Analysis of charge-transfer complex absorption spectra and the applicability of the Mulliken two-state model

Complete absorption spectra, free from interference due to an unbound acceptor and donor, of several charge-transfer complexes have been examined. These spectra serve as a significant test to the applicability of the Mulliken two-state model for this series of complexes. The appearance of multiple new absorption bands and the trend in the observed oscillator strength of the CT transitions and the localized excitation within the complex spectrum require significant expansions of Mulliken's simple two-state model. Multiple new CT absorption bands are frequently encountered and their appearance has been explained in terms of multiple ion-pair states participating in CT interactions. The relative positions of these bands are predicted based on the gas-phase ionization potentials of the donors and the electron affinity of the acceptor in excellent agreement with the observed spectra achieved. The integrated absorption intensities of the localized bands within each CT complex spectrum have been measured and used as an indicator of the extent on intensity borrowing by the CT transition. These measurements show that for the better donors used in this study, i.e, hexamethylbenzene and pentamethylbenzene, intensity borrowing is not significant. However, for the poorer donors used, an increasing contribution of the LE within the CT absorption bands is observed. A multi-state model, which includes the localized excited state of the acceptor,as well as additional ion-pair states, is the minimum required to describe these complexes.