Photoluminescence under selective excitation of thiophene oligomers: relationship to microcrystalline structure

The photoluminescence of thin films of various sexithiophene oligomers has been studied by using excitations near the absorption edge. This emission shows at least two overlapping spectra with very different features. The position of the 0-0 line of the low-energy emission is clearly dependent on the nature and microcrystallinity of the oligomer under study, in contrast with the high-energy one. An identification of the crystalline and molecular levels involved is suggested by using Frenkel exciton theory in the limiting cases of strong and weak intermolecular coupling. Spectral effects due to the reduced size of the grains are discussed.