Two novel two-photon polymerization initiators with extensive application prospects

Article abstract of DOI:10.1016/j.cplett.2004.02.099

Two novel two-photon polymerization initiators, 10-ethyl-3-E-(4-(N,N ^′-di-n-butylamino)styryl)phenothiazine 5 and 10-ethyl-3,7-E,E-bis(4-(N,N^′-di-n-butyl amino)styryl) phenothiazine 6, have been efficiently synthesized with room temp

Full text of DOI:10.1016/j.cplett.2004.02.099

Chemical Physics Letters 388 (2004) 325–329
www.elsevier.com/locate/cplett
Two novel two-photon polymerization initiators
with extensive application prospects
a,b,c,
a
, Mingliang Zhang , Xiaoqiang Yu , Guibao Xu , Yan Ren ,
b
b
b
*
Yupeng Tian
b
a
a
b
Jiaxiang Yang , Jieying Wu , Xuanjun Zhang , Xutang Tao ,
a
Shengyi Zhang , Minhua Jiang
b
a
Department of Chemistry, Anhui University, Longhe route, Hefei 230039, PR China
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
b
c
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China
Received 13 December 2003; in final form 22 February 2004
Published online:
Abstract
0
Two novel two-photon polymerization initiators, 10-ethyl-3-E-(4-(N,N -di-n-butylamino)styryl)phenothiazine 5 and 10-ethyl-3,7-
0
E,E-bis(4-(N,N -di-n-butyl amino)styryl)phenothiazine 6, have been efficiently synthesized with room temperature solid phase Wittig
reaction. Two initiators were found to exhibit good single-photon fluorescence emission and their quantum yields, lifetimes and
solvent effects have been studied in detail. Two-photon fluorescence spectra were investigated under 800 nm fs laser pulse and two-
photon absorption (TPA) cross-sections of the initiators have been evaluated by theoretical calculation.Two-photon initiating
polymerization microfabrication experiments have been carried out and artificial defects were made and polymerization mechanism
was also discussed.
Ó 2004 Elsevier B.V. All rights reserved.
1
. Introduction
have attracted considerable attention both in funda-
mental research and practical applications, such as light
waveguides with low-losses [3], thresholdless lasers [4],
high quality reflective lens at all directions [5], photonic
switching, calculating and storing [6].
In the last century, the rapid development of semi-
conductor materials and devices made human obtain
gigantic benefits from controlling and manipulating
electrons. Compared with electrons, photons exhibit
some special advantages. So many pioneers have pro-
posed whether photons could be controlled like elec-
trons? Until 1987, the concept of photonic band gap
To our knowledge, numerous fabrication techniques,
such as lithography [7], holography [8], self-assembly
[9], lay-by-lay chemical vapor deposition [10], colloidal
crystal growth [11], modulated photoeletrochemical
etching [12] and two-photon microfabrication (TPMF)
[13], have been utilized to make inorganic and organic
PCs. In all these methods, TPMF specially attracted
researchers eyes because of perfect spatial resolution
and facile fabricating procedures. That two-photon
absorption (TPA) probability depends quadratically on
the excitation intensity makes photon-induced chemical
reaction only happen in the focus with a volume of the
(
BPG) was provided by John and Yablonovitch [1,2],
which made people believe photons could be manipu-
lated with some special materials, often referred to as
photonic crystal (PC). That is a non-light-absorbing
material with refractive index modulation at periodici-
ties comparable to the optical wavelength, which influ-
ence the behavior of photons in an analogous manner to
the influence of the crystal lattice on the behavior of
electrons in semiconductors. In the past decades, PCs
3
order of the cube of the excitation wavelength (k ) with
tightly focusing an excitation beam. In addition, the
longer wavelength excitation light (Vis–NIR) can easily
permeate most of media without any damage. At
*
Corresponding author. Fax: +86-551-5107342.
E-mail address: yptian@mars.ahu.edu.cn (Y. Tian).
0
009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.02.099

326
Y. Tian et al. / Chemical Physics Letters 388 (2004) 325–329
present, Two-photon initiation polymerization (TPIP)
is the most important approach of TPMF [14]. Two
forms of TPIP have emerged: one approach involved
traditional UV polymerization initiators and high
power laser pulses to realize TPIP [15], another ap-
proach utilized high efficient initiators and common
laser pulses to do that [13,16–18]. Considering of
practical applying prospects, the latter should be ad-
vocated. However, the presently reported new efficient
TPIP initiators are very limited and rather intricate
synthesis steps were involved generally. It is very sig-
nificant work to explore new structures and facile
synthesis methods for TPIP initiators. In this study, we
have described two novel TPIP initiators that were
synthesized with simple operations and high yields,
which possessed potentials in massive industrial man-
ufacture in future.
3. Results and discussion
3.1. Molecular designing and synthesis
The phenothiazine and its derivatives were studied
extensively for their excellent photoelectric functions
[19]. Based on molecular designing consideration, the
phenothiazinyl was introduced to our target molecules
according to our synthesis route (Scheme 1). Due to
relatively high chemical sensitivity of hydrogen on
10-position N atom of phenothiazine, ethylation reac-
tion assured success of the following derivative steps
and, meanwhile, increased charge intensity of the target
molecules. Then Vilsmeier reaction was used to intro-
duce active functional groups, formyls, at 3- or/and
6-position of 10-ethylphenothiazine. Molecular p-elec-
tron conjugated system was enlarged by forming
–
CH@CH– with Wittig reaction. [4-(N,N-di-n-butyla-
mino)benzyl]triphenylphosphonium iodide was selected
as phosphonium salt because of strong electron-donat-
ing ability and good solubility in most organic solvents
of 4-(N,N-di-n-butylamino)benzyl moiety.
Similar N-alkylation reaction could be carried out
under many conditions [20]. In our experiment, common
reagents and facile operations were involved, mean-
while, high yield (92%) of 1 was acquired. We have done
many experiments of Vilsmeier single- and double-
formylation reactions in different conditions and, at last,
the 10-ethylphenothiazine can be quantificationally
2
. Experimental
2
.1. Materials and synthesis
All chemicals were available commercially and every
solvent was purified as conventional methods before use.
All synthesis processes and characterizations were de-
scribed in detail in supporting information.
2
.2. Instruments and measurements
Microanalyses (C,H,N) were performed using a Per-
kin Elmer 240B elemental analyzer. IR spectra were
obtained on a Nicolet NEXUS 870 spectrometer (KBr
1
pellets). HNMR spectra were obtained on a Bruker
Advance/DMX 500 NMR spectrometer in CDCl (TMS
3
as internal standard). Mass spectrum was determined
with a Micromass GCT-MS (EI source). Absorption
spectra were recorded on a UV-265 spectrophotometer.
The photoluminescence spectra were collected on a
Perkin Elmer LS55 spectrofluorimeter. Two-photon
fluorescence (TPF) spectra were measured using streak
camera (C5680-01, Hamamatsu) and imaging spectro-
graph (C5094, Hamamatsu). The pump laser beam came
from a mode-locked Ti:sapphire laser system operating
at 800 nm, pulse duration 200 fs, repetition rate of
7
6 MHz (Coherent Mira900-D).The TPIP experiments
were carried out on the same laser system. Cyclic vol-
tammetry were carried out on EG & GPAR mode 283
electrochemical system. A platinum-disk working elec-
trode, a platinum-wire auxiliary electrode, and an Ag/
AgCl reference electrode were used in a three-electrode
configuration. The concentration of the samples in
acetonitrile was 1.0 mmol/L and tetrabutylammonium
perchlorate (0.1 mol/L) was used as supporting electro-
lyte with 50 mV/s scan speed.
Scheme 1. Strategy of synthesis of the target compounds.

Y. Tian et al. / Chemical Physics Letters 388 (2004) 325–329
327
translated to 2 or 3. In CHCl3 reaction solvent, the yield
of 10-ethyl-3-formylphenothiazine 2 is as high as 97%,
and the yield of 10-ethyl-3,7-biformylphenothiazine 3 is
rescent (SPF) and absorption spectra in eight different
solvents were collected (Table 1) and the influence of
solvent on fluorophore was discussed with Lippert
equation [25]. The results indicated that the fluorescence
of the compounds were tightly related to solvent po-
larity and another fact was proved, which is the com-
pounds possessed highly delocalized conjugated system.
Quantum yields (U) of 5 and 6 were determined with
reference method [26] and experimental results revealed
that all quantum yields were bigger than 60% (Table 1).
Meanwhile, SPF lifetimes (s) in five solvents were mea-
sured on the Edinburgh FLS920 fluorometer with a
Hydrogen flash lamp (pulse duration <1 ns) and radi-
9
1% in N,N-dimethyl formamide (DMF). Compared
with analogous literatures [21], our synthesis approach is
high efficient and facile, no rigorous experimental
conditions, no expensive reaction reagents and no sep-
aration with column chromatography. [4-(N,N-di-n-
butylamino)benzyl]triphenylphosphonium iodide 4 was
prepared by one pot reaction [22], which is di-n-buty-
laniline, formaldehyde, potassium iodide, triphenyl
phosphine and reaction solvents placed together in a
sealed reaction container and stirred at room tempera-
ture for a few days.
ative kinetic constants (K ) and nonradiative kinetic
r
The Wittig reactions were carried out under a few re-
action systems, such as NaOH/CH OH, C H ONa/
constants (K ) were calculated.
nr
Two-photon fluorescence (TPF) spectra of 5 and 6 in
three solvents with 5.0 ꢀ 10ꢁ moL/L concentrations
were recorded on Ti: sapphire laser at 800 nm with
200 fs pulse width and 76 MHz frequency (Table 1 and
Fig. 2). The squared dependence of induced fluorescence
power and incident laser intensity was observed and the
log–log plot of the fluorescence signal vs. excited light
power provided direct evidence for two-photon excited
process (the inset of Fig. 2) [27]. However, with the in-
creasing of incident irradiances, the power of emitting
fluorescence deviated from the second-order power law
and decreased gradually. This deviation may be attrib-
uted to some other nonlinear optical processes, such as
stimulated emission [28], excited-state absorption [29]
and ground-state depletion [30]. TPA cross-sections (r)
of 5 and 6 were evaluated based on the two-state model
[31]. The state dipoles and transition dipole moments
were obtained with ZINDO programme and were di-
rectly used in the calculations. Assuming 800 nm linear
polarized light was used, the calculated r values of 5 and
3
2
5
3
C2H5OH, (CH3)3COK/THF, (CH3)3COK/(CH3)3COH,
CH3)3COK/DMF, NaH/THF, NaOH/solvent-free.
(
Synthetically considering reaction conditions, reagents,
purification processes and yields, the room temperature
solid phase reaction was selected as the optimal synthesis
route. As we known, room temperature solid phase
Wittig reaction was utilized at the first time [23], which
agrees with the principles of ‘green chemistry’ [24]. Al-
dehyde 2 or 3, phosphonium salt 4 and NaOH were
placed in a dry mortar and manually milled for several
minutes. Obvious change of reaction mixture exhibited
occurrence of the reaction and the reaction process was
investigated with TLC. The resulting mixture was added
into water and effectively extracted by petroleum ether, in
0
which the products (10-ethyl-3-E-(4-(N,N -di-n-butyl-
amino)styryl) phenothiazine 5 and 10-ethyl-3,7-E,E-bis
0
(
sess good solubility.
4-(N,N -di-n-butylamino)styryl) pheno thiazine 6) pos-
ꢁ
50
4
ꢁ1
3
.2. Single-photon and two-photon fluorescence
6 are 646.74 and 418.09 ꢀ 10
cm s photon , re-
spectively, which indicated both compounds exhibited
strong TPA at 800 nm laser pulses.
During synthesis and dealing process, our target
compounds were found to emit strong fluorescence in
nature light although all starting materials have not. The
acetone solution and solid phase of 5 and 6 were irra-
diated with 365.0 nm ultraviolet light and bright green
yellow fluorescence was observed from both solid and
solution (Fig. 1). The data of their single-photon fluo-
3.3. Two-photon initiating polymerization microfabrica-
tion
In our TPIP experiments, three-dimensional cross-
linked woodpile periodic microstructures were fabri-
cated with negative resins system, which contained
oligomer (bisphenyl A epoxide dimethylacrylate) and
0.5% compound 5 or 6 as initiator and a little 1,2-di-
chloroethane (increasing the compatibility and control-
ling the viscosity). The same mode-locked Ti: sapphire
laser that was used in the TPF measurements was uti-
lized for TPMF at 800 nm. The laser was tightly focused
via an objective lens (50 ꢀ, NA ¼ 0.45), and the focal
point was modulated in the gel resin that is on a xyz-step
monitorized stage controlled by computer. The pulse
energy after being focused by the objective lens was
ꢂ20 mW. The polymerization results were observed
Fig. 1. Solution (a) and solid (b) luminescent photographs of target
compounds 5 and 6 under 365.0 nm ultraviolet irradiance.

328
Y. Tian et al. / Chemical Physics Letters 388 (2004) 325–329
Table 1
The date of absorption spectra, SPF and TPF with solvent effects
8
ꢁ1
8
ꢁ1
)
Solvent
k
abs- max (nm)
k
em- max (nm)
U
s (ns)
K
r
ꢀ 10 (s
)
K
nr ꢀ 10 (s
kem of TPF
a
Benzene
382
02
463
491
0.68
0.70
0.97
1.35
7.01
5.19
3.30
2.22
4
Toluene
381
01
460
488
0.65
0.67
4
Benzyl alcohol
382
02
468
494
0.61
0.63
2.15
1.73
2.84
3.64
1.81
2.14
489
503
4
CH
2
Cl
2
383
02
472
496
0.69
0.70
1.20
1.42
5.75
4.93
2.58
2.11
480
498
4
DMF
386
03
477
499
0.72
0.74
1.68
1.65
4.29
4.48
1.67
1.58
488
502
4
Acetone
Ethanol
Methanol
388
98
479
493
0.67
0.67
3
378
99
467
496
0.68
0.69
1.44
1.54
4.72
4.48
2.22
2.01
3
379
00
471
498
0.66
0.65
4
a
In every grid, the up is the data of 5 and the down is the data of 6.
Fig. 2. TPF spectra of 5 and 6 in DMF; the inset is Log–Log linear of
squared dependence of induced fluorescence signal and incident irra-
diance intensity.
with a polarization microscope (Opton, Germany) and
the photographs of microstructures were shown in
Fig. 3.
Fig. 3a show a 1.0 ꢀ 1.0 mm polymerization region
with three layers stereo structure. The distance of every
two neighboring polymerization streaks is 10.0 lm. The
polymerization reaction did not take place in the part
out of the laser focus, where could be easily washed out
by tetrahydrofuran. The polymerization solid skeletons
are uniform and orderly and the width is about 3 lm
(
the inset of Fig. 3a). The microstructures could keep
stability for a long time under normal condition and no
volume shrinkage and distortion happened. The possible
reason was that polymerization reaction in the skeletons
was very complete and the inner structure was compact
and rigid. To investigate the influence of oligomer on
Fig. 3. Micrographs of microstructures fabricated via TPIP with the
novel initiators. The size of every grid is 10.0 ꢀ 10.0 lm.

Y. Tian et al. / Chemical Physics Letters 388 (2004) 325–329
329
TPIP reaction, methyl methylacrylate was selected as
monomer. Experimental results revealed our novel ini-
tiators also could effectively realize TPIP in this resin
Science Foundation of China (50272001, 50325311,
20071001, 50173015), Natural Science and Person with
Ability Foundation of Anhui Province (03044701).
(
Fig. 3b). In order to further explore the fabrication of
practical devices, we designed and introduced defects for
the functional microstructures. Fig. 3c showed that a
simple defect was introduced successfully, which con-
tented the microfabrication requirements of applied
devices.
The TPIP mechanisms of these novel initiators are
still unclear. According to Cumpston et al. [13], the
strong donor substituents on molecular conjugated
system make the electron very rich in the molecule, and
after one- or two-photon photoexcitation these chro-
mophores would be able to transfer an electron even to
relatively weak acceptors, and this process could be used
to activate various chemical reactions, such as poly-
merization. In 5 and 6, heterocycle phenothiazine moi-
ety possessed relative high electron density, and the
conjugation systems were enlarged with Wittig reac-
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Acknowledgements
This work was supported by a grant for the State Key
Program of China (G1998061402), the National Natural