Squarilium dyes, their infrared and resonance Raman spectra and possible use in redox reactions for solar-energy conversion

Infrared and resonance Raman spectra of squarilium dyes I–VI of the type [graphic omitted] are presented and some vibrational modes of the central 4-membered ring are assigned. Methyl viologen dication (MV2+) is reduced photocatalytically by IV in DMSO + H2O (95/5, v/v) with a quantum yield ϕ of (1.8 ± 0.4)× 10–3 at 406.7 nm, if EDTA is present. Oxidation products of MV2+ behave similarly with ϕ=(4.7 ± 0.6)× 10–2 at 457.9 nm in DMF + H2O (2/1, v/v). IV˙+ seems to oxidize H2O nearly reversibly.