Substituent effect on photophysical properties of bi-1,3,4-oxadiazole derivatives in solution

Article abstract of DOI:10.1016/j.molstruc.2016.01.012

A series of phenyl substituted bi-1,3,4-oxadiazole derivatives were designed and synthesized; the effect of substituent on the photophysical properties and molecular electronic structures was fully studied by the combination of experimental techniques and theoretical calculations. Compared to parent compound without any substituent (BOXD), fluoro-substituent shows little effect on the absorption and emission spectra, whilst a little larger spectral red-shift could be observed for methoxy-, nitro-substituted derivatives and thienyl-substituted bi-1,3,4-oxadiazole (TBOXD). These spectral changes can be well explained by theoretically calculated HOMO and LUMO energy level changes. All these molecules show high fluorescence quantum yield except for nitro-substituted derivative in dilute solutions. The quantum yield of BOXD changes with the concentration and exhibits a high value at the concentrated solution. This work revealed the influence of substituent on the photophysical properties of bi-1,3,4-oxadizaole derivatives in dilute solutions and provided guidance for designing molecules with potential application.

Full text of DOI:10.1016/j.molstruc.2016.01.012

Journal of Molecular Structure 1109 (2016) 239e246
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Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Substituent effect on photophysical properties of bi-1,3,4-oxadiazole
derivatives in solution
,
,
, b, *
Fangyi Chen a 1, Taiji Tian a 1, Chengxiao Zhao a, Binglian Bai c, Min Li a, Haitao Wang a
a Key Laboratory of Automobile Materials (MOE) & College of Materials Science and Engineering, Jilin University, Changchun 130012, China
b Institute of Theoretical Chemistry, Jilin University, Changchun 130023, China
c College of Physics, Jilin University, Changchun 130012, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of phenyl substituted bi-1,3,4-oxadiazole derivatives were designed and synthesized; the effect
of substituent on the photophysical properties and molecular electronic structures was fully studied by
the combination of experimental techniques and theoretical calculations. Compared to parent compound
without any substituent (BOXD), fluoro-substituent shows little effect on the absorption and emission
spectra, whilst a little larger spectral red-shift could be observed for methoxy-, nitro-substituted de-
rivatives and thienyl-substituted bi-1,3,4-oxadiazole (TBOXD). These spectral changes can be well
explained by theoretically calculated HOMO and LUMO energy level changes. All these molecules show
high fluorescence quantum yield except for nitro-substituted derivative in dilute solutions. The quantum
yield of BOXD changes with the concentration and exhibits a high value at the concentrated solution.
This work revealed the influence of substituent on the photophysical properties of bi-1,3,4-oxadizaole
derivatives in dilute solutions and provided guidance for designing molecules with potential application.
© 2016 Elsevier B.V. All rights reserved.
Received 30 September 2015
Received in revised form
8 January 2016
Accepted 9 January 2016
Available online 11 January 2016
Keywords:
Bi-1, 3, 4-oxadiazole derivatives
Photophysical properties
Substituent effect
DFT
CAM-B3LYP
1. Introduction
materials such as fluorescence and charge carrier mobility are not
only decided by the molecular electronic structure but also by the
In recent years, organic fluorescent materials have been studied
extensively, not only to explore their fundamental optical and
electrical properties, but also to identify their potential applications
in optoelectronic field, such as organic light-emitting diodes
(OLED) [1e5], organic light-emitting field-effect transistors (OLE-
FETs) [6e10], organic solid-state lasers [11e15] and organic fluo-
rescent sensors [16e19]. Although there have been a lot of reports
focusing on the organic fluorescent materials, many problems are
still not solved. For instance, most molecules show a high fluores-
cence yield in their dilute solutions while exhibit an opposite sit-
uation in aggregated or condensed state [20e22]. It is reported that
the fluorescence quenching may be caused by certain interactions
among the adjacent molecules [22e24]. Moreover, some molecules
have high charge carrier mobility for the existence of large extent of
packing modes and orientation [22,26,27]. How the molecular
structure influences the photophysical properties and the mecha-
nism is yet to be clear, which seriously restrict the applications of
the organic light-emitting materials. In order to explore these
problems, much more systematic research should be done. Thor-
oughly understanding the substituent effect is the basis to obtain
high efficient solid state light-emitting materials and provides
guidance for the molecular design of organic light-emitting
materials.
Among the various
p-conjugated systems, the phenyl
substituted 1,3,4-oxadiazole (OXD) unit is an ideal mode to study
the photophysical properties due to their tunable energy gap, high
fluorescence quantum yield, good thermal and chemical stabilities
[28e31]. However, systematically exploring the influence of sub-
stituent on the photophysical properties, the solid state molecular
packing, and further more the solid state light-emission properties
of 1,3,4-oxadiazole derivatives are still missing. Recently, we have
designed and synthesis a series of phenyl substituted bi-1,3,4-
oxadiazole (OXD) derivatives, where both the type and the posi-
tion of the substituents are widely changed (Scheme 1). Here we
will focus on the synthesis and photophysical properties of these
p-p interactions but show a very low fluorescence [25]. It is difficult
to achieve the high fluorescence and the high charge carrier
mobility simultaneously. The properties of organic light-emitting
* Corresponding author.
E-mail address: haitao_wang@jlu.edu.cn (H. Wang).
These authors contributed equally to this work.
1
http://dx.doi.org/10.1016/j.molstruc.2016.01.012
0022-2860/© 2016 Elsevier B.V. All rights reserved.

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F. Chen et al. / Journal of Molecular Structure 1109 (2016) 239e246
Scheme 1. Synthetic routes to a series of bi-1,3,4-oxadiazole derivatives.
bi-1, 3, 4-oxadiazole derivatives in dilute solution. In order to get
deep insight into the nature of the photophysical properties,
experimental study and theoretical calculations were carried out
jointly.
0.011 mol) were added. Yield >70%, melting point: 273e275 ꢀC. 1H
NMR (300 MHz, DMSO), (ppm, from TMS): 8.17 (d, J ¼ 6.82 Hz, 2H),
7.72 (m, J ¼ 8.04 Hz, 3H). FT-IR (KBr, pellet, cmꢁ1): 3434, 3064,
2923, 2853, 2350, 1621, 1602, 1585, 1540, 1476, 1467, 1415, 1399,
1334, 1306, 1286, 1261, 1151, 1081, 1067, 1027, 993, 953, 907, 782,
707, 686. Elemental analysis: Found: C, 66.55; H, 3.34; N, 19.33.
Calc. for C16H8N6O6: C, 66.20; H, 3.47; N, 19.30%.
2. Experimental
2.1. Molecular design and synthesis
2.1.2. 2,2-bis(2-fluorophenyl)-bi-1,3,4-oxadiazole(BOXD-o-F)
2-Fluorobenzoyl hydrazine (2.43 g, 0.016 mol) and oxalyl chlo-
ride (0.9 g, 0.007 mol) were added. Yield >70%, melting point:
215e217 ꢀC. 1H NMR (300 MHz, CDCl3), (ppm, from TMS): 8.22 (t,
J ¼ 8.12 Hz, 1H), 7.64 (ddd, J ¼ 7.30, 5.07, 1.58 Hz, 1H), 7.35 (td,
J ¼ 15.54, 8.30 Hz, 1H). FT-IR (KBr, pellet, cmꢁ1): 3434, 3084, 2921,
1614,1588,1541, 1468, 1436, 1269, 1231, 1161,1153, 1120, 1062, 1085,
995, 952, 874. Elemental analysis: Found: C, 59.24; H, 2.36; N, 17.33.
Calc. for C16H8F2N4O2: C, 58.90; H, 2.47; N, 17.17%.
In designing a molecule, various substituents (-C4H3S (thio-
phene), -F, -OCH3, -NO2) with a range of electron-withdrawing and
electron-donating properties and different substituted positions
(Ortho-, o-; Meta-, m-; Para-, p-) were selected to assess their ef-
fects on these molecules (the molecular structures were shown in
Scheme 1). In nomination, -F, -OCH3 and -NO2 indicate the sub-
stituents in the phenyl rings, -o-, -m-, and -p- indicate the
substituted position, for example BOXD-o-F indicates that the
phenyl ring in BOXD was ortho-substituted by -F; BOXD-6 [32] is a
molecule that was para-substituted by -OC6H13 in BOXD; TBOXD
represents a thienyl-substituted bi-oxadiazole derivative. All these
compounds were obtained through the synthetic route in Scheme
1a, except for TBOXD in Scheme 1b. As shown in Scheme 1,
firstly, the oxalyl chloride was slowly dropped into 50 mL tetra-
hydrofuran (THF) solution of the corresponding hydrazide deriva-
tive. The mixture was stirred at room temperature for about 10 h
and the resulting solution was filtered, yielding oxalyl acid dihy-
drazide. The crude product was purified by washing with boiling
ethanol. Secondly, oxalyl acid dihydrazide was dissolved in 50 mL
phosphorous oxychloride (POCl3). The mixture was stirred and
heated to reflux for about 40 h. After the reaction, the resulting
solution was cooled to room temperature and poured into 1000 mL
ice water. At last, the product was collected by filtration and
recrystallized from ethanol or dimethyl sulfoxide (DMSO) for
further 1H NMR, FT-IR measurements, and elemental analysis. The
original 1H NMR spectra of these bi-oxadiazole derivatives can be
found in the supporting information (Figs. S1eS7).
2.1.3. 2,2-bis(3-fluorophenyl)-bi-1,3,4-oxadiazole (BOXD-m-F)
3-Fluorobenzoyl hydrazine (2.45 g, 0.016 mol) and oxalyl chlo-
ride (1.0 g, 0.008 mol) were added. Yield >70%, melting point:
214e216 ꢀC. 1H NMR (300 MHz, DMSO), (ppm, from TMS): 8.01 (dd,
J ¼ 17.34, 8.42 Hz, 2H), 7.76 (dd, J ¼ 14.03, 7.79 Hz, 1H), 7.62 (t,
J ¼ 8.49 Hz, 1H). FT-IR (KBr, pellet, cmꢁ1): 3434, 2921, 2852, 2316,
1617, 1593, 1545, 1485, 1466, 1424, 1383, 1335, 1314, 1292, 1274,
1208, 1164, 1145, 1090, 1065, 1010, 1002, 968, 954, 923, 878, 869,
833, 806, 728, 677. Elemental analysis: Found: C, 59.27; H, 2.39; N,
17.29. Calc. for C16H8F2N4O2: C, 58.90; H, 2.47; N, 17.17%.
2.1.4. 2,2-bis(4-fluorophenyl)-bi-1,3,4-oxadiazole (BOXD-p-F)
4-Fluorobenzoyl hydrazine (2.44 g, 0.016 mol) and oxalyl chlo-
ride (1.0 g, 0.008 mol) were added. Yield >70%, melting point:
275e278 ꢀC.1H NMR (300 MHz, DMSO), (ppm, from TMS): 8.24 (dd,
J ¼ 8.62, 5.38 Hz, 2H), 7.54 (t, J ¼ 8.78 Hz, 2H). FT-IR (KBr, pellet,
cmꢁ1): 3427, 2920, 2850, 1608, 1556, 1492, 1476, 1299, 1287, 1225,
1159, 1099, 1083, 1011, 953, 847, 828, 813, 786, 737, 696. Elemental
analysis: Found: C, 59.24; H, 2.51; N, 17.25. Calc. for C16H8F2N4O2: C,
58.90; H, 2.47; N, 17.17%.
2.1.1. 2,2-bis(phenyl)-bi-1,3,4-oxadiazole (BOXD)
Benzoyl hydrazine (3 g, 0.022 mol) and oxalyl chloride (1.4 g,

F. Chen et al. / Journal of Molecular Structure 1109 (2016) 239e246
241
2.1.5. 2,2-bis(2-methyloxyphenyl)-bi-1,3,4-oxadiazole (BOXD-o-
OCH3)
2-Methoxybenzoyl hydrazine (2.02 g, 0.012 mol) and oxalyl
and the standard 6-31G** basis set was employed.
3. Results and discussion
chloride (0.75 g, 0.006 mol) were added. Yield >70%, melting
point:193e196 ꢀC.1H NMR (300 MHz, DMSO), (ppm, from TMS):
8.00 (dd, J ¼ 7.76, 1.67 Hz, 1H), 7.70 (ddd, J ¼ 8.95, 7.46, 1.73 Hz, 1H),
7.35 (d, J ¼ 8.39 Hz, 1H), 7.21 (t, J ¼ 7.91 Hz, 1H), 3.97 (s, 3H). FT-IR
(KBr, pellet, cmꢁ1): 3434, 3084, 2950, 2843, 1604, 1583, 1498, 1473,
1452,1438,1328,1286,1265,1184,1164,1135,1081,1058,1047,1010,
949, 857, 798, 777, 767, 749, 704, 671, 583, 533. Elemental analysis:
Found: C, 62.01; H, 3.83; N, 16.07. Calc. for C18H14N4O4: C, 61.71; H,
4.03; N, 15.99%.
3.1. Spectroscopic studies
The photophysical properties of all these compounds were
measured in the tetrahydrofuran (THF) solution with the concen-
tration of 1 ꢂ 10ꢁ5 M except BOXD-p-NO2 in 1 ꢂ 10ꢁ6 M. As shown
in Fig. 1a and Table 1, the parent compound BOXD, which is without
any substituent, shows an intense absorption at 297 nm. The
fluoro-substituted homologs show little changes in absorption
spectra compared with BOXD: the absorption maximum (labs) is
295, 297 and 297 nm for BOXD-o-F, BOXD-m-F, and BOXD-p-F,
respectively. The absorption band width of BOXD-m-F, and BOXD-
p-F is equivalent to BOXD, while in the case of BOXD-o-F, it becomes
much wider. Nitro- and methoxy-substituents have a momentous
impact on the absorption spectra. For BOXD-p-NO2, it red-shifts
about 17 nm in wavelength (labs ¼ 314 nm). For methoxy-
substituted derivatives, the para-substituted one (BOXD-6) [32]
can make a red-shift by about 23 nm, and ortho-substituted one
(BOXD-o-OCH3) shows two absorption bands, one is close to
318 nm and the other one is at 284 nm. Replacing the benzene with
2.1.6. 2,2-bis(4-nitrophenyl)-bi-1,3,4-oxadiazole (BOXD-p-NO2)
4-Nitrobenzoyl hydrazine (2.57 g, 0.014 mol) and oxalyl chloride
(0.9 g, 0.007 mol) were added. Yield >70%, melting point
>320 ꢀC.1H NMR (300 MHz, DMSO), (ppm, from TMS): 8.51 (d,
J ¼ 8.67 Hz, 2H), 8.44 (d, J ¼ 8.68 Hz, 2H). FT-IR (KBr, pellet, cmꢁ1):
3434, 3107, 2923, 2853, 2351, 1954, 1609, 1549, 1529, 1498, 1482,
1472, 1413, 1383, 1342, 1313, 1291, 1259, 1161, 1114, 1081, 1012, 998,
954, 868, 855, 761, 713, 673, 666, 626. Elemental analysis: Found: C,
50.92; H, 2.02; N, 22.12. Calc. for C16H8N6O6: C, 50.54; H, 2.12; N,
22.10%.
2.1.7. 2,2-bis(thienyl)-bi-1,3,4-oxadiazole (TBOXD)
2-Thiophenhydrazine (2.5 g, 0.018 mol) and oxalyl chloride
(1.11 g, 0.009 mol) were added. Yield >70%, melting point:
268e271 ꢀC.1H NMR (300 MHz, DMSO), (ppm, from TMS): 8.09 (d,
J ¼ 4.94 Hz, 1H), 8.05 (s, J ¼ 3.13 Hz, 1H), 7.38 (t, J ¼ 7.12, 1H). FT-IR
(KBr, pellet, cmꢁ1): 3430, 3114, 2924, 2853, 1732, 1627, 1581, 1484,
1471, 1433, 1417, 1343, 1322, 1261, 1228, 1151, 1078, 1067, 1032, 953,
940, 862, 852, 803, 751, 723, 669, 654. Elemental analysis: Found: C,
47.31; H, 1.87; N, 18.43; S, 21.26. Calc. for C12H6N4O2S2: C, 47.67; H,
2.00; N, 18.53; S, 21.21%.
2.2. Characterizations and computational details
1H NMR spectra were recorded with a Mercury-300BB 300 MHz
spectrometer, using CDCl3 or DMSO as a solvent and tetrame-
thylsilane (TMS) as an internal standard (
absorption spectra were recorded on a Shimazu UV-2550 spec-
trometer, and photoluminescence was measured on Per-
d
¼ 0.00 ppm). UVevis
a
kineElmer LS 55 spectrometer. The room-temperature
luminescence quantum yields in solutions were determined rela-
tive to quinine sulfate acid aqueous solution (0.546) and calculated
according to the following equation: Funk
¼
Fstd (Iunk/Aunk)(Astd/
Istd)(hunk
/
hstd)2, where Funk is the radiative quantum yield of the
sample; Fstd is the radiative quantum yield of the standard; Iunk and
Istd are the integrated emission intensities of the sample and
standard, respectively; Aunk and Astd are the absorptions of the
sample and standard at the excitation wavelength respectively;
hunk and hstd are the indexes of refraction of the sample and stan-
dard solutions (pure solvents were assumed), respectively. FTIR
spectra were recorded with a PerkineElmer spectrometer (Spec-
trum One B) and the sample was in the form of a pressed tablet
with KBr.
In theoretical calculations, the ground state geometry of each
molecule was optimized at density functional theory (DFT) level;
and the excitation energy of these compounds was calculated at
time-dependent DFT level. Considering the charge transfer char-
acter might be involved in this system, a range-separated hybrid
functional (CAM-B3LYP) was applied. In order to compare with
experimental observations, the excitation energy was computed in
tetrahydrofuran with a PCM model. All these calculations were
carried out with Gaussian 09 software package (version A.02) [33]
Fig. 1. a): Normalized UVevis absorption spectra of bi-1,3,4-oxadiazole derivatives in
THF solution. b): Normalized fluorescence spectra of 1,3,4-oxadiazole derivatives in
THF solution.

242
F. Chen et al. / Journal of Molecular Structure 1109 (2016) 239e246
Table 1
conformers in solution. These results could support our hypothe-
sis that the existence of two different conformers is responsible for
the feature of double or broader absorption peak in ortho-
substituted derivatives. In addition, we have tried to growth sin-
gle crystal in different conditions to obtain different conformers,
but so far only the twist conformer in which the dihedral angle
between the benzene ring and the oxadiazole ring is 10.53ꢀ has
been observed for BOXD-o-OCH3 (Figure S8 a); while planar
conformer can be found in other compounds including BOXD-o-F
(Figure S9 b) [34,35].
Photophysical characteristics of all the bi-1,3,4-oxadiazole derivatives.
Compound
labs (nm)a
lem (nm)b
lex (nm)c
F
(%)d
BOXD
297
295
297
297
284,318
320
322
365
365
364
366
377
386
385
395
305
303
303
303
324
320
332
310
73.5
78.9
79.8
61.9
92.5
84.6
74.9
24.7
BOXD-o-F
BOXD-m-F
BOXD-p-F
BOXD-o-OCH3
BOXD-6
TBOXD
BOXD-p-NO2
314
The fluorescence spectra of these 1,3,4-oxadiazole derivatives
are shown in Fig. 1b and all of them were measured at their
maximum excitation wavelength, respectively. BOXD shows an
intense emission at 365 nm. Similarly to the absorption spectra,
fluoro-substituent also has little effect on the fluorescence emission
spectra. The emission maxima (lem) are 365, 364 and 366 nm for
BOXD-o-F, BOXD-m-F and BOXD-p-F, respectively, which are very
close to BOXD (lem ¼ 365 nm). The emission band width also can be
retained well in these fluoro-substituted derivatives. Nitro- and
methoxy-substituents also have higher impact on the fluorescence
emission spectra. For BOXD-p-NO2, it red-shifts about 30 nm in
wavelength (lem ¼ 395 nm). For methoxy-substituted derivatives,
it red-shifts about 21 nm in wavelength for the BOXD-6
a
absorption maximum in THF solution (1 ꢂ 10ꢁ5 M).
b
Emission maximum in THF soAlution (1 ꢂ 10ꢁ5 M) except BOXD-p-NO2 in 1 ꢂ
10ꢁ6 M.
c
Excitation maximum in THF solution (1 ꢂ 10ꢁ5 M) except BOXD-p-NO2 in 1 ꢂ
10ꢁ6 M.
d
Quantum efficiency in THF solution (1 ꢂ 10ꢁ6 M) except BOXD-6 in 5 ꢂ 10ꢁ6 M.
a thiophene ring can make a large red-shifts about 25 nm (TBOXD,
labs ¼ 322 nm).
The broad absorption band (BOXD-o-F) or double bands (BOXD-
o-OCH3) observed for these ortho-substituted derivatives may be
ascribed to the fact that there is a stable twist conformer besides
the planar conformer due to the steric effect. The higher-energy
absorption band might be assigned to those molecules with twist
conformation, while the lower-energy band is from the planar
conformation. This hypothesis can be further confirmed by variable
temperature UVevis absorption spectroscopic study and theoret-
ical calculations. Fig. 2 shows the non-normalized absorption
spectra of BOXD-o-OCH3 at variable temperature in THF solution
(~10ꢁ5 M) and the normalized result is shown in Fig. S8. It can be
clearly found that the intensity of the two peaks both increased
with the temperature increased, but the relative increase in in-
tensity at 284 nm is bigger than that at 318 nm. It supports our
hypothesis that the existence of two different conformers and the
planar conformation will gradually change to the twist conforma-
tion with the increasing temperature. The theoretical calculation
has shown that the global energy minimum conformation of BOXD-
o-OCH3 is a planar one (Fig. 3). In order to search for a twist
conformer as observed in the crystal structure (Figure S9 a), a rigid
scan on the twist angle between the benzene ring and the oxa-
diazole ring has been carried out with both M062x/6-31G**and
CAM-B3LYP/6-31G** methods. As shown in Fig. 3, the energy goes
up with the increasing torsion angle and yields a lowest energy at
both 0ꢀ and 150ꢀ, confirming the possibility to have two-type
(
lem ¼ 386 nm) and 12 nm for BOXD-o-OCH3 lem ¼ 377 nm).
(
TBOXD, which replaces the benzene with a thiophene ring, exhibits
a 20 nm red-shifts in its maximum (lem ¼ 385 nm). The maximum
emission peaks of all the derivatives in THF dilute solution
(~10ꢁ5 M) are summarized in Table 1.
All the derivatives show high quantum yields except nitro-
substituted derivative (BOXD-p-NO2) which displays a low quan-
tum yields (24.7%) in the THF dilute solution (~10ꢁ6 M). The
quantum yields are list in Table 1 (The photophysical parameters
used in calculation of the quantum yields are summarized in
Tables S2eS3). The value for the parent compound BOXD is 73.5%.
In fluoro-substituted derivatives, quantum yields (BOXD-o-F:
78.9%; BOXD-m-F: 79.8%; BOXD-p-F: 61.9%) have a comparable
value with BOXD and change with the position of fluoro-
substituent. BOXD-o-OCH3 exhibits the highest quantum yield
among these compounds (F: 92.5%) while for BOXD-6, the quan-
tum yield is about 84.6% in 5 ꢂ 10ꢁ6 M which is only lower than
BOXD-o-OCH3. TBOXD shows a moderate value of 74.9%. Thus, the
introduction of methoxy-substituents in ortho-position and fluoro-
substituents in both ortho- and meta-positions of benzene can
significantly improve the fluorescence quantum yield. What's
more, quantum yield of BOXD was found depend on the concen-
tration of solution. As Fig. 4 shows, the quantum yield of BOXD is
73.5% in 1 ꢂ 10ꢁ6 M and rapidly decreased with the concentration
increased which could be attributed to the fluorescence quenching
caused by the molecular aggregates. The smallest quantum yield is
25.0% in about 2 ꢂ 10ꢁ5 M. This point is obviously smaller than
others, which could be caused by the instrumental error. With the
concentration continue to increased, the quantum yield gradually
increased. When the concentration is about 8 ꢂ 10ꢁ4 M, the
quantum yield has the maximum value (66.0%) in concentrated
solution. This phenomenon is similar to that observed in BOXD-6
[36] (which were reported that it is caused by the transition of
different type of aggregates in different concentration solution.)
The final decrease (from 8 ꢂ 10ꢁ4 to 1 ꢂ 10ꢁ3 M) might be due to
serious re-absorption of these molecular aggregates observed in
the thick solution. BOXD shows such interesting concentration-
dependent properties and high photoluminescence quantum
yields in concentrated solution, which suggests that its possible
application as high quantum yield materials in device.
Fig. 2. UVevis absorption spectra of BOXD-o-OCH3 in THF solution (~10ꢁ5 M) at
different temperature.
We have also inspected the solvatochromic behavior of BOXD-o-
OCH3 in solvents of different polarity. The absorption and

F. Chen et al. / Journal of Molecular Structure 1109 (2016) 239e246
243
Fig. 3. The curve of relative energy and torsion angle of BOXD-o-OCH3 computed with M062x/6-31G**and CAM-B3LYP/6-31G** methods.
fluorescence emission spectra of BOXD-o-OCH3 are present in Fig. 5.
As shown in Fig. 5a, the spectrum from non-polar cyclohexane
(CHEX) exhibits the maximum absorbance intensity at 315 nm. The
ethanol (ETO) spectrum is highly red shift to 323 nm. The spectra in
tetrahydrofuran (THF), acetonitrile (ACN) and chloroform (CHL) lie
between those in CHEX and ETO. There is only 8 nm red-shift found
indicating that the electronic and structural nature of BOXD-o-
OCH3 in the ground state and Franck-Condon excited state, which
are responsible for absorption, do not vary much during excitation.
The fluorescence spectra of BOXD-o-OCH3 in different solvents are
measured at its maximum excitation wavelength respectively. In
CHEX, it shows an intense emission at 364 nm with vibrational fine
structure. The fine structure is somewhat visible in solvents with
moderate polarity (THF and CHL), but it is totally lost in highly polar
solvents (ACN and ETO). It is found that the emission maxima in-
crease along with increasing of empirical solvent polarity param-
eter ENT and form linear relationship (Fig. S10). The emission spectra
shows a large red-shift (about 40 nm) in their maximum which
indicates that intramolecular charge transfer did not occur in the
photo excitation process, but occurred during the excited-state
molecular geometry relaxation in solutions and reorganization of
the solvent molecules. This point has also been demonstrated in
our previous work [32]. As the molecular dipole moment (or the
local dipole moment in our system) is larger than the ground state,
the energy gap between the excited state and the ground state will
become narrow, and the maximum emission peak would be red
shift.
Fig. 5. a): UVevis absorption spectra of BOXD-o-OCH3 (normalized) in different sol-
vents (~10ꢁ5 M). b): Fluorescence spectra of BOXD-o-OCH3 (normalized) in different
solvents (~10ꢁ5 M) (CHEX: cyclohexane; CHL: chloroform; THF: tetrahydrofuran; ACN:
acetonitrile; ETO: ethanol).
4. Theoretical calculations
4.1. Molecular geometries
In order to reveal the electronic structure and the natural pho-
tophysical properties of these bi-1,3,4-oxadiazole derivatives,
theoretical calculations are carried out at the density functional
theory level. These calculations predicted planar structure for all
Fig. 4. concentration-dependent photoluminescence quantum yields of BOXD in THF.

244
F. Chen et al. / Journal of Molecular Structure 1109 (2016) 239e246
these molecules indicating that oxadiazole ring shows great
conjugating ability with benzene ring. For the para-substituted
derivatives, there is only one stable conformer; while for the
ortho- and meta-substituted derivatives, two types of stable con-
formers could be obtained (Fig. S11, denoted as 1 and 2). The energy
differences between those different conformers are in the range of
1e8 kJ/mol. As for BOXD-o-F and BOXD-o-OCH3, the energy of the
conformer in which the substituent on the benzene ring is at the
same side with the oxygen atom in the oxazole ring (BOXD-o-F-1
and BOXD-o-OCH3-1) is lower than those at the different sides
(BOXD-o-F-2 and BOXD-o-OCH3-2); While for BOXD-m-F, the two
conformers share basically the same energy, indicating that the
ortho-substituents might have much more steric effect. The bond
length and twist angles in these bi-1,3,4-oxadiazole derivatives do
not change a lot (Table S1), which suggests that the different type of
substituents have very little influence on the conjugation between
benzene and oxadiazole rings.
fluro- and methoxy-substituted derivatives (0.07 eV and 0.23eV),
while a little bit larger for nitro-substituted derivatives (0.36 eV).
For TBOXD, the LUMO energy decreased by 0.15 eV and the HOMO
energy increased by 0.17 eV, so the energy gap decreased 0.31 eV.
4.3. Electronic state transitions
In order to get insight into the excited state properties of these
1,3,4-oxadiazole derivatives, time-dependent density functional
theory calculations were also carried out. The data for excitation
energy, oscillator strength and composition of the orbital transi-
tions are listed in Table 2. The calculated results reveal that S0eS1
transition is highly allowed in almost all the molecules except for
nitro-substituted derivative, in which S0eS1 transition is forbidden;
the mainly allowed transition is S0eS3 transition. So the one main
peak observed in the UVevis absorption spectra in BOXD, and fluro-
and methoxy-substituted derivatives can be attributed to the
transition from ground state to S1 state. As compared with the
experimental results listed in Table 2, the excitation energy for the
first excited by theoretical calculations gave satisfied agreement for
the energy difference is within 0.2 eV. An inspection of the
component orbital transitions makes it clear that the electron
transition from the ground state to the first excited state mainly
consisted of H/L transitions, with a little contribution from H-
1/Lþ1 or other transitions, so the absorption for S1 should be
4.2. Frontier orbitals
Fig. 6 presents the electronic density of frontier molecular or-
bitals of BOXD (others can be obtained in Fig. S12). As can be seen,
almost all these derivatives share the same characteristics of the
HOMO and LUMO orbitals. The HOMO and LUMO orbitals are of
bonding and anti-bonding character, respectively. Other occupied
orbitals, H-1~H-2, mainly consist of -bonding orbitals within each
aromatic ring (benzyl and 1,3,4-oxadiazole rings); on the contrary,
the unoccupied orbitals, Lþ1~Lþ2, are mainly consisted of the
antibonding orbitals. In addition, the wave functions are almost
delocalized over the whole -systems for orbitals HOMO and
p-
p
mainly assigned as a
only occupied a small percentage (such as H-4/L in BOXD) for
most of these molecules. Since the EH-L is basically the same for all
p-p* type transition. The n-p* type transition
p-
D
these molecules, it is not surprising to find that there is only small
variation observed in the absorption spectra. A little larger change
p
LUMO, while in other orbitals, the electron density are more or less
localized on the 1,3,4-oxadiazole unit or the benzene rings. Table 2
and Fig. 7 show the HOMO-LUMO energy levels and energy gap of
all these bi-1,3,4-oxadiazole derivatives. When compared with
BOXD, fluro-substituent shows little effect on the energy level of
these frontier orbitals: there are almost no changes in ortho- and
para-substituted derivatives (0e0.11 eV), but a little bit larger
change for meta-substituted molecules (0.21e0.25 eV). Nitro-
substituted derivative shows a significant decrease in both HOMO
and LUMO energies (1.18 eV and 0.82 eV). Different from others,
methoxy-substituted derivative increases the energy level of both
HOMO and LUMO. Since the HOMO and LUMO energy decreased or
increased simultaneously, the change of energy gap is very small for
of DEH-L for TBOXD and BOXD-p-NO2 (~0.3 eV) might be the reason
for the red shift in absorption spectra.
5. Conclusion
A series of bi-1,3,4-oxadiazole derivatives with different sub-
stituents were successfully synthesized in a facile rote. Our study
shows that the influences of different substituents are different
from each other. The molecules with nitro- and methoxy-
substituents as well as the thienyl group (TBOXD) exhibit a large
red-shift in the absorption and emission maximum compared with
BOXD, while the molecules with fluoro-substituent do not show
significant influence. All the compounds exhibit very high fluo-
rescence quantum yield in dilute solution except for nitro-
substituted molecule. What's more, quantum yield of BOXD
changes with the concentration and exhibits a high value at the
thick solution. Theoretical calculations can explain the experi-
mental phenomenon very well. According to the calculation results,
the excellent fluorescent properties of these bi-1,3,4-oxadiazole
derivatives may be attributed to the great rigidity of the phenyl
substituted bi-oxadiazole backbones and the highly allowed
p-p*
type transition from the ground state to the first singlet excited
state. Compared with fluro-substituent, which shows very little
influence on the energy levels of HOMO and LUMO, methoxy- and
nitro-substituents show a little larger effect, however, the changes
of energy gap are still not quite distinct due to the synchronic
change of HOMO and LUMO energy. Differently, replacement of the
phenyl ring by thiophene ring can decrease the LUMO energy,
while increase the HOMO energy so that the energy gap shrink
much more obviously. This combinational theoretical and experi-
mental study has revealed the detail mechanism of the substituent
effect on the electronic structure and spectra, providing an effective
guidance to finely tune the molecular energy levels and energy gap,
and further the photophysical properties.
Fig. 6. Electron density diagrams of molecular orbitals of BOXD computed with CAM-
B3LYP/6-31G** method.

F. Chen et al. / Journal of Molecular Structure 1109 (2016) 239e246
245
Table 2
Excitation energy, oscillator strength and calculated HOMO-LUMO energy states of all the bi-1,3,4-oxadiazole derivatives.a
Compound
EL (eV)
EH (eV)
△EH-L (eV)
Excitation energy
(eV)
f
Main component of
transition
E (kJ/mol)b
Calc.
4.25
Exp.
4.18
BOXD
ꢁ0.87
ꢁ0.91
ꢁ0.98
ꢁ7.80
ꢁ7.80
ꢁ7.90
6.93
1.35
1.41
1.36
H-4/ L
2.1%
7.5%
88.1%
2.9%
5.1%
87.9%
2.3%
5.7%
87.4%
6.5%
86.2%
2.0%
3.1%
4.7%
87.3%
2.5%
7.3%
87.8%
2.9%
15.0%
75.4%
2.9%
12.5%
79.2%
10.8%
85.5%
2.1%
ꢁ2584605.1
ꢁ3105599.7
ꢁ3105595.7
H-1 / Lþ1
H / L
BOXD-o-F-1
BOXD-o-F-2
6.90
6.92
4.28
4.27
4.21
4.21
H-3 / Lþ1
H-1 / Lþ1
H / L
H-3 / Lþ
H-1 / Lþ1
H / L
BOXD-m-F-1
BOXD-m-F-2
ꢁ1.11
ꢁ1.11
ꢁ8.01
ꢁ8.05
6.90
6.94
4.27
4.29
4.18
4.18
1.36
1.34
H-1 / Lþ1
ꢁ3105616.2
ꢁ3105616.6
H / L
H-4 / L
H-3 / Lþ1
H-1 / Lþ1
H / L
H-4 / L
H-1 / Lþ1
H / L
H-2 / L
H-1 / Lþ1
H / L
H-2 / L
H-1 / Lþ1
H / L
BOXD-p-F
ꢁ0.96
ꢁ0.44
ꢁ0.59
ꢁ7.82
ꢁ7.40
ꢁ7.29
6.86
6.96
6.70
4.24
4.19
4.03
4.18
4.37
3.90
1.34
0.91
1.26
ꢁ3105622.5
ꢁ3185545.0
ꢁ3185538.0
BOXD-o-OCH3-1
BOXD-o-OCH3-2
TBOXD
ꢁ1.02
ꢁ2.05
ꢁ7.63
ꢁ8.62
6.61
6.57
3.97
4.04
3.85
3.95
1.27
1.75
H-1 / Lþ1
ꢁ4269254.5
ꢁ3658067.1
H / L
BOXD-p-NO2
H-8 / Lþ2
H-1 / Lþ1
H / L
H / Lþ2
14.8%
74.3%
6.8%
a
As for BOXD-p-NO2, electron transitions from the ground state to the first and second singlet excited state are forbidden, so the data presented in the table were the result
of electron transition from the ground state to the third single excited state.
b
The sum of electronic and zero-point energies in kJ$molꢁ1 calculated with CAM-B3LYP/6-31G** method.
Fig. 7. HOMO-LUMO energy states of all the bi-1,3,4-oxadiazole derivatives (1: BOXD; 2: BOXD-o-F-1; 3: BOXD-o-F-2; 4: BOXD-m-F-1; 5: BOXD-m-F-2; 6: BOXD-p-F; 7: BOXD-o-
OCH3-1; 8: BOXD-o-OCH3-2; 9: TBOXD; 10: BOXD-p-NO2; Additional numbering (1 and 2) was involved to distinguish two stable conformers found in meta- and orth-substituted
derivatives: 1 stands that the substituent on the benzene ring is at the same side with the oxygen atom on the oxadiazole ring; 2 stands that the substituent on the benzene ring is at
the different side with the oxygen atom on the oxadiazole ring).
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