Enhancement of acid photogeneration through a para-to-meta substitution strategy in a sulfonium-based alkoxystilbene designed for two-photon polymerization

Article abstract of DOI:10.1021/cm2030075

This contribution reports on the synthesis and the photochemical behavior of two new sulfonium-based photoacid generators (PAGs). We demonstrate that a para-to-meta substitution of a methyl (p-cyanobenzyl) sulfonium group in a 4-alkoxystilbene core strongly influences the photodissociation efficiency of the PAGs and leads to an increase of the quantum yield for acid generation by a factor 2.4. This substantial effect, which was also corroborated by a reactivity enhancement in cationic photopolymerization, is assigned to the modulation of the electronic interaction between two low lying excited states whose energy gap is strongly influenced by this substitution effect. Moreover, it was found that the position of the sulfonium moiety hardly affects the two-photon absorption properties of these push-pull chromophores. By the two-photon fabrication of microstructures, we finally show the potential use of the meta derivative as cationic two-photon initiator.

Full text of DOI:10.1021/cm2030075

Article
pubs.acs.org/cm
Enhancement of Acid Photogeneration Through a Para-to-Meta
Substitution Strategy in a Sulfonium-Based Alkoxystilbene Designed
for Two-Photon Polymerization
Rongjie Xia,† Jean-Pierre Malval,*,‡ Ming Jin,*,† Arnaud Spangenberg,‡ Decheng Wan,† Hongting Pu,†
Thomas Vergote,‡,§ Fabrice Morlet-Savary,‡ Helene Chaumeil,§ Patrice Baldeck,∥ Olivier Poizat,⊥
́
̀
and Olivier Soppera‡
†Institute of Functional Polymer Materials, Tongji University, 1239 Siping Road, 200092 Shanghai, China
‡Institut de Sciences des Mater
iaux de Mulhouse, LRC CNRS 7228, Universite de Haute Alsace, 15 Rue Jean Starcky, 68057
Mulhouse, France
§Laboratoire de Chimie Organique et Bioorganique EA 4566, Universite
Mulhouse, France
́
́
́
de Haute Alsace, ENSCMu, 3 Rue Alfred Werner, 68093
∥Laboratoire Interdisciplinaire de Physique, UMR CNRS 5588, Universite
́
Joseph Fourier, 38402 Saint Martin d’Her
̀
es, France
⊥Laboratoire de Spectrochimie Infrarouge et Raman, UMR CNRS 8516, Universite
́
des Sciences et Technologies de Lille, 59655
Villeneuve d’Ascq Cedex, France
S
* Supporting Information
ABSTRACT: This contribution reports on the synthesis and the photochemical behavior of
two new sulfonium-based photoacid generators (PAGs). We demonstrate that a para-to-meta
substitution of a methyl (p-cyanobenzyl) sulfonium group in a 4-alkoxystilbene core strongly
influences the photodissociation efficiency of the PAGs and leads to an increase of the
quantum yield for acid generation by a factor 2.4. This substantial effect, which was also
corroborated by a reactivity enhancement in cationic photopolymerization, is assigned to the
modulation of the electronic interaction between two low lying excited states whose energy gap
is strongly influenced by this substitution effect. Moreover, it was found that the position of the sulfonium moiety hardly affects
the two-photon absorption properties of these push−pull chromophores. By the two-photon fabrication of microstructures, we
finally show the potential use of the meta derivative as cationic two-photon initiator.
KEYWORDS: two-photon absorption, photoacid generators, microfabrication
photon active sensitizer, such as coumarin,23 phenothiazine,24
1. INTRODUCTION
or thioxanthone,25 with a commercial PAG, such as onium salts.
Photoacid generators (PAGs) constitute an important class of
The acid generation generally proceeds through a photo-
cationic photoinitiators that occupy a strategic position in the
field of the photoresists.1,2 The applications of PAGs have been
induced electron transfer (PET) from the excited sensitizer to
the PAG. However, the global kinetics of this bimolecular
extensively developed in various research domains such as
microelectronics,3 photosensitized resins,4−6 3D microfabrica-
method is intrinsically limited by diffusion. Therefore, an
integrated approach can be alternatively proposed.25−29 It is
based on the molecular association of the acid generator
functionality into the structure of the two-photon active
chromophore. This appealing strategy has been nicely
exemplified by Perry et al.,26−28 who designed a sulfonium-
based bis[(diarylamino)styryl]benzene derivative that exhibits
both high quantum yield for acid generation (∼ 0.5) and large
two-photon absorption (2PA) cross sections (δ). It should be
noted that in this D−π−D system, the dimethylsulfonium
substituents were localized in the meta position, with respect to
the amino groups. More recently, Belfield et al.29 developed a
1,8-bis(4′-styryl)fluorene with two para-substituted diphenyl-
tion, and high density optical data storage.7 Commercial PAGs
are mainly photoactivated through direct excitation at wave-
lengths ranging from VUV to close-UV.3,8−14 This irradiation
mode enables to reach submicrometric resolutions, according
to the Rayleigh diffraction limit. However, because of the
nonselective excitation of the photoinitiator, undesirable side
reactions consecutive to the excitation of PAGs byproduct
should also occur.1,2,5,10 Moreover, a severe restriction in the
choice of the resin can be necessary as a result of the bulk inner
filter, which thus hampers the extension to new applications
such as the 3D-microfabrication. To achieve the specific
excitation of the PAGs with a concomitant improvement of
spatial resolution, the two-photon activation process constitutes
an alternative solution.4,7,15−19 In this case, two approaches are
mainly adopted. The first one, which is derived from one-
photon sensitizing methods,20−22 consists of associating a two-
Received: February 27, 2011
Revised: December 14, 2011
Published: December 14, 2011
© 2011 American Chemical Society
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Fluorohub from IBH, operating at 1 MHz. The detection was based on
a R928P type photomultiplier from Hamamatsu with high sensitivity
photon-counting mode. The decays were fitted with the iterative
reconvolution method on the basis of the Marquardt−Levenberg
algorithm.33 Such a reconvolution technique34 allows an overall-time
resolution down to 0.2 ns. The quality of the exponential fits was
checked using the reduced χ2 (≤1.2).
The steady-state anisotropy measurements were performed in a
glassy matrix of ethanol at 77 K. The samples are placed in a 5-mm
diameter quartz tube inside a Dewar filled with liquid nitrogen. Two
Glan-Thompson polarizers are placed in the excitation and emission
beams. The anisotropy r is determined as follows: r = (IVV − gIVH)/
(IVV + 2gIVH), where g = (IHV)/(IHH), I is the fluorescence intensity,
the subscripts denote the orientation (horizontal H or vertical V) of
the excitation and emission polarizers, respectively, and g is an
instrumental correction factor. The proper calibration of the setup was
checked using a standard method35 with rhodamine 101 in glycerol.
The cyclic voltammetry experiments36 (using a computer-controlled
Radiometer Voltalab 6 potentiostat with a three-electrode single-
compartment cell; the working electrode was a platinum disk; a
saturated calomel electrode (SCE) used as a reference was placed in a
separate compartment) were performed at 300 K, in N2-degassed
acetonitrile with a constant concentration (0.1 M) of (n-Bu)4BF4.
Ferrocene was used as an internal reference.
sulfonium groups at both extremities of the PAGs. Even if this
molecular architecture is similar to the previous one (i.e., A−π−
A) and the exergonic character of the intramolecular PET is
assumed, the quantum yield for acid generation drops to a value
6 times lower. This dramatic decrease in acid generation
efficiency should be first ascribed to the leaving-group
propensity of the substituents on the sulfonium subunit.
Indeed, Sullivan et al.30 showed that a methyl group cleaved
more easily than a phenyl one. Moreover, as a result of the
extension of electronic conjugation, the positioning of the
sulfonium substituent should influence the acid generation
efficiency as well as the two-photon absorption cross section.
To our knowledge, such combined effect has not been clearly
rationalized, even if it could open up some relevant routes for
the design of highly sensitive two-photon activable PAGs. In
this context, we investigate herein the photochemical behavior
of a 4-ethoxystilbene derivative in which a methyl(p-
cyanobenzyl)sulfonium is substituted in either the 4′ position
(EtO-PS) or the 3′ position (EtO-MS) (see Scheme 1). The p-
Scheme 1. Molecular structures of EtO-PS and EtO-MS
The photoacid generation quantum yields were measured using
rhodamine B base as acid indicator.37 A low pressure Hg lamp
equipped with band-pass filters was used as light source and the
photon doses at 313 and 351 nm were determined by ferrioxalate
actinometry.38 Acetonitrile solutions of EtO-PS were irradiated at 351
nm, and solutions of EtO-MS were irradiated at 313 nm. The
absorbance at excitation wavelength was greater than 2.5 to assume a
total absorption of the incident photons. In parallel with the
determination of the photoacid generation quantum yields, the
photolysis quantum yields were evaluated by plotting the decrease
of the absorbance at the maximum absorption wavelength (λabs) in
function of the time of irradiation. The photolysis quantum yield was
then derived from the slope at t = 0, corrected by the photon flux and
38
the extinction coefficient at λabs
.
cyanobenzyl moiety was chosen because of its better leaving
propensity as the methyl group.30,31 Linear and nonlinear
spectroscopic analyses of these new PAGs are presented in
close connection with their reactivity in one- and two-photon
cationic photopolymerization.
The concentration of generated acid upon two-photon excitation
was also titrated using rhodamine B base as an acid indicator. All the
experiments were carefully carried out in the dark to avoid
photogeneration of acid due to parasite light. Typically, a solution of
acetonitrile with the PAG (5 × 10−4 M) was two-photon irradiated for
15 min. An excess of rhodamine B (2 × 10−5 M) was then added to the
sample. The concentration of the photogenerated acid was
spectrophotometrically evaluated from the absorbance of protonated
rhodamine B base. Each measurement was reproduced three times,
which leads to an average value of H+ concentration.
The two-photon absorption cross sections were measured at 730
nm by using open-aperture Z-scan method39,40 with a femtosecond
mode-locked Ti:sapphire laser (Spectra-Physics, Mai Tai; pulse
duration ∼ 100 fs; repetition rate, 80 MHz; wavelength range, 690−
1020 nm). After passing through a beam expander (×4), the laser
beam is focused using an f = 10 cm lens and passed through a quartz
cell (1 mm optical path length). The position of the sample cell is
varied along the laser-beam direction (z-axis) using a Z-step motorized
stage controlled by a computer. At constant incident excitation, the
local power density within the sample is changed, and the
corresponding transmitted laser beam, T(z), recorded with a silicon
photodetector (Ophir PD300), is monitored in connection with the z-
position of the cell. The on-axis peak intensity of the incident pulses at
the focal point, I0, ranged from 35 to 50 GW cm−2. If we assume that
the linear absorption of the sample is negligible at working wavelength
and that the laser exhibits a Gaussian beam profile in space and time,
the nonlinear absorption coefficient β can be calculated from the curve
fitting to the experimental transmittance with the following equation:
2. EXPERIMENTAL SECTION
2.1. Materials. All chemicals for synthesis were purchased from
Sinopharm Chemical Reagent Co., Ltd., TCI, Alfa Aesar, or J&K
Chemical, and they were used without further purification unless
otherwise specified. Methylene chloride and acetonitrile were dried by
calcium hydride refluxing for 24 h. All the solvents employed for
photophysical measurements were Fluka spectroscopic grade. The
cycloaliphatic diepoxide monomer, namely, 3,4-epoxycyclohexylmeth-
yl-3,4-epoxycyclohexanecarboxylate, was supplied by Aldrich.
1
2.2. General Techniques. H NMR was performed on Bruker-
400 M to confirm the structures, and tetramethylsilane (TMS) was
used as internal standard. High resolution MS was measured on an
Agilent Technologies 6510 (Q-TOF) spectrometer using a dual ESI
source.
The absorption measurements were carried out with Perkin-Elmer
Lambda 2 or Shimadzu 2500 double beam UV−vis spectropho-
tometers. Steady-state fluorescence and phosphorescence spectra were
collected from a FluoroMax-4 spectrofluorometer. Emission spectra
are spectrally corrected, and fluorescence quantum yields included the
correction due to solvent refractive index and were determined relative
to quinine bisulfate in 0.05 M sulfuric acid (Φ = 0.52).32
Fluorescence lifetimes were obtained using a nano-LED emitting at
344 nm as an excitation source, with a nano-LED controller module,
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Scheme 2. (a) Ph3MePBr, NaH, DMSO, 85°C (b) Pd(OAc)2, N(CH2CH2OH)3, 120°C (c) 4-cyanobenzyl bromide, CF3SO3Ag,
CH2Cl2, −20°C (d) KPF6 (3 equiv), water/acetone
(40×) that leads to 400 nm radial spot size (1/e Gaussian). The
average laser power was 220 μW. This entire lithography setup was
purchased from Teem Photonics.
2.3. Synthesis. The synthetic route for the synthesis of both
chromophores is depicted in Scheme 2. Previously reported
procedures were used to prepare 4-ethoxybenzaldehyde44 1 and 3-
bromothioanisole.45
βlI
0
T(z) = 1 −
2
⎛
⎞
⎛
⎞
⎟
⎠
z
⎜
⎟
⎟
⎠
2 2 1 + ⎜
⎜
⎝
⎝
z
0
(1)
where z0 is the coordinate along the propagation direction of the focal
point of the beam and l is the optical path length. The 2PA cross-
section, δ, (in units of 1 GM:10−50 cm4 s photon−1 molecule−1) is then
determined by using the relationship:
4-Ethoxystyrene 2. NaH (60%, 2.8 g, 70 mmol) was suspended in a
solution of methyltriphenylphosphonium bromide (21 g, 60 mmol) in
DMSO (80 mL). This reaction mixture was stirred under N2 at 85 °C
for 60 min to form orange phosphonium ylide. The obtained solution
was then allowed to reach room temperature (RT). 4-Ethoxybenzal-
dehyde (9 g, 60 mmol) was added dropwise. This reaction mixture was
stirred at RT for 2 h under Ar, poured into cold water, and extracted
three times with Et2O. The combined organic layer was extracted with
brine, dried over MgSO4, and evaporated under reduced pressure.
Purification by flash column chromatography (CH2Cl2/petroleum
ether 1:10) afforded pure 4-ethoxystyrene 2 (7 g, 80%) as a colorless
liquid. The spectroscopic data obtained were in agreement with those
of a commercial sample.
4-Ethoxy-4′-methylthiostilbene 3 and 4-Ethoxy-3′-methyl-
thiostilbene 4. A suspension of bromothioanisole (1 equiv), 4-
ethoxystyrene (1 equiv), and Pd(OAc)2 (0.01 equiv) in triethanol-
amine (2 mL per 1 mmol of bromoanisole) was heated under stirring
at 100 °C for 10 h under Ar. The reaction mixture was then poured
into water. The precipitate obtained was filtered and dissolved in
CH2Cl2. Filtration through a short pad of silica gel, followed by a
recrystallization from ethanol/CH2Cl2, afforded the desired stilbenes 3
and 4.
δN d
A
−3
10
β =
(2)
hν
where h is the Planck constant, ν the frequency of the incident laser
beam, NA the Avogadro constant, and d is the concentration of the
chromophore (mol L−1). The rhodamine 6G in methanol41 (16.2
2.4 GM at 806 nm) was used for the calibration of our measurement
technique (see Figure S2 in the Supporting Information) . The
uncertainty in δ is on the order of 20%.
The photopolymerization was monitored in situ by Fourier
transform real-time infrared spectroscopy (FT-RTIR) with an
AVATAR 360 FTIR spectrometer from Nicolet. A drop of the
photocurable formulation is deposited on a KBr pellet then spread out
with a calibrated bar. The aerated film, which exhibits an average
thickness of 40 μm, is irradiated at 365 nm using a Xe−Hg lamp
(Hamamatsu, L8252, 150 W) equipped with a band-pass filter. The
UV light irradiance was maintained to a value of 3.5 mW cm−2. The
conversion rates are derived from the changes in the infrared
absorption bands of the monomer. The regions of interest correspond
to the decrease in the epoxide absorption bands in the 800−900 cm−1
region, which corresponds to the C−H bond stretching vibration of
epoxide and to the increase of the peaks at 1050−1150 cm−1 assigned
to the absorption of the (C−O−C) groups in the poly(ether), which is
produced upon photopolymerization.42,43
Compound 3. Starting from 4-ethoxystyrene (148 mg, 1 mmol)
and 4-bromothioanisole (203 mg, 1 mmol), 3 was obtained (170 mg,
63%) as a white powder. 1H NMR (400 MHz, CDCl3, δppm): 7.42 (d, J
= 8.8 Hz, 2H, H2); 7.40 (d, J = 8.4 Hz, 2H, H2′); 7.23 (d, J = 8.4 Hz,
2H, H3′); 7.02 (d, J = 16.3 Hz, 1H, Hvinyl); 6.91 (d, J = 16.3 Hz, 1H,
Hvinyl(ethoxyphenyl)); 6.88 (d, J = 8.8 Hz, 2H, H3); 4.05 (q, J = 7.0 Hz, 2H,
CH2); 2.50 (s, 3H, S−CH3); 1.42 (t, J = 7.0 Hz, 3H, CH3).
Compound 4. Starting from 4-ethoxystyrene (148 mg, 1 mmol)
and 3-bromothioanisole (203 mg, 1 mmol), 4 was obtained (165.7 mg,
The 3D lithographic microfabrication was performed using a Zeiss
Axio Observer D1 inverted microscope equipped with a frequency
doubled Nd:YAG microlaser Nanolase from JDS Uniphase (λexc = 532
nm; pulse duration, 0.61 ns; maximum pulse energy, 6 μJ; repetition,
11.8 kHz). The two-photon excitable resin was sandwiched between
two coverslips, and then, mounted on a computer-controlled 3D
piezoelectric stage, allowing the translation relative to the laser focal
point. The incident beam was focused through a 0.65 NA objective
1
61.5%) as a white powder. H NMR (400 MHz, CDCl3, δppm): 7.53
(d, J = 8.8 Hz, 2H, H2); 7.44 (t, J = 1.5 Hz, 1H, H2′); 7.34 (dt, J = 7.7
Hz, J = 1.5 Hz, 1H, H6′); 7.29 (t, J = 7.7 Hz, 1H, H5′); 7.23 (d, J = 16.5
239
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Hz, 1H, Hvinyl); 7.12 (dt, J = 7.7 Hz, J = 1.5 Hz, 1H, H4′); 7.06 (d, J =
16.5 Hz, 1H, Hvinyl(ethoxyphenyl)); 6.93 (q, J = 8.8 Hz, 2H, H3); 4.04 (q, J
= 6.9 Hz, 2H, CH2); 2.51 (s, 3H, S−CH3); 1.33 (t, J = 6.9 Hz, 3H,
CH3).
the glassy matrix of ethanol are also superimposed. All
spectroscopic data are listed in Table 1. It is noteworthy that
the absorption spectra of the chromophores are hardly sensitive
to solvent polarity. Both spectra exhibit two well-separated
bands. In the high energy side of the spectra, a distinctive band
located at 230 nm is observed and exhibits a value of anisotropy
close to 0.2 for each chromophore. This band mainly
4-Ethoxy-4′-methyl(p-cyanobenzyl)sulfonium Stilbene Hex-
afluorophosphate (EtO-PS) and 4-Ethoxy-3′-methyl(p-
cyanobenzyl)sulfonium Stilbene Hexafluorophosphate (EtO-
MS). According to the literature,31 4-ethoxy methylstilbenes 3 and 4
were converted into their trifluoromethanesulfonate salts. Then, anion-
exchange of CF3SO3− by PF6− was performed using the Zhou et al.
method,14 leading to the final products EtO-PS and EtO-MS. Both
PAGs are stable in the dark and can be stored several months at room
temperature without any decomposition. They show a good solubility
in organic solvents, such as acetone, tetrahydrofuran, acetonitrile,
methylene chloride, and chloroform.
1
corresponds to the La transition centered on alkoxyphenyl
moiety.46 Whereas the maximum wavelength of this band is
insensitive to the position of the sulfonium substituent, its
intensity clearly decreases by a factor 1.35 upon going from a
meta to para compound. Reciprocally, the longest-wavelength
absorption band located in the 275−415 nm range increases by
a factor 1.5 and undergoes a remarkable redshift by 23 nm.
These effects should be ascribed to a better π orbital overlap
between the donor (i.e., alkoxyphenyl) and acceptor (i.e.,
phenylsulfonium) groups, which is promoted by a more planar
conformation in the case of EtO-PS. Moreover, the anisotropy
recorded within this spectral region exhibits a constant value of
ca. 0.36 0.02 and 0.32 0.03 for para and meta derivatives,
respectively. These constant anisotropies confirm that this
absorption band is mainly dominated by a single transition,
probably with π−π* character, and whose absorption transition
moment is collinear with that of the emission one. Interestingly,
the anisotropy decreases at the red edge of the absorption
spectrum, which indicates the presence of another transition
with a distinct electronic symmetry with respect to that of the
π−π* one. The occurrence of this weakly allowed transition is
also corroborated by the additional weak tail observed in the
low energy side of the absorption spectrum of EtO-MS. In
EtO-PS, the corresponding band is likely hidden by the π−π*
transition, as revealed by the position of the anisotropy drop.
Such a transition should be assigned to the π−σ* transition
with a σ* LUMO located on the sulfur−carbon (S−C) bond.
As previously demonstrated by Saeva et al.,47,48 efficient
photoinduced cleavage of the C−S bond of sulfonium salts
occurs when the lowest excited state is the π−σ* one. In this
case, the main photocleavage properties are dependent on the
energy gap between the σ* LUMO and the next higher lying π*
MO. On the other hand, a crossover between the σ* and π*
states induces a drastic decrease of the photodissociation ability
of the sulfonium salt. According to the anisotropy signal, the
π−σ* transition is lying at lower energy than the π−π*
transition in both the EtO-MS and EtO-PS compounds, which
excludes the presence of such a crossover. However, it is clear
that the energy gap between these two electronic levels is
decreasing upon going from the meta to the para derivative.
Such a spectroscopic effect will have strong consequences on
the photolysis properties of the PAGs, as described hereafter.
Both compounds exhibit very low fluorescence quantum
yields (Φfluo < 10−2) and have short fluorescence lifetimes
below 0.2 ns, which suggests that the lowest singlet excited
states (S1) mainly relax through nonradiative deactivation
pathways, such as the S−C bond cleavage. As depicted in
Figure 2, the absorption spectra of the chromophores undergo
strong changes upon irradiation at 350 nm. Whereas the high
energy band centered at 230 nm is slightly increasing during
irradiation, the longest wavelength absorption band collapses
dramatically in connection with the growth of new bands
located in the 250−310 nm range. Moreover, an isosbestic
point is observed at 284 nm for EtO-MS and at 307 nm for
EtO-PS, which indicates an equilibrium reaction at ground
state. It is to be noted that these nondegassed solutions exhibit
EtO-PS. Starting from 3 (270 mg, 1 mmol), EtO-PS (398 mg, 75%)
1
was obtained as a pale powder. H NMR (400 MHz, CD3CN, δppm):
7.76 (d, J = 8.6 Hz, 2H, H3′); 7.71 (d, J = 8.2 Hz, 2H, H2 benzonitrile);
7.65 (d, J = 8.6 Hz, 2H, H2′); 7.56 (d, J = 8.7 Hz, 2H, H2); 7.39 (d, J =
16.4 Hz, 1H, Hvinyl); 7.36 (d, J = 8.2 Hz, 2H, H3benzonitrile); 7.11 (d, J =
16.4 Hz, 1H, Hvinyl(ethoxyphenyl)); 6.95 (d, J = 8.7 Hz, 2H, H3); 4.90 (d, J
= 12.8 Hz 1H, S−CH2); 4.72 (d, J = 12.8 Hz, 1H, S-CH2); 4.08 (q, J =
7.0 Hz, 2H, O−CH2); 3.19 (s, 3H, S−CH3); 1.37 (t, J = 7.0 Hz, 3H,
C−CH3). 13C NMR (100.6 MHz, CD3CN, δppm): 15.03; 25.91; 50.98;
64.44; 114.38; 115.71; 118.9; 119.47; 124.47; 128.89; 129.54; 129.77;
132.29; 132.44; 133.81; 133.98; 134.12; 145.55; 160.63. HRMS (ESI-
Q-TOF) m/z calcd. for [C25H24NOS]* +: 386.1573. Found: 386.1568.
EtO-MS. Starting from 4 (1 g, 3.7 mmol), EtO-Ms (1.321 g, 67%)
was obtained as a yellow powder. 1H NMR (400 MHz, CD3CN, δppm):
7.87 (d, J = 7.9 Hz, 1H, H4′); 7.79 (t, J = 1.5 Hz, 1H, H2′); 7.71 (d, J =
8.3 Hz, 2H, H2benzonitrile); 7.56 (t, J = 7.9 Hz, 1H, H5′); 7.53 (d, J = 8.7
Hz, 2H, H2); 7.50 (d, J = 7.9 Hz, 1H, H6′); 7.35 (d, J = 8.3 Hz, 2H,
H3benzonitrile); 7.26 (d, J = 16.5 Hz, 1H, Hvinyl); 7.06 (d, J = 16.5 Hz, 1H,
Hvinyl(methoxyphenyl)); 6.95 (d, J = 8.7 Hz, 2H, H3); 4.91 (d, J = 12.9 Hz,
1H, S-CH2); 4.73 (d, J = 12.9 Hz, 1H, S−CH2); 4.08 (q, J = 7.0 Hz,
2H, O−CH2); 3.21 (s, 3H, S−CH3); 1.37 (t, J = 7.0 Hz, 3H, C−CH3).
13C NMR (100.6 MHz, CD3CN, δppm): 15.04; 25.52; 50.70; 64.41;
114.42; 115.70; 118.88; 123.42; 124.15; 128.49; 129.27; 129.75;
129.87; 131.98; 132.47; 132.72; 133.21; 133.71; 133.97; 141.91;
+
160.41. HRMS (ESI-Q-TOF) m/z calcd. for [C25H24NOS]*
386.1573. Found: 386.1568.
:
3. RESULTS AND DISCUSSION
Photophysical Properties of the Photoacid Gener-
ators. Figure 1 shows the absorption spectra of the PAGs in
acetonitrile. The steady-state excitation anisotropies recorded in
Figure 1. Absorption spectra of EtO-PS and EtO-MS in acetonitrile
(full lines). Fluorescence excitation anisotropy (circles) in glassy
matrix of EtOH at 77 K (EtO-PS: λem = 440 nm, EtO-MS: λem = 450
nm).
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Table 1. Summary of Optical Data and Parameters for EtO-PS and EtO-MS
a
b
c
a
b
λabs (nm)
εMAX (M−1 cm−1)
λfluo (nm)
Φfluo
τfluo (ns)
E00 (eV)
ΔGeT (eV)
Φacid
λ
2PA(nm)
δ (GM)
EtO-MS
EtO-PS
324
26 000
36 100
382
388
0.004
0.009
<0.2
3.50
3.24
−1.23
−0.97
0.24
0.10
710
760
68
73
347
<0.2
a
b
c
b
Error is about 10%. Error is about 20%. Free energy calculated from eq 3. The uncertainty in δ is of the order of 20%
Figure 3. Cyclic voltamogramm of EtO-MS in acetonitrile +
(nBu)4NPF6 (0.1 M) on platinum electrode at 100 mV s−1 ([EtO-
PS]: 3 × 10−3 M).
Figure 2. Evolution of the absorption spectra of the PAGs upon
irradiation at 350 nm (solvent: acetonitrile).
rates of photolysis equivalent to those obtained for nitrogen-
degassed solutions. This is a clear indication that the reaction is
insensitive to the presence of oxygen and should thus occur at
the singlet excited level rather than the triplet one. A bond
cleavage mechanism promoted by an intramolecular photo-
induced electron transfer (PET) at S1 state is also supported by
the estimation of the free energy (ΔGeT) associated to this
process. We calculated ΔGeT using Rehm−Weller eq 3:49
the energy gap between the σ* and π* MOs mainly stems from
the strong stabilization of the π* level, as depicted in Scheme 3.
At high potential, both chromophores show a similar
Scheme 3. Evolution of the Two Low-Lying Levels
Disposition upon Going from EtO-PS to EtO-MS
ΔG = E − E −E − C
(3)
eT
ox
red
00
In this formalism, Eox and Ered correspond to the oxidation and
reduction potentials of the donor and acceptor, respectively. E00
is the energy of singlet excited state, which is calculated by
Berlman’s method.50 C is the Coulombic energy term
characterizing the interaction of the radical ion pairs. In
acetonitrile, this term usually makes a small contribution to the
overall energy change for PET and was neglected in the
calculation, as previously assumed for other sulfonium-based
derivatives.14 The values of Eox and Ered are determined from
cyclic voltammetry. Figure 3 displays the cyclic voltamogramm
of EtO-MS in acetonitrile. It is to be noted that EtO-PS has a
similar electrochemical fingerprint. Each compound exhibits an
irreversible electrochemical reduction wave with a peak at
−0.89 V and −0.92 V vs SCE for EtO-MS and EtO-PS,
respectively. These cathodic potentials should be assigned to
the one-electron reduction of the sulfonium group, in good
accordance with those measured for similar methyl(p-
cyanobenzyl)sulfonium cations.31 Such an electrochemical
reduction is generally irreversible because of the carbon−sulfur
σ-bond cleavage leading to the production of a sulfide and a
carbon-centered radical. Interestingly, the change in reduction
potentials within a series of compounds can be considered as a
measure of the energy of the S−C bond σ* orbital. In this
regard, the similar Ered values in EtO-MS and EtO-PS indicate
that the para to meta substitution hardly affects the relative
position of the σ*-LUMO. As a consequence, the decrease of
irreversible wave with a peak at 1.35 V vs SCE, which
corresponds to the oxidation of the electron-rich part of the
chromophores. Interestingly, this oxidation potential is lower
than that of anisole (1.65 V vs SCE); therefore, the vinyl
function should be involved in the stabilization of the generated
dication. According to the redox potentials, ΔGeT values have
been estimated and are listed in Table 1. In both cases, the
intramolecular electron transfer at the S1 state is largely
exergonic, while it is thermodynamically forbidden at the
ground state (ΔGeT > 0). Moreover, this driving force is more
exergonic by 0.26 eV for EtO-MS than for EtO-PS, which is
due to a much lower singlet state energy. The photoacid
generation quantum yields (Φacid) were evaluated using the
method previously reported by Scaiano et al.37 EtO-MS and
EtO-PS were irradiated at 313 and 351 nm, respectively, which
corresponds to the π−π* excitation. The photodecomposition
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Chemistry of Materials
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conversion was recorded for values lower than 5%, so as to
neglect the interferences from secondary photoproducts. As
shown in Figure 4, the concentration of the photogenerated
acid increases linearly with the absorbed photon dose. From the
Figure 5. Conversion vs time curves for cationic photopolymerization
of diepoxide monomer. λexc: 365 nm. Irradiance: 3.5 mW cm−2.
Photoinitiating systems: EtO-PS, 0.4 wt % (circles), EtO-MS, 1.6 wt %
(triangles).
Figure 4. Concentration of photogenerated acid as function of the
absorbed photon in acetonitrile (Circles: EtO-PS. Squares: EtO-MS).
Information). Figure 6 shows the one- (full line) and two-
photon (data points) absorption spectra of the compounds.
linear relationships, it is clear that the slope related to the meta
compound is higher than that for the para one. The quantum
yields for acid generation (Φacid) are gathered in Table 1. We
can then observe that EtO-MS exhibits a remarkable Φacid of ca.
0.24, which is more than twice higher (∼ 2.4) than the value
found for its para isomer. Interestingly, the photolysis quantum
yield of the meta derivative (ca. 0.29) is also 2.6 times larger
than that measured for EtO-PS. Such equivalent ratios first
suggest that the chemical rearrangement consecutive to the
primary photocleavage31,51−53 process has similar kinetics for
both PAGs and that the enhancement in acid generation is
mainly ascribed to a more efficient S−C bond dissociation
ability for EtO-MS. This photocleavage enhancement is a direct
consequence of a better energy separation between the σ*
LUMO and its high lying π* MO. Indeed, a weak gap (Δ in
Scheme 3) promotes the electronic coupling between these two
MOs, which should reduce the dissociative character of the
LUMO.
Figure 6. One- (full lines) and two- (data points) photon absorption
spectra of photoacid generators (solvent: acetonitrile).
Photoinitiated Cationic Polymerization. The more
efficient acid generation observed for EtO-MS, as compared
to EtO-PS, has a direct effect on the cationic photoionitiating
properties of each photoacid generator. Figure 5 shows the FT-
RTIR kinetic curves for difunctional epoxide formulations
containing the PAGs, which are irradiated at 365 nm. The
concentration of the PAGs was adjusted so that each sample
exhibits a similar absorbance at the excitation wavelength.
Whereas the conversion of epoxide hardly reached 25% after
120 s irradiation for the para compound, this conversion is
multiplied by a factor of 2 for the EtO-MS. In the same
manner, the initial rate of photopolymerization for EtO-MS is
1.7 higher than that measured for EtO-PS. Hence, the meta
derivative clearly shows higher photoinitiating reactivity than
the para isomer. It is noteworthy that, even in highly viscous
monomer, we have no leveling effect and a good correlation is
maintained between the acid generation quantum yield and the
cationic photoinitiation reactivity.
The 2PA spectra are plotted against half the excitation
wavelength. The 2PA spectrum of the two photoacid
generators consists of a low intensity band with a maximum
located near 760 and 710 nm for EtO-PS and EtO-MS,
respectively. This band does not coincide with the lowest one-
photon absorption band and should be ascribed to the two-
photon excitation of the weakly allowed S0−S1 transition
(π−σ* type), which is located in the low energy side of the
one-photon absorption spectrum, as previously indicated. The
2PA cross sections at the maximum absorption are of the same
order of magnitude with values close to 70 GM for both PAGs
(see Table 1). Therefore, it is of interest to note that the para-
to-meta substitution strategy does not exhibit detrimental
effects toward the 2PA ability of these push−pull stilbenes but
leads to a sizable increase of the acid generation.
To show that acid generation can be two-photon activated
with EtO-PS and EtO-MS, we measured the acid yield in
function of the excitation intensity at 730 nm. As shown in
Figure 7, the concentration of the photogenerated acid, which
was determined spectrophotometrically with rhodamine B,
increases with the square of the excitation power. This
Two-Photon Absorption Properties. The two-photon
absorption (2PA) spectra of the PAGs have been measured in
the 700−800 nm range using the open aperture Z-scan
technique (see Figures S2 and S3 in the Supporting
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Chemistry of Materials
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Figure 7. Plots of log[H+] vs log[excitation power] at 730 nm.
quadratic relationship is consistent with a photochemical
reaction, which is two-photon activated.
Two-Photon Microfabrication. To demonstrate the
potential applications of our photoacid generators in two-
photon photopolymerization, 2D microstructures were pro-
duced using the same difunctional epoxide formulation as that
previously employed for one-photon photopolymerization. For
this purpose, the resin was excited at 532 nm with a
nanosecond pulse microlaser. At this time-scale excitation, it
should be pointed out that a stepwise two-photon absorption
process is excluded because of the very short lifetimes of singlet
excited states (τfluo < 0.2 ns). Moreover, it has been previously
demonstrated for ionic push−pull systems54 or for porphyrin
derivatives55−57 that a strong enhancement of 2PA cross-
section is observed nearby the one-photon absorption region.
In line with this strategy, we also assume a one-photon
resonance enhancement for our sulfonium photoacid gener-
ators. Figure 8 shows examples of optical transmission
microscopy images of 2D-microstructures written within the
resin containing EtO-MS and using a 40 × 0.65 NA objective.
The patterns were fabricated at low incident power of 220 μW.
These 2D-microstructures first indicate that the meta derivative
can be two-photon activated near its one-photon-allowed
absorption region and also illustrate the potential interest of
EtO-MS in microfabrication domain.
Figure 8. Microstructures fabricated by two-photon induced polymer-
ization upon excitation at 532 nm. (Diepoxide resin with EtO-MS, 1.6
wt %. Power: 220 μW.)
absorption ability of the donor−acceptor stilbene system, which
exhibits δMAX close to 70 GM in the 700−800 nm range. We
finally show that EtO-MS is of potential interest for two-
photon initiating polymerization when excited near its one-
photon absorption band.
ASSOCIATED CONTENT
■
S
* Supporting Information
Additional graphs and figures. This material is available free of
charge via the Internet at http://pubs.acs.org.
CONCLUSION
■
Two positional isomers of a 4-alkoxystilbene substituted with a
methyl (p-cyanobenzyl) sulfonium group have been designed
and synthesized. The photophysical features of these new
photoacid generators have been described and compared with
each other, so as to highlight some relevant structure−
properties relationships in connection with the substitution
strategy. Both systems undergo an S−C bond cleavage at
singlet excited state promoted by an intramolecular PET
process. However, upon going from EtO-PS to EtO-MS, a clear
enhancement of the photolysis efficiency is observed as a result
of the increase of the energy gap between the dissociative σ*
LUMO and the high lying π* MO. The quantum yield of acid
generation is consequently multiplied by a factor of 2.4, which
is nicely corroborated in cationic photopolymerization by a
strong increase in the photoinitiating reactivity. Interestingly,
such a substitution approach hardly affects the two-photon
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: jean-pierre.malval@uha.fr.
*E-mail: mingjin@tongji.edu.cn.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(No. 20902069) and the program for Young Excellent Talents
in Tongji University (No. 2008KJ002) for financial support.
Support from the Agence Nationale de la Recherche “Projet
Blanc: 2PAGmicrofab (ANR-BLAN-0815-03) ” and “Retour
Post-Doc: Nanoquenching (10DOC-009 01) ” are also gratefully
acknowledged.
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