Electron correlation induced transverse delocalization and longitudinal confinement in excited states of phenyl-substituted polyacetylenes

摘要

Electron-electron interactions in general lead to both ground state andexcited state confinement. We show, however, that in phenyl-substitutedpolyacetylenes electron-electron interactions cause enhanced delocalization ofquasiparticles in the optically excited state from the backbone polyene chaininto the phenyl groups, which in turn leads to enhanced confinement in thechain direction. This co-operative delocalization--confinement lowers theenergy of the one-photon state and raises the relative energy of the lowesttwo-photon state. The two-photon state is slightly below the optical state inmono-phenyl substituted polyacetylenes, but above the optical state indi-phenyl substituted polyacetylenes, thereby explaining the strongphotoluminescence of the latter class of materials. We present a detailedmechanism of the crossover in the energies of the one- and two-photon states inthese systems. In addition, we calculate the optical absorption spectra over awide wavelength region, and make specific predictions for the polarizations oflow and high energy transitions that can be tested on oriented samples. Withinexisting theories of light emission from $\pi$-conjugated polymers, strongphotoluminescence should be restricted to materials whose optical gaps arelarger than that of trans-polyacetylene. The present work show thatconceptually at least, it is possible to have light emission from systems withsmaller optical gaps.