Intramolecular electron transfer in the reductive chromophore-quencher complex [(bpy)Re(CO)3(py-PTZ)

Abstract

Three metal complexes containing the reductive quencher ligand py-PTZ, ((bpy)Re(CO)/sub 3/(py-PTZ))/sup +/ and ((bpy)/sub 2/Ru/sup II/L(py-PTZ))/sup n+/ (where L = Cl/sup -/ (n = 1), PTZ = phenothiazine and acetonitrile ( n = 2)), were prepared and their redox and spectral properties investigated. The chromophore-quencher complex have essentially the same photophysical properties as their corresponding pyridine analogues at 77K in a 4:1 (v/v)) ethanol/methanol glass, but their properties are profoundly different in fluid solution. For the Re complex in fluid solution excitation of the Re..-->.. ..pi..*(bpy) metal to ligand charge-transfer (MLCT) chromophore at 355 nm is followed by rapid (<10 ns) appearance of transient absorption (TA) features at 350 and 500 nm consistent with formation of the charge-separated state ((bpy/sup.-/)Re(CO)/sub 3/(py-PTZ/sup.+))/sup +*/. Picosecond TA experiments monitored at 500 nm show that the PTZ/sup .+/ site grows in within approx.200 ps in polar organic solvents following laser excitation at 355 nm. The transient behavior observed leads to the conclusion that initial excitation of the MLCT chromophore is followed by rapid intramolecular electron-transfer quenching with k/sub q/(RT) = 4.8 x 10/sup 9/ s /sup -1/ in acetonitrile to give the charge-separated excited state ((bpy/sup .-)Re(CO)/sub 3/(py-PTZ/sup.+))/sup +/, which, in turn, decays tomore » the ground state with k/sub 2/ = 4.0 x 10/sup 7/ s/sup -1/. 36 references, 11 figures, 2 tables.« less