ESIPT inspired fluorescent 2-(4-benzo[d]oxazol-2-yl)naphtho[1,2-d]oxazol-2- yl)phenol: Experimental and DFT based approach to photophysical properties

Article abstract of DOI:10.1016/j.tet.2012.11.095

ESIPT inspired fluorescent 2-(4-benzo[d]oxazol-2-yl)naphtho[1,2-d]oxazol-2- yl)phenol was synthesized from 1-amino-3-(1,3-benzoxazol-2-yl)naphthalen-2-ol. Photophysical behavior of the synthesized compound was studied using UV-visible and fluorescence spectroscopy in polar and non-polar solvents. The synthesized naphthoxazolyl benzoxazole is fluorescent and very sensitive to the micro-environment. It shows a single absorption and dual emission in non-polar solvents with large Stokes shift originating from Excited State Intramolecular Proton Transfer while in polar solvents only a single short wavelength emission is observed. Experimental absorption and emission wavelengths are in good agreement with those predicted using the Time-Dependent Density Functional Theory (TD-DFT) [B3LYP/6-31G(d)]. The largest wavelength difference between the experimental and computed absorption maxima was 16 nm (acetonitrile) and 7 nm (ethyl acetate, THF, and 1,4-dioxane) in the short and long wavelength regions, respectively. A largest difference of 25 nm was observed for the short wavelength emission in DMF and 22 nm for the longer wavelength emission in chloroform.

Full text of DOI:10.1016/j.tet.2012.11.095

Tetrahedron 69 (2013) 1767e1777
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
ESIPT inspired fluorescent 2-(4-benzo[d]oxazol-2-yl)naphtho[1,2-d]
oxazol-2-yl)phenol: experimental and DFT based approach to
photophysical properties
,
,
b
,
y
a,
,
,
,
,
y
Kiran R. Phatangare a y, Vinod D. Gupta a z, Abhinav B. Tathe a y, Vikas S. Padalkar a y, Vikas S. Patil a
,
Ponnadurai Ramasami b , Nagaiyan Sekar
a Tinctorial Chemistry Group, Department of Dyestuff Technology, Institute of Chemical Technology, N.P. Marg, Matunga, Mumbai 400 019, Maharashtra, India
,
*
*
b
ꢀ
Computational Chemistry Group, Department of Chemistry, University of Mauritius, Reduit, Mauritius
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2012
Received in revised form 1 November 2012
Accepted 27 November 2012
Available online 6 December 2012
ESIPT inspired fluorescent 2-(4-benzo[d]oxazol-2-yl)naphtho[1,2-d]oxazol-2-yl)phenol was synthesized
from 1-amino-3-(1,3-benzoxazol-2-yl)naphthalen-2-ol. Photophysical behavior of the synthesized
compound was studied using UVevisible and fluorescence spectroscopy in polar and non-polar solvents.
The synthesized naphthoxazolyl benzoxazole is fluorescent and very sensitive to the micro-environment.
It shows a single absorption and dual emission in non-polar solvents with large Stokes shift originating
from Excited State Intramolecular Proton Transfer while in polar solvents only a single short wavelength
emission is observed. Experimental absorption and emission wavelengths are in good agreement with
those predicted using the Time-Dependent Density Functional Theory (TD-DFT) [B3LYP/6-31G(d)]. The
largest wavelength difference between the experimental and computed absorption maxima was 16 nm
(acetonitrile) and 7 nm (ethyl acetate, THF, and 1,4-dioxane) in the short and long wavelength regions,
respectively. A largest difference of 25 nm was observed for the short wavelength emission in DMF and
22 nm for the longer wavelength emission in chloroform.
Keywords:
Benzoxazole
Naphthoxazole
ESIPT
Fluorescence
Solvatochromism
TD-DFT
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
the molecule.6 Occurrence of this second red shifted emission makes
such molecules interesting, and they find a wide range of applica-
Phototautomerization from Excited State Intramolecular Proton
Transfer (ESIPT) is of prime importance in biological and chemical
systems1 and as a result, there have been experimental and theo-
retical interests.2 Photoexcitation leads to a shift in electron density
that facilitates proton migration from a donor atom, usually oxygen,
to a nearby acceptor atom, either oxygen or nitrogen, culminating in
ESIPT. In molecules with an oxygen donor, the ground state is the enol
tions as photochromic dyes,7 laser dyes,8 fluorescent probes,9
sensors,10 organic electroluminescence optical materials,11 and
photostabilizers12 as well as in fluorescence recording techniques.13
The design, synthesis, and photophysical characterization of
ESIPT organic molecules are of contemporary interest.14 ESIPT is
largelycharacterized bythe energy level changes of E
*
/K , and only
*
small energy barrier or even barrierless is allowed. This could be
justification for the fact the most ESIPT molecules are of small sizes.
Molecules possessing acidic and/or basic groups are known to ex-
perience an enhancement of acidity and/or basicity upon excitation.
Specifically, molecules having an acidic group and basic site hydro-
gen bonded in the ground state can undergo an ultrafast ESIPT, from
the acidic to the basic site.15 Amongst the large range of systems
undergoing ESIPT, compounds such as benzoxazoles and naph-
thoxazoles form an important class.16 There are reports describing
the dependence of fluorescence on solvent polarity17,20 as well as
computations regarding the geometry of the different conformers.18
Therefore, taking into consideration the applications and
importance of ESIPT inspired molecules and in continuation of
our research work on fluorescent materials,19,20 herein, we report the
synthesis of novel ESIPT inspired fluorophore, 2-((4-benzo[d]oxazol-
form (E), and the emissive excited state is the keto form (K ). The enol
*
(E)eketo (K) phototautomerism takes place in many instances via
five-,3 six-,4 or rarely seven-member5 quasi transition state. There-
fore, a dual fluorescence band could be observed as the absorption is
from E/E
*
(E
*
is excited state enol form) and emission is from K /K.
*
A large apparent Stokes red shift was observed in fluorescence
due to the difference in energy between absorption by the original
(enol) tautomer and emission from the resultant (keto) tautomer of
* Corresponding authors. Tel.: þ230 403 7507; fax: þ230 465 6928 (P.R.); tel.:
þ91 22 33611111/2707; fax: þ91 22 33611020 (N.S.); e-mail addresses: ramchemi@
intnet.mu (P. Ramasami), n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com (N.
Sekar).
y
Tel.: þ91 22 3361 1111/2707; fax: þ91 22 3361 1020.
z
2-yl)naphtho[1,2-d]oxazol-2-yl)phenol
8 and its photophysical
Tel.: þ230 403 7507; fax: þ230 465 6928.
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.11.095

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K.R. Phatangare et al. / Tetrahedron 69 (2013) 1767e1777
properties andTime-DependentDensity FunctionalTheory (TD-DFT)
explorations.
the solvatochromism and solvatofluorism behaviors of the mole-
cule were studied by measuring electronic absorption and emission
spectra in solvents of different polarities.
2. Methodology
2.2. Computational methods
2.1. Synthesis strategy
The different conformers of compound 8 and their tautomers
involved are illustrated in Fig. 2 The ground state (So) geometry of
the conformers and tautomers of compound 8 in their Cs symmetry
were optimized using the tight criteria in the gas phase using Den-
sity Functional Theory (DFT).23 The functional used was B3LYP. The
B3LYP method combines Becke’s three parameter exchange func-
tional (B3)24 with the nonlocal correlation functional by Lee, Yang,
and Parr (LYP).25 The basis set used for all atoms was 6-31G(d), the
latter has been justified in the literature14b,26a,b for the current in-
vestigation. The vibrational frequencies at the optimized structures
were computed using the same method to verify that the optimized
structures correspond to local minima on the energy surface.26c
The vertical excitation energies at the ground state equilibrium
geometries were calculated with TD-DFT.27 The low-lying first
singlet excited state (S1) of each conformer was relaxed using the
TD-DFT to obtain its minimum energy geometry. The difference
between the energies of the optimized geometries at the first sin-
glet excited state and the ground state was used in computing the
emissions.28 Frequency computations were also carried out on the
optimized geometry of the low-lying vibronically relaxed first ex-
cited state of conformers. All the computations in solvents of dif-
ferent polarities were carried out using the Polarizable Continuum
Model (PCM).29 All electronic structure computations were carried
out using the Gaussian 09 program.30
The synthetic scheme for the preparation of 2-((4-benzo[d]
oxazol-2-yl)naphtho[1,2-d]oxazol-2-yl)phenol
8
is shown in
Scheme 1. 3-(1,3-Benzoxazol-2-yl)naphthalen-2-ol 3 was prepared
by the reported procedure21 from 3-hydroxynaphthalene-2-
carboxylic acid 1 and 2-aminophenol 2 in the presence of PCl3 in
chlorobenzene at 133e135 ꢀC. Sulfanilic acid was diazotized to get
4-sulfobenzenediazonium chloride 5, which was further coupled
with 3 to obtain azo compound 6. This azo compound 6 was re-
duced by using sodium dithionate at pH 8e922 to get 1-amino-3-
(1,3-benzoxazol-2-yl)naphthalene-2-ol 7, which was confirmed by
FTIR, 1H NMR, mass spectral analysis and its Mþ1 peak was found to
be at 277.1. Compound 7, on treatment with salicylic acid in the
presence of phosphorus trichloride in chlorobenzene at reflux
temperature (133e135 ꢀC)21 for 15e18 h gave 2-(4-benzo[d]oxazol-
2-yl)naphtho[1,2-d]oxazol-2-yl)phenol 8. The synthesized com-
pound 8 was purified by column chromatography and was con-
firmed by FTIR, 1H NMR, and mass spectral and elemental analyses.
Compound 8 showed a distinct stretching vibrational peak for eOH
in FTIR at 3352e3367 cmꢁ1 and 1H NMR showed the phenolic eOH
signal at
eOH peak disappeared in D2O exchange with an additional peak
appearing at 4.80e4.82 ppm. Mass spectral data for compound 8
d 10.56 ppm and confirmed by D2O exchange. The phenolic
d
show Mþ1 peak at 379.1, which is in good agreement with its
molecular weight of Mþ1 species. Absorption and emission spectra
were measured to investigate its photophysical properties and also
2.3. Relative quantum yield calculations
The quantum yields of compound 8 in different solvents were
calculated by using tinopal and fluorescein as reference standards
in different solvents using the comparative method.31 Absorption
and emission characteristics of the standards and for compound 8
in polar as well as non-polar solvents were measured at different
concentrations at (2, 4, 6, 8, and 10 ppm level). Emission intensity
values were plotted against absorbance values and linear plots
were obtained. Gradients were calculated for compound 8 in each
solvent and for the standards. All the measurements were done by
keeping the parameters constant such as solvent and slit width.
Relative quantum yields of synthesized compound 8 in different
solvents were calculated by using Eq. 1.31
!
ꢀ
ꢁ
hX2
h2St
GradX
GradSt
FX
¼
FSt
(1)
where: FX: quantum yield of synthesized compound; FSt: quantum
yield of standard sample; GradX: gradient of synthesized com-
pound; GradSt: gradient of standard sample; hX: refractive index of
solvent used for synthesized compound; hSt: refractive index of
solvent used for standard sample.
Since the molecule has two emissions, the fluorescence quantum
efficiency at each emission was calculated using a standard having
similar emission at the respective wavelength (for short wavelength
emission tinopal (
emission fluorescein (
F
¼0.81 in DMF32) and for long wavelength
F
¼0.91 in methanol33)) was used.
3. Results and discussion
3.1. ESIPT phenomenon
Scheme 1. Synthesis of 2-(4-benzo[d]oxazol-2-yl)naphtho[1,2-d]oxazol-2-yl)phenol.
Reagents and conditions: (a) PCl3, chlorobenzene, reflux (133e135 ꢀC), 4 h, 79%; (b)
NaNO2, HCl, 0e5 ꢀC, 45 min, 98%; (c) methanol/water (80:20), pH 8e9, 0e5 ꢀC, 3 h,
90%; (d) Na2S2O4, NaOH, H2O, 90 ꢀC, 1.5 h, 60%; (e) PCl3, chlorobenzene, reflux
(133e135 ꢀC), 15e16 h, 58%.
The synthesized compound 8 contains an acidic eOH group at
the 2-position with respect to naphtho[1,2-d]oxazole ring. The

K.R. Phatangare et al. / Tetrahedron 69 (2013) 1767e1777
1769
location of acidic eOH group and ]Ne groups is such that there is
an existence of the intramolecular hydrogen bonding in the ground
state. On excitation, the ]Ne moiety of phenylnaphtho[1,2-d][1,3]
oxazole ring becomes strongly basic and eOH becomes strongly
acidic as shown by the changes in the Mulliken charge distribution
in ground and excited states (Supplementary data, Table S4). This
and the K4-Enol, where the probability of proton transfer happens
to be very remote. Compound 8 can coexist in various conformers
with their tautomers in solution. DFT computations showed that in
all solvents, the keto form is less stable than the enol form, which is
preferred in the ground state (Supplementary data, Table S1 and
Fig. S1). Among the four enol forms, K1-Enol is the most stable
one. The K1-Enol and K2-Enol forms are more stable in polar sol-
vents than in non-polar solvents due to solvation. All the keto forms
are prohibitively higher in energy such that they do not exist. The
absence of a carbonyl stretching frequency in FTIR (Supplementary
data, Fig. S19) indicates that keto is not the preferred configuration
in the ground state. The absence of long wavelength absorption is
also indicative of the non-existence of the keto form as the ab-
sorptive species.
leads to the ESIPT, i.e., excited state enol form (K1-Enol
the excited state keto form (K1-Keto ) (Fig. 1). Absorption of radi-
ation leads to the excitation of the ground state enol (K1-Enol) and
it goes to the excited enol (K1-Enol ). This K1-Enol form either
undergoes ESIPT to form the excited state keto form (K1-Keto ) or
emits a radiation and returns to the ground state enol form (K1-
Enol). The excited state keto form (K1-Keto ) emits radiation and
*
) converts to
*
*
*
*
*
comes to the ground state keto form (K1-Keto). In this way the
molecule shows a single absorption around 325 nm (lmax) and dual
emission at 411 and 517 nm.
3.3. Absorption and solvatochromism study
Compound 8, which exists exclusively as the K1-Enol form
showed a prominent and intense absorption in all solvents of dif-
ferent polarities, accompanied by two more weak shoulder-like less
intense absorptions. The experimental values of absorption max-
ima together with the molar absorptivity and the vertical excitation
values (computed absorption) obtained from the TD-DFT compu-
tation are summarized in Table 1. It has been observed that there is
close agreement between the experimental lmax and computed
vertical excitations.
The largest difference in wavelength between the computed and
experimental absorption maxima was 16 nm in acetonitrile for
shorter wavelength region. In the case of the long wavelength re-
gion it was 10 nm in THF as well as in 1,4-dioxane. In all solvents,
the prominent intense absorption can be assigned to HOMOꢁ1 to
LUMO transition and the other less intense absorption arises from
HOMOeLUMO transition. The frontier molecular orbital (FMO) 98
is HOMO and FMO 99 is LUMO in all cases (Supplementary data,
Table S5). It is also observed that the appended benzoxazolyl ring
acts as an acceptor; HOMOeLUMO transition is accompanied by the
transfer of electrons toward the pending benzoxazolyl ring. It is
observed that the electron density is distributed on the naph-
thoxazole and 2-hydroxy phenyl ring in HOMO. On excitation to the
LUMO with a contribution of 67%, the electron density is redis-
tributed equally on all the three ring systems (naphthoxazole,
benzoxazole, and 2-hydroxy phenyl ring), which all are in one
plane. As an example, in the case of non-polar solvent like chlo-
roform HOMO/LUMO (67%) transition is responsible for vertical
excitation located at 378 nm with oscillator strength (f) 0.3978
(Fig. 6a), which corresponds to the experimentally observed ab-
sorption (shoulder peak) at 373 nm. The HOMOꢁ1 to LUMO (65%)
(Supplementary data, Table S5) transition is responsible for the
vertical excitation at 336 nm with oscillator strength (f) 0.6865,
which corresponds to the experimentally observed prominent ab-
sorption peak at 325 nm. For polar solvents like DMF, HOMO-
/LUMO (67%) transition is responsible for vertical excitation
located at 377 nm with oscillator strength (f) 0.4066 (Fig. 6b), which
corresponds to the experimentally observed absorption (shoulder
peak) at 379 nm. The HOMOꢁ1 to LUMO (65%) (Supplementary
data, Table S5) transition is responsible for the vertical excitation
at 336 nm with oscillator strength (f) 0.6635, which corresponds to
the experimentally observed prominent absorption peak at 325 nm
(Table 1 and Fig. 3).
Fig. 1. Excited State Intramolecular Proton Transfer (ESIPT) pathway.
3.2. Rotamers and conformers and their ground state energy
Compound 8 has a fused oxazole ring (at 1- and 2-position of the
naphthalene ring), which is rigid and a free benzoxazole ring at the
3-position of the naphthalene ring that can rotate. The
naphthalene-fused oxazole ring has a free phenyl group having the
ortho hydroxyl group participating in the proton hopping at the
excited state.34 In the ground state the molecule can exist in
equilibrium between different conformers arising from rotamerism
and their tautomers (Fig. 2). The normal planar syn form (denoted
as K1-Enol) allows an intramolecular hydrogen bond between the
acidic eOH group and the basic nitrogen atom of the naphthox-
azole ring. The K1-Enol conformer can undergo proton transfer to
form its tautomer (denoted as K1-Keto). The conformer K1-Enol
can undergo rotamerization around the appended benzoxazolyl
group generating the K2-Enol conformer, which on proton transfer
leads to the K2-Keto form. In addition to these two enol forms
(K1-Enol and K2-Enol), the two other possibilities are the K3-Enol,
A similar trend is observed in all other solvents (Supplementary
data, Figs. S7eS15). This confirms that the experimentally observed
short wavelength absorption in the range 320e325 nm is due to the
HOMOꢁ1 to LUMO transition and the shoulder peak observed at
long wavelength in the range 370e375 nm is due to the HOMO to
LUMO transition (Supplementary data, Table S5). The sol-
vatochromism study reveals that the absorption wavelength is

1770
K.R. Phatangare et al. / Tetrahedron 69 (2013) 1767e1777
Fig. 2. Conformers and tautomers of compound 8.
Table 1
Observed UVevisible absorption and computed absorption of compound 8 in different solventsa
Solvent
Experimentalb
TD-DFTc
lEmxapxtðnmÞ
Molar absorptivity
Vertical excitation
nm
f
Assignmentf
(a) L molꢁ1 cmꢁ1
eV
MeOH
EtOH
ACN
322d
370e
322d
370e
319d
370e
325d
379e
325d
376e
325d
373e
322d
370e
322d
370e
325d
370e
12,814
5746
11,680
5557
11,831
5746
18,673
9374
21,206
10,622
11,264
5746
11,794
6010
11,642
6199
17,955
9526
335
376
336
376
335
376
336
377
336
377
336
378
336
377
336
377
336
377
3.69
3.29
3.69
3.29
3.69
3.29
3.68
3.28
3.68
3.28
3.68
3.27
3.69
3.28
3.68
3.28
3.68
3.26
0.6279
0.3825
0.6412
0.3897
0.6334
0.3865
0.6635
0.4066
0.6710
0.3993
0.6865
0.3978
0.6582
0.3842
0.6667
0.3936
0.6904
0.3806
HOMOꢁ1/LUMO (65%)
HOMO/LUMO (67%)
HOMOꢁ1/LUMO (65%)
HOMO/LUMO (67%)
HOMO-1/LUMO (65%)
HOMO/LUMO (67%)
HOMOꢁ1/LUMO (65%)
HOMO/LUMO (67%)
HOMOꢁ1/LUMO (65%)
HOMO/LUMO (67%)
HOMOꢁ1/LUMO (65%)
HOMO/LUMO (67%)
HOMOꢁ1/LUMO (65%)
HOMO/LUMO (67%)
HOMOꢁ1/LUMO (65%)
HOMO/LUMO (67%)
HOMOꢁ1/LUMO (65%)
HOMO/LUMO (67%)
DMF
DCM
CHCl3
EtOAc
THF
Dioxane
a
Analyses were carried out at room temperature (25 ꢀC).
b
c
d
e
f
Experimentally observed lmax
TD-DFT computations were carried out with the use of optimized structures at B3LYP method with 6-31G(d) basis set.
lmax
.
.
Shoulder peak.
Only major contributions are presented; f¼oscillator strength.
insensitive to solvent polarity. However, the absorption intensity is
very sensitive to the solvent polarity. The intensity (both computed
and observed) of short wavelength absorption is always nearly
twice the intensity of long wavelength absorption. The decreasing
order of absorption intensity in the short wavelength region is
DCM>DMF>1,4-dioxane>MeOH>ACN>EtOAc>EtOH>THF>CHCl3
and in case of long wavelength the order is DMF>1,4-dioxane,
THF>EtOAc>CHCl3¼ACN¼MeOH¼EtOH (Table 1, Fig. 3) The exci-
tation spectra mentioned in Fig. 3b were obtained by using shorter
(398e436 nm) and longer wavelength (510e544 nm) emissions of
the respective solvents.
solvents (methanol, ethanol, acetonitrile, and N,N-dimethylforma-
mide) a single emission was observed. It is well known that the long
wavelength emission in ESIPT molecules is attributed to the excited
state keto tautomer originating from the excited state enol tauto-
mer.14a The driving force behind the proton transfer is the associ-
ated intrinsic extra stabilization of the keto tautomer at the excited
state. This aspect is confirmed by the TD-DFT computations that the
excited state keto form has lower energy in non-polar solvents
(Table 2) and this reveals that the stabilization is solely intrinsic and
not due to the polar environment. The geometry consideration
(Section 3.2) favors the keto form in the excited state. TD-DFT
computations in the gas phase also reveal that the excited state
keto tautomer is more stable (by 9.84 kcal molꢁ1) and this again
confirms that the stability of the keto tautomer in the excited state
is intrinsic in nature. The energy considerations are also in favor of
a single emission in polar environment. The TD-DFT calculation
reveals that there is hardly any difference between the energy of
3.4. Emission and solvatofluorism study
The solvatofluorism study reveals that compound 8 showed
dual emission in non-polar solvents (dichloromethane, chloroform,
ethyl acetate, THF, and 1,4-dioxane) while in case of the polar

K.R. Phatangare et al. / Tetrahedron 69 (2013) 1767e1777
1771
Fig. 3. (a) Absorption spectra of compound 8 in different solvents. (b) Excitation spectra of compound 8 in different solvents.
Table 2
Excited state energy difference between K1-Enol
*
and K1-Keto
*
in kcal molꢁ1a
Solvent
EEꢂ nol ꢁ EKꢂ etoðkcal molꢁ1
MeOH
1.21
EtOH
1.34
ACN
1.18
DMF
1.16
DCM
2.29
CHCl3
3.53
EtOAc
2.99
THF
2.58
Dioxane
6.03
Þ
a
Excited state energy were computed using TD-DFT with B3LYP method and 6-31G(d) basis set.
optimized enol and keto tautomers at the excited state. The dif-
ference between the energies of the optimized geometries at the
first singlet excited state and the ground state gives emissions for
all the solvents (Supplementary data, Tables S6 and S7).
for shorter emission but longer emission considerably decreases as
solvent polarity increases (Supplementary data, Fig. S5).
The cyclic four level photophysical process involving excited
state proton transfer (K1-Enol/K1-Enol
*
/K1-Keto /K1-Keto) is
*
Quantum yield is sensitive to the solvent polarity
(Supplementary data, Fig. S4). It is observed that the quantum yield
is higher in non-polar solvents than in polar solvents and this can
be attributed to the contribution coming from the long wavelength
emission. The quantum efficiency was much higher in DMF
exemplified in Fig. 6a and b for chloroform and DMF, respectively.
3.5. Structural properties of compound 8
The enol form on photoexcitation at the first excited singlet state
undergoes redistribution of charge densities leading to a different
geometry in terms of bond angle and bond distance favoring
a proton transfer and ultimately the excited state keto form, which
becomes an emissive species. This aspect is evident from the TD-
DFT computations. The N1eH13, O3eH13, O3eC16, C11eC15
bond distances and Mulliken charges on N1, H13, and O3 of the
ground and excited state optimized geometries of the enol form as
well as the excited state geometry of keto form are given in
Supplementary data, Tables S2eS4, respectively. As a representa-
tive example, structural views of the molecule (ground and excited
state enol as well as keto forms) in chloroform using Gaussian
compatible CYLview software35 are presented in Fig. 7.
(F
¼0.117) than in any other polar solvent. In DMF, compound 8
shows red shift in emission spectra attributed to the deprotonation
of eOH group because of the higher basicity of DMF than any other
polar solvent. Therefore eH will not be available for the ESIPT
(Fig. 3) and hence shows single emission. Similarly, compound 8
has the exceptionally higher quantum yield in chlorinated hydro-
carbon solvents (chloroform and DCM) than in oxygenated solvents
(ethyl acetate, 1,4-dioxane, and THF; Table 3 and Fig. 5). The short
wavelength emission occurs in all cases with a narrow range of
Stokes shift (75e79 nm) except in DMF (111 nm). It is interesting to
note that the short wavelength emission is much red shifted in DMF
(Fig. 4 and Table 3). Solvent polarity does not affect the Stokes shift

1772
K.R. Phatangare et al. / Tetrahedron 69 (2013) 1767e1777
Table 3
Observed and computed emission of compound 8 in various solvents along with its quantum yield and excitation valuesa
Solvent
Experimental (short emission)
Observed
Stokes shift Fb
TD-DFT (short Excitation
Experimental (long emission)
Observed
Stokes shift Fd
TD-DFT (long Excitation
Total quantum
emission)c
(nm)
(nm) (shorter
emission)
emission)c
(nm)
(nm) (longer yielde
emission)
emission (nm) Dl (nm)
emission (nm) Dl (nm)
MeOH
EtOH
ACN
401
400
398
436
400
403
400
400
79
78
79
111
75
78
78
78
75
0.079 411
296
296
296
298
272
270
298
300
306
d
d
d
d
d
d
d
d
d
d
d
0.079
0.077
0.063
0.117
0.413
0.434
0.169
0.266
0.284
0.077 411
0.063 411
0.117 411
0.175 408
0.182 406
0.102 407
0.135 408
0.146 401
d
d
d
d
d
d
DMF
d
d
d
DCM
CHCl3
EtOAc
THF
514
504
512
520
525
189
179
190
198
200
0.238 514
0.252 526
0.067 521
0.131 510
0.138 544
270, 349
290, 350
296, 344
347
Dioxane 400
354
a
Analyses were carried out at room temperature (25 ꢀC).
For short wavelength emission, tinopal was used as reference standard.
Computed using TD-DFT method with the B3LYP/6-31G(d).
b
c
d
e
For long wavelength, fluorescein was used as reference standard.
Summation of short wavelength and long wavelength quantum yield.19b
slightly longer than the K1-Enol ground state.34 The O3eH13 bond
ꢁ
ꢁ
distance increases by 0.006 A form K1-Enol (0.990 A) to K1-Enol
*
ꢁ
(0.996 A) as reported in Supplementary data, Table S2, which
means that in excited state (K1-Enol ) the H13 approaches near to
*
the N1 via hydrogen bonding for transfer of proton (ESIPT). The
ꢁ
bond distance C11eC15 decreases from K1-Enol (1.446 A) to K1-
ꢁ
ꢁ
Enol
*
(1.419 A) by 0.027 A while O3eC16 bond distance decreases
ꢁ
ꢁ
ꢁ
by 0.015 A from K1-Enol (1.350 A) to K1-Enol
further decreased by 0.074 A in case of K1-Keto
*
(1.335 A), which is
ꢁ
ꢁ
*
(1.261 A)
(Supplementary data, Table S3). In addition to this, the bond angle
H13eO3eC16 increases by 0.6ꢀ from K1-Enol (108.8ꢀ) to K1-Enol
*
(109.4ꢀ). In this way, H13 is approaches nearer to N1 in K1-Enol
*
and further transfer to N1 leads to K1-Keto
*
with N1eH13 bond
ꢁ
distance 1.023 A and O3eH13 hydrogen bonding distance of
1.934 A. The K1-Keto returns to ground state K1-Keto conformer
with N1eH13 bond distance of 1.051 A and O3eH13 hydrogen
bonding distance of 1.680 A (Supplementary data, Table S2). As H13
ꢁ
*
ꢁ
ꢁ
Fig. 4. Emission spectra of compound 8 in various solvents.
approaches to O3 the K1-Keto hydrogen bonding distance de-
*
ꢁ
ꢁ
creases by 0.254 A from 1.934 to 1.680 A (Supplementary data,
Table S2) and it immediately converts to the K1-Keto conformer
and then to the K1-Enol. In all the other solvents the N1eH13 hy-
drogen bond distance is shorter in K1-Enol than that of the K1-
*
Enol conformer while the O3eH13 hydrogen bond distance is
lower in ground state than that of excited state (Fig. 8a and b).
In the case of K1-Enol , as the solvent polarity increases the
*
N1eH13 hydrogen bonding distance increases from 1.772 to
ꢁ
1.789 A (Supplementary data, Table S2, Fig. 8a), O3eH13 bond
ꢁ
length decreases from 0.998 to 0.995 A in case of K1-Enol
*
(Supplementary data, Table S2, Fig. 8b), and O3eC16 bond length
ꢁ
increases from 1.332 to 1.338 A (Supplementary data, Table S3,
ꢁ
Fig. 10a) but bond length C11eC15 decreases from 1.422 to 1.416 A
(Supplementary data, Table S3, Fig. 10b). The Mulliken charges on H
and
(Supplementary data, Table S4, Fig. 9a and b) while they decrease
for N atom indicating that in K1-Enol form, H13 is in close prox-
imity of N atom, which is favorable for ESIPT. Further the charges
again decrease for K1-Keto . From K1-Keto to K1-Keto Mulliken
O
in all solvent increase from K1-Enol to K1-Enol
*
*
*
*
Fig. 5. Day light and UV light photographs of compound 8 in various solvents.
charges for O and N atom decrease while for the H atom it increases
(Supplementary data, Table S4, Fig. 9c and d), which supports for
the ESIPT process.
Molecular planarity is the major factor necessary for ESIPT fa-
cilitating the proton transfer in the excited state from O3 to N1. In
the case of chloroform N1eH13 hydrogen bonding distance de-
3.6. Effect of solvent polarity on ground and excited state
dipole moment
ꢁ
ꢁ
ꢁ
creases by 0.019 A from K1-Enol (1.802 A) to K1-Enol
while the charge on H13 and O3 was increased by 0.002jej and
0.009jej from K1-Enol to K1-Enol , respectively. The N/H hydro-
gen bonding distance in HBO is 1.815 A, which is found to be
*
(1.783 A)
*
As the solvent polarity increases, dipole moment increases in
the case of K1-Enol and K1-Keto in both the ground and excited
ꢁ

K.R. Phatangare et al. / Tetrahedron 69 (2013) 1767e1777
1773
Fig. 6. (a) Representation of the UVevisible absorption and emission of compound 8 in chloroform. The vertical excitation related calculations were based on optimized ground
state geometry and the emission based calculations were based on optimized excited state geometry at B3LYP/6-31G(d). The HOMO and LUMO energy levels in the excited state are
different from those in the ground state. The FMO involved in the vertical excitation (UVevisible absorption: two blue lines and emission: two red lines). (b) Representation of the
UVevisible absorption and emission of compound 8 in DMF. The vertical excitation related calculations were based on optimized ground state geometry and the emission based
calculations were based on optimized excited state geometry at B3LYP/6-31G(d). The HOMO and LUMO energy levels in the excited state are different from those in the ground state.
The FMO involved in the vertical excitation (UVevisible absorption: two blue lines and emission: one red line).
states. The ratio of dipole moment in excited to ground state in
different solvents is plotted against the solvent function ðETNÞ
(Table 4, Fig. 11a and b). The solvent polarity function has been
taken from the literature.36 The calculated dipole moment ratio for
shorter emission is 1.101 and is 2.231 for longer emission. K1-Keto
shows the highest dipole moment in both the states, in the excited
state it ranges from 8.044 to 9.899 D while in the ground state it
ranges from 5.531 to 8.255 D. K1-Enol shows the lower dipole
moment in both the states, in the excited state it ranges from 2.726
to 3.259 D and in the ground state it ranges from 2.802 to 3.817 D.
measured on standard melting point apparatus and are un-
corrected. The FTIR spectra were recorded on a Fourier Transform IR
instrument (ATR accessories). 1H NMR and 13C NMR spectra were
recorded on Varian Cary Eclipse, MR-300 MHz, USA, NMR spec-
trometer operating at 300 and 75 MHz, respectively, using TMS as
an internal standard. Mass spectra were recorded on Finnigan Mass
Spectrometer instrument. The absorption spectra of the com-
pounds were recorded on a Spectronic Genesys 2 UVevisible
spectrophotometer; fluorescence emission spectra were recorded
on Varian Cary Eclipse fluorescence spectrophotometer using
freshly prepared solutions. Quantum yields were calculated by
using tinopal (
F
¼0.81 in DMF32) for shorter emission and fluores-
4. Conclusion
cein (
standards.
F
¼0.91 in methanol33) for longer emission as reference
To conclude, this paper reports the synthesis of the fluorescent
2-((4-benzo[d]oxazol-2-yl)naphtho[1,2-d]oxazol-2-yl)phenol. This
compound is well characterized by 1H NMR, 13C NMR, and mass
spectroscopy. It shows a single prominent absorption and emission
in solvents, single emission in polar solvents, and dual emission in
non-polar solvents. The highest quantum efficiency was observed
5.2. Experimental procedure
5.2.1. 3-(1,3-Benzoxazol-2-yl)naphthalen-2-ol (3). A mixture of 2-
aminophenol 2 (1.09 g, 0.01 mol) and 3-hydroxynaphthalene-2-
carboxylic acid 1 (1.88 g, 0.01 mol) was refluxed (133e135 ꢀC) in
in chloroform (
F
¼0.434). The absorption and emission wave-
lengths were computed using TD-DFT and they are in good agree-
ment with the experimental results. To the best of our knowledge,
only a few naphthalene-based ESIPT molecules are described in the
literature and this report is the first one that uses the computa-
tional chemistry to gain further insight into the ESIPT process at the
molecular level.
21
chlorobenzene (10 mL) in the presence of PCl3 (2 g, 1.3 mL,
0.01 mol) for 4 h. After completion of reaction, the solid product
was precipitated out and was filtered to give crude product 3. Yield
2.06 g (79%), which was further recrystallized from ethanol, mp:
171e173 ꢀC (lit. mp: 171e172 ꢀ 21b).
C
5.2.2. 4-{[3-(1,3-Benzoxazol-2-yl)-2-hydroxynaphthalen-1-yl]dia-
zenyl}benzenesulfonic acid (6). Sulfanilic acid 4 (2.07 g, 0.012 mol)
was dissolved in water (10 mL) containing sodium carbonate of
(0.62 g) at 50e55 ꢀC. A solution of NaNO2 (0.84 g, 0.012 mol) in
water (10 mL) was added to sulfanilic acid solution in water and
then this mixture was added slowly to the concd HCl (4 mL) at
0e5 ꢀC for 30 min and further stirred at this temperature for 1.5 h to
get 4-sulfobenzenediazonium chloride salt 5. This was further
5. Experimental section
5.1. General
All commercial reagents were purchased and used without
purification and all solvents were reagent grade. The reaction was
monitored by TLC using on 0.25 mm silica gel 60 F254 precoated
plates, which were visualized with UV light. Melting points were
coupled with the 3-(1,3-benzoxazol-2-yl)naphthalene-2-ol
3

1774
K.R. Phatangare et al. / Tetrahedron 69 (2013) 1767e1777
Fig. 7. Ground and excited state optimized structural conformations of compound 8 in chloroform. These figures were obtained from Gaussian compatible CYLview software.35
(2.61 g, 0.010 mol) at 0e5 ꢀC at pH 8e9 for 3 h and then at room
temperature for 1 h in methanol/water mixture (80:20). Then re-
action mixture was neutralized with acetic acid (9e10 mL) at pH 7,
methanol was removed under reduced pressure and then filtered
out to afford 4-{[3-(1,3-benzoxazol-2-yl)-2-hydroxynaphthalen-1-
yl]diazenyl}benzenesulfonic acid 6. Yield 4.0 g, (90%). Recrystal-
lized from ethyl alcohol, mp: 234 ꢀC.
5.2.3. 1-Amino-3-(1,3-benzoxazol-2-yl)naphthalen-2-ol (7). The azo
compound 6 (4.45 g, 0.010 mol) was heated in water containing
Fig. 8. Plot of N1eH13 and O3eH13 bond distances in K1-Enol against solvents.

K.R. Phatangare et al. / Tetrahedron 69 (2013) 1767e1777
1775
Fig. 9. Plot of Mulliken charges on H, N, and O atoms of K1-Enol in ground and excited states.
Fig. 10. Plot of O3eC16 and C11eC15 bond distance in K1-Enol and K1-Enol
*
versus solvents.
Table 4
Dipole moment (
m
in Debye) of K1-Enol in ground state and excited state in various solvents
me Enol
mg Enol
me Keto
mg Keto
mg K1-Enola
me K1-Enol
*
mg K1-Ketoa
me K1-Keto
*
Experimental
b
b
Medium
ETN
MeOH
EtOH
ACN
DMF
DCM
CHCl3
EtOAc
THF
0.775
0.654
0.460
0.386
0.309
0.259
0.228
0.207
0.164
3.810
3.791
3.815
3.817
3.775
3.663
3.509
3.575
3.626
3.254
3.240
3.258
3.259
3.229
3.259
3.051
3.093
3.126
0.854
0.855
0.854
0.854
0.855
0.890
0.869
0.865
0.862
8.237
8.190
8.250
8.255
8.250
7.866
7.470
7.641
7.771
9.891
9.870
9.896
9.899
9.850
9.712
8.044
8.044
9.662
1.201
1.205
1.200
1.199
1.194
1.235
1.077
1.053
1.243
1.101 (Short wavelength emission)
2.231 (Long wavelength emission)
Dioxane
a
Dipole moments were obtained at the optimized ground state geometry.
Dipole moments were obtained at the optimized excited state geometry.
b
NaOH (3.51 g, 0.088 mol) at 50 ꢀC and sodium dithionate (3.78 g,
0.025 mol) was added at 90 ꢀC in portions over period of 1 h. The
mixture was heated at 90 ꢀC at pH 8e9 until the color of azo dis-
appeared (1.5 h).21 The progress of the reaction was monitored by
TLC. After completion of reaction, the reaction mass was neutral-
ized up to pH 7, filtered and washed well with water 2e3 times, and
dried well to afford crude 1-amino-3-(1,3-benzoxazol-2-yl)naph-
thalen-2-ol 7. Yield 1.65 g (60%). Crude product was recrystallized
in ethyl alcohol, mp: 206e208 ꢀC.
5.2.4. 2-((4-Benzo[d]oxazol-2-yl)naphtho[1,2-d]oxazol-2-yl)phenol
(8). 1-Amino-3-(1,3-benzoxazol-2-yl) naphthalen-2-ol 7 (0.010 mol),

1776
K.R. Phatangare et al. / Tetrahedron 69 (2013) 1767e1777
Fig. 11. Plots of dipole moment (
m
) versus solvent polarity function ðETNÞ.
21
2-hydroxybenzoic acid (0.010 mol), and PCl3 (0.015 mol) were
refluxed in chlorobenzene (130e133 ꢀC) (6 mL) until completion of
reaction (16e18 h) and was confirmed by TLC. After cooling the re-
action mass, solid product was filtered out and recrystallized from
chloroform to afford corresponding compound 8. Yield 2.19 g (58%),
which was purified by column chromatography using hexane/ethyl
acetate mixture as eluant (8:2), mp: 208e210 ꢀC.
Anal. Calcd for C24H14N2O3: C, 76.18; H, 3.73; N, 7.40. Found: C,
76.03; H, 3.55; N, 7.23.
Acknowledgements
K.P. is thankful to UGC-CAS for providing research fellowship
under Special Assistance Programme (SAP). N.S. and V.G. are
thankful to the 5th UGC-TEC Consortium agreement for financial
support. A.T. is grateful to UGC for research support under Major
Research Project as well as UGC-CSIR fellowship. V.P. is thankful for
postdoctoral fellowship from the Principal Scientific Adviser (PSA),
Govt. of India. The authors acknowledge the useful comments from
anonymous reviewers and the editor to improve the manuscript.
5.3. Spectral data of compounds 7 and 8
5.3.1. 1-Amino-3-(1,3-benzoxazol-2-yl)naphthalen-2-ol
3510, 3452, 3357 (nNeH OeH), 3028 (nAromatic Stretching), 1615,
1585, 1477 (nC N ring stretching), 1222, 1132 (nCeO stretching), 750
(7). FTIR
,
CeH
] ]
C, C
(
nAromatic CeH out of plane bending) cmꢁ1
.
1H NMR (DMSO-d6 300 MHz)
d
¼6.85 (d, J¼7.9 Hz, 1H, AreH),
Supplementary data
7.36e7.43 (m, J¼7.8, 8.2, 1.8 Hz, 5H, AreH), 7.48 (dd, J¼8.2, 1.9 Hz,
1H, AreH), 7.92 (dd, J¼8.5, 1.6 Hz, 1H, AreH), 8.73 (s, 1H, AreH),
9.96 (s, 2H, -NH2), 10.12 (s, 1H, -OH) ppm.
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2012.11.095.
13C NMR (DMSO-d6 75 MHz)
d
¼111.5, 113.3, 119.9, 124.6, 125.9,
126.7, 127.2, 127.5, 129.2, 136.6, 145.9, 149.5, 153.9, 162.4.
MS (m/z) 277.1 (Mþ1, 99%), 276.1 (75%), 275 (30%), 246.1 (18%),
124.1 (20%).
References and notes
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73.87; H, 4.21; N, 10.03.
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5.3.2. 2-((4-Benzo[d]oxazol-2-yl)naphtho[1,2-d]oxazol-2-yl)phenol
(8). FTIR 3367 (nOeH stretching), 3050 (nAromatic
stretching), 1636,
CeH
1581, 1537 (nC
733 (nAromatic -CH out of plane bending vibration) cmꢁ1
1H NMR (CDCl3, 300 MHz)
]
C, C
]N ring stretching), 1243, 1209, 1168 (nCeO stretching),
.
d
¼6.92 (dd, J¼7.8, 7.2 Hz, 1H, AreH),
7.02 (d, J¼8.4 Hz, 1H, AreH), 7.10 (dd, J¼10.8, 8.1 Hz, 1H, AreH), 7.18
(d, J¼7.8 Hz, 1H, AreH), 7.45e7.52 (m, J¼7.5, 2.7, 1.8 Hz, 2H, AreH),
7.62 (dd, J¼7.2, 1.8 Hz, 1H, AreH), 7.75 (m, J¼7.2, 1.5 1H, AreH), 7.92
(m, J¼8.1, 1.5, 1.8 Hz, 1H, AreH), 8.10 (d, J¼8.1 Hz, 1H, AreH), 8.32
(dd, J¼8.1, 1.5 Hz, 1H, AreH), 8.50 (d, J¼8.2 Hz, 1H, AreH), 8.74 (s,
1H, AreH), 10.55 (s, 1H, AreOH) ppm.
1H NMR (D2O exchange) (CDCl3, 300 MHz)
d¼4.82 (s, 1H,
HeOD), 6.92 (dd, J¼7.8, 7.2 Hz, 1H, AreH), 7.0 (d, J¼8.4 Hz, 1H,
AreH), 7.11 (dd, J¼10.8, 8.1 Hz, 1H, AreH), 7.18 (d, J¼7.8 Hz, 1H,
AreH), 7.45e7.52 (m, J¼7.5, 2.7, 1.8 Hz, 2H, AreH), 7.62 (dd, J¼7.2,
1.8 Hz, 1H, AreH), 7.75 (m, J¼7.2, 1.5 1H, AreH), 7.92 (m, J¼8.1, 1.5,
1.8 Hz, 1H, AreH), 8.11 (d, J¼8.1 Hz, 1H, AreH), 8.32 (dd, J¼8.1,
1.5 Hz, 1H, AreH), 8.50 (d, J¼8.2 Hz, 1H, AreH), 8.74 (s, 1H,
AreH) ppm.
13C NMR (DMSO-d6, 75 MHz)
d
¼111.4 (strong), 112.1, 113.0, 119.8
(strong), 123.0, 125.4, 125.7, 126.3, 126.5, 127.1, 123.3, 128.2, 129.7,
130.3, 130.9, 136.4, 140.1, 142.6, 144.1, 149.5, 154.4, 162.3 ppm.
MS (m/z): 379.1 (Mþ1, 100%), 380.1 (25%).

K.R. Phatangare et al. / Tetrahedron 69 (2013) 1767e1777
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