Ground- and Excited-State Dynamics of Aluminum and Gallium Corroles

Abstract

The steady-state absorption and emission spectra and the temporal fluorescence decay profiles of two metallocorroles, Al(tpfc)(py)(n) and Ga(tpfc)(py)(n) (n = 1,2), have been measured in a noncoordinating solvent, benzene, in a coordinating solvent, pyridine, and in mixed benzene-pyridine solutions. The ground-state spectra reveal that an equilibrium between the pentacoordinate corrole (n = 1) and the hexacoordinate corrole (n = 2) is established in the mixed benzene-pyridine solutions. The ground-state equilibrium constants are 135 M(-1) and 1.0 M(-1) at 295 K for the Al and Ga species, respectively. The excited-state radiative and nonradiative decay constants of the pentacoordinate and the hexacoordinate species have been obtained from measurements of the fluorescence quantum yields and monoexponential fluorescence decay times in pure benzene and pure pyridine. Temporal fluorescence decays of the gallium system in a mixed benzene-pyridine solution are biexponential due to dissociation of the hexacoordinate species in the excited state leading to the establishment of a dissociation-association equilibrium. The rate constants for the pyridine association and dissociation processes for the gallium corrole in the excited state have been measured, k(a)* = 2.3 x 10(8) M(-1) s(-1) and k(d)* = 2.9 x 10(8) s(-1), respectively, leading to a value for the excited-state association equilibrium constant of K(a)* = 0.78 M(-1).