Photophysics of π-conjugated metal-organic oligomers: Aryleneethynylenes that contain the (bpy)Re(CO)₃Cl chromophore

A comprehensive study of a series of four monodisperse, metal-organic π-conjugated oligomers of varying length is reported. The oligomers are based on the aryleneethynylene architecture, and they contain a 2,2′-bipyridine-5,5′-diyl (bpy) metal binding unit. The photophysical properties of the free oligomers and their complexes with the (L)Re^I(CO)₃X chromophore (where L = the bpy-oligomer and X = Cl or NCCH₃) were explored by a variety of methods including electrochemistry, UV-visible absorption, variable temperature photoluminescence (PL), transient absorption (TA), and time-resolved electron paramagnetic spectroscopy (TREPR). The absorption of the free oligomers and the metal complexes is dominated by the π,π* transitions of the π-conjugated oligomers. The free oligomers feature a strong blue fluorescence that is quenched entirely in the (L)Re^I(CO)₃X complexes. The metal-oligomers feature a weak, relatively long-lived red photoluminescence that is assigned to emission from both the ³π,π* manifold of the π-conjugated system and the dπ Re → π* bpy-oligomer metal-to-ligand charge transfer (³MLCT) state. On the basis of a detailed analysis of the PL, TA, and TREPR results an excited-state model is developed which indicates that the oligomer-based ³π,π* state and the ³MLCT states are in close energetic proximity. Consequently the photophysical properties reflect a composite of the properties of the two excited-state manifolds.