PHOTOCHEMICAL GENERATION OF RADICAL CATIONS FROM THIOPHENE OLIGOMERS

The photoexcitation process for a series of thiophene oligomers nT, from terthiophene to sexithiophene, has been analyzed in dichloromethane solution using laser flash photolysis. The immediately formed excited triplet states 3nT*, which have been characterized by their visible absorption spectra, show a lifetime on the order of a few tens of microseconds. The corresponding radical cations nT.+ have been obtained either by electron transfer from 3nT* to an electron acceptor or by direct photogeneration from nT using high excitation energy. The absence of an oxygen effect together with triplet sensitization experiments indicate that nT.+ cations are formed directly from the singlet state 1nT*. The absorption spectra of these radical cations, which greatly vary with conjugation length n, are in agreement with literature data for nT.+ generated chemically in solution. The process of radical cation formation by photoionization from 1nT* and its second-order decay mechanism are analyzed. The molecular properties of radical cations are discussed in terms of charge-transport properties of the corresponding solid-state materials.