, 3H), 3.66 (s, 3H).
13C NMR (600 MHz, CDCl3) d: 153.23, 152.40, 142.32, 142.25,
138.51, 138.25, 137.90, 131.89, 131.01, 130.92, 129.67, 129.36,
128.67, 128.15, 127.84, 127.72, 127.60, 127.44, 127.31, 125.67,
124.93, 122.24, 121.86, 121.81, 121.50, 121.47, 114.87, 111.34,
111.14, 110.48, 57.78, 57.51.
760, 820, 830 and 830 nm, respectively. The concentration
was c = 1 ꢀ 10ꢁ3 mol lꢁ1
.
2.2. Synthesis and structure
The synthetic approaches for 1 and 2 and for 3 and 4 are shown
in Schemes 1 and 2, respectively.
Scheme 2 Synthesis of 3 and 4: (a) PPh3, CCl4, BPO, reflux; (b) THF, 4-bromobenzaldehyde, grinding, RT; (c) THF, 20, grinding, RT;
(d) vinylpyridine, tri-o-tolylphosphine, palladium(II) acetate, triethylamine, reflux.
480
N e w J . C h e m . , 2 0 0 5 , 2 9 , 4 7 9 – 4 8 4

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b = 97.797(5)1, Z = 2, U = 1838.2(15) A3, T = 293(2) K,
N-(4-{(1E,8E)-4-[(E)-4-(Diphenylamino)styryl]-2,5-dimethoxy-
styryl}phenyl)-N-phenylbenzeneamine (2). Compound 2 was
obtained by a similar method for 1. It forms orange needle-
like crystals (yield 38%). Anal. calcd for C48H40N2O2: C,
85.18; H, 5.96; N, 4.14; found: C, 85.69; H, 6.02; N, 4.19%.
1H NMR (600 MHz, CDCl3) d: 7.45 (d, 4H, J = 8.45), 7.41 (s,
1H), 7.38 (s, 1H), 7.28 (q, 6H, J = 8.03 Hz), 7.14 (d, 11H, J =
8.14 Hz), 7.07 (m, 11H), 3.93 (s, 6H). 13C NMR (600 MHz,
CDCl3) d: 152.96, 149.09, 148.69, 133.70, 130.76, 129.80,
128.92, 128.17, 125.94, 125.16, 124.46, 123.19, 110.50, 57.92.
Dc = 1.216 g cmꢁ3, R1 = 0.0648, wR2 = 0.1199 for I 4 2s(I).z
3. Results and discussion
3.1. Structure and characterization
Selected bond lengths and bond angles of 1 are given in Table
1. The bond lengths of all single bonds are significantly shorter
than the typical single bonds, whereas the double bonds are
significantly longer than typical double bonds. The bond
lengths of the phenyl ring and carbazole ring are all of normal
aromatic character. These structural features indicate that 1
has a p-conjugated system although the planarity is not very
good. Indeed, the dihedral angles between two carbazole rings
and their adjacent phenyl rings are 126.3 and 51.81, respec-
tively. The dihedral angles between the central phenyl ring and
the adjacent two phenyl rings are 113.9 and 55.71, respectively.
The apparently centrosymmetrical 1 is, in fact, rather asymme-
trical, as illustrated in Fig. 1. In the crystal structure, the
chemical surroundings of the H atoms on C27 and C28 are
different, which may be responsible for the different chemical
shifts of the hydrogens on C27 and C28 in the 1H NMR
spectra, as well as explain the different numbers of carbon
atoms, 32 versus 16, in the 13C NMR spectra. These character-
1,4-Bis{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}-2,5-bisdimethoxy-
benzene (3). At room temperature, 4-bromobenzaldehyde (1.85 g,
0.01 mol), 10 (3.16 g, 0.004 mol), NaOH (8.00 g, 0.2 mol) and
THF (2 ml) were added into a mortar. The mixture was ground
for 5 min, then poured into distilled water (200 ml), neutralized
with dilute hydrochloric acid, and extracted with dichloro-
methane. The organic layer was dried over anhydrous magnesium
sulfate. After filtration, the solvent was removed at reduced
pressure and the residue was dissolved in dichloromethane. The
resulting solution was chromatographed on silica gel using ethyl
acetate–petroleum ether (1 : 20) as eluent. Then the isolated green
crystals were added into a mixture of tri-o-tolylphosphine (1.96 g,
6.44 mmol), vinylpyridine (3.48 ml, 32.24 mmol), palladium(II)
acetate (0.18 g, 0.8 mmol) and redistilled triethylamine (100 ml).
The mixture was refluxed in an oil bath under nitrogen, giving an
orange solution after heating and stirring for 36 h. The solvent
was removed under reduced pressure and the residue was dis-
solved in methylene chloride, washed three times with distilled
water, and dried with anhydrous magnesium sulfate. Then it was
filtered and concentrated. The resulting solution was chromato-
graphed on silica gel using ethyl acetate as eluent. The final
product was obtained as a red solid (yield 34%). Anal. calcd for
C38H32N2O2: C 83.21, H 5.84, N 5.11; found: C 83.97, H 5.95, N
5.23%. 1H NMR (CDCl3, 600 MHz) d: 8.58 (4H, q, J = 5.2 Hz),
7.56 (3H, q, J = 10.9 Hz), 7.44 (3H, t, J = 7.6 Hz), 7.36 (7H, m),
7.15 (1H, t, J = 9.1 Hz), 7.02 (4H, d, J = 16.2 Hz), 6.80 (2H, d,
J = 7.3 Hz), 6.75 (2H, t, J = 12.3 Hz), 6.66 (2H, t, J = 12.8 Hz),
3.90 (3H, s), 3.54 (3H, s). 13C NMR (600 MHz, CDCl3) d: 152.26,
151.64, 146.26, 146.10, 139.89, 139.57, 136.35, 134.49, 134.28,
131.34, 131.24, 130.97, 129.90, 128.89, 128.50, 128.29, 128.26,
127.75, 127.53, 127.43, 127.21, 127.14, 125.38, 122.30, 122.26,
114.82, 114.34, 110.31, 57.71, 57.44.
1
istics in the H NMR and 13C NMR spectra have also been
observed for 3. In contrast, the 1H NMR and 13C NMR
spectra of 2 and 4 indicate that they are centrosymmetric.
1
The H NMR spectra of 1 and 2 are given in the ESI.
3.2. One- and two-photon fluorescence
The photophysical properties of these compounds are summar-
ized in Table 2. The linear absorption, one-photon fluorescence
and two-photon fluorescence spectra of these compounds are
shown in the ESI. The linear absorption and fluorescence
spectra in DMF for 1 are depicted in Fig. 2.
As shown in Table 2, the absorption maxima are not sig-
nificantly different, while the fluorescence maxima are slightly
red-shifted and the fluorescent lifetimes are lengthened upon
increasing the polarity of the aprotic solvent for 2, 3 and 4. This
can be explained by a possibly higher polarity for the excited
states than for the ground states, since solvatochromism is
associated with energy level lowering. Increased dipole–dipole
interactions between the solute and solvent lower the energy
levels greatly.30,31 In the case of benzyl alcohol, the possibility of
hydrogen bonding between the solvent and solute molecules will
push the excitation energy even lower.32 For 1, the absorption/
fluorescence maxima show no obvious change with an increase
in polarity of the aprotic solvent. The maxima (Table 2) of the
two-photon fluorescence show slight red-shifts compared to the
one-photon fluorescence in the same solvent for each com-
pound. This can be ascribed to the effect of re-absorption as
the linear absorption band has a slight overlap with the emission
band and our two-photon fluorescence experiments were carried
out on concentrated solutions that made re-absorption a sig-
nificant process.33 The absorption/fluorescence maxima of 2
show an obvious red-shift, and the one-photon quantum yield
indicate an obvious increase compared to 1 in each solvent. This
can be explained by the facts that the structure of 2 is centro-
symmetric and possesses excellent planarity, compared to 1, and
that the electron donating strength of diphenylamino is greater
than that of carbazolyl. Compared to 3, the fluorescence max-
imum of 4 shows a slight red-shift and the one-photon quantum
yield shows no obvious change compared to 3 in each solvent.
This implies that the substituents on the O atom have no
significant influence on the properties for these two molecules.
1,4-Bis{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}-2,5-bisdodecyl-
oxybenzene (4). This was obtained by a similar approach as for
3 (yield 39%). Anal. calcd for C60H76N2O2: C 84.11, H 8.88,
1
N 3.27; found: C 84.79, H 8.97, N 3.35%. H NMR (CDCl3,
600 MHz) d: 8.59 (4H, d, J = 4.0 Hz), 7.57 (10H, m), 7.41 (4H,
d, J = 4.7 Hz), 7.36 (1H, s), 7.33 (1H, s), 7.19 (1H, s), 7.15 (3H,
d, J = 2.2 Hz), 7.05 (2H, d, J = 16.2 Hz), 4.09 (4H, t, J = 6.4
Hz), 1.91 (4H, m), 1.58 (4H, m), 1.41 (32H, m), 0.88 (6H, t, J =
6.8 Hz). 13C NMR (600 MHz, CDCl3) d: 151.25, 149.62,
138.69, 135.11, 133.29, 128.22, 127.47, 126.97, 125.39, 124.25,
120.92, 114.58, 114.39, 110.71, 69.62, 31.92, 29.72, 29.68, 29.64,
29.58, 29.50, 29.46, 29.38, 26.32, 22.68, 14.10.
X-Ray crystallography
The X-ray diffraction data of 1 were collected using a Bruker
P4 four-circle diffractometer with MoKa radiation. The num-
ber of measured reflections was 4161, of which 3936 were
independent (Rint = 0.0354). The structure was solved
by direct methods and refined with least-squares techniques
using SHELXL-97 programs.29 Crystal data for 1: C48
H36N2O2, FW = 672.79, monoclinic system, space group Pc,
m = 0.074 mmꢁ1, a = 8.777(5), b = 21.279(5), c = 9.934(5) A,
z CCDC reference number 261654. See http://www.rsc.org/suppdata/
nj/b4/b415354e/ for crystallographic data in .cif or other electronic
format.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 4 7 9 – 4 8 4
481

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Table 1 Selected bond lengths (A) and bond angles (1) for molecule 1
C1–N1
C1–C2
1.368(8)
1.3900
1.3900
C16–(17
C16–C19
1.375(9)
C32–C33
C33–C34
1.374(10)
1.393(10)
1.383(10)
1.423(10)
1.366(10)
1.3900
1.468(10)
1.381(10)
1.339(9)
C1–C6
C2–C3
C17–C18
C19–C20
C34–C35
C34–N2
1.3900
1.3900
C3–C4
C4–C5
C20–C21
C21–C22
1.462(10)
1.399(10)
1.413(11)
1.367(11)
1.390(9)
C35–C36
C37–C38
1.3900
1.3900
C5–C6
C6–C7
C21–C26
C22–C23
C37–C42
C37–N2
1.3900
1.435(9)
1.3900
1.3900
1.390(7)
1.3900
1.3900
C7–C8
C7–C12
C22–O1
C23–C24
C38–C39
C39–C40
1.373(11)
1.406(11)
1.469(11)
1.354(10)
1.378(10)
1.424(10)
1.413(9)
C8–C9
C9–C10
1.3900
1.3900
C24–C25
C24–C29
C40–C41
C41–C42
1.3900
1.3900
C10–C11
C11–C12
1.3900
1.3900
C25–O2
C25–C26
C42–C43
C43–C44
1.422(7)
1.3900
1.3900
C12–N1
C13–C18
1.372(8)
1.373(10)
1.381(9)
1.402(9)
1.379(9)
1.391(10)
C27–O2
C28–O1
C43–C48
C44–C45
1.3900
1.3900
C13–C14
C13–N1
C29–C30
C30–C31
1.329(10)
1.480(11)
1.374(9)
C45–C46
C46–C47
1.3900
1.3900
C14–C15
C15–C16
C31–C32
C31–C36
C47–C48
C48–N2
1.386(10)
1.360(8)
118.4(8)
120.2(8)
120.0
N1–C1–C6
C2–C1–C6
C3–C2–C1
C1–C6–C7
C12–C7–C6
C7–C8–C9
C18–C13–C14
C17–C16–C19–C20
C16–C19–C20–C21
C19–C20–C21–C26
C23–C24–C29–C30
109.9(6)
120.0
120.0
C16–C17–C18
C15–C16–C19
C13–C18–C17
C20–C19–C16
C26–C25–C24
C25–C26–C21
C32–C31–C36
C11–C12–N1–C13
C24–C29–C30–C31
C29–C30–C31–C36
C2–C1–N1–C13
120.4(8)
C32–C33–C34
C35–C34–C33
C39–C40–C41
C42–C41–C40
C47–C46–C45
N2–C48–C47
N2–C48–C43
C18–C13–N1–C1
C38–C37–N2–C34
C33–C34–N2–C37
C29–C30–C31–C32
122.2(7)
121.0(7)
125.3(8)
119.7(9)
121.1(8)
118.5(8)
5.0(10)
107.1(6)
105.0(7)
120.0
120.0
120.0
129.9(6)
110.1(6)
ꢁ51.6(11)
ꢁ6.7(10)
ꢁ51.5(11)
ꢁ152.0(11)
118.7(8)
ꢁ149.6(8)
177.6(7)
36.6(12)
46.0(15)
7.9(18)
31.9(16)
ꢁ8.8(10)
the ground state. oj is the excitation energy of the excited state
|ji. The TPA cross section is given by orientational averaging
over the two-photon absorption probability:
3.3. Two-photon absorption cross section
The transition intensity for one-photon absorption (OPA) is
described by an oscillator strength:
h
i
X
X
dtpa
¼
F ꢀ SaaSbꢂb þ G ꢀ SabSaꢂb þ H ꢀ SabSbꢂa
ð3Þ
2of
3
2
dop
¼
jh0jmajf ij X
ð1Þ
ab
a
where the coefficients F, G and H are related to the incident
radiation. For linearly polarized light, F, G and H are 2, 2 and
2, but for the circularly polarized case, they are ꢁ2, 3 and 3,
respectively. In this work, we only consider the results with a
linearly polarized laser beam. The summation goes over the
molecular axes a, b = {x, y, z}.
where ma is the electric dipole moment operator, of denotes the
excitation energy of the excited state |fi, |0i denotes the ground
state, and the summation is performed over a, the molecular x,
y and z axes.
In terms of the sum-over-state formula, the two-photon
matrix element for the two-photon resonant absorption of
two photons with identical energy is written as:
The TPA cross section directly comparable with the experi-
mental measurements is defined as:
"
#
X
4p2a50a o2gðoÞ
h0jmajjihjjmbj f i h0jmbjjihjjmaj f i
ꢀ
ꢀ
Sab
¼
þ
ð2Þ
stpa
¼
dtpa
ð4Þ
oj ꢁ of
2
oj ꢁ of
2
15c0
Gf
j
where a0 is the Bohr radius, c0 is the speed of light, a is the fine
structure constant, o is the photon energy of the incident light,
g(o) denotes the spectral line profile (here it was assumed to be
a d function). Gf is the lifetime broadening of the final state,
which was assumed to be 0.1 eV.19
Where |0i and |fi denote the ground state and the final state,
respectively. |ji denotes all the intermediate states, including
The GAUSSIAN-98 program package36 was utilized to
optimize the molecular geometrical structure with the hybrid
density functional theory (DFT/B3LYP) and a 6-31G basis set.
The program DALTON37 was used to calculate the one-
photon absorption strength and TPA cross sections of these
molecules with response function theory in the random phase
approximation. The maximum absorption strength for the
one-photon case takes place in the first excited state. The
maximum TPA cross sections for compounds 1, 2 and 3 occur
for the second excited state and are 1.54, 2.36 and 0.73 ꢀ 10ꢁ48
cm4 s photonꢁ1, respectively. For 4, due to the large number of
atoms, we were unable to optimize completely the structure
and hence were unable to obtain the two-photon cross section.
For the first excited state, the TPA cross sections are smaller
Fig. 1 Crystal molecular structure of 1.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 4 7 9 – 4 8 4
482

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Table 2 Data from the absorption and the one- and two-photon fluorescence spectra, showing the solvent effects
(1a) b
(1f) b
max
(2f) b
Compound
Solvent
ea
l
/nm
e
res/104
l
/nm
tc/ns
Fd
l
/nm
max
max
1
DMF
37.6
9.1
401
402
408
403
404
426
427
431
421
423
415
419
429
420
423
421
425
433
426
428
7.35
6.79
6.83
6.84
6.63
5.52
5.26
4.30
4.20
4.70
4.36
6.02
5.51
6.62
5.78
3.52
5.38
4.87
5.54
5.00
458
458
462
458
458
510
496
500
494
492
518
512
520
510
506
520
514
522
512
508
1.36
1.33
1.23
1.31
1.27
1.46
1.33
1.27
1.30
1.21
1.67
1.56
1.42
1.52
1.41
1.65
1.43
1.52
1.38
1.35
0.48
0.44
0.54
0.42
0.41
0.67
0.60
0.71
0.58
0.59
0.66
0.63
0.76
0.60
0.66
0.65
0.61
0.75
0.58
0.63
466
462
468
464
462
516
506
510
503
500
CH2Cl2
Benzyl alcohol
THF
13.1
7.58
4.806
37.6
CHCl3
DMF
2
3
4
a
CH2Cl2
Benzyl alcohol
THF
9.1
13.1
7.58
4.806
37.6
CHCl3
DMF
CH2Cl2
Benzyl alcohol
THF
9.1
13.1
7.58
4.806
37.6
CHCl3
DMF
525
529
CH2Cl2
Benzyl alcohol
THF
9.1
13.1
7.58
4.806
CHCl3
e is the relative permittivity measured at 20 1C.34 eres is the corresponding molar absorption coefficient. l(m1aax) , l(m1fa)x and l(m2af)x are the one-photon
b
c
d
absorption, one-photon fluorescence and two-photon fluorescence maxima peak, respectively. t is the one-photon fluorescence lifetime. F is the
one-photo quantum yield determined using Coumarin 307 as the standard.35
compared to the maximum values but are finite. Therefore, the
fluorescent spectra by TPA for the first excited state can still be
observed.
microscope (Olympus, BX-51), as illustrated in Fig. 3. Fig. 3
shows a 0.08 ꢀ 0.06 mm polymerization region with a two
dimensional structure. The distance between two neighboring
polymerization streaks is about 8 mm.
The photopolymerization mechanism of these initiators is
still unknown. According to Cumpston et al.2 strong donor
substituents would make the conjugated system electron-rich,
and after one- or two-photon photoexcitation, the molecule
would be able to transfer an electron even to relatively weak
acceptors; this process could be used to activate the polymer-
ization reaction. In order to demonstrate this process, we
performed ab initio calculations at the time-dependent hybrid
density functional theory B3LYP level coded in the GAUS-
SIAN-98 program package for 2, showing that the first excited
state is the charge-transfer (CT) state with an excitation energy
l = 541 nm. When the molecule is irradiated by the 820 nm
laser, it can be expected that the molecule will simultaneously
absorb two photons and be excited to the first excited state (the
CT state). For a better understanding of the charge-transfer
process, the charge density difference between the ground and
the CT states in a molecule of 2 in the gas phase is depicted in
Fig. 4.38 It is clear that upon excitation, charge is mainly
3.4. Two-photon photopolymerization
Our two-photon-initiated photopolymerization experiments
have been carried out using 1–4 as initiators, with wavelengths
of 760, 820, 830 and 830 nm, respectively. The 2D lattice was
fabricated with a negative resin system, which contained an
oligomer (bisphenyl A epoxide dimethylacrylate), 5% of the
initiator and a small amount of dichloroethane (to increase the
compatibility and control the viscosity). The same mode-
locked Ti:sapphire laser as that used in the two-photon fluor-
escence measurements was used for two-photon microfabrica-
tion. The lasing source was tightly focused via an objective lens
(ꢀ40, NA = 0.65) and the focal point was focused on the
sample film place on the xy-step motorized stage controlled by
computer. The pulse energy, after being focused by the objec-
tive lens, was B20 mW. The polymerized solid skeleton was
obtained after the unreacted liquid mixture had been washed
out. The fabricated lattice was observed through a polarization
Fig. 2 Absorption and fluorescence spectra of 1. Curves a and b are
the linear absorption and one-photon fluorescence spectra, respec-
tively, with c = 1 ꢀ 10ꢁ5 mol lꢁ1 in DMF. Curve c is the two-photon
fluorescence spectra with c = 1 ꢀ 10ꢁ3 mol lꢁ1 in DMF.
Fig. 3 Optical micrograph of the lattice fabricated via two-photon
polymerization using 2 as an initiator.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 4 7 9 – 4 8 4
483

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14 D. R. Larson, W. R. Zipfel, R. M. Williams, S. W. Clark, M. P.
Bruchez, F. W. Wise and W. W. Webb, Science, 2003, 300, 1434.
15 B. A. Reinhardt, L. L. Brott, S. J. Clarson, A. G. Dillard, J. C.
Bhatt, R. Kannan, L. Yuan, G. S. He and P. N. Prasad, Chem.
Mater., 1998, 10, 1863.
16 G. S. He, J. D. Bhawalkar, C. F. Zhao and P. N. Prasad, Appl.
Phys. Lett., 1995, 67, 2433.
17 J. D. Bhawalkar, G. S. He and P. N. Prasad, Rep. Prog. Phys.,
1996, 59, 1041.
18 J. E. Ehrlich, X. L. Wu, L.-Y. Lee, Z.-Y. Hu, H. Roeckel, S. R.
Marder and J. Perry, Opt. Lett., 1997, 22, 1843.
Fig. 4 Density difference between the charge-transfer and ground
states of 2 in the gas phase. The light and dark areas represent the
electron loss and gain regions, respectively, upon excitation.
19 M. Albota, D. Beljonne, J.-L. Bredas, J. E. Ehrlich, J.-Y. Fu, A.
A. Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder, D.
McCord-Maughon, J. W. Perry, H. Rockel, M. Rumi, G. Sub-
ramaniam, W. W. Webb, X.-L. Wu and C. Xu, Science, 1998, 281,
1653.
20 M. Rumi, J. E. Ehrlich, A. A. Heikal, J. W. Perry, S. Barlow, Z.
Hu, D. M. Maughon, T. C. Parker, H. Rockel, S. Thayumanavan,
S. R. Marder, D. Beljonne and J.-L. Bredas, J. Am. Chem. Soc.,
2000, 122, 9500.
distributed at the two terminals of 2, indicating that it can give
away its electron to its surroundings. This result is in accord
with the conclusion of Cumpston et al.2 However, whether the
photoinduced electron-transfer reaction is energetically feasi-
ble needs further theoretical investigation.
21 Y.-X. Yan, X.-T. Tao, Y.-H. Sun, G.-B. Xu, C.-K. Wang, J.-X.
Yang, X. Zhao and M.-H. Jiang, J. Solid State Chem., 2004, 177,
3007.
4. Conclusions
22 M. Sigalov, A. Ben-Asuly, L. Shapiro, A. Ellern and V. Khodor-
kovsky, Tetrahedron Lett., 2000, 41, 8573.
23 Z. Kotler, J. Segal, M. Sigalov, A. Ben-Asuly and V. Khodor-
kovsky, Synth. Met., 2000, 115, 269.
Four symmetrical two-photon photopolymerization initiators
were synthesized and characterized. The crystal structure of 1
has been determined by X-ray single crystal diffraction analy-
sis, showing an asymmetric character. A similar asymmetry can
24 J. Segal, Z. Kotler, M. Sigalov, A. Ben-Asuly and V. Khodor-
kovsky, Proc. SPIE-Int. Soc. Opt. Eng., 1999, 3796, 153.
25 M. Silagov, A. Ben-Azuly, L. Shapiro and V. Khodorkovsky,
Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. B, 2000, 25, 443.
26 X.-M. Wang, Y.-F. Zhou, W.-T. Yu, C. Wang, Q. Fang, M.-H.
Jiang, H. Lei and H.-Z. Wang, J. Mater. Chem., 2000, 10,
2698.
27 Y. Ren, X.-Q. Yu, D.-J. Zhang, D. Wang, M.-L. Zhang, G.-B.
Xu, X. Zhao, Y.-P. Tian, Z.-S. Shao and M.-H. Jiang, J. Mater.
Chem., 2002, 12, 3431.
1
be observed for 3. For 2 and 4, the H NMR and 13C NMR
spectra indicate that they are centrosymmetric. The photophy-
sical results illustrate that the four compounds are all good
two-photon absorbing chromophores and effective two-photon
photopolymerization initiators. The wavelengths for initiating
two-photon polymerization reactions of 1, 2, 3 and 4 are 760,
820, 830 and 830 nm, respectively. A possible mechanism is
based on a charge-transfer process when the laser irradiation
is applied.
28 B. Wang and M. R. Wasielewski, J. Am. Chem. Soc., 1997, 119,
12.
29 G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, University of Gottingen, Germany, 1997.
30 P. Fromherz, J. Phys. Chem., 1995, 99, 7188.
Acknowledgements
31 U. Narang, C. F. Zhao, J. D. Bhawalkar, F. V. Bright and
P. N. Prasad, J. Phys. Chem., 1996, 100, 4521.
32 C.-K. Wang, P. Macak, Y. Luo and H. Agren, J. Chem. Phys.,
2001, 114, 9813.
33 C. Wang, Y. Ren, Z.-S. Shao, X. Zhao, G.-Y. Zhou, D. Wang,
Q. Fang and M.-H. Jiang, Nonlinear Opt., 2001, 28, 1.
34 CRC Handbook of Chemistry and Physics, ed. D. R. Lide, CRC
Press, Boca Raton, FL, 73rd edn., 1992–1993.
This work was supported by financial support from the
National Natural Science Foundation of China (grant no.
50323006, 50325311, 10274044) and the Swedish International
Development Cooperation Agency (SIDA). The authors also
thank Prof. Xiao-Ming Chen for his help in revising the
manuscript.
35 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991.
36 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, Jr.,
R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D.
Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V.
Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.
Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala,
Q. Cui, K. Morokuma, P. Salvador, J. J. Dannenberg, D. K.
Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman,
J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.
Nanayakkara, M. Challacombe, P. M. W. Gill, B. G. Johnson, W.
Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon,
E. S. Replogle and J. A. Pople, GAUSSIAN 98 (Revision A.11),
Gaussian, Inc., Pittsburgh, PA, 2001.
37 T. Helgaker, H. J. Aa. Jensen, P. Jørgensen, J. Olsen, K. Ruud, H.
Agren, A. A. Auer, K. L. Bak, V. Bakken, O. Christiansen, S.
Coriani, P. Dahle, E. K. Dalskov, T. Enevoldsen, B. Fernandez,
C. Hattig, K. Hald, A. Halkier, H. Heiberg, H. Hettema, D.
Jonsson, S. Kirpekar, R. Kobayashi, H. Koch, K. V. Mikkelsen,
P. Norman, M. J. Packer, T. B. Pedersen, T. A. Ruden, A.
Sanchez, T. Saue, S. P. A. Sauer, B. Schimmelpfennig, K. O.
Sylvester-Hvid, P. R. Taylor and O. Vahtras, DALTON, a mole-
cular electronic structure program, Release 1.0, 1997.
38 P. Flukiger, H. P. Luthi, S. Portmann and J. Weber, MOLEKEL
4.0, Swiss Center for Scientific Computing, Manno, Switzerland,
2000 .
References
1
2
M. Saruo, O. Nakamura and S. Kawata, Opt. Lett., 1997, 22, 132.
B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer, J. E.
Ehrlich, L. L. Erskine, A. A. Keikal, S. M. Kuebler, I.-Y. S. Lee,
D. M. Maughon, J. Qin, H. Rockel, M. Rumi, X. L. Wu, S. R.
Marder and J. W. Perry, Nature (London), 1999, 398, 51.
S. Kawata, H.-B. Sun, T. Tanaka and K. Takada, Nature
(London), 2001, 412, 697.
W. Zhou, S. M. Kuebler, K. L. Braun, T. Yu, J. K. Cammack, C.
K. Ober, J. W. Perry and S. R. Marder, Science, 2002, 296, 1106.
D. A. Parthenopoulos and P. M. Rentzepis, Science, 1989, 245,
843.
3
4
5
6
7
J. H. Strickler and W. W. Webb, Opt. Lett., 1991, 16, 1780.
A. S. Dvornikov and P. M. Rentzepis, Opt. Commun., 1995, 119,
341.
8
9
K. D. Belfield, Y. Liu, R. A. Negres, M. Fan, G. Pan, D. J. Hagan
and F. E. Hernandez, Chem. Mater., 2002, 14, 3663.
J. E. Ehrlich, X. L. Wu, L.-Y. Lee, Z.-Y. Hu, H. Roeckel, S. R.
Marder and J. Perry, Opt. Lett., 1997, 22, 1843.
10 G. S. He, G. C. Xu, P. N. Prasad, B. A. Reinhardt, J. C. Bhatt and
A. G. Dillard, Opt. Lett., 1995, 20, 435.
11 J. D. Bhawalkar, N. D. Kumar, C. F. Zhao and P. N. Prasad,
J. Clin. Laser Med. Surg., 1997, 15, 201.
12 W. Denk, J. H. Strickler and W. W. Webb, Science, 1990, 248, 73.
13 C. Xu, W. R. Zipfel, J. B. Shear, R. M. William and W. W. Webb,
Proc. Natl. Acad. Sci. USA, 1996, 93, 10763.
484
N e w J . C h e m . , 2 0 0 5 , 2 9 , 4 7 9 – 4 8 4