Photoreduction of chloranil by benzhydrol and related compounds. Hydrogen atom abstraction vs sequential electron-proton transfer via quinone triplet radical ion-pairs

Article abstract of DOI:10.1021/ja970271i

The photoreduction of chloranil (Q) to the hydraquinone (QH₂) in benzene by benzhydrols and by related arylmethanols has been investigated. The products of photooxidation of the benzhydrols are benzophenones, in lieu of formation of benzpinacols. Three distinct mechanisms of oxidation-reduction have been identified from quantum yield determinations and laser flash photolysis experiments, including quinone triplet quenching via H-atom and electron transfer paths. Direct excitation of ground state quinone complexes has also been investigated. The quenching of the triplet state of the quinone by benzhydrol proceeds normally (k(q) = 1.3 x 10⁶ M^-1 s^-1) and gives semiquinone radical (QH^., λ(max) = 435 nm) and the benzhydryl radical (λ(max) = 535 nm). The latter intermediate decays by pseudo-first-order kinetics through hydrogen atom transfer with ground state quinone (Q). Triplet quenching by bis(4-methoxyphenyl)methanol proceeds at a more rapid rate (k(q) = 5.5 x 10⁹ M^-1 s^-1) leading to an intermediate that is identified as the chloranil radical anion (λ(max) = 450 nm). A similar intermediate is observed on Q quenching by 1-naphthylmethanol and acenapthenol with the appearance of an accompanying naphthalene radical cation absorption (ca. 670 nm). The radical ion transients, which are assigned to contact ion-pairs (triplet excited complexes) of the quinone and the various electron donors, decay to semiquinone radicals (QH^.) by first-order processes occurring in the 100 ns time regime. The transient behavior is interpreted in terms of a hydrogen atom transfer mechanism for photoreduction with benzhydrol and, for the more robust electron donors, a mechanism involving electron transfer followed by proton transfer between geminate radical ions. For the electron transfer donors, ground state charge-transfer (CT) complexes can be observed (λ(max) ca. 500 nm). Selective CT excitation leads to quinone photoreduction with reduced quantum yield. The results are discussed in terms of the time resolution of sequential electron/proton transfer steps for photogenerated ion-pairs, the occurrence of one photon-two electron transfer photoredox mechanisms and the kinetically distinct pathways for decay of singlet and triplet intimate radical ion-pairs.

Full text of DOI:10.1021/ja970271i

8788
J. Am. Chem. Soc. 1997, 119, 8788-8794
Photoreduction of Chloranil by Benzhydrol and Related
Compounds. Hydrogen Atom Abstraction vs Sequential
Electron-Proton Transfer via Quinone Triplet Radical
Ion-Pairs
Guilford Jones, II,*,† Nandini Mouli,† William A. Haney,† and
William R. Bergmark*,‡
Contribution from the Department of Chemistry, Boston UniVersity, Boston Massachusetts 02215,
and Department of Chemistry, Ithaca College, Ithaca, New York 14850
ReceiVed January 27, 1997X
Abstract: The photoreduction of chloranil (Q) to the hydroquinone (QH2) in benzene by benzhydrols and by related
arylmethanols has been investigated. The products of photooxidation of the benzhydrols are benzophenones, in lieu
of formation of benzpinacols. Three distinct mechanisms of oxidation-reduction have been identified from quantum
yield determinations and laser flash photolysis experiments, including quinone triplet quenching via H-atom and
electron transfer paths. Direct excitation of ground state quinone complexes has also been investigated. The quenching
of the triplet state of the quinone by benzhydrol proceeds normally (kq ) 1.3 × 106 M-1 s-1) and gives semiquinone
radical (QH•, λmax ) 435 nm) and the benzhydryl radical (λmax ) 535 nm). The latter intermediate decays by
pseudo-first-order kinetics through hydrogen atom transfer with ground state quinone (Q). Triplet quenching by
bis(4-methoxyphenyl)methanol proceeds at a more rapid rate (kq ) 5.5 × 109 M-1 s-1) leading to an intermediate
that is identified as the chloranil radical anion (λmax ) 450 nm). A similar intermediate is observed on Q quenching
by 1-naphthylmethanol and acenapthenol with the appearance of an accompanying naphthalene radical cation absorption
(ca. 670 nm). The radical ion transients, which are assigned to contact ion-pairs (triplet excited complexes) of the
quinone and the various electron donors, decay to semiquinone radicals (QH•) by first-order processes occurring in
the 100 ns time regime. The transient behavior is interpreted in terms of a hydrogen atom transfer mechanism for
photoreduction with benzhydrol and, for the more robust electron donors, a mechanism involving electron transfer
followed by proton transfer between geminate radical ions. For the electron transfer donors, ground state charge-
transfer (CT) complexes can be observed (λmax ca. 500 nm). Selective CT excitation leads to quinone photoreduction
with reduced quantum yield. The results are discussed in terms of the time resolution of sequential electron/proton
transfer steps for photogenerated ion-pairs, the occurrence of one photon-two electron transfer photoredox mechanisms,
and the kinetically distinct pathways for decay of singlet and triplet intimate radical ion-pairs.
The photoreduction of carbonyl compounds ranks among the
CIDNP, CIDEP, and flash photolysis techniques have also been
applied to the problem of atom vs electron transfer mechanisms
in the photoreduction of carbonyl compounds.5,6 Coupled
electron/proton transfer sequences have played a role as well
in the transformation of other functional groups (e.g., photo-
chemical addition or substitution involving stilbene7 or iminium
functional groups).8 Only rarely has it been possible to observe
directly sequential steps of electron and proton transfer that
appear to be important to these carbonyl photoreduction
mechanisms. For example, Manring and Peters9a and Mataga
et al.9b using picosecond pulsed laser techniques observed the
evolution of ion-pairs into radical pairs on photolysis of
benzophenone and N,N-dialkylanilines.
best known of organic photochemical processes.1 The focus
of early mechanistic studies was the photoreduction of ben-
zophenone by benzhydrol,2 for which a mechanism involving
hydrogen atom transfer to the triplet state of the ketone, followed
by the coupling or disproportionation of resulting radicals was
readily discerned.3 Many variations on this theme have now
appeared, usually involving the carbonyl compound participating
as an electron acceptor and various reducing agents leading to
charge transfer in a primary quenching step. For example, in
an extensive series of studies, Wagner and his co-workers,4 using
quantum yield, product, and substituent effect probes have
distinguished the differing paths of photoreduction for acetophe-
nones and benzophenones by arenes (e.g., toluenes) and
identified structural features that dictate the changeover from
H-atom abstraction to electron transfer for these systems.
Adding to studies of the electron transfer photochemistry of
the high potential quinone chloranil (Q) with cyclopolyenes,10
arenes,11 and pinacols12 we have investigated quinone reactions
† Boston University.
‡ Ithaca College.
(5) Wong, S. K. J. Am. Chem. Soc. 1978, 100, 5488.
(6) Roth, H. D.; Manion, M. L. J. Am. Chem. Soc. 1975, 97, 6886.
(7) Lewis, F. D. Acc. Chem. Res. 1986, 19, 401.
X Abstract published in AdVance ACS Abstracts, September 15, 1997.
(1) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cum-
mings: Menlo Park, CA, 1978; Chapter 10.
(8) Mariano, P. S. Acc. Chem. Res. 1983, 16, 130.
(2) Moore, W. M.; Hammond, G. S.; Fess, R. P. J. Am. Chem. Soc. 1961,
83, 2789.
(3) (a) Gilbert, A.; Baggott, J. Essentials of Molecular Photochemistry
CRC Press: Boston, 1991; pp 302-328. (b) Schuster, D. I.; Karp, P. B. J.
Photochem. 1980, 12, 333.
(4) (a) Wagner, P. J.; Truman, R. J.; Puchalski, A. E.; Wake, R. J. Am.
Chem. Soc. 1986, 108, 7727. (b) Wagner, P. J.; Thomas, M. J.; Puchalski,
A. E. J. Am. Chem. Soc. 1986, 108, 7739. (c) Wagner, P. J.; Leavitt, R. A.
J. Am. Chem. Soc. 1973, 95, 3669.
(9) (a) Manring, L. E.; Peters, K. S. J. Am. Chem. Soc. 1985, 107, 6452.
(b) Miyasaka, H.; Morita, K.; Kamada, K.; Nagata, T.; Kiri, M.; Mataga,
N. Bull. Chem. Soc. Jpn. 1991, 64, 3229.
(10) Jones, G., II; Haney, W. A. J. Phys. Chem. 1986, 90, 5410.
(11) (a) Jones, G., II; Haney, W. A.; Phan, X. T. J. Am. Chem. Soc.
1988, 110,1922. (b) Jones, G., II; Mouli, N. J. Phys. Chem. 1988, 92, 7174.
(12) Perrier, S.; Sankararaman, S.; Kochi, J. K. J. Chem. Soc., Perkin
Trans 2 1993, 825. See, also: Gaillard, E.; Whitten, D. G. Acc. Chem.
Res. 1996, 29, 296.
S0002-7863(97)00271-0 CCC: $14.00 © 1997 American Chemical Society

Chloranil Photoreduction by Benzhydrol and Related Compounds
J. Am. Chem. Soc., Vol. 119, No. 38, 1997 8789
with benzhydrol and related arylmethanols with the objective
of relating the mechanism of formal two-electron reduction of
the quinone (Q f QH2) to the classic example of benzophenone
photoreduction. The strongly oxidizing properties of chloranil
and its spectral features are uniquely suited for examination of
photoredox processes under several modes of photoexcitation.
For example, using nanosecond laser flash photolysis methods,
we can observe ion-pairs derived from Q and simple arenes
(e.g., 1,4-dimethylnaphthalene and related donors, represented
schematically as DH2) in a nonpolar solvent (benzene).11 Thus,
for these systems that show a ground state interaction (for high
concentrations of quinone and electron donor), the lowest triplet
of the charge-transfer (CT) complex is accessible through
quenching of free triplet chloranil (eq 1).
anisms for biological systems.19,20 For special consideration is
a class of photoredox reactions that involve a net two electron
reduction-oxidation per initiating photon (e.g., reduction of
indigo dyes or flavins by amines).21
In the present work we report the photoreduction of chloranil
in the presence of benzhydrols of variable oxidation potential
with other arylmethanols substituted with naphthalene moieties
(NCH2OH and ANOH). Of particular interest is the observation
of ion-pair intermediates and the direct measurement of intrapair
proton transfer rate constants including kinetic isotope effects.
The results also provide a comparison of the behavior of triplet
and singlet excited complexes (radical ion-pairs) and further
reveal mechanistic steps for a one photon-two electron redox
system.
3Q + DH2 f
3(Q•-, DH2
)
(1)
•+
Results
triplet excited complex (TEC)
Chloranil (Q) Photoreduction by Hydrogen Atom Donors.
On irradiation of 5.0 mM 2,3,5,6-tetrachloro-2,5-cyclohexadi-
ene-1,4-dione (Q) and 500 mM Ph2CHOH in benzene at 366
nm,22 benzophenone and tetrachlorohydroquinone (QH2) were
obtained as the exclusive products (HPLC). These products
were identified by comparison with authentic samples; the
routine oxidation product, benzpinacol,3 was unobserved. The
quantum yields for steady irradiation at 366 nm (low conversion)
were benzophenone, 0.24, QH2, 0.31, and benzpinacol, < 0.01.
Apparent quantum yields were diminished at higher degrees of
conversion (>10%) due to the known tendency of QH2 to act
as an inhibitor and unproductive quencher of quinone excited
states.16
3(Q•-, DH2•+) f 3(QH•, DH•)
(2)
Several unusual and noteworthy features emerge from laser
flash photolysis experiments on this system. Electron transfer
intermediates (radical ion spectra) that result from the quenching
of quinone triplets (eq 1) can be observer directly in the
convenient time domain of 50-500 ns following the laser pulse.
These species which can be generated in a nonpolar solVent
(typically benzene) are shown to evolve subsequently into
radical species, the result of intra-ion-pair proton transfer (eq
2). Using straightforward photokinetic techniques, it is possible
Laser flash photolysis of the Q/benzhydrol system was carried
out using the frequency tripled output of a Nd/YAG laser (355
nm, typically 80 mJ/per 7 ns pulse) and the detection system
previously described.23 In Figure 1 the absorption due to
phototransients is depicted for several time intervals following
the laser pulse. The triplet state of the quinone (λmax ) 510
nm), apparent in early times, was replaced by the superimposed
absorptions of the semiquinone radical (QH•, λmax ) 435 nm)24
and the benzophenone ketyl (λmax ) 535 nm).26 Apparent in
Figure 1 also is the dramatic difference in rate of disappearance
of the two radical species suggesting a rapid decay path for the
ketyl that competes with conventional radical recombination.
(13) (a) Kobashi, H.; Funabaski, M.; Kondo, T.; Morita, T.; Okada, T.;
Mataga, N. Bull. Chem. Soc. Jpn. 1984, 57, 3557. (b) Levin, P. P.; Kuz’min,
V. A. IzV. Akad. Nauk, Ser. Khim. 1992, 3, 572. (c) Levin, P. P.; Pluzhnikov,
P. F.; Kuz’min, V. A. Khim. Fiz. 1989, 8 1504. (d) Levin, P. P.; Kokrashvili,
T. A.; Kuz’min, V. A. IzV. Akad. Nauk SSSR 1983, 284. (e) Petrushenko,
K. B.; Vokin, A. I.; Turchaninov, V. K.; Gorshkov, A. G.; Frolov, Y. L.
IzV. Akad. Nauk SSSR 1985, 267.
(14) Tahara, T.; Hamaguchi, H. J. Phys. Chem. 1992, 96, 8252.
(15) Johnston, L. J.; Scaiano, J. C.; Wilson, T. J. Am. Chem. Soc. 1987,
109, 1291.
(16) (a) Becker, H.-D.; Turner, A. B. In The Chemistry of the Quinoid
Compounds; Patai, S.; Rappoport, Z., Eds.; John Wiley: New York, 1988;
Vol. 2, p 1351. (b) Inouye, H.; Leistner, E. In The Chemistry of the Quinoid
Compounds; Patai, S.; Rappoport, Z., Eds.; John Wiley: New York, 1988;
Vol. 2, p 1294.
(17) Carlson, B. W.; Miller, L. L. J. Am. Chem. Soc. 1985, 107, 479.
(18) Bridge, N. K.; Porter, G. Proc. Roy. Soc. 1958, 276.
(19) Miller, L. L.; Valentine, J. R. J. Am. Chem. Soc. 1988, 110, 3982.
(20) Shugichi, F.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am. Chem.
Soc. 1987, 109, 305.
(21) (a) Kellett, M. A.; Whitten, D. G.; Gould, I. R.; Bergmark, W. R.
J. Am. Chem. Soc. 1991, 113, 358. (b) Schanze, K. S.; Lee, L. Y. C.;
Giannotti, C.; Whitten, D. G. J. Am. Chem. Soc. 1986, 108, 2646. (c) Traber,
R.; Kramer, H. E. A.; Hemmerich, P. Pure Appl. Chem. 1982, 54, 1651.
(22) For a discussion of a contribution of absorption due to a complex
of Q and solvent benzene, see ref 11a.
to extract rate constants for intrapair proton transfer and for
back electron transfer with the ion pair and to establish struc-
ture-reactivity relationships for radical ion reactivity with vari-
ous Q/donor combinations.11 The observation of triplet excited
complexes (TEC) on laser photolysis of Q/arenes is important
and not an isolated incident, as suggested by the identification
of similar long-lived ion-pair intermediates for several quinones
with a variety of electron donors (other arenes, anilines, and
nitrogen heterocycles13,14). These triplet excited complexes have
been structurally characterized by resonance Raman spectros-
copy which reveals a highly radical ionic character.14
There is a long standing interest in the steps involved in a
net two-electron reduction of quinones, in part due to their
importance in biological redox processes.16,17 The issue of
hydrogen atom vs electron transfer (or hydride) as a primary
step in quinone reductions was raised some years ago,18 and
many studies have been directed to differentiating these mech-
(23) Malba, V.; Jones, G., II; Poliakoff, E. D. Photochem. Photobiol.
1985, 42, 451.
(24) Gschwind, R.; Haselbach, E. HelV. Chim. Acta 1979, 62, 941.
(25) (a) Sankararaman, S.; Haney, W. A.; Kochi, J. K. J. Am. Chem.
Soc. 1987, 109, 5235, 7824. (b) Shida, T. Electronic Absortion Spectra of
Radical Ions; Elsevier: New York, 1988; p 254.

8790 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Jones et al.
Table 1. Rate Constants for Quenching of Triplet Chloranil by
Hydrol Donors, Absorption Data, and Formation Constants for
Ground State Q/Donor Complexes in Benzene Solution
kqa (M-1 s-1
)
λ
max (CT) (nm) KCT, M-1
ꢀ
CT
(CH3)2CHOH
Ph2CHOH
Ph2CDOH
An2CHOH
NCH2OH
ANOH
6.6 × 106
1.3 × 107
0.70 × 107
5.5 × 109
4.0 × 109
8.1 × 109
470
500
510
3.6
0.52
200
665
a Obtained from 0.5 mM Q in benzene. Values are (15%.
Figure 1. Transient spectra resulting from flash photolysis of 0.5 mM
chloranil (Q) and 0.6 M benzhydrol (Ph2CHOH) in benzene. Peak
maxima are associated with the intermediate triplet quinone, the
semiquinone radical (QH•), and the benzhydryl radical, Ph2C•(OH).
Delay times are 0.25-3.85 µs following a 355 excitation pulse
(descending curves at 500-550 nm, 600 ns intervals).
Figure 3. Decay kinetics for the QH• transient resulting from flash
photolysis of 0.5 mM chloranil and 0.6 M benzhydrol in benzene.
donor alcohol, isopropyl alcohol, are shown in Table 1. For
these measurements, the transient at 510 nm was monitored as
a function of donor concentration and lifetime quenching plots
obtained. Preparation of deuterated benzhydrol (Ph2CDOH)
allowed determination of the isotope effect on bimolecular
quenching (kH/kD ) 2.0 ( 0.2).
Figure 2. Decay kinetics for the 535 nm transient (benzhydryl radical)
resulting from flash photolysis of 0.5 mM chloranil and 0.6 M
benzhydrol in benzene (λexc ) 355 nm).
Chloranil Photoreduction with Electron Donors. Chloranil
also could be photoreduced with the benzhydrol donor substi-
tuted with electron donating groups, [(p-MeOC6H4)2CHOH
(dianisylmethanol or An2CHOH)]. Again, hydroquinone (QH2)
and substituted benzophenone An2CdO were the exclusive
products; the quantum yields for photolysis of a benzene solution
with 5.0 mM Q and 10 mM An2CHOH at 366 nm were 0.063
(An2CdO) and 0.051 (QH2). Flash photolysis experiments
revealed that although there were apparent similarities in the
overall photoreduction, the mechanism in fact involved a
different path. The triplet state of Q was quenched at a very
high rate by An2CHOH (Table 1). Notably also, triplet
quenching now results in a phototransient at 450 nm (Figure
4), characteristic of the quinone radical-anion.11,13a,24 The
corresponding anisole-type radical cation would be expected
to absorb in the same region, about 440 nm.25 These absorptions
are diminished rapidly to give a weakly absorbing transient at
435 nm (QH•), which persists well into the 10 ms regime.
Although the overlapping absorptions are difficult to distinguish,
the single exponential decay profile for absorption at 450 nm
(Figure 5) is fully consistent with the assignment of the radical
ion intermediates Q•- and An2CHOH•+ decaying as a bound
pair. The radical species, An2C•OH, which would result (along
with QH•) from proton transfer between primary electron transfer
products (Q•-, An2CHOH•+) is observed at several hundred ns
following the laser flash but is rapidly depleted (or absent),
depending on quinone concentration, due to reaction with ground
state Q (vide supra).
Scheme 1
Notably, the disappearance of the ketyl transient followed first-
order kinetics (Figure 2, correlation coefficient ) 0.98); the
decay rates for the 535 nm transient were also dependent on
the concentration of ground state Q. Thus, the lifetimes of the
decay monitored at 535 nm were obtained for concentrations
of Q ) 0.25-1.5 mM, and from a plot of 1/τ ) 1/τo + kq[Q],
the value of the rate constant for presumed bimolecular trapping
of the ketyl by Q was calculated (k ) 1.1 × 109 M-1 s-1. Other
reactions of ketyls with ketones have been measured to be 3.6
× 105 M-1 s-1).27 The decay of the transient absorbing at 435
nm (QH•) followed second-order kinetics. From a plot of the
data (r ) 0.984), k/ꢀ ) 2.1 × 106 cm s-1 could be calculated,
and using ꢀ ) 7300 M-1 cm-1 for the semiquinone radical,24
the rate constant for bimolecular reaction of QH• was obtained
(1.5 × 109 M-1 s-1). The mechanism proposed for Q
photoreduction-benzhydrol oxidation is shown in Scheme 1.
Rate constants for the quenching of quinone triplet with
various donors including benzhydrol and a prototype hydrogen
The replacement of substituted benzhydrol by arylmethanols
having naphthalene moieties led to a somewhat different display
of the ion-pair structures. A broad featureless component of
the phototransient spectra was introduced suggesting the
(26) Inbar, S.; Linschitz, H.; Cohen, S. G. J. Am. Chem. Soc. 1982, 104,
1979.
(27) Demeter, A.; Berces, T. J. Phys. Chem. 1991, 95, 1228

Chloranil Photoreduction by Benzhydrol and Related Compounds
J. Am. Chem. Soc., Vol. 119, No. 38, 1997 8791
According to the scheme, the rate constant for decay and the
lifetime for the triplet ion-pair are related as kd ) 1/τ ) kH +
kisc, involving the separate steps of intra-ion-pair proton transfer
and decay to the ground state, respectively. Rate constants can
therefore be calculated as follows
kH ) Φ(QH•)/τ
(3)
(4)
kisc ) [1 - Φ(QH•)]/τ
where Φ(QH•) is the quantum yield for the conversion of triplet
ion-pairs to radical pairs, computed as previously described for
Q complexes of substituted naphthalene (hydrocarbon) donors.11
For this measurement the decay of species at 450 nm was
monitored following a 355 nm laser pulse as shown in Figure
5. Transient absorbances associated with quinone anion (ca.
100 ns decay time) and radical (ca. 50 ms decay) were
extrapolated to zero time, and the ratio of transients taken,
assuming extinction coefficients for Q•- (ꢀ450 ) 9300 M-1
Figure 4. Transient spectrum from 355 nm laser flash of 0.5 mM
chloranil and 5.0 mM An2CHOH in benzene. The curves are recorded
180 ns, 2.65 µs, and 3.85 µs after the flash in descending order.
cm-1 24 and QH• (ꢀ435 ) 6600 M-1 cm-1 12b
) . The data
)
including the lifetimes of the ion-pair intermediates, the quantum
efficiencies for radical formation, and the derived rate constants
are shown in Table 2. For the deuterated dianisylmethanol, An2-
CDOH, an isotope effect on the intra-ion-pair proton transfer
was obtained: kH/kD ) 1.37 ( 0.24.
Quinone Photoreduction on Direct Irradiation of Ground
State Complexes. An additional distinguishing feature for the
arylmethanols that are better electron donors is the observation
of ground state complexes with Q and charge-transfer (CT)
absorption bands.28 The intervention of complexes is illustrated
for the dianisylmethanol donor in Figure 7 and other absorption
data with computed complexation constants provided in Table
1. With the combination of Q and concentrations of donor
sufficiently high for ground state interaction, another option for
photoexcitation of the pair was available. Under conditions in
which benzene solutions were nearly saturated with An2CHOH
(0.3 M) and [Q] ) 2.5 mM, samples were irradiated at 436 nm
(Hg lamp), a wavelength where free quinone absorbs poorly.
From the equilibrium and absorptivity data, it was possible to
estimate that virtually all the light under these conditions was
absorbed by the complex. Redox products, hydroquinone and
benzophenone, were again obtained, but quantum yields were
significantly reduced: An2CdO, 0.029; QH2, 0.036. Also, laser
photolysis at 355 nm resulted in negligible transient absorption
at >20 ns time delays consistent with excitation of quinone
complexes and rapid decay of the resultant singlet radical ion-
pairs (eq 5).28
Figure 5. Decay of the 450 nm transient (Q•-) on flash photolysis of
0.5 mM chloranil and 5.0 mM An2CHOH in benzene.
characteristic appearance of naphthalene radical cations (λmax
) 650-690 nm11,24). This feature decays rapidly along with
Q•- and is shown in Figure 6 for Q triplet quenching by
1-naphthylmethanol (NCH2OH). The first-order decays found
for radical ion transients (monitored at 670 nm for the
naphthalenes and at 450 nm for the benzhydrols) are shown in
the Figure 6 inset and in Table 2. Data at both wavelengths fit
first-order decay curves, and the resulting rate constants, 1.1 ×
107 and 3.1 × 107 s-1, are the same within experimental error,
consistent with the decay of a bound pair of ions.
It was possible to monitor conveniently the growth and decay
of the radical species resulting from electron/proton transfer
(note transients at longer time scales in Figure 7). Thus, the
species NCH•OH was observed to grow in at 370 nm; inspection
of the early time domain revealed a first-order growth of this
transient (Figure 7, insert) with a τ ) 120 ns. This mirrored
the corresponding decay of the radical ion (670 nm). Con-
comitant with the formation of the donor-derived radical was a
shift in the Q•- absorption at 450 to 435 nm, characteristic of
the radical, QH•. This later radical transient disappeared at a
rate that depended on Q concentration (most noticeably at [Q]
> 1.0 mM). Monitoring the 370 nm transient ([Q] ) 0.5 mM)
fit second-order decay, consistent with a recombination process
of NCH•OH and QH• with a rate constant of 1.3 × 109 M-1
s-1. The radical species observed for acenapthenol (ANOH)
(390 nm) was also shown to appear (τ ) 95 ns) concomitant
with decay of the TEC intermediate.
1[Q•-, ArCH(OH)R•+] f [Q, ArCH(OH)R]
Discussion
(5)
The photolysis of chloranil in the presence of the benzhydrols
(arylmethanols) takes a course that differs from the conventional
route to benzpinacol for carbonyl compounds and alcohols.3 The
photoredox products (hydroquinone and ketone) are, however,
those that have been observed in many cases of quinone
photoreduction by alcohols.16 The reaction is facilitated by the
interception of radicals by ground state quinone, according to
the mechanism of Scheme 1. This step can proceed in one of
several ways including H-atom or electron transfer.29 The
(28) Singlet excited CT complexes exhibit lifetimes generally in the 0.1-
1.0 ns range. For a review of time resolved measurements, see: Jones, II,
G. In Photoinduced Electron Transfer Fox, M. A., Chanon, M., Eds.;
Elsevier Science Publishers: Amsterdam, 1989; Vol. A.
(29) Reaction of Q and benzhydryl radicals leading to hemiacetal adducts
is a possibility which has some precedence in thermal quinone oxidations
(ref 30).
Kinetics of Intra-Ion-Pair Proton Transfer for Chloranil
Complexes. The alternative mechanism for quinone triplet
quenching via electron transfer with arylmethanols is shown in
Scheme 2. The key steps involve the formation and decay of
the intimate radical ion-pair of presumed triplet multiplicity.28

8792 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Jones et al.
Figure 6. Absorption spectra of transients obtained from flashing 0.59 mM chloranil and 56.7 mM naphthylmethanol in benzene at delay times
20, 100, and 200 ns. Insets are the decay curves monitored at 450 and 670 nm. Data points have been fit to exponential decay curves.
s-1 for benzene, 25 °C) for the hydrols that are more effective
Table 2. Lifetimes of Radical Ion-Pair Intermediates (TEC),
electron donors36 signals the change in mechanism. The
Quantum Yields, and Rate Constants for Intra-Ion-Pair Proton
Transfer for Excited Complexes of Chloranil and Hydrol Donorsa
appearance of transients that are assigned to the chloranil radical
τ, ns
Φ(QH•)
10-6 kH, s-1
10-6 kisc, s-1
anion (λmax ) 450 nm) and the naphthalene radical cation (λmax
) 650-690 nm) is consistent only with a mechanism for
photoreduction which employs electron transfer as a primary
step. The first-order decay in the 100 ns regime for the radical
ion species followed by the appearance of a second longer lived
intermediate (435 nm, QH•) further confirms the mechanism
(Scheme 2).
An2CHOH
An2CDOH
NCH2OH
ANOH
130
120
110
95
0.15
0.12
0.34
0.11
1.3
0.91
2.5
6.4
7.4
6.6
9.9
1.1
a Obtained from 0.5 mM solutions of Q in benzene. Values are
(15%.
A feature that confers stability on the ion-pair transients is
the large charge-transfer component between donor and acceptor
moieties. That is, these intermediates are true Mulliken ion-
pairs,37 a characteristic that is not necessarily shared by other
triplet “exciplexes” that have larger contributions from electronic
configurations that are local in character.38 Chloranil-alkyl-
benzene triplet excited complexes show a substantial radical
ion character as shown by transient resonance Raman spectros-
copy.14 The absence of local excited state character, the
Coulombic stabilization of an electrostricted pair (in nonpolar
solvent), and the imposition of a somewhat larger S1-T1 singlet-
triplet splitting may contribute to the stability of the quinone
triplet excited complexes (TECs).11b
The TEC species participate in proton transfer within the
solvent cage yielding a radical pair that dissociates readily. The
lower magnitude of the quantum yield for photoreduction and
the modest quantum yields of radicals on adoption of the
electron transfer mechanism show that intrapair proton transfer
is a modest competitor for intersystem crossing (note kH and
Kisc values in Table 2). Comparison of kinetic acidities for
several radical-cation systems is warranted. The proton transfer
step proceeds somewhat more slowly (from 3 × 102 to 2 × 107
M-1 s-1) for bimolecular transfer to pyridine bases from arene
radical cations in acetonitrile.39 Faster proton transfers have
mechanism of hydrogen atom transfer has been proposed
previously,31 based on pulse radiolysis and flash photolysis
experiments in which hydroxyalkyl radicals react with ground
state (benzo)quinone. Alternatively, electron transfer trapping
of the arylmethanol radicals appears feasible (eq 6) and would
be appropriately fast (kobd >109 M-1 s-1).
Q + ArC•(OH)R f [Q•-, ArC+(OH)R]
(6)
Formation of an ion-pair in benzene solvent (followed by a
final proton transfer step or ionic addition29) thus would be
attractive, given the oxidative properties of chloranil32 and the
strongly reducing characteristics of (OH)- and (OR)-substituted
carbon radicals.33 The other prominent examples of sequential
two-electron reduction (reduction with amines21) require also
reducing (aminoalkyl) radicals.33 In the present case the
interception of ArC•(OH)R radicals in the follow up step for
reduction of the quinone can be directly observed. The primary
step of H-atom transfer for Q and benzhydrol (Scheme 1)
proceeds for benzhydrol (and isopropyl alcohol) at a normal
(although fast) rate for hydrogen abstraction,1,3 consistent with
an accessible n,π* triplet for chloranil.34
The significant aspects of the present work concern the
observation of ion-pair intermediates for quinone reduction in
benzene and the temporal resolution in the 100 ns regime for
the critical intra-ion-pair proton transfer step. The high rate of
triplet quenching (near the diffusion limit of ca. 5 × 109 M-1
(34) The assignment of the configuration of the chloranil triplet has varied
but most evidence favors a low lying π,π* triplet state.35 Consistent with
the notion of a close lying, accessible and a reactive n,π* triplet are the
various observations of Paterno-Buchi cycloadditions of Q (for a review
of the observations, see: Jones, II, G. In Organic Photochemistry; Padwa,
A., Ed.; Marcel Dekker, Inc.: New York, 1982; Vol. 5).
(35) Bunce, N. J.; Ridley, J. E.; Zerner, M. C. Theor. Chim. Acta 1979,
45, 283.
(30) (a) Walling, C.; Zhao, C.; El-Taliawi, G. M. J. Org. Chem. 1983,
48, 4919. (b) Camaioni, D. M.; Franz, J. A. J. Org. Chem. 1984, 49, 1607.
(31) (a) Land, E. J.; Swallow, A. J. J. Biol. Chem. 1970, 245, 1890. (b)
Bensasson, R.; Land, E. J. Trans. Faraday Soc. 1971, 67, 1904.
(32) The reduction potential for Q in acetonitrile solvent is 0.01 V vs
SCE: Mann, C. K. Barnes, K. K. Electrochemical Reactions in Non-aqueous
Systems; Marcell Dekker, Inc.: New York, 1970.
(36) Oxidation potentials for model compounds are as follows: for benzyl
alcohol, 1-methylnaphthalene, acenaphthene, and anisole, >2.3, 1.53, 1.36,
and 1.65 V vs SCE, acetonitrile (ref 32).
(33) (a) Wayner, D. D. M.; McPhee, D. J.; Griller, D. J. Am. Chem.
Soc. 1988, 110, 132. (b) Griller, D.; Lossing, F. P. J. Am. Chem. Soc. 1981,
103, 1586.
(37) Mulliken, R. S.; Person, W. B. Molecular Complexes; Wiley-
Interscience: New York, 1969.
(38) O’Connor, D. V.; Ware, W. R. J. Am. Chem. Soc. 1979, 101, 121.

Chloranil Photoreduction by Benzhydrol and Related Compounds
J. Am. Chem. Soc., Vol. 119, No. 38, 1997 8793
Figure 7. Absorption spectra of transients obtained from flashing 1.0 mM chloranil and 25 mM naphthylmethanol in benzene. The inset represents
the time evolution of the decay monitored at 375 nm.
Scheme 2
Figure 8. Development of charge-transfer absorption on addition of
An2CHOH to chloranil (1.0 mM) in benzene.
measured for An2CHOH, can be compared with that determined
been observed sometimes (k ) 1.5 × 109 s-1 for intra-ion-pair
transfer from diphenylmethylamine to benzophenone9a and k
) 5.4 × 109 s-1 for intra-ion-pair transfer from benzophenone
and alkylanilines).9b In related experiments on chloranil in
combination with arenes (e.g., 1-methylnaphthalene), kH values
ranging from 0.4 × 106 to 4.2 × 106 correlated with the charge
density associated with the aromatic ring position next to the
acidic (CH3) site.
The kinetic isotope effect for quenching of Q triplets, kq(H)/
kq(D) ) 1.9 ( 0.20, shows C-H bond breaking and therefore
H-atom abstraction for Ph2CHOH. On the other hand, the value
of kq(H)/kq(D) ) 1.1 ( 0.10 for An2CHOH indicates that H
transfer is not the rate-determining step. Wagner4 similarly
found negligible isotope effects on triplet quenching when
electron transfer was involved. On the other hand, the kinetic
isotope effect for the proton transfer step, kH/kD ) 1.4 ( 0.24
for Q and 1-methylnaphthalene system (kH/kD ) 3.4).11b
Maximum kinetic isotope effects have been obtained for systems
with symmetrical transition states.40 The smaller magnitude
found for intrapair proton transfer in An2CHOH suggests that
proton transfer in the triplet ion-pair of proceeds via an earlier,
less symmetrical transition state. This is consistent with the
enhanced stability associated with benzhydryl radical relative
to the naphthylmethyl. Another isotope-lowering effect in this
system may be the enforced nonlinearity of the transition state
for proton transfer associated with a sandwich-like arrangement
in the contact ion-pair. (For an earlier depiction see Scheme 5
in ref 11a.) Manring and Peters9a also obtained a value of 1.4
for the isotope effect for the proton transfer within the radical
ion-pair for benzophenone and dimethylaniline.
Of interest also is the contrasting behavior of the CT complex
of Q and An2CHOH. Under “CT irradiation” conditions, a
(39) Schlesener, C. J., Amatore, C.; Kochi, J. K. J. Am. Chem. Soc. 1984,
106, 7472.
(40) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic
Chemistry; Harper & Row: New York, 1987; pp 236-238.

8794 J. Am. Chem. Soc., Vol. 119, No. 38, 1997
Jones et al.
radical ion species of singlet multiplicity is expected.28 The
diminution of quantum yield for photoreduction under these
conditions is consistent with the exceedingly rapid back electron
transfer step for the singlet ion-pair (ps) that has been observed
in the recent study of Q and durene that included a picosecond
time resolved measurement.41 Photoreduction based on the
electron transfer mode therefore depends on a moderate rate of
intra-ion-pair proton transfer that successfully competes with a
(back) electron transfer for radical ion-pairs generated on Q
triplet bimolecular quenching. The back electron transfer step
is prohibitively fast for singlet ion-pairs produced on direct
excitation of the Q/arylmethanol CT complex.
In the present example of a two electron reduction-oxidation
that is apparently driven by excitation by a single photon, the
circumstances are dictated by the secondary reaction of a
reducing radical derived from the benzhydrols with the ground
state of chloranil. This pattern of reactivity is now familiar for
photoreductions that employ amines21 but is less well understood
for simple alcohols that are themselves less readily oxidized.
We are continuing studies directed to a better understanding of
redox photoprocesses that employ sequential steps of electron
and proton transfer.
generated via many laser pulses (usually from 10 to 100) and plotted
either as ln ∆OD vs time or 1/∆OD vs time (linear least squares
analysis). All samples were photolyzed at room temperature using
purified anhydrous benzene as solvent with argon sparging (usually 5
min/mL). Photoexcitation was carried out by irradiation into the
chloranil absorption (330 nm) using the third harmonic of the Nd:YAG
laser (355 nm). Rates of quenching of the Q triplet were determined
by measurement of the pseudo-first-order decay of the triplet transient
at 515 nm with varying concentrations of hydrol donors. Lifetimes
were determined from 400 digitized signal averaged data points (> 3
decades, r > 0.990) and plotted: τo/τ ) 1 + kqτo[donor] (τo ) 5.7 ms
for Q in benzene). At least three separate determinations were averaged
with a variance of 15%. Decay of the QH• radical was fit to second-
order plots, the slopes of which yielded k/ꢀ ) 2.1 × 106 cm s-1 for
flash photolysis of 0.5 mM Q and 0.6 M benzhydrol.
Steady State Photolysis. For determination of product quantum
yields previously described an apparatus was used, the essential
components of which are an Oriel 500 W lamp, a B and L monochro-
mator (9.6 nm bandpass), and a rhodamine B quantum counter.43
Ferrioxalate actinometry was used for calibration of the apparatus.44
Typically a 2 mL solution of the sample was placed in a quartz cuvette
with magnetic stir bar, sealed with a septum, and then sparged with
argon before irradiation in the monochromator apparatus with magnetic
stirring. The products were analyzed (an average of three runs) by
HPLC with reference to a calibration curve of integrated area versus
concentration determined for the pure substance (authentic samples of
benzophenones and hydroquinone, QH2).
Experimental Section
Benzhydrol and benzpinacol (Aldrich) were purified by recrystal-
lization from ethanol. Benzophenone and 4,4′-dimethoxybenzophenone
were recrystallized from methanol and petroleum ether. Tetrachloro-
hydroquinone (Aldrich) was purified by recrystallization from glacial
acetic acid, followed by washing with distilled water, drying under
vacuum, and then vacuum sublimation. Baker HPLC grade benzene
was purified by fractional distillation from P2O5 and then stored over
molecular sieves. Chloranil (Q) purchased from Aldrich was further
purified by recrystallization from purified benzene (vide supra) followed
by vacuum sublimation at 2.5 T (ca. 135 °C). The purified quinone
gave rise to the characteristic shoulder in benzene (ꢀ330 ) 2200 M-1
cm-1).11a Acenapthenol (ANOH) (mp 147-148 °C) and 1-naphthyl-
methanol (mp 61-62 °C) (Aldrich) were recrystallized from hexanes.
Spectrograde isopropyl alcohol was fractionally distilled.
Benzhydrol-d and Bis(4-methoxyphenyl)methanol-d. Specifically
monodeutereated samples of the title alcohols were obtained by hydride
reduction of benzophenone and 4,4′-dimethoxybenzophenone following
procedures for reduction leading to undeuterated benzhydrols.42 Ben-
zophenone (0.02 mol) was allowed to react with 0.44 g (0.01 mol) of
LiAlD4 in dry ether. The mixture was refluxed for 10 min and poured
onto a mixture of 1 mL concentrated sulfuric acid and 20 g of crushed
ice. The ether layer was separated, washed, dried, and evaporated to
give white crystals which were recrystallized from ethanol/water,
yielding 2.0 and 2.7 g of Ph2CDOH and An2CDOH, respectively. NMR
spectra confirmed the position of the deuterium label (absence of peak
at 4.5 δ, CDCl3) and showed >95% deuterium incorporation.
Laser Flash Photolysis. The apparatus described elsewhere in
detail23 is comprised of a Quantel YG-581 Nd:YAG laser (with
frequency doubling and tripling capability) interfaced with a LeCroy
Tr8818 100-megasample/s digitizer with an Apple Macintosh IIci
computer. Other components include an Oriel 150-W xenon monitoring
lamp, Instruments SA H-20 monochromator with attached Kinetic
Systems stepping-motor controller for microcomputer controlled wave-
length selection from 200-800 nm, and an RCA 4840 photomultiplier
tube. Monitoring and laser beams were configured perpendicularly with
a cell path length for the monitoring beam of 2 cm and that for the
laser beam of 1 cm.
The HPLC system used was a Rainin/Gilson HP/HPX instrument
with binary solvent delivery system with a Rainin Microsorb high
performance C18 reversed phase column and a microprocessor. Sample
injections were made via a Rheodyne 725 injector; a KRATOS 757
variable wavelength UV-vis detector was used. The HPLC solvent
system was 100% Baker HPLC grade methanol vs a mixture of 90%
Millipore water (MilliQ system with ion and carbon filters) and 10%
methanol. The solvents were sparged with argon and maintained under
argon throughout the analytical runs. Column operating conditions were
1.2 mL/min at 3500 psi, while the detector was set at a wavelength of
330 nm and a sensitivity of 0.05 absorbance units full scale. Retention
times in minutes were benzophenone, 12.5; tetrachlorohydroquinone,
5.2; chloranil, 9.5; and 4,4′-dimethoxybenzophenone, 11.5. An HP
3380A plotting integrator was used for recording sample absorbances
and retention times as well as quantifying peaks through their integrated
areas relative to an internal standard or with respect to a calibration
curve of integrated area versus concentration. Irradiation times were
generally 10-60 min. The reported quantum yields are an average of
duplicate independent measurements and were reproducible to within
(15%.
Formation Constants for Charge-Transfer Complexes. For three
hydrol donors ground state association with chloranil was detected in
terms of long wavelength absorption bands that were not ascribable to
the sum of spectra for Q and donor (Figure 8 ). Association constants,
Kct, and extinction coefficients for the complexes, ꢀct, could be obtained
using the Benesi-Hildebrand spectrophotometric method.45 The change
in optical density (OD) was recorded for dilute Q solutions with added
excess hydrol donor. Analysis of the slopes and intercepts of plots of
[Q]/∆OD vs 1/[hydrol] (r > 0.995) gave values of Kct and ꢀct, the data
shown in Table 1 with CT absorption band maxima (analysis
wavelengths).
Acknowledgment. The authors thank the Office of Basic
Energy Sciences of the Department of Energy for support of
this work. We thank also Valentine Vullev, Mohammad Farahat,
and William Petit for technical assistance.
Transient spectra were obtained using one to five laser pulses for
sampling at each monitoring wavelength (typically increments of 2-5
nm). Decay curves were obtained from a collection of data points
JA970271I
(43) Jones, G., II; Becker, W. G. J. Am. Chem. Soc. 1983, 105, 1276.
(44) Murov, S. L. Handbook of Photochemistry; Marcel Dekker: New
York, 1973; pp 119-123.
(45) Foster, R. Organic Charge-transfer Complexes; Academic Press:
New York, 1969.
(41) Kobashi, H.; Funabashi, M.; Kondo, T.; Morita, T.; Ukada, T.;
Mataga, N. Bull. Chem. Soc. Jpn. 1984, 57, 3557.
(42) Stewart, R. J. Am. Chem. Soc. 1957, 79, 3057.