Toward organic photohydrides: Excited-state behavior of 10-methyl-9-phenyl-9,10-dihydroacridine

Article abstract of DOI:10.1021/jp401770e

The excited-state hydride release from 10-methyl-9-phenyl-9,10- dihydroacridine (PhAcrH) was investigated using steady-state and time-resolved UV/vis absorption spectroscopy. Upon excitation, PhAcrH is oxidized to the corresponding iminium ion (PhAcr

Full text of DOI:10.1021/jp401770e

Article
pubs.acs.org/JPCB
Toward Organic Photohydrides: Excited-State Behavior of 10-Methyl-
9-phenyl-9,10-dihydroacridine
Xin Yang,† Janitha Walpita,† Dapeng Zhou,† Hoi Ling Luk,‡ Shubham Vyas,‡ Rony S. Khnayzer,†
Subodh C. Tiwari,§ Kadir Diri,§ Christopher M. Hadad,‡ Felix N. Castellano,† Anna I. Krylov,§
and Ksenija D. Glusac*,†
†Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403,
United States
‡Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, United States
§Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, United States
S
* Supporting Information
ABSTRACT: The excited-state hydride release from 10-
methyl-9-phenyl-9,10-dihydroacridine (PhAcrH) was inves-
tigated using steady-state and time-resolved UV/vis absorption
spectroscopy. Upon excitation, PhAcrH is oxidized to the
corresponding iminium ion (PhAcr+), while the solvent
(acetonitrile/water mixture) is reduced (52% of PhAcr+ and
2.5% of hydrogen is formed). The hydride release occurs from
the triplet excited state by a stepwise electron/hydrogen-atom
transfer mechanism. To facilitate the search for improved
organic photohydrides that exhibit a concerted mechanism, a computational methodology is presented that evaluates the
thermodynamic parameters for the hydride ion, hydrogen atom, and electron release from organic hydrides.
Scheme 1. Excited State Hydride Release from PhAcrH: (a)
the Overall Photochemical Reaction in Acetonitrile and pH
0.65 H2O Mixture (V:V = 1:1); (b) Proposed Mechanism
INTRODUCTION
■
The reduced form of nicotinamide-adenine dinucleotide
(NADH) is a biological cofactor that performs hydride transfer
reactions to various substrates, and these reactions are driven by
the stabilization of the aromatized NAD+ product.1 Given its
biological significance, the mechanism of hydride transfer from
NADH and its analogues has been extensively studied.2−10
These studies have shown that the overall hydride release
occurs either by a concerted single-step process2−5 or by one of
the sequential mechanisms, such as electron−proton−elec-
tron6−9 or electron−hydrogen atom transfers.2,10 The overall
mechanism can be controlled by varying the one-electron
reduction potential of the donor and acceptor or the pKa value
of the radical cation of the NADH analogue.5,11
If the hydride transfer from the NADH analogue is driven
photochemically, the desired reduction of the substrate could
be powered using the photon’s energy. Such a photochemical
event can, in turn, be used to generate solar fuels, by reducing
protons to hydrogen12−18 or carbon dioxide to methanol,19 via
simple earth-abundant organic structures. Motivated by this
perspective, we present here a study of the excited-state hydride
release from one of the widely used NADH analogues, a
dihydroacridine derivative PhAcrH (Scheme 1a). We previously
investigated the photochemistry of a hydroxylated analogue
(PhAcrOH), which was shown to undergo a fast (τ = 108 ps)
release of the hydroxide ion in protic solvents.20 These results
have provided initial evidence that the acridine framework
exhibits a tendency to undergo the excited-state heterolytic
bond cleavage to generate the aromatic PhAcr+ product upon
excitation. In the absence of protic solvation, PhAcrOH
exhibited no photochemistry and its excited state decayed to
the ground state by an S1 → T1 → S0 sequence of intersystem
crossing steps.
This study investigates the hydride transfer from the
photoexcited PhAcrH to the solvent (acidified acetonitrile/
water mixture). We present steady-state and time-resolved (fs−
ns range) UV/vis absorption experiments and the supporting
Special Issue: Michael D. Fayer Festschrift
Received: February 19, 2013
Revised: May 4, 2013
© XXXX American Chemical Society
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density functional theory (DFT) calculations showing that the
photochemical hydride release indeed occurs from the excited
PhAcrH, and that the mechanism involves a sequential e−H
transfer process, as shown in Scheme 1b. These results are
discussed in terms of thermodynamic driving forces for the
excited-state processes.
routinely with variation typically below 5% at a certain Ar flow
rate (Figure S1, Supporting Information). A solution of 0.25
mM PhAcrH in acetonitrile and pH 0.50 water (1:1) mixture
was prepared in a custom built quartz reactor capped with a
PTFE septum (the solution volume was 54 mL; the headspace
volume was 18 mL). After irradiation, a 100 μL sample from
the mixture headspace was injected into the GC using a
Hamilton airtight syringe and the amount of hydrogen was
quantified. The baseline hydrogen detection was done with the
exact same conditions and components but in the absence of
PhAcrH.
EXPERIMENTAL SECTION
All chemicals were purchased from commercial suppliers and
used without further purification unless otherwise noted. H
■
1
NMR spectra were recorded on a Bruker Avance 300 MHz
system. UV/vis absorption spectra were recorded on an Agilent
8453 UV Spectrophotometer in a 1 cm quartz cell. GC analyses
were performed by means of a Shimadzu GC-14A gas
chromatograph equipped with a TCD column. GC-MS analysis
was done with a Shimadzu QP 5050 gas chromatograph mass
spectrometer.
H2O2 Detection. Hydrogen peroxide was detected by using
the triiodide method.21 A solution of 0.08 mM PhAcrH was
prepared in acetonitrile:water (1:1) mixture and irradiated
(vide supra) for 12 min. The irradiated sample was treated with
−
a solution of excess NaI, and the formation of I3 was
monitored on the basis of UV/vis absorption (λmax at 290 nm =
Synthesis. PhAcrH(D) was synthesized according to the
5.2 × 103).21
literature under modified conditions.7
Femtosecond Transient Absorption Experiment. The
800 nm laser pulses were produced at a 1 kHz repetition rate by
a mode-locked Ti:sapphire laser (Hurricane, Spectra-Physics).
The output from the Hurricane was split into pump (85%) and
probe (10%) beams. The pump beam (800 nm) was sent into
an optical parametric amplifier (OPA-800C, Spectra Physics) to
obtain 305−310 nm excitation sources. The probe beam was
focused into a horizontally moving CaF2 crystal for white light
continuum generation between 350 and 800 nm. The flow cell
(Starna Cell Inc. 45-Q-2, 0.9 mL volume with 2 mm path
length), pumped by a Fluid Metering RHSY Lab pump
(Scientific Support Inc.), was used to prevent photodegradation
of the sample. After passing through the cell, the continuum
was coupled into an optical fiber and input into a CCD
spectrograph (Ocean Optics, S2000). The data acquisition was
achieved using in-house LabView (National Instruments)
software routines. The group velocity dispersion of the probing
pulse was determined using nonresonant optical Kerr effect
(OKE) measurements. Sample solutions were prepared at a
concentration needed to have an absorbance of 1.0 at the
excitation wavelength.
Nanosecond Transient Absorption Experiment. The
nanosecond laser flash photolysis experimental setup utilized
for measurements in this paper is described in detail
elsewhere.22 Briefly, a Nd:YAG laser (Spectra Physics LAB-
150-10) was used as the excitation source with an excitation
wavelength of 266 nm. All of the solutions utilized in these
experiments were made such that the absorptivity at 266 nm
was 1. Transient absorption spectra were recorded using a
Roper ICCD-Max 512T digital intensified CCD camera with
up to 2 ns temporal resolution. The single wavelength kinetic
measurements were recorded using a PMT connected to an
oscilloscope, which was directly connected to a computer that
runs a custom LabView control and acquisition program.
Computational Methods. All thermochemical calculations
were performed using the wB97X-D functional.23,24 The
solvation free energies were computed using the CPCM
model.25 Excited-state calculations were performed using
TDDFT/TDA with various functionals. All calculations were
performed using the Q-Chem electronic structure package.26
Additional information is available in section 7b of the
Supporting Information.
10-Methyl-9-phenyl-9,10-dihydroacridine (PhAcrH).
PhAcr+ (450 mg, 1.2 mmol) was suspended in 20 mL of
ethanol. Sodium borohydride (NaBH4) (90 mg, 2.4 mmol) in
ethanol was added dropwise over a period of 5 min. The
resulting colorless solution was refluxed for 2 h. Ethanol was
evaporated in vacuo at room temperature, and the crude
product was extracted with dichloromethane (3 × 40 mL). The
organic layer was dried over CaCl2 and recrystallized from
ethanol to give PhAcrH as white crystals (276 mg, 85%). H-
NMR (300 MHz, CD3CN): 3.38 (s, 3 H), 5.23 (s, 1 H), 6.9−
7.3 (m, 13 H).
The deuterated compound (PhAcrD) was prepared by the
same procedure with NaBD4 and purified by recrystallization.
1H-NMR (300 MHz, CD3CN): 3.36 (s, 3 H), 6.8−7.4 (m, 13
H).
Steady State UV/vis Absorption Experiment. Solutions
of ∼0.1 mM PhAcrH were prepared in acetonitrile:water (1:1)
mixture at different pH values. For oxygen-free experiments, the
samples were degassed for 45 min with argon prior to the
experiments. For the experiments in the presence of oxygen,
the samples were prepared under atmospheric conditions. As
the irradiation source, a medium pressure Hg arc lamp
(Hanovia PC 451050) was used and the excitation wavelengths
were controlled below 350 nm using a short pass filter (Asahi
Spectra USA Inc.). All irradiations were performed at room
temperature. The conversion of PhAcrH to PhAcr+ was
monitored using UV/vis spectrophotometry at different time
intervals.
GC Hydrogen Detection. Shimadzu GC-8A was operated
with ultrahigh purity argon as carrier gas and a 5 Å molecular
sieve column (Restek) to separate gas mixtures. This GC was
customized with two injection ports, the first for syringe
injections and the second for automatic injections from a 500
μL sample loop directly linked to a Schlenk line. The detector
was calibrated against known amounts of H2 gas. In principle,
500 μL of 10% H2 balanced Ar certified gas standard (Praxair)
was injected at different pressures using a home-built Schlenk
line linked to a pressure gauge. The area under the hydrogen
peak was then plotted against the calculated number of the
moles of hydrogen injected to get the calibration constant of
the detector. This constant was then verified by syringe
injections of different volumes of 25% H2 balanced Ar certified
gas standard (Praxair), equilibrated to atmospheric pressure
and placed in an airtight vial. The calibration was performed
1
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RESULTS AND DISCUSSION
■
The photochemical behavior of PhAcrH is affected by the
molecular oxygen and by the pH of the solution. In the
presence of oxygen, efficient photooxidation of PhAcrH to
PhAcr+ was observed using UV/vis absorption spectroscopy
(Figure S2, Supporting Information), and similar results were
reported previously for acridine27−30 and other NADH
derivatives.31−33 While H2O2 is the likely coproduct in this
reaction, the iodide test detected only minor amounts of H2O2
(Figure S2c, Supporting Information), possibly due to the
reaction of PhAcrH with triiodide (Supporting Information).34
In the absence of oxygen and at neutral pH, photooxidation of
PhAcrH to PhAcr+ did not occur. Instead, other photoproducts
are formed (Figure S3, Supporting Information), possibly
dimers formed upon bimolecular coupling of PhAcr radicals
formed upon irradiation (Figure S3, Supporting Information).35
Importantly, a decrease of the solution pH in the absence of
oxygen leads to the formation of increasing amounts of PhAcr+
(Figure S4, Supporting Information). Figure 1a shows an
Figure 2. (a) Femtosecond transient absorption spectra of 1 mM
PhAcrH in ACN and pH 0.65 H2O mixture (V:V = 1:1). (b) Kinetics
at λexc = 310 nm.
the 26 ps lifetime is currently not known; it is likely due to the
solvation dynamics of PhAcrH in the excited state. The 1.6 ns
decay is accompanied by a growth of the new transient with
λmax = 550 nm (2.25 eV). A similar species was observed in
transient absorption spectra of PhAcrOH in aprotic solvation20
and was assigned to the triplet excited state. The intersystem
crossing (τ = 1.6 ns for PhAcrH and 1.4 ns for PhAcrOH20) is
faster than expected for a molecule that lacks heavy atoms.
However, similar findings were reported previously, where the
relatively fast intersystem crossing was explained by transitions
between states with different electronic configurations (El-
Sayed rule).38 It is likely that the similar mechanism operates in
the case of PhAcrH.
Additional support for the assignment of the 550 nm (2.25
eV) intermediate to the T1 state of PhAcrH is provided by
time-dependent density functional theory (TD-DFT) calcu-
lations (Figure S6, Supporting Information). The computed
maximum in the T1 absorption spectrum is at 2.13 eV, which is
slightly red-shifted (by 0.12 eV) relative to the experimental
maximum. This is within error bars of the theoretical method
employed, as confirmed by the differences between computed
and observed absorption maxima of other species (Table 1).
These results provide additional support for the assignment of
the 550 nm band to the T1 state of PhAcrH.
Figure 1. Photoirradiation of 0.1 mM PhAcrH in ACN and pH 0.65
H2O mixture (V:V = 1:1), in the absence of O2, λexc = 220−350 nm.
(a) UV/vis absorption changes upon irradiation. (b) Yield percentage
of H2, detected using GC as a function of pH. (The inset in part b
shows the GC signals for H2 in the absence (blue) and in the presence
(red) of 0.25 mM PhAcrH.)
example of such behavior recorded at pH 0.65, showing that
52% of PhAcr+ is formed at the end of photoirradiation. Head
space analysis via gas chromatography of the irradiated solution
shows that the hydrogen is formed in this process, as one would
expect for the photoreduction of water by PhAcrH. However,
the quantitative analysis revealed that the yield of hydrogen
relative to the starting PhAcrH is only 2.5% at pH 0.65 (Figure
1b), suggesting that the photoreduction by PhAcrH involves
mostly reduction of the cosolvent acetonitrile, possibly
generating protonated ethyl amine, which is known to be
formed upon reduction of acetonitrile either chemically by
molecular hydrogen and hydrides36 or electrochemically.37
Unfortunately, the measurements could not be achieved in
purely aqueous solution, due to the low solubility of PhAcrH in
water. Our attempts to replace acetonitrile with other
cosolvents, such as tetrahydrofuran, were not successful (the
irradiation of the sample generated >50% hydrogen, but the
PhAcrH converted to a product that was not PhAcr+). This
setback can most likely be avoided in the future by the use of
water-soluble photohydrides.
Table 1. Positions of the First Peak (eV/nm) in the
Calculated and Experimental Spectra of Different PhAcrH
Species
position of the first peak (eV/nm)
calculation
experiment
difference
PhAcrH(cis2)
PhAcr+
4.62/268
3.20/388
2.74/453
2.26/549
2.13/583
4.29/289
2.91/426
2.48/500
1.89/655
2.25/550
0.33
0.29
0.26
0.37
0.12
PhAcr•
PhAcrH•+
3[PhAcrH]*
In addition to the formation of the T1 state at 550 nm,
another transient is formed at 430 nm (2.88 eV). The
formation of this intermediate competes with intersystem
crossing and is pH-dependent. Femtosecond transient
absorption spectra of PhAcrH in ACN:water mixtures were
obtained at several pH values (Figure 3). At pH 7, the initially
formed S1 state decays with a concurrent formation of an
intermediate with absorption at 550 nm, which is assigned to
the T1 state of PhAcrH. As the solution pH is lowered, another
process competes with intersystem crossing. This process leads
to a decrease in the absorption at 550 nm and the formation of
a new transient with absorption in the 420−440 nm range. The
possibility of excited-state protonation of the PhAcrH S1 state
To investigate the mechanism of photoreduction by PhAcrH,
femtosecond and nanosecond UV/vis transient absorption
experiments were collected. The initially formed transient is
assigned to the singlet excited state of PhAcrH and exhibits a
broad absorption in the visible range (brown spectrum in
Figure 2a). The S1 state exhibits a two-component decay with
lifetimes of τ1 = 26 ps and τ2 = 1.6 ns (Figure 2b). The origin of
C
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Figure 4. (a) Nanosecond transient absorption spectra of 0.18 mM
PhAcrH in ACN and pH 0.65 H2O mixture (V:V = 1:1), in the
absence of O2, λexc = 266 nm. (b) Kinetics of different concentrations
of PhAcrH in ACN and pH 0.65 H2O mixture (V:V = 1:1) at 580 nm:
0.8 mM (pink), 1.6 mM (blue), 3.2 mM (red). (c) UV/vis absorption
spectrum of 1 mM PhAcr+ (black) and PhAcr radical (pink) in ACN.
(d) Kinetics of PhAcrH(D) in ACN and pH 0.65 H2O mixture (V:V =
1:1) at 580 nm.
Figure 3. Femtosecond transient absorption spectra of 1 mM PhAcrH
The acceptor of the hydrogen atom released from PhAcrH+
is either the solvent or another molecule of PhAcrH+. To
determine the nature of the accepting species, the kinetics of
PhAcrH+ were investigated as a function of PhAcrH
concentration. The lifetime τ3 of PhAcrH+ at 580 nm decreases
as the PhAcrH concentration increases, as shown in Figure 4b
(the lifetime decreases from 264 μs at low concentrations to 93
μs at high concentration). This behavior is indicative of a
bimolecular process, in which the overall hydrogen atom from
PhAcrH+ is released by an electron transfer to another
PhAcrH+ coupled with a release of the proton to the solvent
(second equation presented below). On the basis of these
experimental findings, we propose that the photochemical
oxidation of PhAcrH to PhAcr+ occurs in the following
sequence of steps:
in ACN:H2O (1:1) mixture (the water pH was varied as shown). λexc
=
310 nm.
was excluded, since TD-DFT calculations predict the
+
absorption of protonated ground-state PhAcrH2 to appear at
254 nm, which is below the spectral range covered in our
transient absorption experiment. In addition, the formation of
+
protonated PhAcrH2 in the S1 state was not considered, since
the 420−440 nm intermediate exhibits a lifetime longer than 1
μs. We postulate that this intermediate is an excimer of
PhAcrH, consistent with the previous studies of excimers
formed from aromatic compounds.39,40 The PhAcrH excimer
eventually decays to the ground monomeric state, and no
photochemical hydride release occurs via this pathway.
3
1[PhAcrH]* → [PhAcrH]*
(kisc = 0.63 ns−1)
(kET = 3.3 μs−1)
The T1 state of PhAcrH transfers an electron to the solvent
with a τ3 = 0.3 μs lifetime (Figure 4d and Figure S5, Supporting
Information) to generate PhAcrH•+, with its characteristic
absorption in the 500−800 nm range.6,7 The dynamics of
PhAcrH•+ exhibit two components: (i) decay with a τ4 = 18 μs
lifetime, possibly due to the electron recombination to generate
PhAcrH in the ground state; (ii) decay with τ5 = 202 μs, which
is assigned to the transfer of a hydrogen atom to generate an
iminium ion PhAcr+. The assignment of this decay to the H-
atom transfer is based on the following findings: (i) the
PhAcrH•+ decay is coupled with the growth of a band at 425
nm (Figure 4a), which is assigned to PhAcr+ (absorbs at 425
nm, black spectrum in Figure 4c); (ii) the deprotonation of
PhAcrH•+ was not observed; the deprotonation is expected to
generate neutral PhAcr• radical with its absorption band at 500
nm (purple spectrum in Figure 4c),20 which was not detected
in the transient absorption spectrum of PhAcrH (Figure 4a);
(iii) the kinetic isotope effect was observed for the lifetime τ3
when deuterated PhAcrD sample was investigated (Figure 4d),
with kH/kD ∼ 3 (the value is a rough estimate, since the time
scale of the experiment is insufficient for accurate determi-
nations of these rate constants). The presence of the kinetic
isotope effect is indicative of a hydrogen-atom transfer
process.2,41
3[PhAcrH]* → PhAcrH•+ + e−
2PhAcrH•+ → PhAcr+ + PhAcrH + H+
5H+ + 4e− + CH3CN → CH3CH2NH3+
2H+ + 2e− → H2
It is interesting to compare the photochemical behavior of
PhAcrH with the previously reported photochemistry of
PhAcrOH.20 Despite similar electronic structures, the S1 state
of PhAcrOH releases OH− in a single fast step, while PhAcrH
releases hydride ion from its T1 state by a two-step mechanism.
To evaluate the driving force for each of these photochemical
events, we employ a simple Forster cycle,42 which is frequently
̈
applied to evaluations of excited-state acidities of aromatic
alcohols.43 Using the excitation energies of PhAcrH/PhAcrOH
and PhAcr+, as well as evaluated ΔG values for the
corresponding ground-state reactions, one finds that the
Gibbs free energy change for the excited state release of
OH− ion from PhAcrOH is ΔGOH* = −24.2 kcal/mol, while
this value is ΔGH* = −13.7 kcal/mol for the one-step proton
reduction by excited PhAcrH (Scheme 2 and section 7a in the
D
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that the photoreduction will take place, it does increase its
likelihood. Computational methods are a useful tool to screen
model systems, where the calculated thermodynamic parame-
ters for the proton reduction can be obtained from the
following calculated quantities: (i) one-electron oxidation
potentials of organic hydrides; (ii) Gibbs free energy for the
release of a hydride ion from the organic hydride in solution
(i.e., PhAcrH → PhAcr+ + H−); (iii) Gibbs free energy for the
release of a hydrogen atom from the organic hydride in solution
(i.e., PhAcrH → PhAcr• + H•); (iv) excitation energies of the
organic hydride and the corresponding iminium cation.
Scheme 2. Gibbs Free Energy Profile for Photochemical
Processes of PhAcrH in the Presence of a Proton in Aqueous
Solution
To facilitate the search for photohydrides that act via a
concerted mechanism, we developed a computational protocol
that allows us to evaluate the driving forces for hydride ion,
hydrogen atom, and electron release from organic hydrides
(section 7b, Supporting Information). In our approach, the gas-
phase electronic energy differences are evaluated by wB97X-D/
6-311(+,+)G(2df,p)23,24 and thermodynamic corrections at the
wB97X-d/6-31+G(d,p) level of theory, while the solvation
energetics for all species except hydride ion were evaluated
using the CPCM model.25 Due to computational challenges
associated with the solvation energy for the hydride ion,48 this
value was obtained using the experimental reduction potential
for the H/H− couple and the appropriate thermodynamic
cycle.49 The resulting value is ΔGsolv = −68.265 kcal/mol.
Using this energy, we arrive to the absolute free energy of
hydride in acetonitrile Gabs(H−/ACN) = −402.9 kcal/mol. This
is quite far (9.8 kcal/mol off) from the recently reported value
(−412.7 kcal/mol).48 However, it is close to another reported
value (−404.7 kcal/mol)50,51 (section 7b3, Supporting
Information). Additional corrections due to deficiencies in
calculated solvation energies of cationic species52 and spin
contamination were made using model compound AcrH2 as an
“internal reference”53 (section 7b4, Supporting Information).
Using this approach, the ΔG values for hydride ion, hydrogen
atom, and electron release from PhAcrH were evaluated, as
presented in Table 2. The calculated values are in excellent
Supporting Information). This breakdown shows that the
simple Forster cycle cannot be used to explain the difference in
̈
the photochemical behavior of PhAcrOH and PhAcrH: both
processes are thermodynamically favored, while only the
concerted hydroxide release from excited PhAcrOH was
observed experimentally. In contrast, the one-step proton
reduction to hydrogen by excited PhAcrH does not occur;
instead, the reaction proceeds via electron transfer followed by
H-atom transfer. The difference in behavior between PhAcrOH
and PhAcrH likely arises due to different barriers for the two
photochemical processes. In the case of PhAcrOH, the
hydrogen bonding between the −OH group of PhAcrOH
and the protic solvent (methanol) facilitates the heterolytic C−
OH bond cleavage. The lack of such interaction between the
C−H group of PhAcrH and the solvent makes the one-step
hydride transfer less likely to occur. Scheme 2 presents energies
for different photochemical pathways of PhAcrH estimated
from published electrochemical measurements in aqueous
solution44−47 (more detail is available in section 7a, Supporting
Information). It is interesting to note that the hydrogen
formation from PhAcrH in the ground state is slightly uphill, by
only 10.1 kcal/mol. This result suggests that the excitation by
UV photon provides much more energy than is required for
this process and that, in theory, this process could be driven by
low-energy red photons. We note that the driving force for the
electron release from the T1 state of PhAcrH presented in
Scheme 2 suggests that this process is not thermodynamically
favorable, even though we observe the formation of the
PhAcrH•+ in the experiment. Two possible explanations are
suggested for this discrepancy: (i) the derivation of
thermodynamic parameters was performed using aqueous
solvation, while the actual experiment was performed in the
acetonitrile:water mixture; this solvent mixture possibly alters
the ΔG values reported in Scheme 2; (ii) it is possible that the
radical cation is formed before the T1 state is populated,
possibly from the T2 state of PhAcrH.
Table 2. Gibbs Free Energy for Electron, Proton, and
Hydrogen Transfer Processes of PhAcrH in Acetonitrile
ΔG (kcal/mol)
PhAcrH•+ + e−
PhAcr+ + H−
PhAcr• + H•
experiment
calculation
126.1
126.3
76.3
74.1
69.6
67.9
agreement with the experimental energies, suggesting that this
approach can be used for the computational screening of a large
series of organic hydrides. Time-dependent computational
methods can then be used to evaluate the excited-state
energetics and spectroscopic properties of reaction intermedi-
ates.
Hydride release via stepwise mechanisms is not desirable for
the following reasons: (i) the radicals generated after each step
are reactive and can undergo unwanted chemistry; (ii) the
stepwise processes are more energy-demanding, making it
unlikely to drive such photochemistry using low-energy visible
photons. Thus, it is advantageous to tune the electronic
properties of organic hydride donors to enable the concerted
process. The first step toward this goal is the development of
chemical systems that exhibits thermodynamically favorable
excited-state reduction of protons, while the ΔG values for the
excited-state electron and hydrogen-atom transfer should be
positive. While this thermodynamic condition does not ensure
CONCLUSIONS
■
This manuscript describes a study of the photochemical hydride
release from an organic hydride, PhAcrH. Using time-resolved
spectroscopy, we find that the hydride release occurs via a
stepwise electron-hydrogen atom transfer process. The photo-
induced concerted hydride transfer was not observed, even
though it is thermodynamically favored. To facilitate the search
for organic photohydrides that can undergo a concerted
hydride release, we developed a computational methodology
for evaluation of the relevant thermodynamic parameters.
E
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, computational methodology, TA
laser setup, supplementary figures, and results. This material
is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kglusac@bgsu.edu.
Notes
(15) Curtin, P. N.; Tinker, L. L.; Burgess, C. M.; Cline, E. D.;
Bernhard, S. Structure−Activity Correlations among Iridium(III)
Photosensitizers in a Robust Water-Reducing System. Inorg. Chem.
2009, 48, 10498−10506.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(16) Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B.
Hydrogen Evolution Catalyzed by Cobaloximes. Acc. Chem. Res. 2009,
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This work was supported by National Science Foundation
(CHE-1055397 CAREER award to K.D.G.). A.I.K. acknowl-
edges support from the Department of Energy through the DE-
FGO2-05ER15685 grant and from the Humboldt Foundation
(Bessel Research Award). R.S.K. was supported by a McMaster
Fellowship.
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