Optical processes in ultrathin oligothiophene films on well-defined substrates

The influence of structural order of thin oligothiophene films on their optical properties has been investigated. For this purpose ultrathin films (7-40 Angstrom) of three oligothiophenes (alpha nT) with different chain lengths (n = 4-6) and of endcapped quaterthiophene (EC4T) were prepared by vapour deposition onto the basal planes of highly oriented pyrolytic graphite and of AE(111). Contrary to films prepared on glass which show only broad structureless spectra, the photoluminescence (PL) and reflection spectra of highly ordered films on smooth substrates exhibit sharp vibronic lines (FWHM(0-0)less than or equal to 50 cm(-1)). Thus a detailed analysis of the optical transitions and the respective vibronic progressions is possible, and a comparison with matrix isolated molecules is performed. For alpha 6T the PL proceeds mainly via traps, and only a small PL component resonant to the absorption is observed. Ln contrast, no traps are found for alpha 4T. The reflection spectra of alpha 4T and alpha 6T both show a splitting of 165 cm(-1) and 120 cm(-1), respectively, for the first excited electronic level which is interpreted as Davydov splitting. The evolution of the PL spectrum of alpha 4T as a function of layer thickness is studied and attributed to a Herzberg-Teller coupling of the only weakly allowed emitting transition. The consequences and the general relevance of these findings are discussed. (C) 1998 Elsevier Science B.V.