Solvent effects as probes of the dynamical factors controlling sub-nanosecond photoprocesses: The case of W(CO)₅pyridine

Quantum yields for the formation of W(CO)₅S (S=solvent) from W(CO)₅L (L=pyridine, other bases) are solvent and wavelength dependent. The reactions are determined by processes occurring in the femtosecond to short picosecond time domain in competition with vibrational relaxation. Tools are needed to probe the dynamical events in this short time domain. The aim would be the detail of mechanistic analysis that is achieved in study of the slower reactions from vibrationally equilibrated excited states using structure-reactivity correlation and transition state theory. In this review, the case study of W(CO)₅pyridine is used to suggest ways to use solvent effects and pressure dependence for understanding of dynamic processes. Special attention is given to two 'solvent cage effects': the classic cage effect related to viscosity and the recently recognized thermal cage effect related to solvent bulk thermal conductivity. The special utility of dynamical parameters of the bulk solvent as aids in analysis of fast dynamics in photochemistry is emphasized. The potential utility of molecular dynamic simulation is explored.