Ultrafast Molecular Dynamics in Complexed Trans -

Ultrafast molecular dynamics in large molecular systems have been studied using either pump/probe time -of-flight mass spectrometry, or laser-induced fluorescence with time-correlated single photon counting technique. Intramolecular vibrational energy redistribution (IVR) and photoisomerization reaction in alkane-solvated and substituted trans-stilbenes are presented. Absent, restricted, and dissipative IVR were identified in stilbene-hexane_1 van der Waals complex. The dramatic increase of the density of states upon hexane solvation shifts the threshold for an efficient IVR to much lower excess-energy ( ~300 cm^{-1} for the dissipative regime) in comparison with the bare molecule (~1170 cm^ {-1}). The effects of symmetry, density of states, and number of atoms on IVR were studied by using 4-methoxystilbene, 4,4^' -dimethoxystilbene, and 2-phenylindene as model systems. The solvation and structural effects on the isomerization reaction in trans-stilbene is even more dramatic. For the first time, the lowering of the isomerization barrier ( ~700 +/- 100 cm ^{-1}) in trans-stilbene upon one -hexane complexation is observed experimentally upon and compared with that in the bare molecule (~ 1250 +/- 100 cm^ {-1}). Furthermore, the isomerization rate above the barrier in stilbene-(hexane)_1 complex is slightly slower than that in bare trans -stilbene. As the excess-vibrational energy exceeds the calculated binding energy of the (1:1) complex, the vibrational predissociation channel become open and was accounted for by using a simple kinetic model. Further decrease in the isomerization rate in stilbene-hexane_{n } clusters was observed as the number of solvent molecules (n <= 5) increases. As the cluster size increases, the rate becomes less dependent on the excess energy in the S_1 -state and the separation of the barrier must await theoretical calculations. The solvent polarity (in stilbene-hexanenitrile _{n} complexes) and isotope effects (in stilbene-hexane-d_{14} complex) on the overall isomerization reaction were also examined. Similar studies were carried out on stilbene-(ethane)_{n} clusters which has a lower binding energy than stilbene -(hexane)_{n} clusters which, in turn, make the fragmentation of the cluster more severe. On the other hand, interesting observations were made on the reaction dynamics in a designed set of substituted stilbenes. 4-Methoxystilbene, 4,4^' -dimethoxystilbene, 4,4^' -dihydroxystilbene, trans-beta -1-cyclohexene-styrene and 2-phenylindene were used as model systems to study the role of electron conjugation and the torsional motion around C_{e} -C_{ph} single bond in the isomerization reaction in trans-stilbene.