Direct measurement of the photochemical ring opening in 1,3-cyclohexadiene by picosecond time-resolved UV resonance Raman

Ultrafast Dynamics of a Molecular Switch from Resonance Raman Spectroscopy: Comparing Visible and UV Excitation

Resonance Raman spectroscopy probes the ultrafast dynamics of a diarylethene (DAE) molecular switch following excitation into the first two optical absorption bands. Mode-specific resonance enhancements for Raman excitation at visible (750-560 nm) and near-UV (420-390 nm) wavelengths compared with the calculated and experimental off-resonance Raman spectrum at 785 nm reveal different Franck-Condon active vibrations for the two electronically excited states. The resonance enhancements at visible wavelengths are consistent with initial motion on the first excited-state that promotes the cycloreversion reaction, whereas the enhancements for excitation at near-UV wavelengths highlight motions involving conjugated backbone and phenyl ring stretching modes that are orthogonal to the reaction coordinate. The results support a mechanism involving rapid internal conversion from the higher-lying state followed by cycloreversion on the first excited state. These observations provide new information about the reactivity of DAE derivatives following excitation in the visible and near-UV.

The Ultrafast Pathway of Photon-Induced Electrocyclic Ring-Opening Reactions: The Case of 1,3-Cyclohexadiene

The photochemically induced electrocyclic ring-opening reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene serves as a prototype for many important reactions in chemistry and in biological systems. Based on experimental and computational studies, a detailed picture of the reaction has now emerged: Excitation to the Franck-Condon region places the molecule on a steeply repulsive part of the 1B potential energy surface, which propels the molecule in exactly the conrotatory direction that conforms to the Woodward-Hoffmann rules of orbital symmetry. Bypassing a cusp in a symmetry-breaking direction, the wave packet enters the 2A state within 55 fs. It continues to move directly toward the 2A/1A conical intersection, where it crosses in approximately 80 fs to the ground state. This article summarizes the published experimental and theoretical work to describe the current understanding of the reaction while pointing to important questions that remain to be addressed.

Spectroscopy and femtosecond dynamics of the ring opening reaction of 1,3-cyclohexadiene

The early stages of the ring opening reaction of 1,3-cyclohexadiene to form its isomer 1,3,5-hexatriene, upon excitation to the ultrashort-lived 1 1B2 state, were explored. A series of one-color two-photon ionization/photoelectron spectra reveal a prominent vibrational progression with a frequency of 1350 cm(-1), which is interpreted in a dynamical picture as resulting from the ultrafast wave packet dynamics associated with the ring opening reaction. Photoionization in two-color three-photon and one-color four-photon ionization schemes show an ionization pathway via the same ultrashort-lived 1 1B2 state, and in addition, a series of Rydberg states with quantum defects of 0.93, 0.76, and 0.15, respectively. Using those Rydberg states as probes for the reaction dynamics in a time-resolved pump-probe experiment provides a direct observation of the elusive 2 1A1 state that has been implicated as an intermediate step between the initially excited 1 1B2 state and the ground electronic state. The rise and decay times for the 2 1A1 state were found to be 55 and 84 fs, respectively.

Ultraviolet resonance Raman spectroscopy of bacteriorhodopsin

Ultraviolet resonance Raman spectra of bacteriorhodopsin have been obtained using 229 nm excitation from a hydrogen-shifted neodymium yttrium aluminum garnet (Nd: YAG) laser. High signal-to-noise spectra are observed exhibiting vibrational bands at 762, 877, 1011, 1175, 1356, 1552 and 1617 cm-1 which are assigned to scattering from tryptophan and tyrosine side chains. This demonstrates the feasibility of using UV resonance Raman spectroscopy to monitor aromatic amino acid structural changes during the bacteriorhodopsin photocycle.