Exciplex formation and energy transfer in mixed films of phthalocyanine and perylene tetracarboxylic diimide derivatives

The absorption and emission properties of co-evaporated and bilayer phthalocyanine-perylene tetracarboxylic diimide (PTCD) thin films have been examined in search for new emitters. The identification of exciplex emission in co-evaporated thin solid films of N,N'-bis(neopentyl)-3,4,9,10-perylenebis(dicarboximide) (BNPTCD) and titanyl(IV) phthalocyanine (TiOPc) is reported for the first time. The efficiency of the exciplex emission is correlated with a high degree of mixing in the co-evaporated mixed film. The homogeneity of the emission from the surface of the mixed film is visualized using the global imaging technique. The chemical detection of the mixing in the film is visualized using Raman global imaging. The fact that exciplex formation is not a common occurrence when mixing these materials is illustrated with the emission properties of co-evaporated mixed chloroindiumphthalocyanine (ClInPc)-BNPTCD films. In ClInPc-BNPTCD mixed films there is no exciplex emission and in turn energy transfer is the main pathway for internal relaxation. For comparison, bilayer films of BNPTCD/Pc were fabricated and studied as model systems of two separated phases with a single interface. The effect of thermal annealing on film structure, molecular organization, and phase separation in the mixed films was investigated using transmission and reflection-absorption (RAIRS) infrared spectroscopy. The infrared data and AFM topographical images provide information that allows one to correlate film structure with the observed emission properties.