Structure-property relationships for tailoring phenoxazines as reducing photoredox catalysts

Article abstract of DOI:10.1021/jacs.7b12074

Through the study of structure-property relationships using a combination of experimental and computational analyses, a number of phenoxazine derivatives have been developed as visible light absorbing, organic photoredox catalysts (PCs) with excited state reduction potentials rivaling those of highly reducing transition metal PCs. Time-dependent density functional theory (TD-DFT) computational modeling of the photoexcitation of N-aryl and core modified phenoxazines guided the design of PCs with absorption profiles in the visible regime. In accordance with our previous work with N,N-diaryl dihydrophenazines, characterization of noncore modified N-aryl phenoxazines in the excited state demonstrated that the nature of the N-aryl substituent dictates the ability of the PC to access a charge transfer excited state. However, our current analysis of core modified phenoxazines revealed that these molecules can access a different type of CT excited state which we posit involves a core substituent as the electron acceptor. Modification of the core of phenoxazine derivatives with electron-donating and electron-withdrawing substituents was used to alter triplet energies, excited state reduction potentials, and oxidation potentials of the phenoxazine derivatives. The catalytic activity of these molecules was explored using organocatalyzed atom transfer radical polymerization (O-ATRP) for the synthesis of poly(methyl methacrylate) (PMMA) using white light irradiation. All of the derivatives were determined to be suitable PCs for O-ATRP as indicated by a linear growth of polymer molecular weight as a function of monomer conversion and the ability to synthesize PMMA with moderate to low dispersity (dispersity less than or equal to 1.5) and initiator efficiencies typically greater than 70% at high conversions. However, only PCs that exhibit strong absorption of visible light and strong triplet excited state reduction potentials maintain control over the polymerization during the entire course of the reaction. The structure-property relationships established here will enable the application of these organic PCs for O-ATRP and other photoredox-catalyzed small molecule and polymer syntheses.

Full text of DOI:10.1021/jacs.7b12074

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE
Structure-Property Relationships for Tailoring
Phenoxazines as Reducing Photoredox Catalysts
Blaine McCarthy, Ryan Pearson, Chern-Hooi Lim, Steven
M. Sartor, Niels H. Damrauer, and GARRET M. MIYAKE
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12074 • Publication Date (Web): 07 Mar 2018
Downloaded from http://pubs.acs.org on March 7, 2018
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 39
Journal of the American Chemical Society
1
2
3
4
5
Structure-Property Relationships for Tailoring Phenoxazines as Reducing
Photoredox Catalysts
6
7
8
9
Blaine McCarthya,b, Ryan Pearsona,b, Chern-Hooi Lima,b, Steven Sartor b, Niels Damrauer b,
Garret M. Miyakea,b,c
aDepartment of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
b
c
States of America. Department of Chemistry and Biochemistry, Materials Science and
Engineering, University of Colorado, Boulder, Colorado, 80309-0215, United States of America
*Corresponding author, E-mail: garret.miyake@colostate.edu
Abstract
Through the study of structure-property relationships using a combination of experimental
and computational analyses, a number of phenoxazine derivatives have been developed as visible
light-absorbing, organic photoredox catalysts (PCs) with excited state reduction potentials rivaling
those of highly reducing transition metal PCs. Time-dependent density functional theory (TD-
DFT) computational modeling of the photoexcitation of N-aryl and core modified phenoxazines
guided the design of PCs with absorption profiles in the visible regime. In accordance with our
previous work with N,N-diaryl dihydrophenazines, characterization of non-core modified N-aryl
phenoxazines in the excited state demonstrated that the nature of the N-aryl substituent dictates
the ability of the PC to access a charge transfer excited state. However, our current analysis of core
modified phenoxazines revealed that these molecules can access a different type of CT excited
state which we posit involves a core substituent as the electron acceptor. Modification of the core
of phenoxazine derivatives with electron donating and electron withdrawing substituents was used
to alter triplet energies, excited state reduction potentials, and oxidation potentials of the
phenoxazine derivatives. The catalytic activity of these molecules was explored using
organocatalyzed atom transfer radical polymerization (O-ATRP) for the synthesis of poly(methyl
methacrylate) (PMMA) using white light irradiation. All of the derivatives were determined to be
1
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 39
1
2
3
4
5
6
7
8
9
suitable PCs for O-ATRP as indicated by a linear growth of polymer molecular weight as a
function of monomer conversion and the ability to synthesize PMMA with moderate to low
dispersity (dispersity less than or equal to 1.5) and initiator efficiencies typically greater than 70%
at high conversions. However, only PCs that exhibit strong absorption of visible light and strong
triplet excited state reduction potentials maintain control over the polymerization at low monomer
conversions. The structure-property relationships established here will enable the application of
these organic PCs for O-ATRP and other photoredox-catalyzed small molecule and polymer
syntheses.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Introduction
Increased interest in photoredox catalysis for small molecule and macromolecular
synthesis during the last decade has led to the development of new catalytic transformations using
mild conditions.1 Critical to these advancements has been the use of ruthenium complexes—which
were first demonstrated as photoredox catalysts (PCs) several decades ago2—and iridium
complexes, both of which are capable of absorbing visible light to initiate electron or energy
transfer reactions from their reactive photoexcited states. The impressive performance of these
catalysts arises from their redox stability and photophysical properties including visible light
absorption and formation of excited states that can engage in both reductive and oxidative electron
transfers.1,3 Despite the success of ruthenium and iridium-containing PCs, the use of these
transition metal complexes in catalysis has some limitations. For example, the use of ruthenium
and iridium-containing PCs in polymer synthesis can limit the application scope for the materials
synthesized since these complexes can be challenging to remove from the polymer matrix leading
to contamination of the final polymer product. Polymeric materials contaminated with transition
metals may not be suitable for biomedical or electronic applications which limits the broad use of
2
ACS Paragon Plus Environment

Page 3 of 39
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
these synthetic methods. As such, the development of organic PCs is desirable.4 While organic
PCs that are strong excited state reductants have recently been reported,5 it is imperative to
establish the structure-property relationships and molecular design principles that govern the
photophysical and catalytic capabilities of these organic molecules to develop new PCs, expand
their use in photoredox catalysis, and enable the development of new transformations.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Our work in the field of photoredox catalysis originated with our interest in
organocatalyzed atom transfer radical polymerization (O-ATRP), a method to synthesize well-
defined polymers using organic PCs activated by UV or visible light. The O-ATRP method
originated with the application of UV light-absorbing N-phenyl phenothiazine as an organic PC
for the polymerization of vinyl monomers in 2014.5b In this system, N-phenyl phenothiazine is
proposed to access a highly reducing excited state and operate via an oxidative quenching pathway
analogously to previously used iridium catalyzed ATRP systems.6 We have investigated
perylene,5e N,N-diaryl dihydrophenazines,5f and N-aryl phenoxazines5 as PCs for O-ATRP.
Perylene was the first example of a visible light-absorbing PC for O-ATRP, but it is less efficient
compared to these other PC families. N,N-diaryl dihydrophenazines absorb visible light to access
highly reducing excited states with computationally predicted triplet excited state reduction
potentials (E0*T1,calc[2PC•+/3PC*]) that can exceed -2.0 V vs SCE. Furthermore, it has been shown
that N,N-diaryl dihydrophenazines bearing electron poor or highly conjugated N-aryl substituents
access charge transfer (CT) excited states, a property that is proposed to engender superior
performance compared to related systems that do not access such states.7 The CT excited states of
these molecules are analogous to the metal to ligand charge transfer (MLCT) excited states of
transition metal complexes. Similar to how transition metal complexes transfer electron density
from an electron-rich metal center to an electron-deficient ligand to access MLCT states, N,N-
3
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 39
1
2
3
4
5
6
7
8
9
diaryl dihydrophenazines transfer electron density from the electron-rich tricyclic phenazine core
to one or both N-aryl “ligand(s)” to access intramolecular CT excited states if the N-aryl
substituents are sufficient electron acceptors (exhibit a low-lying π*). Following this work, we
explored structurally similar N-aryl phenoxazines, which typically absorb UVA light to access
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
excited states with similar reduction potentials to those of N,N-diaryl dihydrophenazines (E0*
T1,calc
~ -1.8 to -2.0 V vs SCE). Computational modeling of N-aryl phenoxazines predicted that
derivatives bearing N-naphthyl substituents possess spatially separated singularly occupied
molecular orbitals (SOMOs) in the triplet excited state, a feature that is characteristic of CT
species.7 These N-naphthyl phenoxazine derivatives were found to be effective PCs for O-ATRP.
Despite the excellent performance of N-naphthyl phenoxazine PCs in O-ATRP, we aimed
to design visible light-absorbing derivatives because UV light can initiate undesirable side
reactions in ATRP.8 We reasoned that visible light-absorbing phenoxazines could be realized
through extending the conjugation on the phenoxazine core via installation of biphenyl core
substituents.5g For organic light emitting diodes (OLEDs), core modified phenoxazines were
reported for use as donor-acceptor molecules.9 Guided by this precedence for synthetic
modification of the phenoxazine core, we synthesized visible light-absorbing PC 1 via installation
of biphenyl groups at the 3- and 7- positions of the phenoxazine core of 2 (Figure 1A). Indeed, PC
1 absorbs visible light and catalyzes the O-ATRP of methacrylate monomers to synthesize
polymers of target MWs with improved control compared to the non-core modified parent
compound (2). In addition, PC 1 exhibits robust catalytic performance for O-ATRP carried out in
flow reactors10 and in varied reaction irradiation conditions.11 More recently, we demonstrated that
PC 1 was also effective in catalyzing small molecule transformations such as atom transfer radical
additions, substitution trifluoromethylations, and dual photoredox/nickel catalyzed C-N and C-S
4
ACS Paragon Plus Environment

Page 5 of 39
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
cross-coupling reactions that were previously exclusive to transition metal complexes.12 Given that
the phenoxazine core structure can be synthetically modified and that 1 exhibits excellent catalytic
performance for numerous transformations, we envisioned a versatile scaffold for the development
of superior organic PCs using core modified N-aryl phenoxazines.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
While our previous work with PC 1 demonstrated the ability to alter the absorption properties
of N-aryl phenoxazines, broadening the reaction scope and increasing the selectivity of reactions
catalyzed by these molecules may require further alteration of their redox properties. In photoredox
catalysis employing ruthenium and iridium PCs, alteration of the redox properties of these catalysts
has been achieved by changing the electronics of the ligand set.1,13 Analogous to transition metal
PCs, we hypothesized that alteration of the electronics of the phenoxazine scaffold via
modification of the N-aryl and core substituent(s) would enable the redox properties of the parent
catalyst to be tuned. In this work, we investigate this hypothesis as well as characterize the
absorption properties and ability of core modified phenoxazines to access charge transfer excited
states. We experimentally and computationally characterize the properties of four previously
reported phenoxazines (PCs 1-3 and 6) and fifteen new N-aryl core modified phenoxazines (PCs
4, 5 and 7-19) to identify structure-property relationships for tailoring organic PC properties
(Figure 2). In addition, these new phenoxazine derivatives are employed as PCs for photoredox-
catalyzed polymer synthesis via O-ATRP to demonstrate their catalytic activity and to understand
how differences in PC properties manifest in differences in catalytic performance in O-ATRP. To
the best of our knowledge, this is the first report that characterizes the structure-property
relationships of phenoxazine derivatives in the context of photoredox catalysis, and we envision
that the design principles established herein will encourage the discovery of photoredox-catalyzed
transformations using these PCs.
5
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 39
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. (A) Our previous work with phenoxazine PCs focused on non-core modified PCs and
core modified PC 1. (B) Proposed general photoredox cycle that proceeds via an oxidative
quenching pathway. 1PC: ground state PC; PC*: excited state PC, which can be either in the singlet
(1PC*) or triplet (3PC*) excited state; 2PC•+: radical cation of the photoredox catalyst.
Results and Discussions
Desirable PC features can be identified with guidance from the photophysical and redox
processes involved in a photoredox catalyzed oxidative quenching cycle (Figure 1B). PCs that
absorb visible light are preferred over ultraviolet (UV) light absorbers as UV light can cause
undesirable side reactions. Moreover, PCs that possess high molar extinction coefficients (ε)
absorb light more efficiently which may increase the population of the reactive excited state
catalyst. In general, upon light absorption the PC is promoted to a singlet excited state (Sn, n ≥ 1),
whereupon, in accordance with Kasha’s rule, it quickly relaxes via internal conversion (IC) to S1.
From S1, triplet states (Tn, n ≥ 1) can be accessed via intersystem crossing (ISC). Similar to the
singlet manifold, higher energy triplets (Tn) relax via IC to the lowest energy T1. Electron transfer
6
ACS Paragon Plus Environment

Page 7 of 39
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
processes pertinent to photoredox catalysis can occur from either the S1 or T1 state; however, the
longer-lived T1 state is typically invoked to be responsible for photoredox catalysis when its
initiation requires bimolecular collision events.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The ability to access an excited state with CT character in which electron density has been
shifted from a “donor” moiety to an “acceptor” moiety in the molecule has been identified in both
transition metal and organic PCs (Figure 1B). Polypyridyl ruthenium and iridium complexes are
known to access metal to ligand charge transfer (MLCT) T1 excited states where energy wasting
charge recombination is slowed, leading to improved catalytic efficiency.2,14 Analogous to the
MLCT states of transition metal complexes, several of the organic molecules that our group7 and
others15 have investigated are capable of accessing CT excited states. Regardless of whether the
PC exhibits CT character in the excited state, it can transfer an electron to a substrate acting as an
electron acceptor (A) and enable further reaction. The ability of the photoexcited catalyst (PC*) to
transfer an electron is partially dictated thermodynamically by the excited state reduction potential
(E0*). Following the oxidative electron transfer reaction, the catalytic cycle is completed when the
oxidized PC (radical cation [2PC•+]) is reduced back to the ground state (1PC) with an electron
donor (D).
7
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 39
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Schematic representation of the subsets of catalysts compared in this work to identify
structure-property relationships. The absorption profiles of several phenoxazines were investigated
to determine the effects that core modification has on photon absorption in these molecules in
order to design visible light-absorbing derivatives (top left). The ability of non-core modified and
core modified phenoxazines to access CT excited states was evaluated computationally and
spectroscopically (top right). Alteration of the reduction potentials of N-aryl phenoxazines via
mono- and di-core substitutions was explored; key comparisons within this group included
investigating the effects of electron withdrawing groups (highlighted in blue) and electron
donating groups (highlighted in green).
The four major steps of the catalytic cycle (Figure 1B), 1. light absorption, 2. access of a
CT excited state, 3. excited state reduction potential, and 4. oxidation potential and redox
reversibility, were used as criteria to investigate how structural modification of organic excited
state reductants impact PC properties. The fundamental questions guiding investigation of these
catalyst properties were: Ⅰ. how does core modification affect photoexcitation events in N-aryl
phenoxazines and how does extending the conjugation of the phenoxazine core affect the
absorption profiles of these molecules?; Ⅱ. how does core modification affect the ability of N-aryl
8
ACS Paragon Plus Environment

Page 9 of 39
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
phenoxazines to access CT excited states?; and Ⅲ. how does alteration of the electronics of the
core substituents affect the excited and ground state redox properties and redox reversibility of
core modified phenoxazines? To answer these questions, we designed and investigated three series
of phenoxazine derivatives (Figure 2). Comparison of PCs within each of these series was used to
test our hypotheses corresponding to each of the aforementioned questions.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Light Absorption. In our initial design of PC 1, we hypothesized that installation of aryl
core substituents would stabilize the π* orbitals involved in photon absorption, allowing for
photoexcitation using lower energy visible light (Figure 1A). This hypothesis was corroborated by
observing that installation of biphenyl substituents onto the core of PC 2 red-shifts the maximum
wavelength of absorption (λmax,abs) from 323 nm to 388 nm (PC 1) and the absorption profile tails
into the visible regime. Very recently, a similar approach was applied to develop visible light
absorbing phenothiazine based PCs.16 Given that PCs that absorb visible light rather than UV light
are more desirable for minimization of side reactions,1b,8 we were driven to better understand how
these structural modifications fundamentally affect photon absorption. Herein, we disclose studies
on how systematic core modification alters the energies of the π* orbitals of phenoxazine PCs,
which in turn affects the wavelength of light absorbed and energies of the S1 and T1 states (Figure
2). Furthermore, the energies of these states dictate the reducing power of the PC in each respective
state.
Time-dependent density functional theory (TD-DFT) calculations at the CAM-B3LYP/6-
31+g(d,p) level of theory were employed to understand the orbitals involved in the λmax,abs of
photoexcitation for phenoxazine derivatives with increasing aryl conjugation on the core, namely
non-core modified PC 3, phenyl core modified PC 4, and biphenyl core modified PC 5 (Figure 3).
We note that 2-naphthalene is used as the N-aryl substituent for the PCs investigated in this work,
9
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 39
1
2
3
4
5
6
7
8
9
rather than the 1-naphthalene substitution used previously in order to expand our overall
understanding of these types of systems and the non-core substituted derivative (3) was shown to
synthesize polymers with low dispersity (Ɖ) via O-ATRP.5g
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. (Top) Time-dependent density functional theory calculations of orbitals involved in
photoexcitation of PCs 3, 4, and 5 at their corresponding λmax, abs. Computationally predicted
percentage contribution of corresponding orbitals involved in photoexcitation at the λmax, abs are
also presented. The zero point on the orbital axis is defined as the electron in a vacuum. (Bottom)
10
ACS Paragon Plus Environment

Page 11 of 39
Journal of the American Chemical Society
1
2
3
4
5
UV-vis spectra of each PC acquired in N,N-dimethyl acetamide (DMAc) with the observed λmax,
abs and εmax, abs shown.
6
7
8
9
As can be seen with PCs 3, 4, and 5, regardless of aryl core substituents, the highest occupied
molecular orbital (HOMO) is localized on the phenoxazine core with lowest energy excitation
involving these πHOMO electrons (Figure 3). Further, these aryl core substituents do not appreciably
perturb the energy of the πHOMO, as demonstrated by the similar calculated energies of -5.57 eV, -
5.49 eV, and -5.46 eV for PCs 3, 4, and 5, respectively. However, aryl core substituents do
significantly change the energies and nature of the low-lying πLUMO orbitals. For PC 3, which lacks
core substituents, the energies of unoccupied π orbitals involved in photoexcitation are -0.99 eV
(πLUMO+1), -0.79 eV (πLUMO+3), and -0.63 eV (πLUMO+4). PC 3 was experimentally determined to
absorb primarily UVA light (λmax,abs = 318 nm, Figure 3), which is computationally predicted to
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
be the result of excitation from πHOMO into the aforementioned high-lying orbitals, namely πLUMO+1
,
πLUMO+3, and πLUMO+4, with respective contributions of 37%, 16%, and 23%. In PC 4, core
substitution with phenyl groups introduces a new lower-lying πLUMO+1 orbital (-1.43 eV) with
character inclusive of the phenoxazine core’s π* with extended conjugation into the phenyl
substituents. Excitation from πHOMO into this low-lying πLUMO+1 (69% contribution) is corroborated
by the observed red-shift of λmax,abs to 367 nm. A similar trend is observed for PC 5, but with a
notably larger effect, where biphenyl substituents exert greater stabilization on the phenoxazine
core’s π* to yield an even lower lying πLUMO (-1.74 eV). Thus, the observed λmax,abs is red-shifted
even further to 384 nm (contributed by 78% πHOMO - πLUMO transition). These analyses and
observations highlight the importance of aryl core substituents for the design of visible light-
absorbing PCs.
In addition to red-shifting the λmax, abs, core modification also increases the εmax of PC 3 from
8,040 M-1cm-1 to 18,300 M-1cm-1 (PC 4) and 25,900 M-1cm-1 (PC 5), improving the efficiency of
11
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 39
1
2
3
4
5
6
7
8
9
photoexcitation (Figure 3, bottom). This observation motivated us to determine trends in how
structural modifications affect the efficiency of photon absorption in phenoxazine PCs. As a simple
phenoxazine scaffold, phenyl-10-phenoxazine (PC 6) was decorated with a single phenyl (7) or
biphenyl (8) core substituent. Systematically increasing the aryl conjugation on the core position
increases the εmax value from 6,580 M-1cm-1 in PC 7, to 10,300 M-1cm-1 in PC 8 (Figure 4A). A
further increase of εmax was achieved with installation of a second biphenyl core substituent (PC
9) which more than doubles the εmax of PC 8 (εmax = 24,000 cm-1M-1 for 9). These data are supported
by higher values in the computationally predicted oscillator strength (f) for these PCs (Table S2).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. (A) UV-vis absorption spectra acquired in DMAc for N-phenyl phenoxazines. (B) UV-
vis absorption spectra for N-2-naphthyl phenoxazines with electron withdrawing, neutral, and
donating core substituents. The light grey area indicates the UV regime of the electromagnetic
spectrum.
Previously, we showed that N-2-naphthyl phenoxazine (3) exhibits superior catalytic
performance in O-ATRP (as indicated by producing polymeric material with lower Ɖ, higher I*,
and with a more linear growth of polymer molecular weight as a function of conversion) compared
to N-phenyl phenoxazine (6), which we proposed is due to the ability of non-core modified N-
12
ACS Paragon Plus Environment

Page 13 of 39
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
naphthyl phenoxazines to access CT excited states.5g The better performance of PC 3, motivated
investigation of the effects of core modification on the absorption profiles of N-2-naphthyl
phenoxazines (Figure 4B). Core modification with electron donating 4-methoxyphenyl groups (PC
11) leads to a 4 nm blue-shift relative to PC 4, while core modification with aryl electron
withdrawing substituents has the opposite effect, and the λmax,abs of PCs 12 (R= 4-cyanophenyl)
and 13 (R= 4-trifluoromethylphenyl) are red-shifted 44 nm and 21 nm, respectively (Figure 4B
and Figures S64-S66). In fact, the cyano-containing PC 12 exhibits the most red-shifted absorption
spectrum of the phenoxazines investigated, being the only PC with a λmax,abs in the visible regime
(λmax,abs = 411 nm, ε = 22,300 cm-1M-1). Exceptions to these trends are observed in the absorption
profiles of PCs 14, 15, and 16 (Figures S67, S62, and S63, respectively, and Table S3). PC 14,
which possesses electron donating diphenyl amino core substituents, does not exhibit a blue shift
in absorption compared to PC 4. Installation of more highly conjugated phenanthracenyl (15) and
pyrenyl (16) core substituents does not lead to a red shift of the λmax,abs compared to PC 4. In these
cases, in-depth evaluation of the destabilizing or stabilizing effects of aryl core-substituents on
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
πcore (HOMO) and π*core (LUMOs) have to be considered to determine the net effect on λmax,abs
.
Ultimately, the design of core modified phenoxazines enabled twelve visible light-absorbing N-
aryl phenoxazine PCs to be realized with red-shifted λmax,abss compared to non-core modified
derivatives, with one derivative exhibiting a λmax,abs = 411 nm (PC 12). In addition, these
derivatives exhibit εmaxss typically greater than 10,000 cm-1M-1, with one derivative exhibiting a
εmax of 37,700 cm-1M-1 (PC 14).
Access of Charge Transfer Excited State. Previously, we observed that N,N-diaryl
dihydrophenazine PCs, which are structurally similar to N-aryl phenoxazines, are capable of
accessing CT excited states.7 In the lowest excited state of these compounds, CT occurs from the
13
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 14 of 39
1
2
3
4
5
6
7
8
9
electron rich dihydrophenazine core (electron donor) to the N-aryl substituents (electron acceptor),
given that the substituents possess a low-lying π* orbital (e.g. 2-naphthyl). We empirically
observed that N,N-diaryl dihydrophenazine PCs that exhibit CT character perform better in O-
ATRP, which we posit is due, in part, to minimization of unproductive back electron transfer.7 In
our previous work with N-aryl phenoxazines we observed a similar trend. Specifically, PCs with
N-naphthyl substituents (PCs 1, 2, and 3) are computationally predicted to possess spatially
separated SOMOs in the triplet excited state (for the SOMOs of PC 3 see Figure 3) and these PCs
(1, 2, and 3) perform better in O-ATRP compared to those that do not exhibit this feature (PC 6).5g
However, experimental characterization of the nature of CT in phenoxazine PCs was only recently
investigated using PC 1.12 In the current work, CT character in core and non-core modified N-
phenyl and N-2-naphthyl phenoxazines is explored.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
To investigate the nature of the excited states of N-aryl phenoxazines, the properties of 3PC*,
1PC*, and 1PC for 3 and 6 were explored. Electrostatic potential (ESP)-mapped electron density
diagrams were used to predict the distribution of electron density for PCs 3 and 6 in their ground
and triplet excited states (Figure 5A). In PC 6, similar electron density distributions (depicted in
red) are observed in both 1PC and 3PC*, which indicates a lack of CT character. We reason that the
N-phenyl substituent in PC 6 does not possess a π* that is low enough in energy to accept an
electron from the phenoxazine core in the excited state, thus preventing CT. On the contrary, PC
3, which possesses a lower-lying 2-naphthyl π* orbital, exhibits a shift in electron density for 1PC
versus the 3PC*, suggesting that the T1 state has CT character with transfer of electron density to
the 2-naphthyl substituent and depletion on the phenoxazine core.
For these same molecules, fluorescence studies enable exploration of CT character (Figure
5B). The observed emission in PCs 3 and 6 is likely from the S1 state since the emission is not
14
ACS Paragon Plus Environment

Page 15 of 39
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
quenched by the presence of oxygen, a known triplet quencher. On one hand, PC 6 exhibits a small
Stokes shift, ∆λ, (difference between its λmax,abs [324 nm] and λmax,ems [392 nm]) in DMAc [∆λ =
68 nm (0.66 eV)] with a sharp and featured emission peak. In contrast, PC 3, exhibits a larger
Stokes shift in DMAc [∆λ = 191 nm (1.46 eV)] and a broad and featureless emission profile, a
hallmark of a CT emission.7,15 These data suggest that the S1 state of PC 3, possessing significant
CT character, is qualitatively different from that of PC 6.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 5. (A) Electrostatic potential (ESP)-mapped electron density of phenoxazine PCs 3, 5, 6,
and 9 are shown for the singlet ground state (left) and triplet excited state (right). (B) Overlays of
the absorption profiles acquired in DMAc (dark blue) and emission profiles acquired in DMAc
(turquoise) with the experimentally determined Stoke Shifts (∆λ) are shown for each PC. (C)
Photographs of the PCs dissolved in solvents with increasing polarity, as indicated by their
15
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 16 of 39
1
2
3
4
5
6
7
8
9
increasing Reichardt parameter (ET(30) in kcal/mol). From left to right: 1-hexene (ET(30) = 32.4
kcal/mol), THF (ET(30) = 37.4 kcal/mol), , and DMAc (ET(30) = 42.9 kcal/mol). The maximum
wavelength of emission (λmax,em) of PCs 3, 5, 6, and 9 acquired in each of these solvents is shown
below the corresponding scintillation vial in the photograph. For the full emission spectra of PCs
3, 5, 6, and 9 in these solvents, please see Figures S89-S92 in the supporting information.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Further support for the assignment of CT character in the excited state is the observance of
fluorescence solvatochromism with changing solvent polarity.17 The intramolecular transfer of
electron density in CT molecules typically creates a larger molecular dipole in the excited state
(compared to the ground state), which is more stabilized in polar solvents, leading to lower energy
emission. To qualitatively and quantitatively determine the presence or absence of solvatochromic
behavior in PCs 6 and 3, the emission of these PCs was studied in three solvents (1-hexene,
tetrahydrofuran [THF], and DMAc ) of varying polarity, as indicated by their different Reichardt
polarity parameters, ET(30), expressed in kcal per mole (ET(30) = 32.4, 37.4, 42.9, respectively,
Figure 5C).17 While PC 3 clearly exhibits solvatochromic behavior, as indicated by a change in
the color of its emission in different solvents, PC 6 qualitatively appears to emit the same color.
Moreover, the wavelength of emission for PC 6 was measured in these three solvents and found to
shift by no more than 6 nm or 0.05 eV (Figure S89). In contrast, the λmax,ems for PC 3 red-shifts 66
nm (0.40 eV) going from 1-hexene to THF and another 22 nm (0.11 eV) going from THF to
DMAc (Figure S90). The emission profiles, Stokes shifts, and solvatochromic behavior of PCs 6
and 3 are analogous to that of N,N-diphenyl and N,N-dinaphthyl dihydrophenazines, respectively,
suggesting that non-core modified N-aryl phenoxazines must also be designed with an N-aryl
substituent possessing a low-lying π* to yield PCs with accessible CT states.
After gaining an understanding of the nature of CT in 3 and 6, we were motivated to determine
if core modification affects the ability of N-aryl phenoxazines to access CT excited states (Figure
5). To do this, we performed a similar analysis of PCs 5 and 9, which are core modified derivatives
16
ACS Paragon Plus Environment

Page 17 of 39
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
of PCs 3, and 6, respectively. We initially hypothesized that the excited states of these core
modified derivatives would mimic that of the parent molecules such that PC 5 accesses an excited
state with CT character and exhibits similar emission to PC 3 while PC 9 exhibits no CT character.
However, in comparing PC 5 in the ground state and T1 state, the ESP diagrams show that electron
density has moved from the phenoxazine core in the ground state to one of the biphenyl core
substituents in the triplet excited state. This observation is qualitatively different from PC 3, in
which CT is predicted to be toward the N-naphthyl substituent in the triplet excited state. PC 5
exhibits bathochromic emission behavior similar to PC 3, but to a lesser extent. For PC 5,its λmax,ems
red-shifts from 1-hexene [ET(30) = 32.4 kcal/mol] to THF [ET(30) = 37.4 kcal/mol] by 20 nm and
exhibits another 17 nm red-shift in DMAc [ET(30) = 42.9 kcal/mol, Figure S91]). In addition, PC
5 exhibits a smaller ∆λ (82 nm, 0.57 eV in DMAc) than PC 3, suggesting a smaller energy
difference between the initially excited Sn state and the relaxed S1 state. In concert, these findings
refute our original hypothesis by indicating that PC 5 can access a CT excited state that is
qualitatively distinct from PC 3. This conclusion is corroborated by the emission data of PC 9. To
our surprise, the λmax,ems, ∆λ, and solvatochromic behavior of PC 9 is similar to that of PC 5
indicating that it can access a CT excited state despite bearing an N-phenyl group rather than an
N-naphthyl group (Figure 5 and Figure S92). These observations support the notion that photon
emission in PCs 5 and 9 is CT in nature but is tied to the phenoxazine core and its biphenyl
substituents.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Excited State Reduction Potential. An inherent challenge in the design of visible light-
absorbing PCs lies in lowering the energy of photoexcitation without sacrificing excited state
reducing power. Ideally, one would like to lower the energy of the Sn state for visible excitation
while maintaining similar energies for the S1 or T1 state required for excited state redox chemistry
17
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 18 of 39
1
2
3
4
5
6
7
8
9
with similar reactivity. This principle was applied to design visible light-absorbing phenoxazine
PCs that are still strongly reducing in their excited states. For example, non-core modified PC 3
absorbs in the UV regime, but core modified PC 5 absorbs in the visible regime, indicating that
photoexcitation is to higher energy Sn states for PC 3 compared to PC 5. Since the T1 state is
typically assigned as the reactive species in photoredox catalyzed transformations requiring
bimolecular collisions, the triplet excited state reduction potentials of these PCs (E0*T1,calc) were
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
computationally calculated (Figure 6). The E0*
was determined to be -1.90 V vs SCE5g for
T1, calc
UV-light absorbing PC 3 and somewhat lower (-1.70 V vs SCE) for visible light-absorbing PC 5
(Figure 6). Even though the E0*T1, calc is lower for PC 5, this value is still on par with some of the
most reducing transition metal PCs such as fac-Ir(ppy)3 which exhibits a triplet excited state
reduction potential of -1.73 V vs SCE.1b This example demonstrates that the strong reducing power
of N-aryl phenoxazines in the excited state can still be maintained after core modification is used
to lower the energy of light required for photoexcitation.
18
ACS Paragon Plus Environment

Page 19 of 39
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Structures, calculated triplet excited state reduction potentials (E0*T1,calc[PC•+/3PC*]),
2
calculated oxidation potential of PC•+ (E0 =E0[2PC•+/1PC]), and calculated triplet energies (ET)
ox
of phenoxazines investigated in this study. Catalysts colored in grey are UVA-light absorbing and
catalysts colored in blue are visible light-absorbing.
In addition to designing visible-light absorbing PCs that are strong excited state reductants,
we sought to design PCs with a range of excited state reduction potentials. The triplet excited state
reduction potential (E0*T1) is defined as E0*[2PC•+/3PC*] = E0[2PC•+/1PC] - ET where ET is the triplet
energy (the difference in energy between T1 excited state and S0 ground state) and E0
=
ox
1
2
E0[2PC•+/1PC] is the potential to oxidize PC to PC•+. Thus, for catalysts with similar triplet
energies, those that are easier to oxidize will be more strongly reducing in the excited state. We
envisioned that the excited state reduction potentials (E0*s) of N-aryl core modified phenoxazines
could be tuned through alteration of the electronics of the core substituents which would in turn
19
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 20 of 39
1
2
3
4
alter the values of E0 , the singlet energy (ES), or ET and lead to alteration of the singlet and triplet
ox
5
6
E0*s.
7
8
9
To investigate the range of E0*s that can be achieved with the phenoxazine PC scaffold,
core modified PCs bearing electron donating and electron withdrawing groups (4, 5, and 7-19)
were synthesized and their triplet excited state reduction potentials (E0*T1, calc) were calculated
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
using computationally determined values for ET and E0 (Figure 6). It is important to note that
ox
aryl core substituents were chosen for the design of these derivatives in pursuit of making them
visible light absorbers (vide supra). Core modification of PCs 6 and 3 with aryl substituents (4, 5,
7-9, and 11-19) lowers the computationally calculated ETs compared to the parent compounds, but
with ETs still exceeding 2.0 eV in all cases (Figure 6). Core modification of PCs 6 and 3 with
electron withdrawing trifluoromethyl (13, 19) or cyano (12) substituents, or highly conjugated (15,
16) substituents yields PCs with lower ETs that are also more difficult to oxidize. Consequently,
these PCs exhibit more positive values of E0*T1, calc rendering them less reactive PCs in the excited
state. We reason that derivatives bearing electron withdrawing groups exhibit these redox
properties because electron withdrawing core substituents shift electron density away from the
phenoxazine core making them more challenging to oxidize to the 2PC•+. Conversely, installation
of electron donating methoxy (11, 18) or diphenyl amino (14) core substituents leads to PCs with
higher ETs and are easier to oxidize (exhibit lower E0ox values), making them stronger excited state
reductants (as indicated by their more negative E0*s).
In addition to computationally calculating the triplet excited state reduction potentials, we
experimentally determined the singlet excited state reduction potentials (E0*S1, exp[2PC•+/1PC*])
using a combination of fluorescence spectroscopy and electrochemistry (Table 1). This analysis is
particularly useful for PCs capable of accessing CT excited states in which the donor and acceptor
20
ACS Paragon Plus Environment

Page 21 of 39
Journal of the American Chemical Society
1
2
3
4
likely exhibit minimal electronic coupling as the S1 and T1 states in these systems are nearly
5
6
7,18
degenerate such that the singlet E0* approximates the triplet E0*
.
The singlet energies
S1
T1
7
8
9
determined from fluorescence emission (which correspond to emission from the S1 state and are
denoted as ES1, in Table 1) were found to be smaller than the computationally calculated T1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
exp
energies in only two cases (for PC 10 and PC 16). We note that the differences in these cases are
0.02 eV and 0.10 eV, which is within the expected error of ±0.2 eV for DFT calculations. We also
note that while S1 and T1 energies are close to degenerate for species that exhibit extensive charge
transfer, the S1 and T1 energy splitting gets increasing larger as the extent of charge transfer
decreases.
The experimentally determined singlet excited state reduction potential (E0*S1, exp) values
were found to follow the same trends as the computationally calculated triplet reduction potentials
(E0*T1, calc). Namely, PCs bearing electron donating core substituents were more strongly reducing
in the singlet excited state and typically more easily oxidized in the ground state while the opposite
trends were observed for PCs bearing electron withdrawing core substituents. We note that the E1/2
(2PC•+/1PC)values determined computationally (E0 ) are systematically ~0.2 eV less positive than
ox
the values determined experimentally which demonstrates the utility of these computational
calculations as a predictive tool for this application. From the computational and experimental
data we demonstrate that core modified derivatives exhibit a wide range of excited state reduction
potentials with experimentally determined E0*
values spanning from -1.54 V to -2.25 V vs.
S1,exp
SCE and computationally predicted E0*
values spanning from -1.42 V to -2.01 V vs. SCE.
T1,calc
Notably, E0*S1,exp values of PCs 11 and 14 were experimentally determined to be -1.81 V and -1.83
V vs SCE, respectively, with E0*T1,calc values of -1.91 V and -1.88 V vs SCE, respectively, making
them some of the strongest visible light-absorbing organic excited state reductants reported to date.
21
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 22 of 39
1
2
3
4
For example, these PCs are on par with highly reducing organic PCs such as UV-absorbing phenyl
5
6
phenothiazine (E0*
= -2.10 V vs SCE)4c and visible light-absorbing N,N-diaryl
S1,exp
7
8
9
dihydrophenazines (E0*T1,calc ~ -2.2 V vs SCE).5f
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Redox Properties of Core Modified Phenoxazines.
0
0
*
E0
E
E
*
PC
em ES1, exp ET1, calc
E1/2
(eV)c (2PC•+/1PC)
(V vs. SCE)d (V vs. SCE) c (V vs. SCE)e (V vs. SCE)c
ox
S1, exp
T1, calc
λmax
(eV)b
(2PC•+/1PC) (2PC•+/1PC*) (2PC•+/3PC*)
(nm)a
N-phenyl PCs
7
8
427
472
471
412
476
2.90
2.63
2.63
3.01
2.60
2.26
2.24
2.14
2.43
2.14
0.65
0.64
0.62
0.60
0.69
0.47
0.53
0.42
0.42
0.54
-2.25
-1.99
-2.01
-2.41
-1.91
-1.79
-1.71
-1.72
-2.01
-1.60
9
18
19
N-(1-naphthyl) PCs
1f
506
532
2.45
2.33
2.11
2.35
0.65
0.64
0.42
0.47
-1.80
-1.69
-1.70
-1.88
10
N-(2-naphthyl) PCs
4
514
466
532
508
467
2.41
2.66
2.33
2.44
2.65
2.37
2.53
2.20
2.66
2.23
2.13
2.28
2.04
2.16
2.18
2.18
2.30
2.09
0.64
0.63
0.52
0.69
0.72
0.54
0.67
0.66
0.63
0.44
0.43
0.37
0.62
0.58
0.30
0.46
0.45
0.40
-1.77
-2.03
-1.81
-1.75
-1.93
-1.83
-1.86
-1.54
-2.03
-1.80
-1.70
-1.91
-1.42
-1.58
-1.88
-1.72
-1.85
-1.69
5
11
12
13
14g 522
15g 489
16g 564
17
466
aEmission wavelength measured using fluorescence spectroscopy in DMAc. See supporting information
b
c
for more details. Singlet energies were calculated using the maximum wavelength of emission. DFT
calculations performed at the uM06/6-311+G(d,p)//uM06/6-31+G(d,p) level of theory with CPCM-
d
described solvation in aqueous solvent. All measurements were performed in a 3-compartment
electrochemical cell with an Ag/AgNO3 reference electrode in MeCN (0.01 M) and 0.1 M NBu4PF6
electrolyte solution. DMAc was used to solvate the PCs and in the working electrode compartment while
platinum was used as both the working and counter electrodes. E (V vs. SCE) = E (V vs Ag/AgNO3 [0.01
e
M]) + 0.298 V. Singlet excited state reduction potentials were calculated using the singlet energies
(estimated from the maximum wavelength of emission) and the E1/2. fValues for the properties of PC 1 were
g
taken from reference 5g. Due to the extensive molecular structure, frequency calculations of these
compounds were computed at the uM06/6-31G(d,p)/CPCM-H2O level of theory.
22
ACS Paragon Plus Environment

Page 23 of 39
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Oxidation Potential of PC and Redox Reversibility. After the photoexcited catalyst
reduces a substrate in an oxidative quenching catalytic cycle, the PC is oxidized to a radical cation,
2PC•+. To complete the catalytic cycle, the 2PC•+ must be reduced to regenerate 1PC. Therefore, the
efficiency of the catalytic cycle is reliant to a large degree on the ability of the 2PC•+ to engage in
rapid and reversible electron transfer reactions. Previously, it was shown that PC 1 exhibits a
reversible cyclic voltammogram in DMAc for the 2PC•+/1PC redox couple and that electron transfer
reactions involving this PC and the electrode are diffusion-limited.12 To evaluate electron transfer
reactions involving the phenoxazine derivatives explored in this work, CV was performed to
analyze their oxidation potentials (Table 1) and redox reversibility (Figure 7 and Figures S43-57).
The oxidation potential of each PC was approximated from its half wave potential, where
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
E0 [2PC•+/1PC] ≈ E1/2[2PC•+/1PC].19 In particular, we sought to determine whether installation of
ox
electron donating, electron withdrawing, or highly conjugated core substituents on phenoxazine
PCs affects the reversibility of electron transfer processes. Additionally, the cyclic voltammograms
of the PCs were acquired at different scan rates to gain insight into the kinetics of electron transfer
processes involving these PCs.
PCs with electron donating core substituents transfer electron density to the oxidized
phenoxazine core and thus stabilize formation of the 2PC•+, such that they are more easily oxidized
(E1/2= ~0.50 V to 0.60 V vs. SCE for PCs 11 and 14) compared to those bearing electronically
neutral core substituents or no core substituents (E1/2 = ~0.60 V to 0.69 V vs SCE for PCs 4, 5, 7-
10 and 15-18). These PCs are observed to exhibit reversible cyclic voltammograms when acquired
in DMAc (Figures S43-S48 and Figures S51-S56). A representative example is the cyclic
voltammogram of PC 11, in which the difference between the anodic and cathodic peak potentials
(∆Ep) is 75 mV (compared to the theoretical value of 59 mV for a reversible system) and the ratio
23
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 24 of 39
1
2
3
4
5
6
7
8
9
of peak anodic current (ipa) to peak cathodic current (ipc) is 1.07 (compared to a theoretical value
of 1 for a reversible system) when acquired with a scan rate of 0.05 V/s (Figure 7A, blue trace).
The reversibility of this redox couple is maintained at different scan rates (0.02 V/s to 0.10 V/s)
across which the ∆Ep (65- 78 mV) and ipa/ipc values are also consistent (1.06-1.07).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 7. Cyclic voltammograms (CVs) of PCs 11 (A) and 13 (B) acquired at different scan rates
0.02 V/s, 0.05 V/s, 0.08 V/s and 0.10 V/s.
Although 2PC•+s with electron withdrawing core substituents (PCs 12, 13 and 19) are more
destabilized (E1/2 > 0.69), reversible CVs are still observed for these species (Figures S49, S50 and
S57). For example, the most oxidizing 2PC•+ (the radical cation of PC 13) exhibits a reversible CV
with a ∆Ep of 78 mV in DMAc (Figure 7B). The ipa/ ipc could not be accurately determined for this
catalyst in DMAc due to the proximity of its oxidation potential to that of the solvent window.
Therefore, CV with PC 13 was also conducted in THF for which the ipa/ ipc is 1.10 at the same scan
rate of 0.05 V/s (Figure S57). To evaluate whether electron transfer reactions between the PC and
the electrode are faster than the rate of diffusion of the PC, the cyclic voltammograms were
acquired at different scan rates ranging from 0.02V/s to 0.10 V/s. A linear relationship between
the ipa and the square root of the scan rate (ν1/2) is observed for both PCs 11 and 13 (Figure 7,
insets). This observation indicates that the electron transfer process involving these PCs and the
24
ACS Paragon Plus Environment

Page 25 of 39
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
electrode is fast and is diffusion limited. Overall, these studies demonstrate that regardless of the
nature of the substituents installed onto the phenoxazine core, all of the derivatives explored herein
engage in reversible and diffusion-limited electron transfer reactions with an electrode.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Application of Core Modified Phenoxazines as PCs in O-ATRP. After establishing structure-
property relationships for the phenoxazines presented in this work, we aimed to better understand
how differences in the phenoxazines manifest in differences in catalytic performance in O-ATRP.
We first compared the performance of non-core modified phenoxazines 3 and 6 (Figure 6), and
then extended this comparison to core modified derivatives of these molecules. Next, we compared
the performance of all the N-2-naphthyl core modified phenoxazines (4, 5, and 11-17, Figure 6) in
O-ATRP. Through this study, we provide insight for the development of improved O-ATRP
catalysts (vide infra).
While these PCs are likely capable of catalyzing a range of transformations, we originally
designed them for application in O-ATRP. A suitable PC for O-ATRP must be both strongly
2
reducing in the excited state and sufficiently stable and oxidizing as PC•+ in order to synthesize
well-defined polymers (Figure 8).20 Specifically, O-ATRP initiators and the alkyl bromide
polymer chain-end group possess reduction potentials of ~ -0.6 V to -0.8 V vs SCE, requiring a
strong excited state reductant for the reaction to occur without addition of sacrificial electron
2
donors.21 Additionally, the PC•+ must be sufficiently stable and more oxidizing than the
propagating radical of the polymer chain end, for which E0 ~ -0.8 V vs SCE, to deactivate the
propagating chain.5f Since these catalysts likely operate via an outer-sphere electron transfer (O-
SET) pathway (more specifically that of dissociative electron transfer),5g Marcus theory predicts
that the redox properties of these PCs (and those of the electron donor or acceptor) will directly
relate to the rates of activation and deactivation in O-ATRP (Figure 8). Importantly, the synthesis
25
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 26 of 39
1
2
3
4
5
6
7
8
9
of well-defined polymers via any controlled radical polymerization requires a low concentration
of radical species in solution, and in ATRP this is maintained by a fast rate of deactivation relative
to the rates of activation and propagation.22 If an O-ATRP PC possesses the required redox
properties for efficient reversible deactivation, then the polymerization is expected to proceed with
linear reaction kinetics that are first order with respect to monomer consumption and a linear
growth of polymer molecular weight (MW) as a function of monomer conversion. As such,
polymers of target MWs (quantified by initiator efficiency, I* = Mn(theo)/Mn(actual), where Mn is the
number average MW) and low dispersity (Ɖ = Mw/Mn, where Mw is the weight-average MW) can
be synthesized.22 Advances in controlled radical polymerization methodologies such as ATRP
over the last few decades have enabled the synthesis of polymers with near quantitative I*s with
moderate (Ɖ < 1.3) to low (Ɖ < 1.1) dispersity.23 Guided by these standards, the success of these
phenoxazines to efficiently catalyze O-ATRP was evaluated. In particular, it was determined
whether O-ATRP employing these PCs demonstrates linear growth of polymer MW as a function
of conversion and produces polymeric material with Ɖ < 1.3, and high I* (for an ideal O-ATRP I*
= 100 %).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
26
ACS Paragon Plus Environment

Page 27 of 39
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Figure 8. Reaction scheme for the polymerization of MMA (top) and proposed mechanism of O-
ATRP using alkyl halide initiators (bottom) are shown. In the proposed mechanism of O-ATRP,
initiation occurs once the excited state organic PC reduces the alkyl halide bond of the initiator or
halide-capped polymer chain-end (Figure 8). This reaction generates a carbon centered radical on
the initiator or polymer chain end that can propagate via reaction with monomers. Meanwhile, the
same reduction event also yields a bromide anion, which we propose associates with the oxidized
2PC•+ to form the 2PC•+Br- ion pair.7a We hypothesize this ion pair is responsible for deactivating
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
the polymer chain by reinstalling the halide end group and regenerating the ground state PC,
which can re-enter the catalytic cycle.
Previously, we observed that the O-ATRP of methyl methacrylate (MMA) catalyzed by N-
phenyl phenoxazine (6) using UV-light yields polymers of less predictable MW (I* = 137%) and
relatively high Ɖ (Ɖ = 1.48) compared to N-2-naphthyl phenoxazine 3 (I* = 77%, Ɖ = 1.11). The
comparatively better performance of PC 3 in O-ATRP is further exemplified by the linear growth
of polymer MW with respect to monomer conversion with close agreement to the theoretical
growth of MW for a well-controlled polymerization (Figure 9A).15 In contrast, O-ATRP catalyzed
by 6 exhibits less linear growth of polymer MW throughout the polymerization with more
appreciable deviation from theoretical MW values (Figure 9B). Notably, the O-ATRP catalyzed
by 6 suffers from poor control at low monomer conversions (at 24% conversion I* = 41%). In
addition, the Ɖ of the PMMA synthesized using PC 6 is higher and more erratic throughout the
polymerization. The superior performance of PC 3 compared to PC 6 in O-ATRP demonstrates
the importance of the N-aryl substituent in designing non-core modified N-aryl phenoxazine PCs.
Due to the similar absorption and redox properties of PCs 3 and 6, we posit that the significant
difference in their catalytic performance is a result of differences in the nature of these molecules
in the excited state. Namely, the N-naphthyl substituted PC (3) exhibits a polar, CT excited state
while the N-phenyl substituted PC (6) exhibits a less polar, locally excited state (vide supra).
Future photophysical investigation will be aimed at understanding the connection between the
ability of these molecules to access CT excited states and their catalytic performance in O-ATRP.
27
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 28 of 39
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 9. Plots of growth of the experimentally measured number average molecular weight (Mn)
as a function of monomer conversion (blue squares) with theoretical values (blue dotted line). The
dispersity of each corresponding sample is shown as a function of monomer conversion (orange
diamonds). Data for the aliquots of each polymerization taken after eight hours are shown above
each plot. All polymerizations (A-D) were run with [1000]:[10]:[1] of [MMA]:[DBMM]:[PC] in
an equal volume of DMAc as that of MMA. Polymerizations catalyzed by PC 3 (A) and PC 6 (B)
were performed in a UV-nail apparatus while polymerizations catalyzed by PC 5 (C) and PC 9 (D)
were run in the presence of white-light LEDs. The polymerization data acquired for PC 3 were
acquired from reference 5g. Additional experimental details are provided in the Supporting
Information.
After furthering our understanding of the performance of non-core modified phenoxazines
3 and 6 in O-ATRP, we compared biphenyl core modified derivatives of these molecules (5 and
9). Characterization of the absorption properties, nature of the excited state, and redox properties
of these molecules revealed that the nature of the core substituents, rather than the N-aryl
substituent, more greatly dictates the properties of these PCs. In particular, 5 and 9 exhibit similar
absorption profiles (λmax = 389 nm and 389 nm, respectively and ε = 25,900 and 24,000,
respectively) and, in contrast to their parent molecules, both absorb in the visible regime.
28
ACS Paragon Plus Environment

Page 29 of 39
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Investigation of the nature of 5 and 9 in the excited state revealed that they both access similar CT
excited states which likely involve one of their biphenyl core substituents. This property again
stands in contrast to the non-core modified molecules for which 3 (the parent molecule of 5)
exhibits CT involving the N-naphthyl substituent and 6 (the parent compound of 9) does not exhibit
CT character in the excited state. Furthermore, these core modified derivatives exhibit redox
properties that are very similar to each other, but distinct from that of 3 and 6 (Table 1).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Given the similar overall properties of PCs 5 and 9, we hypothesized that they would
exhibit similar performance in O-ATRP, as quantified by the parameters described above. To test
this hypothesis, the O-ATRP of MMA was performed employing 5 and 9 as catalysts in the
presence of white-light LEDs (Figure 9C and D, for emission of white light LEDs see Figure S41).
O-ATRP with both PCs demonstrates linear growth of polymer MW with respect to monomer
conversion with close agreement to theoretical MW values throughout the entire polymerization.
As a result, 5 and 9 are able to synthesize PMMA with nearly quantitative I* at high monomer
conversions, a marked improvement over the performance of PCs 3 and 6. In addition, O-ATRP
catalyzed by these core modified derivatives exhibits a lowering of Ɖ throughout the
polymerization, with Ɖ values rivaling that of PC 3 at high conversions. In summary, the
performance of 5 and 9 remains similar to each other through the entire polymerization and is
distinct from the performance of their parent molecules. Together, these studies highlight the
importance of determining which structural motifs lead to improved catalytic properties and
performance within each subset of non-core modified and core-modified N-aryl phenoxazines.
To further investigate our overarching hypothesis that catalyst structure dictates catalyst
properties (specifically their absorption and redox properties) which in turn affects catalyst
performance, the N-2-naphthyl core modified phenoxazines (4, 5, and 11-17) were used as
29
ACS Paragon Plus Environment