Excited-state relaxation of ultrathin MePTCDI films interacting with inorganic substrates

Abstract Ultrathin organic layers (0.01–50 ML) of the perylene dye MePTCDI (N,N′-dimethylperylene-3,4: 9.10-bis(dicarboximide)), UHV-deposited on He-cooled inorganic substrates (Ag(111); Si(111): H; quartz) are studied in situ by cw and ps time-resolved fluorescence techniques. Emphasis is put on the unambiguous separation of the two competing relaxation pathways: via interaction of the electronically excited state with the substrate and via intermolecular interaction within the organic film. We have been able to analyze the substrate interaction separately by reducing the coverage into the submonolayer regime, whereupon energy transport within the organic film is suppressed successively. The distance dependence of the molecule-substrate interaction is studied by using spacer (Ar) layers of variable thickness (0–300 A). Fluorescence lifetime shortening of more than three orders of magnitude is observed as a function of spacer thickness on metal substrates and is quantitatively explained by the classical image dipole theory (CPS).