Properties and dynamics of the sigma(M'-Re)pi* excited state of photoreactive dinuclear L(n)M'-Re(CO)(3)(alpha-diimine) (L(n)M'=Ph(3)Sn, (CO)(5)Mn, (CO)(5)Re; alpha-diimine=bpy', iPr-PyCa, iPr-DAB) complexes studied by time-resolved emission and absorption spectroscopies

The photophysics and photochemistry of the metal-metal bonded complexes L(n)M'Re(CO)(3)(alpha-diimine) (L(n)M'=Ph(3)Sn, (CO)(5)Re, (CO)(5)Mn; alpha-diimine = bpy', iPr-PyCa, iPr-DAB) have been studied. According to the time-resolved emission (80 K) and absorption (room temperature) spectra, the lowest excited state has a (3) sigma(M'-Re)pi* character. It is a bound state, which can only be populated by surface crossing from optically excited MLCT states. Homolysis of the metal-metal bond from the sigma pi* state is promoted by nucleophilic and chlorinated solvents. Exceptional in this respect is the complex Ph(3)SnRe(CO)(3)(bpy'), which is nearly photostable in non-chlorinated solvents. The lifetime of the (3) sigma pi* state decreases in the order alpha-diimine = bpy' > iPr-PyCa > iPr-DAB > pAn-DAB. This trend is mainly determined by the energy gap law. The L(n)M' dependence is more complicated because of an additional deactivating effect of an excited state distortion which depends on L(n)M'. At 80 K, the lifetime is determined by the weak coupling to the ground state; at room temperature by dissociation of M'-Re (with the exception of Sn-Re).