Synthesis and Photophysics of Thioindigo Diimines and Related Compounds

We report the synthesis and comprehensive
characterization of diamine
and diimine derivatives of the fluorescent compound thioindigo. Diamines 1 were obtained by metal-mediated amine condensation reaction
with thioindigo. Oxidation of the products of the coupling reaction
provided the diimines 2. X-ray crystal structures, cyclic
voltammetry, and spectral and photophysical data of the compounds
are presented. X-ray crystal structures demonstrate a planar structure
for the diimine derivatives and a twisted conformation for the diamines.
The diamine compounds 1 absorb in the UV (λmax 324–328 nm), significantly blue-shifted from the
absorption spectrum of thioindigo. Diamines 1 exhibit
moderate fluorescence (ΦF = 0.25, 0.045). A transient
triplet state is observed in laser flash photolysis (LFP) experiments
of 1, with lifetimes 1 order of magnitude longer than
those of thioindigo. The diimine compounds 2 absorb at
longer wavelengths (λmax 495–510 nm) than
the diamines but are still slightly blue-shifted from thioindigo,
with molar extinction coefficients 17–70% higher compared to
thioindigo. The diimine compounds are not emissive, and LFP studies
indicate transient species with microsecond lifetimes. Quenching experiments
and transient absorption spectra are consistent with trans–cis
isomerization.