Synthesis and photophysical studies of oxazole rings containing compounds as electron accepting units

Article abstract of DOI:10.1016/j.saa.2012.10.005

A series of oxazole derivatives, 5-methyl-2-(p-methylphenyl)-4-acetyl oxazole (MMPAO), 5-methyl-2-(p-methoxyphenyl)-4-acetyl oxazole (MOPAO), 5-methyl-2-(p-N,N′-dimethylamino-phenyl)-4-acetyl oxazole (MDMAPAO) and 5-methyl-2-(p-N,N′-diphenylaminophenyl)-4

Full text of DOI:10.1016/j.saa.2012.10.005

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 256–262
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and photophysical studies of oxazole rings containing
compounds as electron accepting units
Caihong Zhang a, Lifeng Li a, Hongjuan Wu a, Zhixun Liu a, Junfen Li a, Guomei Zhang a,
Guangming Wen a, Shaomin Shuang a, Chuan Dong a, , Martin M.F. Choi b,
⇑
⇑
a School of Chemistry and Chemical Engineering, and Center of Environmental Science and Engineering Research, Shanxi University, Taiyuan 030006, PR China
b Department of Chemistry, Hong Kong Baptist University, 224 Waterloo Road, Kowloon Tong, Hong Kong Special Administrative Region
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
Four compounds with oxazole and
benzene rings have been
synthesized.
The ICT was observed with the
enhancing of the electron-donor
ability.
5-Methyl-2-(p-N,N-diphenylamino
phenyl)-4-acetyl oxazole shows
obvious ICT.
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of oxazole derivatives, 5-methyl-2-(p-methylphenyl)-4-acetyl oxazole (MMPAO), 5-methyl-2-(p-
methoxyphenyl)-4-acetyl oxazole (MOPAO), 5-methyl-2-(p-N,N0-dimethylamino-phenyl)-4-acetyl oxa-
zole (MDMAPAO) and 5-methyl-2-(p-N,N0-diphenylaminophenyl)-4-acetyl oxazole (MDPAPAO) have
been synthesized and studied to compare their photophysical properties. The UV–visible absorption
spectra of MDMAPAO and MDPAPAO are bathochromatically shifted as compared to that of MMPAO
and MOPAO. The fluorescence emission of MDPAPAO is very sensitive to the polarity of solvents. The
magnitude of change in the dipole moment was calculated using the Lippert–Mataga equation. MDPA-
Received 20 April 2012
Received in revised form 22 September
2012
Accepted 4 October 2012
Available online 13 October 2012
Keywords:
PAO shows the highest change in the dipole moment (Du = 13.3D) than that of the other three oxazole
Oxazole ring
Fluorescence
Charge transfer
Interaction
derivatives. The spectral properties including fluorescence quantum yield and lifetime were determined
in solvents with different polarities. MDPAPAO displays the highest fluorescence quantum yield and life-
time, following a bi-exponential fluorescent decay fashion. Our result demonstrates that the excited state
of MDPAPAO possesses the property of intramolecular charge transfer.
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
and has formed an indispensable building block of photochemistry
[2]. In ICT, a charge separation may result from the transfer of charge
Since the first observation of dual emission in N,N0-dimethylami-
nobenzonitrile by Lippert et al. [1], photoinduced intramolecular
charge transfer (ICT) has become the object of much interest in the
past few decades. ICT is a fundamental process of the excited states
from a donor (D) to an acceptor (A) within a pertinent molecule upon
photoexcitation. Aromatic compounds, especially those containing
five-membered heteroaromatic units, possessing electron donor–
acceptor chromophores and exhibiting dual fluorescence are of par-
ticular interest because the additional fluorescence is usually associ-
ated with the ICT state [3–8]. Among the p-conjugated compounds
with five-membered heteroaromatic rings, pyrrole and thiophene
[9–12] are typical electron-donating units while imidazole [13], thi-
⇑
Corresponding authors. Tel./fax: +86 351 7018613 (C. Dong), tel.: +852
34117839; fax: +852 34117348 (M.M.F. Choi).
E-mail addresses: dc@sxu.edu.cn (C. Dong), mfchoi@hkbu.edu.hk (M.M.F. Choi).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.10.005

C. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 256–262
257
azole [14,15], triazole [16], oxadiazole [17,18], and thiadiazole [19]
MMPAO: m.p. 84–85 °C
IR(KBr),
/cmꢀ1: 1598.9, 1560.3 1500.5, 1419.5, 825.5, 1678.0,
usually possess electron-accepting property owing to the electron-
withdrawing ability of the imine group. In addition, oxazole possess-
ing the electron-accepting five-membered ring is another good can-
didate of p-conjugated compound.
t
1647.1 1174.6 1382.9.
1H NMR, d: 2.40 (s, 3H), 2.60 (s, 3H), 2.71 (s, 3H), 7.30 (d, 2H),
7.29 (d, 2H).
Oxazoles are considered as an important class of heterocyclic
compounds since they have been used as efficient luminophores
for liquid and plastic scintillators, laser dyes, organic light-emitting
materials and as fluorescent probes for biological systems [20,21].
Oxazoles are associated with anti-bacterial, anti-fungal, anti-
inflammatory and anti-tumoral activities and can be used as pep-
tide mimics or enzyme inhibitors [22–24]. The physicochemical
and photophysical properties of some oxazole derivatives were
extensively studied in the past year because of their potential
applications. However, most of the studies only focus on the
hydrogen bonding capability of oxazole or the excited state intra-
molecular proton transfer; unfortunately, their ICT character is
not so prominent [25,26]. In order to enhance the ICT character
of oxzaoles, better electron-donating moiety could be introduced
to the oxazoles. In here, some 2-phenyloxazole derivatives such
as 5-methyl-2-(p-methylphenyl)-4-acetyl oxazole (MMPAO),
5-methyl-2-(p-methoxyphenyl)-4-acetyl oxazole (MOPAO), 5-
methyl-2-(p-N,N0-dimethylaminophenyl)-4-acetyl oxazole (MDM-
APAO) and 5-methyl-2-(p-N,N0-diphenylaminophenyl)-4-acetyl
oxazole (MDPAPAO) are successfully synthesized and reported.
The ICT of these 2-phenyloxazole derivatives were studied in detail
and they possess interesting electron-donating groups on their p-
benzene rings which indeed could enhance their ICT character.
The effect of solvent polarity on the absorption and fluorescence
emission spectra of the 2-phenyloxazole derivatives was investi-
gated and the charge transfer property of these compounds was
systematically studied.
MOPAO: m.p. 86–88 °C
IR(KBr), t
/cmꢀ1: 1614.3, 1598.9, 1502.4, 1458.1, 837.0, 1681.8,
1614.3, 1170.7, 1382.9.
1H NMR, d: 2.56 (s, 3H), 2.67 (s, 3H), 3.88 (s, 3H), 6.95 (d, 2H),
7.98 (d, 2H).
MDMAPAO: m.p. 131–133 °C
IR(KBr),
t
/cmꢀ1: 1598.9, 1614.3, 1512.1, 1429.2, 815.8, 1674.1,
1357.8.
1H NMR, d: 2.58 (s, 3H), 2.66 (s, 3H), 3.04 (s, 3H), 6.74 (d, 2H),
7.89 (d, 2H).
MDPAPAO: m.p. 132–134 °C
IR(KBr),
t
/cmꢀ1: 690–710, 730–770, 1500, 1600, 790–840,
1685.7, 1600–1635, 1176.5, 2910–2950.
1H NMR, d: 7–8 (m, 14H), 2.2 (s, 3H), 2.8(s, 3H).
MS, m/z: 368.22, 325.11, 42.92, 76.98, 167.11.
Instrumentation
All UV absorption spectra were recorded on a Shimadzu UV-265
UV–visible absorption spectrophotometer (Tokyo, Japan). Fluores-
cence spectra were taken on a Hitachi F4500 spectrofluorometer
(Tokyo, Japan). Both excitation and emission slits were set at
5 nm. All the experiments are carried out at 20 1 °C. The fluores-
cence lifetime measurements were performed on an Edinburgh
FLS920 fluorescence lifetime spectrometer (Livingston, UK) operat-
ing in the time-correlated single photon counting mode. The exci-
tation source was a hydrogen lamp (Edinburgh Instruments). The
lifetime values were obtained from the re-convolution fit analysis
using a double-exponential decay function of the decay profiles
with F900 analysis software. The goodness of fit was evaluated
by the reduced v2 value (close to 1 in all case).
It is well known that fullerene (C60) can act as an electron accep-
tor when interacting with a conjugated heteroaromatic compound
as an electron donor. In order to compare the ICT property of the
2-phenyloxazole derivatives, the inter-molecular interaction of
MDPAPAO with fullerene (C60) was also investigated. The experi-
mental results should provide solid evidence to support the elec-
tron-donating ability of MDPAPAO. Finally, it is our wish that
these fundamental data could provide valuable information for
understanding the ICT character of these new 2-phenyloxazole
derivatives.
Fluorescence quantum yields
The fluorescence quantum yields were obtained using quinine
bisulfate in 0.050 M sulfuric acid as standard and calculated on
the basis of Eq. (1) [29,30]:
FuAsn2u
FsAun2s
/uf ¼ /sf
ð1Þ
Experimental
where F represents the corrected fluorescence peak area, A is the
absorbance at the excitation wavelength, n is the refractive index
of the solvent used, uf is the fluorescence quantum efficiency and
the superscripts and subscripts ‘s’ and ‘u’ refer to standard and un-
known, respectively.
Materials
All solvents including cyclohexane, diethyl ether, ethyl acetate,
acetonitrile and ethanol of analytical-grade were purchased from
Beijing Chemical Plant (Beijing, China). All chemicals and reagents
were used as received unless otherwise stated.
Results and discussion
Synthesis of compounds with oxazole ring
Absorption and fluorescence emission characteristics
Scheme 1 outlines the synthesis routes for the new oxazole
derivatives. In essence, p-benzaldehyde containing various substi-
tuted groups were allowed to react with 3-(hydroxyimino)pen-
tane-2,4-dione in HCl gas saturated with glacial acetic acid at 0–
5 °C to obtain the substituted oxazole intermediates [27,28]. The
intermediates were reduced to its corresponding target oxazole
derivatives by Zn powder in glacial acetic acid for 5 h at 40–
50 °C. The reaction mixture was purified by column chromatogra-
phy on silica gel to obtain 77%, 51%, 24% and 22% yields of MMPAO,
MOPAO, MDMAPAO and MDPAPAO, respectively. Their melting
points, IR, 1H NMR and MS data were shown as following:
Fig. 1a depicts the absorption spectra of MMPAO, MOPAO,
MDMAPAO and MDPAPAO in ethyl acetate. All these oxazole com-
pounds display strong absorption bands in UV region with their
absorption peak maxima (kmax) at 272, 277, 318 and 330 nm,
respectively. The kmax of the oxazole compounds follows the trend:
MMPAO < MOPAO < MDMAPAO < MDPAPAO, closely relating to
the electron-donating ability of the substituents on the parent
oxazole: p-methylphenyl < p-methoxyphenyl < p-N,N0-dimethyl-
aminophenyl < p-N,N0-diphenylaminophenyl. Larger red-shifts in
the spectra were observed for MDMAPAO and MDPAPAO as com-

258
C. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 256–262
O
C
O
C
R
CHO
+
H3C
H3C
H3C
H3C
NOH
HCl
N
Zn
Cl-
OH
N
R
R
O
O
C
O
C
O
H3C
C
CH3
1. MMPAO
R=-CH3
R=-N(CH3)2
2. MOPAO
4. MDPAPAO
R=-OCH3
3. MDMAPAO
R=--N(Ph)2
Scheme 1. The synthesis routes of MMPAO, MOPAO, MDMAPAO and MDPAPAO.
Table 1
(a)
The absorption and fluorescence emission of compounds in various solvents.
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
kem½ (cmꢀ1
)
D )
tstoke (cmꢀ1
3
Solvent
kab (nm)
kem (nm)
1
2
1. MMAPO
MMPAO
n-Hexane
2. MOAPO
272
271
273
271
271
271
–
–
–
–
–
–
–
–
–
–
–
–
3. MDMAPAO
4. MDPAPAO
4
Diethyl ether
Ethyl acetate
Dichloromethane
Acetonitrile
Ethanol
MOPAO
n-Hexane
278
276
279
277
278
277
–
–
Diethyl ether
Ethyl acetate
Dichloromethane
Acetonitrile
Ethanol
345
347
351
347
354
4486
4531
4505
4522
4996
7246
7024
7611
7153
7953
250
300
350
400
MDMAPAO
Wavelength (nm)
n-Hexane
315
316
320
317
318
321
–
–
Diethyl ether
Ethyl acetate
Dichloromethane
Acetonitrile
Ethanol
368
370
373
376
377
3605
3880
4473
4223
4736
4851
4628
(b)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
3619
3640
6
1
1.n-Hexane
2.Diethyl ether
3.Ethyl acetata
4.Dichloromethane
5.Acetonitrile
6.Ethanol
MDPAPAO
n-Hexane
300, 324
310, 326
300, 326
300, 328
301, 332
301, 333
385
408
419
448
452
468
3895
4064
4136
3912
4176
4317
4890
6165
6809
8167
7996
8662
Diethyl ether
Ethyl acetate
Dichloromethane
Acetonitrile
Ethanol
– Not detected.
polarities. It was found that the absorption band has two peaks,
the maximum absorption wavelength and the relative intensity
of the longer absorption at around 330 nm was obvious depen-
dence on the solvent polarity, whereas the shorter one at around
300 nm showed very weak dependence. The clearly observed dual
absorption peaks could be readily assigned to the locally state and
the ICT state, suggesting the ground state ICT interaction of MDPA-
PAO. By comparison, there is only one peak for MDMAPAO and the
maximum absorption wavelength red-shifted slightly from n-
cyclohexane to ethanol.
250
300
350
400
450
Wavelength(nm)
Fig. 1. (a) UV–visible absorption spectrum of MMPAO, MOPAO, MDMAPAO and
MDPAPAO in ethyl acetate. The concentrations are 0.15 mM. (b) Normalized UV–
visible absorption spectrum of MDPAPAO in n-cyclohexane, diethyl ether, ethyl
acetate, dichloromethane, acetonitrile, and ethanol.
Table 1 lists the fluorescence emission peak maximum (kem) of
the oxazole compounds in different solvents. MMPAO does not
show fluorescence in these organic solvents while the other oxa-
zole compounds possess good fluorescence properties in the UV re-
gion as depicted in Fig. 2. These oxazole compounds display
solvatochromic red-shift with increasing the solvent polarity. The
shift is largest for MDPAPAO, smaller for MDMAPAO and smallest
for MOPAO. The strong solvatochromic red-shift of MDPAPAO
infers that it possesses an asymmetrically charge distributed state
derived from the ICT of its fluorescent state. In addition, the emis-
sion bands display good normal Gaussian shape and their half
pared to MMPAO and MOPAO, indicating that the dimethylamino
and diphenylamino moieties show stronger electron-donating
ability than that of the methyl and methoxy moieties. In addition,
the kmax of the oxazole compounds in other solvents of various
polarities such as n-cyclohexane, diethyl ether, ethyl acetate,
dichloromethane, acetonitrile and ethanol are summarized in
Table 1. It was found that the kmax of MMPAO and MOPAO do
not change much with the solvent polarity whereas it is more pro-
nounced for MDMAPAO and MDPAPAO. Fig. 1b shows the absorp-
tion spectra of MDPAPAO in organic solvents with different

C. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 256–262
259
Rettig and Zander state that ICT emission could be observed if
the molecular system fulfill the conditions of Eqs. (2) and (3)
[31,32]:
(a)
1
1.0
0.8
0.6
0.4
0.2
0.0
1. Diethyl ether
2. Ethyl acetate
3. Dichloromethane
4. Acetonitrile
5. Ethanol
5
EICT ꢀ ELE < 0
ð2Þ
EICT ꢁ IPD ꢀ EAA þ ECoul þ Esol
ð3Þ
v
EICT and ELE are the energies of the lowest ICT state and the lowest
locally excited state, respectively. IPD and EAA are the ionization po-
tential and electron affinity potential of the donor and acceptor
moieties, respectively. ECoul is the Coulomb energy and Esolv is the
interaction energy between the charge-separated molecule and
the polar solvent. The EAA is more or less the same for the oxazole
compounds but the IPD increases from MMPAO to MOPAO to MDM-
APAO and then to MDPAPAO. It is well known that the red-shift in
absorption spectrum of a compound depends on the electron-
donating ability of its moiety; in other words, the larger the IPD,
the larger the red-shift of the absorption spectrum. For MDPAPAO,
the triphenylamine moiety serves as an excellent electron donor
300
350
400
450
Wavelength (nm)
(b)
when it links to the oxazole ring to form a D-p-A structure; as a re-
sult, this provides a good basis for MDPAPAO to possess ICT charac-
ter especially in the polar solvents. Further evidences including the
changes in fluorescence emission spectrum, dipole moment and
fluorescence lifetime with solvent polarity, and the interaction of
MDPAPAO with C60 strongly support that MDPAPAO obviously pos-
sesses an ICT excited state. The details will be discussed in the sub-
sequent sections.
1
5
1.0
1. Diethyl ether
2. Ethyl acetate
3. Dichloromethane
4. Acetonitrile
5. Ethanol
0.8
0.6
0.4
0.2
0.0
Effect of solvent polarity on emission spectra and dipole moment
As the spectral characteristics of the oxazole compounds de-
pend on the type of solvents, the effect of solvent polarity on their
kem and Stokes shift are studied. Fig. 3 displays the plots of (a) 1/
kem against the solvent polarity parameter ET(30) and (b) Stokes’
350
400
450
shift against the polarity parameter f(e, n). The plots of 1/kem vs.
Wavelength (nm)
ET(30) are linear for the oxazole compounds, demonstrating that
the solvent polarity is directly related to the emission peak of the
oxazole compounds. In addition, the slope of the plot for MDPAPAO
is steeper than that of MOPAO and MDMAPAO, indicating that the
solvent polarity has more significant effect on MDPAPAO. These re-
sults suggest that the ICT state of MDPAPAO is more stable in polar
solvents [33].
(c)
1
6
1.0
0.8
0.6
0.4
0.2
0.0
1. n-Cyclohexane
2. Diethyl ether
3. Ethyl acetate
4. Dichloromethane
5. Acetonitrile
6. Ethanol
To determine the excited state dipole moment of the oxazole
compounds, the Stokes shifts (
Dtstoke) were plotted against the
polarity parameter ( f( , n)) as shown in Fig. 3b using the Lip-
D
e
pert–Mataga Eqs. (4) and (5) [34]:
2
ta
ꢀ
tf ¼ ½2ðle
ꢀ
lgÞ =4
p
e
0hcq3
ꢂ
D
fð
e
; nÞ þ const
ð4Þ
ð5Þ
D
fð
e
; nÞ ¼ ð
e
ꢀ 1Þ=ð2
e
þ 1Þ ꢀ ðn2 ꢀ 1Þ=ð2n2 þ 1Þ
where
t
a and
t
f are the wavenumbers corresponding to the absorp-
g is the magni-
tude of the change in the dipole moment of the excited state
350
400
450
500
550
600
tion and emission maxima of the compounds. le
ꢀ
l
Wavelength (nm)
and ground state, h, c, e0,
q correspond to the Plank’s constant
(6.6 ꢃ 10ꢀ34 JS), velocity of light in the vacuum (3.0 ꢃ 108 msꢀ1),
permittivity of vacuum (8.85 ꢃ 10ꢀ12 V Cꢀ1 mꢀ1) and Onsagar cav-
ity radius (in meter), respectively. Using the assumption of Lippert,
Fig. 2. Normalized fluorescence spectra of: (a) MOPAO, (b) MDMAPAO and (c)
MDPAPAO in n-cyclohexane, diethyl ether, ethyl acetate, dichloromethane, aceto-
nitrile, and ethanol. The concentrations are 0.5
280, 315 and 325 nm, respectively.
lM. The excitation wavelengths are
the Onsagar cavity radius
q can be derived from the length of the
molecule multiplied by a reduction factor of 0.8 [35], giving the va-
lue 4.21, 4.32 and 4.57 Å for MOPAO, MDMAPAO and MDPAPAO,
bandwidths (kem½) increases significantly with the increase in sol-
vent polarity. The Stokes shift ( stoke) for MDPAPAO is very pro-
D
t
respectively.
e and n are the dielectric constant and the refractive
nounced in polar solvents such as dichloromethane, acetonitrile
and ethanol as displayed in Table 1. Again, all these data suggest
that polar solvents could exhibit significant effect on the excited
state of MDPAPAO.
index of the solvent, respectively [36].
Fig. 3b displays that the plot of Stokes shift (
D
t
stoke) vs. (
lg for MOP-
AO, MDMAPAO and MDPAPAO were calculated as 3.9, 5.4 and 13.3,
Df(e, n))
is linearly related. The change in dipole moment le
ꢀ

260
C. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 256–262
all these compounds and it is well known that many carbonyl com-
pounds present a low-lying n–
ꢄ state. The energy required for n–
ꢄ and ꢄ electronic transitions are so close that n and can
form mixing states [37]. The high fluorescence quantum yield of
MDPAPAO can be explained by a predominance of
ꢄ character
and n–
ꢄ caused by the N atom and aryl in the triphenylamine
moiety.
(a)
p
30
28
26
24
22
p
p
–p
p
p–p
p
MOAPAO
Fluorescent lifetime studies
MDMAPAO
MDPAPAO
The fluorescent decay of MOPAO, MDMAPAO and MDPAPAO
were determined in different solvents (diethyl ether, ethyl acetate,
dichloromethane, acetonitrile, and ethanol). Table 3 summarizes
the lifetimes of these compounds in various solvents. It was found
that MOPAO and MDMAPAO show a single exponential decay
whereas MDPAPAO follows a bi-exponential fluorescence decay,
inferring the existence of two different emissive states for MDPA-
PAO, i.e., the locally excited state (LE) and charge transfer state
32
36
40
44
48
52
ET(30) (Kcal/mol)
(b)
10000
(ICT) [38]. The average lifetime (
s) is calculated and summarized
in Table 3 using the following Eq. (6) [39]:
s
¼ ða1s21 þ a2s22Þ=ða1s1 þ a2s2
Þ
ð6Þ
MOAPAO
MDMAPAO
MDPAPAO
8000
6000
4000
where s1 and s2 are the lifetime values of the two emissive states
and a1 and a2 are called the pre-exponential factors, which give
the abundance of each emissive state.
-1
−1
v
(cm )
Δυ (cm )
stoke
stoke
The inter-molecular charge transfer process of MDPAPAO with C60
Among the wide variety of donor–acceptor systems, C60 is par-
ticularly appealing as an electron acceptor because of its three-
dimensional structure, delocalized p-electrons within the spherical
0.0
0.1
0.2
0.3
carbon framework, small reorganization energy, low reduction po-
tential, and absorption extending over most of the visible region.
These unique physical and chemical properties make C60 a promis-
ing candidate for investigating its electron transfer processes with
electron-donor compounds [40–43]. Fig. 4a displays the UV–visible
absorption spectrum of C60 in the presence of various concentra-
tions of MDPAPAO. C60 has a strong absorption band at about
450–650 nm in the visible light region, deriving from the symme-
try-forbidden electronic transitions and the symmetry-allowed
vibronic transitions [44,45]. Since MDPAPAO does not absorb at
wavelengths longer than ꢅ450 nm, the C60 absorption band at
450–650 nm was chosen to investigate the CT interaction between
MDPAPAO and C60. The absorption intensity of C60 decreases with
the increase in concentration of MDPAPAO, attributing to the for-
mation of inter-molecular CT complex [32]. By contrast, there is al-
most no change in spectral characteristic and absorption intensity
of C60 in the presence of the other three oxazole compounds
(MMPAO, MOPAO and MDMAPAO), inferring that they have weak
Δ f(ε, n)
Fig. 3. Effect of solvent polarity on the emission peak maxima and Stokes shift of
MOPAO, MDMAPAO and MDPAPAO in n-cyclohexane, diethyl ether, ethyl acetate,
dichloromethane, acetonitrile, and ethanol. Plots of: (a) 1/kem (ꢃ1000 cmꢀ1) vs.
ET(30) and (b) Stokes shift (ꢃ1000 cmꢀ1) vs. f(
e, n).
respectively using the slopes of Eq. (4) and the Onsagar cavity ra-
dius p. The change in dipole moment for MDPAPAO was obviously
larger than that of MOPAO and MDMAPAO. These results are in
consistent with the solvent polarity dependence of the fluores-
cence spectra, possibly attributing to the stronger charge transfer
effect from the electron rich donor to acceptor moieties in the ex-
cited state of MDPAPAO.
Fluorescence quantum yield
Table 2 summarizes the fluorescence quantum yields of MOP-
AO, MDMAPAO and MDPAPAO were determined in different sol-
vents: diethyl ether, ethyl acetate, dichloromethane, acetonitrile
and ethanol. The fluorescence quantum yields do not change sig-
nificantly with the change of solvent polarity for the investigated
compounds. The fluorescence quantum yields of MDPAPAO were
found to be generally higher than that of MOPAO and MDMAPAO
in all the solvents. A methyl carboxylate group is also present in
or no interaction with C60
.
Fig. 4b depicts the fluorescence quenching of C60 on MDPAPAO.
The maximum excitation wavelength is 330 nm, but C60 have
Table 3
The fluorescence lifetime of oxazole compounds.
Solvent
MOAPO
(ns)
MDMAPAO
(ns)
MDPAPAO
1(ns)
s
s
s
s
2(ns)
s(ns)
Table 2
Diethyl ether
Ethyl acetate
Dichloromethane
Acetonitrile
Ethanol
1.79
4.53
1.70
3.92
2.31
1.64
1.87
3.68
1.91
1.66
1.90
4.00
6.12
4.61
3.72
3.91
8.17
7.97
7.25
7.15
3.20
4.55
7.23
5.61
3.72
Fluorescence quantum yield of the oxazole compounds.
Solvent
MOPAO
MDMAPAO
MDPAPAO
Diethyl ether
Ethyl acetate
Dichloromethane
Acetonitrile
Ethanol
0.019
0.134
0.051
0.056
0.053
0.016
0.030
0.044
0.029
0.019
0.886
0.659
0.972
0.486
0.264
s1, s2 are the fluorescence decay of the locally excited state (LE) and the charge
transfer state (ICT), respectively [29,38].
the fourth column [39].
s is the average lifetime of MDPAPAO in

C. Zhang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 256–262
261
physical properties were investigated in detail. The results show
that MMPAO, MOPAO and MDMAPAO have lower fluorescence
quantum yields. Their quantum yield, fluorescence lifetime and
spectral characteristics are less dependent on the solvent polarity.
By contrast, MDPAPAO displays larger changes in fluorescence
emission spectrum, dipole moment and quantum yield in different
solvent polarity. The bi-exponential fluorescence decay behavior of
MDPAPAO indicates that the triphenylamine moiety on MDPAPAO
can enhance its ICT character as well as increase its fluorescence
quantum yield. In addition, C60 could efficiently quench the fluo-
rescence emission of MDPAPAO, inferring that C60 can inhibit the
ICT of MDPAPAO. The special ICT properties of MDPAPAO could
possibly facilitate its use as a laser dye, fluorescent probe and lumi-
nescent material in the many fields.
(a)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
1
9
500
600
700
Acknowledgements
Wavelength (nm)
The work described in this paper was supported by the National
Nature Science Foundation (Nos. 21175086 and21105060) and the
Science Foundation of Shanxi Province (2012011007-2).
(b)
600
500
400
300
200
100
0
1.8
1.6
1.4
1.2
1.0
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.
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New 2-phenyloxazoles MMPAO, MOPAO, MDMAPAO, and
MDPAPAO have been successfully synthesized and their photo-

262
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