Stretching the Bisalkyne Raman Spectral Palette Reveals a New Electrophilic Covalent Motif.

Small heteroaryl-diyne (Het-DY) tags with distinct vibrational frequencies, and physiologically relevant cLog P were designed for multiplexed bioorthogonal Raman imaging. Pd-Cu catalyzed coupling, combined with the use of Lei ligand, was shown to improve overall yields of the desired heterocoupled Het-DY tags, minimizing the production of homocoupled side-products. Spectral data were in agreement with the trends predicted by DFT calculations and systematic introduction of electron- rich/poor rings stretched the frequency limit of aryl-capped diynes (2209-2243 cm-1 ). The improved Log P of these Het-DY tags was evident from their diffuse distribution in cellular uptake studies and functionalizing tags with organelle markers allowed the acquisition of location-specific biological images. LC-MS- and NMR-based assays showed that some heteroaryl-capped internal alkynes are potential nucleophile traps with structure-dependent reactivity. These biocompatible Het-DY tags, equipped with covalent reactivity, open up new avenues for Raman bioorthogonal imaging.