INTERSYSTEM-CROSSING DYNAMICS IN HETERODINUCLEAR POLYPYRIDYL-BRIDGED COMPLEXES

The excited-state properties of [(L)(4)M-II(dpp)Ru-II(bpy)(2)](PF6)(n) (bpy = 2,2'-bipyridine; dpp = (2,3-bis(2-pyridyl)pyrazine); (L)(4)M-I I = (NH3)(4)Ru-II (n = 4) Or (CN)(4)Fe-II (n = 0)) have been investiga ted by steady-state and time-resolved picosecond and nanosecond transi ent luminescence and absorption spectroscopic methods. The [(NH3)(4)Ru -II(dpp)Ru-II(bpy)(2)](4+) complex is nonemissive in room-temperature H2O solution. Picosecond laser flash photolysis of [(NH3)(4)Ru-II(dpp )Ru-II(bpy)(2)](4+) reveals the formation of a trans lent sp cc ies th at decays exponentially (tau = 290 (+/-80) ps) independent of excitat ion wavelength. The excited-state behavior of [(NC)(4)Fe-II(dpp)Ru-II( bpy)(2)] display a complicated dependence on excitation wavelength. F ollowing 591-nm pulsed laser excitation into the (1)[(d pi)(Fe)(5)(pi )(dpp)(1)]<--(1)[(d pi)(Fe)(6)] metal-to-ligand charge transfer (MLCT) band, a short-lived transient is formed with tau < 80 ps. Laser excit ation at 354.7 nm into the (1)[(d pi)(Ru)(5)(pi)(bpy/dpp)(1)]<--(1)[( d pi)(Ru)(6)] MLCT transition produces a transient species with tau > 200 ns. One explanation for the wavelength-dependent photophysics of t he [(NC)(4)Fe-II(dpp)Ru-II(bpy)(2)] is that the Fe-II metal center und ergoes a low-to high-spin interconversion following (1)[(d pi)(Ru)(5)( pi)(bpy/dpp)(1)]<--(1)[(d pi)(Ru)(6)] excitation, a process not acces sible via direct (1)[(dst)(Fe)(5)(pi)(dpp)(1)]<--(1)[(d pi)(Ru)(6)] e xcitation.