A new electron-transfer donor for photoinduced electron transfer in polypyridyl molecular assemblies

A synthetic procedure has been devised for the preparation of the reductive quencher ligand 4-methyl-4'-(N-methyl-p-tolylaminomethyl)-2,2'-bipyridine (dmb-tol), which contains toluidine covalently bound to 2,2'-bipyridine. Wh en bound to Re-I in [Re-I(dmb-tol)(CO)(3)Cl], laser flash Re-I --> dmb meta l-to-ligand charge-transfer (MLCT) excitation at 355 nm in CH3CN at 298 +/- 2 K is followed by efficient, rapid (<5 ns) appearance of a transient with an absorption feature at 470 nm. The transient spectrum is consistent with formation of the redox-separated state, [Re-I(dmb(-)-tol(+))(CO)(3)Cl], wh ich returns to the ground state by back election transfer with k(ET) = (1.0 5 +/- 0.01) x 10(7) s(-1) (tau = 95 +/- 1 ns) at 298 +/- 2 K. Rapid, effici ent quenching is also observed in the Ru-II complex [Ru(4,4'-(C(O)NEt2)(2)b py)(2)(dmb-tol)](2+). Based on transient absorption measurements, a rapid e quilibrium appears to exist between the initial metal-to-ligand charge-tran sfer excited state and the redox-separated state, which lies at higher ener gy. Decay to the ground state is dominated by back electron transfer within the redox-separated state which occurs with k > 4 x 10(8) s(-1) at 298 +/- 2 K.