Merostabilization in radical ions, triplets, and biradicals. 6. The excited state behaviour of para-substituted tetraphenylethylenes

Abstract

The half-wave oxidation and reduction potentials, measured by cyclic voltammetry, of tetraphenylethylene and a series of para-substituted (cyano, methyl, and methoxy) derivatives are found to correlate with σ+ and σ− values respectively. The deviations from these correlations that are observed for one of the derivatives (unsymmetrically substituted) are attributed to merostabilization.The nature of the first excited singlet state of these compounds is investigated by spectroscopic and photochemical means. Quantum yields for the direct cis–trans photoisomerization demonstrate that this provides the dominant pathway for decay of the singlet state. The π,π* triplet energies are estimated from a qualitative investigation of the relative efficiencies of triplet-sensitized cis–trans isomerization as a function of sensitizer triplet energy.