Crystal structure, spectroscopic properties, and DFT studies of 2-(2-hydroxyphenyl)benzo[d]oxazole-6-carbaldehyde

Article abstract of DOI:10.1080/15421406.2015.1069465

The title compound 2-(2-hydroxyphenyl)benzo[d]oxazole-6-carbaldehyde (1a) was synthesized and characterized by single-crystal X-ray diffraction. Compound 1a possesses intramolecular C-H···O and O-H···N hydrogen bonds, which generate S(5) and S(6) rings, r

Full text of DOI:10.1080/15421406.2015.1069465

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Crystal structure, spectroscopic
properties, and DFT studies of 2-(2-
hydroxyphenyl)benzo[d]oxazole-6-carbaldehyde
Kew-Yu Chen
To cite this article: Kew-Yu Chen (2016) Crystal structure, spectroscopic properties, and DFT
studies of 2-(2-hydroxyphenyl)benzo[d]oxazole-6-carbaldehyde, Molecular Crystals and Liquid
Crystals, 625:1, 212-220, DOI: 10.1080/15421406.2015.1069465
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1069465
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Date: 22 March 2016, At: 17:32

MOL. CRYST. LIQ. CRYST.
, VOL. , –
http://dx.doi.org/./..
Crystal structure, spectroscopic properties, and DFT studies of
-(-hydroxyphenyl)benzo[d]oxazole--carbaldehyde
Kew-Yu Chen
Department of Chemical Engineering, Feng Chia University, Taichung, Taiwan, ROC
ABSTRACT
KEYWORDS
Benzothiazole derivatives;
benzoxazole derivatives;
DFT calculations; ESIPT; X-ray
diffraction
The title compound 2-(2-hydroxyphenyl)benzo[d]oxazole-6-carbal
dehyde (1a) was synthesized and characterized by single-crystal
X-ray diffraction. Compound 1a possesses intramolecular C–H···O
and O–H···N hydrogen bonds, which generate S(5) and S(6) rings,
respectively. Intermolecular π–π stacking is observed in the crystal
structure, which links a pair of molecules into a cyclic centrosymmetric
dimer. The crystal structure is further stabilized by three different inter-
molecular C–H···O hydrogen bonds, which link the molecules into a
continuous three-dimensional framework. Its spectroscopic properties
and complementary density functional theory (DFT) calculations are
also reported.
1. Introduction
The excited-state intramolecular proton transfer (ESIPT) reaction of 2-(benzo[d]oxazol-2-
yl)phenol and its derivatives has been investigated using the femtosecond fluorescence up-
conversion technique [1]. The ESIPT reaction involves transfer of a phenolic hydroxyl pro-
ton to an imine nitrogen through an intramolecular six-membered-ring hydrogen-bonding
configuration [2–6], as depicted in Fig. 1. The resulting proton-transfer tautomer (keto-
form) shows great differences in structure and electronic configuration from its correspond-
ing ground state, i.e., a large Stokes shifted K∗→K fluorescence. This unique photophys-
ical property has found a number of important applications such as probes for solvation
dynamics [7–9] and biological environments [10,11], chemosensors [12–16], fluorescence
microscopy imaging [17], photochromic materials [18], nonlinear optical materials [19],
near-infrared fluorescent dyes [20], and organic light-emitting devices [21–25]. To expand
the scope of benzoxazole-based chromophores available for designing systems for highly flu-
orescent dyes and organic optoelectronic materials, the present research reports the syn-
thesis of a 2-(benzo[d]oxazol-2-yl)phenol derivative 2-(2-hydroxyphenyl)benzo[d]oxazole-
6-carbaldehyde (1a) as well as its X-ray structure, optical properties, and complementary
density functional theory (DFT) calculations. The results offer the potential to synthesize
2-(benzo[d]oxazol-2-yl)phenol derivatives with extended molecular architectures and pho-
tophysical properties.
CONTACT Kew-Yu Chen
Taichung, Taiwan, ROC.
kyuchen@fcu.edu.tw
Department of Chemical Engineering, Feng Chia University, 
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOL. CRYST. LIQ. CRYST.
213
Figure . Characteristic four-level photocycle scheme of the ESIPT process.
2. Experimental
2.1. Chemicals and instruments
The starting materials such as 2-hydroxybenzaldehyde (7), 6-amino-m-cresol (6a), lead (IV)
acetate, acetic acid, acetic anhydride, N-bromosuccinimide (NBS), azobisisobutyronitrile
(AIBN), and hexamethylenetetramine (HMTA) were purchased from Merck, ACROS and
Sigma-Aldrich. Column chromatography was performed using silica gel Merck Kieselgel si
60 (40–63 mesh).
1H and 13C NMR spectra were recorded in CDCl3 on a Bruker 500 MHz. Mass spectra
were recorded on a VG70-250S mass spectrometer. The absorption and emission spectra were
measured using a Jasco V-570 UV-Vis spectrophotometer and a Hitachi F-7000 fluorescence
spectrophotometer, respectively. The single-crystal X-ray diffraction data were collected on a
Bruker Smart 1000CCD area-detector diffractometer.
2.2. Synthesis and characterization
... Synthesis of 1a
Hexamethylenetetramine (HMTA) (1.2 g, 8.6 mmol) was added to a solution of 3a (1.4 g,
4.1 mmol) in 50 mL of CHCl3. The mixture was refluxed for 8 hr. After the mixture was
cooled, the residual precipitate was filtered and dried. Next, to 10 mL of glacial acetic acid and
10 mL of water, dried salt was added and refluxed for 2 hr. After it was cooled, the mixture was
neutralized with aqueous NaOH solution and was extracted with CH2Cl2. The organic layer
was dried with anhydrous MgSO4 and evaporated. The mixture of 1a and 2a was dissolved
in a suspension of 0.3 g of KOH in 50 mL of CH2Cl2. The mixture was gently boiled for 4
hr. After it was cooled, the solution was poured into water and neutralized with aqueous HCl
solution. The mixture was extracted with CH2Cl2 and dried with anhydrous MgSO4. After
the solvent was removed, the crude product was purified by silica gel column chromatography
with eluent ethylacetate/n-hexane (1/4) to afford 1a. 1H NMR (500 MHz, CDCl3, in ppm) δ
11.18 (s, 1H), 10.06 (s, 1H), 8.08 (s, 1H), 8.02 (d, J = 8.6 Hz, 1H), 7.91 (d, J = 8.3 Hz, 1H), 7.82
(d, J = 8.3 Hz, 1H), 7.48 (t, J = 7.6 Hz, 1H), 7.11 (d, J = 8.4 Hz, 1H), 7.02 (t, J = 7.6 Hz, 1H).
13C NMR (125 MHz, CDCl3, ppm): 190.69, 165.97, 159.21, 149.27, 145.16, 134.65, 133.90,
127.73, 127.51, 119.83, 119.45, 117.67, 111.19, 109.76. MS (EI, 70 eV): m/z (relative intensity)
239 (M+, 100). HRMS calcd for C14H9NO3, 239.0582; found, 239.0586. Yellow parallelepiped

214
K.-Y. CHEN
crystals suitable for the crystallographic studies reported here were isolated over a period of
5 weeks by slow evaporation from a dichloromethane solution.
... Crystal structural determination
A single crystal of the title compound with dimensions of 0.58 mm × 0.10 mm × 0.08 mm was
selected. The lattice constants and diffraction intensities were measured with a Bruker Smart
˚
1000CCD area detector radiation (λ = 0.71073 A) at 110(2) K. An ω-2θ scan mode was used
for data collection in the range of 3.19o ࣚ θ ࣚ 29.18o. A total of 4643 reflections were collected
and 2430 were independent (Rint = 0.0232), of which 1664 were considered to be observed
with I > 2σ(I) and used in the succeeding refinement. The structure was solved by direct
methods with SHELXS-97 [26] and refined on F2 by full-matrix least-square procedure with
Bruker SHELXL-97 packing [27]. All non-hydrogen atoms were refined with anisotropic ther-
mal parameters. The hydrogen atoms refined with riding model position parameters isotrop-
ically were located from difference Fourier map and added theoretically. At the final cycle of
2
refinement, R = 0.0548 and wR = 0.1012 (w = 1/[σ2(Fo ) + (0.0465P)2 + 0.0000P], where
2
2
P = (Fo + 2Fc )/3), S = 1.000, (ꢀ/σ)max = 0.001, (ꢀ/ρ)max = 0.242, and (ꢀ/ρ)min = −0.262 e
−3
˚
A . Crystallographic data for compound 1a have been deposited with the Cambridge Crys-
tallographic Data Center as supplementary publication number CCDC 1044863. Copies of
these information can be obtained free of charge from the Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 1223 336 033; E-mail: deposit@ccdc.cam.ac.uk).
2.3. Computational methods
The Gaussian 03 program was used to perform the ab initio calculation on the molecu-
lar structure [28]. Geometry optimization for compound 1a was carried out with the 6-
31G∗∗ basis set to the B3LYP functional. After obtaining the converged geometries, the
TD-B3LYP/6-31G∗∗ was used to calculate the vertical excitation energy, and the emission
energy was obtained from TDDFT/B3LYP/6-31G∗∗ calculations performed on S1 optimized
geometries.
3. Results and discussion
Scheme 1 shows the synthetic route and the chemical structures of 1a and 1b. In brief, the
synthesis of 1a started from a condensation of 2-hydroxybenzaldehyde (7a) and 6-amino-
m-cresol (6a), followed by the acetylation of benzoxazol (5a), giving an ester compound 4a.
Next, the bromination of 4a allowed for efficient synthesis of benzyl bromide (3a), followed
by the oxidation of the bromo adduct to give an aldehyde compound 2a. Finally, the protected
hydroxyl group was deprotected by KOH to give 1a. The presence of a single carbonyl sub-
stituent of 1a can be verified by the presence of a signal at δ 10.06 ppm and seven signals at
δ 6.9–8.1 ppm in the 1H NMR spectrum. Detailed synthetic procedures and product charac-
terization are provided in the Experimental section. To confirm its structure, a single crystal
of 1a was obtained from a dichloromethane solution, and the molecular structure was deter-
mined by X-ray diffraction analysis. Moreover, its X-ray structure is compared with that of 1b
[29].
Figure 2 shows the ORTEP diagram of 1a. Compound 1a, as well as compound 1b, crys-
tallizes in the monoclinic space group P21/c (Table 1). The complete molecule of 1a is nearly
planar, as indicated by the key torsion angles (Table 2). The maximum deviations from the
˚
˚
mean plane through the non-H atoms are 0.024(1) A for atom C(5) and 0.029(1) A for atom

MOL. CRYST. LIQ. CRYST.
215
Scheme . The synthetic route and the chemical structures for a and b.
C(14). The dihedral angle between the mean plane of the phenol ring and the mean plane of
the benzoxazol ring is 1.4(2)o, which is slightly larger than that in 1b (3.5(2)o). Compound 1a
possesses an intramolecular O–H···N hydrogen bond, which generates an S(6) ring motif. The
dihedral angle between the mean plane of the S(6) ring and the mean plane of the oxazol ring
o
o
˚
is 0.8(2) . This, together with 2.671(2) A of O(2)···N distance and 151(2) of O(2)–H(2A)–N,
strongly supports the S(6) ring formation. In good agreement with this observation, the 1H
NMR spectrum (in CDCl3) revealed a significantly downfield hydroxyl signal at 11.18 ppm.
The parameters of the hydrogen bond of 1a (Table 3) are slightly different from those of 1b
o
˚
(2.623(2) A of O(2)···N distance and 150(2) of O(2)–H(2A)–N). Additionally, there is a weak
o
˚
intramolecular C–H···O hydrogen bond in 1a (2.825(2) A of C(5)···O(1) distance and 100 of
Figure . The molecular structure of 1a, showing the atom-labeling scheme. Displacement ellipsoids are
drawn at the % probability level. Blue and green dashed lines denote the intramolecular C–H···O and
O–H···N hydrogen bonds.

216
K.-Y. CHEN
Table . Crystallographic data for compounds 1a and 1b.
Compound
1a
1b
Chemical formula
Formula weight
Crystal system
Space group
CHNO
.
Monoclinic
P/c
.()
.()
.()

.()

.()

.
CHNOS
.
Monoclinic
P/c
.()
.()
.()

.()

.()

.
˚
a (A)
˚
b (A)
˚
c (A)
α (°)
β (°)
γ (°)

˚
Volume (A )
Z
Dcalc (g cm−
)
μ (mm−
)
.
.
F


Crystal size (mm)
θ range (°)
`. × . × .
. × . × .
.–.
.–.
Index ranges
– ࣚ k ࣚ 
– ࣚ l ࣚ 
Reflections collected
Independent reflections (Rint
Refinement method on F
GOF on F
– ࣚ hࣚ 
–  ࣚ k ࣚ 
– ࣚ l ࣚ 

 (.)
Full-matrix least-squares
.
– ࣚ h ࣚ 

)
 (.)
Full-matrix least-squares
.
R [I > σ (I)]
wR [I > σ (I)]
R (all data)
.
.
.
.
.
.
wR (all data)
Residual (e A
.
. and –.
.
. and –.
−
˚
)
˚
Table . Comparison of the experimental and optimized geometric parameters of 1a (A and °).
X-ray
DFT
˚
Bond lengths (A)
O()-C()
O()-C()
O()-C()
O()-C()
N-C()
N-C()
C()-C()
.()
.()
.()
.()
.()
.()
.()
.()
.()
.()
.
.
.
.
.
.
.
.
.
.
C()-C()
C()-C()
C()-C()
Bond angles (°)
C()-O()-C()
C()-N-C()
.()
.()
.()
.()
.()
.()
.
.
.
.
.
.
O()-C()-C()
C()-C()-C()
C()-C()-C()
O()-C()-C()
Torsion angles (°)
C()-O()-C()-N
C()-O()-C()-C()
O()-C()-C()-C()
C()-C()-C()-O()
.()
–.()
–.()
.
− .
− .
.()
.

MOL. CRYST. LIQ. CRYST.
217
o
˚
Table . Hydrogen-bond geometry (A, ).
D–H···Cg
d(D–H)
d(H···Cg)
d(D···Cg)
ࢬDHCg
O()–H(A)···N
.()
.
.
.
.
.()
.
.
.
.
.()
.()
.()
.()
.()
()




C()–H(A)···O()
C()–H(A)···O()a
C()–H(A)···O()b
C()–H(A)···O()c
Symmetry codes: (a) x, –y–/, z–/; (b) –x+, –y, –z–; (c) x, –y+/, z+/.
C(5)–H(5A)–O(1)), forming an S(5) motif, which may lead to an increase in the O(2)–N dis-
tance as compared to that of compound 1b (see above). The bond-length and angle patterns
of 1a are typical, and the values of geometrical parameters are very close to those determined
for 1b.
Figure 3 shows the molecular packing of 1a in the crystal unit cell. The crystal structure
is stabilized by intermolecular π–π interactions, which links a pair of molecules into a cyclic
centrosymmetric dimer. Pertinent measurements for these π···π interactions are: centroid–
centroid distances of 3.8532(9) (red dashed lines, symmetry code: 1–X, –Y, –Z) and 3.6420(10)
˚
A (blue dashed line, symmetry code: 1–X, –Y, –Z). The crystal packing is further stabilized by
three different intermolecular C–H···O hydrogen bonds (Table 3); one between the H(10A)
atom and the hydroxyl oxygen atom of a neighboring molecule (green dashed line in Fig. 3,
symmetry code: x, –y–1/2, z–1/2), a second between the H(11A) atom and one carbonyl oxy-
gen atom of another neighboring molecule (pink dashed line in Fig. 3, symmetry code: –x+1,
–y, –z–1), and a third between the H(13A) atom and another carbonyl oxygen atom of a third
neighboring molecule (cyan dashed line in Fig. 3, symmetry code: x, –y+1/2, z+1/2), respec-
tively. As a result, the intermolecular π–π and C–H···O interactions link the molecules into
a continuous three-dimensional framework.
Figure 4 shows the steady-state absorption and emission spectra of 1a in cyclohexane. The
absorption spectra are characterized by a lowest-lying band maximized at 336 nm with a
molar absorptivity value of 1.6 × 104 M−1 cm−1, which makes its assignment to the π →
π∗ transition unambiguous. Two higher energy absorption bands are also observed at 292
and 304 nm, respectively. As for the steady-state emission, compound 1a exhibits a sole,
anomalously long wavelength emission (515 nm) in cyclohexane. Figure 4 clearly shows a
Figure . A packing view of 1a. Blue and red dashed lines denote the intermolecular π–π interactions.
Green, pink, and cyan dashed lines denote three different intermolecular C–H···O interactions, respectively.
Cg (red circle), Cg (green circle), and Cg (blue circle) are the centroids of the O/N/C–C, C–C, and
C–C rings, respectively.

218
K.-Y. CHEN
Figure . Normalized absorption (black line) and emission (green line) spectra of 1a in cyclohexane.
far-separation of the energy gap between the 0-0 onset of the absorption and emission. The
Stokes shift of the emission, defined by peak (absorption)-to-peak (emission) gap in terms of
frequency, is determined to be as large as 10344 cm−1. Accordingly, the assignment of 515 nm
emission to a proton-transfer tautomer emission is unambiguous, and ESIPT takes place from
the phenolic proton to the imine nitrogen, forming the keto-tautomer species.
To gain more insight into the molecular structure and electronic properties of 1a, quantum
chemical calculations were performed using density functional theory (DFT) at the B3LYP/6-
31G∗∗ level. The geometric parameters (bond lengths, bond angles, and torsion angles) were
compared with experimental data (Table 2). One can clearly see that there are no signifi-
cant differences between the experimental and DFT/B3LYP calculated geometric parameters.
Therefore, we can conclude that basis set 6-31G∗∗ is suited in its approach to the experimental
results.
Figure 5 shows the HOMO and LUMO of enol and keto form of 1a. It is apparent that the
first excited states for both enol and keto forms are a dominant π → π∗ transition from the
HOMO to the LUMO. The HOMO of the enol-form (E) is delocalized mainly on the phenol
Figure . Selected frontier molecular orbitals involved in the excitation and emission of 1a.

MOL. CRYST. LIQ. CRYST.
219
ring, while the LUMO is mostly delocalized on the benzoxazole ring, which indicates that the
excitation from E to E∗ should involve intramolecular electron density transfer from phenol
ring to benzoxazole ring. With completion of the ESIPT (GSIPT: ground-state intramolecu-
lar proton transfer) reaction, the energy level of the LUMO (HOMO) decreased from −2.20
(−5.49) to −2.33 (−6.17) eV. In addition, the absorption and emission spectra of 1a were
calculated by time-dependent DFT (TD-DFT) calculations (Franck–Condon principle). The
calculated excitation (fluorescence) wavelength for the S0 → S1- (S1 → S0) transition is 345
(522) nm, which is very close to the experimental results.
4. Conclusions
A
2-(benzo[d]oxazol-2-yl)phenol derivative, 2-(2-hydroxyphenyl)benzo[d]oxazole-6-
carbaldehyde (1a), was synthesized and characterized by single-crystal X-ray diffraction. The
crystal belongs to monoclinic, space group P21/c, with a = 7.4609(5), b = 12.4804(8), c =
3
˚
˚
11.4804(6) A, α = 90°, β = 94.037(5)°, γ = 90°, V = 1066.34(11) A , and z = 4. Compound
1a possesses an intramolecular six-membered-ring hydrogen bond, from which excited-state
intramolecular proton transfer takes place, resulting in a proton-transfer tautomer emission
of 515 nm in cyclohexane. Furthermore, the single-crystal X-ray structure determinations
described here have brought to light many interesting properties between 1a and 1b in
the solid phase, including π···π stacking and intra- and intermolecular hydrogen bonding
interactions. The results offer the potential to synthesize new 2-(benzo[d]oxazol-2-yl)phenol
derivatives with extended molecular architectures and optical properties.
Acknowledgment
The authors appreciate the Precision Instrument Support Center of Feng Chia University for providing
the fabrication and measurement facilities.
Funding
The project was supported by the Ministry of Science and Technology (MOST 104-2113-M-035-001)
in Taiwan.
References
[1] Hsieh, C. C., Cheng, Y. M., Hsu, C. C., Chen, K. Y., & Chou, P. T. (2008). J. Phys. Chem. A, 112,
8323.
[2] Chen, K. Y., Hsieh, C. C., Cheng, Y. M., Lai, C. H., & Chou, P. T. (2006). Chem. Commun., 42,
4395.
[3] Satam, M. A., Raut, R. K., Telore, R. D., & Sekar, N. (2013). Dyes Pigm., 97, 32.
[4] Luo, M. H., Tsai, H. Y., Lin, H. Y., Fang, S. Y., & Chen, K. Y. (2012). Chin. Chem. Lett., 23, 1279.
[5] Satam, M. A., Raut, R. K., & Sekar, N. (2013). Dyes Pigm., 96, 92.
[6] Fang, T. C., Tsai, H. Y., Luo, M. H., Chang, C. W., & Chen, K. Y. (2013). Chin. Chem. Lett., 24, 145.
[7] Chen, W. H., & Pang, Y. (2010). Tetrahedron Lett., 51, 1914.
[8] Mahapatra, A. K., Maiti, K., Sahoo, P., & Nandi, P. K. (2013). J. Lumin., 143, 349.
[9] Xie, L., et al. (2012). Dyes Pigm., 92, 1361.
[10] Maupin, C. M., et al. (2011). J. Am. Chem. Soc., 133, 6223.
[11] Lim, C. K., et al. (2011). Dyes Pigm., 90, 284.
[12] Hong, W. H., Lin, C. C., Hsieh, T. S., & Chang, C. C. (2012). Dyes Pigm., 94, 371.
[13] Huang, Q., Yang, X. F., & Li, H. (2013). Dyes Pigm., 99, 871.

220
K.-Y. CHEN
[14] Prabhu, S., Saravanamoorthy, S., Ashok, M., & Velmathi, S. (2012). J. Lumin., 132, 979.
[15] Patil, V. S., Padalkar, V. S., Tathe, A. B., & Sekar, N. (2013). Dyes Pigm., 98, 507.
[16] Lin, W. C., Fang, S. K., Hu, J. W., Tsai, H. Y., & Chen, K. Y. (2014). Anal. Chem., 86, 4648.
[17] Santos, R. C., et al. (2011). Tetrahedron Lett., 52, 3048.
[18] Ito, Y., Amimoto, K., & Kawato, T. (2011). Dyes Pigm., 89, 319.
[19] Ashraf, M., et al. (2012). Dyes Pigm., 95, 455.
[20] Ikeda, S., et al. (2010). J. Org. Chem., 75, 8637.
[21] Li, Y., et al. (2012). J. Lumin., 132, 1010.
[22] Chuang, W. T., et al. (2011). J. Org. Chem., 76, 8189.
[23] Xu, H., et al. (2012). J. Lumin., 132, 919.
[24] Tang, K. C., et al. (2011). J. Am. Chem. Soc., 133, 17738.
[25] Fang, S. K., Tsai, H. Y., Hu, J. W., & Chen, K. Y. (2014). Int. J. Photoenergy, 2014, Article ID 124753,
9 pages. doi:10.1155/2014/124753.
[26] Sheldrick, G. M. (1997). SHELXS97, A Program for Automatic Solution of Crystal Structure, Uni-
versity of Göttingen: Germany.
[27] Sheldrick, G. M. (1997). SHELX97, A Program for Crystal Structure Refinement, University of Göt-
tingen: Germany.
[28] Frisch, M. J., et al. (2003). Gaussian 03, Gaussian, Inc.: Pittsburgh, PA.
[29] Chen, K. Y., Fang, T. C., Chang, M. J., Tsai, H. Y., & Luo, M. H. (2011). Acta Cryst., E67, o2862.