Reaction mechanisms and structural characterization of the reactive intermediates observed after the photolysis of 3-(hydroxymethyl)benzophenone in acetonitrile, 2-propanol, and neutral and acidic aqueous solutions

Article abstract of DOI:10.1021/jo1024249

Nanosecond time-resolved resonance Raman (ns-TR³) spectroscopy was employed to investigate the photoinduced reactions of 3-(hydroxymethyl) benzophenone (1) in acetonitrile, 2-propanol, and neutral and acidic aqueous solutions. Density functional theory calculations were utilized to help the interpretation of the experimental spectra. In acetonitrile, the neutral triplet state 1 [denoted here as (m-BPOH)³] was observed on the nanosecond to microsecond time scale. In 2-propanol this triplet state appeared to abstract a hydrogen atom from the solvent molecules to produce the aryphenyl ketyl radical of 1 (denoted here as ArPK of 1), and then this species underwent a cross-coupling reaction with the dimethylketyl radical (also formed from the hydrogen abstraction reaction) to form a long-lived light absorbing transient species that was tentatively identified to be mainly 2-(4-(hydroxy(3- (hydroxymethyl)phenyl)methylene)cyclohexa-2,5-dienyl)propan-2-ol. In 1:1 H ₂O:CH₃CN aqueous solution at neutral pH, (m-BPOH) ³ reacted with water to produce the ArPK of 1 and then underwent further reaction to produce a long-lived light absorbing transient species. Three photochemical reactions appeared to take place after 266 nm photolysis of 1 in acidic aqueous solutions, a photoreduction reaction, an overall photohydration reaction, and a novel photoredox reaction. TR³ experiments in 1:1 H₂O:CH₃CN aqueous solution at pH 2 detected a new triplet biradical species, which is associated with an unusual photoredox reaction. This reaction is observed to be the predominant reaction at pH 2 and seems to face competition from the overall photohydration reaction at pH 0.

Full text of DOI:10.1021/jo1024249

ARTICLE
pubs.acs.org/joc
Reaction Mechanisms and Structural Characterization of the Reactive
Intermediates Observed after the Photolysis of
3-(Hydroxymethyl)benzophenone in Acetonitrile, 2-Propanol, and
Neutral and Acidic Aqueous Solutions
Jiani Ma,† Ming-De Li,† David Lee Phillips,*,† and Peter Wan‡
†Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong S.A.R., P. R. China
‡Department of Chemistry, Box 3065, University of Victoria, Victoria, British Columbia, Canada V8W 3V6
S Supporting Information
b
ABSTRACT: Nanosecond time-resolved resonance Raman
(ns-TR3) spectroscopy was employed to investigate the photo-
induced reactions of 3-(hydroxymethyl)benzophenone (1) in
acetonitrile, 2-propanol, and neutral and acidic aqueous solu-
tions. Density functional theory calculations were utilized to
help the interpretation of the experimental spectra. In acetoni-
trile, the neutral triplet state 1 [denoted here as (m-BPOH)3]
was observed on the nanosecond to microsecond time scale. In
2-propanol this triplet state appeared to abstract a hydrogen atom
from the solvent molecules to produce the aryphenyl ketyl radical of 1 (denoted here as ArPK of 1), and then this species underwent a
cross-coupling reaction with the dimethylketyl radical (also formed from the hydrogen abstraction reaction) to form a long-lived light
absorbing transient species that was tentatively identified to be mainly 2-(4-(hydroxy(3-(hydroxymethyl)phenyl)methylene)cyclohexa-
2,5-dienyl)propan-2-ol. In 1:1 H2O:CH3CN aqueous solution at neutral pH, (m-BPOH)3 reacted with water to produce the ArPK of 1
and then underwent further reaction to produce a long-lived light absorbing transient species. Three photochemical reactions appeared to
take place after 266 nm photolysis of 1 in acidic aqueous solutions, a photoreduction reaction, an overall photohydration reaction, and a
novel photoredox reaction. TR3 experiments in 1:1 H2O:CH3CN aqueous solution at pH 2 detected a new triplet biradical species, which
is associated with an unusual photoredox reaction. This reaction is observed to be the predominant reaction at pH 2 and seems to face
competition from the overall photohydration reaction at pH 0.
’ INTRODUCTION
quantum yield of about 0.6 to produce predominantly 3-formyl-
benzhydrol (2) (95%) and small amounts of 3-formylbenzophe-
none (3) (5%) (see Scheme 1).15
Many benzophenone (BP)-containing compounds have been
studied to elucidate their photophysics and photochemistry under a
variety of conditions.1ꢀ8 Some commonly observed processes have
included Norrish Type I and II reactions and intermolecular
hydrogen abstraction reactions as well as photoinduced electron
transfer under some conditions.9ꢀ13 Photolysis of a number of BP-
containing compounds in organic solvents such as 2-propanol leads
to formation of a ketyl radical via hydrogen abstraction from the
solvent molecule by the photoexcited ketone species. The ketyl
radical can undergo further reactions to produce pinacols or simple
reduction of the ketone.9ꢀ13 Recently, Wan and co-workers re-
ported a new kind of photochemical reaction that appears to occur
for meta-substituted BP in aqueous solution, in which a formal
water-assisted photoredox reaction reduces the ketone to its alcohol
and a meta-substituted alcohol moiety is oxidized to its aldehyde (or
ketone), while this reaction was not detected for para-substituted
derivatives.14ꢀ19 The 3-(hydroxymethyl)benzophenone (1) com-
pound was used as a prime example of this new formal water-assisted
photoredox reaction. The photochemical reactions of 1 in acidic
(pH < 3) aqueous solution was observed to be very efficient with a
Nanosecond transient absorption experiments carried out for
1 in 1:1 H2O:CH3CN gave a spectrum composed of two bands
(λmax = 325 and 525 nm) that could be attributed to the triplet
excited state of the molecule because the triplet state of BP
exhibits a similar spectrum.15 The transient species was found to
be quenched by lowering the pH of the aqueous solution, and
this suggested that a protonated triplet state could lead to the
observed products under acidic conditions. A preliminary
mechanism was proposed, and this is displayed in Scheme 2.15
The mechanism in Scheme 2 indicates that protonation of the
excited state of 1 results in formation of an intermediate with a
positive charge at the meta position (4), similar to an inter-
mediate suggested to be involved in the photodeuteration of
3-methylbenzyophenone18 and an intermediate suggested by
Wirz and co-workers for the acid-catalyzed photohydration of
Received: December 8, 2010
Published: April 05, 2011
r
2011 American Chemical Society
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Scheme 1. Overall Formal Water-Assisted Photoredox Reaction and Products Produced from Photolysis of 1 in Acidic (pH < 3)
Aqueous Solution15
Scheme 2. Preliminary Mechanism Proposed by Wan and Co-workers15 for the Water-Assisted Photoredox Reaction Seen for 1
BP.20 The intermediate 4 then can convert into intermediate 5
that then goes on to form product 2. The general water-assisted
photoredox reaction observed after photolysis of 1 in aqueous
acidic conditions, and its reaction mechanism constitute an
unusual and important pathway that is different from the typical
pathways of BP that are often employed as triplet sensitizers or
hydrogen abstractors. This interesting photoredox reaction still
needs verification of its mechanism and reactive intermediates. In
particular, it is important to obtain some structural characteriza-
tion of the reactive intermediates to better elucidate the identity
and properties of the intermediates and associated reaction
mechanism.
When potential reactive intermediates exhibit electronic absorp-
tions in a similar wavelength region and have similar lifetimes, it can
be very difficult to clearly identify and distinguish different inter-
mediates from one another by time-resolved absorption spectroscopy
methods.21ꢀ25 For reactions where this happens, it is helpful to utilize
time-resolved vibrational spectroscopic methods that are able to use
the structural information in vibrational frequencies to clearly identify
and characterize different intermediates. We have recently used time-
resolved resonance Raman (TR3) spectroscopy to study the photo-
chemistry of BP and BP-containing compounds and obtained new
insight into the identity, structure, and properties of different
intermediates involved in the reactions of these kinds of molecular
systems.26ꢀ30 In this paper, we report TR3 spectroscopic and density
functional theory (DFT) study of the intermediates associated with
Figure 1. ns-TR3 spectra of the intermediates observed after 266 nm
the reactions after photolysis of 1 in varying solvents. The work here
photolysis of 1 in pure MeCN using a 319.9 nm probe wavelength at
aims to verify the photoredox reaction observed after photolysis of 1
various time delays indicated next to the spectra. The asterisks (*) mark
regions affected by solvent subtraction artifacts and/or stray light.
in acidic (pH < 3) aqueous solution by Wan15 and provides structural
information for the reactive intermediates associated with the reac-
tion. To our knowledge, this is the first vibrational spectroscopic
characterization of the intermediates observed after photolysis of 1.
the intermediates observed after 266 nm photolysis of 1 in MeCN
with different delay times indicated near the spectra. Two species
were detected in MeCN. The first species has strong Raman bands at
969, 1169, 1227, and 1543 cmꢀ1 and is attributed to the neutral triplet
’ RESULTS AND DISCUSSION
state 1 [denoted here as (m-BPOH)3] mainly due to two reasons.
First, the ns-TR3 spectra of this species exhibit a great similarity
with previously observed spectra of the triplet state of BP and
A. Nanosecond TR3 Spectroscopy of 1 in Acetonitrile (MeCN)
and in 2-Propanol Solutions. Figure 1 shows the ns-TR3 spectra of
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Figure 2. Schematic depiction of the optimized structures of the ground
state of 1 (top) and the (m-BPOH)3 (bottom) species obtained from
B3LYP/6-311G** DFT calculations. Selected bond lengths (Å) and
bond angles (deg) are labeled in the structures.
(S)-ketoprofen.26ꢀ30 Second, a comparison between the predicted
Raman spectrum from the DFT calculations for the (m-BPOH)3
species and the 5 ns time delay TR3 spectrum (shown in Figure 2S of
the Supporting Information) reveals that the calculated normal
Raman spectrum displays reasonably good agreement with the
TR3 spectrum with some differences in the relative intensity patterns
that can be attributed to the experimental spectrum being resonantly
enhanced while the calculated spectrum is a normal Raman spec-
trum. Figure 2 displays a simple diagram of the optimized geometry
and selected structural parameters obtained from (U)B3LYP/6-
311G** calculations for the ground state of 1 and the (m-BPOH)3
species. The time dependence of the intensity of the 1226 cmꢀ1
Raman band of (m-BPOH)3 in the ns-TR3 spectra could be fit with a
single exponential function with a decay time constant of ∼113 ns,
and a plot of this is shown in Figure 3S of the Supporting Information.
Most of the Raman bands observed from ns-TR3 spectra of the
(m-BPOH)3 intermediate are due to vibrations associated with
the ring CꢀC stretching and CꢀH bending motions for the
800ꢀ1800 cmꢀ1 region. For instance, the 1543 cmꢀ1 Raman
band is mainly due to a ring CꢀC stretch mode while the
1227 cmꢀ1 Raman band is mostly due to the carbonyl CdO
stretching mode, and the 969 cmꢀ1 Raman feature has contribu-
tions mainly from the CꢀC deformation mode. For aromatic
carbonyl compounds, the electronic configuration of the T1 state
determines the T1 state’s reactivity toward the hydrogen abstrac-
tion reaction with hydrogen donor reagents.31ꢀ36 In general, a T1
state with mainly nꢀπ* character will have a high efficiency for
hydrogen abstraction reaction, but a triplet state with a πꢀπ*
character has little if any reactivity for the hydrogen abstraction
reaction. Previous studies determined that the frequency of the
CꢀO stretch mode appears in the 1400ꢀ1600 cmꢀ1 range for a
πꢀπ* character triplet state and in the 1200ꢀ1400 cmꢀ1 region
for a typical nꢀπ* character triplet state. The CꢀO stretching
frequency for (m-BPOH)3 was seen at 1236 cmꢀ1, and this
indicates that (m-BPOH)3 has nꢀπ* character. Therefore, a high
efficiency for hydrogen abstraction reaction in certain solvents is
expected. Another species that has a characteristic Raman band at
Figure 3. ns-TR3 spectra observed after 266 nm photolysis of 1 in
2-propanol solution using a 319.9 nm probe wavelength at various time
delays indicated next to the spectra. The asterisks (*) mark regions
affected by solvent subtraction artifacts and/or stray light.
1584 cmꢀ1 was also observed after 10 ns. Clear identification of
this species at 1584 cmꢀ1 is difficult because of the weak intensity
of the only clear fingerprint band and will not be considered
further here.
Figure 3 displays the ns-TR3 spectra of intermediates ob-
served after 266 nm photolysis of 1 in 2-propanol solution at
various time delays. The first species appearing at early times has
its most intense bands at 967, 1229, and 1546 cmꢀ1 and is
assigned to (m-BPOH)3 on account of the very high similarity of
the 5 ns spectrum from Figure 3 to the (m-BPOH)3 spectrum
recorded in MeCN (see Figure 1). As the (m-BPOH)3 Raman
signal is quenched, a new species with Raman bands at 989,
1176, 1481, 1584, and 1697 cmꢀ1 is observed in Figure 3.
Because 2-propanol is a strong hydrogen donor solvent, (m-
BPOH)3 can easily abstract a hydrogen atom from 2-propanol to
produce an arylphenyl ketyl radical (denoted hereafter as ArPK
of 1), and the second species in the TR3 spectra is tentatively
attributed to the ArPK of 1 species. Figure 4 displays a
comparison of the TR3 spectrum obtained at the time delay of
90 ns (top) selected from Figure 3 with the DFT-calculated
Raman spectrum (bottom) for the ArPK of 1 intermediate with
its structure displayed near the top of the spectrum. The DFT-
calculated Raman spectrum for the ArPK of 1 intermediate is in
good agreement with the ns-TR3 spectrum in Figure 4. The
Raman band at 987 cmꢀ1 in Figure 3 for the ArPK of 1
intermediate was fit with a Lorentzian band shape to obtain
the time dependence of this species, and the kinetic obtained is
displayed in Figure 4S of the Supporting Information. Under the
experimental conditions employed here, the ArPK of 1 inter-
mediate had a growth time constant of ∼36 ns and a decay time
constant of ∼614 ns.
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Figure 4. Comparison of the ns-TR3 spectrum of the ArPK of 1 radical
species acquired at 90 ns time delay after photolysis of 1 in 2-propanol
solution (top) to the calculated normal Raman spectrum (bottom) of
the ArPK of 1 species with the schematic structure shown in the top of
the figure.
Besides (m-BPOH)3 and ArPK of 1 intermediates, a third
species appeared at 1654 cmꢀ1 at longer time delays in Figure 3.
The hydrogen abstraction reaction produces both a dimethyl
ketyl (DMK) radical and an ArPK of 1 radical. Previous studies of
BP29 and its derivatives in 2-propanol have observed that the
ArPK radical species can cross-couple with the DMK radical
mainly at the para-position to form a long-lived light-absorbing
transient species (LAT). Similarly, the third species seen in
Figure 3 can be attributed mostly to the p-ArPKꢀDMK of 1
intermediate. Figure 5S of the Supporting Information displays a
comparison of the experimental ns-TR3 spectrum at 10 μs (top)
obtained from ns-TR3 in 2-propanol to the calculated spectrum
for the p-ArPKꢀDMK of 1 species (bottom) with its structure
shown next to the spectrum. Inspection of Figure 5S shows that
the calculated p-ArPKꢀDMK of 1 normal Raman spectrum
exhibits reasonable agreement with the experimental TR3 spec-
trum and supports the assignment of the third species to the p-
ArPKꢀDMK of 1 intermediate.
Figure 5. ns-TR3 spectra of the intermediates observed after 266 nm
photolysis of 1 in neutral aqueous solution (MeCN:H2O, 1:1) obtained
using a 319.9 nm probe wavelength at various time delays indicated next
to the spectra. The asterisks (*) mark regions affected by solvent
subtraction artifacts and/or stray light.
Scheme 3. Reaction Mechanism Proposed by Wirz and Co-
workers for the Photohydration of BP in Acidic Aqueous
Solutions20
Time-dependent density functional theory (TD-DFT) calcu-
lations were also done for the ArPK of 1 species, and these results
are displayed in Table 1S of the Supporting Information. The
calculations predict that the ArPK of 1 species has an intense
electronic transition at 325 nm which is close to the probe
wavelength used in the ns-TR3 experiments, and this is consistent
with the high intensity of the Raman bands of the ArPK of 1
intermediate in the ns-TR3 spectra.
intermediate and/or the closely related protonated form of this
intermediate (see Scheme 3 for the equilibrium between these
species). The spectra of the second species with its intense bands
at 988, 1177, 1273, 1478, 1582, and 1696 cmꢀ1 are very similar to
the spectra observed for the ArPK of 1 radical seen in the
2-propanol solution (see Figure 3) and can similarly be attributed
to the ArPK of 1 radical. The comparison of ns-TR3 spectrum
acquired at 600 ns after photolysis of 1 in 1:1 MeCN:H2O
solvent with that of 1 in neat 2-propanol acquired at 600 ns shows
fairly good agreement in the Raman shift frequencies. This
suggests that the second species observed in the neutral aqueous
solution is the ArPK of 1 intermediate. This ArPK of 1 radical
observed in water could be produced by an analogous hydrogen
B. Nanosecond TR3 Spectroscopy of 1 in Neutral and Acidic
Aqueous Solutions. Figure 5 displays the ns-TR3 spectra acquired
after photolysis of 1 in neutral aqueous solution (MeCN:H2O,
1:1) at different time delays. Like the ns-TR3 spectra obtained in
2-propanol solution, three species are observed in the ns-TR3
spectra shown in Figure 5. The spectra of the first species in
Figure 5 with intense Raman bands at 969, 1169, 1227, and
1543 cmꢀ1 are very similar to the spectra observed for the (m-
BPOH)3 species seen in MeCN and 2-propanol solutions (see
Figures 1 and 3) and can be attributed to the (m-BPOH)3
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Figure 6. ns-TR3 spectra of intermediates obtained after 266 nm photolysis of ∼3 ꢁ 10ꢀ4 M 1 in an acidic (pH = 2) aqueous solution (MeCN:H2O,
1:1) in open air (a) and oxygen-purging conditions (b) obtained using a 319.9 nm probe wavelength at various time delays indicated next to the spectra.
The asterisks (*) marks regions affected by solvent subtraction artifacts and/or stray light.
abstraction reaction from water which is abundant in the aqueous
system; however, this may not be easy because this is probably
a very endergonic process for water (about ΔG = þ85 kJ/mol
and ΔH = þ28 kJ/mol for BP based on data from references 37
and 38). It is useful to note that photohydration reaction was
observed for BP in acidic aqueous solutions by Wirz and co-
workers20 (see the proposed mechanism in Scheme 3 for the
photohydration of BP in water) so this may also be occurring in
aqueous environments for the system of interest here. Examina-
tion of Scheme 3 shows that the (m-BPOH)3 intermediate is in
equilibrium with its protonated form. It is conceivable that
hydrated electrons present in the aqueous solution could react
with the protonated form of the (m-BPOH)3 intermediate to
form the ArPK of 1 radical. The protonation of the triplet state
BP was observed to be very fast for BP (on the order of several
hundred picoseconds), and the hydrated electron was also ob-
served on the hundreds of picoseconds to one or two nanose-
conds in acidic aqueous solutions.20 A fast two-step process (e.g.,
protonation of the (m-BPOH)3 intermediate followed by reac-
tion with an hydrated electron) could possibly lead to apparent
fast conversion of the (m-BPOH)3 intermediate into the ArPK of 1
radical as observed in the ns-TR3 spectra of Figure 5 after photolysis
of 1 in neutral aqueous solution (MeCN:H2O, 1:1). At present, it is
not clear which possible mechanism is mainly responsible for the
production of the ArPK of 1 radical in neutral aqueous solutions,
and further experimental and theoretical work is needed to better
understand the complex interplay between different reaction path-
waysinneutralaqueoussolutions.ThethirdspeciesseeninFigure5
appears at long time delays and can be tentatively assigned to some
sort of LAT species formed from some reaction of the ArPK of 1
radical with water or other intermediate present in the solution.
Because the signal of the third species in the TR3 spectra is relatively
weak, we shall not attempt to make a definitive assignment here.
Figure 6a shows the ns-TR3 spectra of intermediates ob-
tained after 266 nm photolysis of 1 in an acidic (pH = 2)
aqueous solution (MeCN:H2O, 1:1). The photochemical be-
havior of 1 under this condition is obviously different from that
for the analogous experiments done in neutral aqueous solu-
tion. In Figure 6a new Raman bands appear at 962 and
1518 cmꢀ1. Since the difference between the experiments done
in pH = 2 MeCN:H2O (1:1) and neutral aqueous solutions is
the acid concentration, the new species may result from a
reaction catalyzed by the presence of the acid in aqueous
solutions. In order to study a possible acid catalysis effect, ns-
TR3 experiments were also conducted in acidic aqueous solu-
tions at the pH values of 4 and 0, and their ns-TR3 spectra are
presented in Figure 7. There are at least three photochemical
reactions that appear to occur for the 266 nm photolysis of 1 in
acidic aqueous solutions. The first type of reaction is the
photoreduction reaction that was the predominant reaction
observed for analogous experiments performed in neutral
aqueous solutions. In acid aqueous solutions, the intensity of
the ArPK of 1 radical Raman bands is much smaller than that in
the neutral solution27 so the probable contribution from this
photoreduction reaction to the ns-TR3 spectra obtained under
acidic aqueous conditions can be considered to be small. An
overall photohydration reaction may also take place in moder-
ate acidic aqueous solution similar to the photohydration
reaction observed for BP in acidic aqueous solutions by Wirz
and co-workers20 so this may be a second type of reaction that
may be seen in the spectra of Figures 6 and 7. Scheme 3 shows
the proposed reaction mechanism for the photohydration of BP
in acidic aqueous conditions.20 The photoredox reaction re-
ported by Wan and co-workers for the photolysis of 1 in acidic
solutions is a third type of reaction that may be seen in the
spectra of Figures 6 and 7.
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Figure 7. The 319.9 nm probe wavelength ns-TR3 spectra of intermediates obtained after 266 nm photolysis of ∼3 ꢁ 10ꢀ4 M 1 in an acidic aqueous
solution (MeCN:H2O, 1:1) with pH = 4 (a) and pH = 0 (b) conditions at various time delays indicated next to the spectra. The asterisks (*) mark regions
affected by solvent subtraction artifacts and/or stray light.
Comparison of the spectra shown in Figures 6 and 7 demon-
strates that under pH = 0 and 2 conditions the intensity of the
Raman band at 1584 cmꢀ1 is stronger than that under pH = 4
conditions. The spectra at 5 ns delay time in the pH = 2 and pH = 0
aqueous solutions suggest that the Raman band at 1584 cmꢀ1 has
higher intensity in the pH = 0 solution. On the other hand, the
intensities of the new Raman bands at 962 and 1518 cmꢀ1 are
much stronger in the pH = 2 aqueous solution than in the pH = 4
and pH = 0 solutions. According to the study of Wirz and co-
workers, the efficiency of the photohydration of BP in acidic
aqueous solutions reached a maximum at the pH value of 0.21
On the basis of the preceding results, the strong Raman band at
1584 cmꢀ1 may have contributions from transient species associated
with both the photoredox reaction and the photoreduction
reaction leading to the overall photohydration reaction, while
the Raman bands at 962 and 1518 cmꢀ1 probably have mainly
contributions from intermediates involved in the photoredox
reaction. Because we could not adequately sort out the different
contributions from the two reactions to the Raman band at
1584 cmꢀ1, a quantitative estimate of the rate constant for the
photohydration reaction and the photoredox reaction is not
given here.
The signal associated with the photoredox reaction could also
be detected in the pH = 4 acid aqueous solution. Analysis of the
spectra shown in Figure 7a shows that in the pH = 4 aqueous
solution the relative intensity of the Raman bands at 962 and
1518 cmꢀ1 were apparently lower than that in the pH = 2
solution, which indicates that the photoredox reaction is not so
favorable in the pH = 4 aqueous solution as that in the pH = 2
aqueous solution. Because the photoredox reaction of 1 in acidic
aqueous solution is thought to be acid-catalyzed one would
expect a higher reaction conversion under more acidic conditions.
Wan and co-workers15 also mentioned that decreasing the pH value
gave a higher yield of 3-formylbenzhydrol, the final product of the
photoredox reaction consistent with the above trend of the
intensity of the Raman bands at 962 and 1518 cmꢀ1 associated
with a reactive intermediate leading to the photoredox products.
A later study19 reported that the photoredox reaction was not
obvious in very high acidic aqueous solutions with pH = 0 and
below. Examination of spectra in Figure 7b suggests that the
photoredox reaction can still be observed in a pH = 0 aqueous
solution but the efficiency of the photoredox reaction was
markedly decreased compared to that in the pH = 2 aqueous
solution using the criterion of the Raman features at 962 and
1518 cmꢀ1 as marker bands for the photoredox reaction. The
relative intensities of these marker bands as a function of pH of
the acidic aqueous solutions indicate there may be significant
competition from some side reactions in pH = 0, which is
probably the photohydration reaction. Inspection of Scheme 3
suggests that the protonation of the carbonyl oxygen is an
integral part in the photohydration reaction of BP in acidic
aqueous solution. As mentioned previously, the efficiency of the
photohydration reaction in acid aqueous solutions appears to
reach a maximum at a pH value of 0. If the photohydration
reaction is more favorable than the photoredox reaction in a
pH = 0 aqueous solution, the efficiency of the photoredox
reaction will become decreased relative to the corresponding
reaction in pH = 2 aqueous solution. This seems consistent with
our present results and previous observations of Wirz and co-
workers20 for BP and Wan and co-workers15,19 for 1 in acidic
aqueous solutions.
There are four intermediate species involved in the proposed
mechanism by Wan and co-workers for the photoredox reaction
of 1 after photolysis in acidic aqueous solutions. It is necessary to
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Figure 8. Time dependence of the 962 cmꢀ1 Raman band of the species related to the photoredox reaction obtained in the ns-TR3 measurements with
(a) ∼3 ꢁ 10ꢀ4 M of 1 in open air, (b) ∼3 ꢁ 10ꢀ4 M of 1 with oxygen purging (with ∼18 ns growth time constant and ∼60 ns decay time constant), (c)
∼6 ꢁ 10ꢀ4 M of 1, (d) ∼9 ꢁ 10ꢀ4 M of 1 in pH = 2 MeCN:H2O (1:1) fit with a two-exponential function. See text for more details.
determine which species appear in the ns-TR3 spectra under
acidic aqueous conditions and are associated with the photo-
redox reaction. To study the quenching effect by oxygen, ns-TR3
experiments were also performed with oxygen being bubbled
through the sample solution, and these spectra are displayed in
Figure 6b. Plots of the integrated areas for the new species using a
marker band at 962 cmꢀ1 as a function of time delay were obtained in
both open air condition and oxygen bubbling condition and shown
in Figure 8a,b. The lifetime of this species in the oxygen-purging
experiment is ∼60 ns, which is obviously shorter than that under
the corresponding open air conditions with a lifetime of ∼430 ns.
This suggests that this new species observed in acidic aqueous
solutions has some triplet character, which would be likely
associated with intermediates 4 or 5 in Scheme 2 rather than
with species 7.
The DFT calculations for the species 4 and 5 were performed
and compared with the spectrum of the new species observed in
the ns-TR3 experiments performed under conditions associated
with the observation of the intriguing photoredox reaction. The
spectrum obtained in the pH = 2 aqueous solution at 120 ns time
delay from Figure 6a was selected as being diagnostic of the
intermediate leading to the photoredox product. A comparison
of this experimental spectrum to the calculated Raman spectra
for the species 4 and 5 is shown in Figure 9. Inspection of
Figure 9 suggested that the new intermediate species observed in
moderately acidic aqueous solutions shows more similarity with
the species 5 than with the species 4. This reasonable agreement
between the TR3 spectrum of the new intermediate species (mid-
dle spectrum of Figure 9) and the DFT calculated normal Raman
spectrum of the intermediate 5 (bottom spectrum of Figure 9)
and the triplet character deduced from the oxygen quenching ex-
periment leads us to attribute this new species (this is associated
with the photoredox reaction) to the triplet biradical intermedi-
ate 5. Results from TD-DFT (UB3LYP/6-311G**) calculations
for the intermediate species 4 and 5 provide further support for
this assignment. These computational results are displayed in
Table 2S of the Supporting Information, and the absorption spectra
deduced from the calculations are shown in Figure 7S of the
Supporting Information. Examination of the computational
results indicates that the triplet biradical species 5 has a higher
oscillator strength near the probe laser wavelength (319.9 nm)
used in the ns-TR3 experiments whereas the predicted transient
absorption spectrum of 4 displays a relatively weak oscillator
strength near the 319.9 nm probe laser wavelength. In this case,
the species 4 may not easily be detected in the ns-TR3 experi-
ments with a 319.9 nm probe wavelength because it has a very
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ARTICLE
attributed to a water-assisted process. This suggests that the
photoredox reaction is assisted by the water molecules and does
not involve diffusional encounters with other molecules of 1.
We did not observe obvious Raman bands for the product
3-formylbenzhydrol in the 319.9 nm probe ns-TR3 experiments
under the experimental conditions, which is anticipated because
the probe wavelength of 319.9 nm used in our experiments may
not be a suitable wavelength for probing the signal of 3-for-
mylbenzhydrol. TD-DFT calculation results for 3-formylbenz-
hydrol are shown in Table 3S of the Supporting Information, and
these results indicate that there is very little absorption for
3-formylbenzhydrol at 336 nm which is near the probe wave-
length used in the ns-TR3 experiments. This is consistent with
not observing any Raman signal of 3-formylbenzhydrol in the
319.9 nm probe ns-TR3 experiments on account it does not have
appreciable absorption at 319.9 nm.
’ CONCLUSION
Figure 9. The spectrum obtained in the pH = 2 aqueous solution at 120
ns time delay from Figure 7a was selected as being diagnostic of the
intermediate leading to the photoredox reaction. A comparison of this
experimental spectrum (middle) to those calculated normal Raman
spectra of species 4 (top) and 5 (bottom) is shown.
Our TR3 study here observed formation of the neutral triplet
state 1 [denoted as (m-BPOH)3 ] on the nanosecond to micro-
second time scale in acetonitrile. In the strong hydrogen donor
solvent, 2-propanol, (m-BPOH)3 abstracted a hydrogen atom
from the solvent molecule to form the arylketyl radical of 1(denoted
as ArPK of 1) and then reacted via a cross-coupling reaction with
the dimethylketyl (DMK) radical (also formed from the hydro-
gen abstraction reaction) to produce a long-lived light absorbing
transient (LAT) species 2-(4-(hydroxy(3-(hydroxymethyl)phe-
nyl)methylene) cyclohexa-2,5-dienyl)propan-2-ol that was de-
noted as p-ArPKꢀDMK of 1. In 1:1 H2O:CH3CN aqueous
solution at near neutral pH, the (m-BPOH)3 species appeared to
react in some way to produce the ArPK of 1 that underwent further
reaction to produce a longer lived intermediate. Three photo-
chemical reactions appeared to occur for the 266 nm photolysis
of 1 in acidic aqueous solutions, the photoreduction reaction, an
overall photohydration reaction, and an unusual photoredox re-
action. When the TR3 experiments were performed in 1:1 H2O:
CH3CN acidic aqueous solution at pH 2, the photoredox
reaction appeared to be the predominant one. This is consistent
with the reported results of Wan that 3-formylbenzhydrol (2)
was observed with a high quantum yield of about 0.6 from the
photoredox reaction under similar conditions.15 A new inter-
mediate in TR3 experiments in 1:1 H2O:CH3CN acidic aqueous
solution at pH 2 with Raman bands at 962 and 1518 cmꢀ1 is
associated with the photoredox reaction and is identified as the
triplet biradical species 5 from comparison of the TR3 spectra to
the predicted Raman spectrum of 5 from DFT calculations and
oxygen-quenching experiments. The lack of significant substrate
concentration dependence on the growth and decay times of
species 5 indicates that the photoredox reaction is mainly a water-
assisted process which does not involve diffusional encounters
with other molecules of 1. The photoredox reaction appears to
experience some competition from the overall photohydration
reaction in aqueous solution at pH 0.
Table 1. Dynamics of the Intermediate Species 5 Obtained in
pH = 2 Aqueous Solutions (MeCN:H2O, 1:1) with Different
Concentrations of 1 (see text for details)
concentration
growth time
decay time
of 1 (M)
constant (ns)
constant (ns)
3 ꢁ 10ꢀ4
6 ꢁ 10ꢀ4
9 ꢁ 10ꢀ4
16
10
10
430
440
430
small Raman signal due to its weak resonance enhancement.
Thus, the species observed in our ns-TR3 spectra in pH = 2 acid
aqueous solution is most consistent with and assigned to the
triplet biradical intermediate species 5 (see Scheme 2).15
The effect of the substrate concentration on the resulting
photochemical behavior of 1 was studied. Ns-TR3 experiments
were done for the 266 nm photolysis of 1 with the concentrations
of ∼6 ꢁ 10ꢀ4 M and ∼9 ꢁ 10ꢀ4 M in pH = 2 aqueous solutions,
and these spectra are shown in Figure 6S of the Supporting
Information. The Raman band area at 962 cmꢀ1 was integrated,
and the corresponding kinetic is shown in Figure 8c,d. The
respective growth time constant and decay time constant for the
intermediate 5 species with concentrations of ∼3 ꢁ 10ꢀ4 M, ∼6
ꢁ 10ꢀ4 M, and ∼9 ꢁ 10ꢀ4 M in pH = 2 aqueous solutions are
given in Table 1. This is consistent with the study by Wan that the
photoredox reaction was unaffected by dilution and was observed
down to the lowest concentration of ∼10ꢀ6 M of 1.15 The lack of
a strong concentration dependence on the lifetime constant
upon the concentration of 1 suggests that we are mainly
observing the photoredox reaction that does not involve diffu-
sional encounters with other molecules of 1 in moderately acidic
aqueous solutions. The isotopic effect experiments in D2O/
CH3CN conducted found that no benzhydrol proton was
observed.19 This result also demonstrated that the methine
proton of the final product of the photoredox reaction arises
from water rather than from the substrate molecule. Conse-
quently, the hydrogen migration in the photoredox reaction was
’ EXPERIMENTAL AND COMPUTATIONAL METHODS
The 3-(hydroxymethyl)benzophenone (m-BPOH) compound was
synthesized following the methods detailed previously in the literature,16
and the basic route is also shown in Scheme 1S of the Supporting
Information. Briefly, the first synthesis step combined 3-methylbenzo-
phenone with 1.2 equiv of N-bromosuccinimide and a catalytic amount
of benzoyl peroxide in 50 mL of benzene solvent. This reaction mixture
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ARTICLE
was refluxed for 14 h, and then the mixture was washed twice with 25 mL
of distilled water and subsequently dried with anhydrous MgSO4. The
solvent was then removed in vacuum. The crude 3-benzoylbenzyl bromide
was then combined with 5 equiv of CaCO3 in a 100 mL of H2O:dioxane
(1:1, V:V) solution and then refluxed for 17 h. After conventional purification,
the m-BPOH was obtained utilizing chromatography (silica gel, 1:1 ethyl
acetateꢀhexane). The purity of the m-BPOH sample from the 1H NMR
the comparison of the calculated results with the experimental spectra.
All the calculations presented in this paper were performed using
the Gaussian 0339 program suite installed on the High Performance
Computing cluster at the Computer Centre in The University of
Hong Kong.
’ ASSOCIATED CONTENT
1
spectra was estimated to be greater than 97%. H NMR (400 MHz,
CDCl3) δ: 7.81ꢀ7.79 (m, 3H), 7.73ꢀ7.72 (d, 1H), 7.63ꢀ7.58 (m, 2H),
S
Supporting Information. Figure 1S presents UVꢀvis
b
7.51ꢀ7.47 (m, 3H), 4.77 (s, 2H), 1.62 (s, 1H).
absorption spectra of 1 in pure MeCN, Figure 2S shows
comparison of the experimental ns-TR3 spectrum of the first
species observed in MeCN at 5 ns time delay to the calculated
normal Raman spectrum of (m-BPOH)3, Figure 3S shows the
time dependence of the 1226 cmꢀ1 Raman band of 3(m-BPOH)
obtained from ns-TR3 measurements in MeCN, Figure 4S shows
the time dependence of the 987 cmꢀ1 Raman band of the ArPK
of 1 obtained in the ns-TR3 measurements in 2-propanol, Figure
5S shows the comparison of the experimental TR3 spectrum at
10 μs obtained from the ns-TR3 spectra in 2-propanol solution to
the calculated spectrum for the p-ArPKꢀDMK of 1 species,
Figure 6S shows ns-TR3 spectra of ∼6 ꢁ 10ꢀ4 M and ∼9 ꢁ 10ꢀ4
M of intermediates observed after 266 nm photolysis of 1 in a
pH = 2 MeCN:H2O (1:1) solution. Tables 1S, 2S, and 3S show
the excited state energies and oscillator strengths from TD-DFT
calculations for the ArPK of 1 radical, the transient species 4 and
5, and the final product of the photoredox reaction 3-formyl-
benzhydrol (2). Cartesian coordinates, total energies, and vibra-
tional zero-point energies for the optimized geometry from the
(U)B3LYP/6-311G** calculations for the compounds and inter-
mediates considered in this paper are given. This material is
available free of charge via the Internet at http://pubs.acs.org.
Spectroscopic grade acetonitrile (MeCN), isopropyl alcohol (IPA),
perchloric acid (HClO4), and deionized water were used as solvents for
the experiments reported here. Unless noted, the sample solutions of m-
BPOH employed in the ns-TR3 experiments in MeCN, IPA, and mixed
solvents were all with the concentration of ∼3 ꢁ 10ꢀ4 M. All of the
mixed solvent ratios are of volume ratio. There were four sets of sample
solutions in 1:1 MeCN:H2O with final pH values of 0, 2, 4, and 7 using
HClO4 to adjust the pH value as needed.
UVꢀvis experiments were done to determine reasonable pump and
probe excitation wavelengths used in the nanosecond time-resolved
resonance Raman (ns-TR3) experiments. Figure 1S displays the UVꢀvis
spectrum of m-BPOH in MeCN with the 266 nm pump and 319.9 nm
probe wavelengths used in the ns-TR3 experiments indicated above the
spectrum. Ns-TR3 experiments were performed using an experimental
apparatus and method described previously in our laboratory,26ꢀ30,36
and only a brief description is given here. The fourth harmonic of a Nd:
YAG nanosecond pulsed laser supplied the 266 nm pump wavelength
and the third anti-Stokes hydrogen Raman-shifted laser line from the
second harmonic of a second Nd:YAG laser supplied the 319.9 nm
probe wavelength used in the ns-TR3 experiments. The 266 nm pump
pulse excited the sample to initiate the photochemical reactions and the
319.9 nm probe pulse interrogated the sample and the intermediate
species produced by the pump pulse. The laser beams were lightly focused
and aligned so that they were overlapped onto a flowing liquid stream of
sample. The diameter of the pump beam was adjusted to be slightly
larger than that of the probe beam at the overlapping volume in the liquid jet
in order to minimize the ground-state normal Raman signal. A pulse delay
generator was employed to electronically control the time delay between
the pump and probe laser beams from the two different Nd:YAG lasers
operated at a repetition rate of 10 Hz. The Raman scattered light was
acquired using a backscattering geometry and then detected by a liquid
nitrogen-cooled charge-coupled device (CCD) detector. The TR3 signal
was acquired for 10 s by the CCD before being read out to an interfaced
PC computer, and 10 scans of the signal were accumulated to produce a
resonance Raman spectrum. The ns-TR3 spectra presented in this paper
were obtained from subtraction of an appropriately scaled probe-before-
pump spectrum from the corresponding pumpꢀprobe resonance
Raman spectrum to remove nontransient bands. Pure MeCN Raman
bands were employed to calibrate the Raman shifts of the ns-TR3 spectra
with an estimated accuracy of 5 cmꢀ1. A Lorentzian function was used to
integrate the Raman bands of the species of interest in the ns-TR3
spectra inordertodeterminetheir areasandelucidatethe growthanddecay
time constants of the species observed in the experiments.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: phillips@hku.hk; tel: 852-2859-2160; fax: 852-2957-
1586
’ ACKNOWLEDGMENT
The research was supported by grants from the Research
Grants Council of Hong Kong (HKU 7035/08P) to D.L.P. D.L.
P. thanks the Croucher Foundation for the Award of a Senior
Research Fellowship and the University of Hong Kong for the
Outstanding Researcher Award. Support from the University
Grants Committee Areas of Excellence Scheme (AoE/P-03/08)
and the Special Equipment Grant (SEG HKU/07) is also grate-
fully acknowledged.
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