Picosecond time-resolved infrared spectroscopic investigation into electron localisation in the excited states of Re(I) polypyridyl complexes with bridging ligands

Mono- and binuclear complexes of (Re(CO)₃Cl) with dipyrido[2,3-a:3′,2′-c]-6,7-dimethylphenazine (ppbMe₂) were synthesised and their photophysical properties probed using picosecond time-resolved infrared spectroscopy (TRIR). Excitation of these complexes in solution at 400 nm produces short-lived excited states. The IR spectrum of the excited state of the mononuclear [Re(CO)₃Cl(ppbMe₂)] have ν(CO) bands shifted to higher wavenumber relative to those of the ground state. This is consistent with formation of a ³MLCT excited state. The IR spectrum of the excited state of the bimetallic [(Re(CO)₃Cl)₂(μ-ppbMe₂)] shows the formation of two distinct groups of ν(CO) bands. This is interpreted as the formation of two distinct Re sites arising from a localised MLCT state with formally oxidised Re centre and a formally reduced bridging ligand. The ν(CO) bands of the adjacent Re centre are affected by the reduction of the bridging ligand. On the IR timescale the excited state structure is best formulated as [Cl(CO)₃Re^II(μ-ppbMe₂˙-)Re^I(CO)₃Cl].