Photoaddition of water and alcohols to the anthracene moiety of 9-(2′-hydroxyphenyl)anthracene via formal excited state intramolecular proton transfer

Article abstract of DOI:10.1021/ja039078g

The title compound undergoes efficient photoaddition of a molecule of a hydroxylic solvent (H₂O, MeOH, (Me)₂CHOH) across the 9- and 10-positions of the anthracene moiety to give isolable triphenylmethanol or triphenylmethyl ether type products. The reaction is believed to proceed via a mechanism involving water-mediated formal excited state intramolecular proton transfer (ESIPT) from the phenolic OH to the 10-position of the anthracene ring, generating an o-quinone methide intermediate that is observable by nanosecond laser flash photolysis, and is trappable with nucleophiles. A "water-relay" mechanism for proton transfer seems plausible but cannot be proven directly with the data available. Irradiation in deuterated solvents led to incorporation of one deuterium atom at the methylene position in the photoaddition product, and partial deuterium exchange of the 10-position of recovered starting material, consistent with the proposed formal excited state proton transfer mechanism. The deuterium exchange and photoaddition reach maximum quantum efficiency at ～5 M water (in CH₃CN or CH ₃OH), with no reaction observed in the absence of a hydroxylic solvent, demonstrating the sensitivity of this type of ESIPT to solvent composition.

Full text of DOI:10.1021/ja039078g

Published on Web 06/05/2004
Photoaddition of Water and Alcohols to the Anthracene
Moiety of 9-(2′-Hydroxyphenyl)anthracene via Formal Excited
State Intramolecular Proton Transfer
Mitchel Flegel, Matthew Lukeman, Lawrence Huck, and Peter Wan*
Contribution from the Department of Chemistry, Box 3065, UniVersity of Victoria,
British Columbia, Canada, V8W 3V6
Received October 15, 2003; E-mail: pwan@uvic.ca
Abstract: The title compound undergoes efficient photoaddition of a molecule of a hydroxylic solvent (H2O,
MeOH, (Me)2CHOH) across the 9- and 10-positions of the anthracene moiety to give isolable triphenyl-
methanol or triphenylmethyl ether type products. The reaction is believed to proceed via a mechanism
involving water-mediated formal excited state intramolecular proton transfer (ESIPT) from the phenolic OH
to the 10-position of the anthracene ring, generating an o-quinone methide intermediate that is observable
by nanosecond laser flash photolysis, and is trappable with nucleophiles. A “water-relay” mechanism for
proton transfer seems plausible but cannot be proven directly with the data available. Irradiation in deuterated
solvents led to incorporation of one deuterium atom at the methylene position in the photoaddition product,
and partial deuterium exchange of the 10-position of recovered starting material, consistent with the proposed
formal excited state proton transfer mechanism. The deuterium exchange and photoaddition reach maximum
quantum efficiency at ∼5 M water (in CH3CN or CH3OH), with no reaction observed in the absence of a
hydroxylic solvent, demonstrating the sensitivity of this type of ESIPT to solvent composition.
Introduction
Excited state intramolecular proton transfer (ESIPT) processes
continue to be a subject of widespread research efforts, years
after the field was opened up with the pioneering work of
Weller.1,2 ESIPT arises when a molecule contains both acidic
and basic functionalities that experience simultaneous enhance-
ment in both their acidity and basicity, respectively, when
electronically excited, permitting the transfer of a proton from
the acidic group to the basic group. The systems that have
received the most attention are those in which the acidic group
is an OH or NH acid and the basic group is a heteroatom, such
as a carbonyl oxygen or heterocyclic nitrogen atom. The key
feature of these systems is that both the forward proton transfer
(to give the tautomer) and the subsequent reverse proton transfer
(to regenerate the starting material) are extremely fast processes,
usually occurring on the subpicosecond time scale.
not required to mediate the proton transfer). Since this discovery,
we have reported a similar ESIPT that is operative in systems
in which the proton acceptor is a 1-naphthyl group.4 In these
systems, dihydrobenzoxanthene products were photogenerated.
For example, ESIPT from the phenol OH to the 7′-aromatic
carbon atom in 3 led to formation of quinone methide 4, which
underwent electrocyclic ring closure to give the isolable 5 with
reasonably high quantum efficiency (Φ ) 0.20). In addition to
this pathway, ESIPT to the 2′-carbon is operative, as evidenced
by deuterium incorporation in recovered starting material. Both
routes required that a sufficient amount of water in the solvent
mixture (∼5 M) to reach maximum efficiency.
More recently, we have reported a new type of ESIPT (from
a phenol OH) to a carbon atom that is part of an aromatic ring.3
ESIPT in 1-OD gave quinone methide 2-2′D, which underwent
reverse proton transfer to give 1-2′D (eq 1). The reaction was
observed to proceed in a variety of solvents and in the solid
state, indicating that the proton transfer is intrinsic (water is
Our current interest in the photochemistry of o-hydroxybiaryls
led us to consider derivatives bearing an anthracenyl substituent.
Hydroxy-substituted phenylanthracenes have received consider-
able attention recently for their ability to form hydrogen-bonded
networks with cavities capable of including guest molecules.5
(1) Weller, K. Naturwissenschaften 1955, 42, 175.
(2) For reviews of ESIPT, see: (a) Ireland, J. F.; Wyatt, P. A. H. AdV. Phys.
Org. Chem. 1976, 12, 131. (b) Klo¨pffer, W. AdV. Photochem. 1977, 10,
311. (c) Arnaut, L. G.; Formosinho, S. J. J. Photochem. Photobiol A: Chem.
1993, 75, 1. (d) Formosinho, S. J.; Arnaut, L. G. J. Photochem. Photobiol.
A: Chem. 1993, 75, 21. (e) Ormson, S. M.; Brown, R. G. Prog. React.
Kinet. 1994, 19, 45. (f) Le Gourrierec, D.; Ormson, S. M.; Brown, R. G.
Prog. React. Kinet. 1994, 19, 211.
(3) (a) Lukeman, M.; Wan, P. Chem. Commun. 2001, 1004. (b) Lukeman, M.;
Wan, P. J. Am. Chem. Soc. 2002, 124, 9458.
(4) Lukeman, M.; Wan, P. J. Am. Chem. Soc. 2003, 125, 1164.
9
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J. AM. CHEM. SOC. 2004, 126, 7890-7897
10.1021/ja039078g CCC: $27.50 © 2004 American Chemical Society

Photoaddition to Anthracene Moiety via ESIPT
A R T I C L E S
The interest in using these derivatives in the engineering of such
crystals is primarily because the 90° twist angle between the
two aryl rings leads to predictable and well-defined crystal
geometries. Potential photoreactivity in these systems could lead
to a disruption of the geometry of the crystal networks and their
guest binding properties. We chose to study 9-(2′-hydroxyphen-
yl)anthracene (6), which was readily prepared from com-
mercially available 9-bromoanthracene. It was anticipated that
a formal ESIPT from the phenol OH to one or more aromatic
carbon positions of the anthracenyl moiety might lead to similar
reactivity as observed for 1 and 3, namely deuterium incorpora-
tion and photocyclization. We report that a formal ESIPT in 6
does not give rise to ring-closed products as expected, but
instead gives products resulting from photoaddition of either a
water or an alcohol molecule across the 9- and 10-positions of
the anthracene ring system. While the photohydration of alkenes
and alkynes via ESIPT is well-known,6 to our knowledge, this
is the first report of the photoaddition of a hydroxylic solvent
to an aromatic ring system resulting from an ESIPT process.
Figure 1. UV-vis spectra showing loss of anthracene chromophore (250
and 365 nm) of 6 with photolysis time. Photolysis was carried out in a
Rayonet RPR-100 reactor with 16 lamps (350 nm) (1:9 H2O-CH3CN, argon
saturated). Representative traces are shown with the corresponding irradia-
tion times given in the figure. Spectral data for wavelengths longer than
300 nm are expanded 10-fold.
similar changes as observed in aqueous CH3CN, indicating that
the reaction requires the mediation of a hydroxylic solvent.
Addition of 5 M water to the CH3OH solution led to a 25%
enhancement of the reaction that gives rise to the absorption
loss, indicating that water is better at mediating the reaction
than CH3OH.
In an effort to isolate and characterize the products of the
photoreaction, a preparatory scale photolysis was carried out
(∼10-3 M, 1:9 H2O-CH3CN, for solubility a higher ratio of
CH3CN cosolvent was used; 350 nm). A single photoproduct
was formed in 58% yield and was chromatographically separated
from the unreacted starting material. NMR analysis is consistent
with assignment of this to the photohydration product 7 (eq 3).
Results and Discussion
Product Studies. We first probed the photoreactivity of 6
by UV-vis spectrophotometry, by irradiating quartz cells
containing the substrate (∼10-6 M) in 1:9 (v/v) H2O-CH3CN
(deoxygenated) in a Rayonet RPR-100 reactor (350 nm lamps).
Traces were recorded at regular time intervals throughout the
course of the irradiation, with representative traces shown in
Figure 1. Irradiation led to a decrease in the intensity of the
absorption maxima at 384, 364, 346, 330, and 254 nm, indicating
loss of the anthracene chromophore. The absorption bands
associated with the phenol chromophore (220 and 280 nm)
remain, suggesting that this moiety is not lost in the transforma-
tion. No new absorption bands were observed to grow in,
suggesting that benzoxanthene-type products that would arise
from photocyclization4 are not formed. Irradiation of 6 in
anhydrous CH3CN did not lead to any observable changes in
the absorption spectrum. Irradiation in neat CH3OH led to
(5) (a) Aoyama, Y.; Endo, K.; Anzai, T.; Yamaguchi, Y.; Sawaki, T.;
Kobayashi, K.; Kanehisa, N.; Hashimoto, H.; Kai, Y.; Masuda, H. J. Am.
Chem. Soc. 1996, 118, 5562. (b) Aoyama, Y.; Endo, K.; Kobayashi, K.;
Masuda, H. Supramol. Chem. 1994, 4, 229-241. (c) Endo, K.; Ezuhara,
T.; Koyanagi, M.; Masuda, H.; Aoyama, Y. J. Am. Chem. Soc. 1997, 119,
499. (d) Endo, K.; Sawaki, T.; Koyanagi, M.; Kobayashi, K.; Masuda, H.;
Aoyama, Y. J. Am. Chem. Soc. 1995, 117, 8341. (e) Kobayashi, K.;
Koyanagi, M.; Endo, K.; Masuda, H.; Aoyama, Y. Eur. J. Chem. 1998, 4,
417.
(6) (a) Isaks, M.; Yates, K.; Kalanderopoulos, P. J. Am. Chem. Soc. 1984,
106, 2728. (b) Kalanderopoulos, P.; Yates, K. J. Am. Chem. Soc. 1986,
108, 6290. (c) Fischer, M.; Wan, P. J. Am. Chem. Soc. 1998, 120, 2680.
(d) Fischer, M.; Wan, P. J. Am. Chem. Soc. 1999, 121, 4555. (e)
Brousmiche, D. W.; Xu, M.; Lukeman, M.; Wan, P. J. Am. Chem. Soc.
2003, 125, 12961.
This product would arise from nucleophilic trapping (by water)
of quinone methide 8 formed by an envisioned formal ESIPT
process from the phenol OH to the 10-position. The addition
of water to the anthracene ring essentially converts this
chromophore into three isolated phenyl rings, explaining the
loss of the characteristic anthracene absorption (and growth in
absorption at wavelengths shorter than 220 nm) observed in
the UV-vis studies. Extending the irradiation time allowed
quantitative generation of 7, with no observable side products.
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A R T I C L E S
Flegel et al.
Quantitative dehydration of 7 (to refurnish 6) was achieved by
12 h reflux in toluene. Irradiation of solutions of 6 in methanol
or 2-propanol (in 1:2:5 ROH:H2O:CH3CN) gave ethers 9 and
10, respectively, with high efficiency. These reactions could
also be taken to quantitative conversion with shorter photolysis
times than those used for the quantitative formation of 7. This
is likely due to the enhanced nucleophilicity of the alcohols
relative to water7 which allows for more efficient trapping of
8, minimizing thermal tautomerization back to 6. Trapping of
8 by hydride was also achieved: irradiation of 6 in the presence
of NaBH4 gave 11 in good yield (>50%). No reaction was
observed when 6 was irradiated in neat CH3CN.
ESIPT processes for 1 and 3 that did not give isolable
products were implicated by irradiating the substrates in D2O
and analyzing the recovered starting material for deuterium
exchange. To probe whether such processes are also available
for 6, irradiations were carried out as described above, but with
the H2O portion replaced with D2O. Irradiation of 6 (∼10-4
M, 10 min) gave 7 with one deuterium incorporated at the
methylene position (7-10D) in 70% yield (eq 4). Starting
Figure 2. Plot showing percent yield of 7-10D (0), 6-10D (2), and 8-10D
(b) present in photolysate following irradiation of 6 in solvents of varying
D2O concentration (∼10-3 M, argon saturated, 350 nm lamps, 10 min).
The yield of 8-10D was calculated as the sum of 7-10D and 6-10D.
amounts of D2O (in CH3CN) were irradiated. The yields of
6-10D and 7-10D generated in these runs were determined by
1H NMR, and results are presented in Figure 2. Since both of
these products arise most likely from intermediate 8-10D, the
sum of the yields of these two products (also included in the
plot in Figure 2) can be taken as a measure of the yield of
formation of this intermediate. The efficiency of quinone
methide generation (formation of 8-10D) shows a strong
dependence on water content, with minimal reaction at low
water concentrations. The yield of 8-10D increased sharply as
more D2O was added to the solvent mixture, reaching a
maximum at approximately 5 M D2O. Addition of more D2O
beyond 5 M led to a gradual decrease in the yield of quinone
methide, probably due to the enhancement of competing
processes, such as ESPT to water to yield the phenolate 6-.
The partitioning of intermediate 8-10D to 6-10D and 7-10D is
also apparently influenced by the concentration of D2O; as the
water concentration is increased past 5 M, the quinone methide
gives more of the deuterium-exchanged starting material 6-10D
at the expense of the photohydrated product 7-10D. This result
is in keeping with expectations since formation of 6-10D is
expected to proceed via a water-catalyzed tautomerization
mechanism involving several molecules of water, whereas the
nucleophilic attack of 8-10D to give 7-10D in principle only
requires a single water molecule.
material from this run which constituted the remaining 30% of
the material present showed deuterium exchange (∼70%) at the
carbon atom at the 10-position (6-10D) with no exchange
observed at any other position. This suggests that the formal
ESIPT in 6 can only occur to the 10-position to give 8 (or
8-10D). Therefore, formation of 6-10D most likely occurs via
tautomerization of 8-10D.
The methyl ether derivative 12 was studied in a manner
similar to that for 6. This derivative lacks an acidic proton and
thus cannot undergo excited state proton transfer (ESPT) or
ESIPT processes. Solutions of 12 irradiated under the same
conditions as described for 6 did not yield any photohydration
products, and the recovered substrate did not show deuterium
exchange when D2O was used as the cosolvent. Thus, an acidic
phenolic proton is required for reaction in these systems,
consistent with the operation of a formal ESIPT process.
The study of the effect of water concentration on previously
reported ESIPT reactions in our group has uncovered important
mechanistic details. For example, the absence of any water effect
on the efficiency of ESIPT of 1 led to the conclusion that the
ESIPT in this system was intrinsic,3 whereas the ESIPT of 3
required water mediation.4 To probe the effect of water content
on the proposed ESIPT of 6, solutions containing varying
The quantum yield for photogeneration of 8-10D in 10 M
D2O-CH3CN was estimated by comparison with the deuterium
incorporation reaction of dibenzosuberene.8 The sum of 7-10D
and 6-10D was used to approximate the quinone methide
efficiency, and a value for the quantum yield of 0.09 ( 0.01
was obtained (Table 1). This value is similar to efficiencies of
quinone methide formation estimated for 13 and 3.4 The quantum
efficiency in H2O would be expected to be somewhat higher
due to the operation of a primary isotope effect in the proton
transfer to obtain 8.
Fluorescence Measurements. The anthracene chromophore
is well-known to exhibit strong and structured fluorescence
emission;10 thus use of fluorescence spectrophotometry was
expected to be a valuable tool for probing the photoreactivity
(8) Budac, D.; Wan, P. J. Org. Chem. 1992, 57, 887.
(9) Melhuish, W. H. J. Phys. Chem. 1960, 64, 762.
(10) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules;
Academic Press: New York, 1971; p 363.
(7) Kevill, D. N.; Anderson, S. W. J. Org. Chem. 1991, 56, 1845.
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Photoaddition to Anthracene Moiety via ESIPT
A R T I C L E S
Table 1. Photophysical and Photochemical Parameters for 6
solvent
Φra
Φfb
τfc
neat CH3CN
4 M D2O (in CH3CN)
0.00
0.09
0.50 ( 0.03
∼0.25
5.9
a Quantum yield for formation of 7-10D and 6-10D (number shown is
sum) from 6 in the solvent indicated. Estimated error is (10% of the quoted
value. Measured relative to the quantum yield for deuterium incorporation
reported for dibenzosuberene.8 b Fluorescence quantum yield measured
relative to the reported fluorescence quantum yield of quinine bisulfate in
0.1 N H2SO4.9 c Fluorescence lifetime as measured by single photon
counting. Fluorescence decay was first order. Estimated error is (0.2 ns.
Figure 4. Stern-Volmer plot of fluorescence quenching vs water
concentration (in CH3CN) for 6.
shown in Figure 4. At low water concentrations, the fluorescence
quenching appears to show a squared dependence on water
concentration. Nonlinear fluorescence quenching by water has
been previously shown for related phenols that can undergo
water-mediated ESIPT processes, and has been associated with
the involvement of water clusters in the ESIPT process.6c,d At
water concentrations above 5 M, there is significant negative
deviation from the binomial behavior. This deviation is indica-
tive of a change in the mechanism of fluorescence quenching.
It is important to note that the water concentration at which the
deviation becomes pronounced (5 M) coincides with the
concentration of water at which the formation of 8 reaches
maximum efficiency, as described in the product studies. On
the basis of this correlation, we assign the fluorescence
quenching at concentrations lower than 5 M as due to the
increasing efficiency of the formal ESIPT reaction to give 8.
The quenching of the fluorescence at higher water concentrations
is probably due to competitive formation of phenolate 6- via
ESPT to solvent (a known process for phenols at high water
content). No new emission band is observed at high water
concentrations indicating that 6-, if formed, shows only
anthracene emission like the normal form, or is nonemissive.
To test for these possibilities, we attempted to record directly
the fluorescence spectrum of 6- (formed by dissolving 6 in 1:1
H2O-CH3CN, pH 12). This solution gave only weak fluores-
cence that is identical to the neutral form with no other emission
bands, supporting our assertion that 6- does not give unique
fluorescence.
Figure 3. Representative traces for 6 showing quenching of fluorescence
emission (in CH3CN) with added water (λex ) 365 nm). Concentrations
are given in the figure.
of 6. Representative fluorescence excitation and emission spectra
are shown in Figure 3. Strong and structured emission was
observed for 6 with maxima at 391 and 412 nm and shoulders
at 435 and 465 nm. The Stokes shift in the fluorescence emission
is very slight, and is much smaller than that observed for the
unsubstituted parent 9-phenylanthracene (13).10 It has been
shown for 13 through both free-jet laser induced fluorescence
studies11 and ab initio molecular orbital calculations12 that the
equilibrium biaryl twist angle changes from 90° in the ground
state to 55-60° in S1. It is this twisting on excitation that is
largely responsible for its sizable Stokes shift. The minimal
Stokes shift observed for 6 suggests that little or no twisting
takes place on excitation, presumably because of the additional
steric crowding imparted by the hydroxyl substituent (of the
phenol). Thus, the emission is essentially that of unsubstituted
anthracene, indicating that the emissive state is not influenced
by the phenol moiety.
The fluorescence quantum yield for 6 (in neat CH3CN) was
measured to be 0.50 ( 0.03 (Table 1), which is essentially the
same as for 13 (Φf ) 0.49 in cyclohexane).10 The fluorescence
lifetime of 6 in neat CH3CN was measured to be 5.9 ( 0.2 ns
(Table 1), also similar to that reported for 13 (τf ) 6.5 ns in
cyclohexane).10 Both results indicate that addition of the
hydroxyl group to the 9-phenylanthracene chromophore does
little to affect its photophysical characteristics unless water is
added to the solvent mixture.
Addition of water to the solvent led to efficient quenching
of the fluorescence emission (Figure 3). A Stern-Volmer plot
of the quenching of the steady-state fluorescence by water is
Nanosecond Laser Flash Photolysis (LFP). We have been
able to successfully characterize a large variety of photogener-
ated quinone methide intermediates by LFP. For example, LFP
of 14 in 1:1 H2O-CH3CN allowed direct detection of o-quinone
(11) Werst, D. W.; Gentry, W. R.; Barbara, P. F. J. Phys. Chem. 1985, 89, 729.
(12) Sakata, K.; Hara, K. Chem. Phys. Lett. 2003, 371, 164.
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A R T I C L E S
Flegel et al.
Figure 6. Chem 3D (MOPAC; AM1) minimized ground state structures
of substrate 6 and proposed quinone methide intermediate 8. The dihedral
angle between the phenol and the anthracene ring of 6 is about 90°. The
distance between the phenol OH and the anthracene 10-position (site of
protonation) of 6 is 4.203 Å, which is too far for a simple direct proton
transfer.
important spectral region was obscured by substrate bleaching,
the region that is visible shows absorption characteristics
expected for 8: absorption between 280 and 550 nm and a
maximum (or shoulder) at 440-450 nm. Thus we are confident
in assigning this transient absorption to that of 8 based on its
similarity to the known absorbance and lifetime characteristics
of the related quinone methide 15.
Figure 5. Transient absorption spectra recorded by LFP of 6 in 5 M H2O
(in CH3CN) taken 0.3 (dashed line) and 15 µs (solid line) after laser flash
(λex ) 266 nm). Data between 330 and 390 nm are obscured due to
bleaching of the substrate.
methide 15 that is formed by net photodehydration (eq 5).13
Mechanism of Reaction. Our data are fully consistent with
formation of the critical quinone methide intermediate 8 on
photolysis of 6 in aqueous CH3CN or water-alcohol solutions.
It is informative to consider the changes in molecular geometry
that are required in this transformation. Chem 3D (MOPAC,
AM1) minimized ground state structures are shown in Figure 6
for 6 and 8. The two aromatic rings of the starting compound
6 are essentially orthogonal to each other (dihedral angle ∼90°).
The distance between the phenolic OH and the carbon-10 on
the anthracene ring (position of protonation) is 4.203 Å, which
is too far for a simple direct proton transfer from the phenol.
Thus, unlike the ESIPT reported for 1,3 compound 6 does not
react without the mediation of a hydroxylic solvent. Moreover,
reaction of 6 involves substantial molecular distortions: (i) the
originally orthogonal phenol ring twists by about 90° such that
it becomes somewhat coplanar with the central ring of the
“anthracene”; (ii) the anthracene ring puckers out of planarity
as the 10-position is protonated. Another important consideration
in the discussion of mechanism is the sequence of events that
results in the formal proton transfer that occurs when 6 forms
8 (these two are tautomeric structures, as would be required in
an ESIPT process, formal or otherwise). We have so far
designated this as a formal ESIPT, but a discussion of how this
actually occurs would be important in any mechanistic discus-
sion.
The timing of the events discussed above are probably very
fast (picosecond or faster time scale), and a detailed analysis
of all the steps would be beyond the scope of this work. Indeed,
our LFP studies (nanosecond time resolution) were only able
to discern the “instantaneous” formations of triplet-triplet
absorption and a long-lived transient assignable to quinone
methide 8. However, these results coupled with fluorescence
quenching studies clearly point to the singlet state of 6 as the
reactive state. This is in accord with a number of other
photoreactions of phenols in aqueous solution reported by our
group.6,13
The transient showed a strong absorption band at 350 nm and
a weaker and broader band at 450 nm, with both bands decaying
to baseline with a lifetime of 5-10 s. The quinone methide
intermediate 8 formed by formal ESIPT reaction of 6 in this
study bears a structural resemblance to quinone methide 15 and
is therefore expected to possess similar absorption character-
istics. The steric demands on 8 (vide infra) are more severe
than those for 15, so we were expecting the former to decay
faster.
LFP of 6 in 1:9 H2O-CH3CN under nitrogen-purged condi-
tions gave rise to a transient absorption between 280 and 550
nm (Figure 5). Absorption between 330 and 390 nm could not
be reliably measured due to bleaching of the absorption of the
substrate 6 in this region by the laser flash. Two transients were
readily identifiable. A short-lived species (τ ) 250 ns) was
detected that showed an absorption maximum at 430 nm and
absorbed across the entire spectral range measured, as shown
in Figure 5. Saturating the solution with oxygen reduced the
lifetime of this species to 50 ns. This transient is assigned to
the triplet state of 6 on the basis of its sensitivity to oxygen
and its spectral similarity to the known T-T absoprtion of 13
(λmax ) 428 nm).14 The second transient that was detected is
too long-lived for the decay rate to be reliably measured by the
available experimental setup, indicating that the lifetime is on
the order of milliseconds or greater. Attempts to detect this
intermediate by conventional UV-vis techniques failed, indicat-
ing that the lifetime is shorter than several seconds. Thus, we
estimate that the lifetime of 8 is between 10 ms and 1 s, which
is in line with expectations for this quinone methide. While an
A provisional mechanism that is consistent with all current
experimental data for the observed photochemistry of 6 is
presented in Scheme 1. Once formed, quinone methide 8 is
transformed to 7 (hydration product) or returns to 6 via
(13) Diao, L.; Yang, C.; Wan, P. J. Am. Chem. Soc. 1995, 117, 5369.
(14) Carmichael, I.; Helman, W. P.; Hug, G. L. J. Phys. Chem. Ref. Data 1987,
6, 239.
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Photoaddition to Anthracene Moiety via ESIPT
A R T I C L E S
Scheme 1
at the carbon 10-position to give 8. The whole process could
be concerted, via 17. This simple mechanism is consistent with
most of the experimental data. One interesting point is that the
reaction is observed at very low water content (maximal
efficiency at about 5 M H2O, in CH3CN, Figures 2 and 4). It is
known that the excited state dissociation of simple phenols to
bulk water requires water concentrations in excess of 10 M for
appreciable ionization, whereas the much more acidic cyano
naphthols (in S1) can dissociate at much lower water content
or even in alcohols.6,15 Therefore, the inference here is that the
anthracenyl phenol 6 is a strong photoacid, although we have
no direct measurement of its excited state pKa. It is interesting
to note that direct photolysis of an authentic sample of phenolate
ion of 6 (generated at pH 12, a pH in which 8 is known to be
stable if formed) gave no observable loss of the anthracene
absorption band which is indicative of lack of photohydration;
only broadening of the absorption was observed, suggesting
photodecomposition of the phenol moiety only. This implies
that excited state phenolate from 17 (after deprotonation of the
phenol moiety) is not identical to that obtained by simple direct
excitation of the ground state phenolate. For example, they differ
in the nature of their counterions (hydronium ion vs alkali metal
ion) and medium acidity and therefore would be expected to
have different reactivities. A more speculative mechanism for
formation of 8 involves a concerted deprotonation of the phenol
with twisting motion of the phenol ring, coupled with proton
transfer at the anthracene 10-position, mediated by water
(structure 17 but with the mediating water molecules coupled
in a “wire”, in a “water-relay” mechanism). However, definitive
proof of this “water-relay” pathway would require additional
enolization. These two reaction pathways from 8 are really not
surprising apart from the isolation of the stable anthracene
hydrates and alcohol adducts.
In order for 6 to proceed to to 8, the following must occur:
(i) a formal proton transfer from the phenol OH to the carbon
atom at the 10-position of the anthracene moiety, the latter
resulting in a puckering of the anthracene ring geometry, and
(ii) twisting about the biaryl bond to (or toward) planarity. These
two events may be coupled (i.e., concerted) or stepwise.
Fluorescence results indicate that in neat CH3CN, a solvent that
is unable to mediate the proton transfer and where there is no
photochemistry, the excited state geometry remains highly
twisted, as only emission from the anthracene chromophore was
observed, with essentially no Stokes shift. Charge transfer from
the phenol ring to the anthracene ring in the excited state is
thought to be the driving force for the ESIPT, but this cannot
occur if the two rings are in a perpendicular arrangement. The
situation is changed when water (or alcohol) is present in the
solvent mixture. Now the formal ESIPT reaction proceeds
readily, as determined by the product studies and fluorescence
quenching results.
One simple pathway for the overall reaction is as follows.
The added water mediates the ionization of the phenol, to give
rise to an electronically excited phenolate, which then twists to
a more planar form 16. The more planar form can then protonate
(15) Tolbert, L. M.; Solntsev, K. M. Acc. Chem. Res. 2002, 35, 19.
9
J. AM. CHEM. SOC. VOL. 126, NO. 25, 2004 7895

A R T I C L E S
Flegel et al.
studies along the lines presented by Leutwyler and co-workers16
for their demonstration of operation of an “ammonium wire”
for conducting ESIPT in 7-hydroxyquinoline.
was complete, the mixture was removed from the ice bath and stirred
for an additional 1 h at room temperature. The reaction was quenched
with 100 mL of H2O, and the mixture was extracted with 2 × 50 mL
of CH2Cl2. The combined organic phases were dried over MgSO4 and
filtered, and the solvent was removed under reduced pressure to give
6 as an off-white solid (0.89 g, 90%). Yellow crystals were obtained
on recrystallization from hot toluene, mp ) 186-188 °C (ref 18, 178-
The identification of a formal ESIPT process for 6 shows
that the generality of ESIPT from phenol to aromatic ring
carbons can be extended to anthracenyl substituents and provides
an impetus for studying other aromatic systems. The photohy-
dration reaction of the anthracenyl moiety is the first example
of the photohydration of an aromatic system via ESIPT.
Although a similar type of photohydration is available via the
ESIPT processes reported for 13 and 3,4 the resulting hydrates
would be much less stable and expected to revert to substrate
quickly. Addition of hydroxylic solvents to alkenes and alkynes
is a reaction of enormous importance in conventional organic
chemistry, but addition to aromatic moieties is not commonly
encountered. Aromatic hydrates generally cannot survive the
harsh conditions necessary for their generation via thermal
methods. This work demonstrates that addition of hydroxylic
solvents to aromatic ring systems can indeed be achieved
photochemically under mild conditions (low temperatures,
neutral pH) to give isolable products. The observation that only
the derivative with the acidic phenol OH group (6, but not 12)
is reactive demonstrates the unique ability of phenols to bring
about the phototransformation of attached aromatic rings (via
ESIPT). Of additional interest in this system is the apparently
unique role of water in mediating the ESIPT reaction. While
the ESIPT was also observed in CH3OH and 2-PrOH, the highest
efficiencies were observed when water was present in solution,
presumably because of its superior ability to mediate the proton
transfer.
1
180 °C). H NMR (500 MHz, acetone-d6) δ 7.09 (ddd, 1H, J ) 7.0,
7.0, 1.0 Hz), 7.15 (dd, 1H, J ) 8.0, 1.0 Hz), 7.19 (dd, 1H, J ) 7.5, 2.0
Hz), 7.39 (ddd, 2H, J ) 7.5 Hz, 7.5 Hz, 1.0 Hz), 7.44 (ddd, 1H, J )
7.5, 7.5, 1.5 Hz), 7.48 (ddd, 2H, J ) 7.5, 7.5, 2.0 Hz), 7.64 (dd, 2H,
J ) 9.0, 1.0 Hz), 7.88 (s, 1H, exchanges with D2O), 8.10 (d, 2H, J )
8.5 Hz), 8.59 (s, 1H). 13C NMR (acetone-d6) δ 116.9, 120.5, 125.7,
125.9, 126.2, 127.4, 127.5, 129.2, 130.3, 131.5, 132.6, 133.3, 134.2,
156.5; HRMS, calculated for C20H14O 270.1045; observed 270.1050.
UV-Vis Studies. Solutions (10-6-10-5 M) of 6 in the appropriate
solvent system were prepared in 1.0 cm quartz cuvettes (3.0 mL) and
deoxygenated by bubbling with a stream of argon gas (syringe needle)
for 5 min, and irradiated in a RPR 100 photochemical reactor containing
16 lamps (350 nm). A merry-go-round apparatus was employed.
Cooling was achieved with a fan. The irradiation was stopped at regular
time intervals and UV-vis traces were recorded.
Product Studies. Solutions (∼10-3 M) were prepared in a 100 mL
quartz tube and purged with argon 10 min prior to and continuously
during irradiation in a RPR 100 photochemical reactor containing 16
lamps (350 nm). Cooling was achieved with an internal coldfinger (∼15
°C). To the photolysate was added 100 mL of H2O, and the resulting
mixture was extracted with CH2Cl2 and dried over MgSO4; the solvent
was removed under reduced pressure.
Photolysis of 6 in 1:9 H2O-CH3CN. A solution of 53 mg of 6 in
80 mL of 1:9 H2O-CH3CN was irradiated for 1.5 h (350 nm), and the
photoproducts were analyzed by 1H NMR which showed a mixture of
7 and 6 in a 60:40 ratio. Compound 7 was isolated chromatographically
on silica gel (1:9 ethyl acetate-hexanes). 1H NMR (500 MHz, acetone-
d6) δ 4.15 (dd, 2H, J ) 26, 20 Hz), 6.39 (s, 1H, exchanges in D2O),
6.56 (m, 2H), 6.74 (d, J ) 8 Hz), 6.99 (ddd, 1H, 9 Hz, 6 Hz, 3 Hz),
7.25 (m, 4H), 7.34 (m, 2H), 7.75 (m, 2H), 9.85 (s, exchanges in D2O).
13C NMR (acetone-d6): δ 34.7, 78.2, 117.8, 119.4, 127.1, 127.6, 127.7,
127.9, 128.5, 128.7, 131.6, 134.6, 142.7, 155.9. IR (film), 3515 (m),
3305 (s, br), 3064 (m), 1583 (s), 1486 (s), 1451 (s) 1233 (s). MS (m/
z), 270 (M+ - H2O). The compound dehydrates to 6 on heating; thus
no melting point was recorded.
Photolysis of 6 in 1:9 D2O-CH3CN. A solution of 7 mg of 6 in 60
mL of 1:9 D2O-CH3CN was irradiated for 10 min, and the photo-
products were analyzed by 1H NMR which showed a mixture of 7-10D
and 6 in a 65:35 ratio. The area of the peak corresponding to the 10-
position of 6 (δ 8.59) was reduced by 70%, indicating a 30:70 ratio of
6 and 6-10D present in the sample, giving an overall product ratio of
65:10:25 of 7-10D:6:6-10D. Runs at different D2O concentrations were
performed in the same way. Results from these runs appear in Figure
2.
Experimental Section
1
General. H NMR spectra were recorded on Bruker AC300 (300
MHz) and AVANCE 500 (500 MHz) instruments. MS were recorded
on a Kratos Concept H spectrometer (EI). UV-vis spectra were
recorded on a Varian Cary 50 instrument. CH3CN was dried over CaH2
and distilled prior to use. Other solvents were reagent grade and used
as received.
Materials. 9-Bromoanthracene, 2-methoxyphenylboronic acid,
Pd(PPh3)4, and BBr3 (1.0 M solution in CH2Cl2) were purchased from
Aldrich.
9-(2′-Methoxyphenyl)anthracene (12). A solution of 9-bromoan-
thracene (7.01 g, 27.3 mmol) and Pd(PPh3)4 (0.32 g, 0.28 mmol) in 90
mL of toluene was added to a solution of 2-methoxyphenylboronic
acid (4.97 g, 32.7 mmol) in 80 mL of ethanol, followed by addition of
K2CO3 (8.30 g, 60.0 mmol). The mixture was stirred and heated to
reflux for 20 h under N2 and allowed to cool to room temperature. To
the reaction mixture was added 0.5 M NaOH solution (200 mL), and
the mixture was extracted with 2 × 100 mL of CH2Cl2. The organic
phases were combined, dried over MgSO4, and filtered, and the solvent
was removed under reduced pressure to yield 12 as an off-white solid
(6.97 g, 90%); mp 181-182 °C (ref 17, 177-179 °C). Purification
was achieved by recrystallization from a mixture of toluene and ligroin.
1H NMR (300 MHz, acetone-d6) δ 3.96 (s, 3H), 7.16-7.64 (m, 10H),
8.10 (d, 2H), 8.58 (s, 1H); HRMS calculated for C21H16O 284.1201;
observed 284.1202.
Photolysis of 6 in 1:2:5 CH3OH:H2O:CH3CN. A solution of 32
mg of 6 was dissolved in 80 mL of a 1:2:5 CH3OH:H2O:CH3CN
solution and irradiated for 45 min. 1H NMR analysis showed quantita-
1
tive conversion to 9. H NMR (500 MHz, acetone-d6) δ 2.86 (s, 3H),
4.24 (d, 1H, J ) 21 Hz), 4.32 (d, 1H, J ) 21 Hz), 6.65 (ddd, 1H, J )
7.5, 7.5, 1.5 Hz), 6.70 (dd, 1H, J ) 8.5, 1.0 Hz), 6.98 (d, 1H, J ) 7.5
Hz), 6.99 (ddd, 1H, J ) 7.0, 7.0, 1.5 Hz), 7.24 (dd, 2H, J ) 8, 8 Hz),
7.28 (ddd, 2H J ) 7.0, 7.0, 1.5 Hz), 7.37 (dd, 2H, J ) 7.5, 1 Hz), 7.47
(dd, 2H, J ) 7.5 Hz, 1.5 Hz), 9.17 (s, 1H, exchanges in D2O).
Compound gives 6 on heating.
9-(2′-Hydroxyphenyl)anthracene (6). A solution of BBr3 (1.0 M
in CH2Cl2) (18 mL, 18 mmol) was added dropwise over a period of 20
min to a stirring solution of 12 (1.05 g, 3.68 mmol) dissolved in 50
mL of CH2Cl2 under N2 and cooled in an ice bath. Once the addition
Photolysis of 6 in 1:2:5 2-Propanol-H2O-CH3CN. A solution
of 35 mg of 6 in a solution of 1:2:5 2-PrOH-H2O-CH3CN was
1
irradiated for 45 min to give 10 in near quantitative yield. H NMR
(16) Tanner, C.; Mance, C.; Leutwyler, S. Science 2003, 302, 1736.
(17) Terao, Y.; Wakui, H.; Nomoto, M.; Satoh, T.; Miura, M.; Nomura, M. J.
Org. Chem. 2003, 68, 5236.
(18) Rice, J.; Cai, Z.-W. J. Org. Chem. 1993, 58, 1415.
9
7896 J. AM. CHEM. SOC. VOL. 126, NO. 25, 2004

Photoaddition to Anthracene Moiety via ESIPT
A R T I C L E S
(300 MHz, acetone-d6) δ 7.50 (d, 6H, J ) 7 Hz), 3.30 (dt, 1H, J ) 7,
7 Hz), 4.35 (s, 2H), 6.55 (m, 1H), 6.75 (m, 1H), 7.00 (dd, 1H, J ) 7,
7 Hz), 7.10-7.50 (m, 9 H), 9.85 (s, 1H, exchanges in D2O). Warming
a sealed NMR tube to 80 °C for 2 h led to 35% conversion back to 6,
concomitant with release of 2-propanol.
and excitation wavelength of 266 nm. Pulse power was adjusted to 20
mJ. Flow cells (7 mm path length) were used and solutions were
bubbled with either nitrogen or oxygen for 10 min prior to excitation.
Optical densities (ODs) at 266 nm were ∼0.3.
Steady-State and Time-Resolved Fluorescence Measurements.
All solutions (OD ) 0.1 at excitation wavelength) were purged with
argon or nitrogen for 5 min prior to analysis. Steady-state fluorescence
spectra were obtained on a Photon Technology International (PTI)
A-1010 Quanta-Master luminescence spectrometer. Fluorescence quan-
tum yields were determined by comparison of the integrated emission
bands with that of quinine bisulfate (Φf ) 0.55 in 0.1 M H2SO4),9
followed by correction for differences in index of refraction. Measure-
ments were made at three different concentrations to exclude possible
aggregation affects. Fluorescence lifetimes were measured on a PTI
LS-1 instrument using the time-correlated single-photon-counting
method with 10 000 counts.
Photolysis of 6 in the Presence of NaBH4. A solution of 32 mg of
6 and 1.0 g of NaBH4 in 80 mL of 1:3 H2O-CH3CN was irradiated
for 1 h. The reaction was quenched with 100 mL of 10% HCl solution
to decompose excess NaBH4 prior to standard workup. 1H NMR
analysis indicated ∼60% conversion to 11, along with minor amounts
of other unidentified products. Product 11 (oil) was isolated from the
1
product mixture on silica gel (10% ethyl acetate-hexanes). H NMR
(500 MHz, acetone-d6) δ 4.03 (d, 1H, 19 Hz), 4.30 (d, 1H, 19 Hz),
5.81 (s, 1H), 6.65 (ddd, 1H, J ) 7.5, 7.5, 1.5 Hz), 6.72 (dd, 1H, J )
7.5, 1.5 Hz), 6.90 (dd, 1H, J ) 8.0, 1.0 Hz), 6.97 (ddd, 1H, J ) 8.0,
7.5, 2 Hz), 7.14 (ddd, 2H, J ) 7.5, 7.5, 2 Hz), 7.18 (ddd, 2H, J ) 7.0,
7.0, 1.5 Hz), 7.34 (dd, 2H, J ) 7.5, 1 Hz), 7.38 (dd, 2H, J ) 7.0, 1.5
Hz), 8.64 (s, 1H, exchanges in D2O). IR (film) 3500 (m), 3427 (m,
br), 1588 (m), 1485 (s), 1451 (s). HRMS, calculated for C20H16O
272.1201; observed 272.1203.
Acknowledgment. Support of this work was provided by the
National Sciences and Engineering Research Council (NSERC)
of Canada. M.L. thanks NSERC for a postgraduate scholarship.
Laser Flash Photolysis (LFP). All LFP studies were conducted at
the University of Victoria LFP facility employing a Spectra Physics
Quanta-Ray YAG laser, model GCR-11, with a pulse width of 10 ns
JA039078G
9
J. AM. CHEM. SOC. VOL. 126, NO. 25, 2004 7897