Microviscosity and wavelength effects on radical cage pair recombination

This study probed two aspects of the reactivity of geminate radical cage pairs formed by photolysis of (Cp′ = η⁵-C₅H₄CH₃). The first aspect studied examined whether the bulk viscosity has any predictive power in determining the magnitude of the cage recombination efficiency (F_cP). Although there is a clear relationship between the magnitude of F_cP and viscosity for systems where the bulk viscosity of the solution is altered by the addition of a non-macroscopic viscosity enhancer, the relationship is unclear for systems where the bulk viscosity is altered using polymeric viscosity enhancers. For this investigation, F_cP values were measured using femtosecond pump-probe transient absorption spectroscopy. The results clearly indicate that bulk viscosity can change drastically without affecting F_cP in systems containing small amounts of added polymers. The bulk viscosity is thus a poor parameter for predicting the cage effect in such systems. The second investigation looked at the effect of the photochemical excitation energy on F_cP for the [Cp′(CO)₃Mo, MoCp′(CO)₃] cage pair in hexane. The results showed that F_cP increased when the wavelength of irradiation was changed from 546 nm to 436 nm, and then remained constant as the wavelength of irradiation was changed from 436 nm to 404 nm to 366 nm. These results are somewhat surprising because the recombination efficiencies for diatomic and triatomic molecules have been shown to decrease monotonically with increasing excitation energy. Two explanations are offered for the reverse wavelength dependence observed in this study. The first explanation invokes the different dynamic behaviors of the two excited states involved at the selected wavelengths, and the second invokes the different speeds of radical separation following irradiation at the selected wavelengths.