Electronic interactions between a quaternary pyridyl-β-diketonate and anionic clay nanosheets facilitate intense photoluminescence

Abstract

Electrostatic interactions between a quaternary pyridyl-β-diketonate and anionic charged nanosheets were observed to produce a highly emissive dispersion in a rich water solution. A greater fluorescence quantum yield of approximately 50% was obtained when a luminogenic β-diketonate, 1-(4-methoxyphenyl)-3-(3-hydroxyethyl-pyridinium bromide)-1,3-propandione (prepared by the Claisen condensation reaction and subsequent quaternization), was molecularly dispersed and enclosed by a couple of atomically flat ultrathin (approximately 1.0 nm) silicate sheets of anionic layered clay. By accommodating β-diketonate into a narrow interlamellar space (approximately 0.4 nm distance), the molecular motion was suppressed, as confirmed by a smaller non-radiative relaxation rate constant, which was obtained by time-resolved luminescence and quantum yield measurements. Because the dense packing of β-diketonate quenched the excited state, the isolation of luminogens by the co-adsorption of photochemical inert cations (tetramethylammonium and benzylammonium) was prevented by concentration quenching. A lower quantum yield was obtained by expanding the interlayer distance above 1.0 nm by co-adsorbing a photo-inactive water-soluble polymer, poly(vinylpyrrolidone). Therefore, the fixation and spatial separation of β-diketonate in the narrow interlayer space was determined to be essential for obtaining strong emission.