OFF-DIAGONAL INTERACTIONS, BOND DENSITY CORRELATION, AND THEIR EFFECTS ON THE EXCITONS IN CONJUGATED POLYMERS

The effects of electron-electron interactions with both diagonal and o ff-diagonal parts on the excitons in conjugated polymers are studied, and it is found that the bond-charge interaction W and the bond-site i nteraction X affect the excitons oppositely: the former suppresses the excitation energy of excitons whereas the latter increases it except for the 2A(g) state. We find that the screening (originating from the bond density correlation effect of pi electrons), which controls the b ond correlation, is a reason that the binding energy of the exciton is reduced. Our calculation shows that the off-diagonal interactions aff ect the singlet exciton with small binding energy even at normal scree ning, but for large exciton energy, such effects are negligible.