Theoretical Investigation of the Intermolecular Charge-Transfer Interactions of Poly-p-Phenylene Oligomers

A composite molecule (CM) model is used to investigate the nature of intermolecular interactions in the excited states of pi-conjugated oligomers of poly-(p-phenylene) (PPP). The dependence of the charge-transfer (CT) interactions in low-lying excited states as a function of intermolecular distance, conjugation length, and the translation shifts is examined. We demonstrate how the amplitude of CT mixing in two low-lying excited states of PPP dimer is sensitive to various aspects of molecular packing. The largest CT mixing in the two excited states is consistently obtained for cofacial packing. However, from a detailed examination of molecular orbital shapes and interactions, it is seen that specific arrangements promote the CT mixing in two low-lying transitions. With cofacial arrangements, it shows a larger percentage of CT in the first excited states than the corresponding second excited states for phenyl rings on the top of C-sp2-C-sp2 single bonds. This work illustrates the importance of molecular packing-CT mixing relationship of functional pi-conjugated materials.