Tetracationic bis‐triarylborane 1,3‐butadiyne as a combined fluorimetric and Raman probe for simultaneous and selective sensing of various DNA, RNA and proteins

A new bis-triarylborane tetracation (4-Ar2B-3,5-Me2C6H2)-C identical withC-C identical withC-(3,5-Me2C6H2-4-BAr2 [Ar = (2,6-Me2-4-NMe3-C6H2)+] (24+) shows distinctly different behaviour in its fluorimetric response than that of our recently published bis-triarylborane 5- (4-Ar2B-3,5-Me2C6H2)-2,2'-(C4H2S)2-5'-(3,5-Me2C6H2-4-BAr2) (34+). Single-crystal X-ray diffraction data on the neutral bis-triarylborane precursor 2N confirm its rod-like dumbbell structure, which is shown to be important for DNA/RNA targeting and also for BSA protein binding. Fluorimetric titrations with DNA/RNA/BSA revealed the very strong affinity of 24+ and indicated the importance of the properties of the linker connecting the two triarylboranes. Using the butadiyne- rather than a bithiophene linker resulted in an opposite emission effect (quenching vs enhancement), and 24+ bound to BSA 100 times stronger than 34+. Moreover, 24+ interacted strongly with ss-RNA, and circular dichroism (CD) results suggest ss-RNA chain-wrapping around the rod-like bis-triarylborane dumbbell structure like a thread around a spindle, a very unusual mode of binding of ss-RNA with small molecules. Furthermore, 24+ yielded strong Raman/SERS signals, allowing DNA or protein detection at ca. 10 nM concentrations. The above observations, combined with low cytotoxicity, efficient human cell uptake and organelle-selective accumulation make such compounds intriguing novel lead structures for bio-oriented, dual fluorescence/Raman-based applications.