Photophysical and Electrochemical Investigations of the Fluorescent Probe, 4,4′-Bis(2-benzoxazolyl)stilbene

In solution, 4,4′-bis­(2-benzoxazolyl)­stilbene
(BBS) was
found to exhibit consistently high absolute fluorescence quantum yields
(Φfl ≥ 0.88) and a monoexponential lifetime,
both independent of BBS concentration. The BBS steady-state and time-resolved
photophysics were investigated by different techniques to understand
the various deactivation pathways. Nonradiative deactivation of BBS
singlet excited state by intersystem crossing was found to be negligible.
Other than fluorescence, the excited state of BBS was found to be
deactivated by trans–cis photoisomerization. At low concentrations (≈5 μg/mL),
UV spectroscopy and laser flash photolysis showed concordant results
that the photoinduced cis isomer
gradually replaced the original absorption spectrum of the pure trans isomer. However, at high concentrations (≈0.2
mg/mL), 1H NMR and DOSY measurements confirmed that irradiating
BBS at 350 nm induced a conversion from the trans-BBS into its cis isomer by photoisomerization.
It was further found that the stilbene moiety of both isomers was
photocleaved. The resulting photoproduct was an aldehyde that was
oxidized under ambient conditions to its corresponding carboxylic
acid, i.e., 4-(1,3-benzoxazol-2-yl)­benzoic acid. The structure of
the photoproduct was unequivocally confirmed by X-ray diffraction.
Spectroscopic investigation of BBS showed a limited photoisomerization
after irradiation at 350 nm of a trans solution.
The BBS electrochemistry showed irreversible oxidation, resulting
in an unstable and highly reactive radical cation. Similarly, the
cathodic process was also found to be irreversible, giving rise to
a radical anion and showing its n-doping character.