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

Picosecond absorption spectroscopy of 4-(N-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(12) s(-1))in acetonitrile (CH3CN) and less than 4 ps (k = (2.5 +/- 0.7) x 10(11) 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)double dagger, to a cation probe pair (L double dagger M) (k = (2.5 +/- 0.5) x 10(11) s(-1) in CH3CN and k = (3.3 +/- 0.5) x 10(11) s(-1) in BuCN) and to a solvent separated cation probe pair (L double dagger/S/M) (k = (1.4 +/- 0.5) x 10(10) s(-1) in CH3CN and k = (4 +/- 0.5) x 10(10) 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.