Optical processes in ultrathin oligothiophene films on well-defined substrates Original Research Article

The influence of structural order of thin oligothiophene films on their optical properties has been investigated. For this purpose ultrathin films (7–40 Å) of three oligothiophenes (α 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 Ag(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 (FWHM0−0 ⩽ 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 α6T the PL proceeds mainly via traps, and only a small PL component resonant to the absorption is observed. In contrast, no traps are found for α4T. The reflection spectra of α4T and α6T 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 α4T 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.