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 bis‐triarylborane tetracation (4‐Ar(2)B‐3,5‐Me(2)C(6)H(2))‐C≡C−C≡C‐(3,5‐Me(2)C(6)H(2)‐4‐BAr(2) [Ar=(2,6‐Me(2)‐4‐NMe(3)‐C(6)H(2))(+)] (2(4+)) shows distinctly different behaviour in its fluorimetric response than that of our recently published bis‐triarylborane 5‐(4‐Ar(2)B‐3,5‐Me(2)C(6)H(2))‐2,2′‐(C(4)H(2)S)(2)–5′‐(3,5‐Me(2)C(6)H(2)‐4‐BAr(2)) (3(4+)). Single‐crystal X‐ray diffraction data on the neutral bis‐triarylborane precursor 2 N 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 2(4+) 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 2(4+) bound to BSA 100 times stronger than 3(4+). Moreover, 2(4+) 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, 2(4+) 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.