Caging and geminate recombination following photolysis of triiodide in solution

A survey of caging and geminate recombination dynamics following the UV photolysis of I₃ ^- in a series of polar solvents is presented. Transient absorption in both the near-IR and UV was measured out to delays of 0.4 ns, probing evolution of the nascent product and recombined reactants, respectively. The fate of photolysis fragments is suggested to be determined shortly after the act of bond fission. Kinetic analysis shows caged fragments either recombine directly and vibrationally relax within a few picoseconds or produce long-lived complexes of unknown structure that decay exponentially in ∼40 ps, and both routes lead to ground-state I₃ ^-. The persistent complex exhibits a near-IR absorption spectrum that is broadened and red-shifted relative to free I₂ ^-. A very shallow and slow residual component of recombination may be associated with encounters of geminate pairs that initially escape the solvent cage. The choice of solvent strongly effects the probability and dynamics of caging, but not the decay rate of complex caged pairs. This is not altered by varying the temperature of an isobutyl alcohol solution from 5 to 45 °C. The results are discussed in an effort to illuminate the role played by the solvent in triiodide recombination in solution.