Carbazole-based 1D and 2D hemicyanines: Synthesis, two-photon absorption properties and application for two-photon photopolymerization 3D lithography

Article abstract of DOI:10.1039/b609309d

One and two dimensional (1D and 2D) carbazole based hemicyanines, where methyl pyridinium, methyl indolium and methyl benzothiazolium were used as acceptor group, were synthesized by Knoevenagel condensation. One-photon absorption, fluorescence and two-photon fluorescence spectra were investigated. The experimental results indicated that the different ionic acceptors affect their one-photon and two-photon properties. Among them, 2D methyl pyridinium carbazole derivatives exhibited low quantum yields and large two-photon absorption cross sections more than 1600 GM. The synthesized compounds were used as photoinitiator of two-photon photopolymerization (TPP), and three-dimensional (3D) microstructure was successfully fabricated by TPP 3D lithography. They could be utilized as effective two-photon polymerization photoinitiators. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b609309d

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Carbazole-based 1D and 2D hemicyanines: synthesis, two-photon
absorption properties and application for two-photon
photopolymerization 3D lithography
Jie Gu,a Wang Yulan,ab Wei-Qiang Chen,a Xian-Zi Dong,a Xuan-Ming Duan*ac
and Satoshi Kawatac
Received (in Montpellier, France) 30th June 2006, Accepted 22nd September 2006
First published as an Advance Article on the web 5th October 2006
DOI: 10.1039/b609309d
One and two dimensional (1D and 2D) carbazole based hemicyanines, where methyl pyridinium,
methyl indolium and methyl benzothiazolium were used as acceptor group, were synthesized by
Knoevenagel condensation. One-photon absorption, fluorescence and two-photon fluorescence
spectra were investigated. The experimental results indicated that the different ionic acceptors
affect their one-photon and two-photon properties. Among them, 2D methyl pyridinium
carbazole derivatives exhibited low quantum yields and large two-photon absorption cross
sections more than 1600 GM. The synthesized compounds were used as photoinitiator of two-
photon photopolymerization (TPP), and three-dimensional (3D) microstructure was successfully
fabricated by TPP 3D lithography. They could be utilized as effective two-photon polymerization
photoinitiators.
phores were involved in two-photon absorption studies.14–19
Introduction
Meanwhile, some efforts have proved that 2D and multi-
In the past decade, two-photon absorption (TPA) has
than the corresponding 1D charge transfer compounds.20–22
attracted increasing attention due to its various applications
dimensional charge transfer system tend to realize larger d
such as optical data storage, optical power limiting, laser up-
On the other hand, hemicyanine system was widely investi-
conversion, two-photon laser scanning fluorescence micro-
gated as second-order nonlinear optical materials in related to
scopy and photodynamic therapy.1–5 As one of the most
their large dipole moment and good stability for off-diagonal
important applications of TPA, 3D micro-fabrication based
undesirable losses of transparency in the visible region.23
on two-photon photopolymerization (TPP) has been attract-
tensor distribution to first hyperpolarizibitity (b) and without
ing a great deal of interest due to its possible application in the
Although a number of reports recently showed that 2D
fields of micro-electromechanical system (MEMS) and 3D
intramolecular charge transfer (2D-ICT) organic salts have
photonic devices.6–8 Depending on the desired application,
larger second-order nonlinearities,24,25 few efforts have been
made to investigate TPA of 2D-ICT organic salts.17,18
Since carbazole is very easy to be modified at the 3- and
6-position to form a C2v symmetric molecule, here, we report
C2v symmetric carbazole-based 2D-ICT salts by introducing
ionic groups at the 3- and 6-position of carbazole ring, to
elongate p-conjugation system compared to its 1D analogue.
Within this context, we investigate one-photon and two-photon
photophysical properties of 1D-ICT organic salts (1a–e) and
2D-ICT organic salts (2a–e), where the pyridinium, indolium
and benzothiazolium cations were used as acceptor group, the
N-alkyl-carbazolyl as donor group and a carbon–carbon
double bond as p-conjugated bridge. Furthermore the
V-shaped 2D-ICT salt 2c was used in TPP 3D lithography as
a photoinitiator.
chromophores combining a large TPA cross section (d) and
high TPP initiating efficiency, which can favor 3D micro-
fabrication processes based on TPP, are needed.
On one hand, some principles for designing TPA materials
had been established, while many structural designs of mole-
cules have been estimated for this purpose. Prasad et al.9–11
have pointed out that 1D D–p–A type molecules containing
fluorene or dithienothiophene as the rigid p-conjugate back-
bone have large d. Marder et al.12,13 have demonstrated that
symmetric stilbenes containing donor (D) or acceptor (A)
groups linked by a p conjugate bridge (D–p–D or A–p–A)
exhibit large d. The structures D–p–A–p–D and A–p–D–p–A
are also characterized by large d. Ray and Leszczynski have
found the d of ionic chromophores to be larger than those of
the corresponding neutral molecules.14 Some ionic chromo-
Experimental
a Laboratory of Organic Nanophotonics, Technical Institute of Physics
and Chemistry, Chinese Academy of Sciences, Beijing, 100080, P. R.
China. E-mail: xmduan@mail.ipc.ac.cn; Fax: +86 01 82543597;
Tel: +86 10 8254596
Materials
Carbazole, sodium hydride, trichlorophosphine oxide, methyl
iodine, ethyl bromide, pentyl bromide, 2-methyl benzothia-
zole, 4-methyl pyridine, methyl tosylate, phenyl hydrazine,
methyl isopropyl ketone, methyl acrylate and all solvents were
b Department of Chemistry, Beijing University of Chemical
Technology, Beijing, 100029, P. R. China
c Department of Applied Physics, Osaka University, Yamadaoka 2-1,
Suita, Osaka, 565-0871, Japan
ꢀc
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obtained from Beijing Chemical Reagent Company and used
without further purification. Dipentaerythritol hexaacrylate
(DEP-6A, trade name: Light Acrylate DEP-6A) was obtained
from Kyoeisha Chemical Co., Ltd, Japan.
literature methods.26–28 The compounds 1a–e and 2a–e are
obtained by Knoevenagel condensation.27 The typical proce-
dure for 1a–e and 2a–e is: salt 5 (10 mmol) and aldehyde 3 (10
mmol) were added to a 50 mL flask with 25 mL ethanol,
followed by catalytic piperidine (1.0 mL). The resulting mix-
ture was allowed to stir overnight. After concentration, the
residue was recrystallized from acetonitrile–diethyl ether,
further recrystallization gives the desired product. When a
pentyl group was introduced at the 9-position, the solubility
increased and further recrystallization is needed to afford
purified products.
Instruments and measurements
1H NMR spectra were recorded on a Varian Gemini-300
spectrometer using CD3OD as solvent and all shifts are
referenced to TMS. The fine splitting of pyridinyl or phenyl
ring patterns is ignored and the signals are reported as simple
doublets, with J values referring to the two most intense peaks.
Mass spectra were measured on Shimadzu LCMS2010.
Elemental analyses were performed on Flash EA1112.
UV-Visible spectra were obtained on a Shimadzu UV-2550
UV-Vis spectrophotometer. All steady-state fluorescence spec-
tra were measured on a Hitachi F2500 spectrofluorometer.
Two-photon induced excited fluorescence (TPEF) spectra
were recorded on SD2000 spectrometer (Ocean Optical),
excited by a mode-locked Ti-sapphire femtosecond laser
(Tsunami, Spectra-Physics) with an oscillating wavelength,
pulse width and repetition rate of 780 nm, 80 fs and
82 MHz, respectively.
3-[2-(1-Methylpyridinium)vinyl]-9-methyl-carbazole tosylate
(1a). Yield: 41%. Mp 241–242 1C. 1H NMR (CD3OD,
d/ppm): 8.52 (d, 2 H, J = 6.8 Hz), 8.43 (s, 1 H), 8.14 (d, 1
H, J = 7.9 Hz), 8.03 (d, 1 H, J = 16.5 Hz), 8.00 (d, 2 H, J =
6.8 Hz), 7.87 (d, 1 H, J = 7.9 Hz), 7.71 (d, 2 H, J = 7.9 Hz),
7.56 (d, 1H, J = 8.6 Hz), 7.52 (m, 2 H), 7.30 (d, 1 H, J = 16.5
Hz), 7.27 (t, 1 H, J = 8.6 Hz), 7.22 (d, 2 H, J = 7.9 Hz), 4.19
(s, 3 H), 3.90 (s, 3 H), 2.34 (s, 3 H). Anal. calcd for
C28H26N2O3S C, 71.46; H, 5.57; N, 5.95; found: C, 71.29; H,
5.62; N, 5.99%.
Two-photon initiated photopolymerization
3-[2-(1-Methylpyridinium)vinyl]-9-ethyl-carbazole tosylate
(1b). Yield: 31%. Mp 241–243 1C. 1H NMR (CD3OD,
d/ppm): 8.52 (d, 2 H, J = 6.8 Hz), 8.43 (s, 1 H), 8.14 (d,
1 H, J = 7.9 Hz), 8.03 (d, 1 H, J = 16.5 Hz), 8.00 (d, 2 H, J =
6.8 Hz), 7.87 (d, 1 H, J = 7.9 Hz), 7.71 (d, 2 H, J = 7.9 Hz),
7.56 (d, 1H, J = 8.6 Hz), 7.52 (m, 2 H), 7.30 (d, 1 H, J = 16.5
Hz), 7.27 (t, 1 H, J = 8.6 Hz), 7.22 (d, 2 H, J = 7.9 Hz), 4.46
(q, 2 H), 4.18 (s, 3 H), 2.34 (s, 3 H), 1.42 (t, 3 H). Anal. calcd
for C29H28N2O3S: C, 71.87; H, 5.82; N, 5.78; found C, 71.39;
H, 5.79; N, 5.98%.
The same mode-locked Ti-sapphire laser was used in two-
photon polymerization 3D lithography. The photocurable
resin (PR1) was prepared by mixing 49.9 wt% of methyl
acrylate as monomer, 50.0 wt% of DEP-6A as cross-linker
and 0.1 wt% of 2c as photoinitiator. The lasing source was
tightly focused by a 100ꢁ oil-immersion objective lens with a
high numerical aperture (N. A. = 1.4, Olympus). The focal
point was focused on the liquid photopolymerisable resin
which was placed on the xyz-step motorized stage controlled
by a computer. After laser fabrication, the unpolymerized
resins were washed out using ethanol. The obtained micro-
structures were characterized with scanning electron micro-
scopy (SEM; Hitachi S-4300FEGd).
3-[2-(1-Methylpyridinium)vinyl]-9-pentyl-carbazole tosylate
(1c). Yield: 15%. Mp 241–242 1C. 1H NMR (CD3OD,
d/ppm): 8.52 (d, 2 H, J = 6.8 Hz), 8.43 (s, 1 H), 8.14 (d,
1 H, J = 7.9 Hz), 8.03 (d, 1 H, J = 16.5 Hz), 8.00 (d, 2 H, J =
6.8 Hz), 7.87 (d, 1 H, J = 7.9 Hz), 7.71 (d, 2 H, J = 7.9 Hz),
7.56 (d, 1H, J = 8.6 Hz), 7.52 (m, 2 H), 7.30 (d, 1 H, J = 16.5
Hz), 7.27 (t, 1 H, J = 8.6 Hz), 7.22 (d, 2 H, J = 7.9 Hz), 4.46
(t, 2 H), 4.18 (s, 3 H), 2.34 (s, 3 H), 1.88 (m, 2 H), 1.37 (m, 4
H), 0.88 (t, 3 H). Anal. calcd for C32H34N2O3S ꢂ H2O: C, 70.56;
H, 6.66; N, 5.14; found: C, 70.90; H, 6.85; N, 4.92%.
Synthesis
The synthetic routes for 1a–e and 2a–e are outlined in Scheme 1.
1,4-Dimethyl-pyridinium tosylate (5), 1,2,3,3-tetramethyl-
indolium iodide (6), 1,2-dimethyl benzothiazolium iodide (7),
N-alkyl-carbazole, N-alkyl-carbazolyl-3-aldehyde (3), N-alkyl-
carbazolyl-3,6-dialdehydes (4) were synthesized according to
Scheme 1 Synthetic route to 1a–e and 2a–e. Reagents and conditions: (i) POCl3/DMF, ClCH2CH2Cl, reflux; (ii) salts 5–7, piperidine (catalytic
amount), ethanol, reflux.
ꢀc
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3-[2-(10,30,30-Thimethyl-indolium-20-yl)vinyl]-9-pentyl-carba-
zole iodide (1d). Yield: 45%. Mp 119–122 1C; 1H NMR
(CD3OD, d/ppm): 0.89 (m, 3 H), 1.39 (m, 4 H,), 1.61 (m, 2
H), 1.92 (s, 6 H), 4.18 (m, 2 H), 4.49 (s, 3 H), 7.37 (m, 1 H),
7.68 (m, 8 H), 8.22 (d, J = 8.7 Hz, 1 H), 8.28 (d, J = 7.6 Hz, 1
H), 8.67 (d, J = 15.9 Hz, 1 H), 8.94 (s, 1 H); MS (m/z): 421.6
[M-I]+; Anal. calcd for C30H33IN2 ꢂ H2O: C, 63.60; H, 6.23;
N, 4.94.; found: C, 63.44; H, 6.10; N, 5.00%.
3,6-Bis[2-(N-methylbenzothiazolium-2-yl) vinyl]-9-pentyl-car-
bazole diiodide (2e). 65% yield. Mp 252–254 1C. 1H NMR
(DMSO-D6, d/ppm): 0.81 (t, J = 6.0 Hz, 3 H), 1.30 (m, 4 H),
1.80 (m, 2 H), 4.40 (s, 3 H), 4.54 (m, 2 H), 7.77 (t, J = 7.6 Hz,
2 H), 7.88 (dd, J = 8.8 Hz, J = 7.7 Hz, 4 H), 8.11 (d, J = 15.7
Hz, 2 H), 8.23 (d, J = 8.4 Hz, 2 H), 8.31 (d, J = 8.6 Hz, 2 H),
8.42 (d, J = 16.2 Hz, 2 H), 8.44 (m, 2 H), 9.04 (s, 2 H). MS
(m/z): 587.3 [M-2I]+, 712.5 [M-I]+. Anal. calcd for
C37H35I2N3S2: C, 52.93; H, 4.20; N, 5.00; found: C, 52.93; H,
4.33; N, 4.96%.
3-[2-(10-Methylbenzothiazolium-20-yl)vinyl]-9-pentyl-carbazole
iodide (1e). Yield: 55%. Mp 233–235 1C. 1H NMR (DMSO-D6,
d/ppm): 0.87 (t, J = 6.4 Hz 3 H), 1.31 (m, 2 H), 1.57 (m, 2 H),
1.71 (m, 2 H), 3.90 (t, J = 7.0 Hz, 2 H), 4.22 (s, 3 H), 6.94 (m, 1
H), 7.13 (d, J = 8.0 Hz, 1 H), 7.30 (m, 4 H), 7.42 (dd, J1 = 7.2
Hz, J2 = 7.5 Hz, 1 H), 7.66 (s, J = 8.7 Hz, 1 H), 8.10 (d, J =
15.3 Hz, 1 H), 8.27 (d, J = 8.4 Hz, 1 H), 8.38 (d, J = 15.5 Hz, 1
H), 8.57 (d, J = 7.6 Hz, 1 H), 9.25 (s, 1 H). MS (m/z): 411.5
([M-I]+).
Results and discussion
One-photon optical properties
The normalized one-photon absorption and fluorescence spec-
tra of 1c–e and 2c–e are shown in Fig. 1. The absorption
spectra of 2c–e exhibit one weaker shoulder peak but the
spectra of 1c–e do not. The lowest excited band and the second
excited band of 2c–e observed are derived from the strong
interaction between two branched components similar to 1c–e.
The absorption maximum of 1D-ICT system is red shifted
3,6-Bis[2-(1-methylpyridinium)vinyl]-9-methyl-carbazole di-
1
tosylate (2a). Yield: 37%. Mp 4280 1C. H NMR (CD3OD,
d/ppm): 8.64 (d, 4 H, J = 6.4 Hz), 8.59 (s, 2 H), 8.14 (d, 2 H,
J = 15.8 Hz), 8.13 (d, 4 H, J = 6.4 Hz), 7.97 (d, 2 H, J = 8.6
Hz), 7.71 (d, 4 H, J = 7.8 Hz), 7.66 (d, 2 H, J = 8.6 Hz), 7.49
(d, 2 H, J = 15.8 Hz), 7.21 (d, 4 H, J = 7.8 Hz), 4.28 (s, 6 H),
3.97 (s, 3 H), 2.33 (s, 6 H). MS (m/z): 588.4 [M-171.0]+, 208.8
[M-2 ꢁ 171.0]2+, 171.0 [tosylate]ꢃ (calcd for C43H41N3O6S2
759.2). Anal. calcd for C43H41N3O6S2 ꢂ (H2O)0.5: C, 67.17; H,
5.51; N, 5.46; found C, 67.14; H, 5.43; N, 5.43%.
3,6-Bis[2-(1-methylpyridinium)vinyl]-9-ethyl-carbazole
di-
tosylate (2b). 31% yield. Mp 245–246 1C. 1H NMR (CD3OD,
d/ppm): 8.61 (d, 4 H, J = 6.5 Hz), 8.58 (s, 2 H), 8.12 (d, 2 H,
J = 15.9 Hz), 8.09 (d, 4 H, J = 6.5 Hz), 7.91 (d, 2 H, J = 8.6
Hz), 7.71 (d, 4 H, J = 8.6 Hz), 7.62 (d, 2 H, J = 8.6 Hz), 7.40
(d, 2 H, J = 15.9 Hz), 7.21 (d, 4 H, J = 8.6 Hz), 4.46 (q, 2 H),
4.18 (s, 3 H), 2.34 (s, 3 H), 1.42 (t, 3 H). Anal. calcd for
C44H43N3O6S2 ꢂ (H2O)2: C, 65.24; H, 5.85; N, 5.19; found C,
65.60; H, 6.05; N, 5.37%.
3,6-Bis[2-(1-methylpyridinium)vinyl]-9-pentyl-carbazole dito-
1
sylate (2c). 14% yield. Mp 256–259 1C. H NMR (CD3OD,
d/ppm): 8.61 (d, 4 H, J = 6.5 Hz), 8.58 (s, 2 H), 8.12 (d, 2 H,
J = 15.9 Hz), 8.09 (d, 4 H, J = 6.5 Hz), 7.91 (d, 2 H, J = 8.6
Hz), 7.71 (d, 4 H, J = 8.6 Hz), 7.62 (d, 2 H, J = 8.6 Hz), 7.40
(d, 2 H, J = 15.9 Hz), 7.21 (d, 4 H, J = 8.6 Hz), 4.43 (t, 2 H),
4.25 (s, 6 H), 2.31 (s, 6 H), 1.87 (m, 2 H), 1.37 (m, 4 H), 0.87
(t, 3 H). MS (m/z): 644.5 [M-171.0]+, 237.0 [M-2 ꢁ 171.0]2+
,
171.0 [tosylate]ꢃ (calcd for C47H49N3O6S2 815.3). Anal. calcd
for C47H49N3O6S2 ꢂ (H2O)0.5: C, 68.42; H, 6.11; N, 5.09; found
C, 68.12; H, 6.05; N, 5.20%.
3,6-Bis[2-(10,30,30-thimethyl-indolium-20-yl)vinyl]-9-pentyl-car-
bazole diiodide (2d). 72% yield. Mp 4300 1C. 1H NMR
(CD3OD, d/ppm): 0.91 (m, 3 H), 1.41(m, 4 H), 1.61 (m, 2 H),
1.94 (s, 12 H), 4.31 (m, 2H), 4.57 (s, 6H), 7.65 (m, 6 H), 7.84
(m, 6 H), 8.25 (d, J = 8.7 Hz, 2 H), 8.67 (d, J = 15.7 Hz, 2 H),
9.49 (s, 2 H). MS (m/z): 605.6 [M-2I]+, 732.6 [M-I]+. Anal.
calcd for C43H47I2N3: C, 60.08; H, 5.51; N, 4.89; found: C,
60.85; H, 5.65; N, 4.88%.
Fig. 1 Normalized Vis-UV spectra (a) and fluorescence spectra (b) of
1c–e and 2c–e. Excited wavelength at 500 nm for 1c/2c, 530 nm for
1d/2d and 1e/2e.
ꢀc
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from 437 to 495 nm as the acceptor changed from pyridinium
(1c) to indolium (1d). The same phenomenon is observed for
the 2D system, with an absorption maximum of 458 nm for 2c,
495 nm for 2e, and 510 nm for 2d.
As shown in Fig. 1(b), the fluorescence of indolium com-
pounds show a red shift compared to pyridinium compounds.
It is worth noting that the fluorescence spectrum of 2d is red
shifted 5 nm compared to 1d for indolium derivatives, however
another two couples exhibit nearly the same fluorescence
spectra. With regard to the indolium cation, the acceptor is
so strong that it affects the degree of intramolecular charge
transfer of the 2D compound in its excited state. The energy of
the 2D-ICTs is in the order of 2c 4 2e 4 2d, which means that
indolium is the strongest acceptor and pyridinium is the
weakest among these three cations.
Fig. 2 The normalized two-photon induced fluorescence emission
spectra of 1c–d and 2c–e excited at 820 nm.
The fluorescence quantum yields were measured at concen-
trations of B1–2 ꢁ 10ꢃ6 M according to the following
expression with fluorescein in 0.1 M aqueous NaOH solution
as a reference standard (F = 0.90) (eqn (1)).29,30
ꢀ
ꢁꢀ
ꢁꢀ ꢁꢀ
ꢁ
shifted 14–20 nm which may be due to the effect of reabsorp-
tion in the higher concentration of 10ꢃ4 M. Fig. 3 presents the
relationship of input power and output fluorescence intensity
under the two-photon induced excitation. The fluorescence
intensity is nearly linear to input power I2in with very good
fixing (R = 0.9999), which provides clear evidence to confirm
that the fluorescence is derived from two-photon induced
excitation of these compounds.
ArðlrÞ IrðlrÞ n2s
Fs
Fr
Fs ¼ Fr
ð1Þ
AsðlsÞ IsðlsÞ n2r
F is the quantum yield, n is the refractive index, I(l) is the
relative intensity of exciting light at wavelength l, A(l) is the
absorbance of solution at the exciting wavelength l, and F is
the integrated area under the emission spectrum. Subscripts s
and r refer to the sample and reference solution, respectively.
All the one-photon photophysical data are listed in Table 1.
The excitation and emission show no significant dependence
on chain length at the substituted carbazolyl nitrogen. The
quantum yields of 1D compounds are larger than those of the
corresponding 2D compounds in methanol. The quantum
yields can be improved using high viscosity solvents,31 those
of 2c and 2d in glycerol increase to 7.7 and 7.6% from 1.3 and
0.7% in methanol, respectively.
TPA cross sections were determined by the two-photon-
induced excited fluorescence (TPEF) method.32 It is assumed
that the quantum efficiencies after two-photon excitation are
the same as those after one-photon excitation. The TPA cross
sections are obtained via the TPEF method with calibration
against fluorescein with a known Fd value in aqueous NaOH
solution (pH 11) at concentrations of 1.0 ꢁ 10ꢃ4 M for the
measurement by using femtosecond pulse laser as the excita-
tion resource. The samples were dissolved in solvents at
concentrations of B1–2 ꢁ 10ꢃ4 M. To ensure that the
measured signals were solely due to TPA, the dependence of
TPEF on the incident intensity was verified in each case to be
quadratic.
Two-photon absorption properties
The two-photon induced fluorescence emission spectra of 1c–d
and 2c–e are presented in Fig. 2. Compare to their one photon
excited fluorescence, all TPEF of these compounds are red
Table 1 Photophysical data of 1 and 2 at room temperature in methanol
TPAd
max
l
abs/nm
emaxa/Mꢃ1 cmꢃ1
l
em/nm
STb/cmꢃ1
Fc
l
/nm
Fde
52.6
df/GM
1a
1b
1c
1d
435
435
437
495
27 800
26 100
25 800
42 300
568
568
568
575
5383
5383
5278
2811
0.11
880
880
880
—
880
ND
800
810
800
800
—
470
490
456
0.096
0.090
0.019
0.034g
0.061
0.011
0.013
0.013
0.077g
0.007
0.076g
0.039
47.0
41.0
—
—
6.9g
203g
ND
1737
1740
1650
1495g
—
1e
2a
2b
2c
474
456
456
458
23 700
57 550
55 430
57 500
569
568
568
569
3522
4324
4324
4259
ND
19.1
22.6
21.5
115g
2d
510
495
40 300
581
570
2396
—
36g
5.9
880
840
474g
151
2e
87 100
2658
a
b
c
Molar absorption coefficient. Stokes shift ST = (1/labs) ꢃ (1/lem). Fluorescence quantum yield in methanol determined relative to fluorescein
d
e
f
in 0.1 M NaOH. The wavelength of the TPA maximum in the range 790–880 nm. Two photon emission cross section. TPA cross section,
1 GM = 10ꢃ50 cm4 s photonꢃ1
g
.
Measured in glycerol. ND not detected.
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Fig. 3 The relationship of input intensity and output intensity during
the TPA cross section measurements for 1c. The solid squares are
experimental spots and solid line is the fitting line.
Fig. 4 The TPA cross sections of 1c–d and 2c–e at different wave-
lengths.
Then the TPA cross section d were calculated on the basis of
the following expression (eqn (2))
Cr nr Fs Fr
ds ¼ d
ð2Þ
r Cs ns Fr Fs
d is TPA cross section, C and n are the concentration and
refractive index of the sample solution, F is the integrated area
under the TPEF spectrum, and F is the fluorescence quantum
yield. Subscripts s and r refer to the sample and reference
solution, respectively.
The shifts of TPA cross section of 1c–d and 2c–e upon
excitation by wavelengths from 790–880 nm were presented in
Fig. 4. The TPA maxima below 880 nm appear at around
800 nm for 2a–c with the TPA cross sections d = 1737, 1740,
1650 GM, respectively; 880 nm for 2d with d = 474 GM and
840 nm for 2e with d = 151 GM, which are contrasted to the
shoulder peak as the second intramolecular charge transfer
state. It is clearly shown that the second ICT state gives great
contributions to TPA spectrum for 2D-ICT molecules. The
TPA excited emission maximium and the TPA cross section
maximium of these compounds were summarized in Table 1.
Fig. 5 SEM image of 3D diamond photonic crystal microstructure.
calculations,35 we found that stilbazolium cations are easily
excited to obtain a diradical conformation in their excited
state. The high TPP initiating efficiency of 2c should be due to
the diradical form of the stilbazolium structure.
Conclusions
Two-photon initiated photopolymerization
In conclusion, C2v symmetric carbazole-based 2D hemicyanines
compounds as well as their 1D analogues were synthesized, and
their one photon absorption, steady state fluorescence, TPA
and TPEF were investigated. The results showed that the 2D
systems possessed larger TPA cross sections than the 1D
systems although the 1D systems have higher fluorescence
quantum yields. The threshold of TPP for the C2v symmetric
carbazole-based 2D stilbazolium compound, 3,6-bis[2-(1-
methylpyridinium) vinyl]-9-pentyl-carbazole ditosylate (2c)
was determined to be 3.0 mW and it was successfully used as
TPP photoinitiator for TPP microfabrication.
As shown above, compound 2c possesses a large TPA cross
section in the range of 780–820 nm which is widely used in 3D
laser microlithography. In order to investigate the two-photon
photosensitivity of chromophore 2c, the threshold energy of
polymerization was evaluated by analyzing the polymer spots
produced by single-spot exposures. The threshold power for 2c
in PR1 is 3.0 mW.
The microstructure with high spatial resolution was success-
fully fabricated by using induced laser power of 10 mW with
exposure time of 2 ms. A microstructure of diamond-lattice
photonic crystal33 with a single-layer 8 ꢁ 8 lattice was success-
fully fabricated, the SEM image of this is illustrated in Fig. 5.
Usually, 1–5wt% of photoinitiator in resins have been used for
photopolymerization,34 however, only 0.1wt% of 2c was
mixed in our experiments. This result clearly indicated that
2c is a photoinitiator with high TPP initiating efficiency. In our
previous study on stilbazolium cations using semiempirical
Acknowledgements
This work was supported in part from the ‘‘One Hundred
Overseas Talents Program’’ of the Chinese Academy of
Sciences (CAS) and the ‘‘Nonlinear Nanophotonics’’ Project
of Japan Science and Technology Agency (JST).
ꢀc
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68 | New J. Chem., 2007, 31, 63–68