Aggregation Behavior of Mixed Fluorocarbon and Hydrocarbon Molecules in Aqueous Organic Solvents. Nonideality and Ideality of Mixing

Abstract

The emission spectra of naphthalene labeled hydrocarbons (NpHCn) and fluorocarbons (NpFCn) and anthracene labeled hydrocarbons (AnHCn) and fluorocarbons (AnFCn) in aqueous organic mixed solvents were investigated. The fluorescence spectrum of NpHC 16 in dimethyl sulfoxide-water (DMSO-H20) is dominated by excimer emission due to formation of aggregates. Addition of a fluorocarbon compound with chain length of seven perfluoromethylenes results in a reduction in the excimer emission and an enhancement of monomer emission, suggesting formation of coaggregates. In contrast, addition of a perfluorocarbon compound with chain length of 12 perfluoromethylenes results in no change in the fluorescence spectrum of NpHC16. Similarly, NpFC7 and NpFC12 in DMSO-H20 also exhibit excimer emission exclusively. Fluorescence studies show that hydrocarbon compounds coaggregate with NpFC7 but do not coaggregate with NpFC12. Selective excitation of the naphthalene moiety in the mixture solution of NpHC16 with AnFC7 or in the mixture solution of NpFC7 with AnHC12 exclusively leads to the emission of the anthracene moiety, suggesting that efficient energy transfer between the naphthalene and the anthracene chromophores occurs. On the other hand, for the mixture of NpHC16 with AnFC12 or NpFC12 with AnHC12, no energy transfer was detected. All these observations indicate that fluorocarbon compounds with short chains can mix with hydrocarbon compounds ideally, while those with long chains behave nonideally. The mixed critical aggregation concentrations of NpHC16 with NpFC7 and with NpFC12 are measured as a function of overall composition of the mixtures, which give insight into the magnitude of the nonideality of the mixings.