Formation and decay of molecular exciplexes in the vapor phase: excitation energy, isotope, and temperature dependence of rate processes in the tetracyanobenzene/p-xylene system

The formation and decay of the tetracyanobenzene (TCNB)/p-xylene gas-phase exciplex have been investigated by transient and steady-state fluorescence. The results suggest that the major radiationless transition of the gas-phase TCNB/p-xylene exciplex is S1 → S0 internal conversion, whose rate is controlled by the low-frequency interchromophore vibration as well as high-frequency intrachromophore vibrations. The important intrachromophore accepting modes are argued on valence and experimental grounds, to be methyl C-H stretching vibrations in p-xylene, C-N triple bond stretching vibrations in TCNB, and skeletal stretching vibrations in p-xylene and TCNB. The minor role of the ring hydrogen stretching vibrations in S1 → S0 internal conversion of the exciplex is ascribed to the presence of these good accepting modes. Finally, the radiative decay rate of the exciplex fluorescence has been found to increase with deuteration of methyl hydrogens in p-xylene. The result is consistent with the proposition that the ionization potential of p-xylene-d3 is greater than that of p-xylene-d0 due to smaller hyperconjugative interaction of CD relative to CH. 17 figures, 2 tables