Excited states absorptions in oligomers of PPV

Abstract

Recent experimental studies based on pump-probe spectroscopy of phenyl-based polymers reveal quite different relaxation dynamics of low and high energy even parity states reached by photoinduced absorption (PA). One class of even parity states ( mA g) experiences ultrafast internal conversion to the lowest singlet state (1B u), whereas the other class ( kA g) in violation of the Vavilov–Kash's rule undergoes a different pathway. We therefore investigate theoretically the nature of higher excited states, both even as well as odd parity states, for various sizes of oligomers of poly(para-phenylene vinylene) (PPV). We study Third Harmonic Generation (THG) of oligomers of PPV within a rigid band correlated Pariser–Parr–Pople (P–P–P) model Hamiltonian, using the π-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 reconfirm the nature of ( mA g) states, which are responsible for the low energy PA band. We also show that in THG three kinds of one-photon states 1B u, jB u and nB u (where j < n) appear whereas two kinds of even parity states 2A g and mA g appears. While mA g absorptions appear just after the nB u, the 2A g absorption peak appears immediately after the jB u absorptions. The later absorption features were not seen in case of linear chain materials like polydiaccetylene (PDA).