Suppression of chromophore coupling in thin films by chemical substitution of a perylene tetracarboxylic acid diimide

Thin films (1–100 nm) of 1,6,7,12-tetrachloro-N,N′-dimethyl perylene tetracarboxylic acid diimide (Cl4MePTCDI) were prepared on glass and quartz substrates by drop coating from solution and by physical vapor deposition under variation of substrate temperature and deposition rate. Optical absorption spectra were measured in situ in dependence on average film thickness during film growth under variation of deposition rate and substrate temperature and following film preparation ex situ in dependence on sample temperature. The results are compared to those obtained earlier [J. Phys. Chem., B 1999, 103, 3078] at thin films of unsubstituted N,N′-dimethyl perylene tetracarboxylic acid diimide (MePTCDI). In contrast to unsubstituted MePTCDI no significant chromophore coupling could be detected in films of Cl4MePTCDI. A relative high optical emission signal for thin films of Cl4MePTCDI with characteristics quite similar to the solution spectrum clearly confirmed this interpretation. A twisted structure of the molecule as indicated earlier [J. Phys. Chem., 1992, 96, 7988] is discussed as the basis of changes in the solid state structure relative to unsubstituted MePTCDI which has a planar aromatic system and grows in crystalline films.