Fluorescence enhancement of a probe molecule in solution by the container molecule cucurbituril (CB) is reported for the first time. The fluorescence of the probe 2-anilinonaphthalene-6-sulfonate (2,6-ANS) in aqueous Na2SO4 solution is found to increase by a maximum factor of 5.0 upon addition of cucurbituril. This fluorescence enhancement is the result of the formation of a hostguest inclusion complex, in which the guest 2,6-ANS is incorporated inside the cavity of the host, cucurbituril. Measurement of the enhancement as a function of cucurbituril concentration yielded a value of the equilibrium constant (K) of 52 ± 10 M1. It is proposed that the mode of inclusion involves the phenyl group of the 2,6-ANS, because of the relatively small size of the cucurbituril cavity. It is further proposed that the observed enhancement is a result of loss of rotational mobility of the phenyl ring relative to the naphthyl fluorophore of 2,6-ANS upon inclusion of the phenyl ring. Since the name cucurbituril is derived from the Latin word for "pumpkin," this fluorescent host-guest complex is referred to as a "molecular Jack O'Lantern," with the 2,6-ANS serving as the candle.Key words: hostguest chemistry, fluorescence, cucurbituril, inclusion compounds.
A solid precipitate is formed immediately upon the mixing of aqueous salt solutions of the fluorescence
probe 1-anilino-8-naphthalenesulfonate (1,8-ANS) and the cage compound cucurbituril. The resulting solid
compound is highly fluorescent, with a maximum emission at ca. 478 nm. Single crystals of this compound
were obtained and studied by X-ray crystallography. The crystal structure indicates a rather novel solid
fluorescent compound, described as a lattice inclusion compound, in which the smaller 1,8-ANS molecules
have cocrystallized in the lattice of the cucurbituril cages. This structure is in effect a highly ordered solid
solution, in which the fluorescent 1,8-ANS molecules are well separated from each other by the larger
cucurbituril molecules. This separation of the fluorescent probe molecules by the nonabsorbing (in the visible
range) cucurbituril spacers prevents interaction of the 1,8-ANS fluorophores and results in a highly fluorescent
solid.
Xanthones with amino substituents were synthesized to diminish the photoreactivity of the xanthone chromophore with DNA, with the objective of using these molecules to study their binding dynamics with DNA. The aminoxanthones showed a strong solvatochromic effect on their singlet and triplet excited-state photophysics, where polar solvents led to a decrease of the energies for the excited states. Quenching of the triplet excited states by nitrite anions was used to determine the binding dynamics, and a residence time in the microsecond time domain was estimated for the bound 2-aminoxanthone with DNA. The quenching experiments performed showed that this methodology will not be applicable to study the binding dynamics of a wide variety of guests with DNA.
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