Fluorescence decays in van der waals complexes of 9-cyanoanthracene studied in supersonic free jets

Abstract

van der Waals (vdW) complexes of 9-cyanoanthracene (9CNA) with ligands of various types have been generated in supersonic free jets and studied by laser-induced fluorescence (l.i.f.) spectroscopy and nanosecond fluorescence lifetime measurements. Depending on the nature of the ligand molecules, four different types of decay have been observed. (i) After the complexation of 9CNA with water, ammonia and ethanol, the emission decay process is unaltered in the complex's vibrationless and vibrationally excited states. (ii) Carbon disulphide and triethylamine (TEA) form vdW complexes with 9CNA whose fluorescence lifetimes are less than one half of that of the uncomplexed 9CNA. Furthermore, the intramolecular vibrational excitation of the 9CNA moiety in these two complexes appears to lead to almost total fluorescence quenching, since no l.i.f. excitation bands are observed at the expected wavelengths. This contrasts sharply with the vdW clusters mentioned above. (iii) For complexes with benzene, much longer lifetimes (36–48 ns) than that of the uncomplexed 9CNA are found, although no new type of emission is detected. (iv)N,N-Dimethylaniline (DMA) yields an exciplex with a lifetime of 110 ns, which might be formed directly from the vdW complex in the ground state. the effect of complexation on the fluorescence lifetimes is also discussed in relation to the fluorescence quenching observed in solution.