R.Y. Iliashenko et al. / Journal of Photochemistry and Photobiology A: Chemistry 298 (2015) 68–77
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the same benzene ring. Having (semi) helical molecular shape they
undergo partial excited state planarization, which compresses
helix to definite extent resulting in their experimentally observed
abnormal fluorescence characteristics. Similar class of heteroar-
omatic compounds–ortho-analogs of the well-known organic
fluorophore widely applicable in liquid an plastic scintillating
devices, 1,2-bis(5-phenyl-1,3-oxazol-2-yl) benzene (POPOP) was
studied in our research team during last decades: their synthesis
[21,22] and X-ray structural analysis [23] was reported together
with molecular modeling, spectral data [24,25] and photophysical
experiments [26,27].
1.5 g (0.0057 mol) of 2-(20-carboxyphenyl)-5-phenyl-1,3-oxa-
zole [21,23] was boiled in 25 ml of thionyl chloride for 3 h until
deflation of hydrogen chloride was finished, then SOCl2 excess was
removed in vacuo. Resulting acyl chloride was dissolved in 25 ml of
pyridine and 0.37 g of hydrazine sulfate (0.003 mol) was added. The
reaction mixture was boiled for 4 h, and then poured into 250 ml of
cold distilled water. The deposited precipitate was filtered off,
washed with water and dried on air. Final cyclization was made at
its boiling in 25 ml of phosphorus oxychloride for 3 h, then cooled
reaction mixture was poured in 200 g of ice. Final compound was
purified by column chromatography (silica gel/benzene) and
crystallized from heptane.
The present communication is devoted to the new representa-
tive of the ortho-POPOP family composed of the increased number
of heteroaromatic units (7 except 5), 2,5-bis[2-(2-phenyl-1,3-
oxazol-5-yl) phenyl]-1,3,4-oxadiazole, compound 1:
2,5-bis[2-(2-phenyl-1,3-oxazol-5-yl) phenyl]-1,3,4-oxadiazole,
1, colorless needles, yield 0.94 g (63%), m.p. ꢀ152–154 ꢁC.
1H NMR (500 MHz, DMSO-d6):
d 8.17 (d, J = 7.8 Hz, 2H), 7.82 (t,
J = 7.7 Hz, 2H), 7.79 (d, 2H, coupling constant is hardly detectable
owing to overlap with the singlet peak at 7.78 ppm), 7.78 (s, 2H),
7.69 (t, J = 7.6 Hz, 2H), 7.58 (d, J = 7.8 Hz, 4H), 7.43 (t, J = 7.6 Hz, 4H),
7.37 (t, J = 7.3 Hz, 2H) ppm.
13C NMR (126 MHz, DMSO-d6)
d 164.57, 158.83, 151.81, 132.84,
131.48,131.34,130.02,129.54,129.35,127.43,126.95,124.67,124.44,
122.32 ppm.
Molecular structure of 1 was confirmed by X-ray structural
analysis (Table 1).
Enlargement of the potential
p-conjugation length is not the
The following computer software was used to treat the X-ray
data: CrysAlis CCD and CrysAlis RED (Oxford Diffraction, 2008),
SHELXS-2013 and SHELXL-2013 [34] for direct structure solving
and refinement, ORTEP-3 [35] and PLATON [36] for structure
visualization.
Atomic coordinates and crystallographic parameters have been
deposited to the Cambridge Crystallographic Data Center (CCDC
1,020,369).
only question to resolve while synthesizing and studying of
compound 1 spectral behavior and photophysics. The parent HSS-
fluorophore of this series, ortho-POPOP, is practically a low-
solvatochromic compound. Owing to the fact, that more electron-
withdrawing oxadiazole cycle is included in 1 molecule, we expect
intensification of the electron density redistribution during the
electronic excitation and thus – appearance of higher sensitivity to
solvent polarity.
NMR spectra were measured on Brucker Avance III 500 spec-
trometer.
Excited state planarization of the ortho-POPOP molecule
requires high-amplitude intramolecular motions, this determines
potential sensitivity of this compound also to media viscosity
[26,28]. The title fluorophore 1 possesses two ortho-substituted
benzene rings, thus its intramolecular rotors are characterized by
higher moments of inertia, so we expect for it more pronounced
sensitivity to viscosity in comparison to other ortho-POPOPs.
Traditionally, fluorescent monitoring of viscosity is based on
compounds with intramolecular rotor moieties [29–31], however
in most cases the proposed fluorophores (including commercially
available ones [32]) are highly sensitive to media polarity as well.
This makes fluorescent sensing of viscosity on their background
less reliable, than it could be in the case of compounds with lower
solvent polarity effects. Moreover, the above mentioned fluores-
cent viscosity probes realize the principles of intensometry and/or
lifetime sensing: intensity of their fluorescence increases in more
viscous surrounding without changing of color, simultaneous
increase demonstrate fluorescence lifetimes. Compound 1 was
expected to vary color of its fluorescence with viscosity, which
makes it more convenient for various analytical applications.
Electronic absorption and steady-state fluorescence spectra
were measured on Hitachi U-3210 spectrophotometer and Hitachi
850 spectrofluorimeter in rectangular quartz cells with the dye
solution layer thickness of 10 mm. Quinine sulfate in 0.5 M aqueous
sulfuric acid was used as reference standard for estimating
quantum yields [37].
Fluorescence decay and time-resolved fluorescence spectra
were measured on home-composed sub-nanosecond kinetic
spectrometer, consisting of a MDR-12 monochromator (LOMO,
Russia), a TimeHarp 200 TCSPC device, a PLS 340-10 picosecond
LED driven by a PDL 800-B device (PicoQuant GmbH, Germany)
and a Hamamatsu H5783P PMT (Hamamatsu, Japan).
N
Normalized Reichardt solvent polarity index ET [38] was
applied for qualitative characterization of the polarity-dependent
properties of the title compound.
3. Quantum-chemical modeling
Theoretical modeling was performed in DFT/TD-DFT schemes
using the b3lyp hybrid potential [39] with the cc-pvdz basis [40].
Calculations of the molecular geometry and electron density
redistribution upon electronic excitation of 1 were made with
Gaussian-09 (release B.01 [41] – for the ground state molecular
2. Materials and methods
Compound 1 was synthesized by the following general scheme: