Multibranched benzylidene cyclopentanone dyes with large two-photon absorption cross-sections

Article abstract of DOI:10.1039/b604695a

Multibranched benzylidene cyclopentanone dyes with a triphenylamine core were synthesized. Their two-photon optical properties were characterized using a Ti:sapphire femtosecond laser. The results showed that the two-photon absorption cross-sections (δ) o

Full text of DOI:10.1039/b604695a

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www.rsc.org/njc | New Journal of Chemistry
Multibranched benzylidene cyclopentanone dyes with large two-photon
absorption cross-sections
Jie Wu,ab Yuxia Zhao,*a Xue Li,ab Mengquan Shi,a Feipeng Wu*a and
Xiangyun Fangc
Received (in Durham, UK) 31st March 2006, Accepted 2nd June 2006
First published as an Advance Article on the web 15th June 2006
DOI: 10.1039/b604695a
Multibranched benzylidene cyclopentanone dyes with a triphenylamine core were synthesized.
Their two-photon optical properties were characterized using a Ti:sapphire femtosecond laser.
The results showed that the two-photon absorption cross-sections (d) of these compounds were
increased with increasing numbers of branches. The largest d value of the three-branched
compound 3 was 3298 GM which was almost six times larger than that of a model compound,
2,5-bis-[4-(dimethylamino)benzylidene]cyclopentanone (BDMA). The sensitizing efficiencies of
three new dyes in photopolymerization systems were all higher than that of BDMA matching with
the commercial initiator 4,40-dimethyldiphenyliodinium hexafluorophosphate (Omnicat 820). The
two-photon polymerization initiated by the bimolecular system composed of compound 3 and
Omnicat 820 was investigated under femtosecond laser pulses.
a photoinitiator or a photosensitizer used in photopolymer-
1
Introduction
ization, both a large d value and a high initiating or sensitizing
efficiency are simultaneously required. Most reported TPA
compounds can not be used as photoinitiators directly because
their large molecular conjugated structures supply a good
stabilizing system for generated free radicals, cations or an-
ions. In addition, most of them also can not be matched very
well with commercially available highly efficient photoinitia-
tors as photosensitizers. Hence leading research groups in
microfabrication and high-density optical data storage using
the TPP technique always use commercially available UV
photocurable materials.2,21,22
Two-photon absorption (TPA) is a process in which a mole-
cule absorbs two photons simultaneously under intense laser
pulses when the sum of energies of the two absorbed photons
matches well with the energy gap between an allowed excited
state and the ground state. Compared with one-photon ab-
sorption (OPA) process, the wavelength of absorbed photons
in TPA is longer and the transition probability of TPA
depends quadratically on the excitation intensity, which pro-
vide high penetration depth and spatial resolution of the
excitation beam. Due to these characteristics, research on
organic compounds with large two-photon absorption cross-
sections (d) is one of the hot topics in the field of functional
materials for their potential applications, such as three-dimen-
sional microfabrication,1,2 high-density optical data storage,3
optical power limitation,4,5 localized photodynamic therapy,6
and two-photon laser scanning fluorescence imaging.7,8
Benzylidene cyclopentanone dyes have been extensively
investigated as highly efficient photosensitizers in common
UV photopolymerization.23–25 They are generally combined
with the commercially available photoinitiators 4,40-dimethyl-
diphenyliodinium hexafluorophosphate (Omnicat 820) or
o-chlorohexaarylbisimidazoles (HABI) as highly efficient
photoinitiating composites.23–25 In this paper, we incorporated
a 2-[4-(dimethylamino)benzylidene]cyclopentanone chromo-
phore (DMA) with a triphenylamine moiety, synthesized
multibranched compounds, and investigated their TPA prop-
erties, photosensitizing efficiencies and suitability in TPP.
In recent years, a variety of TPA compounds including
dipoles,9 quadrupoles,10–12 multibranched compounds,13–15 and
dendrimers16 have been synthesized and their structure–property
relationships have been investigated.10–20 The results of these
studies reveal that the d value of a compound will be increased
by elongating the molecular conjugated system for charge
transfer or incorporating multi-dipole or quadrupole TPA
chromophores into one molecular structure. Though many
compounds with large TPA properties have been reported few
are available for two-photon photopolymerization (TPP). For
2
Experimental
2,5-Bis[4-(dimethylamino)benzylidene]cyclopentanone (BDMA)
and 2-[4-(dimethylamino)benzylidene]cyclopentanone (DMA)
were prepared according to the literature procedures.23,26,27
Benzaldehyde derivatives a–c were synthesized following the
literature.19 4,40-Dimethyldiphenyliodinium hexafluoropho-
sphate (Omnicat 820, from TH-UNIS Insight Co. Ltd),
2-phenoxyethyl acrylate (SR339, from Sartomer Co. Ltd),
pentaerythritol triacrylate (SR444, from Sartomer Co. Ltd)
and epoxy acrylate (CN124A80, from Sartomer Co. Ltd) were
used as received. Other A.R. grade solvents were used after
a Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences, Beijing, 100080, P. R. China. E-mail:
yuxia.zhao@mail.ipc.ac.cn. E-mail: fpwu@mail.ipc.ac.cn;
Tel: +86-10-82543571
b Graduate school of Chinese Academy of Sciences, Beijing, 100049,
P. R. China
c Laboratory of Ultra-Fast Laser, Technical Institute of Physics and
Chemistry, Chinese Academy of Sciences, Beijing, 100080, P. R.
China
ꢀc
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Scheme 1 Synthetic routes to compounds 1–3.
purification and dried with common methods. Optical char-
acterizations were characterized by using solutions of the
compounds in chloroform. UV-visible absorption spectra were
measured on a Jasco V-530 spectrophotometer. One-photon
fluorescence measurements were carried out on a Hitachi
F-4500 fluorescence spectrophotometer. 1H NMR spectra
were obtained on a Bruker DPX 400 spectrometer. Elemental
analyses were preformed using a FLASH EA1112 elemental
analyzer. Mass spectra were recorded using a BIFLEXIII
MALDI-TOF mass spectrometer.
purified by chromatography over a silica gel column using
chloroform as the eluent to afford 2. Yield: 43%. Anal. Calc.
for C48H45N3O2: C, 82.85; H, 6.52; N, 6.04. Found: C, 83.09;
1
H, 6.80; N, 5.85%. H NMR (400 MHz; CDCl3; Me4Si) dH:
7.58 (s, 2H), 7.57 (d, J 8.7, 4H), 7.54 (s, 2H), 7.53 (d, J 8.2,
4H), 7.36 (t, J 7.8, 2H), 7.17 (m, 7H), 6.85 (d, J 8.7, 4H), 3.10
(s, 8H), 3.08 (s, 12H). TOF-MS: m/z 695.3 [M + H], 718.3
[M + Na].
The synthetic routes for compounds 1–3 are shown in
Scheme 1 and details are given below. The chemical structures
of compounds 1–3 and BDMA are shown in Fig. 1.
2-[4-(Dimethylamino)benzylidene]-5-[4-(diphenylamino)
benzylidene]cyclopentanone (1)
4-(N,N-Diphenylamino)benzaldehyde (a) (0.24 g, 0.88 mmol)
and DMA (0.19 g, 0.88 mmol) were dissolved in ethanol (7 ml)
and heated to reflux. After sodium hydroxide (0.02 g) dis-
solved in ethanol (2 ml) was added as catalyst, the colour of
the mixture turned to red immediately and a precipitate
separated out. The reaction was continued for 1 h. After
cooling, the precipitate was filtered off, washed with a small
amount of ethanol and purified by chromatography over a
silica gel column using chloroform as the eluent to afford 1.
Yield: 62%. Anal. Calc. for C33H30N2O: C, 84.22; H, 6.42; N,
5.95. Found: C, 84.35; H, 6.71; N, 5.69%. 1H NMR (400
MHz; CDCl3; Me4Si) dH: 7.57 (s, 1H), 7.56 (d, J 8.4, 2H), 7.52
(s, 1H), 7.49 (d, J 8.7, 2H), 7.31 (t, J 7.8, 4H), 7.16 (d, J 7.8,
4H), 7.12 (t, J 7.4 2H), 7.07 (d, J 8.7, 2H), 6.80 (d, J 8.4 2H),
3.09 (s, 4H), 3.07 (s, 6H). TOF-MS: m/z 470.4 [M + H], 493.4
[M + Na].
Compound 2
Benzaldehyde derivative b (0.3 g, 1.0 mmol) and DMA (0.5 g,
2.3 mmol) were dissolved in ethanol (14 ml) and heated to
reflux. After sodium hydroxide (0.04 g) dissolved in ethanol (4
ml) was added as catalyst, the colour of the mixture turned to
red immediately and a precipitate separated out. The reaction
was continued for 1.5 h. After cooling, the precipitate was
filtered off, washed with a small amount of ethanol and
Fig. 1 Chemical structure of compounds 1–3 and BDMA.
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Compound 3
The same Ti:sapphire laser used in TPEF measurement was
used for two-photon photopolymerization experiments. The
photocurable resin films were prepared by spin-coating the
mixed resin solution containing SR 339, SR 444 and CN
124A80 as monomers, dye 3 as sensitizer, Omnicat 820 as
initiator and chloroform as solvent onto glass substrates and
dried in a vacuum oven overnight to evaporate off the chloro-
form. The film thickness was controlled to be about 50 mm.
The laser was tightly focused via an objective lens (40ꢂ, NA =
0.45) into the sample. The average power of the laser beam
was 20 mW. The sample was fixed on an xyz-step motorized
stage (P-762.3L nanopositioner) controlled by a computer.
Polymerization patterns were recorded with an Olympus
IX-70 microscope.
Benzaldehyde derivative c (0.16 g, 0.48 mmol) and DMA (0.48
g, 2.23 mmol) were dissolved in ethanol (20 ml) and heated to
reflux. After sodium hydroxide (0.03 g) dissolved in ethanol (2
ml) was added as catalyst, the colour of the mixture turned to
red immediately and a precipitate separated out. The reaction
was continued for 5 h. After cooling, the precipitate was
filtered off, washed with a small amount of ethanol and
purified by chromatography over a silica gel column using
chloroform as the eluent to afford 3. Yield: 47%. Anal. Calc.
for C63H60N4O3: C, 82.14; H, 6.57; N, 6.08. Found: C, 82.35;
1
H, 6.85; N 6.35%. H NMR (400 MHz; CDCl3; Me4Si) dH:
7.56 (m, 18H), 7.20 (d, J 8.6, 6H), 6.76 (d, J 8.8, 6H), 3.11
(s, 12H), 3.07 (s, 18H). TOF-MS: m/z 920.4 [M + H], 943.4
[M + Na].
3
Results and discussion
3.1 One-photon optical properties
TPA cross-section (d) values of all compounds in chloroform
solution (10ꢁ4–10ꢁ3 M) were determined by using the two-
photon-excited fluorescence (TPEF) technique with femto-
second laser pulses following the experimental protocol
described in detail by Xu and Webb.28 The excitation light
source was a mode-locked Tsunami Ti:sapphire laser (690–880
nm, 80 MHz, o130 fs). Fluorescein in 0.1 M NaOH solution
(10ꢁ4 M) was used as a reference in our experiment since its
absorption and two-photon-excited fluorescence were in a
similar region to the tested samples. The d values of fluorescein
in 0.1 M NaOH solution in the range 690–880 nm has been
reported by Xu and Webb.28 The d of compounds 1–3 and
BDMA were calculated according to eqn (1) by comparing
their TPEF intensities with that of fluorescein under the same
measurement conditions.29
One-photon absorption and fluorescence spectra of 1–3 in
chloroform solution are depicted in Fig. 2 and photophysical
characteristics are listed in Table 1. The absorptions around
300 nm should be due to localized p-p* transitions of
phenylene moieties. The absorptions at longer wavelength
region (around 480 nm) are considered as intramolecular
SsFrjrcr
SrFsjscs
d ¼
dr
ð1Þ
Here, the subscripts r and s stand for the reference and sample,
respectively; S is the integral area of the two-photon-excited
fluorescence; F is the fluorescence quantum yield; j is the
overall fluorescence collection efficiency of the experimental
apparatus, and c is the number density of the molecules in
solution.
In one-photon photopolymerization experiments, the
monomers were a mixture of SR339, SR444 and CN124A80
(mSR339 : mSR444 : mCN124A80 = 1 : 3 : 5). The initiator was
Omnicat 820 while compounds 1–3 and BDMA were used as
sensitizers. A dichloromethane–methanol (9 : 1) mixed solvent
was used. The mixed photocurable solutions containing all the
components were poured onto a glass substrate and dried in a
vacuum oven overnight to evaporate off all solvent. The film
thickness was controlled to be about 0.5 mm. The light source
was an EFOS Lite 50 W miniature arc lamp with a 5 mm
crystal optical fiber and an intensity of irradiation of 20 mW
cmꢁ2. A band-pass filter was inserted to choose light of
400–500 nm for our experiments. A Nicolet 5700 infrared
spectrophotometer was used to monitor the change of the
double bond absorption peak at 6164 cmꢁ1 in the near-IR
region. The polymerization conversion efficiency of the double
bond was calculated by the data processing software series 7-1.
Fig. 2 One-photon absorption (a) and normalized fluorescence spec-
tra excited at 460 nm (b) of compounds 1–3 in chloroform solution.
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Table 1 One- and two-photon optical properties of compounds 1–3 and BDMA in chloroform
(1)
ꢁ5
max
Compound
l
a/nm
10
e
b/Mꢁ1 cmꢁ1
lflmaxa/nm
Dnc/cmꢁ1
Fd
dmaxe/GM
davge/GM
max
1
2
470.5
490.5
492
0.51
0.84
1.19
0.65
546.4
550.0
552.6
535.4
2952
2205
2229
2759
0.11
0.15
0.16
0.13
781
2475
3299
526
374
1052
1375
247
3
BDMA
466.5
a
b
l(m1a) x, lflmax are the one-photon absorption, one-photon fluorescence maxima peak, respectively. emax is the corresponding molar absorption
c
d
coefficient. Dn is the Stokes shift. F is the one-photon fluorescence quantum yield determined using fluorescein in 0.1 M NaOH as the
standard.
e
dmax, davg are the maximum and average two-photon absorption cross-section, respectively, in the range 700–880 nm.
charge-transfer absorption bands.30,31 The absorption peaks
(l(m1a) x) of the three compounds occur at 470.5, 490.5 and 492
(1)
nm, respectively. The red-shift of l
with increasing num-
max
bers of branches shows that the central triphenylamine moiety
participates in the conjugation. However, the only 2 nm
increase from 2 to 3 indicates that the cross-conjugation effect
becomes relatively unimportant when adding more
branches.32 The fluorescence spectra of three compounds are
very similar and their emission peaks (lflmax) also show a red-
shift from 1 to 3. The decrease of the Stokes shift from 1 to 2
and 3 may be due to a decreased excited-state charge transfer
in multibranched structures induced by the cross-conjugation
effect. The fluorescence quantum yield (F) of 1–3 was mea-
sured in chloroform solutions and fluorescein in 0.1 M NaOH
was used as a standard (F = 0.90).33 The increase of F from 1
to 2 and 3 could be also due to their decreasing excited state
charge transfer.
3.2 Two-photon absorption
Using the TPEF method, we measured the two-photon-excited
spectra of compounds 1–3 and BDMA in the range 700–880
nm (shown in Fig. 3(a)). All of them present a clear TPA band
around 780–880 nm. In the range 700–780 nm, the d values of
1 and BDMA are very similar to each other. However, from
800 to 880 nm, d of 1 are almost one order of magnitude larger
than those of BDMA, which should be due to the larger
contribution of two phenyls to the intramolecular charge
transfer compared to two methyls. The TPA abilities of
compounds 2 and 3 are somewhat larger than that of 1. The
maximum TPA cross-sections (dmax) of 1–3 are 781, 2474 and
3298 GM, respectively. These values are comparable with the
data of other strong two-photon absorption compounds mea-
sured by the same method.34–36 The ratio of dmax for the three
compounds is 1 : 3.2 : 4.2 (for each branch chromophore in
1–3, the ratio is 1 : 1.6 : 1.4). To avoid the possible error
induced by one-wavelength data, we calculated the average
TPA cross-sections (davg) of 1–3 in the range 700–880 nm,
which were 374, 1052 and 1375 GM, respectively, with a ratio
of 1 : 2.8 : 3.7 (for each branch chromophore in 1–3, the ratio
is 1 : 1.4 : 1.2). Obviously, both 2 and 3 present a significant
enhancement for their TPA compared to 1. Previous research
results attributed such enhancements in multibranched com-
pounds to interbranch vibronic coupling and electronic cou-
pling. Chung et al. considered that the electronic coupling
between the branches was a main reason.13 However, Macak
et al. revealed by theoretical study that the enhancement was
mainly caused by interbranch vibronic coupling.18 In our data
(1 : 1.6 : 1.4 maximum, 1 : 1.4 : 1.2 average), it was noted that
Fig. 3 (a) Two-photon excited fluorescence spectra of compounds
1–3 and BDMA in chloroform. (b) Logarithmic plots of TPA induced
fluorescence vs. excitation power for compound 3 at 800 nm.
the enhancement of d values decreased with increasing num-
bers of branches, which coincided with the trend of the cross-
conjugation effect. We suppose that the contribution of elec-
tronic coupling is a main reason for the enhancement of TPA
in compound 2. In addition, such contribution decreases with
increasing numbers of branches. It is difficult to evaluate the
extent of vibronic coupling from our results.
Theoretically, the TPA induced fluorescence intensity of
compounds is proportional to the square of the excitation
intensity. This relationship was examined and confirmed in
our measurement for all compounds. As an example, we
ꢀc
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Fig. 4 One-photon polymerization conversion of photocurable resins
containing dye/Omnicat 820 bimolecular combinations: [dye] = 1 ꢂ
10ꢁ4 mol kgꢁ1, [Omnicat 820] = 4 ꢂ 10ꢁ2 mol kgꢁ1
.
present the result of compound 3 in Fig. 3(b) and a slope
of 1.96 was obtained in the logarithmic plots of the fluores-
cence intensities induced by TPA vs. excitation intensities at
800 nm.
3.3 Photopolymerization sensitizing efficiency
In one-photon photopolymerization experiments, we investi-
gated the sensitizing efficiencies of dyes 1–3 and BDMA to the
initiator Omnicat 820. When the dye/Omnicat 820 bimolecular
combination was exposed to 400–500 nm light, active aryl
radicals were generated due to the electron transfer between
excited dyes and Omnicat 820. These radicals start a chain
process in acrylate monomers. Through monitoring the rela-
tive change of the double bond absorption of acrylate
monomers at 6164 cmꢁ1 in the near-IR region, the photo-
polymerization sensitizing efficiencies of different dyes could
be determined. The double bond conversion vs. time curves of
photocurable resin films containing different dyes with the
same molar concentration as sensitizer are displayed in Fig. 4.
For comparison, a resin film with only Omnicat 820 and no
dyes was also investigated under the same experimental con-
ditions. The results show that all photosensitizers have a
significant benefit on the efficiency of the polymerization
reaction. Though the chain process for the resin film in the
absence of dye also occurs after a long time of irradiation, the
reaction is very slow, especially in the first 10 min. The
addition of 1–3 or BDMA can greatly enhance the conversion
rate and speed of reaction. Because the absorbance of 1 is
lower than that of BDMA in the range 400–500 nm (see data in
Table 1), its faster reaction speed indicates it has a higher
photosensitizing quantum yield compared to BDMA. For 2
and 3, though they have higher absorbance, their reaction
speeds are somewhat even larger than that of BDMA, which
also show they have higher photosensitizing efficiency. The
similar conversion curves of 2 and 3 may due to reaction
saturation considering their high extinction coefficient and
photosensitizing efficiency.
Fig. 5 Micrographs of microstructures fabricated via TPP with dye 3
and Omnicat 820 as the initiating system.
3.4 Two-photon photopolymerization
To explore the application of compounds 1–3 in two-photon
polymerization, three-dimensional cross-linked woodpile per-
iodic microstructures were fabricated in a mixed resin system
containing 3 and Omnicat 820 as the bimolecular initiating
system. Fig. 5 shows the optical micrographs of two polymer-
ization patterns. Due to the different refractive indices of
exposed and unexposed materials, both two-layer (Fig. 5(a))
and four-layer (Fig. 5(b)) cross-linked woodpile microstruc-
tures are very clearly observed. In each layer, the distance
between two proximate streaks is 10 mm. The polymerization
reaction did not take place in areas out of the laser focus,
which could be easily washed out with suitable solvents. The
experimental results showed our novel dyes can have large
potential applications in TPP.
4
Conclusions
In this work we have synthesized novel multibranched benzy-
lidene cyclopentanone dyes with a triphenylamine core using a
very convenient method and studied their linear and nonlinear
optical properties systemically. The maximum TPA cross-
ꢀc
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sections obtained with femtosecond laser pulses was as high as
3298 GM, which was comparable with the data of other strong
two-photon absorption compounds measured by the same
method. In addition, we observed a cooperative enhancement
effect of TPA in two- and three-branched dyes 2 and 3.
Compared with the well-known dye BDMA, all three com-
pounds presented larger TPA cross-sections and higher sensi-
tizing efficiency matching with the commercial UV initiator
Omnicat 820. Primary microfabrication experiments have
been carried out and revealed our dyes/co-initiator system
could effectively realize TPP. Owing to their large TPA cross-
section and high sensitizing efficiency, these dyes would have
extensive application prospects in microfabrication and high-
density optical data storage.
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National Science Foundation of China (50173031 and
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New J. Chem., 2006, 30, 1098–1103 | 1103