Radical cage effects: A method for measuring recombination efficiencies of secondary geminate radical cage pairs using pump-probe transient absorption methods

A method is reported for measuring the recombination efficiency of secondary geminate radical cage pairs. The procedure involves measuring the recombination efficiency for primary geminate recombination (F-c1) using pump-probe laser methods and measuring the apparent (or net) recombination efficiency (F-cP) for all geminate pairs (primary and secondary) using steady-state irradiation methods. A mathematical relationship between F-cP, F-c1, and F-c2 (where F-c2 is the recombination efficiency for secondary geminate recombination) is derived and demonstrated using the photolysis reactions of the [(CpR)Mo(CO)(3)](2) molecules, where CpR = eta(5)-C5H4CH3 and eta(5)-C5H4(CH2)(2)C(O)NCH3(CH2)(n)CH3 (n = 3, 8, 13, 18). As an example of the results obtained using the new method, it was found that F-c1 = 0.43 and F-c2 = 0.68 for the molecule with CpR = eta(5)-C5H4CH2CH2N(CH3)C(O)(CH2)(18)CH3. The value of F-c2 decreased as the side-chain on the Cp ring got shorter; F-c2 is equal to 0.0 for the molecules with n = 3 and for CpR = eta(5)-C5H4CH3. It is hypothesized that a longer side-chain prevents facile diffusion of the radicals out of the secondary cage, whereas the smaller side-chains permit more facile diffusion apart of the radicals. A general conclusion is that the reactions of large radicals in particular may be especially impacted by secondary geminate cage recombination.