Ionic Photodissociation and Picosecond Solvation Dynamics of Contact Ion Pairs

Article abstract of DOI:10.1021/j100277a017

We report picosecond spectroscopic studies of the ionic photodissociation dynamics of malachite green leucocyanide (MGCN).The photoexcitation of MGCN in polar solvents forms malachite green (MG(1+)) carbonium ion and cyanide (CN(1-)).The lowest excited state of MGCN has ionic dissociation lifetimes of 0.1-5 ns which are controlled by solvation of an ionic transition state.Classical dielectric solvation models give reasonable transition state parameters for low-viscosity aprotic solvents and alcohol solvents.A higher energy excited state of MGCN rapidly dissociates into ions after radiationless conversion.The recombination yields of the vibrationally excited contact ion pair depend on dielectric constant and are related to vibrational relaxation and longitudinal dielectric relaxation.The initial contact ion pair has MG(1+) in a tetrahedral geometry which subsequently reorganizes to form planar MG(1+) with a delocalized charge.The times of this conversion are observed by transient absorption spectroscopy, and in aprotic solvents they increase with increasing dielectric constant from 6 to 13 ps.This rate behavior in aprotic solvents is characteristic of an activation energy defined by the solvent energetics.However, charge reorganization rates in alcohol solvents are determined by alcohol monomer reorientation times.This is an unusual solvent effect in which a specific solvent motion controls a transition state barrier crossing.The individual steps of ionic dissociation yield an experimental model for solvent effects in nucleophilic reactions.We give a brief, critical review of solvent dielectric relaxation and its application to charge reorganization phenomena.