Exploiting Charge-Transfer States for Maximizing Intersystem Crossing Yields in Organic Photoredox Catalysts

Article abstract of DOI:10.1021/jacs.8b01001

A key feature of prominent transition-metal-containing photoredox catalysts (PCs) is high quantum yield access to long-lived excited states characterized by a change in spin multiplicity. For organic PCs, challenges emerge for promoting excited-state intersystem crossing (ISC), particularly when potent excited-state reductants are desired. Herein, we report a design exploiting orthogonal π-systems and an intermediate-energy charge-transfer excited state to maximize ISC yields (Φ_ISC) in a highly reducing (E⁰? = -1.7 V vs SCE), visible-light-absorbing phenoxazine-based PC. Simple substitution of N-phenyl for N-naphthyl is shown to dramatically increase Φ_ISC from 0.11 to 0.91 without altering catalytically important properties, such as E⁰?.

Full text of DOI:10.1021/jacs.8b01001

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Exploiting Charge Transfer States for Maximizing
Intersystem Crossing Yields in Organic Photoredox Catalysts
Steven M. Sartor, Blaine G. McCarthy, Ryan M Pearson, GARRET M. MIYAKE, and Niels H. Damrauer
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01001 • Publication Date (Web): 29 Mar 2018
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Exploiting Charge Transfer States for Maximizing Intersystem
Crossing Yields in Organic Photoredox Catalysts
Steven M. Sartor,† Blaine G. McCarthy,‡ Ryan M. Pearson,‡ Garret M. Miyake,‡ Niels H. Damrauer*,†
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†Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States
‡ Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States
Supporting Information
ABSTRACT: A key feature of prominent transitionꢀmetal conꢀ
taining photoredox catalysts (PCs) is high quantum yield access to
longꢀlived excited states characterized by a change in spin multiꢀ
plicity. For organic PCs, challenges emerge for promoting excitꢀ
edꢀstate intersystem crossing (ISC), particularly when potent exꢀ
cited state reductants are desired. Herein, we report a design exꢀ
ploiting orthogonal πꢀsystems and an intermediateꢀenergy charge
transfer excited state to maximize ISC yields (ΦISC) in a highly
reducing (E0* = ꢀ1.7 V vs SCE), visible light absorbing phenoxaꢀ
zineꢀbased PC. A simple substitution of Nꢀphenyl for Nꢀnaphthyl
is shown to dramatically increase ΦISC from 0.11 to 0.91 without
Figure 1. Absorption (dashed lines)10ꢀ11 and emission (solid lines)
of 1 (blue) and 2 (red) in deaerated RT DMAc.
altering catalytically important properties, such as E0*.
nor does it alter the reduction potential from the triplet exited
state, which has been reported as ꢀ1.70 and ꢀ1.72 V vs SCE for 1
and 2, respectively.10ꢀ11 Notably, these excited state reduction
potentials are potent even by comparison to wellꢀknown Ir(III)
complexes such as facꢀIr(ppy)3 (ꢀ1.73 V vs SCE). The current
work seeks to uncover the photophysical mechanism for this stark
discrepancy in ΦISC, such that it may be exploited and further
explored in comparable OPC systems.
The recent resurgence of photoredox catalysis was facilitated
by discrete PCs whose lowest energy excited states are formed in
significant yields and are sufficiently longꢀlived to efficiently
engage in outerꢀsphere electron transfer in competition with innerꢀ
sphere radiative and nonꢀradiative decay processes.1 This property
can confer distinct benefits for photoredox catalysis: making posꢀ
sible high conversion efficiencies, allowing for conditions emꢀ
ploying small coꢀcatalyst concentrations, making feasible catalyꢀ
sis via activated electron transfer, and decreasing catalyst loadꢀ
ing.2 Because long lifetimes are commonly enabled by spin interꢀ
conversion in the excited state, initial PCs were chromophores
incorporating transition metals (e.g Ru or Ir) where ΦISC is apꢀ
proximately unity due to the heavy atom effect.3
Fig. 1 shows electronic absorption for 1 and 2 in DMAc. The
spectra are highly similar, possessing comparable λmax and molar
extinction. These data were modeled using TDꢀDFT (see SI for
details) which reveals that the prominent lowestꢀenergy feature at
388 nm for both PCs is almost completely described as a single
interꢀorbital transition originating in a phenoxazineꢀlocalized
HOMO. The acceptor orbitals (LUMO for 1 and LUMO +1 for 2)
are qualitatively the same and indicate a wavefunction delocalized
over the phenoxazine core plus the adjacent rings of both biphenyl
substituents (Fig. S5). Neither involves the nearly perpendicular
Nꢀaryl substituent. We next turned to consider the catalyticallyꢀ
relevant lowestꢀenergy excited states. Fig. 2A,B shows selected
spectra obtained for 1 and 2 using nanosecond transient absorpꢀ
tion (NSTA) spectroscopy. The long and strongly oxygen sensiꢀ
tive lifetimes (1200 ꢀs (1) and 480 ꢀs (2)12) is highly suggestive
that the lowestꢀenergy excited state in these systems is triplet in
nature.
The scarcity of Ru and Ir has motivated exploration of PCs
built from abundant materials. Those incorporating first row tranꢀ
sition metals similarly exploit the heavy atom effect for lifetime
extension.4 Organic PCs (OPCs) are attractive by way of extendꢀ
ing reaction scope5 and alleviate metal contamination concerns.6
However, unlike metalꢀcontaining PCs, optimization of ΦISC in
OPCs must be tied to design. For excited state oxidants, high ΦISC
can be achieved using carbonyls5 or by incorporation of heavy
halogens.7 Unfortunately, these strategies are often impractical for
highly reducing chromophores6, 8 due to the electron withdrawing
character of these functional groups which can lessen the reducing
power of the OPC or impact its chemical stability.2c, 9
Both compounds possess quite similar NSTA spectra, consistꢀ
ing of a prominent ground state bleach and a largely unfeatured
excited state absorption (ESA). DFT calculations within the triplet
manifold offer additional insight. Figure 2C shows the singly
occupied molecular orbitals (SOMOs) determined for the geomeꢀ
tryꢀoptimized lowestꢀenergy triplet of 2. The SOMOs for 1 are
highly similar (Fig. S6). For both PCs, the lower SOMO is domiꢀ
nated by phenoxazine π character bearing strong resemblance to
the aforementioned ground state (GS) HOMO (Fig. S5). In conꢀ
trast, the higher SOMO (in both molecules) shows π character
Recently, we reported a highly reducing, phenoxazineꢀbased
OPC (2 herein) which possesses a notably high ΦISC = 0.91 and a
long triplet lifetime of 480 ꢀs in room temperature (RT) N,Nꢀ
dimethylacetamide (DMAc).10 It was soon thereafter noted that
high ΦISC is not shared by all structurally similar PCs. In particuꢀ
lar, replacement of Nꢀnaphthyl with Nꢀphenyl (1) results in ΦISC
=
0.11. This is surprising, since the Nꢀsubstituent does not impact
the initial and final excited states in both compounds (vide infra),
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Figure 3. Speciesꢀassociated emission spectra for 2 in deaerated
RT DMAc generated by a globally fit biꢀexponential model of the
TCSPC data.14 See SI for details.
Figure 2. Selected spectra from NSTA of 1 (A) and 2 (B)10 in
deaerated RT DMAc. (C) Lower and higherꢀlying triplet SOMOs
of 2 (camꢀb3lyp/6ꢀ31g(d,p)/CPCMꢀDMA level of theory).
character involving the phenoxazine and a single biphenyl subꢀ
stituent. This assignment is strongly supported by photoluminesꢀ
cence measurements of a chemical analogue possessing only a
single biphenyl substituent while being otherwise identical (called
1a; see SI). Notably, while these compounds possess quite differꢀ
ent absorption profiles,11 the emission profiles of 1 and 1a are
remarkably similar in both DMAc and tetrahydrofuran (Fig. S14),
corroborating the symmetryꢀbreaking CT assignment of the S1 in
shifted to one side, including just one outer ring of the phenoxaꢀ
zine core plus the adjacent ring from a single biphenyl substituent.
This SOMO is suggestive of doubleꢀbond character between these
two rings on only one side of the PC, a point that is strongly supꢀ
ported by structural changes on that side of the molecule (shortenꢀ
ing of the first interꢀring CꢀC bond concomitant with the decreasꢀ
ing of an associated interꢀring dihedral angle) noted in the comꢀ
parison of the optimized triplet geometry with the optimized GS
(see SI). With SOMOs in mind, the lowest triplet of 1 and 2 is
interpreted as charge transfer (CT), involving a shift of electron
density from the phenoxazine core towards one of the biphenyl
substituents (denoted TCTꢀBiph). We note that this symmetryꢀ
breaking localization of charge is typical of acceptorꢀdonorꢀ
acceptor complexes which possess relatively low quadrupolar
moments, particularly in highly polar solvents, such as DMAc.13
Drawing on these results along with the NSTA data, we summaꢀ
rize that 1 and 2 have highly similar lowest triplet excited states.
Notwithstanding these results, the measured ΦISC 1 and 2 are
highly divergent. Whereas ΦISC for 2 is 91%,10 this sharply conꢀ
trast with 11% reported here for 1, as determined using a tripletꢀ
triplet energy transfer method.10 These observations motivate a
more extensive characterization of photophysics.
1. We refer to it hereafter as SCTꢀBiph
.
In contrast to 1, TCSPC data for 2 show marked spectral evoluꢀ
tion. We have extracted 20 ps and 5.2 ns speciesꢀassociated specꢀ
tra (Fig. 3). Interestingly, the earlyꢀtime spectrum of 2 peaks at
460 nm, a value that is very similar to λmax,em for 1. This is comꢀ
pelling evidence that 2 initially explores an emissive singlet simiꢀ
lar to SCTꢀBiph in 1. At later times, emission in 2 changes dramatiꢀ
cally, culminating in the broad spectrum centered at 505 nm that
is responsible for the red shift in Fig. 1. Consideration of the only
substitutional difference hints at participation by the Nꢀnaphthyl
moiety in 2.
Using Φem along with TCSPC lifetimes, a radiative rate conꢀ
stant (kr) can be quantified (see Table S1 for equations for calcuꢀ
lation of rate constants). This is straightforward for 1, as only one
emissive state is observed. In 2, early emission from Sn (Fig. 3)
makes a minor contribution of order 3.7 % and can be ignored in
the calculation. As seen in Table S1, kr for 1 is a factor of 70 largꢀ
er than in 2. Further, using the measured values of ΦISC along with
the observed emission lifetime, the ISC rate constant (kISC) can be
isolated from other nonꢀradiative pathways (knr). Notably, kISC of
2 is 4.5 times greater than 1.
Emission spectra for 1 and 2 (Fig. 1) originate within the sinꢀ
glet manifold, an assignment that is supported by timeꢀresolved
measurements, and their insensitivity to ambient oxygen (see SI).
No triplet emission has been observed for either compound, under
the employed experimental conditions. Both species have been
reported to exhibit a high degree of solvatochromism,10ꢀ11 indicatꢀ
ing CT character in their emissive states. However, their spectral
profiles are clearly distinct. Whereas that of 1 is unremarkable,
that of 2 is broader with a redꢀshifted λmax,em and a shoulder at ~
460 nm. Importantly, there are also substantial differences in
emission quantum yields: Φem(1) = 80% while Φem(2) = 2.3%
(Table S1).
These moleculeꢀspecific kr and kISC values have a marked imꢀ
pact on catalytically important properties. To better understand
their origins, femtosecond transient absorption (FSTA) experiꢀ
ments pumping at 400 nm were conducted. In 1 (Fig. 4A), the
data reveal early processes that complete within ~ 10 ps. The
remaining spectral features (see SI for discussion) decay uniformꢀ
ly with a longer component (> 2 ns) that cannot be fully resolved
with this experiment. This includes the 620 nm ESA, which is
expected to involve π*←π* originating
An initial effort to characterize dynamics was made using time
correlated singlet photon counting (TCSPC). In addition to kinetic
information, the timeꢀevolution of spectral features can be asꢀ
sessed (see SI for details). 1 is again unremarkable. Under certain
experimental conditions involving polarization of the 400 nm
pump, a 350 ps rotational diffusion contribution is observed, but
under no circumstances are spectral shifts or profile changes seen
(see Figs. S9 and S11). The spectrum decays to baseline with a
time constant of 2.87 ns, assigned to the lifetime of the emissive
S1 (Table S1).
in the excess charge density on the biphenyl. Given the TCSPC
lifetime as well as spectral assignments (see SI), these latter TA
features are assigned to SCTꢀBiph
.
We next turn to 2 (Fig. 4B). At early times the TA spectrum is
very similar to that of 1, consistent with the GS absorption comꢀ
monality between these PCs. After this, however, striking evoluꢀ
tion occurs, which includes the emergence of a broad ESA (~ 470
nm) over ~ 20 ps that then remains for the experiment duration.
This evolution corroborates TCSPC evidence that 2 initially ocꢀ
cupies an excited state analogous to SCTꢀBiph in 1 but then rapidꢀ
Given the aforementioned fluorescence solvatochromism for 1,
we anticipate that the emitting state has symmetryꢀbreaking CT
2
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Figure 5. Energy level diagram of 1 and 2, with two alternative
ISC pathways (A) and (B) shown for 2. Straight arrows are radiaꢀ
tive pathways whereas wavy arrows are nonꢀradiative. Dashed
lines reflect less likely pathways between states. The time conꢀ
stants that are shown for processes after formation of the lowestꢀ
energy singlet are derived from the coefficients tabulated in Table
S1 as described there. Generally, energies are derived from the
absorption or emission λmax. TCTꢀNaph (grey) is not observed, and
its energy is approximated. The yꢀaxis * indicates energies deterꢀ
mined computationally.
Figure 4. Selected spectra for 1 (A) and 2 (B) from FSTA experꢀ
iments in deaerated RT DMAc. (C) A lateꢀtime TA spectrum of 2
(dashed line, right axis) and the simulated TA spectrum with the
relevant redoxꢀderived data (solid lines, left axis).
Here, there are two ISC pathways (denoted (A) and (B) in Fig.
5) which cannot be definitely distinguished at this time without
further magneticꢀfield dependent measurements. For (A), the rateꢀ
limiting step is SCTꢀNaph→TCTꢀNaph ISC. These states should have
comparable energies as the orthogonal orbital systems limit direct
exchange interactions between the unpaired electrons. While spin
orbit coupling between SCTꢀNaph TCTꢀBiph is not expected to be large
given the common orbitals, the nearꢀdegeneracy could increase
the electronic coupling of the states via, for example, hyperfine
interactions,18 thus contributing to the speed of the overall SCTꢀNaph
to TCTꢀBiph reaction. In (B), ISC directly couples SCTꢀNaph with the
longꢀlived TCTꢀBiph. To understand why this might be fast, we
draw on literature showing that CT states with perpendicular geꢀ
ometries have been shown to have greatly enhanced kISC to a loꢀ
cally excited triplet19 because the large change in orbital angular
momentum facilitates spinꢀorbit coupling between reactant and
product states. Due to the high degree of spatial separation of
SOMOs generally required to facilitate ISC via (A),20 (B) is conꢀ
sidered to be the more likely of the two pathways.
ly relaxes to an S1 unique to 2. Interestingly, the beginnings of
evolution on the nanosecond timescale is observable, in which a
broad ESA emerges at redder wavelengths (Fig S15), correspondꢀ
ing to the spectral features seen in longer time NSTA data (Fig.
2). This is consistent with triplet state formation.
The low degree of electronic coupling between the naphthyl
substituent that is ~ 90° (GS geometry) relative to the phenoxaꢀ
zine core enables assignment of this unique S1 in 2 using absorpꢀ
tion spectra of respective ions in a redox pair. The spectrum and
molar extinction of the cation (~ 14,400 Mꢀ1 cmꢀ1 at 490 nm) was
determined by spectroelectrochemical bulk electrolysis of 2 (see
SI for details). The anionic spectrum was produced by chemical
reduction of naphthalene using sodium,15 and the molar extinction
ε(λ) was estimated using relevant literature16 (~ 3,500 Mꢀ1 cmꢀ1 at
465 nm) (see SI for details). The sum of these redoxꢀderived data
is used to generate a simulated FSTA spectrum, which is comꢀ
pared with a longꢀtime FSTA spectrum of 2 in Fig. 4C. These data
agree well, particularly with respect to the ~ 475 nm absorptive
feature. The S1 of 2 is thus assigned and denoted as SCTꢀNaph
.
These findings show that the introduction of a CT state with
perpendicular geometry at an appropriate energy can dramatically
improve ΦISC for phenoxazineꢀbased OPCs, and, importantly, this
state has minimal impact on other photophysical parameters key
to photocatalysis.
By way of summarizing, an energy level diagram is presented
in Fig. 5. In 1, excitation to the FranckꢀCondon singlet (SFC) is
followed by solvent reorganization and interꢀring conformational
relaxation as SCTꢀBiph is formed and thermalized on a < 10 ps timeꢀ
scale. That state can in principle engage in ISC but this process is
in competition with efficient photoluminescence driven by a large
ASSOCIATED CONTENT
kr. The result is a high Φem concurrent with low ΦISC
.
Additional photophysical, computational, electrochemical, and
data analysis details. This material is available free of charge via
the Internet at http://pubs.acs.org.
For 2, there is an important difference inasmuch as SCTꢀBiph is
only transiently populated prior to formation of an intermediate
SCTꢀNaph. This has significant consequences that are, we believe,
inherent to the geometry of the CT excited state that bears strong
resemblance to a radical pair. First, the perpendicular anion and
cation πꢀsystems significantly decrease kr connecting SCTꢀNaph with
the GS. This phenomenon is seen in compounds like 9,9ꢀ
bianthryl.17 The second consequence concerns the overall reaction
SCTꢀNaph → TCTꢀBiph, which competes effectively against kr and knr.
AUTHOR INFORMATION
Corresponding Author
*Email: Niels.Damrauer@colorado.edu
Notes
The authors declare no competing financial interests.
3
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J. W.; Hawker, C. J.; Read de Alaniz, J., J. Org. Chem. 2016, 81, 7155ꢀ
7160. (b) Koyama, D.; Dale, H. J. A.; OrrꢀEwing, A. J., J. Am. Chem. Soc.
2018, in press.
(10) Du, Y.; Pearson, R. M.; Lim, C.ꢀH.; Sartor, S. M.; Ryan, M. D.;
Yang, H.; Damrauer, N. H.; Miyake, G. M., Chem. Euro. J. 2017, 23,
10962ꢀ10968.
(11) McCarthy, B. G.; Pearson, R. M.; Lim, C.ꢀH.; Sartor, S. M.;
Damrauer, N. H.; Miyake, G. M., J. Am. Chem. Soc. [Online early access].
DOI: 10.1021/jacs.7b12074. Published Online: Mar 7, 2018.
https://pubs.acs.org/doi/10.1021/jacs.7b12074 (accessed Mar 27, 2018).
(12) Because triplet energetics and solvation are expected to be very
similar for these two molecules (vide infra), this 2.5 difference in lifetime
may indicate participation by other excited states in determining electronic
coupling for nonꢀradiative decay.
(13) Terenziani, F.; Painelli, A.; Katan, C.; Charlot, M.; BlanchardꢀDesce,
M., J. Am. Chem. Soc. 2006, 128, 15742ꢀ15755.
(14) Note that the reported time constants in this figure arise from the fit
of a single measurement, whereas the values in Table S1 originate in the
average of multiple measurements.
(15) DMAc and other solvents considered (e.g. dimethoxyethane) possess
a solvent window that precludes the use of electrochemical techniques.
(16) Shida, T.; Iwata, S., J. Am. Chem. Soc. 1973, 95, 3473ꢀ3483.
(17) (a) Nakashima, N.; Murakawa, M.; Mataga, N., Bull. Chem. Soc. Jpn.
1976, 49, 854ꢀ858. (b) Becker, H. D.; Langer, V.; Sieler, J.; Becker, H. C.,
J. Org. Chem. 1992, 57, 1883ꢀ1887.
(18) (a) Levanon, H.; Norris, J. R., Chem. Rev. 1978, 78, 185ꢀ198. (b)
Dance, Z. E.; Mi, Q.; McCamant, D. W.; Ahrens, M. J.; Ratner, M. A.;
Wasielewski, M. R., J Phys Chem B 2006, 110, 25163ꢀ73.
ACKNOWLEDGMENT
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An NSF GRFP supported S.M.S. and B.G.M. G.M.M acknowlꢀ
edges support from CSU, CU Boulder, and grant R35GM119702
of the NIH. We gratefully acknowledge the use of XSEDE superꢀ
computing resources (NSF TGꢀCHE170050).
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