Synthesis and photophysical properties of ruthenium(II) bis(2,2′:6′,2"-terpyridine) complexes constructed from a diethynylated-thiophene residue

A set of mono-, di- and trinuclear ruthenium(II) bis(2,2':6',2"-terpyridine) complexes has been built around a central 2,5-diethynylated-thiophene unit. The terminal metal complexes luminesce in solution at room temperature, with the emission lifetimes being considerably longer than that recorded for the parent compound. The central thiophene unit, which is resistant to attack by singlet molecular oxygen, operates as an insulator and prevents full electron delocalisation over the polytopic ligand at the pi-radical anion stage. There is, however, improved electronic communication between the terminals at the triplet level and the dinuclear complex possesses significantly better photophysical properties than are exhibited by the other complexes. Critical comparison with the corresponding 1,4-diethynylated-phenylene bridged dinuclear complex shows that the central thiophene fragment provides for improved stabilisation of the triplet state. This behaviour is interpreted in terms of the relative ability of the complex to adopt cumulene-type structures. This favourable perturbation of the photophysical properties, taken in conjunction with well-developed synthetic protocols, holds promise for the construction of extremely long dinuclear complexes built around poly(thiophene)-based connectors.