Picosecond Transient Absorption as Monitor of the Stepwise Cation-Macrocycle Decoordination in the Excited Singlet State of 4-(N-Monoaza-15-crown-5)-4'-cyanostilbene
Résumé
Picosecond absorption spectroscopy of 4-(/V-monoaza-15-crown-5)-4'-cyano stilbene, a polar fluoroionophore of the stilbene-crown series, shows the formation of an intramolecular charge-transfer state (TICT state), in less than 1 ps (k > 10’2 s~’)in acetonitrile (CH3CN) and less than 4 ps (k = (2.5 ± 0.7) x 10” s-1) in butyronitrile (BuCN). When this fluoroionophore is fully engaged in a calcium complex, its electronic excitation leads sequentially from the Franck—Condon state (LM)*, to a cation probe pair (L*M) (k = (2.5 ± 0.5) x 10n s_1 in CH3CN and k = (3.3 ± 0.5) x 10" s_l in BuCN) and to a solvent separated cation probe pair (L*/S/M) (k = (1.4 ± 0.5) x 1010 s_1 in CH3CN and k = (4 ± 0.5) x 1010 s_1 in BuCN). The transient absorption spectra of these three intermediates on the decoordination pathway have been obtained. The increasing charge transfer of the three states is controlled by the ligand exchange in the coordination sphere of the calcium cation.