Picosecond time-resolved Stokes and anti-Stokes Raman studies on the photochromic reactions of diarylethene derivatives

The cyclization and cycloreversion reactions of diarylethene derivatives have been studied with picosecond time-resolved Stokes and anti-Stokes Raman spectroscopies. The cyclization reaction of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (DMTF) is found to occur within 4 ps to produce the vibrationally excited closed forms in the ground electronic (S₀) state. The time constant of the vibrational relaxation toward a thermal equilibrium with solvent molecules is estimated to be about 10 ps. The cycloreversion reaction of 1,2-bis(3,4-dimethyl-5-phenyl-2-thienyl)perfluorocyclopentene (DMPTF) also generates the vibrationally excited open forms in the So state within 4 ps, which decay on a picosecond time scale. The picosecond time-resolved anti-Stokes Raman spectra of DMPTF show two vibrational bands assignable to the C=C stretching modes of the cyclopentene and thiophene moieties of the generated open forms. The Raman intensity arising from the cyclopentene moiety relative to that from the thiophene moiety becomes smaller with the delay time, indicating that part of the excess energy generated via the cycloreversion reaction is localized on the C=C stretching mode of the cyclopentene moiety. This result suggests that the C=C stretching mode of the cyclopentene moiety is one of the promoting or the accepting modes in the cycloreversion reaction.