CATION-CONTROLLED PHOTOPHYSICS IN A RE(I) FLUOROIONOPHORE

Alkali and alkaline earth metal cations have a profound effect on the photophysics of the d-pi (Re) --> pi* (bpy) metal-to-ligand charge-transfer (MLCT) excited state in crown ether substituted complex 1. The unique cation-induced effects arise because of the influence of the "crowned" cation on the energy of the ligand-to-ligand charge-transfer (LLCT) excited state that is formed by charge transfer from the donor nitrogen to the electron-deficient metal center in the MLCT state [GRAPHICS] (where bpy = 2,2'-bipyridine and A = the amine donor). In the absence of cations, the energy of the LLCT state lies below the MLCT state. As a result, k(ic) is large and the LLCT state provides a path for rapid radiationless decay of the MLCT state. However, due to the ion-dipole interaction between the cation and the donor nitrogen in cation-crown complexes (1...M(n+)), the energy of the LLCT state is higher than that of the MLCT state. Under these conditions, the MLCT state decays primarily by "normal" radiative and nonradiative decay paths. As a result, the MLCT emission yield is larger for the 1...M(n+) complexes compared to that for 1. Emission quantum yield and lifetime data for the 1...M(n+) complexes are interpreted by a kinetic model in which the rate-determining step for excited-state decay involves dissociation of the cation from the macrocycle. The crown-substituted complex 1 provides a paradigm for a MLCT-based cation sensor.