Excited-state intramolecular proton transfer of naphthalene-fused 2-(2'-Hydroxyaryl)benzazole family

Article abstract of DOI:10.1021/jp904370t

Intramolecularly hydrogen-bonded organic compounds often exhibit fluorescence emission at considerably longer wavelengths than typical fluorescence as a result of excited-state intramolecular proton transfer (ESIPT). The structure-property relationship of such ESIPT molecules, however, remains obscure. The present article reports the excited-state dynamics of a new family of ESIPT molecules, 2-(2'-hydroxynaphthyl)benzazoles 1-3, based on steady-state and time-resolved spectroscopy measurements. In comparison with the parent compound HBO, all three compounds 1-3 exhibited absorption bands at longer wavelengths and emitted fluorescence from the excited keto-tautomer K* at shorter wavelengths, indicating that the introduction of a naphthalene ring increases the energy gap between the ground and excited states for the keto-tautomer despite the expansion of the aromatic ring. Time-resolved fluorescence spectra revealed dual emission for compounds 1 and 3, consisting of two distinct fluorescence bands originating from K* and the excited rotamer E'*, whereas 2 exhibited fluorescence only from, the K* state. In the transient absorption spectra, both the T-T absorption band and the ground slate absorption band of the Z-keto tautomer were observed for 1, whereas only the T-T absorption band was observed for 2 and only the Z-keto tautomer band was observed for 3.

Full text of DOI:10.1021/jp904370t

J. Phys. Chem. A 2010, 114, 1603–1609
1603
Excited-State Intramolecular Proton Transfer of Naphthalene-Fused
2-(2′-Hydroxyaryl)benzazole Family
Toshiyuki Iijima, Atsuya Momotake, Yoshihiro Shinohara,† Tomoo Sato,
Yoshinobu Nishimura, and Tatsuo Arai*
Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
ReceiVed: May 11, 2009; ReVised Manuscript ReceiVed: NoVember 24, 2009
Intramolecularly hydrogen-bonded organic compounds often exhibit fluorescence emission at considerably
longer wavelengths than typical fluorescence as a result of excited-state intramolecular proton transfer (ESIPT).
The structure-property relationship of such ESIPT molecules, however, remains obscure. The present article
reports the excited-state dynamics of a new family of ESIPT molecules, 2-(2′-hydroxynaphthyl)benzazoles
1-3, based on steady-state and time-resolved spectroscopy measurements. In comparison with the parent
compound HBO, all three compounds 1-3 exhibited absorption bands at longer wavelengths and emitted
fluorescence from the excited keto-tautomer K* at shorter wavelengths, indicating that the introduction of a
naphthalene ring increases the energy gap between the ground and excited states for the keto-tautomer despite
the expansion of the aromatic ring. Time-resolved fluorescence spectra revealed dual emission for compounds
1 and 3, consisting of two distinct fluorescence bands originating from K* and the excited rotamer E′*,
whereas 2 exhibited fluorescence only from the K* state. In the transient absorption spectra, both the T-T
absorption band and the ground state absorption band of the Z-keto tautomer were observed for 1, whereas
only the T-T absorption band was observed for 2 and only the Z-keto tautomer band was observed for 3.
Introduction
an unusually small Stokes-shifted fluorescence from the excited-
state keto form K* of a naphthalene-fused analogue of HBO,
Intramolecularly hydrogen-bonded organic chromophores
often exhibit the phenomenon excited-state intramolecular proton
(or hydrogen atom) transfer (ESIPT), whereby fluorescence is
emitted at much longer wavelengths than typically observed.1,2
ESIPT molecules have attracted particular attention from the
viewpoint of developing new functional molecules such as
fluorescent probes,3-6 laser dyes,7-9 and photostabilizers.10-16
The ESIPT process involves a phototautomerization reaction
to yield an excited keto form (K*) in the subpicosecond time
region, with the molecule changing from the original enol form
(E) to the keto form on photoirradiation (Figure 1).17-27 The
excited keto form K* relaxes radiatively to give a large Stokes-
shifted fluorescence or nonradiatively to the ground-state keto
form (K), which then undergoes thermal re-enolization to give
the original E form. The fluorescence emitted from K* is
intriguing in that it is located at much longer wavelengths than
typical fluorescence because of structural differences between
the ground and excited states. Therefore, the fluorescence
spectrum of K* usually has no overlap with the absorption
spectrum attributed to E, indicating that no reabsorption of
fluorescence takes place even at high concentrations of the
chromophore.
2-(2′-hydroxynaphthalenyl)benzoxazole 135,36 (Figure 1b). The
Stokes shift is a measure of the structural change that occurs
from the ground-state stable form to the excited-state stable
form. The identification of such an unusually small Stokes shift
from tautomer fluorescence led us to investigate other members
of the family of compound 1; however, the excited-state
dynamics of other 2-(2′-hydroxyaryl)benzazole compounds such
as 237 and 338 remain obscure. Therefore, we investigated the
photochemical behavior of a family of naphthalene-fused 2-(2′-
hydroxyaryl)benzazoles and compared these compounds with
the parent compound HBO. All three compounds 1,35 2, and 3
exhibited fluorescence from the excited keto-tautomer K* at
shorter wavelengths than that observed for HBO as a result of
the introduction of the naphthalene ring. Dual fluorescence was
observed in 135 and 3 but not in 2. Time-resolved studies
revealed that dual emission from compounds 1 and 3 consists
of two distinct fluorescence bands, originating from K* and the
excited rotamer E′*. Transient absorption spectra indicate that
1 undergoes ESIPT, followed by both intersystem crossing and
photoisomerization, whereas 2 undergoes ESIPT, followed by
intersystem crossing, and 3 undergoes ESIPT, followed by
photoisomerization.
The photochemistry of the parent compound of our study,
2-(2′-hydroxyphenyl)benzazole (HBO) (Figure 1a), a typical
ESIPT molecule, has been widely investigated experimentally
and theoretically28-34 because of its chemical stability, structural
simplicity, and facility for chemical modification. Despite the
unique character and potential practical uses of such compounds,
very little is known about the dependence of their spectral
properties on structural differences. Recently, we have reported
Experimental Section
Compounds 1-3 were prepared and purified according to a
reported procedure.35,37-39
2-(2′-Hydroxynaphthalenyl)benzoxazole (1). 1H NMR (270
MHz, DMSO-d6, δ): 12.05 (1H, s, OH), 8.45 (1H, d, J ) 8.1
Hz), 8.09 (1H, d, J ) 8.1 Hz), 7.89-7.94 (3H, m), 7.63-7.59
(1H, m), 7.41-7.51(3H, m), 7.36 (1H, d, J ) 10.8 Hz). Anal.
Calcd for C17H11NO2: C, 78.15; H, 4.24; N, 5.36. Found: C,
78.05; H, 4.36; N, 5.44
* Corresponding author. E-mail: arai@chem.tsukuba.ac.jp.
† Current address: Research Facility Center for Science and Technology,
University of Tsukuba.
10.1021/jp904370t  2010 American Chemical Society
Published on Web 01/04/2010

1604 J. Phys. Chem. A, Vol. 114, No. 4, 2010
Iijima et al.
Figure 1. (a) Energy diagram of 2-(2′-hydroxyphenyl)benzoxazole and (b) chemical structures of 2-(2′-hydroxynaphthyl)benzazoles 1-3.
1
2-(2′-Hydroxynaphthyl)benzimidazole (2). H NMR (270
MHz, DMSO-d6, δ): 12.2 (2H, br s, OH and NH), 8.23 (1H, d,
J ) 10.8 Hz), 8.00 (1H, d, J ) 8.1 Hz), 7.96 (1H, d, J ) 8.1
Hz), 7.91 (1H, d, J ) 8.1 Hz), 7.69-7.65 (2H, m), 7.52 (1H,
t, J ) 8.1 Hz), 7.42 (1H, d, J ) 5.4 Hz), 7.38 (1H, d, J ) 8.1
Hz), 7.24 (2H, m). Anal. Calcd for C17H12N2O: C, 78.44; H,
4.65; N, 10.78. Found: C, 78.29; H, 4.88; N, 10.76
in ethanol to produce the Schiff base. Subsequent treatment with
an oxidant gave the desired benzazoles 1-3. The appropriate
oxidant was different depending on the compounds.
Steady-State Absorption and Fluorescence Spectra. The
absorption, fluorescence and fluorescence excitation spectra of
1-3 were measured in benzene at room temperature (Figure
2). The absorption peak of 1 appeared at λmax ) 361 nm along
with a shoulder at 378 nm, and dual fluorescence was observed
with maxima at 400 and 460 nm (Figure 2a), which are similar
to previous results in toluene.35 The absorption spectra of 2 and
3 had similar profiles, with the exception of a red-shifted peak
apparent in the spectrum of 3 not seen in the spectrum of 2.
Unlike compound 1, the fluorescence spectrum of 2 exhibited
a single maximum at 468 nm (Figure 2b). Compound 3 also
gave dual fluorescence, with peak maxima at 405 and 500 nm,
similar to 1 (Figure 2c). The excitation spectra of all three
compounds closely corresponded to those of the absorption
spectra. The spectroscopic data including Stokes shift
values and the fluorescence quantum yields are summarized in
Table 1.
1
2-(2′-Hydroxynaphthyl)benzothiazole (3). H NMR (270
MHz, DMSO-d6, δ): 11.0 (1H, s, OH), 8.27 (1H, d, J ) 8.1
Hz), 8.16 (2H t, J ) 8.1 Hz), 8.00 (1H, d, J ) 8.1 Hz), 7.91
(1H, d, J ) 8.1 Hz), 7.61-7.47 (3H, m), 7.41-7.34 (2H, m).
Anal. Calcd for C17H11NOS: C, 73.62; H, 4.00; N, 5.05. Found:
C, 73.49; H, 4.16; N, 5.05
Measurement. 1H NMR spectra were measured with a JEOL
EX-270 (270 MHz for H NMR) or a Bruker ARX-400 (400
MHz for H NMR) spectrometer using DMSO-d6 as a solvent
with tetramethylsilane as an internal standard. UV-absorption
and fluorescence spectra were recorded on a Shimadzu UV-
1600 spectrophotometer and on a Hitachi F-4500 fluorescence
spectrometer, respectively. Fluorescence decay measurements
were performed using time-correlated single-photon counting.
1
1
In general, extension of the aromatic ring structure results in
a red shift in both the absorption and fluorescence bands.
Therefore, the absorption and fluorescence bands of 1-3 are
expected to appear at longer wavelengths than those of HBO.
As expected, the absorption maxima were observed at longer
wavelengths for 1-3 as compared with HBO (335 nm).
Results and Discussion
Synthesis. Three derivatives of benzazole 1-3 were prepared
first with a condensation procedure, followed by stirring of
equimolar quantities of 2-hydroxy-1-naphthaldehyde and aniline

ESIPT of Naphthalene-Fused HBO Family
J. Phys. Chem. A, Vol. 114, No. 4, 2010 1605
hydrogen-bonding ability of the solvent increases.41 In the case
of HBO, the E form and its rotamer E′ are in equilibrium in
polar or protic solvents (Scheme 1).42-44 The energy of each
rotamer E and E′ varies with the solvent polarity because of
their changing dipole moments. Therefore, the relative energy
and the resultant relative population of the two rotamers E and
E′ change with solvent polarity.42 This tendency should also
be expected in the naphthyl derivatives 1-3. However, com-
pounds 1 and 3 showed dual fluorescence, even in nonpolar
solvents (in addition to polar solvents, data not shown). This
observation can be explained by the difference in relative energy
of E and E′, which is probably smaller for these compounds
than for HBO. Therefore, the relative population of rotamer E′
for 1 and 3 is measurably larger than that of HBO and,
consequently, the fluorescence of rotamer E′ can be observed
even in benzene. Compound 2 exhibited a single peak in
benzene and dual emission in polar solvents (data not shown),
which is likely to be the case for 2-(2′-hydroxyphenyl)imidazole
(HBI), the parent compound of 2. HBI shows fluorescence with
a single peak at 460 nm in dioxane42 and dual emission peaks
at around 360 and 460 nm in polar protic solvents.45 This
indicates that the relative energies of the E and E′ rotamers of
2 are not very different from those of HBI. In addition, the
fluorescence quantum yield of 2 is considerably higher than the
values of 1 and 3. The reason for this difference is still unclear
but is consistent with the fluorescence quantum yield values of
HBI in various solvents being more than one order of magnitude
greater than those of HBO.42 Abou-Zied and Romesberg et al.
attributed the shorter wavelength component of dual fluorescence
in a polar solvent to the solvated, intermolecularly hydrogen-
bonded rotamer E′.46,47 However, in our case, intermolecular
hydrogen bonding with benzene was not possible, suggesting
that dual fluorescence is not due to solvated species. As in the
case of HBO, compounds 1 and 3 can adopt intramolecularly
hydrogen-bonded rotamers E and E′, and only E* undergoes
ESIPT reaction to form K*. Rotamer E′ does not undergo
ESIPT, and E′* can yield normal emission at shorter wavelengths.
Time-Resolved Fluorescence Spectra. The time-resolved
fluorescence spectra of 1 and 3 suggest the existence of two
distinct fluorescent species, E′* and K*. Figure 3 shows the
time-resolved fluorescence spectra of 1 (Figure 3a), 2 (Figure
3b), and 3 (Figure 3c) in benzene solution. Each spectrum was
normalized with respect to peak intensity and given a time range
determined by the peak channel position of the instrument
response function. In Figure 4a, after the major emission band
at ∼460 nm attributed to K* had decreased, the small emission
band at ∼400 nm remained, albeit at a relatively low intensity,
indicating that the fluorescence lifetime of the 400 nm peak is
longer than that of the 460 nm peak. The Stokes shift for this
band was small (1460 cm-1), and it can therefore be attributed
to the excited enol form because a small Stokes shift suggests
a similar electronic structure between the ground state and the
excited singlet state. However, the lifetime of the excited enol
form is generally in the subpicosecond region because of the
very fast ESIPT reaction. Therefore, this emission species is
not likely to be the “normal” excited enol form E*, but its
rotamer E′*, as shown in Figure 4a, which is produced by
excitation of the ground-state rotamer E′. The ratio of rotamer
Figure 2. UV-absorption (solid line), fluorescence (thin line), and
fluorescence excitation (dashed line) spectra for (a)1, (b) 2, and (c) 3
in benzene at room temperature. Monitoring wavelengths: 480, 490,
and 500 nm for 1, 2, and 3, respectively.
However, the major emission bands resulting from the excited
keto form (K*) of compounds 1 (460 nm), 2 (468 nm), and 3
(500 nm) were located at shorter wavelengths than that of HBO
(508 nm). Although the observation of a smaller Stokes-shifted
fluorescence from K* of 1 as compared with HBO has already
been discussed,35 it is noteworthy that the other naphthalene-
based azoles 2 and 3 also showed small Stokes-shifted fluo-
rescence from the K* form. Therefore, in this case, introduction
of the naphthalene ring to the 2-(2′-hydroxyaryl)benzazole
structure increases the energy gap between the ground and
excited states in the keto-form; however, the mechanism of these
effects still remains unknown.
Whereas all three compounds 1-3 exhibited small Stokes-
shifted fluorescence from the K* species, other fluorescence
properties such as fluorescence lifetime were still dependent on
the individual compounds. For example, compounds 1 and 3
showed dual emission with low fluorescence quantum yields
(Φf ) 10-3 for 1 and ) 10-4 for 3; Table 1), but compound 2
exhibited a single peak with efficient fluorescence (Φf ) 10-1).
The parent compound HBO is known to exhibit a single
fluorescence peak in nonpolar solvents such as cyclohexane25
and toluene35 but exhibits dual emission, at around 360 and 500
nm, in polar solvents such as acetonitrile, alcohol,25 DMSO,
and water.40 This phenomenon is generally accepted to be due
to the E form existing as a single form in nonpolar or low-
polar solvents; the stability of the E form decreases as the
1
E′ to E in its ground state is too small to detect by H NMR
spectroscopy, and it mostly exists as the “original” enol form
E. E′* allows a longer fluorescence lifetime than that of E*
because, unlike E*, E′* does not undergo ESIPT reaction.
Consequently, the emission efficiency from E′* is probably
much higher than that of K*, resulting in the dual fluorescence

1606 J. Phys. Chem. A, Vol. 114, No. 4, 2010
Iijima et al.
TABLE 1: Absorption and Fluorescence Maxima, Stokes Shifts, Fluorescence Quantum Yields, Fluorescence Lifetimes, Excited
Triplet State Lifetimes, and Lifetimes for the Z-Keto Tautomer for Compounds 1-3
Abs λmax/nm (λmax/cm-1
)
Fl. λ max/nm (λmax/cm-1
)
Stokes shift/cm-1
Φf
τf/ns (Ar)
τt/µs
τ
Z-keto/µs
1
361, 378
(26 500)
400, 460
(21 700)
4800
2.1 × 10-3
0.02 (0.998)a
1.3 (0.002)a
0.03 (0.999)b
1.6 (0.001)b
0.96 (1.000)b
1.4
0.3
2
3
356
(28 000)
373
468
6600
6800
1.1 × 10-1
2.2 × 10-4
1.3
(21 400)
405, 500
(20 000)
,0.02 (0.999)a
1.4 (0.001)a
,30
88
(26 800)
,0.02 (0.996)c
1.6 (0.004)c
HBO
335
(29 900)
508
(19 700)
10 200
2.0 × 10-2
a 410 nm. b 470 nm. c 500 nm. See the Supporting Information (Figures S1-S5).
SCHEME 1: Original Enol Form E and Its Rotamer E′
of 1-3
assigned to the ground-state Z-keto form, produced by E-Z
photoisomerization from K* (Figure 4a). The band at ∼550 nm
was quenched by oxygen (see also Figure S6 of the Supporting
Information), and thus it can be assigned to the excited triplet
state.
The transient spectra of 2 shown in Figure 5b have absorption
maxima at 400 and 550 nm. Both peaks were quenched by
oxygen, indicating that both peaks belong to the same electronic
state and that the entire spectrum can be attributed to the excited
triplet state. In this case, no transient spectral features originating
from the ground-state Z-keto form were observed.
The transient spectrum of 3 is different from the spectra of
1 or 2 in that it shows a single band at ∼440 nm. Brewer et al.
have reported that the transient absorption bands of 2-(2′-
hydroxyphenyl)benzothiazole (HBT) in aprotic solvents can be
attributed to the ground state Z-keto form and a twisted
conformer, because the change in absorbance (∆O.D.) varies
in the presence of oxygen.49 Similarly, the transient band of 3
is also not quenched by oxygen, indicating the ground-state
Z-keto form (Figure 5c), and does not display any peaks
attributable to the excited triplet state. The lifetimes of the
ground state Z-keto forms of 1 and 3, as shown in Table 1,
were obtained from Figures S6 and S8 of the Supporting
Information, respectively. Interestingly, the bands for the Z-keto
forms of parent compounds HBO (420 nm)31,48 and HBT49 (450
nm) appear at longer wavelengths than those for 1 and 3,
suggesting that the energy gap between the ground and excited
state for the Z-keto-tautomer is larger in 1 and 3 than the energy
gaps of HBO and HBT, despite the expansion of the aromatic
ring.
observed in compound 1. Figure 3b shows the time-resolved
fluorescence spectra of 2. For this compound, only a single peak
was observed, which is consistent with the steady-state fluo-
rescence spectra in Figure 2b. The fact that compound 2 did
not exhibit dual emission suggests that the rotational isomer of
2 is negligible in the ground state in benzene (Figure 4b). The
fluorescence lifetime of 2 is as high as 1 ns, which is much
larger than the lifetimes of 1 and 3 and is consistent with the
high fluorescence quantum yield (Φf ) 0.11) observed for 2 in
comparison with 1 and 3. Figure 3c shows the time-resolved
fluorescence spectra of 3. Raman scattering originating from
the solvent appears at 425 nm in the initial time region. Similar
to the result of compound 1, a long-lifetime emission band at
∼430 nm is observed after the major emission band at ×500
nm had decreased. In the case of 3, the rotamer E′ may also
exist in the ground state (Figure 4c); therefore, fluorescence from
E′* can be observed.
Transient Absorption Spectra. Figure 5 shows characteristic
transient absorption spectra for compounds 1-3. Compound 1
exhibits two transient bands, at around 400 and 550 nm (Figure
5a), which can be assigned to two different species. The
assignment of transient spectra of HBO has already been done
by Stephan and Grellmann.48 They have found that the absor-
bance decay of HBO is composed of two components, one of
which is proton sensitive, peaking at 420 nm, and the other of
which is oxygen sensitive, showing a broad spectra at around
350-600 nm. The former is assigned to the ground-state Z-keto
form, and the latter is assigned to the excited triplet species. In
the case of compound 1, the lifetime of the band at ∼400 nm
was not affected by oxygen, indicating that this band can be
According to the above results, the following scenario is
proposed for the photochemistry of compounds 1-3 in benzene
and possibly other organic solvents (Figure 4). That is,
compound 1 is considered to exist either as an enol form E or
its rotamer E′ in the ground state. On excitation, two distinct
excited species, E* and E′*, are formed, and E* immediately
undergoes ESIPT reaction to give the excited tautomer K*.
Fluorescence is then emitted from E′* and K*, at around 400
and 460 nm, respectively. The excited species K* also undergoes
intersystem crossing to the excited triplet state and photochemi-
cal E-Z isomerization to give the ground-state Z-keto isomer,
of which two transient species can be observed in the transient
absorption spectra. The Z-keto isomer is thermally unstable and
reverts to the original enol form (Figure 4a). Compound 2 does
not adopt a rotamer in the ground state. As such, the observed
fluorescence is only due to K*, which does not undergo

ESIPT of Naphthalene-Fused HBO Family
J. Phys. Chem. A, Vol. 114, No. 4, 2010 1607
Figure 3. Time-resolved fluorescence spectra normalized to peak intensity of (a) 1, (b) 2, and (c) 3 in benzene at room temperature.
Figure 4. Energy diagrams for (a) 1, (b) 2, and (c) 3 in benzene at room temperature.
photoisomerization but instead undergoes intersystem crossing
to produce the excited triplet state. The ground-state keto-
tautomer K immediately returns to the original enol form.
Finally, compound 3 adopts a rotamer in its ground state;
therefore, dual fluorescence is again observed from E′* and K*.
E-Z photoisomerization of K* gives the Z-keto tautomer in the
ground state, whereas the excited triplet state is not produced
in this case.

1608 J. Phys. Chem. A, Vol. 114, No. 4, 2010
Iijima et al.
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In conclusion, the photochemical behavior of naphthalene-
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steady-state and time-resolved spectroscopy. In comparison with
the parent compound HBO, all three compounds showed
absorption bands at longer wavelengths but emitted fluorescence
from the excited keto-tautomer K* at shorter wavelengths,
indicating that the introduction of the naphthalene ring increases
the energy gap between the ground and excited states for the
keto-tautomer despite the expansion of the aromatic ring. Time-
resolved fluorescence spectra revealed dual emission for com-
pounds 1 and 3, consisting of two distinct fluorescence bands
originating from K* and the excited rotamer E′*, whereas
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Acknowledgment. This work was supported by a Grant-in-
Aid for Scientific Research in a Priority Area “New Frontiers
in Photochromism (no. 471) and (no. 19550176) from the
Ministry of Education, Culture, Sports, Science, and Technology
(MEXT), Japan.
Supporting Information Available: This material is avail-
able free of charge via the Internet at http://pubs.acs.org.

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