Efficient initiators for two-photon induced polymerization in the visible range

Article abstract of DOI:10.1016/S0009-2614(02)01073-4

We present two derivatives of biphenyl or fluorene symmetrically substituted, which could initiate the two-photon absorption (TPA) photopolymerization of acrylates in the visible range. Both compounds show broadband TPA between 500 and 650 nm with cross sections of respectively 30 and 80 × 10^-50 cm⁴ s photon^-1 at 532 nm, the photoinitiation wavelength. Threshold energies and dynamic ranges for polymerization were measured in both cases. As expected, the fluorene derivative was slightly more efficient than the biphenyl one. Our initiators appeared to be orders of magnitude more sensitive than the commercial UV ones, commonly used for TPA photopolymerization.

Full text of DOI:10.1016/S0009-2614(02)01073-4

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19 August 2002
Chemical Physics Letters 362 (2002) 291–295
www.elsevier.com/locate/cplett
Efficient initiators for two-photon induced
polymerization in the visible range
a
a
a,
b
C. Martineau , R. Anemian , C. Andraud *, I. Wang ,
ꢀ
b
M. Bouriau , P.L. Baldeck
b
a
Ecole Normale Supeꢀrieure de Lyon, Laboratoire de Steꢀreꢀochimie et Interactions Moleꢀculaires, UMR 5532,
46, alleꢀe d’Italie, 69364 Lyon Ceꢀdex 07, France
b
Laboratoire de Spectromeꢀtrie Physique, Universiteꢀ J.Fourier, UMR 5588, 38402 Saint Martin d’Heꢁres, France
Received 25 February 2002; in final form 19 June 2002
Abstract
We present two derivatives of biphenyl or fluorene symmetrically substituted, which could initiate the two-photon
absorption (TPA) photopolymerization of acrylates in the visible range. Both compoundsꢁ1show broadband TPA be-
tween 500 and 650 nm with cross sections of respectively 30 and 80 ꢀ 10ꢁ50 cm4 s photon at 532 nm, the photoini-
tiation wavelength. Threshold energies and dynamic ranges for polymerization were measured in both cases. As
expected, the fluorene derivative was slightly more efficient than the biphenyl one. Our initiators appeared to be orders
of magnitude more sensitive than the commercial UV ones, commonly used for TPA photopolymerization. Ó 2002
Elsevier Science B.V. All rights reserved.
1. Introduction
layer fabrication [5], led to 3D micro-objects in a
single step with a very high resolution [6,7]. Recent
works on TPA photopolymerization use mainly
femtosecond lasers in the near IR. Two types of
photoinitiators are described : (i) commercial UV-
visible photoinitiators [8,9], (ii) electron-rich mol-
ecules optimized for TPA in this wavelength range
[10,11]. Larger values of TPA cross section ob-
tained for the second class of systems allow lower
powers and shorter times of exposure, which could
lead to a cheaper technology. A wide variety of
laser sources are available at shorter wavelengths
in the visible range (Ar-ion laser or frequency
doubled Nd-YAG laser) which could lead to a
better polymerization resolution and smaller 3D
Recently, two-photon absorption (TPA) pro-
cesses in organic materials were intensively studied
in view of applications for optical limiting [1],
three-dimensional optical data storage [2], fluo-
rescence imaging [3] and microfabrication [4].
These three latter techniques take advantage of the
high spatial resolution of two-photon excitation.
As far as concerned 3D microfabrication, the TPA
initiated polymerization technique, which appears
to be complementary to the well known layer-by-
* Corresponding author. Fax: +33-4-7272-8483.
E-mail address: chantal.andraud@ens-lyon.fr (C. Andraud).
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0009-2614(02)01073-4

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C. Martineau et al. / Chemical Physics Letters 362 (2002) 291–295
Fig. 1. Molecular structures of compounds I and II.
structures. The engineering on TPA initiators in
this wavelength range is just emerging [12].
fluorescence signal on input energy. The TPA
cross-section was determined using 1,4-bis(2-
methylstyryl)benzene as reference [15,16].1
In this paper, we present two molecules, which
are efficient TPA initiators for the polymerization
of triacrylates in the visible range. These photoini-
tiators are two symmet N, N0-bis-(4-methoxyphe-
nyl)-N, N0-diphenyldiamino derivatives (Fig. 1).
They were previously described for TPA optical
limiting applications in the visible range [13,14].
Their D-P-D structure (where D is a donor and P
the conjugated bridge) is known to be well-adapted
for photopolymerization [4]. The moderate conju-
gation in our systems (biphenyl or fluorene deriv-
atives) shifts TPA properties in the visible range. In
this work, we have characterized their TPA cross-
sections, ground-state oxidation potentials and
their threshold energies for the polymerization of
triacrylate monomers. The initiation efficiencies of
these two compounds are compared and discussed
relatively to their physical properties.
The ground oxidation potential of both com-
pounds was measured by cyclic voltammetry ver-
sus Ag/Agþ in acetonitrile/0.1 M ðBu4NþÞðBFꢁÞ.
4
The resins consisted in 3% photoinitiator (I or
II), 70% monomer (tris(2-hydroxyethyl) isocyan-
urate triacrylate with a radical inhibitor level of
100 ppm.) and 27% polymer binder (poly(styrene-
co-acrylonitrile)(75/25)). Films with a thickness of
about 50 lm were obtained by solvent evaporation
from a chloroform solution.
All TPA induced photopolymerization experi-
ments were performed using a frequency doubled
Nd-YAG microlaser Nanolase from JDS Uni-
phase (k ¼ 532 nm, pulse duration 0.5 ns, maxi-
mum pulse energy 4 lJ, repetition rate 6.5 kHz)
focused through a ꢀ5 objective lens. Films were
placed on a 3-axis piezoelectric stage allowing their
translation relative to the laser focal point.
2. Experimental section
3. Results and discussion
Details about the synthesis of molecules I and II
ðN4, N40-bis-(4-methoxyphenyl)-N4, N40-diphenyl-4,
40-diaminobiphenyl and N4, N40-bis-(4-methoxy-
phenyl)-N4, N40-diphenyl-2,7-diamino-9,9-diethyl-
fluorene respectively) are given in the Appendix A.
TPA spectra were obtained by up-conversion
fluorescence measurements using a Nd-YAG laser
pumped optical parametric oscillator supplying 2.6
ns pulses in the 450–650 nm spectral range with 10
Hz repetition rate. The pulse energy was kept low
enough to ensure a quadratic dependence of the
3.1. Physical properties of I and II
3.1.1. Optical properties
These molecules present linear absorption
maxima at kmax ¼ 349 nm and kmax ¼ 374 nm re-
1
rTPA ¼ 62 ꢀ 10ꢁ50 cm4 s photonꢁ1 at 600 nm; the value of
the TPA cross-section has been corrected for a typographical
error.

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C. Martineau et al. / Chemical Physics Letters 362 (2002) 291–295
293
Fig. 2. Optical properties of compounds I and II: (a) UV-visible absorption spectra in dichloromethane; (b) TPA spectra in N,N-
dimethylformamide.
spectively for I and II (Fig. 2), leading to trans-
parent molecules for wavelengths above 400 nm.
Both molecules exhibit broadband TPA prop-
erties in the visible range between 500 and 650 nm
due to a moderate conjugated charge transfer
system as mentioned above. The compound II is
more efficient than I in terms of TPA properties, in
good agreement with an increase of the charge
transfer process within II due to the planarity of
the fluorene core. At the experimental photopoly-
merization wavelength (532 nm), the TPA cross-
sections of I and II are respectively 30 and
80 ꢀ 10ꢁ50 cm4 s photonꢁ1. Although such values
are comparable or sligthly weaker than those
published for synthetic initiators for TPA poly-
merization in the near IR [4,17], they are several
orders of magnitude larger than the TPA cross-
sections of a commercially available resin that
have been used for TPA stereo-lithography [18].
phenyl)-N-phenyl-amino) of the photoinitiator to
the carbonyl acceptor group of the monomer, with
slightly more efficient electronic transfer properties
in II.
3.2. Initiation properties of I and II
Both chromophores are efficient TPA initiators
for acrylates polymerization under 532 nm laser
excitation. The threshold energy of polymerization
is a relevant feature to characterize the initiation
3.1.2. Electrochemical properties
The ground-state oxidation potential is a rele-
vant parameter for the characterization of the
photopolymerization process since in the initiation
step of acrylates radical photopolymerization, a
monoelectronic transfer from the photoinitiator to
the monomer is required [4]. This potential, rela-
tive to Ag/Agþ, was found to be respectively 380
mV for I and 320 mV for II. These low values
show the ability for both molecules to give an
electron from the donor group (N-(4-methoxy-
Fig. 3. Polymer lines fabricated from initiator II using a 1.26
mW average power laser and a 1:8 lm waist beam with speeds
increasing from 90 to 300 lm=s; the corresponding greyscale
profiles are displayed on the bottom.

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C. Martineau et al. / Chemical Physics Letters 362 (2002) 291–295
rffiffiffi
properties. It was evaluated by analyzing lines of
polymer produced by translating the resin across
laser beam at increasing speeds. The average laser
power, beam waist, and scan speeds were 1.26
mW, 1:8 lm and 30–500 lm=s, respectively. A
transmission microscope image of lines written at
increasing speeds is shown on Fig. 3. The contrast
between the polymerized material and the original
resin could be due to a significant increase of the
refractive index upon polymerization as already
observed for other monomers [19]; brighter parts
correspond to high refractive index regions trap-
ping light. The linewidth decreases when the scan
speed increases. This a due to the narrowing of the
exposure dose area that is above threshold. We
used the greyscale profile shown on the bottom
part of Fig. 3 to estimate linewidth values. How-
ever, this profile does not reproduce directly the
physical shape of the solidified structures and
should be considered with care, since it depends
critically on how such phase objects are imaged
through an optical microscope. Assuming a
Gaussian beam profile, the incident energy E (in J/
m2) at a distance d from the middle of the line can
be expressed following the relationship (1), in
which P is the average power of the beam, x0 its
half width at 1=e2 of the intensity maximum and s
the scan speed.
2 P
p x0s
2
2
0
E ¼
eꢁ2d =x
ð1Þ
This expression leads to the relation (2) between
the linewidth W and the scan speed, where Emax is
the incident energy at the waist (d ¼ 0;
qffiffi
old energy.
2
P
Emax
¼
) and Eth the polymerization thresh-
p x0s
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Emax
Eth
W ¼ x0 2 ln
ð2Þ
Fig. 4 displays the changes in the line width as a
function of the scan speed. The solid line corre-
sponds to the data analysis using Eq. (2). From
this model the threshold energies for I and II are
136 J/cm2 and 95 J/cm2, respectively. This is in
agreement with the higher TPA efficiency of initi-
ator II. For both compounds, the threshold energy
is seven orders magnitude lower than for UV
radical generators used as TPA initiators [8].
A further relevant parameter for the effective
fabrication process is the dynamic range of a resin,
defined as the ratio Edamage=Eth, in which Edamage is
the damage energy. This damage energy is ap-
proximatively 2500 J/cm2 for I and II, leading to a
dynamic range of 18 for I and 26 for II. These
values are an order of magnitude higher than those
obtained with commercial UV-photoinitiators
[20]. This enlarges significantly the intensity win-
dow available for fabrication.
4. Conclusion
Two TPA photoinitiators optimized for the
visible range have been synthesized and charac-
terized. These molecules present an efficient TPA
in the 500–650 nm spectral range. They have been
successfully used for acrylates photopolymeriza-
tion without any coinitiator. The polymerization
thresholds were determined by the analysis of
polymer lines produced at variable exposure dose.
The fluorene derivative leads to a lower polymer-
ization threshold than the biphenyl one in agree-
ment with its larger TPA cross-section. For both
compounds, these features are seven orders of
magnitude lower than those of commercial UV
Fig. 4. Width of lines as a function of scan speed using a 1.26
mW average laser power and compounds I and II as photo-
initiators. The data points are fitted according to model de-
scribed in text.