Electrochromism of an aggregating thiapyrylium dye

Electrochromism (electric-field-modulated absorption) spectra are reported for a thiapyrylium dye in a polycarbonate host. When the dye is homogeneously dissolved in the polymer, excitation to the first excited singlet state changes its dipole moment by approximately 6 D (2×10−29 C m). When the dye is aggregated with the host polymer, the structure of the aggregate is believed to consist of layer of the dye associated with layers of the polymer. The electrochromism spectrum indicates that the aggregate possesses two (or more) excitonic states separated by ∼450 cm−1, of which only the lower contributes to the absorption spectrum in the absence of an electric field; application of a field mixes and shifts these states and transfers intensity between them. The near degeneracy is believed to be a consequence of weak Davydov interactions between the exciton states of pairs (or larger stacks) of dye layers.