Effective Utilization of NIR Wavelengths for Photo-Controlled Polymerization: Penetration Through Thick Barriers and Parallel Solar Syntheses

Article abstract of DOI:10.1002/anie.201912484

This contribution details an efficient and controlled photopolymerization regulated by far-red (λ=680 nm) and NIR (λ=780 and 850 nm) light in the presence of aluminium phthalocyanine and aluminium naphthalocyanine. Initiating radicals are generated by photosensitization of peroxides affording an effective strategy that provides controlled polymerization of a variety of monomers with excellent living characteristics. Critically, long wavelength irradiation provides penetration through thick barriers, affording unprecedented rates of controlled polymerization that can open new and exciting applications. Furthermore, a more optimized approach to performing solar syntheses is presented. By combining the narrow Q-bands of these photocatalysts with others possessing complementary absorptions, layered, independent polymerizations and organic transformations may be performed in parallel under a single broadband emission source, such as sunlight.

Full text of DOI:10.1002/anie.201912484

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Accepted Article
Title: Effective utilization of NIR wavelengths for photo-controlled
polymerization – penetration through thick barriers and parallel
solar syntheses
Authors: Zilong Wu, Kenward Jung, and Cyrille Andre Jean Marie
Boyer
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Effective utilization of NIR wavelengths for photo-controlled
polymerization – penetration through thick barriers and parallel
solar syntheses
Zilong Wu,[a] Kenward Jung,*[a] and Cyrille Boyer*[a]
Abstract: This contribution details an efficient and controlled
photopolymerization regulated by far-red (λ = 680 nm) and NIR (λ =
780 and 850 nm) light in the presence of aluminum phthalocyanine
and aluminum naphthalocyanine. Initiating radicals are generated by
photosensitization of peroxides affording an effective strategy that
provides controlled polymerizations of a variety of monomers with
excellent living characteristics. Critically, the long wavelength
irradiation provides penetration through thick barriers, affording
unprecedented rates of controlled polymerization that can open new
and exciting applications. Furthermore, a more optimized approach to
performing solar syntheses is presented. By combining the narrow Q-
bands of these PCs with others possessing complementary
absorptions, layered, independent polymerizations and organic
Previous investigations examining phthalocyanines for
photodynamic therapy have revealed their capacity to
photosensitize (hydro)peroxides to form radical species under
long wavelength irradiation (Scheme 1);[12] a capability that has
yet to be exploited for photochemical syntheses. We reasoned
that combining this initiation pathway with reversible addition
fragmentation chain transfer (RAFT) polymerization would yield
an effective controlled polymerization process regulated by longer
wavelengths. The surprising efficiency of this process affords
unprecedented rates of controlled polymerization in the order of
minutes even through barriers. Moreover, the narrow absorptions
of the model AlPc and AlNc compounds provide a novel approach
to utilizing sunlight for chemical syntheses; in combination with
PCs possessing complementary absorptions in the visible
spectrum, we demonstrate the ability to perform multiple
independent photochemical reactions in parallel under a single
broadband emission source.
transformations may be performed in parallel under
broadband emission source such as sunlight.
a single
Since Ciamician’s visionary address over a century ago,[1]
photochemistry continues to evolve to fuel innovation across
broad research areas. Alongside the intense focus on utilizing the
Sun as a renewable energy source,[2] light-mediated chemical
syntheses have also received significant attention through the
years.[3] In particular, the advent of photoredox catalysis has
spurred the utilization of lower energy visible wavelengths,
overcoming the non-specificity of high energy ultraviolet
irradiation.[4]
Echoing these developments, a wide range of controlled
photopolymerizations regulated by visible wavelengths have been
developed,[5] creating new opportunities in the preparation of
functional materials.[6] Although visible and UV light mediated
polymerizations are now well established,[7] there have been very
few examples utilizing far-red or near infra-red (NIR) light due to
their lower energy.[8] The enhanced penetration of non-
transparent materials by these wavelengths are promising for the
development of new and exciting applications.[9] Moreover, with
more than 50% of the solar spectrum constituted of longer
wavelengths beyond the visible spectrum,[10] expanding our
photochemical capabilities to far-red and NIR wavelengths
provides a gateway to more optimized usage of sunlight for
photochemical syntheses.[11]
Scheme 1. (A) Long wavelength photosensitization of (hydro)peroxides to
generate radicals under far-red and NIR wavelengths mediated by (B)
aluminum phthalocyanine (AlPc) and (C) aluminum naphthalocyanine (AlNc).
Herein, we report a photoinitiation system regulated by far-
red to NIR light utilizing the strong Q-bands of aluminum
phthalocyanine (AlPc; λmax
napthalocyanine (AlNc; λmax = 783 nm) (Figure S1, Table S1).
=
679 nm), and aluminum
A variety of peroxides were first screened (Table S2);
among the peroxides that only proceeded when irradiated in the
presence of the photocatalysts (PCs), urea hydrogen peroxide
(UHP) provided the fastest rates and was selected for in depth
study. In contrast, the non-stabilized hydrogen peroxide promoted
polymerization in the absence of catalyst or irradiation. The
photoinitiation system displayed excellent temporal control, which
was demonstrated by the absence of monomer conversion in the
dark (Figure 1A, D), even for off periods as long as 24 hours
[a]
Z. Wu, Dr. K. Jung, Prof. C. Boyer
Centre for Advanced Macromolecular Design, Australian Centre for
Nanomedicine, School of Chemical Engineering
The University of New South wales, Sydney, NSW 2052 (Australia)
E-mail: cboyer@unsw.edu.au
Supporting information for this article is given via a link at the end of
the document.
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(Figure S3D); resuming the light exposure reactivated the
polymerization with little to no changes to the propagation rate
(Figure S2, Figure S3A-C, Table S2).[13] Interruptions to the
irradiation had no effect on the controlled behaviour as confirmed
by the linear evolution of Mn with increasing monomer conversion
(Figure 1B, E). Save for some low molecular weight tailing and
high molecular weight shoulders arising from termination
reactions, the molecular weight distributions (MWDs) were mostly
narrow and symmetrical, with clear shifts towards higher
molecular weight occurring under light exposure only (Figure 1C,
F, Figure S3D).
by comparative kinetic studies of MA using a fixed light intensity
of 20 mW cm-2 (Figure 2A, Figure S8A-C). Increasing PC
concentrations from 10 to 50 ppm resulted in a concomitant
increase in the observed propagation rate coefficient (kpapp); the
modest increases observed for AlNc under 850 nm light was
attributed to its significantly lower extinction coefficient at this
wavelength. More significant effects were noted from increasing
the light intensity; at a fixed PC concentration of 50 ppm,
increasing the light intensity from 20 to 40 mW cm-2 resulted in a
app
substantial increase in kp (0.16 min-1) for AlPc (λ = 680 nm),
leading to high monomer conversion (> 70%) in about 10 minutes
(Figure 2B, Figure S8D). Further increasing the light intensity to
100 mW cm-2 afforded the same kpapp (0.16 min-1) for AlNc under
app
780 nm light with the kp increasing to ~ 0.07 min-1 under 850
nm light (Figure 2B, Figure S8E, F), leading to similarly high
conversion (> 70%) in about 15 and 30 minutes, respectively.
Figure 1. Demonstration of temporal control for AlPc and AlNc mediated RAFT
photopolymerization of methyl acrylate under 680 nm (I = 4 mW cm-2), 780 nm
(I = 20 mW cm-2) and 850 nm (I = 55 mW cm-2) irradiation with a reaction
composition consisting of [MA]:[DTPA]:[UHP]:[PC] = 200 : 1 : 0.2 : 0.002 in
DMSO. (A) and (D) ln([M]0/[M]t) versus exposure time according to the
designated ON (white) / OFF (gray) profile; (B) and (E) evolution of Mn and Đ
versus monomer conversion and (C) and (F) evolution of the MWDs generated
in the ON/OFF experiments.
app
Figure 2. (A) Evolution of apparent propagation rates (kp
) of MA
polymerizations with increasing AlPc and AlNc concentrations under 680, 780
and 850 nm light at a fixed intensity of 20 mW cm-2. (B) Evolution of kpapp ’s of
MA photopolymerizations with increasing light intensity (5-100 mW cm-2) using
fixed AlPc and AlNc concentrations under 680, 780 and 850 nm light.
Monomer compatibility was verified by polymerizing methyl
acrylate (MA), methyl methacrylate, N-isopropylacrylamide and
N-isopropylmethacrylamide as model monomers (Table S3;
Figure S4) and were observed to proceed as expected with good
control demonstrated under all three wavelengths. Chain
extensions also proceeded in a predictable manner with a clean
shift of MWD towards higher molecular weight (Figure S5D-I).
Excellent chain end fidelity as confirmed via a combination of 1H
NMR analyses (Figure S6, Table S4) and close match of the RI
and UV gel permeation chromatography (GPC) chromatograms
(Figure S5A-C), permitted higher degrees of polymerization (DP)
ranging from 50 to 5,000, which resulted in relatively narrow and
symmetric MWDs (Đ < 1.5) (Figure S7, Table S5). The effect of
the PC concentration was examined under the three wavelengths
The fast polymerization rates indicated unprecedented
capability to polymerize behind barriers in comparison to previous
studies.[8d, 8j, 8o, 14] Photopolymerizations performed through paper
(thickness ~ 0.10 and 0.2 mm) proved to be successful under all
three wavelengths (Figure 3A, Figure S9D-F). Despite significant
reductions to the light intensity (Table S6), high monomer
conversions (> 80 %) were achieved in about 25 and 40 mins for
AlPc (680 nm) and AlNc (780 nm), respectively (Table S7).
Polymerizations performed through chicken skin (1 mm),
mediated by AlNc under NIR wavelengths incurred less penalties
app
to the kp in comparison to paper, hinting at the superiority of
these wavelengths through biological barriers (Figure 3A, Figure
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S9D-F). Surprisingly, AlPc mediated polymerizations under 680
nm light proceeded through pig skin with relatively fast rates even
through thicknesses as high as 5 mm and afforded high
conversions (> 80%) in less than 30 minutes of exposure time.
Increasing the thickness to 15 mm saw the polymerization rates
plummet for all three wavelengths as expected. Interestingly, the
limited absorption of 850 nm light by AlNc was offset by its
superior penetration through the 15 mm thick barrier (Table S6)
and enabled the fastest rates of polymerizations among the
studied PCs. The advantage of NIR wavelengths were epitomized
by polymerizations performed through an opaque blue
polypropylene barrier (2.5 mm) wherein only the NIR wavelengths
(λem = 780 nm and 850 nm) were able to generate polymers
(Table S7 #19-21). Critically, fast kinetics, that appeared to be
relatively unimpeded by the barrier, were achieved using 850 nm
lower layers, enabling independent reactions to be performed in
a
simultaneous manner (Figure 4A, Scheme S3). To
demonstrate this concept, we employed fac-[Ir(ppy)3], eosin Y
(EY), AlPc and AlNc as PCs with complementary absorptions to
optimize the usage of broadband solar emission (AM 1.5G, 1000
W m-2) (Figure 4B).
light with a kp (kp PP = 0.053 min-1 vs kp direct = 0.069 min-1)
(Figure S10). These results showcase the high efficiency of the
PCs in utilizing longer wavelengths to facilitate controlled
photopolymerizations even through thick barriers.
app
app
app
Figure 3. RAFT photopolymerizations of MA mediated by AlPc and AlNc under
far-red (λmax = 680 nm; I = 40 mW cm-2) and NIR (λmax = 780 nm; I = 100 mW
cm-2 and λmax = 850 nm; I = 100 mW cm-2) light through barriers. (A) Comparison
of apparent propagation rate coefficients (kpapp). (B) Kinetic comparison of RAFT
photopolymerizations performed through 15.0 mm thick pig skin. (C) Photo
detailing the experimental setup used for polymerizing through 5.0 mm pig skin.
Having confirmed the high efficiency of this process under
low energy wavelengths, we considered the application of this
current system for sunlight driven synthesis. The incorporation of
complementary dyes to maximize light harvesting has become an
established strategy in the development of dye-sensitized solar
cells; however, such a concept has not been applied to
photochemical syntheses.[15] We envisioned the use of a similar
strategy to perform chemical syntheses under broadband
emissions with one key difference; PCs with narrow but
complementary absorptions may be superposed in segregated
reaction vessels, allowing unused photons to pass through to the
Figure 4. (A) Schematic of layered independent reactions ordered by (top to
bottom) fac-Ir(ppy)3 (λmax = 390 nm), eosin Y (EY; λmax = 540 nm), AlPc (λmax
=
678 nm) and AlNc (λmax = 783 nm) irradiated by simulated sunlight (AM 1.5G).
(B) Comparison of simulated AM 1.5G emission (top) and the normalized
absorption spectra of layered PCs. (C) Kinetic comparison of
photopolymerizations mediated by the layered PCs.
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The principle of segregating the PCs affords the opportunity
to perform independent chemical syntheses using a single
emission source; four independent polymerization reactions were
stacked in the order of: (1) an Ir(ppy)3 mediated PET-RAFT
polymerization,[16] (2) an EY and triethylamine (TEA) mediated
PET-RAFT polymerization,[17] (3) AlPc and (4) AlNc mediated
photopolymerizations using red and NIR light respectively (Figure
4C, Table S8, Figure S11). These photopolymerizations,
mediated by different segments of the simulated solar emission
proceeded in a controlled manner in accord with our previous
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organic transformations as demonstrated by the successful
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[7]
[8]
thiol-ene[19]
)
and
2
polymerization reactions performed
simultaneously in segregated reaction vessels (Table S9, Figure
S12).
This contribution details a fast and highly efficient controlled
photopolymerization process regulated by far-red and NIR
wavelengths by
exploiting the
Q-bands of
model
metallophthalocyanines. The enhanced penetration capabilities
afforded by the low energy wavelengths, in conjunction with the
efficiency of this process, promoted unprecedented
polymerization rates (order of minutes) through synthetic and
biological (chicken and pig skin) barriers. The complementary
absorption characteristics of these PCs to existing catalysts
enables the layered superposition of multiple independent and
simultaneous reactions, providing a model for more optimized
usage of broadband solar emission that can also facilitate
miniaturization of photochemical reactors. Furthermore, the
variety of modifications available for further red-shifting the Q-
band of these PCs[20] is promising for the utilization of even longer
wavelengths in the NIR region and beyond.
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[13]
Keywords: Photopolymerization • NIR • Solar synthesis •
Reversible-deactivation radical polymerization • Photochemistry
[14]
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Entry for the Table of Contents
Communication
Zilong Wu, Kenward Jung* and
Cyrille Boyer*
Page 1 – Page 4
Effective utilization of NIR
wavelengths for photo-
controlled polymerization –
penetration through thick
barriers and parallel solar
syntheses
An efficient far-red and NIR light mediated RAFT photopolymerization utilizing phthalocyanines as a photosensitizer is
presented. The use of longer wavelengths permits polymerization by irradiating through barriers. Furthermore, long
wavelength photochemistry may be used in conjunction with existing photocatalysts to enable more optimized usage of
solar irradiation for chemical syntheses.
This article is protected by copyright. All rights reserved.