Remarkable substituent effects on the photophysics of Pt(4′-X-trpy)Cl+ systems (trpy = 2,2′; 6′,2″-terpyridine)

Article abstract of DOI:10.1016/s0020-1693(97)06082-9

In view of the interest in probing the binding interactions that occur between platinum complexes and biological macromolecules, the aim of this work has been to develop systems that exhibit enhanced excited-state lifetimes and emission yields in fluid solution. The investigation focuses on a series of complexes of the type Pt(4′-X-T)Cl⁺ where 4′-X-T denotes a 4′-substituted derivative of 2,2′; 6′,2″-terpyridine. In all cases the counterion is the non-coordinating ion tetrakis [3,5-bis(trifluoromethyl)phenyl] borate. The substituents employed include electron-withdrawing groups like CN and SO₂Me as well as electron-donating groups like SMe and NMe₂. Within the series of complexes, the first reduction wave ranges over about 0.7 V in DMF. Although the process probably entails ligand reduction, the acceptor orbital appears to have some platinum 6p_z character. Even though electron-donating substituents destabilize the reduced form of the ligand, all substituents induce a red-shift in the charge-transfer (CT) absorption band system that occurs around 400 nm. Furthermore, there is generally an increase in the CT absorption intensity, the emission lifetime and the emission quantum yield in methylene chloride. Thus, at room temperature, the complex with the terpyridine ligand itself is a very poor emitter with an emission lifetime of 10 ns or less, while the Pt(4′-SMe-T)Cl⁺ and Pt(4′-NMe₂-T)Cl⁺ systems exhibit lifetimes of 140 ns and 1.9 μs, respectively. With the electron-donating substituents in particular, the lifetime enhancement reflects a configuration interaction between the original CT state and an intraligand charge-transfer excited state. Substituents also influence a thermally activated pathway to radiationless decay.