Synthesis, structures, and properties of two new two-photon photopolymerization initiators

Article abstract of DOI:10.1246/bcsj.78.300

Two new two-photon photopolymerization initiators, diphenyl-(4-{2-[4-(2- pyridin-4-ylvinyl)phenyl]vinyl]phenyl)amine 3 and 9-(4-{2-[4-(2-pyridin-4- ylvinyl)phenyl]vinyl)phenyl)-9H-carbazole 4, have been synthesized and their crystal structures have been determined. 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 good two-photon absorbing chromophores and effective two-photon photopolymerization initiators. The calculated two-photon absorption cross sections of molecules 3 and 4 for the lowest excited state are 59.3 × 10^-50 and 43.0 × 10^-50 cm⁴ s photon^-1, respectively. Two-photon initiating polymerization microfabrication experiments have been studied and the possible photopolymerization mechanisms have been discussed.

Full text of DOI:10.1246/bcsj.78.300

300 Bull. Chem. Soc. Jpn., 78, 300–306 (2005)
ꢀ 2005 The Chemical Society of Japan
Synthesis, Structures, and Properties of Two New Two-Photon
Photopolymerization Initiators
ꢀ
Yun-Xing Yan, Xu-Tang Tao, Yuan-Hong Sun,1 Wen-Tao Yu, Gui-Bao Xu, Chuan-Kui Wang,1
Hua-Ping Zhao, Jia-Xiang Yang, Xiao-Qiang Yu, Xian Zhao, and Min-Hua Jiang
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China
1Department of Physics, Shandong Normal University, Jinan 250014, P. R. China
Received June 29, 2004; E-mail: yxyan@icm.sdu.edu.cn
Two new two-photon photopolymerization initiators, diphenyl-(4-{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}phen-
yl)amine 3 and 9-(4-{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}phenyl)-9H-carbazole 4, have been synthesized and their
crystal structures have been determined. 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 good two-
photon absorbing chromophores and effective two-photon photopolymerization initiators. The calculated two-photon
absorption cross sections of molecules 3 and 4 for the lowest excited state are 59:3 ꢁ 10ꢂ50 and 43:0 ꢁ 10ꢂ50
cm4 s photonꢂ1, respectively. Two-photon initiating polymerization microfabrication experiments have been studied
and the possible photopolymerization mechanisms have been discussed.
Molecular excitation via the simultaneous absorption of two
photons can lead to improved three-dimensional (3D) control
of photochemical or photophysical processes due to the quad-
ratic dependence of the absorption probability on the incident
radiation intensity, thus furnishing development of improved
3D microfabrication,1–6 ultra-high-density optical data stor-
age,7 biological imaging,8 and the controlled release of biolog-
ically relevant species.9 Although 3D microfabrication has
been illustrated using the two-photon-initiated polymerization
of resins incorporating conventional ultraviolet-absorbing ini-
tiators, the two-photon absorption (TPA) cross sections (ꢀ)
of these initiators are typically very small (ꢀ ꢃ 10 ꢁ 10ꢂ50
cm4 s per photon), and as a result they exhibit low two-photon
sensitivity. Resins containing these initiators can be patterned
only by means of long exposure time and high excitation inten-
sities that frequently result in damage to the structure. Al-
though some approaches to improve the photosensitivity of
photointiator molecules have been reported, such as improving
the two-photon cross section (ꢀ) and the photochemical quan-
tum yield (ꢀ) of the molecule,10–13 effective two-photon pho-
topolymerization initiators are still rare because the photopoly-
merization mechanism and the relationship of the structure/
property for the initiator, which is important for searching
for effective and high photosensitive two-photon photopoly-
merization initiators, remains unclear so far.
fective two-photon photopolymerization initiators. Their cross
sections were calculated to be as high as the so-called AF-50.15
The crystal structures of 3 and 4 were determined to be in the
ꢁ
P21 and P1 space groups, respectively.
Experimental
1
Instruments. The 600 MHz H NMR spectra and 13C NMR
were obtained on a Bruker av600 spectrometer. Elemental analy-
ses were performed using a PE 2400 elemental analyzer. UV–vis–
near-IR spectra were measured on a Hitachi U-3500 recording
spectrophotometer. Melting points were measured on DSC822e
Mettler-Toledo instruments. The steady-state fluorescence spectra
measurements were performed using an Edinburgh FLS 920 spec-
trofluorimeter. A 450 W Xe arc lamp provided a ꢄ400 nm exci-
tation source. Spectra were record between 420 and 700 nm using
a photomultiplier tube as a detector, which was operated in the
single-photon counting mode. The spectral resolution was 0.1
nm. The quartz cuvettes used were of 1 cm path length.
Synthesis. The synthetic approaches of 3 and 4 are shown in
Scheme 1. 4-bromobenzyl(triphenyl)phosphonium bromide and 4-
carbazol-9-ylbenzaldehyde were synthesized according to report-
ed methods.14,16
{4-[2-(4-Bromophenyl)vinyl]phenyl}diphenylamine 1: At
room temperature, 4-(N,N-diphenylamino)benzaldehyde (2.73 g,
0.01 mol), 4-bromobenzyl(triphenyl)phosphonium bromide (7.68
g, 0.015 mol), NaOH (8.00 g, 0.2 mol) and THF (2 mL) were add-
ed into a mortar. The mixture was then ground, and became or-
ange after five minutes. The mixture was poured into 200 mL of
distilled water, neutralized with dilute hydrochloric acid, and ex-
tracted with dichloromethane. The obtained organic layer was
dried over anhydrous magnesium sulfate. The solvent was re-
moved under reduced pressure and the crude product was purified
by column chromatography on silica gel using petroleum ether as
the eluent. After removal of the solvent, a pure green product was
obtained in 75% yield after recrystallization from ethyl acetate.
In our previous work, we had reported an acceptor–ꢁ-donor
(D–ꢁ-A) type of molecule DBASVP that was found to have a
large TPA cross section, and to effectively initiate polymeriza-
tion through two-photon absorption.14 In the present work, we
describe the design and synthesis of two new two-photopoly-
merization initiators, 3 and 4, in which the N,N-dibutylamino
group in DABSVP was replaced by N,N-diphenylamino and
N-carbazoyl groups, respectively. Our results demonstrate that
they were good two-photon absorbing chromophores and ef-
Published on the web February 11, 2005; DOI 10.1246/bcsj.78.300

Y.-X. Yan et al.
Bull. Chem. Soc. Jpn., 78, No. 2 (2005)
301
N
CHO
CHO
N
N
Br
N
a
1
b
3
N
Br
N
N
2
N
4
Scheme 1. (a) THF/4-bromobenzyl(triphenyl)phosphonium bromide, room temperature. b) Vinylpyridine/tri-o-tolylphosphine/
palladium (II) acetate/triethylamine, reflux.
Anal. Calcd for C26H20NBr: C, 73.24; H, 4.69; N, 3.29%. Found:
1
C, 73.31; H, 4.75; N, 3.34%. H NMR (CDCl3, 600 MHz) ꢀ 7.46
(d, 2H, J ¼ 8:3 Hz), 7.36 (m, 4H), 7.27 (t, 4H, J ¼ 7:7 Hz), 7.12
(d, 4H, J ¼ 7:9 Hz), 7.04 (m, 5H), 6.92 (d, 1H, J ¼ 16:2 Hz).
13C NMR (600 MHz, CDCl3) ꢀ 147.78, 147.61, 136.77, 131.78,
131.18, 129.33, 129.09, 127.79, 127.47, 125.81, 124.68, 123.48,
123.21, 120.93.
Diphenyl-(4-{2-[4-(2-pyridin-4-ylvinyl)phenyl]vinyl}phenyl)-
amine 3: First, {4-[2-(4-bromophenyl)vinyl]phenyl}diphenyl-
amine (2.13 g, 5 mmol), tri-o-tolylphosphine (0.65 g, 2.15 mmol),
Fig. 1. Crystal structure of 3.
vinylpyridine (1.16 mL, 10.75 mmol), palladium(II) acetate (0.06
g, 0.27 mmol), and redistilled triethylamine (100 mL) under nitro-
gen were added to a three-necked flask equipped with a magnetic
stirrer, a reflux condenser, and a nitrogen input tube. The reaction
mixture was refluxed in an oil bath under nitrogen. An orange
product was obtained after heating and stirring for 24 h. The sol-
vent was then removed under reduced pressure and the residue
was dissolved in methylene chloride, washed three times with dis-
tilled water, and dried with anhydrous magnesium sulfate. It was
then filtered and concentrated. The orange compound was purified
by column chromatography on silica gel using ethyl acetate/petro-
leum ether (1:1) as an eluent, and crystallized using ethyl acetate,
giving a yield of 60% and mp of 229.0 ꢅC. Anal. Calcd for
C33H26N2: C, 88.00; H, 5.78; N, 6.22%. Found: C, 88.15; H,
Fig. 2. Crystal structure of 4.
(m, 4H), 7.09 (s, 1H), 7.06 (s, 1H). 13C NMR (CDCl3, 600
MHz) ꢀ 149.89, 144.92, 140.78, 137.71, 137.13, 136.29, 135.65,
132.95, 128.84, 128.34, 127.89, 127.52, 127.25, 127.08, 125.97,
125.86, 123.48, 120.88, 120.33, 120.04, 109.80.
1
5.81; N, 6.26%. H NMR (CDCl3, 600 MHz) ꢀ 8.59 (d, 2H, J ¼
4:0 Hz), 7.54 (s, 4H), 7.41 (d, 4H, J ¼ 8:9 Hz), 7.30 (m, 4H),
7.14 (d, 4H, J ¼ 7:7 Hz), 7.11 (s, 1H), 7.07 (t, 4H, J ¼ 7:2 Hz),
7.03 (s, 2H), 7.00 (s, 1H). 13C NMR (CDCl3, 600 MHz) ꢀ
149.55, 147.65, 147.46, 145.25, 138.41, 134.88, 133.33, 131.13,
129.31, 128.98, 127.48, 127.46, 126.72, 126.18, 125.27, 124.62,
123.36, 123.17, 120.90.
Results and Discussion
Crystal Structure. The X-ray diffraction data of single
crystals for compounds 3 and 4 were collected on a Bruker
P4 four-cycle diffractometer with the Mo Kꢂ radiation. Both
structures were solved by direct methods, and refined by
full-matrix least-squares on F2 using SHELXL-97 programs.17
The crystal structures of 3 and 4 are shown in Figs. 1 and 2,
respectively. Selected bond distances and bond angles for
molecules 3 and 4 are listed in Tables 1 and 2, respectively.#
Compound 3 belongs to monoclinic crystal system, P21
space group, M ¼ 450:56, a ¼ 9:5808ð11Þ, b ¼ 8:7128ð13Þ,
9-{4-[2-(4-Bromophenyl)vinyl]phenyl}-9H-carbazole 2: It
was obtained by a similar method for 1. The obtained compound
was green crystals in a yield of 78%. Anal. Calcd for C26H18NBr:
C, 73.59; H, 4.28; N, 3.30%. Found: C, 73.71; H, 4.33; N, 3.35%.
1H NMR (CDCl3, 600 MHz) ꢀ 8.14 (d, 2H, J ¼ 7:8 Hz), 7.72 (d,
2H, J ¼ 7:8 Hz), 7.57 (d, 2H, J ¼ 7:6 Hz), 7.51 (d, 2H, J ¼ 7:6
Hz), 7.42 (q, 6H, J ¼ 7:1 Hz), 7.29 (t, 2H, J ¼ 7:0 Hz), 7.16
(q, 2H, J ¼ 13:9 Hz).
ꢂ
ꢂ 3
c ¼ 29:489ð6Þ A, ꢃ ¼ 93:854ð11Þ, V ¼ 2456:1ð7Þ A , Z ¼ 4,
T ¼ 293ð2Þ K, Dc ¼ 1:218 g cmꢂ3, R1 ¼ 0:0628, wR2 ¼
0:1122 for I > 2ꢄðIÞ. Figure 1 shows that there are two mole-
cules in an asymmetric unit. In one molecule, the dihedral an-
gle between C41–C42–C43–C44–C45–C46 and pyridine ring
is 11.1ꢅ. And the dihedral angle between the former and its ad-
jacent phenyl ring is 9.1ꢅ. The dihedral angles between each
terminal phenyl ring that links to the N4 atom and C49–
C50–C51–C52–C53–C54 are 58.0 and 74.0ꢅ, respectively,
while that between two terminal phenyl rings is 70.6ꢅ. In the
9-(4-{2-[4-(2-Pyridin-4-ylvinyl)phenyl]vinyl}phenyl)-9H-car-
bazole 4: It was obtained by a similar method for 3. The obtained
green crystals were purified by column chromatography on silica
gel using ethyl acetate/petroleum ether (1:1) as an eluent, and
crystallized using ethyl acetate in a yield of 58% and mp of
268.4 ꢅC. Anal. Calcd for C33H24N2: C, 88.39; H, 5.36; N,
6.25%. Found: C, 88.22; H, 5.29; N, 6.17%. 1H NMR (CDCl3,
600 MHz) ꢀ 8.61 (d, 2H, J ¼ 4:3 Hz), 8.17 (d, 2H, J ¼ 7:7
Hz), 7.77 (d, 2H, J ¼ 7:9 Hz), 7.60 (m, 6H), 7.45 (m, 6H), 7.30

302 Bull. Chem. Soc. Jpn., 78, No. 2 (2005)
New Two-Photon Photopolymerization Initiators
ꢅ
ꢂ
Table 1. Selected Bond Lengths (A) and Bond Angles ( ) for 3
C1–N1
C1–C2
C2–C3
C3–C4
C3–C6
C4–C5
C5–N1
C6–C7
C7–C8
C8–C13
C8–C9
C11–C14
C14–C15
1.330(12)
1.349(11)
1.355(13)
1.328(13)
1.476(14)
1.398(13)
1.331(13)
1.336(13)
1.473(13)
1.351(14)
1.385(14)
1.442(11)
1.313(11)
C15–C16
C16–C17
C17–C18
C19–N2
1.484(11)
1.385(11)
1.400(10)
1.404(10)
1.435(10)
1.363(12)
1.355(13)
1.381(11)
1.409(11)
1.465(10)
1.345(10)
1.456(10)
1.366(11)
C41–C46
C44–C47
C47–C48
C48–C49
C49–C50
C52–N4
C51–C52
C55–C56
C55–C60
C57–C58
C60–N4
1.413(11)
1.478(10)
1.325(10)
1.478(10)
1.405(10)
1.429(9)
1.379(11)
1.357(11)
1.387(12)
1.380(13)
1.410(9)
C22–N2
C24–C25
C25–C26
C34–N3
C37–C38
C36–C39
C39–C40
C40–C41
C41–C42
C61–C66
C61–C62
1.375(12)
1.367(11)
N1–C1–C2
C1–C2–C3
C4–C3–C2
C4–C3–C6
C2–C3–C6
C3–C4–C5
C6–C7–C8
C9–C8–C7
C13–C12–C11
C15–C14–C11
C14–C15–C16
C16–C21–C20
C25–C24–C23
C23–C22–N2
C16–C21–C20
C26–C25–C24
C25–C26–C27
C2–C3–C6–C7
C3–C6–C7–C8
124.5(12)
121.3(11)
115.4(10)
134.4(13)
110.2(11)
121.9(12)
122.8(11)
130.9(13)
121.7(11)
126.7(11)
125.8(11)
122.2(10)
118.7(10)
121.0(9)
C26–C27–C22
C33–C28–N2
C29–C28–N2
C31–C30–C29
C30–C31–C32
C28–C33–C32
C35–C34–N3
120.1(11)
119.7(10)
121.9(10)
119.5(12)
121.4(11)
121.5(10)
122.3(10)
115.4(9)
123.6(10)
125.5(10)
127.8(9)
120.5(10)
121.0(10)
120.0(8)
C52–C53–C54
C54–C49–C50
C51–C52–N4
C49–C54–C53
C56–C57–C58
C56–C55–C60
C57–C58–C59
C60–C59–C58
C59–C60–N4
C62–C61–C66
C61–C62–C63
C64–C65–C66
C61–C66–C65
C19–N2–C22
C38–N3–C34
C60–N4–C61
C52–N4–C61
C36–C39–C40–C41
C44–C47–C48–C49
120.3(9)
117.3(8)
121.5(9)
121.8(9)
119.1(10)
120.8(10)
120.7(11)
118.8(10)
119.3(9)
120.4(10)
119.2(12)
120.2(13)
118.9(11)
124.3(8)
115.6(9)
120.5(7)
118.8(7)
ꢂ176:6ð9Þ
ꢂ178:5ð9Þ
C37–C36–C35
N3–C38–C37
C40–C39–C36
C39–C40–C41
C42–C41–C40
C44–C43–C42
C45–C46–C41
C48–C47–C44
C47–C48–C49
C50–C51–C52
C10–C11–C14–C15
C11–C14–C15–C16
122.2(10)
121.8(10)
119.4(10)
173.5(11)
ꢂ173:4ð10Þ
127.6(9)
125.8(9)
121.5(9)
169.2(11)
178.7(10)
other molecule, the dihedral angles between C8–C9–C10–
C11–C12–C13 and the pyridine ring and adjacent phenyl ring
are 10.6 and 22.0ꢅ, respectively. Therefore, molecule 3 is al-
most co-planar. The dihedral angles between each terminal
phenyl ring that links to the N2 atom and C16–C17–C18–
C19–C20–C21 are 58.4 and 65.1ꢅ, respectively, while that be-
tween two terminal phenyl rings is 73.2ꢅ. The bond lengths of
C3–C6, C7–C8, C11–C14, C15–C16, C36–C39, C40–C41,
C44–C47, and C48–C49 are shorter than the typical single-
bond length of C–C. The bond lengths of C6–C7, C14–C15,
C39–C40, and C47–C48 are longer than the typical double-
bond length of C=C. These structural features suggest that
molecule 3 is a highly delocalized ꢁ-electron system.
of C6–C7 and C14–C15 are longer than the typical double
bond length of C=C. The conjugated geometric configuration
reveals that molecule 4 is also a highly delocalized ꢁ-electron
system.
Linear Absorption and Fluorescence Spectra. The pho-
tophysical properties of 3 and 4 are summarized in Table 3.
The one-photon fluorescence spectra and lifetimes of these
two compounds were measured in different solvents with a
concentration of c ¼ 1 ꢁ 10ꢂ5 mol/L. The excited wave-
lengths of 3 and 4 were 400 and 366 nm, respectively. The
two-photon induced emission spectrum could be observed with
a suitable laser beam from a mode-locked Ti:sapphire laser
(Coherent Mira 900F) as the pump source with a pulse dura-
tion of 200 fs, a repetition rate of 76 MHz, and a single-scan
streak camera (Hamamatsu Model C5680-01) together with a
monochromator as the recorder. The TPA induced emission
spectra of two compounds were measured in different solvents
with a concentration of c ¼ 1 ꢁ 10ꢂ2 mol/L. The excited
wavelengths of compounds 3 and 4 were 800 and 760 nm,
respectively. Figures 3, 4, and 5 show the linear absorption,
one- and two-photon fluorescence spectra of compounds 3
and 4 in CHCl3. As shown in Table 3, the maximum absorp-
tion show a slight blue-shift, while the maximum fluorescence
clearly show red-shifts, and lengthened fluorescent lifetimes,
except for benzyl alcohol, along with increase in the polarity
ꢁ
Compound 4 belongs to a triclinic crystal system, P1 space
group, M ¼ 448:54, a ¼ 9:2893ð10Þ, b ¼ 9:6014ð17Þ, c ¼
ꢂ
13:3645ð15Þ A, ꢂ ¼ 92:365ð10Þ, ꢃ ¼ 97:015ð6Þ, ꢅ ¼
ꢅ
ꢂ 3
96:292ð10Þ , V ¼ 1174:1ð3Þ A , Z ¼ 2, T ¼ 293 K, Dc ¼
1:269 g cmꢂ3, R1 ¼ 0:0444, wR2 ¼ 0:1010 for I > 2ꢄðIÞ.
Figure 2 shows that the angle between the pyridine ring and
the adjacent phenyl ring is only 5.9ꢅ. The angle between the
two central phenyl rings is 49.2ꢅ. The dihedral angle between
the carbazole ring and the adjacent phenyl ring is 117.4ꢅ.
Therefore, molecule 4 is not co-planar. The bridge bond
lengths of C3–C6, C7–C8, C11–C14, and C15–C16 are shorter
than the typical single bond length of C–C. The bond lengths

Y.-X. Yan et al.
Table 2. Selected Bond Lengths (A) and Bond Angles ( ) for 4
Bull. Chem. Soc. Jpn., 78, No. 2 (2005)
303
ꢅ
ꢂ
C1–N1
C1–C2
C2–C3
C3–C4
C3–C6
C4–C5
C5–N1
C6–C7
C7–C8
C8–C13
C8–C9
C9–C10
C10–C11
1.333(2)
1.375(2)
1.388(2)
1.389(3)
1.462(2)
1.378(2)
1.322(2)
1.321(3)
1.469(2)
1.389(2)
1.391(2)
1.374(2)
1.391(2)
C11–C12
C11–C14
C12–C13
C14–C15
C15–C16
C16–C17
C16–C21
C17–C18
C18–C19
C19–C20
C19–N2
1.391(2)
1.467(2)
1.371(2)
1.319(2)
1.467(2)
1.388(2)
1.392(2)
1.381(2)
1.378(2)
1.377(2)
1.4326(19)
1.372(2)
1.391(2)
C22–C27
C23–C24
C24–C25
C25–C26
C26–C27
C27–C28
C28–C29
C28–C33
C29–C30
C30–C31
C31–C32
C32–C33
C33–N2
1.411(2)
1.373(3)
1.392(3)
1.372(3)
1.392(2)
1.441(2)
1.400(2)
1.409(2)
1.374(3)
1.392(3)
1.376(2)
1.383(2)
1.392(2)
C20–C21
C22–N2
N1–C1–C2
C1–C2–C3
C2–C3–C4
C2–C3–C6
C4–C3–C6
C5–C4–C3
N1–C5–C4
C7–C6–C3
C6–C7–C8
124.50(19)
119.81(18)
115.80(16)
124.40(18)
119.78(17)
119.83(19)
124.6(2)
126.94(18)
126.12(18)
117.19(16)
122.30(17)
120.51(16)
121.97(17)
120.52(17)
117.51(15)
118.92(16)
123.56(16)
121.67(17)
169.2(2)
C15–C14–C11
C14–C15–C16
C17–C16–C21
C17–C16–C15
C21–C16–C15
C18–C17–C16
C19–C18–C17
C20–C19–C18
C20–C19–N2
127.40(17)
125.26(17)
117.37(15)
120.60(15)
121.99(16)
121.66(16)
119.85(16)
119.21(15)
120.11(15)
120.68(15)
120.91(16)
120.99(17)
129.29(16)
108.83(15)
121.84(17)
117.41(18)
121.8(2)
C25–C26–C27
C26–C27–C22
C26–C27–C28
C22–C27–C28
C29–C28–C33
C29–C28–C27
C33–C28–C27
C30–C29–C28
C29–C30–C31
C32–C31–C30
C31–C32–C33
C32–C33–N2
C32–C33–C28
N2–C33–C28
C5–N1–C1
119.58(18)
118.72(17)
134.48(16)
106.80(14)
118.35(17)
134.68(16)
106.95(15)
119.71(17)
120.55(18)
121.42(19)
117.95(17)
129.15(15)
122.01(16)
108.79(15)
115.36(17)
108.61(13)
125.83(15)
125.55(15)
179.38(18)
178.36(16)
C13–C8–C9
C13–C8–C7
C9–C8–C7
C18–C19–N2
C21–C20–C19
C20–C21–C16
N2–C22–C23
C10–C9–C8
C9–C10–C11
C12–C11–C10
C12–C11–C14
C10–C11–C14
C13–C12–C11
C4–C3–C6–C7
C3–C6–C7–C8
N2–C22–C27
C23–C22–C27
C24–C23–C22
C23–C24–C25
C26–C25–C24
C12–C11–C14–C15
C11–C14–C15–C16
C22–N2–C33
C22–N2–C19
C33–N2–C19
N2–C22–C23–C24
C31–C32–C33–N2
120.61(19)
ꢂ152:1ð2Þ
176.07(17)
ꢂ177:93ð17Þ
Table 3. The Data of Absorption, One- and Two-Photon Fluorescence Spectra with Solvent Effects of 3 and 4
Compds
Solvents
"
ꢆðm1aaxÞ/nm
"res=104
ꢆðm1afxÞ /nm
ꢇ/ns
ꢈ
ꢆðm2aÞx/nm
3
DMF
37.6
9.1
13.1
7.58
4.806
37.6
9.1
13.1
397
399
409
398
401
364
366
375
365
367
5.76
4.80
4.28
5.39
4.66
6.63
6.33
5.92
6.54
6.39
570
540
558
536
520
485
470
482
468
457
2.44
2.19
2.11
1.93
1.74
1.81
1.47
1.60
1.37
1.22
0.84
0.73
0.97
0.66
0.68
0.80
0.66
0.89
0.56
0.58
559
542
574
538
523
487
482
493
478
473
CH2Cl2
Benzyl alcohol
THF
CHCl3
DMF
CH2Cl2
Benzyl alcohol
THF
4
7.58
4.806
CHCl3
ð2Þ
ꢆðm1aaxÞ, ꢆðm1afxÞ , and ꢆ
are one-photon absorption, one-photon fluorescence, and two-photon fluorescence maxima
max
peak, respectively. ꢈ is one–photo quantum yield determined using coumarin 307 ꢅas the standard.21 ꢇ is one-pho-
ton fluorescence lifetime. " is the relative permittivity that was measured at 20 C.22 "res is the corresponding
molar absorption coefficient.
of the solvent for each compound. The fact can be explained
because the excited states of these compounds may possess
higher polarity than that of the ground states, since the solva-
tochromism is associated with energy-level lowering. An in-
creased dipole-dipole interaction between the solute and the
solvent would lead to a great lowering of the energy-level.18,19
For the case of benzyl alcohol, we can easily realize the pos-
sibility of hydrogen bonding formation between the solvent
and solute molecules, which would push the excitation energy
lower.15 It is should noted that both the largest one-photon os-
cillator strength and the maximum two-photon absorption
cross section take place in the same excited state, the charge

304 Bull. Chem. Soc. Jpn., 78, No. 2 (2005)
New Two-Photon Photopolymerization Initiators
strength of diphenylamino group is greater than that of carba-
zolyl group, which increases the charge transfer character.
In addition to the similarities between one-photon fluores-
cence and two-photon fluorescence, the two-photon fluores-
cence peak positions are also independent of the laser wave-
length used. Thus, although the electrons can be pumped to
the different excited states by linear absorption, or two-photon
absorption, due to the different selection rules, they would fi-
nally relax to the same lowest excited state via internal conver-
sion and/or vibrational relaxation.
0.7
0.6
4
3
0.5
0.4
0.3
0.2
0.1
0.0
Two-Photon Absorption Cross-Sections. The transition
intensity for one-photon absorption (OPA) is described by
the oscillator strength,
300
400
500
600
700
800
900
1000
wavelength/nm
X
2!f
2
ꢀop
¼
jh0jꢉꢂjfij ;
ð1Þ
Fig. 3. Linear absorption spectra of 3 and 4 in CHCl3 with
a concentration of c ¼ 1 ꢁ 10ꢂ5 mol/L.
3
ꢂ
where ꢉꢂ is the electric dipole moment operator, !f denotes
the excitation energy of the excited state jfi, h0j denotes the
ground state, and the summation is performed over the molec-
ular x, y, and z axes.
In terms of sum-over-state formula, the two-photon matrix
element for the two-photon resonant absorption of identical
energy is written as
120000
100000
4
3
80000
60000
40000
20000
0
ꢀ
ꢁ
X
h0jꢉꢂjjihjjꢉꢃjfi h0jꢉꢃjjihjjꢉꢂjfi
S
¼
þ
; ð2Þ
ꢂꢃ
!j ꢂ !f =2
!j ꢂ !f =2
j
where h0j and jfi denote the ground state and the final state,
respectively; jji means all of the intermediate states, including
the ground state; !j is the excitation energy of the excited state
jji. The TPA cross section is given by orientational averaging
over the two-photon absorption probability,
400
450
500
550
600
650
700
wavelength/nm
Fig. 4. One-photon fluorescence spectra of 3 and 4 in
CHCl3 with a concentration of c ¼ 1 ꢁ 10ꢂ5 mol/L.
h
i
X
ꢀ
ꢀ
ꢀ
S
ꢂꢃ
ꢃꢂ
ꢀtpa
¼
F ꢁ S
S
þ G ꢁ S
S
þ H ꢁ S
; ð3Þ
ꢂꢂ
ꢂꢃ
ꢃꢃ
ꢂꢃ
ꢂꢃ
140
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 circular case, they are ꢂ2, 3, and 3, respec-
tively. In the present work, we only considered the results with
a linearly polarized laser beam. The summation goes over the
molecular axes ꢂ; ꢃ ¼ fx; y; zg.
120
4
3
100
80
60
40
20
0
The TPA cross section, which is directly comparable with
experimental measurements, is defined as
4p a50ꢂ !2gð!Þ
2
ꢄtpa
¼
ꢀtpa
;
ð4Þ
15c0
ꢃf
425 450 475 500 525 550 575 600 625 650
wavelength(nm)
where a0 is the Bohr radius, c0 the speed of light, ꢂ the fine
structure constant, h! the photon energy of the incident light;
ꢁ
gð!Þ denotes the spectral line profile, and here it is assumed to
be a ꢀ function. ꢊf is the lifetime broadening of the final state,
which is assumed to be 0.1 eV.11
Fig. 5. Two-photon fluorescence spectra of 3 and 4 in
CHCl3 with a concentration of c ¼ 1 ꢁ 10ꢂ2 mol/L.
transfer state, in the lower energy region, because of the asym-
metrical characteristics of the present molecules. Therefore,
we studied the two-photon fluorescence spectra, as described
in the following section.20 Comparing compound 3 with 4,
we can see that replacing of the carbazolyl by the diphenyl-
amino group resulted in an enhancement of the one-photon
quantum yield and a red-shift of the absporption and emission
band. This may provide evidence that the electron-donating
We utilized the GAUSSIAN-98 program package23 to opti-
mize the molecular geometrical structure with hybrid density
functional theory (DFT/B3LYP) and a basis set 6-31G. The
excited energies for two molecules are then calculated by
time-dependent density functional theory. The first excited en-
ergies for molecules 3 and 4 are 466 and 428 nm, respectively.
The program DALTON24 was used to study the two-photon
absorption cross sections of the two molecules. For the first ex-

Y.-X. Yan et al.
Bull. Chem. Soc. Jpn., 78, No. 2 (2005)
305
cited state, the TPA cross sections (ꢄ, 10ꢂ50 cm4 s photonꢂ1
)
for molecules 3 and 4, irradiated with the linearly polarized la-
ser, were 59.3 and 43.0, respectively, which indicate that both
of the two molecules have a relatively large two-photon ab-
sorption cross section under the experimental circumstance.
Therefore, these two molecules can be used as two-photon
polymerization initiators.
Two-Photon Photopolymerization. In our two-photon in-
itiation polymerization experiments, two-dimensional cross-
linked periodic microstructures were fabricated with a nega-
tive resins system, which contained oligomer (bisphenyl A ep-
oxide dimethylacrylate) and 0.5% of compound 3 or 4 as an
initiator and a little 1,2-dichloroethane (increasing the compat-
ibility and controlling the viscosity). The excited wavelengths
for initiating two-photon polymerization reactions of 3 and 4
were 800 and 760 nm, respectively. The same mode-locked
Ti:sapphire laser as that used in the two-photon fluorescence
measurements was used for two-photon microfabircation.
The lasing source was tightly focused via an objective lens
(ꢁ40, NA ¼ 0:65), and the focal point was focused on a sam-
ple film on an xy-step monitorized stage controlled by a com-
puter. The pulse energy after being focused by the objective
lens was ꢄ20 mW. The polymerized solid skeleton was ob-
tained after the unreacted liquid mixture had been washed
out. The fabricated lattice was observed through a polarization
microscope (Olympus, BX-51). Its photograph is illustrated in
Fig. 6.
The photopolymerization mechanisms of these two initia-
tors are 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, these chro-
mophores would be able to transfer an electron even to rela-
tively weak acceptors. This process could be used to activate
the polymerization reaction. In order to demonstrate this proc-
ess, we tried to conduct a theoretical investigation. Our ab ini-
tio calculation using time-dependent hybrid density functional
theory B3LYP level coded in a GAUSSIAN package for the
molecule 3 showed that the first excited state was the charge
transfer (CT) state with an excited energy ꢆ ¼ 466 nm
(ꢄE ¼ 2:6589 eV, f ¼ 1:2563). When the molecule was irra-
diated by a 800 nm laser beam, it could be expected that the
molecule would simultaneously absorb two photons, and be
excited to the first excited state (the CT state). We plotted
Fig. 7. Density difference between the charge-transfer and
ground states of molecule 3 in gas phase. Areas with
chickenwire and dots represent the electron lose and gain,
respectively, upon the excitation.
the charge-density difference between the ground and the CT
states for 3 in the gas phase (see Fig. 7), which was visualized
using the MOLEKEL program.25 Figure 7 shows that upon ex-
citation, charges were mainly transferred from the acceptor
side to the donor side of the molecule. In the CT state, there
were more electrons at the donor side, indicating that the mole-
cule could give away its electron to its surroundings. This pic-
ture seems to support Cumpston’s conclusion.2 However,
whether the photoinduced electron-transfer reaction can be
energetically feasible needs to be further investigated theoret-
ically.
Conclusion
Two new two-photon photopolymerization initiators of 3
and 4 were synthesized and characterized. The one-photon flu-
orescence quantum yields, lifetimes, and solvent effects of the
initiators were studied in detail, and both compounds exhibit a
large delocalized ꢁ-electron conjugated system. The experi-
mental results suggest that they are good two-photon absorbing
chromophores and effective two-photon photopolymerization
initiators. The two-photon absorption cross sections of them
were calculated, and two-photon initiation polymerization ex-
periments were carried out. The possible photopolymerization
mechanism was considered for the case when laser irradiation
is applied.
This work was supported by grants from the financial sup-
port of the state National Natural Science Foundation of China
(grant no. 50323006, 50325311, 10274044) and Swedish Inter-
national Development Cooperation Agency (Sida).
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