Benzophenone sensitization of triplet oxazine and of delayed fluorescence by oxazine in acetonitrile solution

Abstract

Benzophenone has been used as a triplet energy donor to sensitize the production of the triplet state of the oxazine, 2,7-bis (diethylamino)-phenazoxonium chloride, in dilute acetonitrile solution. This enabled the spectrum, lifetime (14.5 µs) and absorption coefficient (1.6 × 10⁴ dm³ mol^–1 cm^–1 at 830 nm) of the triplet oxazine to be determined. The quantum yield of triplet–state production for this oxazine by direct excitation was shown to be < 0.01 but addition of ethyl iodide quenched the oxazine fluorescence by induced intersystem crossing as confirmed in nanosecond laser photolysis experiments in which the oxazine triplet-state transient absorption spectrum was detected in the presence but not in the absence of ethyl iodide. Quenching of triplet benzophenone by oxazine was investigated by means of steady-state luminescence and also transient emission and absorption techniques in deaerated acetonitrile solution at room temperature. A rate constant for triplet-triplet energy transfer of 0.9 ± 0.1 × 10¹⁰ dm³ mol^–1 s^–1 was obtained. Delayed fluorescence was detected, which like the prompt fluorescence exhibits a slight red shift of the maximum owing to increasing reabsorption when the concentration of oxazine increases. The mechanism for this delayed fluorescence is established as involving an interaction between the triplet state of the donor (benzophenone) and the triplet state of the oxazine, i.e. mixed triplet–triplet annihilation.