Inner-sphere and outer-sphere charge-transfer quenching of the uranyl UO22+(VI) luminescence, and kinetics for the tertiary and secondary alcohols activated uranyl emission

Abstract

For the uranyl UO22+(VI) luminescence, the quenching constants (Kq: mM−1) of various σ–donors and π–donors, which have been shown to form electron-donor–acceptor (EDA) complexes with UO22+, were found directly related to the ionization energies (IE: eV) of the electron donors, such that the common logarithm logKq = –0.0932(IE) + 1.671 (R2 = 0.945). The relationship has revealed an inner-sphere mechanism for the quenching resulting from the charge-transfer (a single-electron transfer) from the HOMO of the quenching substance (Q) to a SOMO of the excited state (UO22+)* that takes place in an EDA [Q, (UO22+)*] complex. For some weak electron donors which do not form EDA complexes with UO22+, their Kq values for the quenching of UO22+ luminescence were found significantly much smaller than expected from the logKq versus IE relationship established for the inner-sphere charge-transfer quenching. The observations have indicated an outer-sphere charge-transfer quenching mechanism possibly resulting from the simple collision between the quenching substance and UO22+ without the formation of an EDA complex between them. In the presence of tertiary butyl alcohol (tBuOH) at 0 – 0.015 M (15 mM), the intensity (I) of the emission of UO22+ (0.01 M) at 508 nm (with Ex = 340 nm) in acetone was found to increase linearly as a function of the tBuOH millimolar concentration (mM) (I = 34.2[tBuOH] + 1802, R2 = 0.992). Kinetic analysis has revealed an electronic energy transfer from the excited state of tBuOH (A*), identified by luminescence spectroscopy, to the ground state of UO22+ (U) to promote it to the excited state (U*) (A* + U → A + U*), enhancing the UO22+ luminescence. The excitation energy transfer is believed to follow a Dexter mechanism, with the overall spin of the energy donor and acceptor conserved, followed by an intersystem crossing ([1A*, 1U] → [1A, 1U*] → [1A, 3U*]).