Photoisomerization dynamics of stiff-stilbene in solution

Photoinduced isomerization of 1,1'-bis-indanyliden (stiff-stilbene) in solution was studied with broadband transient absorption and femtosecond Raman spectroscopies, and by quantum-chemical calculations. Trans-to-cis S1 isomerization proceeds over a 600 and 400 cm(-1) barrier in n-hexane and acetonitrile, respectively. The reaction develops on multiple time scales with fast (0.3-0.4 ps) viscosity-independent and slower (2-26 ps) viscosity-dependent components. In the course of intramolecular torsion (which should be the main reaction coordinate) some excited molecules pass through the perpendicular conformation P and reach the cis geometry, to be temporarily trapped there. Subsequently they relax back to P and further to the ground state S0. The cis-to-trans isomerization reveals ultrafast (0.06 ps) oscillatory relaxation followed by 13 ps decay in n-hexane and 2 ps decay in acetonitrile, corresponding to barriers of 800 and 400 cm(-1), respectively. Raman S0 and S1 spectra are reported and discussed. The perpendicular conformation P was not detected, possibly due to its low oscillator strength and short lifetime, or because of strong overlap with hot product spectra. XMCQDPT2 calculations locate a stationary S1 point on the cis side and two perpendicular-pyramidalized stationary points, to be reached from the former over 300 and 680 cm(-1) barrier. Implications for parent stilbene are discussed; in this case we also see evidence for the trans-to-cis adiabatic path, as in stiff-stilbene. Very similar viscosity dependence for the two compounds supports the common isomerization pathway: torsion about the central double bond.