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To make the effects of molecular size on photoinduced electron-transfer (ET) reactions clear, the ET fluorescence quenching of aromatic hydrocarbons by trivalent lanthanide ions M3+ (europium ion Eu3+ and ytterbium ion Yb3+) and the following ET reactions such as the geminate and free radical recombination were studied in acetonitrile. The rate constant k(q) of fluorescence quenching, the yields of free radical (phi(R)) and fluorescer triplet (phi(T)) in fluorescence quenching, and the rate constant k(rec) of free radical recombination were measured. Upon analysis of the free energy dependence of k(q), phi(R), phi(T), and k(rec), it was found that the switchover of the fluorescence quenching mechanism occurs at deltaG(fet) = -1.4 to -1.6 eV: When deltaG(fet) < -1.6 eV, the fluorescence quenching by M3+ is induced by a long-distance ET yielding the geminate radical ion pairs. When deltaG(fet) > -1.4 eV, it is induced by an exciplex formation. The exciplex dissociates rapidly to yield either the fluorescer triplet or the geminate radical ion pairs. The large shift of switchover deltaG(fet) from -0.5 eV for aromatic quenchers to -1.4 to -1.6 eV for lanthanide ions is almost attributed to the difference in the molecular size of the quenchers. Furthermore, it was substantiated that the free energy dependence of ET rates for the geminate and free radical recombination is satisfactorily interpreted within the limits of the Marcus theory.

Chemosensors have attracted considerable attention among the numerous strategies for detecting organic molecules in water. A turn-off mechanism was previously employed for the construction of a cyclodextrin (CD) chemosensor. This mechanism is greatly effective but has several shortcomings. In order to overcome these shortcomings, new fluorescent chemosensors NC0αCD, NC0βCD, and NC0γCD, which were (7-nitrobenz-2-oxa-1,3-diazol-4-yl)amine-modified α-CD, β-CD, and γ-CD, respectively, were prepared. Their guest selectivities were different from those of previously reported CD chemosensors. Here, the mechanism of new CD chemosensors was investigated using nuclear magnetic resonance (NMR) spectroscopy and molecular mechanics calculations. The fluorescence intensity of NC0βCD and NC0γCD slightly decreased and largely increased, respectively, upon the addition of ursodeoxycholic acid as a guest. This is due to the fact that the fluorophore of NC0βCD moved away to the hydrophilic bulk water to form hydrogen bonds between the host and the guest, while the fluorophore of NC0γCD remained located at the primary hydroxy side of the γ-CD unit to form a stable inclusion complex with hydrogen bonds between the fluorophore and the guest. NC0αCD also acted as a turn-on chemosensor for small guests, which could not be detected by the previous CD chemosensors. The motion restriction of the fluorophore through the generation of inclusion complexes could also contribute to increase in fluorescence intensity. [ABSTRACT FROM AUTHOR]

Boron difluoride (BF 2 ) complexes of N -(5-phenyl-2-pyrazinyl)pivalamide and its derivatives having a substituent on the phenyl group were prepared and their spectroscopic properties were investigated. It was found that the BF 2 complexes show fluorescence in solution and the solid state. The origins of the fluorescence properties are discussed on the basis of the fluorescence yields and lifetimes, crystal structures and DFT calculations. The present work would provide a guide to design BF 2 complexes of amidopyrazines exhibiting solid state fluorescence. [ABSTRACT FROM AUTHOR]