A Water‐Soluble Fluorescent 2,5‐bis(arylethynyl)rhodacyclopentadiene for Live‐Cell Imaging and Sensing of DNA, RNA, and Protein (BSA)

A water‐soluble and water‐stable 2,5‐bis(arylethynyl)rhodacyclopentadiene ( 4a ) containing terminal 4‐Me ₃ N–C ₆ H ₄ groups has been synthesized for bioimaging and sensing of biomolecules, and its linear and nonlinear optical properties investigated. Complex 4a exhibits fluorescence from the S ₁ excited state in both organic and aqueous solutions, with no observable phosphorescence from T ₁ at RT. However, competitive intersystem crossing (ISC) to T ₁ leads to potent ¹ O ₂ sensitization. DFT calculations show that the HOMO and LUMO are essentially ligand‐localized with very modest contributions from Rh, leading to slow S ₁ → T ₁ ISC and competitive fluorescence and triplet state formation, typical of such 2,5‐bis(arylethynyl)rhodacyclopentadienes. Complex 4a shows a two‐photon absorption cross‐section of 290 GM at 680 nm. It binds strongly to DNA, RNA, and protein (BSA) with similar affinities, but gives opposite fluorimetric response, the emission being strongly enhanced for BSA but efficiently quenched by DNA/RNA. Compound 4a efficiently enters living human cells and accumulates preferentially in mitochondrial membranes, being nontoxic even at high (10 µM) concentrations. Although 4a is a potent ¹ O ₂ sensitizer in a cuvette, exposure of the complex to intense visible light inside human cell results in fast bleaching of 4a with no effect on cell viability, suggesting local consumption of ¹ O ₂ by the complex.