Interactions of excited-state porphyrin–quinone in reversed micelles studied by time-resolved fluorescence spectroscopy

Abstract

Interactions in reversed micelles of benzyldimethyl-n-hexadecylammonium chloride (BHDC) between excited-state porphyrin (ZnTPP) and quinones [duroquinone (DQ) and anthraquinone-2-sulphonate (AQS)] have been investigated by time-resolved spectroscopy. Essentially dynamic quenching is observed when the quencher DQ is solubilised in the organic phase, although this is affected by the distribution of quencher molecules at the micellar interface.

When the quencher (AQS) is bound at the interface, the decay of the singlet excited state of porphyrin (ZnTPP) also bound at the interface in the absence of water (ω₀=[H₂O]/[surf]= 0) shows two components: one with a mean lifetime 〈τ〉≈ 1 ns, which corresponds to unquenched porphyrin, and a much shorter 〈τ〉= 100 ps, which accounts for the decay of quenched porphyrin. For quencher concentrations in the range of [AQS]= 1–4 mmol dm^–3 the observed decay data may be interpreted in terms of a static active-sphere quenching model, where only bound quencher molecules within a certain volume around the bound porphyrin molecule lead to quenching. The lifetimes obtained in these studies indicate a pseudounimolecular electron-transfer rate constant for P^*_i⋯Q_i of the order of 10¹⁰ s^–1.