Ground and excited state nonlinear optical properties of poly(-para phenylene vinylene)

We investigate theoretically the nature of higher energy excited even as well as odd parity states, for various sizes of oligomers of PPV. We study in detail various nonlinear optical properties, like third harmonic generation (THG), two-photon absorption (TPA), electro absorption (EA) of oligomers of PPV within a rigid band correlated Pariser-Parr-Pople (PPP) model Hamiltonian, using the pi-electron basis and the powerful multireference single and double configuration interaction (MRSDCI) method. We also use single-double-triple-quadruple configuration interaction (SDTQCI) in limited cases. We show that in THG three kinds of one-photon states 1B(u), jB(u) and nB(u) (where J < n) whereas three kinds of even parity states 2A(g), mA(g) and kA(g) appear. The even parity states 2A(g), mA(g) and kA(g) also appear in TPA. While mA(g) absorptions appear just after the nBu, the 2Ag appears immediately after the jB(u) absorptions. A detailed analysis of the wavefunctions of higher energy excited states with conjugation length are presented. We show that beyond certain conjugation length both mAg and kAg states mainly involve excitations between the lowest delocalized valance and conduction bands. Our study indicate middle order exciton binding energy. Finally, we theoretically predict that by optically exciting PPV at one wavelength equivalent to the optical gap for a sufficiently long time to perform nonlinear optical measurement, the values Of X(3) can be enhanced by three orders-of-magnitude. (C) 2008 Elsevier B.V. All rights reserved.