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

Abstract

Sila-and germa uorenes containing alkynyl(aryl) substituents at the 2,7-position are strongly emissive with high quantum yields in organic solvents. Provided they are su ciently 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-diphenylsila uorene (1), 2,7-bis[alkynyl(methoxynaphthyl)]-9,9diphenylgerma uorene (2), 2,7-bis[alkynyl(p-tolyl)]-9,9-diphenylsila uorene (3), and 2,7-bis[alkynyl(muorophenyl)]-9,9-diphenylsila uorene (4) were characterized in aqueous solution and in the presence of various surfactants. Despite a high degree of hydrophobicity, all of these metalla uorenes (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 20 nm). Fold emission enhancements in the presence of various surfactants are highest toward Triton X-100 and CTAB (ranging from 5-25 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 uorophores in biological use. Strikingly, MF quantum yields in the presence of TX-100 and CTAB approach 100% quantum e ciency. 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 metalla uorenes competitive probes for surfactants and their properties and behaviors, and thus could also have important biological applications.

Push-pull behavior of 2,7-disubstituted sila- and germafluorenes

“…2,7-disubstituted sila-and germafluorenes (metallafluorenes, MFs) [17][18][19] (Scheme 1) have many of the above favorable photophysical properties due to the aromatic core with strong 𝜋 − 𝜋 stacking and tunable 2,7-substituents. In previous work, we demonstrated that this small library of 2,7-disubstituted sila-and germafluorenes interact with surfactant micelles, exhibiting impressive quantum yields in the presence of surfactants (approaching 100% quantum efficiency) and appreciable emission fold enhancements (5-25x) [20]. We also demonstrated that these compounds have specificity for lipid structures in yeast cells, colocalizing with the lipid droplet stain Nile Red [21].…”

Group 14 Metallafluorenes for Lipid Structure Detection and Cellular Imaging

“…Proc. 2022, 4, x luminescent in aqueous solution and exhibit high quantum yields and dramatic emi enhancements in the presence of various surfactants (5-25-fold) [10]. These results su that MFs could have biological applications.…”

“…Furthermore, they are non-toxic to cells and can enter cells and colocalize with Nile Red, a lipid probe. In addition, the higher extinction coefficients of MFs and competitive quantum yields [10] make them more sensitive. All of these observations bode well for the application of MFs as lipid probes both in vitro and in vivo.…”

“…All chemicals were of reagent grade and were used as received without further purification. Compounds 1-4 were synthesized as previously described using an appropriate alkynyl(aryl) precursor in a palladium-catalyzed Sonagashira cross-coupling reaction [8,9] and dispensed from stocks in DMSO as previously described [10].…”

“…Recently, we evaluated a small library of sila-and germafluorenes (metallafluorenes or MFs) containing alkynyl(aryl) substituents at the 2,7-position ( [8,9]; Figure 1) for their potential as fluorescent probes of surfactants. These compounds are soluble and luminescent in aqueous solution and exhibit high quantum yields and dramatic emission enhancements in the presence of various surfactants (5-25-fold) [10]. These results suggest that MFs could have biological applications.…”