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'-cyan o stilbene, a polar fluoroionophore of the stilbene-crown series, show s the formation of an intramolecular charge-transfer state (TICT state ), in less than 1 ps (k > 10(12) s(-1))in acetonitrile (CH3CN) and les s 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 dag ger/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 t hree intermediates on the decoordination pathway have been obtained. T he increasing charge transfer of the three states is controlled by the ligand exchange in the coordination sphere of the calcium cation.