Comparative study on the photodynamics of chloromethylanthracenes in a supersonic free jet and solution

Abstract

Fluorescence excitation spectra and fluorescence decays of 9-chloromethylanthracene (CMA) and 9-methyl-10-chloromethylanthracene (MCMA) were measured in a supersonic free jet. Excitation bands associated with the bare molecules were observed only at excess vibrational energies at less than 530 cm -1 for CMA and 400 cm -1 for MCMA. All the bands of the bare CMA molecule yielded fluorescence lifetimes (τ f ) shorter than 3 ns. The τ f of the bare MCMA molecule was 16.6 ns at the electronic origin and it was shortened by increasing the excess energy. The excitation bands attributed to van der Waals (vdW) complexes of MCMA with rare-gas atoms were observed with fluorescence lifetimes longer than those of the bare molecule, whereas fluorescence from vdW complexes of CMA was observed only for large complexes of Kr and Xe atoms. Activation energies for the radiationless transition obtained from the temperature dependence of the fluorescence lifetime in solution were 610–650 cm -1 for CMA and 520–610 cm -1 for MCMA, and large values exceeding 10 11 s -1 were obtained as the frequency factor. These results obtained in jets and solution are discussed from the viewpoint of the effects of medium on the photodissociation and intersystem crossing (ISC) processes.