Substituent Effects on Dynamics at Conical Intersections: Cyclopentadienes

Substituent effects on dynamics at conical intersections are investigated by means of femtosecond time-resolved photoelectron spectroscopy for cyclopentadiene and its substituted analogues 1,2,3,4-tetramethylcyclopentadiene, 1,2,3,4,5-pentamethylcyclopentadiene, and 1,2,3,4-tetramethyl-5-propylcyclopentadiene. By UV excitation to the S(2) (1(1)B(2)) state, the influence of these substitutions on dynamics for the initially excited S(2) (1(1)B(2)) surface and the spectroscopically dark S(1) (2(1)A(1)) surface were investigated. We observed that the dynamics depend only on a small number of specific vibrations. Whereas dynamics at the S(2)/S(1)-conical intersection are independent of substitution at the 5-position, internal conversion dynamics on the S(1) (2(1)A(1)) surface slow down as the inertia of the 5-substituent increases. Contrary to the expectations of simple models of radiationless transitions, an increasing density of states does not lead to faster processes, suggesting that a true dynamical picture of vibrational motions at conical intersections will be required.