Terpyridine isomerism as a tool for tuning red-to-NIR emissive properties in heteronuclear gold(I)–thallium(I) complexes

The polymeric linear chain [AuTl(C6Cl5)2]n reacts with three terpyridine-type ligands substituted with thiophene groups containing N-donor centres in different relative positions (L1, L2 and L3), leading to the Au(I)/Tl(I) complexes [AuTl(C6Cl5)2(L1)]n (1), [{AuTl(C6Cl5)2}2(L2)]n (2) and [AuTl(C6Cl5)2(L3)]n (3). X-Ray diffraction studies reveal that L1 acts as a chelate, while L2 and L3 act as bridging ligands, resulting in different coordination indexes for the thallium(I) centre. These structural differences strongly influence their optical properties, and while compounds 2 and 3 emit near the limit of the visible range, complex 1 emits in the infrared region. DFT calculations have also been carried out in order to determine the origin of the electronic transitions responsible for their optical properties.