Controlling the relaxation pathways by substituent effects in conjugated dienes

Interactions between conjugated polyenes and light play important role in photosynthesis and vision. To understand and to manipulate the conversion of light energy to chemical energy in conjugated polyenes, we investigated the relaxation processes in the simplest conjugated diene, 1, 3-butadiene (BD) shown in Fig. 1, by time-resolved photoelectron spectroscopy using high harmonics (TRPES) [1]. In BD, the ultrafast relaxation from the excited states mainly takes place via the conical intersection (CI) between S 1 and S 0 ((S 1 /S 0 ) CI ), where the hydrogen atoms at one end of the molecule stick out of the molecular plane to form the pyramidalized structure shown in Fig. 2a. The molecular structure at (S 1 /S 0 ) CI motivated us to control the relaxation pathways by substituting the end hydrogen atoms with methyl groups, preventing the pyramidalization by steric hinderance. To compare with the dynamics of BD, the relaxation processes of 1, 3-pentadiene(PD) and 2, 5-dimethyI-2, 4-hexadine (HD) were investigated by TRPES. The relaxation pathway in HD is expected to be different from the others.