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-phenylacetyl ene (PPA) and polydiphenylacetylene (PDPA), respectively. PL is a consequen ce of the occurrence of the two-photon state (2A(g)) above the optically ex cited 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-sub stituted polyacetylenes electron-electron interactions cause enhanced deloc alization of quasiparticles in the optically excited state from the backbon e polyene chain into the phenyl groups. This coulomb enhanced delocalizatio n in the transverse direction leads to confinement in the longitudinal dire ction and causes crossover between the 1B(u) and 2A(g). We further show tha t in the absence of electron repulsion the low energy absorption is x-polar ized whereas the high energy absorpion is predominantly y-polarized. In con trast, in presence of many body electron interaction both of them (low and high energy absorptions) axe x-polarized. Thus photophysics of PPA/PDPA etc . are consequences of true many body effects.