On the nature of electronic excitations in poly(paraphenylenevinylene): A quantum-chemical investigation

Correlated quantum-chemical calculations are performed on phenylenevinylene oligomers containing up to eleven repeat units, to characterize the nature of the electronic excitations relevant for the photophysical properties of the corresponding polymer. The focus is first on the nonlinear optical res ponse of model conjugated chains and the simulation of their frequency-depe ndent (third-harmonic generation, electroabsorption, and two-photon absorpt ion) response. From the assignment of the calculated resonance features, th e excited states dominating the third-order nonlinear polarizability are id entified and their chain-length dependence is investigated. On that basis, we build an essential-state single-chain model (that includes the 1B(u), 2A (g), mA(g), and nB(u) states) and apply it to the interpretation of recent experimental data reported for poly(paraphenylenevinylene) and derivatives. We then examine how the exciton binding energy, here defined as the differ ence between the energies of the charge-separated nB(u) and the strongly op tically allowed 1B(u) excited states, is affected by both intrachain and in terchain polarization effects. (C) 1999 American Institute of Physics. [S00 21-9606(99)30230-0].