Substituent Effects on Dynamics at Conical Intersections: Cyclopentadienes

Substituent effects oil dynamics at conical intersections are investigated by means of femtosecond time-resolved photoelectron spectroscopy for cyclopentadiene and Its Substituted analogues 1,2,3,4-tetramethyl-cyclopentadiene, 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 oil 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 oil 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 oil 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 radiation less 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 he required.