Optical properties of amorphous ultrathin films of perylene derivatives

Ultrathin films of PTCDA and MePTCDI (submono/multilayers with thickness d = 0.01 – 10 ML), UHV-deposited on He-cooled quartz substrate, are studied in situ by cw and ps time-resolved fluorescence spectroscopy. The absorption spectrum, measured via photoexcitation, shows a monomer-like shape independent of film thickness, indicating an amorphous structure. The monotonic red-shift with increasing d reflects enhancement of isotropic intermolecular interaction. The fluorescence of ultrathin films consists of a single, monomeric component (M) exhibiting mirror symmetry to the absorption. With increasing film thickness a lower energy excimer-like component (E) emerges, gaining intensity at the expense of M due to energy transfer to E traps. The two components can be separated by time-resolved spectroscopy. Individual molecular behaviour can be studied in the limit of ultrathin films (< 0.03 ML), when E completely vanishes.