Ultrafast Optical Control of Ring-Opening and Isomerization in 7-Dehydrocholesterol and Stilbene.

Abstract

7-dehydrocholesterol and stilbene are model systems for the molecules used within the fast-growing field of molecular motors and switches. One of the significant problems facing the development of effective motors and switches is the quantum yield of the photoisomerization reactions that these materials undergo. This dissertation describes the use of optical control methods to improve the yields of ring-opening and isomerization of 7-dehydrocholesterol and stilbene following UV excitation. The effects of both single-pulse and two-pulse control schemes using UV and visible excitation pulses were investigated. First, the dependence of the 7-dehydrocholesterol ring-opening on excess vibrational energy was studied by exciting with pump pulses tunable over a broad range of wavelengths. Based on these measurements, it appears that either the vibrational mode in which the excess energy is deposited does not play a role in the ring-opening reaction, or the excess energy is quickly dispersed to the remaining vibrational modes by intramolecular vibrational distribution. Next, the effect of chirp on the dynamics and photoproducts of 7-dehydrocholesterol and cis-stilbene was explored through the use of UV pump pulses with chirp between about 10000 fs2 and 10000 fs2. These experiments showed that there was no measurable change in the amount of photoproduct formed following excitation with these pulses. Finally, a two-pulse pump-repump design was used to first place a population of molecules into the lowest electronic excited state before re-exciting to an even higher state. This technique had little effect on the reaction dynamics of 7-dehydrocholesterol, but successfully modified the isomerization yield for both cis- and trans-stilbene. After the arrival of a visible repump pulse, cis-stilbene showed a small change in reaction dynamics, and the yield of trans-stilbene photoproduct was increased. Trans-stilbene showed a large depletion of the excited state population following repumping, and the yield of cis-stilbene photoproduct was increased. Trans-stilbene also demonstrated a distinct change in the recovery of the ground state population, indicating that the re-excited trans-stilbene molecules utilize a different decay pathway that bypasses the S1 state. These results show that a two-pulse excitation scheme may provide the means to modify the isomerization yield in stilbene based molecular motors.