Do coupling exciton and oscillation of electron-hole pair exist in neutral and charged pi-dimeric quinquethiophenes?

Optical physical properties of neutral and charged quinquethiophene monomer, and neutral and cationic pi-dimeric quinquethiophenes were investigated with density functional theory as well as the two dimensional (2D) site (transition density matrix) and three dimensional (3D) cube (transition density and charge difference density) representations, stimulated by the recent experimental report [T. Sakai et al., J. Am. Chem. Soc. 127, 8082 (2005)]. Transition density shows the orientation and strength of the transition dipole moment of neutral and charged quinquethiophene monomer, and charge difference density reveals the orientation and result of the charge transfer in neutral and charged quinquethiophene monomer. To study if coupling exciton and oscillation of electron-hole pair exist in neutral and cationic pi-dimeric quinquethiophenes, the coupling constants J (coupling exciton of electron-hole pair) and K (coupling oscillation of electron-hole pair) were introduced to the exciton coordinate and momentum operators, respectively, and the 2D and 3D analysis methods were further developed by extending our previous theoretical methods [M. T. Sun, J. Chem. Phys. 124, 054903 (2006)]. With the new developed 2D and 3D analysis methods, we investigated the excited state properties of neutral and cationic pi-dimeric quinquethiophenes, especially on the coupling exciton and oscillation of electron-hole pair between monomers. The 2D results show that there is neither coupling exciton (J=0) nor oscillation (K=0) of electron-hole pair in neutral pi-dimeric quinquethiophenes. For some excited states of cationic pi-dimeric quinquethiophenes, there is no coupling exciton (J=0), but there is coupling oscillation (K not equal0); while for some excited states, there are both coupling exciton and coupling oscillator simultaneously (J not equal0 and K not equal0). The strength of transition dipole moments of pi-dimeric quinquethiophenes were interpreted with 3D transition density, which reveals the orientations of their two subtransition dipole moments. The 3D charge transition density reveals the orientation and result of intermonomer and/or intramonomer charge transfer. The calculated results reveal that excited state properties of neutral pi-dimeric quinquethiophene are significantly different from those of the cationic pi-dimeric quinquethiophenes.