Electron correlation and photo physics of phenyl substituted polyacetylenes

We investigate theoretically photoluminescence (PL) properties of mono and di-phenyl substituted trans-polyacetylene (t-PA), namely, poly-phenylacetylene (PPA) and polydiphenylacetylene (PDPA), respectively. PL is a consequence of the occurrence of the two-photon state (2A_g) above the optically excited state (1B_u). Usually, electron correlation leads to confinement of 1B_u as well as the ground state. We show, in contrast, that in phenyl-substituted polyacetylenes electron-electron interactions cause enhanced delocalization of quasiparticles in the optically excited state from the backbone polyene chain into the phenyl groups. This coulomb enhanced delocalization in the transverse direction leads to confinement in the longitudinal direction and causes crossover between the 1B_u and 2A_g. We further show that in the absence of electron repulsion the low energy absorption is x-polarized whereas the high energy absorption is predominantly y-polarized. In contrast, in presence of many body electron interaction both of them (low and high energy absorptions) are x-polarized. Thus photophysics of PPA/PDPA etc. are consequences of true many body effects.