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 fe atures. The position of the 0-0 line of the low-energy emission is cle arly dependent on the nature and microcrystallinity of the oligomer un der stud;, in contrast with the high-energy one. An identification of the crystalline and molecular levels involved is suggested by using Fr enkel exciton theory in the limiting cases of strong and weak intermol ecular coupling. Spectral effects due to the reduced size of the grain s are discussed.