Picosecond dynamics of photoinduced interligand electron transfer in [Re(MQ+)(CO)3(dmb)]2+ (dmb = 4,4 dimethyl-2,2 bipyridine, MQ+ = N-methyl-4,4 bipyridinium)

Excited-state dynamics of Re(MQ+)(CO)3(dmb)]2+, (dmb = 4,4'-dimethyl-2,2'-bipyridine, MQ+ = N-methyl-4,4'-bipyridinium) was studied by femtosecond time-resolved spectroscopy in the visible spectral region. Excitation at 400 or 330 nm prepares a mixture of Re --> dmb and Re --> MQ+ metal-to-ligand charge-transfer, MLCT, states. The Re --> dmb MLCT state undergoes a dmb*- --> MQ+ interligand electron transfer to produce a relatively long-lived Re --> MQ+ MLCT excited state, which was characterized spectroscopically. The lifetime of this reaction was determined as 8.3 ps in CH3CN. The interligand electron transfer occurs as a nonadiabatic process in the Marcus normal region. The electronic coupling was estimated to lie in the range 20-40 cm(-1). The electron transfer becomes partially adiabatic in ethylene glycol solutions for which the reaction lifetime of 14.0 ps was determined. Depending on the medium relaxation time, the principal control of the electron-transfer rate changes from electron tunneling to solvent relaxation.