Group 14 Metallafluorenes as Sensitive Luminescent Probes of Surfactants in Aqueous Solution

Sila- and germafluorenes containing alkynyl(aryl) substituents at the 2,7- position are strongly emissive with high quantum yields in organic solvents. Provided they are sufficiently soluble in water, their hydrophobic structures have the potential for many biological and industrial applications in the detection and characterization of lipophilic structures. To that end, the emission behaviors of previously synthesized 2,7- bis[alkynyl(biphenyl)]-9,9-diphenylsilafluorene (1), 2,7- bis[alkynyl(methoxynaphthyl)]-9,9-diphenylgermafluorene (2), 2,7- bis[alkynyl(p-tolyl)]-9,9-diphenylsilafluorene (3), and 2,7- bis[alkynyl(m-fluorophenyl)]-9,9-diphenylsilafluorene (4) were characterized in aqueous solution and in the presence of various surfactants. Despite a high degree of hydrophobicity, all of these metallafluorenes (MFs) are soluble in aqueous solution at low micromolar concentrations and luminesce in a common aqueous buffer. Further, the 2,7 substituent makes the emission behavior tunable (up to 30 nm). Fold emission enhancements in the presence of various surfactants are highest toward Triton X-100 and CTAB (ranging from 5 to 25 fold) and are lowest for the anionic surfactants SDS and SDBS. These enhancements are competitive with existing probes of surfactants. Quantum yields in buffer range from 0.11 to 0.34, competitive with many common fluorophores in biological use. Strikingly, MF quantum yields in the presence of TX-100 and CTAB approach 100 % quantum efficiency. MF anisotropies are dramatically increased only in the presence of TX-100, CTAB, and CHAPS. Coupled with the above data, this suggests that MFs associate with neutral and charged surfactant aggregates. Interactions with the anionic surfactants are weaker and/or leave MFs solvent exposed. These properties make metallafluorenes competitive probes for surfactants and their properties and behaviors, and thus could also have important biological applications.