Quenching of electronically excited triplet states by molecular iodine in benzene solution

Abstract

The rate constants for quenching of the triplet states of nine different organic molecules by molecular iodine in benzene solution have been measured using the technique of nanosecond laser photolysis. The five triplet donors with energies much higher than that of the lowest triplet state of iodine (gas-phase value = 121 kJ mol^–1) all quench very efficiently with k_Q=(1.35 ± 0.1)× 10¹⁰ dm³ mol^–1 s^–1, i.e. quenching is close to diffusion-controlled. The iodine quenching constants for the other four triplet states decrease slowly as the triplet energy of the molecule being quenched decreases. Since quenching by iodine of the lowest-energy triplet state studied, β-carotene (E_T= 75 kJ mol^–1), is still quite large (2.5 × 10⁹ dm³ mol^–1 s^–1) it is suggested that quenching is probably due to a combination of energy transfer and quenching caused by charge-transfer interactions. Attempts to confirm this mechanism by studying the quenching of triplet β-carotene by iodine as a function of solvent polarity were inconclusive.