Push-pull acylo-phosphine oxides for two-photon-induced polymerization

Article abstract of DOI:10.1021/ma4010988

The two-step method for the synthesis of acyl-phosphine oxides from aromatic aldehydes was optimized giving the products in 52-97% overall yield. Linear and nonlinear optical properties of series of acyl-phosphine oxides possessing substituents with different electron-donating ability were investigated. Two-photon absorption cross sections (σ₂) of push-pull acyl-phosphine oxide was determined as 9GM via z-scan measurements with femtosecond laser pulses. Using acyl-phosphine oxides possessing dipolar structure as initiators, 3D nanopatterns were successfully fabricated by two-photon initiated polymerization. These compounds also initiate classical photopolymerization when excited with UV radiation.

Full text of DOI:10.1021/ma4010988

Article
pubs.acs.org/Macromolecules
Push−Pull Acylo-Phosphine Oxides for Two-Photon-Induced
Polymerization
Rashid Nazir,† Paulius Danilevicius,‡ David Gray,‡ Maria Farsari,‡,* and Daniel T. Gryko†,§,*
†Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
‡Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology Hellas (FORTH), P.O. Box 1527, 711
10 Heraklion, Crete, Greece
§Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warsaw, Poland
S
* Supporting Information
ABSTRACT: The two-step method for the synthesis of acyl−phosphine
oxides from aromatic aldehydes was optimized giving the products in
52−97% overall yield. Linear and nonlinear optical properties of series of
acyl−phosphine oxides possessing substituents with different electron-
donating ability were investigated. Two-photon absorption cross sections
(σ2) of push−pull acyl−phosphine oxide was determined as 9GM via z-
scan measurements with femtosecond laser pulses. Using acyl−phosphine
oxides possessing dipolar structure as initiators, 3D nanopatterns were
successfully fabricated by two-photon initiated polymerization. These
compounds also initiate classical photopolymerization when excited with
UV radiation.
photoinitiators (TPA PIs), which ensure high writing speeds
and a low polymerization threshold.
INTRODUCTION
■
Two-photon-induced photopolymerization (TPIP) has been
intensively studied recently as the interest in the direct laser
writing of high-resolution three-dimensional (3D) structures
has increased.1 The technique is based on the two-photon
polymerization (2PP) of photosensitive materials; when the
beam of an ultrafast laser is tightly focused within the volume of
such a transparent material, the polymerization process can be
initiated by nonlinear absorption within the focal volume. By
moving the beam focus through the resin in three dimensions,
3D structures can be fabricated. TPIP is a solid free-form
fabrication technique where a resin, usually containing a variety
of acrylate, epoxy, or hybrid materials and a photoinitiator (PI),
is cured inside the focal voxel of the laser. To obtain an efficient
and clean polymerization and therefore high-quality structures,
highly active two-photon absorption (TPA) PI plays a key role.
The method has been exploited in various applications, such as
photonic crystals,2 mechanical devices3,4 and biomolecule
scaffolds.5 Resolution as high as 9 nm has been reported
recently.6
TPIP provides excellent spatial control due to the confine-
ment of the photoactivated polymerization within the focal
volume of the laser beam.7 Moreover, as the material is
transparent to the excitation wavelength (usually 530−800
nm), it is possible to penetrate into the volume of the resin8
and there is reduced scattering. In addition, as the laser pulse
length is in the picosecond or femtosecond range, there are
reduced additional thermal or photochemical side reactions. A
competent TPIP process requires active two-photon absorption
To date, there has been limited research into PIs specially
designed for TPIP. PIs designed for one-photon lithography are
commonly used.9 However, as these PIs have rather low TPA
cross sections (σ2)10 high excitation power and long exposure
time are required, which often result in damage to the
polymeric structures. The development of practical two-photon
absorption photoinitiators (TPA PIs) has been slow due to
their complex syntheses often utilizing expensive catalysts.
These shortcomings have been a critical barrier for further
advances in the promising field of two-photon-induced
photopolymerization (TPIP) technology. Recently several
groups reported good results for α,β-unsaturated ketones as
TPA PIs.10b,c
We aimed to study the possibilities to overcome this
disadvantage, by designing and synthesizing a new generation
of photoinitiators possessing at the same time large σ2 and high
yield of radical photoinitiating. While designing new photo-
initiators we followed well-known principles proving that
dipolar (D−π−A) chomophores typically possess larger σ2
than compounds with uniform electron distribution.11 Herein
we would like to present the new generation of acyl−phosphine
oxide photoinitiators designed toward favorable combination of
Received: May 25, 2013
Revised: September 2, 2013
© XXXX American Chemical Society
A
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used without further purification for next step. Yield of crude product
was 81%.
large two-photon absorption cross section and high yield radical
photoinitiating.
General Procedure for the Synthesis of Benzoyldiphenyl-
phosphine Oxides (3a−3d). A solution of α-hydroxybenzylphos-
phine oxides (0.5 mmol) and MnO2 (870 mg, 10 mmol) in
dichloromethane (10 mL) was stirred at room temperature for 18 h
until the complete of reaction (TLC analysis). The reaction mixture
was filterered through Celite, solvent was concentrated under vacuum,
and the crude product was purified by recrystallization or by column
chromatography.
4-Methoxybenzoyldiphenylphosphine Oxide (MBDPO) (3a).
The crude product was purified by recrystallization from cyclohexane
as white crystals in 99% yield. Its physical properties concur with the
published data.16
4-Dimethylaminobenzoyldiphenylphosphine Oxide
(DMABDPO) (3b). The crude product was purified by recrystalliza-
tion from toluene as yellow crystals in 93% yield. 1H NMR (500 MHz,
CDCl3): δ 8.50 (d, J = 9 Hz 2H), 7.90−7.86 (m, 4H), 7.51−7.44 (m,
6H), 6.64 (d, J = 9 Hz), 3.063 (s, 1H). 13C NMR (500 MHz, CDCl3):
δ 199.2 (carbonyl, d, 327 Hz), 154.6, 133.0, 132.08, 132.06, 132.04,
131.9, 131.6, 130.9, 129.0, 128.5, 128.4, 126.4, 126.0, 110.9, 40.0.
HRMS (ESI): m/z ([M + Na]+) calcd for C21H20NO2PNa, 372.1129;
found, 372.1125.
4-Dihexylaminobenzoyldiphenylphosphine Oxide (DHABD-
PO) (3c). Purification by column chromatography (Hex:EtOAc = 5:1)
yielded the product of as yellow oil in 97% yield. 1H NMR (500 MHz,
CDCl3): δ 8.46 (d, J = 9.0 Hz, 2H), 7.91−7.87 (m, 4H), 7.53−7.43
(m, 6H), 6.58 (d, J = 9.0 Hz, 2H), 3.32 (t, J = 15.5 Hz, 4H), 1.59−1.57
(m, 4H), 1.30−1.25 (m, 4H), 0.89 (t, J = 13.5 Hz, 6H). 13C NMR
(500 MHz, CDCl3): δ 198.0 (carbonyl, d, 329 Hz), 153.0, 133.2,
132.7, 132.0, 132.0, 131.9, 131.8, 131.0, 130.8, 130.7, 129.0, 128.9,
128.5, 128.4, 125.4, 110.7, 51.2, 31.7, 27.2, 26.7, 22.7, 14.1. HRMS
(ESI): m/z ([M + Na]+) calcd for C31H40NO2PNa, 512.27; found,
512.2708.
EXPERIMENTAL SECTION
■
Synthesis. All commercially available compounds were used as
received. All solvents were dried and distilled prior to use.
Transformation and oxygen sensitive compounds were performed
under argon atmosphere. The reaction progress was monitored by
means of thin layer chromatography (TLC) which was performed on
aluminum sheets, coated with silica gel 60 F254 (Merck) or aluminum
oxide 60 F254 (neutral Merck) with detection by a UV lamp. Product
purification was done by means of column chromatography with silica
flash P 60 (40−63 μm, SiliCycle) or aluminum oxide 90 (neutral, 70−
230 mesh, Merck), dry column vacuum chromatography (DCVC)
with silica (MN-Kieselgel P/UV254) or Aluminum oxide (MN-
Aluminumoxid G). Identity and purity of prepared compounds were
1
proved by H NMR and 13C NMR (Varian 500/200 MHz). High-
resolution mass spectra (ESI HRMS) were obtained on MaldiSY-
NAPT G2-S HDMS/GCT Premier, Waters. The following com-
pounds: diphenylphosphine oxide (DPO),12 1c13 and 1d14 were
obtained as described in literature.
General Procedure for the Preparation of α-Hydroxybenzyl-
phosphine Oxides (2a−2d). To solution of aromatic aldehyde (1.0
mmol) and diphenylphosphine oxide (202 mg, 1.0 mmol) in
tetrahydrofuran (10 mL), triethylamine (0.15 mL, 1.0 mmol) was
added dropwise. The reaction was stirred at room temperature for 4 h
(TLC analysis). The solvent was concentrated under vacuum and the
crude product was purified by recrystallization or by column
chromatography.
α-Hydroxy(4-methoxybenzyl)diphenylphosphine Oxide
(2a). The crude product was purified by recrystallization from toluene
as a white crystals in 98% yield. Physical properties concur with the
published data.15
α-Hydroxy(4-dimethylaminobenzyl)diphenylphosphine
Oxide (2b). The crude product was purified by recrystallization from
toluene as yellow crystals in 94% yield. 1H NMR (500 MHz, CDCl3):
δ 7.33−7.80 (m, 10H), 7.015 (dd J1 = 1.5 Hz, J2 = 2 Hz, 2H), 6.53 (d,
J = 8.5 Hz), 5.343 (d, J = 4 Hz), 4.14 (s, 1H) 2.88 (s, 1H). 13C NMR
(500 MHz, CDCl3): δ 150.5, 132.5, 132.4, 132.1, 132.0, 131.8, 131.3,
130.5, 129.6, 128.8, 128.7, 128.4, 128.3, 128.3, 128.2, 112.5, 74.3(d, J =
334.5 Hz),40.8. HRMS (ESI): m/z ([M + Na]+) calcd for
C21H22NO2PNa, 374.1286; found, 374.1286.
9-Ethylcarbazolebenzoyldiphenylphosphine Oxide (CBDPO)
(3d). The crude product was purified by recrystallization from toluene
1
as a yellow crystal with a yield 91%. H NMR (500 MHz, CDCl3): δ
9.50 (d, J = 1.5 Hz, 1H), 8.70 (dd J1 = 1.5 Hz, J2 = 1.5 Hz, 1H), 8.20
(d, J = 8 Hz, 1H), 7.97−7.93 (m, 4H), 7.55−7.41 (m, 10H), 4.39 (q, J
= 22 Hz, 2H), 1.45 (t, J = 14 Hz, 3H); 13C NMR (500 MHz, CDCl3):
δ 202.3 (carbonyl, d, 323 Hz), 143.8, 140.7, 132.29, 132.28, 132.1,
132.0, 131.3, 130.6, 129.6, 129.2, 128.7, 128.6, 128.3, 126.8, 124.9,
123.5, 123.2, 121.2, 120.6, 109.2, 108.6, 37.9, 13.9. HRMS (ESI): m/z
([M + Na]+) calcd for C27H22NO2PNa, 446.1286; found, 446.1287.
Phthanoyl Bis(diphenylphosphine oxide) (PBDPO) (3e). A
solution of 2e (538 mg, 1.0 mmol) and manganese dioxide (MnO2)
(1.74 g, 20 mmol) in 10 mL of dichloromethane (CH2Cl2) was stirred
for 18 h until the complete of reaction (TLC analysis). The reaction
mixture filter through Celite, solvent was concentrated under vacuum
and the crude product was purified by recrystallization with toluene
with yield 64%. Physical properties concur with the published data.17a
4,4-Bis(diethylamino)benzophenone (R). 4,4-Bis-
(diethylamino)benzophenone (R), otherwise known as Michler’s
ketone, was used for reference, as it is used as a two-photon initiator
by many research groups.17b It was obtained from Aldrich and used
without further purification.
z-Scan Technique. In order to investigate the TPA sensitivity of
our compounds, we used the z-scan technique to measure the TPA
cross-section. The z-scan technique is based on the change of the
material’s properties while traversing the focus of a laser beam. To
measure the TPA cross-section, the change in the material’s
transmission was monitored. When approaching the waist of the
beam, the intensity is high enough to induce nonlinearity. If the
sample exhibits TPA, a decrease in light transmittance can be
observed, depending on the sample position with respect to the beam
waist. Fitting the transmittance versus the sample position to the
following equation allows the TPA cross-section to be extracted.18
α-Hydroxy(4-dihexylaminobenzyl)diphenylphosphine
Oxide (2c). Purification by column chromatography (Hex:EtOAc =
1
5:1) yielded the product as yellow crystals with a yield of 91%. H
NMR (500 MHz, CDCl3): δ 7.84−7.34 (m, 10H), 6.98 (d, J = 7.5 Hz,
2H), 6.45 (d, J = 9 Hz, 2H), 5.33 (s, 1H), 3.20 (t, J = 15.5 Hz, 4H),
1.52−1.51 (m, 4H), 1.31−1.29 (m, 4H), 0.89 (t, J = 13.5 Hz, 6H) 13C
NMR (500 MHz, CDCl3): δ 147.2, 133.8, 132.1, 131.8, 131.7, 131.5,
131.4, 131.4, 131.2, 131.1, 131.0, 130.9, 128.9, 128.9, 128.4, 128.3,
128.1, 128.1, 128.0, 127.9, 123.7, 110.4, 72.3 (d, J = 354 Hz), 50.7,
31.1, 26.7, 26.0, 22.1, 13.8. HRMS (ESI): m/z ([M + H]+) calcd for
C31H43NO2P, 492.3031; found, 492.3033.
((9-Ethylcarbazol-3-yl)(hydroxy)methyl)diphenylphosphine
Oxide (2d). The solid was filtered and washed with THF and purified
by recrystallization from toluene as light yellow crystals in 88% yield.
1H NMR (500 MHz, CDCl3): δ 7.99 (s, 1H), 7.95 (d, J = 8 Hz, 1H),
7.88 (q, J = 27.5 4H), 7.57−7.41 (m, 9H), 7.34 (d, J = 8 Hz, 1H), 7.18
(d, J = 15 Hz, 1H), 6.51 (dd J1 = 5.5 Hz, J2 = 6 Hz, 1H), 5.78 (t, J =
11.5 Hz, 1H), 4.40 (q, J = 21 Hz, 2H), 1.28 (t, J = 14 Hz, 3H). 13C
NMR (500 MHz, CDCl3): δ 132.2, 132.2, 132.1, 132.0, 128.7, 128.3,
126.8, 124.9, 121.2, 120.6, 109.2, 108.6, 37.9, 13.9. HRMS (ESI): m/z
([M + Na]+) calcd for C27H24NO2PNa, 448.1442; found, 448.1443.
1,4-Phenylenebis((diphenylphosphoryl)methanol) (2e). To
solution of terephthalaldehyde (134 mg, 1.0 mmol) and diphenyl-
phosphine oxide (404 mg, 2.0 mmol) in tetrahydrofuran (15 mL) was
added dropwise triethylamine (0.30 mL, 2.0 mmol). The reaction was
stirred at room temperature for 24 h (TLC analysis).The reaction
mixture was stirred for 24 h, and during this period, the product
precipitated. The solid was filtered and washed with THF, dried, and
(−q0)n
(n + 1)3/2(1 + x2)n
∞
T(z) =
∑
(1)
n=0
B
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Table 1. Results of Two-Step Synthesis of Acyl−Phosphine Oxides 3a−3d
where q0 = I0LeffNAρ × 10−3σ0λ/hc, I0 is the laser intensity in the waist
RESULTS AND DISCUSSION
■
2
of the beam, which is calculated as I0 = 2E/π3/2τw0 , E is the laser pulse
Both acyl−phosphine oxides and acyl−phosphonates are
known to be used as classical (i.e., one-photon) photoinitiators,
for example, to produce model polymers for dental
applications.21 From the point of targeted dipolar structure
these classes of compounds possess one obvious advantage
the functionality, which is responsible for photoinitiating
activity is at the same time very strong electron-withdrawing
group. In consequence, by adding suitable donor functionality,
one can create push−pull systems.
Two methods are known for the synthesis of α-carbon-
ylphosphine oxides. The classical one, consisting of reaction of
acid chlorides with alkoxyphosphine oxide, is straightforward
but suffers from limited availability of acid chlorides.22 The
alternative, two-step methodology has been developed by Su
and co-workers. Initial addition of aldehydes to diphenylphos-
phine oxide in toluene in the presence of sodium methanolate
as catalyst, gives α-hydroxyphosphine oxides, which are
oxidized to desired acyl−phosphine oxides by vanadyl
acetylacetonate or MnO2.23,24
energy, τ is the pulse duration, w0 is the beam waist radius, Leff is the
effective sample thickness, which is expressed as Leff = (1 − e−αL)/α, α
is the single-photon absorption coefficient, L is the sample thickness (1
mm thick cuvette was used in our experiment), NA is the Avogadro
constant, ρ is the concentration of the sample in mol/L, λ is the
wavelength of the laser, h is the Planck constant, c is the speed of light,
σ2 is two-photon absorption cross-section, x = z/z0, z is sample
position, and z0 is Rayleigh length
The laser source used is a regenerative Ti:sapphire amplifier,
operating at 800 nm, with 250 fs pulses and a repetition rate of 1 kHz.
The laser beam was focused with a lens of 10 mm focal length into a
solution of our compounds in a 1 mm path-length cuvette. The sample
was translated with a motorized stage along the beam propagation axis
and the light transmittance was measured with a photodiode
(DET100A, Thorlabs). The same type of photodiode was used as a
reference where part of the incident beam was sampled before the
sample in order to correct for fluctuations in the laser. The sample
movement, correction of laser energy fluctuations and data collection
was automated with custom-made Lab VIEW software.
TPIP Setup. The TPIP setup used for the fabrication 3D structures
has been described previously.19 In this work, the light source used was
a Ti:sapphire femtosecond laser operating at 800 nm with a repetition
rate of 75 MHz (Femtolasers Fusion). This source is a compact diode-
pumped femtosecond laser oscillator with integrated dispersive mirrors
that precompensate the beam delivery and focusing optics to achieve
sub-20 fs duration pulses into the sample. The laser beam was tightly
focused into the volume of the photosensitive hybrid material using a
high numerical aperture microscope objective lens (100X, N.A. = 1.4,
Zeiss, Plan Apochromat). Sample movement in xyz was achieved using
piezoelectric stages (Physikal Instrumente). The TPIP procedure was
controlled through a computer using the 3DPoli software. The average
laser power needed for the fabrication of 3D structures was 20−40
mW, measured before the objective (average transmission 20%), and
the scanning velocity was 20 μm/s.
We decided to explore the second methodology since its
scope is potentially broader and it allows to start from various,
commercially available aromatic aldehydes. The addition of 4-
methoxybenzaldehyde to diphenylphosphine oxide has been
chosen as model reaction (Table 1). The first step was
performed by following the protocol reported by Wan et al.25
Thus, reaction of aldehyde 1a with Ph2HPO catalyzed by
triethylamine gave α-hydroxyphosphine oxide 2a in 98% yield
(Table 1). The oxidation step proved more difficult. Variety of
oxidants such as chromium trioxide, potassium permanganate,
iodobenzenediaceate, vanadyl acetylacetonate, manganese
dioxide, and pyridinium dichromate were tested. Among
them only MnO2 in DCM gave satisfactory yield without
decomposition of starting material 2a. Subsequently we applied
these optimized protocol to synthesize the other acyl−
phosphine oxides (Table 1). Moderately electron-rich MeO
group has been replaced with stronger donor (Me2N) (Table
The material used for the fabrication of 3D structures is a composite
hybrid material based on zirconium and silicon oxides. Its synthesis has
been described elsewhere.20
C
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1). Its better soluble analogue 3c has been also synthesized
along with carbazole derivative 3d (Table 1).
will be located around 800 nm, which should enable to use a
Ti:sapphire femtosecond laser for nonlinear experiments.
Two-photon absorption was studied via z-scan measure-
ments. Figure 2 shows the normalized laser intensity trans-
Having such efficient methodology in hand we wondered if it
would be able to lead to compounds possessing two acyl−
phosphine oxides moieties. The double addition of tereptha-
laldehyde followed by oxidation led to bis-oxide 3e (possessing
an A−π−A type of structure) in 52% overall yield (Scheme 1).
Scheme 1
Figure 2. z-Scan data from measurements of the compound 3c [0.15
M solution in methanol, 223 GW/cm2 laser intensity], and compound
R [0.25 M solution in methanol, 115 GW/cm2 laser intensity]. Dots
correspond to experimental data points and the curves are the best fit
from eq 1.
mittance dependence on the sample position. The reduction of
the transmittance around the focal spot of the laser beam
indicates TPA in the compounds 3c and R. By fitting this data
with eq 1, we retrieved the TPA cross-section of the
compounds 3c and R, which are the 9GM and 6GM
respectively. Similar value has been obtained for compound
3b, but cross sections of all other compounds were lower than
this value, but not accurately measurable as they are below the
signal-to-noise ratio for this experiment.
The one-photon capability of the synthesized photoinitiators
was confirmed by polymerizing drop-cast films of the material
using a wide-spectrum UV lamp.
Subsequently, linear optical properties of compounds 3a−3e
have been then examined (Figure 1). Compounds 3a and 3e
absorb only UV-radiation, while distinctly yellow dyes 3b-d
have λmax ∼390 nm (Figure 1). This absorption of violet light
allowed us to draw conclusion that their two-photon absorption
Finally, TPIP structuring tests at different laser intensities
and feed rates were performed to evaluate the TPA initiation
efficiency of each initiator. Ideal building parameters for each
initiator were determined by changing the laser intensity, the
feed rate, and the shape size using the same molar PI
concentrations. For the 3D-structure fabrication, we chose the
woodpile photonic crystal geometry.26 This is widely used and
well characterized and allows us to compare the material
structurability and resolution with our previous results, and
those of other groups using TPIP.2b,27 It was possible to
fabricate good quality 3D Woodpile structures using the
compounds 2b, 3b, 3c, and 3d. Figure 3 shows such an example
of 3D-structure fabricated using the zirconium−silicon hybrid
described in20 and 1% wt of the compound 3c as a radical
initiator. The distance between consecutive lines is 500 nm.
The highest lateral resolution achieved was in the order of 300
nm.
It should be pointed out that while two different laser
systems were used for the photopolymerization and the z-scan
measurements (a Ti:sapphire oscillator and a regenerative
Ti:sapphire amplifier, respectively), when the experimental
conditions are taken into account, it can be calculated that the
photon flux density is the similar in both cases and in the order
of 1029photons/s cm2 .This indicates that third order transition
probabilities are the same for TPIP and z-scan.
Figure 1. Absorption spectra of compounds 3a (black line), 3b (green
line), 3c (purple line), 3d (blue line), and 3e (red line) in chloroform.
D
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Figure 3. Woodpile photonic crystals with 500 nm interlayer periodicity fabricated by TPIP using 3c as a radical initiator.
(3) Tian, Y.; Zhang, Y. L.; Ku, J. F.; He, Y.; Xu, B. B.; Chen, Q. D.;
Xia, H.; Sun, H. B. Lab Chip 2010, 10, 2902−2905.
CONCLUSIONS
■
In conclusion, by using two-step protocol and mild oxidizing
agent it is possible to obtain D−A type acyl−phosphine oxides
bearing basic nitrogen atoms. Experimental results clearly
indicate that the acylo-phosphine oxide chromophores exhibit
measurable TPA cross-section only if the strongest electron-
donating groups are present. Using such acyl−phosphine oxide
possessing push−pull structure as initiator, 3D nanopatterns
were successfully fabricated by TPIP. Our results showed that
the electron-donating ability as well as the solubility
significantly affected the two-photon initiation efficiency. This
study constitutes a valuable reference for further research in
designing and synthesis of new TPIP initiators.
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1H NMR and 13C NMR spectra of compounds 2a−2e and 3a−
3e . This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mails: dtgryko@icho.edu.pl and mfarsari@iesl.forth.gr.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research leading to these results has received funding from
the European Community’s Seventh Framework Marie Curie
ITN program TopBio (PITN-GA-2010-264362).
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