PROPERTIES AND DYNAMICS OF THE SIGMA(M'-RE)PI-ASTERISK 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 complexe s 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. Accordin g to the time-resolved emission (80 K) and absorption (room temperatur e) spectra, the lowest excited state has a (3) sigma(M'-Re)pi charact er. It is a bound state, which can only be populated by surface crossi ng from optically excited MLCT states. Homolysis of the metal-metal bo nd from the sigma pi state is promoted by nucleophilic and chlorinate d 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-diimi ne = bpy' > iPr-PyCa > iPr-DAB > pAn-DAB. This trend is mainly determi ned by the energy gap law. The L(n)M' dependence is more complicated b ecause of an additional deactivating effect of an excited state distor tion which depends on L(n)M'. At 80 K, the lifetime is determined by t he weak coupling to the ground state; at room temperature by dissociat ion of M'-Re (with the exception of Sn-Re).