Laser photolysis investigation of induced quenching in photoreduction of benzophenone by alkylbenzenes and anisoles

Abstract

The quenching processes of triplet benzophenone (³BP*) by alkylbenzenes (AB) and anisole derivatives (AD) in benzene (Bz) and a mixture of acetonitrile (ACN) and water (4:1 v/v) have been studied on the basis of rate constants and efficiencies determined by nanosecond laser flash photolysis at 355 nm at 295 K. It was found that (1) the deactivation of ³BP* by ADs in ACN–H₂O (4:1 v/v) was governed by electron transfer (ET) to produce the benzophenone anion (BP^-) and corresponding cation (AD⁺) radicals with efficiencies, α_ET<1 whereas no chemical species were formed in Bz; and (2) photoreduction of ³BP* by ABs resulted in benzophenone ketyl radical (BPK) formation by benzylic hydrogen abstraction (HA) with efficiencies α_HA<1 in Bz and ACN–H₂O (4:1 v/v). The residual efficiency (α_IQ: 1-α_ET or 1-α_HA) was attributed to a bimolecular process with no photochemical product, which was named ‘induced-quenching (IQ)’. The quenching rate constants (k_q) of ³BP* by ADs and ABs were less than the diffusion limits of both Bz and ACN–H₂O (4:1 v/v). The net bimolecular rate constants for the ET, HA and IQ processes were estimated from the k_q values and efficiencies. The rate constants (k_ET and k_IQ) of ET and IQ with AD versus the oxidation potential (E_ox) of AD followed Rehm–Weller behaviour while logarithmic rate constants (k_HA and k_IQ) of HA and IQ by ABs increased linearly with a decrease in the E_ox of AB. It was suggested, for the deactivation mechanism of ³BP* by ABs and ADs (RH), that (1) the IQ process was intersystem crossing (ISC) enhanced by the partial charge–transfer (CT) character of the triplet exciplexes, ³(BP^δ-···RH^δ+)_cage^*; (2) radical ion formation by ET might be accomplished in a polar solvent by further CT interaction in the exciplex; (3) the process of BPK formation was inferred to be H-atom transfer in the exciplex, where the more protic H-atom was readily mobile, rather than ET followed by proton transfer and (4) the loss of efficiencies of photochemical-product formation was derived not from back ET but from the IQ process, inherent to photoreactions, via triplet exciplexes. The deactivation processes of ³BP* by RH are illustrated in Scheme 1.