Excited states absorptions in oligomers of PPV

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 (mAg) experiences ultrafast internal conversion to the lowest singlet state (1Bu), whereas the other class (kAg) 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 (mAg) states, which are responsible for the low energy PA band. We also show that in THG three kinds of one-photon states 1Bu, jBu and nBu (where j < n) appear whereas two kinds of even parity states 2Ag and mAg appears. While mAg absorptions appear just after the nBu, the 2Ag absorption peak appears immediately after the jBu absorptions. The later absorption features were not seen in case of linear chain materials like polydiaccetylene (PDA).