The photophysical properties of a pyrene–thiophene–terpyridine conjugate and of its zinc(ii) and ruthenium(ii) complexes

Abstract

A tripartite supermolecule, comprising a pyrene moiety tethered to a 2,2′:6′,2″-terpyridyl ligand via a 2,5-diethynylated thiophene linker, has been synthesized. This compound is highly fluorescent in solution due to the formation of an intramolecular charge-transfer (CT) state. From consideration of the electrochemical properties, it is concluded that the CT state arises because of charge transfer from pyrene to the central thiophene-based unit. Formation of the CT state involves an increase in dipole moment of ca. 18.5 D. Phosphorescence was not observed but the intermediate population of the triplet state was confirmed by laser flash photolysis. Addition of Zn²⁺ cations results in a drastic decrease in the fluorescence yield while the absorption spectrum exhibits a pronounced red shift. It appears that the dipole is extended upon cation binding, with the zinc terpyridine terminal acting as the electron acceptor. Again, no phosphorescence was apparent at 77 K. Coordination of a ruthenium(II) 2,2′;6′,2″-terpyridyl metallo-fragment to the vacant terpyridine terminal causes the appearance of weak phosphorescence in fluid solution at room temperature. The emitting species has a lifetime of 2.6 μs in deoxygenated acetonitrile at 20 °C. Luminescence, which shows a complex temperature dependence, is attributed to either the lowest-energy metal-to-ligand, charge-transfer triplet localised on the ruthenium(II) complex or to the intraligand CT state. In the later case, spin–orbit coupling effects induced by the ruthenium atom are responsible for promoting emission.