Spin chemical control of photoinduced electron-transfer processes in ruthenium(II)-trisbipyridine-based supramolecular triads

Nanosecond time-resolved absorption studies in a magnetic field ranging fro m zero to 3.0 T have been performed on a series of covalently linked donor- Ru(bipyridine)(3)-acceptor complexes (D-C2+-A(2+)). In these complexes the electron donor is a phenothiazine moiety linked to a bipyridine by a (-CH2- )(p) (p = 1, 4, 5, 7) chain, and the electron acceptor is an N,N'-diquatern ary-2,2'-bipyridinium moiety, linked to a bipyridine by a (-CH2-)(2) chain. On the nanosecond time scale the first detectable photoinduced electron-tr ansfer product after exciting the complex C2+ is the charge-separated (CS) state, D+-C2+-A(+), where an electron of the phenothiazine moiety, D, has b een transferred to the diquat moiety, A(2+). In zero field the lifetime of the CS state is about 150 ns. At low fields (B-0 < 0.5 T) the magnetic fiel d strongly affects the decay kinetics, splitting it up into a major compone nt, the rate constant of which decreases by a factor of about 10 at fields of several 100 mT, and a minor component with an approximately field indepe ndent rate constant. At high fields (B-0 > 0.5 T) the total amplitude of th e CS absorption signal decreases and the relative contribution of the fast decaying component increases. The magnetic field effects can be consistentl y interpreted and quantitatively modeled by taking into account the mechani sms and kinetics of the spin multiplicity changes in the CS state and its p recursor, a short-lived CT state (D-C3+-A(+)) formed upon primary electron transfer from the triplet excited complex to the diquat moiety, Exploiting the magnetic field dependent kinetics, the rate constants of the triplet-si nglet transitions in the two types of linked radical pairs and of all the e lectron-transfer processes following the primary one can be assessed. Magne tic-field-dependent investigations thus can be essential for the understand ing of the complex kinetics in supramolecular systems with sequential cycli c electron transfer.