Luminescence quenching of ordered π-conjugated molecules near a metal surface: Quaterthiophene and PTCDA on Ag(111)

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Abstract
Photoluminescence (PL) spectra were measured for highly ordered films of two planar π-conjugated molecules [quaterthiophene and 3,4,9,10-perylenetetracarboxylicacid-dianhydride (PTCDA)], which were both grown on a Ag(111) surface with thicknesses varied between 1 and 30 monolayers. For both molecules the PL is quenched below the detection limit for two-layer-thick films and increases very steeply for thicker films. These results cannot be explained within the classical image dipole theory, but demonstrate the presence of ultrafast, nonradiative short-range decay processes, such as charge delocalization and tunneling. It is effective for the first chemisorbed molecular layer and the second molecular layer on top of the first. Implications of these findings for luminescence experiments on organic films in close contact to metal surfaces are discussed.