Photochemistry of Selected Redox Systems for H Evolution

H2 evolving systems with Ru(bpy) or Proflavine as photosensitizers and Methylviologen (MV ) or Ru(bpy) acceptors have become very popular as prototypes for probing other suitable candidates for the D,S,A relays and it is useful to digress on the photochemistry of these systems. Both Ru(bpy) and Proflavine sensitized schemes have been the subject of detailed flash photolysis studies. 

Let us consider the Ru(bpy) sensitized reduction of MV. Excitation of Ru(bpy) with a 530 nm laser pulse leads to the formation of MLCT excited state with a lifetime of 620 nsec, in water. 

In the presence of electron acceptors like MV2+, the excited state is oxidatively quenched yielding Ru(bpy) and MV . 

As indicated earlier, the back reaction can be prevented by adding a third con5 onent such as triethanolamine (TEOA) which is capable of reducing Ru(bpy)3 to the 2(+) state. 

With increasing amounts of TEOA, reaction (29) removes Ru(bpy)- in 

---