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 n m 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.