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 (phi(f) 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 (phi(f)=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.