Organocatalyzed Atom Transfer Radical Polymerization Using N-Aryl Phenoxazines as Photoredox Catalysts

Article abstract of DOI:10.1021/jacs.6b08068

N-Aryl phenoxazines have been synthesized and introduced as strongly reducing metal-free photoredox catalysts in organocatalyzed atom transfer radical polymerization for the synthesis of well-defined polymers. Experiments confirmed quantum chemical predictions that, like their dihydrophenazine analogs, the photoexcited states of phenoxazine photoredox catalysts are strongly reducing and achieve superior performance when they possess charge transfer character. We compare phenoxazines to previously reported dihydrophenazines and phenothiazines as photoredox catalysts to gain insight into the performance of these catalysts and establish principles for catalyst design. A key finding reveals that maintenance of a planar conformation of the phenoxazine catalyst during the catalytic cycle encourages the synthesis of well-defined macromolecules. Using these principles, we realized a core substituted phenoxazine as a visible light photoredox catalyst that performed superior to UV-absorbing phenoxazines as well as previously reported organic photocatalysts in organocatalyzed atom transfer radical polymerization. Using this catalyst and irradiating with white LEDs resulted in the production of polymers with targeted molecular weights through achieving quantitative initiator efficiencies, which possess dispersities ranging from 1.13 to 1.31.

Full text of DOI:10.1021/jacs.6b08068

Article
pubs.acs.org/JACS
Organocatalyzed Atom Transfer Radical Polymerization Using N‑Aryl
Phenoxazines as Photoredox Catalysts
Ryan M. Pearson,†,∥ Chern-Hooi Lim,†,∥ Blaine G. McCarthy,† Charles B. Musgrave,†,‡,§
and Garret M. Miyake*,†,§
‡
§
†Department of Chemistry and Biochemistry, Department of Chemical and Biological Engineering, and Materials Science and
Engineering Program, University of Colorado Boulder, Boulder, Colorado 80309, United States
S
* Supporting Information
ABSTRACT: N-Aryl phenoxazines have been synthesized
and introduced as strongly reducing metal-free photoredox
catalysts in organocatalyzed atom transfer radical polymer-
ization for the synthesis of well-defined polymers. Experi-
ments confirmed quantum chemical predictions that, like their
dihydrophenazine analogs, the photoexcited states of phenox-
azine photoredox catalysts are strongly reducing and achieve
superior performance when they possess charge transfer
character. We compare phenoxazines to previously reported
dihydrophenazines and phenothiazines as photoredox catalysts
to gain insight into the performance of these catalysts and
establish principles for catalyst design. A key finding reveals that maintenance of a planar conformation of the phenoxazine
catalyst during the catalytic cycle encourages the synthesis of well-defined macromolecules. Using these principles, we realized a
core substituted phenoxazine as a visible light photoredox catalyst that performed superior to UV-absorbing phenoxazines as well
as previously reported organic photocatalysts in organocatalyzed atom transfer radical polymerization. Using this catalyst and
irradiating with white LEDs resulted in the production of polymers with targeted molecular weights through achieving quan-
titative initiator efficiencies, which possess dispersities ranging from 1.13 to 1.31.
catalysts (PCs) do not possess the reducing power to directly
reduce an alkyl bromide through an outer sphere electron
transfer mechanism. Commonly, supplemental sacrificial elec-
tron donors are required for alkyl bromide reduction through a
reductive quenching pathway. The addition of sacrificial
electron donors, however, introduces potential side-reactions7
that impede the ability to synthesize polymers with low Đ.8
Select strongly reducing iridium9 or copper10 PCs can directly
reduce an alkyl bromide through an oxidative quenching path-
way, and elimination of the sacrificial electron donor can
facilitate the synthesis of well-defined polymers.11 Light medi-
ated CRPs further introduce spatial and temporal control as an
attractive interactive feature for the incorporation of added
synthetic complexity.12 However, the concerns of metal con-
tamination and the sustainability of iridium or ruthenium metal
PCs motivate the use of organic PCs.14,13
In accord with transition metal PCs, few organic PCs are able
to directly reduce an alkyl bromide without the addition of a
sacrificial electron donor.14 Strongly reducing organic catalysts,
including perylene,15 N-aryl phenothiazines,16 and N,N-diaryl
dihydrophenazines17 have been demonstrated as organic PCs
capable of mediating O-ATRP (Figure 1). Continued progress
in this field is required to further understand the mechanism of
INTRODUCTION
■
Atom transfer radical polymerization (ATRP) is the most
used controlled radical polymerization (CRP) for the synthesis
of polymers with controlled molecular weight (MW), disper-
sity (Đ), architecture, and composition.1 Traditionally, metal
catalysts have been employed to mediate the equilibrium
between an alkyl halide and a carbon centered radical, produced
by reduction of the halide, and deter bimolecular termination
pathways.2 Concerns about metal contamination of polymers
intended for biomedical or electronic applications have moti-
vated efforts to lower metal catalyst loadings and enhance
purification methods.3 Although CRPs exist that are mediated
by organic catalysts and which thus entirely circumvent the
issue of metal contamination,4 organic catalysts capable of
mediating an organocatalyzed ATRP (O-ATRP) are limited
because of the required significant reducing power required to
reduce alkyl bromides commonly used for ATRP (∼ −0.6 to
−0.8 V vs SCE).5
Photoredox catalysis presents a strategy to drive chemical
transformations under mild conditions through the generation
of reactive open-shell catalysts via photoexcitation.6 Recently,
work in this field has heavily focused on polypyridal ruthenium
and iridium complexes because such metal complexes efficiently
absorb visible light, possess sufficiently long excited state
lifetimes as a result of metal to ligand charge transfer (MLCT),
and have tunable redox properties. However, most photoredox
Received: August 3, 2016
© XXXX American Chemical Society
A
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this polymerization to realize even more efficient PCs and
access a broader application landscape.
A proposed general photoredox O-ATRP mechanism involves
photoexcitation of the PC to an excited state PC (PC*) which is
capable of reducing alkyl bromides via an oxidative quenching
pathway to generate the active radical for polymerization
propagation, while yielding the radical cation PC (PC•+) and
Br− ion pair complex, PC•+Br− (Figure 1C). Efficient
deactivation is central to the production of well-defined
polymers. Deactivation requires the PC•+Br− complex to be
sufficiently oxidizing relative to the propagating radical to
regenerate the alkyl bromide and ground state PC; subsequent
photoexcitation of the PC reinitiates the catalytic cycle. Here, we
report N-aryl phenoxazines as a new class of PCs for O-ATRP
which produce well-defined polymers with low dispersities.
Through following our maturing catalyst design principles, we
report a visible light phenoxazine PC that produces polymers
with Đ ranging from 1.13 to 1.31 over a range of polymer MWs
while achieving quantitative initiator efficiency (I*).
To accelerate our progress in developing O-ATRP, we
previously used quantum chemical calculations to guide the
discovery and design of strongly reducing diaryl dihydrophe-
nazines PCs for O-ATRP.17 We based our computationally
directed strategy on the hypothesis that photoexcitation of
the PC delivers, through intersystem crossing (ISC) from the
singlet excited state PC (1PC*), a triplet excited state PC
(3PC*) which is responsible for the alkyl bromide reduction.
This hypothesis hinges on the necessity of the photoexcited
species to possess a sufficiently long lifetime for photoredox
catalysis.
Figure 1. (A) O-ATRP mediated by organic PCs using alkyl bromide
initiators and aryl phenoxazines studied in this work. (B) Organic
PCs examined in previous work. (C) Proposed, general photoredox
catalytic cycle of O-ATRP.
Our continued work in this field has been piqued by the
impressively strong excited state reduction potentials (E0* =
E0(2PC•+/PC*)) of N,N-diaryl dihydrophenazines and N-aryl
Figure 2. Geometric reorganization energies and reduction potentials (vs SCE) for 10-phenylphenoxazine, diphenyl dihydrophenazine, and 10-phenylphenothiazine
(bottom) transitioning from the 3PC* to 2PC•+ to 1PC species involved in the proposed mechanism for photoredox O-ATRP. Reduction potentials were
computed here with the improved 6-311+G** basis set compared to 6-31+G** used in the previous report.17 See Supporting Information for further details.
B
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Figure 3. (A) N-Aryl phenoxazines studied in this work along with computed triplet state reduction potentials. (B) Computed triplet state SOMOs
of phenoxazine derivatives.
Table 1. Results of the O-ATRP of MMA Using PCs
1 through 4
the propagating radical (e.g., ∼ −0.8 V vs SCE for methyl
methacrylate), as required for a successful O-ATRP. Lastly,
reports on the photophysical properties of phenoxazines sug-
gested their promise as PCs; the phosphorescence quantum
yield of 10-phenylphenoxazine (1) at 77 K was reported to be
94% with a lifetime as long as 2.3 s.22 These properties
highlight the efficient ISC to the triplet manifold and slow non-
radiative decay attributed to small Franck−Condon vibrational
a
run no. PC conv (%) Mw (kDa) Mn (kDa) dispersity (Đ) I* (%)
1
2
3
4
1
2
3
4
95.6
55.3
78.8
80.2
10.6
9.5
7.2
6.5
1.48
1.45
1.22
1.11
137
85.5
10.8
11.9
8.8
92.6
77.3
10.8
a
3
[MMA]:[DBMM]:[PC] = [1000]:[10]:[1]; 9.35 μmol PC, 1.00 mL
overlap factors between PC* and the ground state.
dimethylacetamide, and irradiated with a 54 W 365 nm light source
for 8 h.
Our analysis of exchanging the sulfur in phenothiazines with
the oxygen in phenoxazines identified several distinct phenom-
ena that alter the physical properties of these molecules and
which we propose manifest in improvements in PC perform-
ance for O-ATRP, qualitatively assessed through analysis of the
polymer product. The significant distinction between these two
systems is the conformation of their heterocyclic rings. The
smaller van der Waals radius of oxygen (1.52 Å) relative to
sulfur (1.80 Å) permits the ground state phenoxazine (e.g.,
PC 1) to access a planar geometry similarly to dihydrophena-
zines (nitrogen, 1.55, Å). In contrast, phenothiazine adopts a
bent boat conformation in its ground state, observable in crystal
structures23 and predicted by our computations (Figure 2).
phenothiazines (∼ −2 V vs SCE), which are even more
reducing than commonly used metal PCs, such as fac-Ir(ppy)3
(−1.73 V vs SCE).18 These strong E0*s and the success of
the diaryl dihydrophenazines in O-ATRP further drew our
attention toward N-aryl phenoxazines as a potential class of
organic PCs for O-ATRP. We hypothesized that phenoxazines
possessed characteristic traits that would distinguish them as
organic PCs and make them successful catalysts for O-ATRP.
Interestingly, these N, S, and O containing heterocycles are
found in biologically relevant molecules19 and organic elec-
tronic applications.20
2
However, upon oxidation to the radical cation state PC•+, all
three PCs adopt a planar conformation.
RESULTS AND DISCUSSION
The consequences of phenothiazine adopting bent con-
formations in the ground and triplet states, but a planar
geometry in the radical cation state, introduce larger structural
reorganizations during electron transfer (ET) as compared to
the consistently planar phenoxazines and dihydrophenazines.
We calculated a structural reorganization penalty associated
with oxidation of the bent 10-phenylphenothiazine triplet
state to the planar radical cation of 8.2 kcal/mol. In contrast,
the triplet and radical cation states of 1 are both planar,
analogous to diaryl dihydrophenazines, which results in a lower
■
DFT calculations predict that N-aryl phenoxazines possess
similarly strong E0*s (∼ −2 V vs SCE) in their lowest lying
triplet excited state as dihydrophenazines and phenothiazines,
which was corroborated experimentally.21 Although dihydro-
phenazines are stronger excited state reductants, the radical
cations of phenoxazines and phenothiazines [E0(2PC•+/PC) =
∼0.5 V vs SCE] are more oxidizing than those of dihydro-
phenazines [E0(2PC•+/PC) = ∼0.0 V vs SCE]; all three classes
of PCs possess an oxidation potential capable of deactivating
C
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a
Table 2. Results of the O-ATRP of MMA Using PCs 3 and 4
run no.
PC
[MMA]:[DBMM]:[PC]
conv (%)
Mw (kDa)
Mn (kDa)
dispersity (Đ)
I* (%)
5
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
[500]:[10]:[1]
[1000]:[10]:[1]
[1500]:[10]:[1]
[2000]:[10]:[1]
[2500]:[10]:[1]
[1000]:[5]:[1]
[1000]:[15]:[1]
[1000]:[20]:[1]
[500]:[10]:[1]
[1000]:[10]:[1]
[1500]:[10]:[1]
[2000]:[10]:[1]
[2500]:[10]:[1]
[1000]:[5]:[1]
[1000]:[15]:[1]
[1000]:[20]:[1]
80.8
78.8
72.2
76.5
78.4
74.6
74.5
80.7
85.1
80.2
68.9
58.2
65.9
70.5
70.9
76.1
5.8
10.8
11.4
18.4
25.9
26.4
8.3
4.9
8.8
1.16
1.22
1.19
1.26
1.31
1.38
1.20
1.19
1.09
1.11
1.25
1.17
1.23
1.35
1.12
1.07
86.1
92.6
116
6
7
9.5
8
14.6
19.8
19.1
6.9
107
9
101
10
11
12
13
14
15
16
17
18
19
20
79.6
75.7
92.9
84.1
77.3
109
5.5
4.6
5.9
5.4
11.9
12.2
14.7
21.2
22.3
9.3
10.7
9.8
112.5
17.3
16.8
8.3
95.2
96.6
85.3
60.6
64.0
6.8
6.1
a
See the Supporting Information for experimental details.
reorganization energy of only 2.4 kcal/mol. As phenoxazine,
dihydrophenazine, and phenothiazine derivatives, possess similar
E0*s (−2.11, −2.25, and −2.03 V, respectively), we expect a
kinetically faster activation (reduction of the alkyl bromide)
in O-ATRP by phenoxazines and dihydrophenazines because of
their lower reorganization energies for ET.
with 1- or 2-naphthalene yielded molecules with spatially
separated SOMOs and thus predicted intramolecular charge
transfer from the heterocyclic ring to the naphthalene sub-
stituent upon photoexcitation and subsequent intersystem
crossing to the triplet state.
All four phenoxazine derivatives were synthesized through
C−N cross-couplings from commercially available reagents
and employed in the polymerization of MMA.21 A screen of
common ATRP alkyl bromide initiators revealed that diethyl
2-bromo-2-methylmalonate (DBMM) served as the superior
initiator to produce polymers with the lowest Đ while achieving
the highest I* (Table S4). To evaluate the PCs, polymer-
izations using DBMM as the initiator were conducted in
dimethylacetamide and irradiated with a 365 nm UV nail curing
lamp (54 W) (Table 1). In accord with diaryl dihydrophena-
zines, N-aryl phenoxazines possessing localized SOMOs (PCs 1
and 2) did not perform as well as the PCs with separated
SOMOs (PCs 3 and 4). Specifically, 1 and 2 produced
poly(methyl methacrylate) (PMMA) with a relatively high Đ
of 1.48 and 1.45, respectively (runs 1 and 2). Polymerization
results with PCs 3 and 4 were superior, and produced PMMA
with lower dispersities (Đ = 1.22 and 1.11, respectively) while
achieving high I*s of 92.6 and 77.3%, respectively (runs 3 and 4).
Further, molecular weight control could be obtained using
either PC through modulation of the monomer (runs 5 to 9 for
PC 3; runs 13 to 17 for PC 4) or initiator (runs 10 to 12 for PC
3; runs 18 to 20 for PC 4) ratios (Table 2). Overall, PC 3
produced PMMA through higher I* (∼80−100%) while PC 4
produced PMMA with lower Đ (as low as 1.07). This
1-naphthalene versus 2-naphthalene substitution effect influenc-
ing high I* or low Đ, respectively was also observed with diaryl
dihydrophenazines.
Polymerization deactivation involves reduction of the planar
phenylphenothiazine radical cation to regenerate the bent
ground state. We calculate a reorganization energy for this ET
of 4.1 kcal/mol. For 1 or diphenyl dihydrophenazine, the same
reduction process requires lower reorganization energies of 2.3
or 2.5 kcal/mol, respectively consistent with the conservation of
the planarity of the cation radical and ground states. Given the
similar ground state oxidation potentials for the phenoxazine
and phenothiazine (0.58 and 0.49 V), the radical cation of 1 is
likely kinetically faster in deactivation, which imparts better
control in O-ATRP (vide infra). How this concept pertains
2
to the less oxidizing dihydrophenazine PC•+ requires further
investigation, although previous results demonstrated that
dihydrophenazines are efficient PCs for O-ATRP.17
Toward the goal of designing phenoxazines as PCs for
O-ATRP we applied the concepts conceived from our previous
study of diaryl dihydrophenazines, which revealed that PCs
with spatially separated singly occupied molecular orbitals
(SOMOs) in their 3PC* state yielded PCs with superior
performance in O-ATRP in regards to achieving the highest I*
and producing polymers with the lowest Đ. As such, we inves-
tigated strongly reducing N-aryl phenoxazines with spatially
separated SOMOs (with the lower lying SOMO localized on
the phenoxazine core and the higher lying SOMO localized on
the aryl substituent) and localized SOMOs (with both SOMOs
localized on the phenoxazine core), to evaluate their perform-
ance as O-ATRP PCs and determine if this concept extends to
phenoxazines (Figure 3).
In the cases of diphenyl dihydrophenazine and 1, we calcul-
ate that neither exhibits spatially separated SOMOs. In contrast,
incorporation of electron withdrawing trifluoromethyl function-
alization on the para position of the N-phenyl substituents of
the dihydrophenazine yielded spatially separated SOMOs
whereas this substitution on phenoxazine (2) results in both
SOMOs localized on the phenoxazine core. However, for both
dihydrophenazines and phenoxazines, N-aryl functionalization(s)
Our analysis of the polymerization of MMA by 3 and 4
showed that both PCs imparted control over the polymer-
ization that is becoming expected from O-ATRP. Specifically, a
linear growth in polymer molecular weight as well as a low
dispersity during the course of polymerization was attained
(Figure 4A and B). Additionally, temporal control was demon-
strated using a pulsed irradiation sequence (Figure 4C−F).
Monomer conversion was only observed during irradiation,
which resulted in a linear increase in number-average MW (Mn)
while producing PMMA with low Đ.
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Figure 4. Plots of molecular weight (Mn, blue) and dispersity (Đ, orange) as a function of monomer conversion for the polymerization of MMA
catalyzed by 3 (A) and 4 (B). Plots of conversion vs time using 3 (C) or 4 (E) (irradiation in white and dark periods in gray) and plots of molecular
weight (Mn, blue) and dispersity (Đ, orange) as a function of MMA conversion using a pulsed-irradiation sequence and PC 3 (D) or 4 (F) (filled
markers are data directly after irradiation while open markers are data directly after the dark period) Conditions for all plots: [MMA]:[DBMM]:
[PC] = [1000]:[10]:[1]; 9.35 μmol PC, 1.00 mL dimethylacetamide, and irradiated with UV-light.
Both PCs also efficiently polymerized other methacrylates,
including benzyl methacrylate (BnMA), isobutyl methacrylate
(BMA), and isododecyl methacrylate (IDMA) (Table S2). As
such, 3 was used to perform chain extension polymerizations
from an isolated PMMA (Mw = 9.9 kDa, Đ = 1.12) macro-
initiator because the ATRP mechanism inherently reinstalls the
bromine chain end group onto the growing polymer chain
(Figure 5). Chain extensions from this PMMA macroinitiator
with MMA, DMA, BnMA, and BMA were successful, both
confirming high bromine chain end group fidelity and allowing
the synthesis of block polymers.
To further establish these naphthalene phenoxazines as
efficient PCs, we next directly compared 3 and 1-naphthalene-
10-phenothiazine as PCs for O-ATRP under our polymer-
ization conditions (Figure S19). Both catalysts exhibited nearly
identical rates of polymerization, achieving 85.1% and 88.4%
monomer conversion after 10 h for 3 and the phenothiazine,
respectively. Additionally, both PCs achieved high I*s of 93.5%
and 95.6%, respectively. However, a significant difference in
polymerization performance was observed when comparing
the Đ of the resulting PMMA. When using 3, PMMA was
produced with Đ = 1.26, while the phenothiazine produced
PMMA with comparatively higher Đ = 1.66, consistent with
previous reports16a,b using this PC.
As inferred above, the higher Đ of the PMMA produced by
the phenothiazine is attributed to the larger reorganization
energies of the phenothiazines. Incorporation of O versus S in
the core of phenoxazines versus the core of phenothiazines
imparts distinct quantitative differences in the electronic
and geometric structures of these molecules that affect their
performance as PCs for O-ATRP. As such, the planarity of
phenoxazines throughout the photoexcitation and ET processes
E
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Figure 5. Chain extension polymerizations from a PMMA macroinitiator (A) with MMA (B), IDMA (C), BMA (D), and BnMA (E). Gel
permeation chromatography traces of the polymers depicted by the chemical structures with corresponding color schemes (F).
causes them to perform more closely to diaryl dihydrophena-
zines as PCs for O-ATRP. We hypothesize that the differences
between these PCs specifically manifest in each of their abilities
to balance the rates of activation and deactivation which results
in the differences observed in the Đ of the resulting PMMA
produced by each PC.
An additional consideration when comparing phenoxazines,
dihydrophenazines, and phenothiazines is that the planar core
of phenoxazines and dihydrophenazines promotes intra-
molecular charge transfer to charge separated SOMOs while
the bent phenothiazine core limits electronic coupling between
the heterocyclic ring and the N-aryl substituent and conse-
Figure 6. X-ray crystal structures of 1-naphthalene substituted planar
quently the ability to form an intramolecular charge transfer
phenoxazine (A) and bent phenothiazine (B). Hydrogen atoms
complex.24 The planar phenoxazine core versus the bent
phenothiazine core can be visualized in the X-ray crystal
Figure 7. ESP mapped electron density of 1PC and 3PC* of 1-naphthalene substituted phenoxazine (A), dihydrophenazine (B), and phenothiazine (C).
F
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3
Figure 8. Properties of PC 5. (A) Structure, computed triplet excited state reduction potential, and ESP mapped electron density of PC* 5. (B)
Computed triplet state SOMOs of PC 5. (C) Plot of Mn and Đ as a function of monomer conversion for the polymerization of MMA by PC 5;
[MMA]:[DBMM]:[5] = [1000]:[10]:[1]; 9.35 μmol PC, 1.00 mL dimethylacetamide, and irradiated with white LEDs. (D) UV−vis spectrum of PC
5 and 1-naphthalene functionalized phenoxazine, dihydrophenazine, and phenothiazine, with color coded structures, and extinction coefficients at
their respective λmax with the visible absorbance spectrum highlighted in white.
a
Table 3. Results of the O-ATRP of MMA Using PC 5
run no.
[MMA]:[DBMM]:[5]
conv (%)
Mw (kDa)
Mn (kDa)
Dispersity (Đ)
I* (%)
21
22
23
24
25
26
27
[500]:[10]:[1]
[1500]:[10]:[1]
[2000]:[10]:[1]
[2500]:[10]:[1]
[1000]:[5]:[1]
[1000]:[15]:[1]
[1000]:[20]:[1]
67.2
75.2
90.9
87.5
89.9
73.8
72.1
4.07
13.7
22.9
27.5
23.0
6.17
4.52
3.64
11.8
17.5
21.3
18.1
5.31
3.76
1.13
1.16
1.31
1.29
1.27
1.16
1.20
99.4
98.0
105
104
101
97.5
103
a
See the Supporting Information for experimental details.
structures of the PCs (Figure 6). The electrostatic potential
(ESP) mapped electron density of the PC* state of these
substituted phenoxazine derivative. Computations predicted
that PC 5, possessing 4-biphenyl core substitutions, would be
an excellent target PC with 3PC* possessing a strong reduction
3
compounds reveal that electron density is transferred to the
naphthalene substituent (red region) in phenoxazine upon
photoexcitation and ISC from 1PC, even more so with
dihydrophenazines, while electron density remains localized
on the phenothiazine core (Figure 7).
We further envisaged that a visible light absorbing pheno-
xazine derivative would provide an even more efficient poly-
merization catalyst, as irradiation of the reaction with high
energy UV-light can initiate nondesirable reaction pathways,
which may increase the Đ of the produced polymer and lower
I*.25 To realize a visible light absorbing PC we explored a core
1
potential and spatially separated SOMOs, while PC would
exhibit an absorbance profile in the visible spectrum. The
visible light absorbing PC 5 was synthesized in high yield from
PC 3 through selective bromination at the 3- and 7-positions
on the phenoxazine core using N-bromosuccinimide followed
by Suzuki cross-coupling.21 A similar synthetic strategy was
recently reported to synthesize thiophene core substituted
phenothiazines for use as visible light absorbing catalysts for
cationic polymerization.26 The absorbance profile of PC 5 was
not only red-shifted (Δλmax = 65 nm versus noncore substituted
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PC 3) into the visible spectrum (λmax = 388 nm), but also
exhibited an extremely enhanced molar extinction coefficient
(ε = 26635 M−1cm−1 at λmax = 388 nm), making it significantly
more efficient at absorbing visible light than the noncore
substituted 1-napthalene functionalized phenoxazine, dihydro-
phenazine, or phenothiazine (Figure 8).
The polymerization performance of PC 5 confirmed our
predictions that it would be an excellent PC for O-ATRP,
demonstrating superior control over the polymerization than
the UV-absorbing phenoxazines or even previously reported
dihydrophenazines. The polymerization of MMA using PC 5
irradiated by white LEDs was efficient and showcased charac-
teristics of a controlled polymerization with a linear increase
in polymer Mn and a low polymer Đ during the course of
polymerization (Figure 8C). Furthermore, the molecular
weight of the polymer could be tailored through manipulation
of either the monomer or initiator loading, while keeping the
polymerization otherwise constant, to produce polymers with Đ
of 1.13−1.31 while achieving quantitative I* (Table 3).
Research Fund (56501-DNI7). Research reported in this
publication was supported by the National Institute of General
Medical Sciences of the National Institutes of Health under
Award Number R35GM119702. The content is solely the
responsibility of the authors and does not necessarily represent
the official views of the National Institutes of Health. BGM is
supported by the University of Colorado Department of
Chemistry and Biochemistry Marian Sharrah Fellowship. CBM
was supported by NSF grant CHE-1214131. We gratefully
acknowledge the use of XSEDE supercomputing resources
(NSF ACI-1053575). The authors would like to thank Steven
Satur (CU Boulder) for technical assistance.
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CONCLUSIONS
■
N-Aryl phenoxazines have proven to be efficient PCs for O-
ATRP that produce polymers with controlled molecular
weights and low dispersity. Through the culmination of com-
putational and experimental results, we report a visible light
absorbing phenoxazine photoredox catalyst that produces
polymers with controlled molecular weights and low dis-
persities, achieving quantitative initiator efficiencies that out-
compete previously reported organic PCs for O-ATRP. The
continued establishment of design principles for PCs capable of
mediating O-ATRP will further expand the scope and impact
of this polymerization methodology, which we foresee will
translate to an additional means for selective small molecule
transformations. Our future work will investigate the intricacies of
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b08068.
■
S
Materials and methods; characterization of photoredox
properties; computational details; polymerization data;
X-ray crystallography data; coordinates of molecular
structures (PDF)
Crystal data for 1-naphthalene phenothiazine (CIF)
Crystal data for 1-naphthalene phenoxazine (CIF)
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AUTHOR INFORMATION
Corresponding Author
*garret.miyake@colorado.edu
■
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Author Contributions
∥R.M.P. and C.-H.L. contributed equally.
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the University of Colorado
Boulder, the Advanced Research Projects Agency-Energy (DE-
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