Pyrazolo[1,5‐a]pyrimidine‐Dioxaborinine Hybrid Dyes: Synthesis and Substituent Effect in the Photophysical Properties

A novel family of pyrazolo[1,5‐a]pyrimidine‐dioxaborinine (PP‐DB) hybrid dyes was synthesized by the direct construction of the dioxaborinine (DB) fragment on the pyrazolo[1,5‐a]pyrimidine (PP) ring, which implies the formation of four new bonds in a one‐pot manner. The dyes’ optical properties were investigated and compared with the starting pyrazolo[1,5‐a]pyrimidines; a study evidencing large fluorescence quantum yields in products (φf up to 69 %) due to an intramolecular charge transfer (ICT) process from the PP core to a ring of DB (PP→DB) that is absent in precursors (φf=0.03–0.30). Time‐dependent density functional theory (TD‐DFT) calculations confirmed the fluorescence process involved in the novel dyes, where their ICT limits the non‐radiative process due to the restricted rotation in the D−A system. The present work provides insight into how phenyl and DB ring incorporation impact the optical properties of this new group of hybrids dyes based on PP‐DB. A novel family of hybrid fluorophores based on the pyrazolo[1,5‐a]pyrimidine‐dioxaborinine (PP‐DB) system is provided by constructing the dioxaborinine moiety on the fused ring in a one‐pot manner.