New insight in the optical properties of ultrathin oligothiophene films by epitaxial preparation

Interesting optical properties are obtained if highly ordered interfaces and thin films of large organic molecules are prepared on an atomically flat and clean substrate. Then the optical spectra are very different from those of conventional films and reveal a variety of fine structures. On the basis of one example, quaterthiophene on Ag(111), it is demonstrated that long-range ordered monolayers with commensurate superstructures can be prepared on a well-defined surface. Even epitaxial growth can be achieved, although this is much more complicated due to the extremely different properties of both components. The optical spectra then reveal a variety of fine structures which represent the coupling to different vibrations and can be assigned in detail. The 0-0 transitions become very sharp (40 cm(-1)) and show no Stokes shift. The absorption spectra indicate a likely Davydov splitting which leads to a gradual reduction of the PL intensity of the 0-0 band for increasing film thickness. (C) 1998 Elsevier Science B.V. All rights reserved.