Exciplex quenching of photoexcited platinum(II) terpyridines: influence of the orbital parentage

Quenching studies involving a range of Leu is bases establish that exciplex quenching can affect the lifetime of the emissive charge-transfer state of a platinum(II) terpyridine. The evidence comes from studies of Pt(trpy)SCN+, where trpy denotes 2,2':6',2"-terpyridine, and a Pt(4'-X-T)Cl+ series where 4'-X-T denotes a 4'-substituted trpy derivative and X is a CN, SMe or NMe2 substituent. Thus, in dichloromethane the quenching rate constant increases with the donor number as the quencher varies from a relatively weak base like acetonitrile or acetone to a stronger donor like DMSO or pyridine. For the thiocyanate complex in particular, the quenching rate increases by almost three orders of magnitude. Within the Pt(4'-X-T)Cl+ series, the rates show a marked variation with the electron-donating ability of the substituent X. Thus, with pyridine as the quencher, the rate constant varies from 3.5 x 10(8) to 1.0 x 10(10) M-1 s(-1) as X changes from NMe2 to CN. Variations in the orbital parentage of the excited state account for the trend because the Lewis acidity of the metal center decreases with the delocalization of the hole onto the ligand. When the rate of exciplex formation is slow, an outer-sphere complex accumulates in solution and the kinetic plots show saturation behavior at high quencher concentrations. (C) 2001 Elsevier Science B.V. All rights reserved.