Involvement of upper excited state S2 in the photophysics of trans-1, 2-diarylethenes due to slow internal conversion to S1

The fluorescence decay of some styrylarenes bearing naphthyl, phenanthryl, anthryl, pyrenyl and pyridyl groups has been measured by both the single photon counting and phase-modulation techniques in the nanosecond and picosecond regions. Generally, two decay components were observed. The most abundant and longer component (in the 1-10 ns range) corresponded to the typical S-1-->S-0 emission of these molecules. A second weaker component (in the 100-800 ps range) was practically coincident with the short component recently observed for some of these compounds by absorption measurements using laser flash photolysis. It was assigned to the emission from an upper excited singlet state, S-2, directly populated at high excitation energies. An investigation of the effect of the exciting and monitoring wavelengths on the fluorescence decay showed that the abundance of the short component increased with the excitation energy. Preliminary quantum-chemical calculations supported the relatively slow internal conversion from S-2 to S-1, so that other relaxation pathways, such as fluorescence from S-2 by-passing S-1 or intersystem crossing or twisting towards the cis isomer, can become competitive and play a role in the deactivation pathways of these flexible molecules,