OPTICAL PROCESSES IN ULTRATHIN OLIGOTHIOPHENE FILMS ON WELL-DEFINED SUBSTRATES

The influence of structural order of thin oligothiophene films on thei r optical properties has been investigated. For this purpose ultrathin films (7-40 Angstrom) of three oligothiophenes (alpha nT) with differ ent chain lengths (n = 4-6) and of endcapped quaterthiophene (EC4T) we re prepared by vapour deposition onto the basal planes of highly orien ted pyrolytic graphite and of AE(111). Contrary to films prepared on g lass which show only broad structureless spectra, the photoluminescenc e (PL) and reflection spectra of highly ordered films on smooth substr ates exhibit sharp vibronic lines (FWHM(0-0)less than or equal to 50 c m(-1)). Thus a detailed analysis of the optical transitions and the re spective vibronic progressions is possible, and a comparison with matr ix isolated molecules is performed. For alpha 6T the PL proceeds mainl y via traps, and only a small PL component resonant to the absorption is observed. Ln contrast, no traps are found for alpha 4T. The reflect ion 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 leve l which is interpreted as Davydov splitting. The evolution of the PL s pectrum of alpha 4T as a function of layer thickness is studied and at tributed to a Herzberg-Teller coupling of the only weakly allowed emit ting transition. The consequences and the general relevance of these f indings are discussed. (C) 1998 Elsevier Science B.V.