Dominance of triplet mechanism over radical pair mechanism in short-lived triplets: hydrogen abstraction reaction of hydroxybenzophenones†

Abstract

The role of the lifetime of triplet precursor states in chemically induced electron spin polarization in photochemically generated free radicals is phenomenologically examined by explicitly including the triplet decay channel in the kinetic scheme. From the general expression, two conditions are derived for the occurrence of dominant triplet mechanism (TM): a reaction-controlled TM and a lifetime-controlled TM. The former holds when the lifetime of the triplet state is very long, and the well-known requirement of the radical generating reaction competing with the electron spin-lattice relaxation of the triplet is met. The lifetime-controlled TM can operate when the lifetime of the triplet is very short and is comparable to its spin-lattice relaxation time. A comparative time-resolved EPR study on benzophenone (BP) and o-, m-, and p-hydroxybenzophenones (HBP) in 2-propanol, as a function of temperature, showed that hydrogen abstraction is the primary photochemical step in both systems. This observation is contrary to the generally accepted belief that HBPs were unreactive towards hydrogen abstraction from alcohols. In spite of their similarity in chemical behaviour, the dynamics of radical generation and subsequent evolution of spin polarization in these two systems are different. In BP, the radical pair mechanism (RPM) dominates. In all the isomers of HBP, predominantly emissive polarization of the solvent radical and the HBP ketyl radical indicated the dominance of TM. The origin of this is attributed to lifetime-controlled TM. Participation of the hydroxyl group of HBP in forming a hydrogen bond with the solvent cage of 2-propanol is proposed to play a major role in the dynamics of the geminate radical pair. †Dedicated with best wishes to Professor B. Venkataraman, on his 76th birthday.