Excited state intramolecular proton transfer (ESIPT) from phenol to carbon in selected phenylnaphthols and naphthylphenols

Article abstract of DOI:10.1021/jo301456y

ESIPT and solvent-assisted ESPT in isomeric phenyl naphthols and naphthyl phenols 5-8 were investigated by preparative photolyses in CH ₃CN-D₂O, fluorescence spectroscopy, LFP, and ab initio calculations. ESIPT takes place only in 5 (D-exchange Φ = 0.3), whereas 6-8 undergo solvent-assisted PT with much lower efficiencies. The efficiency of the ESIPT and solvent-assisted PT is mainly determined by different populations of the reactive conformers in the ground state and the NEER principle. The D-exchange experiments and calculations using RI-CC2/cc-pVDZ show that 5 in S₁ deactivates by direct ESIPT from the OH to the naphthalene position 1 through a conical intersection with S₀, delivering QM 14 that was detected by LFP (τ = 26 ± 3 ns). ESIPT to position 3 in 5 is possible but it proceeds from a less-populated conformer and involves an energy barrier on S₁. In solvent-assisted PT to naphthalene position 4 in 5, zwitterion 17 is formed, which cyclizes to stable naphthofuran photoproducts 9-12. The regiochemistry of the deuteration in solvent-assisted PT was correlated with the NBO charges of the corresponding phenolates/naphtholates 5^--8^-. Combined experimental and theoretical data indicate that solvent-assisted PT takes place via a sequential mechanism involving first deprotonation of the phenol/naphthol, followed by the protonation by H ₂O in the S₁ state of phenolate/naphtholate. The site of protonation by H₂O is mostly at the naphthalene α-position.

Full text of DOI:10.1021/jo301456y

Article
pubs.acs.org/joc
Excited State Intramolecular Proton Transfer (ESIPT) from Phenol
to Carbon in Selected Phenylnaphthols and Naphthylphenols
Nikola Basaric,*,† Nađa Doslic,‡ Jakov Ivkovic,† Yu-Hsuan Wang,§ Jelena Veljkovic,†
́
̌
́
́
́
Kata Mlinaric-Majerski,† and Peter Wan§
́
†Department of Organic Chemistry and Biochemistry, Ruđer Bosk
̌
ovic
́
Institute, Bijenick
̌
a cesta 54, 10000 Zagreb, Croatia
‡Department of Physical Chemistry, Ruđer Bosk
̌
ovic
́
Institute, Bijenicka cesta 54, 10000 Zagreb, Croatia
̌
§Department of Chemistry, University of Victoria, Box 3065 Stn CSC, Victoria BC, V8W 3 V6, Canada
S
* Supporting Information
ABSTRACT: ESIPT and solvent-assisted ESPT in isomeric phenyl naphthols
and naphthyl phenols 5−8 were investigated by preparative photolyses in
CH3CN−D2O, fluorescence spectroscopy, LFP, and ab initio calculations.
ESIPT takes place only in 5 (D-exchange Φ = 0.3), whereas 6−8 undergo
solvent-assisted PT with much lower efficiencies. The efficiency of the ESIPT
and solvent-assisted PT is mainly determined by different populations of the
reactive conformers in the ground state and the NEER principle. The D-
exchange experiments and calculations using RI-CC2/cc-pVDZ show that 5
in S1 deactivates by direct ESIPT from the OH to the naphthalene position 1
through a conical intersection with S0, delivering QM 14 that was detected by
LFP (τ = 26 3 ns). ESIPT to position 3 in 5 is possible but it proceeds
from a less-populated conformer and involves an energy barrier on S1. In
solvent-assisted PT to naphthalene position 4 in 5, zwitterion 17 is formed,
which cyclizes to stable naphthofuran photoproducts 9−12. The regio-
chemistry of the deuteration in solvent-assisted PT was correlated with the NBO charges of the corresponding phenolates/
naphtholates 5‑−8‑. Combined experimental and theoretical data indicate that solvent-assisted PT takes place via a sequential
mechanism involving first deprotonation of the phenol/naphthol, followed by the protonation by H2O in the S1 state of
phenolate/naphtholate. The site of protonation by H2O is mostly at the naphthalene α-position.
and 7-hydroxyquinoline16 and has been used for the probing of
INTRODUCTION
■
structural dynamics in proteins.17
Proton transfer in both ground and excited states is a fundamental
process in chemistry and biology.1 Upon electronic excitation,
The most common acidic group in the ESIPT reactions is
the phenolic OH, whereas the basic site is usually a heteroatom
some organic functional groups exhibit enhanced acidity or
such as carbonyl oxygen or nitrogen of a heterocycle.2,3 Furthermore,
some examples are known wherein the photoprotonation of a
carbon competes with the protonation of heteroatoms.18 How-
ever, ESIPT to carbon usually takes place with much lower
quantum efficiency.19 Consequently, proton transfer (PT) in-
volving a carbon acid or base is often inadvertently overlooked
as a possible excited-state pathway. Nevertheless, we have a
continuing interest in the acid−base chemistry of excited states,
with a particular effort directed at discovering new examples of
carbon photoacids or photobases.20 ESIPT from the phenol
OH to carbon atoms is now recognized as a general reaction as
many examples have been documented by us involving phenol
adjacent to the following rings: phenyl,21 naphthyl,22 anthryl23
terphenyl24 or pyrenyl.25
basicity.2 If these sites are in close proximity, upon electronic
excitation an excited-state intramolecular proton transfer (ESIPT)
can occur.3 ESIPT has been the focus of intensive research
because of its fundamental nature2,3 as well as its many potential
applications.4 According to Kasha, ESIPT occurs via several
reaction mechanisms including intrinsic intramolecular proton
transfer (the true ESIPT), concerted biprotonic transfer, proton-
relay tautomerization, and catalysis of proton transfer.5 Whereas
direct ESIPT has found applications in the synthesis of chromanes,6
sensing,7 laser dyes,8 photostabilizers,9 scintillators,10 long-lived
pH jumps,11 or the photoswitching of polymorphs,12 the
proton-relay mechanism is particularly interesting in biological
systems. Biological proton transfer generally takes place over
long distances that may involve chains of polar amino acids and
H2O molecules.13 Furthermore, light-initiated long-range
proton transfer takes place in bacteriorhodopsin, enabling the
function of a proton pump that moves protons through mem-
brane against the gradient.14 The solvent-relay mechanism of
ESIPT has also been thoroughly investigated for 7-azaindoles15
Special Issue: Howard Zimmerman Memorial Issue
Received: July 14, 2012
© XXXX American Chemical Society
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More recently we reported a very efficient ESIPT reaction in
2-phenyl-1-naphthol (1) that gives QM 2, 4 and zwitterion 3
(eq 1).26 Formation of QMs 2−4 was inferred due to
regiospecific incorporation of deuterium on irradiation in
D2O (D-exchange Φ = 0.7), the highest efficiency ever reported
for such a process. QM 2 was spectroscopically detected by
laser flash photolysis (LFP) (τ = 20 ns). A theoretical investi-
gation showed that ESIPT in aprotic solvents operates via a
conical intersection with the ground state that represents the
dominant deexcitation pathway from the singlet excited state
surface. In water, a H2O-mediated relay mechanism takes place
via a conical intersection, delivering QMs and zwitterions.26
The ESIPT in 1 was compared with the reaction in 2-phenyl-
phenol. Due to the unfavorable conformer distribution in the
S0, ESIPT in 2-phenylphenol takes place with significantly
lower efficiency (D-exchange Φ = 0.041),18,26 whereas ESIPT
in the isomeric 1-phenyl-2-naphthol does not take place at
all.27Herein, we describe a systematic study of ESIPT and
solvent-assisted ESPT in a series of naphthalenes 5−8. This
series of compounds was designed to investigate the influence
of the naphthalene vs naphthol chromophore on the efficiency
and regioselectivity of ESIPT and solvent-assisted ESPT. Two
compounds, 5 and 6, have phenol as the acidic site attached to
the naphthalene chromophore, whereas in 7 and 8 the phenyl is
attached to the naphthol, which is the acidic site. Furthermore,
naphthols 7 and 8 were chosen to investigate whether solvent-
assisted PT can take place from the naphthol OH to the
carbom atoms of the distal phenyl ring. Although not ex-
haustive, the available compounds shed new insights into the
ESIPT process involving carbon bases. The compounds were
investigated by irradiations in the presence of D2O, fluo-
rescence measurements and LFP. Rational explanations for the
regioselectivity and efficiency of the PT processes were
provided by high-level theoretical calculations.
Photochemical Experiments. Irradiations of 5−8 were
first performed in CH3CN and CH3CN−H2O in UV−vis
cuvettes to check for the formation of stable photoproducts.
After each exposure to light, the UV−vis spectra were taken
(see the Supporting Information). Irradiation of naphthols 7
and 8 in CH3CN and CH3CN−H2O resulted in a broadening
of the characteristic 1-naphthol absorption bands, suggesting
formation of new products or decomposition of the samples
(vide infra). Furthermore, irradiations of phenols 5 and 6 in
CH3CN did not change their UV−vis spectra, suggesting that
these molecules in CH3CN do not give stable photoproducts.
However, on irradiation of 5 in CH3CN−H2O new structured
bands at 350−400 nm appeared, clearly indicating formation of
new products absorbing in that wavelength region.
To isolate stable photoproducts, preparative irradiations of 5,
7 and 8 were performed in CH3CN−H2O and CH3OH.
Prolonged irradiations (1−4 h) of 7 and 8 gave only high
molecular weight material and the starting material (which was
recovered), indicating that the changes seen in the UV−vis
spectra were due to decomposition. On the other hand,
irradiation of 5 (1 h, 300 nm, 16 lamps) in CH3CN−H2O (3:1)
resulted in conversion of 58% and gave four naphthofuran
products 9−12 isolated in 3, 12, 15 and 6% yield, respectively
(eq 4). Irradiation in CH3OH (2 h) resulted in lower con-
version of 48%, and gave products 9, and 11−13 in 6, 16, 3 and
7% yield, respectively (eq 5). The structures of products 9 and
10 were determined by comparison of their 1D and 2D NMR
spectra with those published in literature.28,29 The structures of
new naphthofuran derivatives 11−13 were determined with the
help of 2D NMR techniques (COSY, NOESY, HSQC, HMBC,
see the Experimental Section).
To probe for ESIPT involving C-atoms as a basic site, ir-
radiations were performed in CH3CN−D2O mixtures, wherein
incorporation of deuterium at carbon atoms denotes these
pathways. Irradiations were typically performed in CH3CN−
D2O (c(D2O) = 0.3 M) or CH3CN−D2O (2:1, c(D2O) =
14 M). After irradiations and aqueous workup the samples were
analyzed by MS and NMR to determine the extent and position
of deuteration, respectively. Irradiation (8 lamps, 300 nm, 1 h)
of 1, and 5−8 in CH3CN−D2O containing low concentration
RESULTS AND DISCUSSION
■
Synthesis. Naphthalene derivatives 5−8 were prepared in
high yields from the corresponding methoxy derivatives
5OMe−8OMe, which were in turn synthesized by Suzuki
coupling reactions (eqs 2 and 3).
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formed only in traces, isolated in a fraction enriched in 10-D.
NMR indicated that 10-D contained one deuterium at C-6 and
85% D at C-5. In the aromatic part of the spectrum, due to
overlapping of the signals, the position of the deuteration could
not be determined. Naphthofuran derivatives 11-D and 12-D
were isolated in 25% and 20% yield, respectively. 11-D
contained D at the naphthalene position 5 (15%), 6 (100%),
10 (30%), and 11 (74%), whereas 12-D contained D at the
naphthalene positions 5 (18%), 6 (100%), 8/9 (23%), 10
(47%), and 11 (83%). The irradiation experiment in CH3CN−
D2O (3:1) was taken to higher conversion to isolate the deu-
terated products 9−12. On the other hand, the irradiation
taken to low conversions clearly indicate that 5D-1 is formed
first, whereas formation of other derivatives takes place about
5−10 times slower.
Direct ESIPT in 6−8 is not possible due to a larger distance
between the phenol ring and the proton-accepting rings.
Therefore, a higher concentration of D2O (protic solvent) is
required to mediate the ESPT, which eventualy gives rise to
deuteration at the distal sites. Irradiation (1 h) of 6 in CH3CN−
D2O (2:1) resulted in D-incorporation at the naphthalene position
4 (10%, corresponding to 6-D4, eq 7), and position 5/8 (45%).
of D2O (0.3 M) gave rise to deuterium incorporation only in 1
and 5. Irradiation of 1 resulted in very efficient incorporation of
D at the ortho position of the adjacent phenyl ring. After 1 h
irradiation of 1, both ortho H atoms were replaced by D.26
Irradiation of 5 after 1 h gave quantitative exchange at the
naphthalene position 1 (corresponding to 5-D1) and 78% at
the position 3 (5-D3), whereas MS indicated presence of 10%
monodeuterad, 76% dideuterated, and 14% of trideuterated
species. The observed incorporation of D is in accordance with
ESIPT and formation of QMs that on subsequent thermal
tautomerization give rise to deuterated starting molecules (vide
infra). The other option for D-exchange would be efficient
protonation of the naphthalene in the excited state by D2O, as
demonstrated by Shizuka et al. for methoxynaphthalenes.30
However, since no D-exchange takes place on irradiation of
methoxy derivatives 5OMe−8OMe, this pathway probably does
not take place or takes place with insignificant quantum efficiency.
In a protic solvent, ESPT in 1-naphthol gives naphtholate
(pK*a = 0.4).31 In addition, solvent-assisted formal transfer of
the OH proton to the distal carbon atom at the position 5 takes
place with a quantum efficiency of 0.11, as evidenced by D-
exchange experiments.31,32 Photolyses of naphthols 1, 7 and 8
and phenols 5 and 6 in CH3CN−D2O at c(D2O) = 14 M also
gave rise to D-incorporation. Interestingly, the efficiency of the
deuteration at lower D2O concentration (c(D2O) = 0.3 M) for
1 and 5 was 10−20% higher than at c(D2O) = 14 M.
The finding is in accordance with direct ESIPT mechanism of
D-exchange in these derivatives that is partly perturbed on addi-
tion of more H2O (D2O). However, at higher D2O concen-
tration, in addition to direct ESIPT, solvent assisted ESPT gives
rise to D-exchange at the distal positions.
Photolysis of 5 in CH3CN−D2O (2:1) gave rise to D-ex-
change in the molecule, and formation of deuterated products
9-12 (Scheme 1). To characterize all deuterated products pre-
parative irradiation of 5 in CH3CN−D2O (3:1) was performed
(100 mg, 2 h, 16 lamps, 300 nm). After chromatographic
separation on TLC, 30% of 5-D was isolated with D in-
corporated at C-1 (100%), C-3 (18%), C-4 (10%), C-6/7
(13%), and C-5/8 (100%). Naphthofuran 9-D was isolated in
7% yield with D incorporated at C-10 (31%), C-3/4/6 (28%),
and at C-5 (68%). Dihydronaphthalene derivative 10-D was
The MS analysis indicated presence of 34% of deuterated and
13% dideuterated species, and 3% trideuterated species. Irra-
diation of naphthol 7 gave rise to D-exchange at the positions 5
(16%) and 8 (11%), and MS indicated 30% of deuterated and
5% of dideuterated species (eq 8), whereas irradiation of 8 gave
D only at the position 8 (15%), which was in agreement with
the MS indicating 15% of deuterated species (eq 9).
Quantum yields of deuterium incorporation (see the Experimental
Section) were determined by use of a secondary actinometer, D-
exchange in 2-phenylphenol (Φ = 0.041),21b and a primary one,
valerophenone (giving acetophenone in aqueous solvent Φ =
0.65).33 Measurements were performed in triplicate and the
mean value is reported (Table 1). Both actinometers gave
similar values for the quantum yield of D-exchange, with the
mean value within the error of the experiment. From the data it
is clear that the quantum efficiency cannot be correlated with
the direction of proton transfer, that is, excitation of phenol
(5 and 6) vs. excitation of naphthol (1, 7, 8). In addition, the
C
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a
Scheme 1. Main Products Obtained after Irradiation of 5 in CH3CN−D2O (3:1)
a
Compounds with a high percentage of D (> 70%) are underlined.
a
Table 1. Quantum Yields for Deuterium Incorporation
Table 3. Photophysical Properties of 1, and 5−8 in CH3CN
and CH3CN−H2O (1:4)
Φ
b
Φ (CH3CN−
τ (CH CN)/
τ (CH3CN−
b
a
a
3b
1
5
6
7
8
0.73 0.07
0.30 0.05
0.065 0.005
0.02 0.01
∼0.01
Φ (CH3CN)
H2O)
ns
H2O)/ns
c
c
1
0.031 0.001
0.032 0.002
0.3 0.1
(30%)
3.0 0.5
naphthol
1.7 0.1
(20%)
6.1 0.5
naphtholate
c
a
Photolysed in CH3CN−D2O (2:1) at 254 nm (2 lamps, Luzchem
10.0 0.5
c
(50%)
reactor). The actinometer was valerophenone in CH3CN−H2O
(Φ = 0.65),33 and D-exchange in 2-phenylphenol (Φ = 0.041).21
5
6
7
0.04 0.01
0.19 0.03
0.39 0.08
0.020 0.002
0.02 0.001
0.25 0.07
1.3 0.1
0.1 (1.8) phenol
0.13 phenol
b
From ref 26.
0.64 0.02
2.35 0.01
1.6 (8.0)
naphthol
efficiency of D-exchange is significantly higher for 1 and 5
wherein direct ESIPT along the H-bonding trajectory between
the OH(OD) and the π-system is possible.
Fluorescence. Photophysical properties of the studied
phenols and naphthols were characterized by UV−vis and
fluorescence measurements. Fluorescence spectra (see the
Supporting Information), quantum yields of fluorescence and
singlet excited state lifetimes were measured in CH3CN and
CH3CN−H2O (1:4), wherein it was anticipated that the
differences would arise due to solvent-assisted PT (Tables 2 and 3).
0.7 0.1
naphtholate
8
0.44 0.02
0.03 0.01
2.5 0.1
(88%)
4.07 0.01
(12%)
4.9 0.1
naphtholate
a
Determined by use of quinine sulfate in 0.05 M aqueous H2SO4
b
(Φ = 0.53) or fluorene in CH3OH (Φ = 0.68) as ref 36. Measured by
time-correlated single photon timing (SPT) method. From ref 26.
c
UV−vis spectra of 1, 7 and 8 are characterized by the presence
of a broad absorption band, typical for α-naphthols, that gives
Table 2. Spectral Properties of 1, and 5−8
34
1
1
rise to La and Lb states. Naphthylphenol derivatives 5 and 6
absorb at slightly shorter wavelength, corresponding to the typical
naphthalene absorption. The lowest excited state of 1 in CH3CN
can be characterized as 1La.35 Broader fluorescence spectra of 7
and 8 suggest the presence of two emissive states. Since 7 and 8
cannot have conformers with different photophysical proper-
λmax
/
Δυ /
STc
a
b
d
e
f
nm
λmax/nm
cm−1
ΔυHBW/cm−1
λmax/nm
ΔpKa
1
5
6
7
8
292
286
291
304
304
353.5/364
350.0
6774
6394
8166
6509
7319
3881
3440
4421
4440
4873
373 (∼475)
350 (518) 13.0
390 (518) 14.6
371.5
379.0
390 (454)
9.2
1
1
ties, we attribute the finding to La and Lb states. A larger
Stokes shift for 8 (compared to 1 and 7) is probably due to a
better charge delocalization with the phenyl at the naphthol
position 5. In addition, the Stokes shift and the half-bandwidth
is larger for 6 than for 5, suggesting a higher charge-transfer
character in the S1 in 6.
391.0
∼400 (526) 11.4
a
b
Maximum of the absorption spectrum in CH3CN. Maximum of the
c
d
fluorescence spectrum in CH3CN. Stokes shift in CH3CN. Half-
e
bandwidth of the fluorescence spectrum in CH3CN. Maximum of the
fluorescence spectrum in CH3CN−H2O (1:4). The values in
Fluorescence quantum yields for 1 and 5 in CH3CN (Table 3)
are 5−10 times lower than for 6−8. The finding can be
parentheses correspond to the emission from phenolate/naphtholate.
f
pK -pK * estimated by Forster cycle.
̈
a
a
D
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rationalized by operation of efficient ESIPT in 1 and 5, which
lowers Φf, and is not possible in aprotic solvent for 6−8.
Fluorescence decays measured by time-correlated single photon
timing (SPT) were single-exponential for CH3CN solutions of
5−7, giving singlet-excited state lifetimes (Table 3). On the
contrary, fluorescence decays for 1 and 8 were fitted to three-,
and two-exponential function, respectively, probably due to the
presence of several emissive states.
suggests that 6 and 8 are much stronger photoacids than 7.
However, Forster cycle analysis must be taken with some
̈
caution, as a more precise estimation of pKa* requires intrinsic
rate constants of proton transfer.
Laser Flash Photolysis (LFP). LFP was performed to
probe for QMs that are formed by ESIPT or solvent-assisted
PT. The transient spectra were recorded in N2- and O2-purged
CH3CN and CH3CN−H2O solutions (see the Supporting
Information). LFP of N2-purged solution CH3CN of 5 gave rise
to a strong transient absorption with a maximum at 400 nm and
absorbing into the visible region. The transient decayed with
k = (2.3 0.5) × 105 s−1 (τ = 4 1 μs), and was strongly
quenched by O2 (O2-purged CH3CN solution k = (2.8 0.2) ×
107 s−1, τ = 37 2 ns). Based on literature precedent38 and the
observed quenching by O2, the transient was assigned to the
phenol triplet−triplet absorption. In O2-purged solution, in addi-
tion to the triplet, a very-short-lived transient was detected (Figure 2)
Addition of H2O to the CH3CN solution of phenols and
naphthols results in fluorescence quenching due to an opening
of a new excited-state relaxation pathway, PT to solvent.37 To
probe for the influence of a protic solvent on the photophysical
properties of 5−8, fluorescence titrations with H2O were
performed (see the Supporting Information). However,
addition of a small amount of H2O (up to 2 M) to the
CH3CN solution of 5 increased fluorescence, suggesting that
H2O partly blocks the nonradiative deactivation pathway,
ESIPT to carbon at the naphthalene position 1 (vide supra). A
similar behavior was found for 1 and rationalized by efficient
intrinsic ESIPT pathway giving rise to QM 2 that is partly
quenched on addition of a protic solvent.26 Further addition of
H2O (above 2.5 M) to the CH3CN solution of 5 quenches
fluorescence and changes the appearance of the fluorescence
spectra. A new band appears at a longer wavelength that is seen
only in the protic solvent and assigned to the fluorescence of
phenolate formed by adiabatic PT to solvent. Adiabatic PT to
aqueous solvent is also corroborated by SPT measurements.
Single-exponential fluorescence decay in CH3CN solution
becomes two-exponential in CH3CN−H2O (1:4). The decay
component contributing more at the shorter wavelengths can
be assigned to phenol emission, whereas at longer wavelengths,
the component with a negative pre-exponential factor that
dominates corresponds to the phenolate emission.
Figure 2. Transient absorption spectra of 5 in O2-purged CH3CN
solution. Inset: decay at 500 nm.
Addition of H2O to the CH3CN solutions of 6−8 clearly
resulted in fluorescence quenching, particularly for 6 and 8
wherein Φf in CH3CN−H2O (1:4) is more than 10 times lower
than in neat CH3CN. Furthermore, addition of H2O gave rise
to dual fluorescence due to adiabatic formation of fluorescent
phenolate 6‑ and naphtholates 7‑ and 8‑ (Figure 1). Stronger
absorbing at 400−600 nm decaying with k = (4.0 0.4) × 107
s−1, τ = 26 3 ns). Similarly to the transients observed by LFP
of 1,26 the short-lived species could tentatively be assigned to
QM 14. In the transient spectra of 5 taken in CH3CN−H2O
solution, due to a strong signal of the fluorescence stretching
over the whole visible region, the short-lived transient was not
observed. To verify if the observed short-lived transient
corresponds to QM, LFP of 5OMe (that cannot give QMs)
was also undertaken. However, in the LFP spectra of 5OMe
taken in N2-purged CH3CN solution, we detected a signal
corresponding to the triplet at 430 nm (τ ≈ 3 μs) and transient
absorption at λ > 500 nm that was not affected by O2 (in O2-
purged CH3CN, k ≈ 2 × 105 s−1, τ ≈ 5 μs), probably
corresponding to radical-cations formed by two photon
ionization. Therefore, we cannot give a firm assignment of
the short-lived transient observed by LFP of 5, since it may
correspond to radical-cation undergoing a very fast deprotonation.
LFP of CH3CN or CH3CN−H2O solution of 6 gave rise to a
strong transient absorption at 500 nm k = (2−5) × 105 s−1 (τ =
2−5 μs), that was quenched by O2, which we have assigned to
the triplet state. In addition, in CH3CN−H2O, a more per-
sistent transient was detected at 430 nm (O2 purged solution
k = (2.1 0.1) × 104 s−1, τ = 45 2 μs. In comparison with
published spectra,38 the transient was assigned to the phenoxyl
radical, probably formed by two photon ionization, initially
giving a phenol radical-cation and solvated electron. Radical-
cations deprotonate readily in protic solvent, giving phenoxyl
radicals. Therefore, we observed a stronger transient absorption
Figure 1. Normalized fluorescence spectra of 6−8 in CH3CN (full)
and CH3CN−H2O (1:4, dash).
fluorescence quenching for 8, compared to 7, is in line
with higher increase in acidity in S1, probably due to a larger
charge-transfer character of the S1 state in 8. The steady state
absorption and fluorescence spectra in neutral and basic solu-
tion enable approximation of the difference in the pKa values
between the S and S by use of the Forster cycle. The values
̈
1
0
of ΔpKa for 5−8 are compiled in Table 2. The data strongly
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corresponding to phenoxyl radicals in aqueous solution. LFP of
naphthol 8 also gave rise to a signal corresponding to the triplet
absorbing with a maximum at 480 nm (in N2-purged CH3CN,
k = (1.35 0.05) × 106 s−1, τ = 730 30 ns), and a more
persistant transient, probably corresponding to phenoxyl
radicals. The longer-lived transient for 7 absorbed at λ = 325 nm,
400 nm, and had a shoulder at 500 nm, stretching over the visible
region, decayed with k = (2.5 0.5) × 104 s−1, (τ = 40 10 μs).
Similarly, the phenoxyl radical was detected in the LFP of 8 that
in CH3CN−H2O had a maximum at 350 nm and a shoulder at
400 nm decaying with k ≈ 2 × 104 s−1, τ ≈ 50 μs.
chemical reactivity therefore can be explained by the NEER
principle (nonequilibration of the excited-state rotamers).39
Vertical electronic excitations for 5−8 were computed using
RI-CC2/cc-pVDZ. Energies for the first two excited states of
5−8 are compiled in Table 5. The results correspond to the gas
Quantum Chemical Calculations. Additional insight into
the photochemistry of the investigated phenylnaphthols and
naphthylphenols is provided by ab initio calculations.
Specifically, the calculations were aimed to rationalize the
outcome of the D-exchange experiments. Ground-state geom-
etries of several low energy conformers of 5−8 were obtained
by employing the MP2/TZVP method in the gas phase and in
the framework of the polarizable continuum model MP2/
PCM/TZVP (Table 4). With respect to the orientation of the
Table 5. Vertical and Adiabatic Excitations (nm) for 5−8
a
Computed using RI-CC2/cc-pVDZ
state
S5−1
S5−3
S6
A7
A8
S1 285 (CIS1/S0
)
284 (334)
262
280 (308)
265
285 (340)
273
284 (347)
274
S2 268
a
The value in parentheses corresponds to adiabatic excitation energy.
phase excitation and are directly comparable with the experi-
mental results for 5. However, for 6−8, larger deviations
between the experimental values obtained in CH3CN solution
(Table 2) and the calculated results were obtained.
The leading excitations contributing to the S1 state of S5−1
are of ππ* type with charge transfer (CT) character (Figure 3).
Table 4. Electronic Energies of Several Low Energy
Conformers of 5−8 Computed at the MP2/TZVP and MP2/
a
PCM/TZVP Levels
MP2/TZVP
MP2/PCM/TZVP
S1
0.00
3.79
0.00
0.23
2.72
1.80
0.00
0.09
0.00
2.33
0.00
2.32
0.00
2.63
0.00
0.25
1.29
1.28
0.04
0.00
0.00
2.34
0.00
2.32
A1
S5−1
S5−3
A5−1
A5−3
S6
A6
A7
S7
A8
S8
a
The energies are given in kcal/mol relative to the corresponding
minimum energy conformer.
OH group and the adjacent proton accepting ring, we dis-
tinguish between the syn (S) and anti (A) conformers.
Figure 3. Occupied (bottom) and unoccupied (top) molecular orbitals
of the S1 state of S5−1 at the geometry of vertical excitation.
The quantum yields of deuterium incorporation are strongly
affected by the ground state population equilibrium of the syn
and anti conformers. MP2/PCM/TZVP yields four minimum
energy structures of 5. The two lowest energy structures cor-
respond to the syn conformers S5−1 and S5−3. These
structures favor direct ESIPT to the naphthalene position 1
and 3. The A5−1 and A5−3 forms are found 1.29 and 1.28
kcal/mol higher in energy, respectively, resulting in a
Boltzmann population ratio of 0.54:0.34:0.06:0.06 for S5−
1:S5−3:A5−1:A5−3. Contrary to 5, the energetics of 6 is not
sensitive to the orientation of the OH group. However, the
large separation between the OH and the naphthalene ring
makes the direct ESIPT impossible. On the other hand, in 7
and 8 direct ESIPT is in principle conceivable. However, in the
low energy structures A7 and A8, the OH group is directed
away from the second naphthol ring. The syn oriented con-
formers S7 and S8 are found to be more than 2.3 kcal/mol
higher in energy. That is, in 7 and 8, the ground state confor-
mational equilibrium effectively precludes direct ESIPT to the
naphthalene position 8. The obsereved differences in the photo-
The occupied orbitals are delocalized on both rings while the
unoccupied ones are localized on the naphthyl ring. This is in
contrast to 1, where the occupied orbitals are localized on the
naphthol ring and the unoccupied one is completely delocalized.
However, the net effect is the samean effective transfer of
negative charge from one ring, here the phenol, to the other.
This is evident from Figure 4 where the difference in the
electron densities between the S1 and S0 is shown. Note also
the strong redistribution of charge within the naphthyl ring.
The leading excitations contributing to the S1 state of 6 and 8
are shown in the Supporting Information.
The geometry optimization of the S1 state in S5−1 leads to
the stretching of the OH bond, rotation of the phenol ring with
concomitant reduction of the torsional angle between the two
rings from 57.3 to 37.5 degrees, and finally to ESIPT to the
position 1 of the naphthyl ring. These geometrical changes
greatly destabilize S0 and the two states intersect 2.46 eV above
the S0 minimum. The optimization of S5−3 in the S1 state does
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atoms bearing the most negative charge are the preferred sites of
deuterium incorporation in a H2O(D2O)-mediated reaction.
The computed NBO charges on the oxygen atoms of 5‑−8‑
are −0.435, −0.491, −0.559 and −0.602, respectively, and
illustrate the proton affinity of the corresponding RO− groups.
Focusing on 5‑, we see that apart from C-1, which is the target
of direct ESIPT, carbon atoms C-4, C-5, and C-8 are prone to
H2O(D2O) attack. In 6‑, the most probable sites of attack are
again C-4, C-5, and C-8, while in 7‑ these are C-5 and C-8, and
in 8‑ they are C-3 and C-8. Apart form 5‑ where the NBO
charge on oxygen is lower than (more negative), but to some
extent comparable to, the most basic carbon atoms, in 6‑−8‑ RO−
is the most probable site of H2O(D2O) attack. These findings are
in very good agreement with the observed regioselectivity and
lower quantum yields of D-exchange (Table 1).
Photochemical Reaction Mechanisms. With the above
experimental and theoretical results, photochemical reaction
mechanisms for 5−8 can be proposed. Irradiation of 5 in
aprotic solvent (or with low H2O concentration) leads to an
ultrafast efficient deactivation from the S1 via a conical inter-
section with the S0, delivering QM 14, and less efficient de-
activiation, also via a conical section delivering QM 15 (eq 10).
Figure 4. Difference in the electronic densities of 5 between states S1
and S0 evaluated at the electron density surface of S0 with isovalue of
0.01/ Å3. The areas of increased electron density are shown in blue.
not lead to CI with S0 and hence to direct ESIPT. Instead, a
minimum energy structure is found 0.48 eV (11.1 kcal/mol)
below the S1 vertical excitation. However, the barrier
encountered on the ESIPT pathway is very small, less than 0.1
eV. The difference in energy between the vertical and adiabatic
excitation corresponds to the gain in kinetic energy at the S1
minimum. This energy gain is clearly larger than the barrier to
ESIPT and ensures formation of QM 15. In a protic solvent, the
redistribution of the excess kinetic energy into atomic motion
probably gives rise to the deprotonation and formation of
naphthyl-phenolate 5−3‑.
According to the D-exchange experiments, QMs 14 and 15 are
formed in a ratio 5:1, which correlates with the relative energies
of the reactive phenol conformations S5−1 and S5−3. Due to
efficient ESIPT delivering QMs 14 and 15, fluorescence
quantum yield of 5 in CH3CN is low. Addition of protic
solvent (H2O) increases fluorescence due to a hindering of
ESIPT, as seen for 1.26 However, at a high enough H2O
concentration, a new photochemical deacivation pathway from
S1 takes place involving deprotonation of the phenol OH. That
is demonstrated in the fluorescence spectra where an additional
band at longer wavelenghts can be observed, assigned to the
fluorescence from phenolate formed in an adiabatic PT to
solvent. Once phenolate in S1 is populated, the anion under-
goes protonation by H2O at the distal sites, at C-4 and C-8,
giving zwitterion 17 and QM 16 (Scheme 2). Ab initio
calculation strongly indicates that protonation by H2O takes
place after deprotonation of the phenol since regioselectivity of
the D-incorporation correlates with the calculated charges in
the 5−1‑ and not with the neutral S5−3 in S1. The signals in 1H
NMR corresponding to the H-atoms at C-5 and C-8 of the
naphthalene overlap. Therefore, according to the NMR spectra
after D-exchange, we cannot distinguish if photochemical re-
action gives QM 16 or zwitterion 21. However, 21 is probably
not formed since we have not observed formation of products
22 and 23 that would arise from 21.
For 6−8, direct ESIPT is not possible. Furthermore, Table 5
reveals that minima are located on the S1 surfaces of the major
conformers S6, A7, and A8. On the basis of the large difference
between the vertical and adiabatic transitions we expect, however,
that in the presence of proton-accepting solvent, adiabatic de-
protonation takes place. The anions 5‑−8‑ have been optimized
at the same RI-CC2/cc-pVDZ level of theory and the natural
bond orbital (NBO) analysis has been performed on all the
minima. The adiabatic excitation energies of 5−1‑, 6‑, 7‑, 8‑, are
552 nm, 663 nm, 463 and 661 nm, respectively. Table 6 lists the
NBO charges on the carbon atoms. We expect that the carbon
Table 6. NBO Charges on C-atoms in the Optimized S1 State
a
of the Anions (Phenolates and Naphtholates)
C-atom
5−1‑
6‑
7‑
8‑
1
−0.25392
−0.15077
−0.23718
-0.29656
-0.30199
−0.28021
−0.25980
-0.29796
−0.05505
−0.03596
0.02595
−0.19462
−0.24956
−0.27601
-0.30096
-0.29935
−0.27478
−0.25927
-0.32651
−0.02381
−0.04712
0.08277
0.40255
−0.22240
−0.27088
0.03196
0.38543
−0.21576
−0.29003
-0.10835
-0.08504
−0.28490
−0.19643
-0.31488
−0.09540
−0.09619
−0.10233
−0.25461
−0.23494
−0.32406
−0.23390
−0.27045
2
3
4
5
-0.39492
−0.27554
−0.24021
-0.34349
−0.11546
−0.08275
−0.05848
−0.22287
−0.24372
−0.23428
−0.24838
−0.19466
6
7
8
9
10
1′
2′
3′
4′
5′
6′
0.32677
−0.23697
−0.22307
0.33621
−0.23773
−0.27758
−0.17042
−0.26999
−0.22314
−0.23085
We have not detected H2O-adducts 18−20 suggesting that
QMs 14−16 do not react with H2O (or CH3OH) giving adducts
(a known reaction for QMs), or that 18−20 are not stable
products. Probably, QMs 14−16 only undergo tautomerization
a
Notation of C-atoms follow IUPAC nomenclature rules and can be
found in Scheme 2 and eqs 11, 12, and 13. C-atoms marked in bold are
the preferred sites for D-incorporation.
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Scheme 2
to the starting material. However, we cannot firmly rule out a
pathway involving formation of 18−20 followed by a rapid
elimination of H2O. On irradiation in the presence of D2O,
deuterated QMs are formed that tautomerize to 5. However,
due to the primary isotope effect, D favorably remains
regioselectively incorporated in the molecule. Contrary to
14−16 which only tautomerize to 5, zwitterion 17 cyclizes to
stable photoproducts 11 and 12. Furthermore, naphthofuran
12 undergoes thermal or photochemical secondary [1,3]
H-shift yielding 10, whereas naphthofuran 9 is probably
formed from 10 by aromatization due to the trace-impurities
of O2. After irradiation of 5 in CH3CN−D2O only a small
content of D is found in the position 4 of the naphthalene.
The finding indicates that 17 undergoes faster cyclization to
11 and 12 than tautomerization to the starting material.
After irradiation in CH3CN−D2O, 10−12 were isolated with
D-incorporated in the molecules (Scheme 1). As indicated by
D-exchange experiments taken to lower conversions, the fastest
process is formation of 5D-1. That is, the quantum yield for
ESIPT in 5 giving 14 is about ten times higher than solvent-assisted
PT giving 17. Therefore, the isolated product 11 contained a
high percentage of D at the position 11. Furthermore, afer the
first fast D-exchange at the naphthalene position 1, deuterated
5−1D undergoes second photochemical reaction wherein D
enters at the naphthalene position 4 and gives QM 17 that
irreversibly cyclize to 11. Consequently, isolated 11 contained a
high percentage of dideuterated species, wherein one D was
quantitatively incorporated at the position 6 and the other was
at a high percentage at the position 11 (corresponding to 11-
D6 and 11-D11). Similarly, after irradiation in CH3CN−D2O,
product 12 was isolated with one deuterium quantitatively
incorporated at the position 6, and some molecules contained 2
deuteriums, at positions 6 and 11.
Compounds 6−8 are photochemicaly stable in CH3CN since
transfer of the OH proton from the naphthalene or phenol OH
to the distal carbon sites cannot take place without protic
solvent. Therefore, all of 6−8 are characterized by high
fluorescence quantum yields in neat CH3CN, almost an order
of magnitude higher than for 1 and 5. Excitation of 6−8 to S1
results in an increase of dipole moment in these molecules. The
difference in the calculated and measured absorption is pro-
bably due to a neglect of the stabilization by solvent in the
calculations. Stronger polarization of the molecules in S1 leads
to enhanced acidity of the phenol or naphthol OH. Therefore,
excitation to S1 in a protic solvent gives rise to deprotonation
and formation of phenolates/naphtholates, along with fluo-
rescence quantum yields significantly lower than in neat
CH3CN. After deprotonation, the naphtholates or phenolates
in S1 undergo protonation by H2O (D2O) that give QMs.
Phenolate 6‑ is protonated at the naphthalene positions 4, 5,
and 8, yielding QMs 24−26 (eq 11). The regioselectivity of
D-exchange is correlated with the calculated negative charge
in S1 of the phenolate. Similarly, in protic solvent, naphthols 7
and 8 in S1 deprotonate giving naphtholates. Due to a better
charge stabilization with the phenyl at position 5, 8 is more
acidic than 7, which is corroborated by fluorescence measure-
ments. Subsequent protonation of the naphtholate 7‑ and 8‑
gives QMs 27 and 28, or 29, respectively (eqs 12 and 13). Due
to significantly lower quantum efficiency for the solvent-assisted
PT than for the direct ESIPT, QMs in the photochemical
reactions of 6−8 are formed less efficiently than from 1 and 5.
Hence, we were not able to detect 24−29 by LFP.
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Accurate mass determination was performed on an ESI-quadrupole
instrument. A mg/mL solution of the analyte was diluted by a factor of
10−100 and injected by liquid infusion at ∼300−500 nL/min by
syringe pump through a nano ESI source; all solvents used were MS
grade. For the sample analysis, a HPLC equipped with a Diode-Array
detector and a C18(2) column was used. Mobile phase was CH3OH−H2O
(20%). For the chromatographic separations, silica gel (0.05−0.2 mm)
was used. Analytical thin layer chromatography was performed on
SILG/UV254 plates. Irradiation experiments were performed in a
reactor equipped with 2, 8, or 16 lamps with the output at 254 or
300 nm. During irradiations in the reactor, the irradiated solutions
were continuously purged with Ar and cooled by a tap water finger
condenser. Solvents for irradiations were of HPLC purity. Chemicals
(bromonaphthalenes, boronic acids, solution of BBr3) were purchased
from the usual commercial sources or prepared according to the
known procedures. Solvents for chromatographic separations were
purified by distillation.
Suzuki Reaction, General Procedure. A two neck flask
(100 mL) equipped with a condenser and a nitrogen inlet was charged
with a methanol solution (15 mL) of boronic acid (10 mmol) and a
toluene solution (35 mL) of bromonaphthalene (10 mmol). To the
mixture was added anhydrous potassium carbonate (2.73 g, 19.72 mmol)
and tetrakis-triphenylphosphine palladium Pd(PPh3)4 (0.17 g,
0.148 mmol), and the mixture was heated at the temperature of
reflux for 24 h. The next day, to the cooled mixture H2O (50 mL) was
added and the layers were separated. The aqueous layer was extracted
with CH2Cl2 (3 × 30 mL); the organic layers were combined and
dried over anhydrous MgSO4. After filtration, the solvent was removed
on a rotary evaporator to afford crude product that was purified on a
column of silica gel using hexane−CH2Cl2 (3:1) as eluent.
2-(2-Methoxyphenyl)naphthalene (5OMe).40 From 2-methoxy-
phenyl boronic acid (1.5 g, 10 mmol) and 2-bromonaphthalene
(2.07 g, 10 mmol), in the presence of potassium carbonate (2.76 g,
20 mmol) and Pd(PPh3)4 (0.17 g, 0.15 mmol), the reaction furnished
1.92 g (83%) of the product in the form of colorless crystals.
1-(4-Methoxyphenyl)naphthalene (6OMe).41 From 4-methox-
yphenyl boronic acid (1.0 g, 6.6 mmol) and 1-bromonaphthalene
(0.91 mL, 6.6 mmol), in the presence of potassium carbonate (1.82 g,
13.2 mmol) and Pd(PPh3)4 (0.11 g, 0.1 mmol), the reaction furnished
1.11 g (72%) of the product in the form of colorless crystals.
1-Methoxy-4-phenylnaphthalene (7OMe).42 From phenyl
boronic acid (0.77 g, 6.6 mmol) and 1-bromo-4-methoxynaphthalene
(0.91 mL, 6.6 mmol), in the presence of potassium carbonate (1.75 g,
12.6 mmol) and Pd(PPh3)4 (0.11 g, 0.1 mmol), the reaction furnished
0.51 g (34%) of the product in the form of colorless crystals.
1-Methoxy-5-phenylnaphthalene (8OMe).43 From phenyl
boronic acid (0.44 g, 3.6 mmol) and 1-bromo-5-methoxynaphthalene
(0.80 g, 3.4 mmol), in the presence of potassium carbonate (0.94 g,
6.8 mmol) and Pd(PPh3)4 (60 mg, 0.05 mmol), the reaction furnished
610 mg (76%) of the product in the form of colorless crystals.
General Procedure for the Removal of the Methyl Ether
Groups from Phenols with BBr3. Reaction was carried out under
N2-atmosphere in a three-neck round-bottom flask (250 mL) equip-
ped with a septum and a syringe. In the flask was placed methoxy
derivative 5OMe−8OMe (10 mmol), dissolved in dry CH2Cl2 (100 mL)
and the solution was cooled to 0 °C by ice bath. To the cold solution,
a solution of BBr3 in CH2Cl2 (1 M, 30 mL) was added dropwise by use
of a syringe during 1 h. After the addition was completed, the stirring
was continued for 1 h at 0 °C and then at rt overnight. The next day,
H2O (100 mL) was added to the reaction mixture, the layers were
separated, and the aqueous layer was extracted with CH2Cl2 (3 × 50 mL).
The combined organic extracts were dried over anhydrous MgSO4, solid
was removed by filtration, and solvent was removed on a rotary
evaporator. The obtained crude product was purified on a column filled
with silica gel by use of CH2Cl2 and CH2Cl2−EtOAc (4:1) as eluent or
crystallization.
CONCLUSIONS
■
Phenols 5 and 6 and naphthols 7 and 8 undergo ESIPT or
solvent-assisted PT from the phenol/naphthol OH to the
carbon atoms of the adjecent aromatic ring giving quinone
methide intermediates. In addition, solvent-assisted PT in 5 gives
zwitterions that undergo cyclization to naphthofurans 9−12.
Investigation of the photophysical properties and photoreactivity
of 5−8 reveal significantly higher photochemical reactivity in
systems where direct ESIPT is possible. Solvent-assisted PT
takes place with quantum yields that are 5−10 times lower. The
probable mechanism for solvent-assisted PT involves first the
deprotonation of the OH group in the S1 state, followed by
protonation by H2O of carbon atoms of the naphtholate/phenolate
on the same surface. Furthemore, photochemical reactivity is highly
correlated with the conformational distribution in the ground state
representing another example of the operation of the NEER
principle.
EXPERIMENTAL SECTION
■
1
General. H and 13C NMR spectra were recorded on 300, 500, or
600 MHz. The NMR spectra were taken in CDCl3 or DMSO-d6 at rt
using TMS as a reference and chemical shifts were reported in ppm.
For the assignment of signals, 2D homonuclear COSY and NOESY
and heteronuclear HSQC and HMBC techniques were used. Melting
points were determined using a Mikroheiztisch apparatus (and were
not corrected). UV−vis spectra were recorded on a spectrophotometer
at rt. IR spectra were recorded on a spectrophotometer in KBr.
Deuterium content was determined by MS recorded on an instrument
by use of the electrospray ionization technique. The spectra were
obtained in the negative mode in the presence of triethylamine.
2-(2-Hydroxyphenyl)naphthalene (5).44 From 2-(2-
methoxyphenyl)naphthalene (5OMe, 1.64 g, 7.0 mmol) and solution
of BBr3 (1M, 20 mL), the reaction gave 1.54 g of the crude product
that was purified by chromatography on silica gel by use of CH2Cl2
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and CH3OH (1 → 3%) as eluent to afford 1.22 g (80%) of the pure
product in the form of colorless crystals.
H-5), 5.78 (dd, 1H, J = 2.5 Hz, J = 8.7 Hz, H-11), 3.60 (ddd, 1H, J =
2.1 Hz, J = 6.3 Hz, J = 19.4 Hz, H-6), 3.37 (dd, 1H, J = 8.7 Hz, J = 19.4
Hz, H-6); 13C NMR (CDCl3, 125 MHz) δ/ppm 162.5 (s, C−O1),
139.4 (s, C-4b), 135.8 (s, C-10a), 133.2 (s, C-6a), 129.7 (d, C-2),
127.3 (d, C-8/9), 127.0 (d, C-7), 126.2 (d, C-8/9), 124.8 (s, C-4a),
124.5 (d, C-10), 121.0 (d, 2C, C-3, C-4), 113.0 (d, C-5), 110.6 (d,
C-1), 81.1 (d, C-11), 31.7 (t, C-6); HRMS (ESI) calculated for
C16H12O (+Na+) 243.0780, found 243.0780.
HMBC interactions: H-6 and C-5, C-4b, C-10a, C-6a, C-8, H-11
and C-10a, H-5 and C-6, C-4a, C-6a, C-6 and H-5, H-8/9, C-11 and
H-10, H-8/9, H-1 and C-3/4, C-4a, C−O1, H-3 and C-1, C-3/4, C-4a,
C-2, H-2 and C-3/4, C−O1, H-7 and C-10, C-6a, H-8/9 and C-10a.
General Irradiation in CH3CN−D2O. To a solution of naphtha-
lene derivative (5 mg, 0.023 mmol) in CH3CN (38 mL) was added
D2O (12 mL). The solution was purged with Ar for 30 min and
irradiated in a reactor at 254 or 300 nm (16 lamps) for 1 or 10 min.
After irradiation, 50 mL of H2O was added and extractions with
CH2Cl2 (3 × 50 mL) were carried out. The extracts were dried over
anhydrous MgSO4. After filtration and evaporation of the solvent,
crude mixture was analyzed by NMR and MS.
1-(4-Hydroxyphenyl)naphthalene (6).45 From 1-(4-
methoxyphenyl)naphthalene (6OMe, 0.81 g, 3.4 mmol) and solution
of BBr3 (1M, 10 mL), the reaction gave the crude product that was
purified by chromatography on silica gel by use of CH2Cl2 and
CH2Cl2−EtOAc (4:1) as eluent to afford 0.74 g (99%) of the pure
product in a form of colorless crystals.
4-Phenyl-1-naphthol (7).46 From 1-methoxy-4-phenylnaphtha-
lene (7OMe, 0.25 g, 1.1 mmol) and solution of BBr3 (1M, 3.2 mL),
the reaction gave the crude product that was purified by
chromatography on silica gel by use of CH2Cl2 and CH2Cl2−EtOAc
(4:1) as eluent to afford 220 mg (91%) of the pure product in a form
of colorless crystals.
5-Phenyl-1-naphthol (8).47 From 1-methoxy-5-phenylnaphtha-
lene (8OMe, 0.51 g, 2.2 mmol) and solution of BBr3 (1M, 6.5 mL),
the reaction gave the crude product that was purified by chroma-
tography on silica gel by use of CH2Cl2 and (1 → 3%) CH3OH as
eluent to afford 436 mg (90%) of the pure product in a form of
colorless crystals.
Irradiation of 2-(2-Hydroxyphenyl)naphthalene (5). A solu-
tion of 5 (100 mg, mmol) in CH3CN−H2O (3:1, 100 mL) was poured
in a quartz vessel, purged with Ar for 30 min, and irradiated 1 h in a
reactor (16 lamps, 300 nm). After the irradiation, extraction with
CH2Cl2 was carried out (3 × 50 mL). The extracts were dried over
anhydrous MgSO4 and filtered and the solvent was removed on a
rotary evaporator. The crude residue was chromatographed on a thin
layer of silica using CH2Cl2/hexane (1:3) as eluent, to afford pure
photoproducts and regenerate 42 mg (42%) of the starting material.
Benzo[d]naphtho[1,2-b]furan (9).28 Three milligrams (3%);
colorless crystals, 1HNMR (CDCl3, 500 MHz) δ/ppm 8.44 (d, 1H, J =
8.1 Hz, H-10), 7.96−8.02 (m, 3H, H-4, H-6), 7.77 (d, 1H, J = 8.2 Hz,
H-5), 7.70 (d, 1H, J = 8.0 Hz, H-1), 7.63 (dd (t), 1H J = 7.4 Hz, H-9),
7.55 (dd, 1H, J = 7.5 Hz, J = 7.0 Hz, H-8), 7.47 (dd, 1H, J = 7.5 Hz, J =
8.1 Hz, H-2), 7.38 (dd (t), 1H, J = 7.5 Hz, H-3); 13C NMR (CDCl3,
125 MHz) δ/ppm 155.9 (s), 151.9 (s), 133.0 (s), 128.4 (d, C-7), 126.4
(d, C-9), 126.1 (d, C-2), 126.0 (d, C-8), 125.0 (s), 123.2 (d, C-5),
122.8 (d, C-3), 121.3 (s), 120.8 (d), 120.2 (d), 119.1 (s), 118.4 (d,
C-6), 111.7 (d, C-1).
Alternatively, naphthalene derivative (10 mg, 0.046 mmol) was
dissolved in CH3CN (10 mL). The solution was divided into two parts
and poured into two quartz cuvettes. To the first cuvette CH3CN
(5 mL) and D2O (100 μL) were added, and to the second CH3CN
(5 mL) and D2O (5 mL). Both solutions were purged with Ar
(30 min), sealed and irradiated at the same time in a reactor equipped
with a merry-go-round using 8 lamps with the output at 300 nm for
1 h. To the irradiated solutions H2O (20 mL) was added and extrac-
tions with CH2Cl2 (3 × 30 mL) were carried out. The extracts were
dried over anhydrous MgSO4. After filtration and evaporation of the
solvent, crude mixture was analyzed by NMR and MS.
Control experiments were performed by preparing the same solu-
tion as described above that were kept in dark and worked-up with
H2O. After extractions with CH2Cl2 drying over anhydrous MgSO4,
filtration and evaporation of the solvent, the mixture was analyzed by
NMR and MS.
Irradiation of 5 in CH3CN−D2O. To a solution of 5 (100 mg,
0.45 mmol) in CH3CN (80 mL) was added D2O (20 mL). The
solution was purged with Ar for 30 min and irradiated in a reactor at
300 nm (16 lamps) for 2 h. After the irradiation, 50 mL of H2O was
added and extractions with CH2Cl2 (3 × 50 mL) were carried out. The
extracts were dried over anhydrous MgSO4, the solution was filtered
and the solvent removed on a rotary evaporator. The residue was
chromatographed on a thin layer of silica gel using CH2Cl2/hexane
(1:3) as eluent.
5,6-Dihydrobenzo[d]naphtho[1,2-b]furan (10).29 Twelve milli-
1
grams (12%); colorless crystals, HNMR (CDCl3, 500 MHz) δ/ppm
7.65 (d, 1H, J = 7.5 Hz, H-10), 7.50 (d, 1H, J = 8.2 Hz, H-1),
7.49 (d, 1H, J = 7.5 Hz, H-4), 7.18−7.30 (m, 5H), 3.09 (t, 2H, J = 8.0
Hz, H-6), 2.95 (t, 2H, J = 8.0 Hz, H-5); 13C NMR (CDCl3, 125 MHz)
δ/ppm 155.2 (s), 151.7 (s), 135.8 (s), 128.2 (s), 127.9 (d), 127.7 (s),
127.5 (d), 126.7 (d), 123.9 (d), 122.6 (d), 120.4 (d, C-10), 119.0
(d, C-4), 114.0 (s, C-4a), 111.3 (d, C-1), 28.6 (t, C-6), 19.2 (t, C-5).
5a,6-Dihydrobenzo[d]naphtho[2,3-b]furan (11). Fifteen milli-
9-D 7 mg (7%) 31% D at C-10, 28% D at C-3/4/6, 68% D at C-5.
10-D traces (<1%) 1D at C-6 and 85% D at C-5.
11-D 25 mg (25%) 30% D at C-10, 74% D at C-11, 15% D at C-5,
1D (108%) at C-6; MS (ESI): 17% 1D, 27% 2D, 18% 3D, 18% 4D.
12-D 20 mg (20%) 47% D at C-10, 23%D at C-8/9, 18% D at C-5,
83%D at C-11, 1D (110%) at C-6; MS (ESI): 16% 1D, 27% 2D, 23%
3D, 13% 4D, 7% 5D.
5-D 30 mg (30%) 10%D at C-4, 1D at C-1, 1D (107%) at C-5/8,
18% D at C-3, 13% D at C-6/7; MS (ESI): 13% 1D, 32% 2D, 25% 3D,
10% 4D.
Irradiation of 5 in CH3OH. A solution of 5 (110 mg, 0.5 mmol) in
CH3OH (100 mL) was purged with Ar for 30 min and irradiated in a
reactor at 300 nm (16 lamps) for 1 h. After the irradiation, the solvent
was removed on a rotary evaporator. The residue was chromato-
graphed on a thin layer of silica gel using CH2Cl2/hexane (1:5) and
hexane/EtOAc (4:1) as eluent to afford 9 (7 mg, 6%), 11 (18 mg,
16%), 12 (3 mg, 3%), 13 (8 mg, 7%), and 5 (68 mg, 62%).
6a-Methoxy-5,6,6a,11a-tetrahydrobenzo[d]naphtho[1,2-
b]furan (13). Colorless oil; 1HNMR (CDCl3, 500 MHz) δ/ppm 7.52
(d, 1H, J = 7.5 Hz, H-10), 7.29 (dd, 1H, J = 1.0 Hz, J = 7.5 Hz, H-4),
7.26 (t, 1H, J = 7.5 Hz, H-9), 7.19−7.23 (m, 2H, H-8, H-2), 7.08
(d, 1H, J = 7.5 Hz, H-7), 6.95 (dt, 1H, J = 0.7 Hz, J = 7.5 Hz, H-3),
6.79 (d, 1H, J = 8.0 Hz, H-1), 5.60 (s, 1H, H-11), 3.15 (s, 3H, OCH3),
2.70 (dd, 1H, J = 4.5 Hz, J = 15.7 Hz, H-6), 2.49 (ddd, 1H, J = 3.5 Hz,
J = 11.8 Hz, J = 15.7 Hz, H-6), 2.39 (ddd, 1H, J = 3.5 Hz, J = 4.5 Hz,
1
grams (15%); colorless crystals, mp 123−125 °C; HNMR (CDCl3,
500 MHz) δ/ppm 7.48 (d, 1H, J = 7.5 Hz, H-1), 7.22−7.26 (m, 3H,
H-3, H-9, H-7), 7.18 (d, 1H, J = 7.0 Hz, H-10), 7.14 (dd (t), 1H, J =
7.4 Hz, H-8), 6.96 (dd (t), 1H, J = 7.6 Hz, H-2), 6.93 (d, 1H, J = 8.2
Hz, H-4), 6.62 (d, 1H, J = 3.1 Hz, H-11), 5.27 (ddd, 1H, J = 3.1 Hz,
J = 7.1 Hz, J = 15.8 Hz, H-5), 3.27 (dd, 1H, J = 7.1 Hz, J = 14.3 Hz,
H-6), 3.12 (dd (t), 1H, J = 15.0 Hz, H-6); 13C NMR (CDCl3, 125 MHz)
δ/ppm 164.2 (s, C−O1), 140.2 (s, C-1b), 135.0 (s, C-6a), 131.2 (s,
C-10a), 130.4 (d, C-3), 128.9 (d, C-7), 127.4 (d, C-9), 126.8 (d, C-8/
10), 126.7 (d, C-8/10), 124.5 (s, C-1a), 121.5 (d, C-1), 121.2 (d, C-2),
113.9 (d, C-11), 110.7 (d, C-4), 82.3 (d, C-5), 34.2 (t, C-6); HRMS
(ESI) calculated for C16H12O (+H+) 221.0961, found 221.0966.
HMBC interactions: H-6 and C-5, C-1b, C-6a, C-10a, C-7, H-5 and
C-11, C-1b, C-6 and H-7, H-11 and C-5, C-1a, C-10, H-1 and C-11,
C−O1, C-1b, C-3, H-4 and C−O1, C-1, C-1a, H-2 and C-4, C-1a, H-
10 and C-11, H-8 and C-7, C-10a.
5,11a-Dihydrobenzo[d]naphtho[1,2-b]furan (12). Six milli-
1
grams (6%); yellow crystals; mp 92−95 °C; HNMR (CDCl3, 500
MHz) δ/ppm 7.67 (d, 1H, J = 7.5 Hz, H-10), 7.41 (d, 1H, J = 7.5 Hz,
H-4), 7.28−7.33 (m, 2H, H-8, H-9), 7.25 (d, 1H, J = 7.2 Hz, H-7),
7.19 (dd (t), 1H, J = 7.7 Hz, H-2), 6.96 (d, 1H, J = 8.0 Hz, H-1), 6.91
(dd (t), 1H, J = 7.5 Hz, H-3), 6.28 (dd, 1H, J = 2.5 Hz, J = 6.3 Hz,
J
dx.doi.org/10.1021/jo301456y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
J = 12.8 Hz, H-5), 2.19 (ddd, 1H, J = 4.5 Hz, J = 11.8 Hz, J = 12.8 Hz,
H-5); 13C NMR (CDCl3, 125 MHz) δ/ppm 160.7 (s, C−O1), 138.1
(s, C-10a/6a), 134.0 (s, C-10a/6a), 130.5 (d, C-2), 130.3 (d, C-10),
128.2 (d, C-7), 127.9 (d, C-8), 126.7 (d, C-9), 125.9 (s, C-4a), 124.5
(d, C-4), 120.6 (d, C-3), 110.5 (d, C-1), 85.3 (s, C-4b), 85.2 (d, C-11),
50.7 (q), 32.8 (t, C-6), 26.9 (t, C-5); HRMS (ESI) calculated for
C17H16O2 (+Na+) 275.1043, found 275.1050.
Quantum Yields for the D-Incorporation. Solutions of
naphthalene derivatives 5−8 in CH3CN−D2O (3:1), as well as a solu-
tion of valerophenone in CH3CN−H2O (1:1) were freshly prepared
and their concentrations were adjusted to have absorbances 0.4−0.8
at 254 nm. After adjustment of the concentration and measurement of
the corresponding UV−vis spectra, the solutions were filled in quartz
cuvettes (15 mL), purged with a stream of N2 (20 min each), and
sealed with a septum. The cuvettes were irradiated at the same time
in a reactor equipped with a merry-go-round and 2 lamps with the
output at 254 nm for 0.5, 1, and 2 min. After each irradiation, the
samples were taken from the cuvettes by use of a syringe and analyzed
by ESI MS (the D content was determined from the difference of
the intensity of signals of irradiated and nonirradiated sample),
whereas the conversion of valerophenone was analyzed by HPLC.
Quantum yields for the D-exchange was calculated using valerophe-
none actinometer (formation of acetophenone in aqueous media,
Φ = 0.65 0.03).33
purged with nitrogen or oxygen for 20 min prior to measurements.
Absorbances at 266 nm were ∼0.4.
Computational Methods. The ground state equilibrium geom-
etries of 5−8 have been determined with the second order Møller−
Plesset (MP2) method and the triple-ζ valence plus polarization (TZVP)
basis set.48 Solvation effects were estimated with the polarizable
continuum model (PCM)49 using the dielectric constant of CH3CN as
implemented in the Gaussian09 program package.50 The vertical
excitation energies, optimized geometries, and ESIPT reaction paths
have been determined with the second order approximate coupled
cluster (CC2) method with the cc-pVDZ basis set.51 The resolution of
identity (RI) approximation has been used for the evaluation of the
electron repulsion integrals.52 All coupled cluster calculations and the
natural bond orbital (NBO) analysis53 have been performed with the
TURBOMOLE package.54
ASSOCIATED CONTENT
■
S
* Supporting Information
Copies of the 1H and 13C NMR spectra, UV−vis, fluorescence,
LFP, and computational data. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Fax: + 385 1 4680 195. Tel: +385 1 4561 141. E-mail:
nbasaric@irb.hr.
■
Alternatively, 5−8 (5 mg, 0.023 mmol) were dissolved in CH3CN
(32 mL). To the solution was added D2O (12 mL). The solution was
purged with Ar for 30 min and irradiated in a reactor at 254 (16
lamps) for 1 min. During irradiation, the solution was continuously
purged with Ar and cooled by a coldfinger condenser. After irradiation,
50 mL of H2O was added and extractions with CH2Cl2 (3 × 50 mL)
were carried out. The extracts were dried over anhydrous MgSO4.
Under the same conditions, 2-phenylphenol (6 mg, 0.03 mmol),
dissolved in CH3CN−D2O (3:1, 50 mL), was irradiated for 10 min.
After aqueous workup, extraction and removal of the solvent, the
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
These materials are based on work financed by the Croatia
Foundation for Science, Higher Education and Technological
Development of the Republic of Croatia (HRZZ grant no.
02.05/25), the Ministry of Science Education and Sports of the
Republic of Croatia (grants No. 098-0982933-2911 and 098-
0352851-2921), the Natural Sciences and Engineering Research
Council (NSERC) of Canada and the University of Victoria.
1
extent of the deuterium incorporation was determined by H NMR.
Quantum yield for the D-exchange was calculated using D-exchange in
2-phenylphenol as a secondary actinometer (Φ = 0.041 0.03).21
Steady State and Time-Resolved Fluorescence Measure-
ments. The steady state measurements were performed on a
luminescence spectrometer. The samples were dissolved in CH3CN,
or CH3CN−H2O (1:4) and the concentrations were adjusted to have
absorbances at the excitation wavelength (270, 290, 300, or 310 nm) <
0.1. Solutions were purged with nitrogen for 30 min prior to analysis.
The measurements were performed at 20 °C. Fluorescence quantum
yields were determined by comparison of the integral of the emission
bands with the one of fluorene in methanol (Φ = 0.68) or quinine
sulfate in 0.05 M aqueous H2SO4 (Φ = 0.53).36 The measurements
were performed in triplicate and the mean value was reported.
Typically, three absorption traces were recorded (and averaged) as
well as three fluorescence emission traces, exciting at three different
wavelengths. Three quantum yields were calculated and the mean
value was reported.
Fluorescence decay histograms were obtained on an instrument
equipped with a light emitting diode (excitation wavelength 265 or
310 nm), using time-correlated single photon counting technique in
1023 channels. Histograms of the instrument response functions
(using LUDOX scatterer) and sample decays were recorded until they
typically reached 3 × 103 counts in the peak channel. The half width of
the instrument response function was typically ∼1.5 ns. The time
increment per channel was 0.049 or 0.098 ns. Obtained histograms
were fitted as sums of exponential using Gaussian-weighted nonlinear
least-squares fitting based on Marquardt−Levenberg minimization im-
plemented in the software package of the instrument. The fitting
parameters (decay times and pre-exponential factors) were determined
by minimizing the reduced chi-square χ2. An additional graphical
method was used to judge the quality of the fit that included plots of
surfaces (“carpets”) of the weighted residuals vs channel number.
Laser Flash Photolysis (LFP). All LFP studies were conducted
employing a YAG laser, with a pulse width of 10 ns and excitation
wavelength 266 nm. Static cells (0.7 cm) were used and solutions were
DEDICATION
■
This article is dedicated to the memory of Professor Howard E.
Zimmerman.
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