The HOF·CH3CN complex, easily prepared by
passing F2 through aqueous acetonitrile, is an
exceptionally efficient oxygen transfer agent. All types of
sulfides have been oxidized to sulfones
in excellent yields, in a few minutes at room temperature. The
reaction proceeds through the
formation of sulfoxides which at low temperatures of around −75 °C
could be isolated in good yields.
It was demonstrated through a reaction with thianthrene 5-oxide
(20) that HOF·CH3CN is
strongly
electrophilic in nature. Sulfides with an electron-depleted sulfur
atom such as perfluoroalkyl or
aryl sulfides, which could not be well oxidized by any other method,
were also efficiently converted
to the corresponding sulfones in minutes. Thiophenes are generally
hard to oxidize to the
corresponding S-dioxides since the conditions required by
the orthodox oxidants encourage
consecutive typical ene and diene reactions.
HOF·CH3CN requires short reaction times
and low
temperatures, thus enabling the isolation of thiophene dioxides, some
of which could not be made
by any other way. It seems that apart from unprotected amines,
other functional groups such as
aromatic rings, ketones, hydroxyls, and ethers do not interfere, since
the sulfur atom reacts
considerably faster.
Nonaromatic stable polyenes are obtained by dioxygenation of oligothiophenes using HOF⋅CH3CN prepared in situ by passing F2 through aqueous acetonitrile (see scheme). These fully S,S‐dioxygenated derivatives have a considerably smaller HOMO–LUMO gap than the parent compounds; furthermore, in the solid state they tend to form favorable π–π stacking interactions, unlike most non‐oxidized oligothiophenes which adopt the “herringbone” motif.
A comprehensive photophysical and spectroscopic (electronic and Raman) study guided by density functional theory, DFT, CIS, and correlated ab initio calculations has been performed on a series of fully oxidized oligothiophenes with variable chain length, and up to four rings. A comparison with the properties of oligoenes and oligothiophenes is proposed. Absorption, fluorescence, lifetimes, flash-photolysis, phosphorescence, two-photon absorption, Raman, resonance Raman, and thermospectroscopy data are collected and interpreted according to the obtained theoretical results. The interest is focused on the ground electronic state and in the low-lying excited electronic states. Full oxygenation of the sulfur atoms of oligothiophenes results in: (i) restricted inter-ring isomerization such as observed from the absorption spectra; (ii) an effective quenching of fluorescence, and (iii) the appearance of dual emission. The emission features are explained by the interference of a dipole-allowed and a dipole-forbidden singlet excited states leading to simultaneous lighting from a local Frenkel and an intramolecular charge transfer photon-releasing configurations. These two excited states contribute to the broadening of the light emission spectrum. These properties highlight the similarity of these samples to that of oligoenes with comparable number of π-electrons.
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