Photo-induced proton-coupled electron transfer reactions of acridine orange: Comprehensive spectral and kinetics analysis

Article abstract of DOI:10.1021/ja505755k

The triplet excited state of acridine orange (³*AO) undergoes a proton-coupled electron transfer (PCET) reaction with tri-tert-butylphenol (^ttbPhOH) in acetonitrile. Each of the reaction components possesses a spectroscopic signature, providing a rare opportunity to monitor the individual proton transfer, electron transfer, and H ^?-transfer components in parallel via transient absorption spectroscopy. This enhanced optical tracking, along with excited-state thermochemical analysis, facilitates assignment of the mechanism of excited-state PCET reactivity. ³*AO is quenched via concerted proton-electron transfer (CPET) from ^ttbPhOH to form acridine radical (AOH^?) and ^ttbPhO^? (k_CPET = 3.7 × 10⁸ M^-1 s^-1, KIE = 1.3). Subsequently, AOH^? reduces the phenoxyl radical (k_ET = 5.5 × 10⁹ M^-1 s^-1), forming AOH ⁺ and ^ttbPhO^-, followed by proton transfer (k_PT = 1.0 × 10⁹ M^-1 s^-1) to regenerate the starting reactants.

Full text of DOI:10.1021/ja505755k

Communication
pubs.acs.org/JACS
Photo-induced Proton-Coupled Electron Transfer Reactions of
Acridine Orange: Comprehensive Spectral and Kinetics Analysis
Thomas T. Eisenhart and Jillian L. Dempsey*
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, United States
S
* Supporting Information
Here, we describe the detailed mechanistic study of a system
ABSTRACT: The triplet excited state of acridine orange
(3*AO) undergoes a proton-coupled electron transfer
(PCET) reaction with tri-tert-butylphenol (ttbPhOH) in
acetonitrile. Each of the reaction components possesses a
spectroscopic signature, providing a rare opportunity to
monitor the individual proton transfer, electron transfer,
and H•-transfer components in parallel via transient
absorption spectroscopy. This enhanced optical tracking,
along with excited-state thermochemical analysis, facilitates
assignment of the mechanism of excited-state PCET
reactivity. 3*AO is quenched via concerted proton−
electron transfer (CPET) from ttbPhOH to form acridine
radical (AOH•) and ttbPhO• (kCPET = 3.7 × 108 M−1 s−1,
KIE = 1.3). Subsequently, AOH• reduces the phenoxyl
radical (kET = 5.5 × 109 M−1 s−1), forming AOH+ and
ttbPhO−, followed by proton transfer (kPT = 1.0 × 109 M−1
s−1) to regenerate the starting reactants.
that undergoes excited-state PCET followed by a thermal
PCET reaction that yields the reactants in their electronic
ground states. Our model system is based on acridine orange
(AO), a Brønsted base and a mild excited-state oxidant that
undergoes efficient intersystem crossing to a long-lived triplet
state.16,17 Some N-heterocyclic compounds are known to be
photochemically active toward PCET reagents,18−20 and herein
we show that AO conforms to this reactivity pattern.
Employing AO, we photo-trigger the PCET oxidation of tri-
tert-butylphenol (ttbPhOH) in CH3CN and track the reaction
3
intermediates with time-resolved spectroscopy. *AO can react
with ttbPhOH through a concerted pathway or one of several
stepwise PCET reaction pathways (Scheme 1). The strong
Scheme 1. PCET Square Scheme Incorporating Ground- and
Excited-State Reactivity
roton-coupled electron transfer (PCET) processes are
Pcentral to the conversion of small molecules to energy-rich
fuels.1,2 Harvesting solar energy to achieve these challenging
transformations is an increasingly attractive option as the need
for alternative energy resources becomes more pressing.
Investigation of PCET in biological systems and in molecular
models provides a general understanding of PCET processes
between molecules in their electronic ground states,1−6 but
comparatively little is known about PCET reactions of
electronically excited molecules.7−9 In response, we are
undertaking mechanistic studies to provide a comprehensive
understanding of excited-state PCET reactions. Rationalizing
how to integrate light absorption and H+/e− transfer will
facilitate the design of new systems that can effectively couple
energy capture and conversion.
absorption features of AO (λmax = 425 nm, ε = 12 500 M−1
cm−1) and the corresponding acridinium (AOH+, λmax = 495
nm, ε = 31 250 M−1 cm−1) provide spectroscopic handles for
determining the AO protonation states; the oxidation states of
these species can also be assessed optically (Figure S1,
Supporting Information). Analysis of the transient absorption
(TA) spectra and corresponding kinetics traces provides a
complete picture of the light-induced PCET oxidation of
phenol and the subsequent thermal reaction.
In the absence of reactive O−H bonds, TA spectra of AO in
CH3CN (Figure S2) recorded at time intervals between 100 ns
and 100 μs following excitation (λex = 425 nm) reveal three
features: new absorptions centered at 410 and 540 nm
A great deal of research has focused on the thermal PCET
reactions of phenol substrates, in part because of the
importance of tyrosine radicals in biological systems such as
photosystem II and ribonucleotide reductase.1,2,4,5,10 As a
result, the PCET thermochemistry for a great many phenols is
well established, including parameters for concerted proton−
electron transfer (CPET) and stepwise electron transfer−
proton transfer (ET-PT) and proton transfer−electron transfer
(PT-ET) pathways.11 By coupling this information to excited-
state reduction potentials (obtained via modified Latimer
diagrams12,13) and excited-state pKa values (determined from
Forster cycles14,15), a thermochemical picture of excited-state
̈
Received: June 8, 2014
PCET reactions can be developed.8
© XXXX American Chemical Society
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Communication
corresponding to the triplet excited state of (3*AO) and a
bleach centered at 435 nm due to the loss of the ground-state
AO absorption. These transient features decay non-exponen-
tially: 3*AO undergoes first-order decay through a nonradiative
process (knr), reacts through a triplet−triplet interaction (kTT)
to produce the corresponding radical cation (AO+) and radical
anion (AO−), and is quenched by ground-state AO (ks).16,17,21
Transient features and decay rate constants determined from
modeling single-wavelength kinetics traces are similar to those
reported in water at pH 12 (Table S1, Figure S3).17
ttbPhOH, no dominant signal at 495 nm appears. The signal
corresponding to AOH• decays on the 100 μs time scale.
Optical detection of discrete intermediates aids evaluation of
3
H+/e− transfer mechanisms for reactions between *AO with
ttbPhOH or TEMPOH. The mechanisms indicated in Schemes
2 and 3 are consistent with the spectral evidence for reaction
Scheme 2
In the presence of ttbPhOH, we observe accelerated decay of
3
the *AO transient absorption features (410 and 540 nm)
(Figure 1A). Between 100 ns and 10 μs, the blue absorption
Scheme 3
intermediates. 3*AO is quenched by CPET from both ttbPhOH
and TEMPOH to form ttbPhO• and TEMPO•, respectively,
along with AOH•. This reactivity is presumed to occur through
a hydrogen-bonded encounter complex, consistent with
previous reports of photo-induced PCET reactions in organic
solvents.24−26
After formation of the excited-state CPET products, thermal
back-PCET ensues to yield the ground-state reactants.
Thermochemical analysis (see Supporting Information) in-
dicates that ET-PT and CPET processes are both reasonable
pathways for the reaction of ttbPhO• and AOH• because they
circumvent high energy intermediates (in contrast, initial PT
has ΔG°PT = 49.9 kcal mol−1). Spectral observation of the initial
ET product (AOH+) indicates the reaction proceeds via the
stepwise ET-PT pathway. Specifically, the AOH• formed by
excited-state CPET is a moderate reductant that can reduce
ttbPhO• (ΔGE°T = −20.8 kcal mol−1), and accordingly, ET from
AOH• to ttbPhO• proceeds to form AOH+ and ttbPhO−,
detected by the decay of the transient AOH• signal and the
appearance of the AOH+ absorption feature. Similar reactivity
was noted for reactions of phenyl thiyl radical with 9-mesityl-
10-methylacridinium, the former acting as an oxidant in a
photoredox catalysis scheme.27 The products of this sequential
excited-state CPET−thermal ET sequence are equivalent to a
net-PT process, reminiscent of reactivity proposed for
ruthenium−phenol dyads.24,28,29 Subsequently, AOH+ and
ttbPhO− recombine via PT to form the ground-state reactants,
detected by the decay of the AOH+ signal.
Figure 1. Transient difference spectra of (A) 40 μM AO and 1 mM
ttbPhOH and (B) 40 μM AO and 3 mM TEMPOH in CH3CN at
selected time delays after laser excitation. λex = 425 nm, 0.1 mM
[Bu4N][PF6].
feature undergoes a bathochromic shift to 425 nm. This new
feature is consistent with the formation of a neutral acridine
radical (AOH•), the absorption spectrum (λmax = 425 nm, ε ≥
6500 M−1 cm−1) of which was measured independently via
reduction of AOH+ in a spectroelectrochemical cell (Figure
S4). The concurrent growth of a small signal at 400 nm
indicates the formation of ttbPhO• (λmax = 400, ε = 2120 M−1
cm−1).22 In the time interval 10−100 μs, the AOH• signal
decays and a transient bleach centered at 425 nm, assigned to
the loss of AO, appears on the high-energy side of the spectra.
Most notably, the spectra recorded in the time interval 4−100
μs are dominated by the growth of a prominent feature at 495
nm, which is consistent with production of AOH+.
For comparison, we also explored excited-state PCET
reactivity of AO in the presence of 1-hydroxyl-2,2,6,6-
tetramethylpiperidine (TEMPOH), a well-documented PCET
reagent that favors CPET over stepwise mechanisms.11 As for
ttbPhOH, added TEMPOH quenches the triplet absorbance
features of AO at 410 and 540 nm (Figure 1B). The appearance
of AOH• (425 nm) is observed between 100 ns and 10 μs. The
concurrent detection of TEMPO• is not expected (λmax = 463
nm, ε = 10 M−1 cm−1).23 In contrast with the reactivity of
By contrast, the thermal back-PCET reaction of AOH• and
TEMPO• proceeds via CPET to form the ground-state
reactants in the time interval 1−100 μs. This assignment is
consistent with both spectral observations and the thermo-
chemical preference of TEMPO• to react via CPET pathways
(ΔGC°PET = −21.7 kcal mol−1, ΔGE°T = +8.1 kcal mol−1, ΔGP°T =
+50.1 kcal mol−1). The diverging thermal reactivity of AOH•
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Communication
with ttbPhO• vs TEMPO• is attributed to the different
thermochemistry of the two systems.11
The spectral profiles of the AO-derived intermediates allows
for detailed kinetics analyses. The quenching of 3*AO by
ttbPhOH was monitored by single wavelength TA at 560 nm
(Figure 2). Quenching is enhanced with increasing concen-
composite of the association constant for the formation of the
hydrogen-bonded AO−phenol adduct and the first-order rate
constant for intra-adduct CPET. Because of the complexity of
the system, we cannot directly extract KA and a first-order rate
constant as has previously been done in luminescence
quenching experiments.24−26 Interpreting hydrogen bonding
in the context of kinetics analysis is the focus of ongoing study.
These kinetic details, coupled with thermochemical analysis
(see Supporting Information), support the proposed forward
CPET, reverse ET-PT reaction sequence. An activation energy
(ΔG⧧) in the range of +9.9−11.2 kcal mol−1 can be estimated
from the kCPET (based on the range of [ttbPhOH]
investigated).11 Quenching of *AO by ttbPhOH via an ET
3
pathway is inconsistent with this barrier (ΔG°ET = +23.1 kcal
3
mol−1). If PT from ttbPhOH to *AO is not accompanied by
excited-state deactivation (3*AOH+ is formed initially, then
subsequently relaxes to form AOH+), initial PT has a negligible
barrier (ΔGP°T = +1.6 kcal mol−1). Alternatively, if PT is
3
coupled to excited-state deactivation of *AO, the free energy
change for PT (ΔGP°T = −47.6 kcal mol−1) indicates this
pathway is highly exergonic, though the organic framework
does not offer a clear mechanism for coupling relaxation.31
However, for both of these initial PT pathways, the subsequent
ET reaction between AOH+ and ttbPhO− to form the net PCET
products observed spectroscopically is substantially endergonic
(ΔGE°T = +20.8 kcal mol−1), inconsistent with the observed
ΔG⧧. Further, the large positive ΔGE°T for the PT-ET pathway
suggests that the excited-state PT intermediates AOH+ and
ttbPhO− should be detectable, but they are not observed.
Figure 2. Kinetics traces (lines) of 40 μM AO with 0−1000 μM
ttbPhOH in CH3CN and the simulated spectra (markers). λex = 425
nm, λobs = 560 nm, 0.1 M [Bu4N][PF6].
trations of ttbPhOH, but the excited-state decay is non-
exponential. A kinetics model based on Scheme 2 and
accounting for the intrinsic decay pathways of AO (Scheme
S1) was used to describe the photo-induced PCET reaction
between AO and ttbPhOH and subsequent thermal reactivity.
Intrinsic reactivity of 3*AO (above) is described by rate
constants knr, kTT, and kS, respectively. The presence of trace
amounts of AOH+ in solution is accounted for by the
independently measured rate constant kAOH (4 × 104 s−1),
which describes the relaxation of 3*AOH+ (Figure S5). Excited-
state CPET is described by kCPET. Though not detectable at 560
nm, ET from AOH• to ttbPhO• (kET) and PT from AOH+ to
ttbPhO− (kPT) complete the model. Rate constants for these
processes were determined from kinetics traces recorded at 460
nm and are discussed below.
Rather, consistent with transient spectral measurements and the
3
estimated barrier, *AO is quenched by CPET (ΔGC°PET
=
−26.8 kcal mol−1) from ttbPhOH to form ttbPhO• and acridine
radical AOH•.
The appearance and subsequent reactivity of AOH+ were
3
monitored at 460 nm as functions of AO (and, in turn, *AO)
concentration (Figure 3). Simulations of these kinetics traces
Kinetics simulations were performed with a series of
differential equations (Scheme S2) derived from the kinetics
model described in Scheme S1. The rate law for the coupled
reactions was solved numerically with an ordinary differential
equations solver. Vectors describing initial concentrations of
reactants, intermediates, and products, the time interval for
analysis, and rate constants were provided as inputs to solve the
initial value problem. The time-dependent concentration
profiles produced for each species were translated to simulated
TA difference spectra by applying the Beer−Lambert law and
summing the absorbance values for each species. Rate constants
were adjusted iteratively to simulate the kinetics traces for a
series of five samples (0−1000 μM ttbPhOH) to obtain a self-
consistent set of rate constants. Rate constants independently
determined (knr, kTT, kS, kAOH) for intrinsic decay pathways
were fixed at these values in order to give the most accurate rate
constants for the other processes.
Figure 3. Kinetics traces (lines) of 20−80 μM AO with 500 μM
ttbPhOH in CH3CN and the simulated spectra (markers). λex = 425
nm, λobs = 460 nm, 0.1 M [Bu4N][PF6].
per the model described above provides additional insight into
the subsequent thermal ET and PT steps: ET from AOH• to
ttbPhO• (kET) and PT from AOH+ to ttbPhO− (kPT). In this
analysis, rate constants knr, kTT, kS, kAOH, and kCPET determined
from simulations of 560 nm kinetics traces were fixed. The rate
constant for kET is diffusion limited (5.5 × 109 M−1 s−1),
consistent with the 900 mV driving force for ET (ΔGE°T =
−20.8 kcal mol−1), and no KIE is observed, as expected.11
The second-order rate constant determined for excited-state
3
CPET is 3.7 × 108 M−1 s−1. The analogous reaction for *AO
and ttbPhOD is slightly slower (kCDET = 2.9 × 108 M−1 s−1, KIE
= 1.3) (Figure S6). This small, but non-negligible KIE is
consistent with our assignment of CPET.1,30 The second-order
rate constant determined through kinetic simulations is a
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Journal of the American Chemical Society
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Recombination of AOH+ and ttbPhO− via PT occurs with a rate
constant of 1.0 × 109 M−1 s−1, consistent with the large driving
force (8.7 unit pKa difference, ΔG°PT = −11.9 kcal mol−1).11 No
KIE is observed, as anticipated for diffusion-controlled
bimolecular PT and consistent with related studies (Figure
S7).32
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In summary, we found that *AO reacts with ttbPhOH and
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TEMPOH via excited-state CPET pathways. Excited-state
3
CPET reactions of *AO have not, to our knowledge, been
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, spectrophotometric titrations, cyclic
voltammograms, additional transient absorption spectra, addi-
tional kinetics traces, thermochemical calculations, and details
of kinetic modeling. This material is available free of charge via
the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
dempseyj@email.unc.edu
■
(28) Bronner, C.; Wenger, O. S. Phys. Chem. Chem. Phys. 2014, 16,
3617−3622.
Notes
(29) Chen, J.; Kuss-Petermann, M.; Wenger, O. S. Chem.Eur. J.
2014, 20, 4098−4104.
The authors declare no competing financial interest.
(30) Alligrant, T. M.; Alvarez, J. C. J. Phys. Chem. C 2011, 115,
10797−10805.
ACKNOWLEDGMENTS
■
(31) Dempsey, J. L.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc.
2010, 132, 16774−16776.
This work was supported by the University of North Carolina
at Chapel Hill. We thank Prof. Jeffrey J. Warren and Dr. Jay R.
Winkler for insightful discussions and Dr. M. Kyle Brennaman
for experimental assistance. This research made use of
instrumentation funded by the UNC EFRC: Center for Solar
Fuels, an Energy Frontier Research Center supported by the
U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences, under award no. DE-SC0001011.
(32) Irebo, T.; Zhang, M.-T.; Markle, T. F.; Scott, A. M.;
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