Dipyrrolylquinoxaline difluoroborates with intense red solid-state fluorescence

Abstract

A set of organic fluorescent dyes of dipyrrolylquinoxalines (PQs 4–6) and their BF₂ complexes (BPQs 1–3) were synthesized from commercial reagents, and were characterized by their X-ray structural analysis, and optical and electrochemical properties. BPQs 1–3 showed intense broad absorption in the visible region in the solution-state. In comparison with that of PQs 4–6, there is an over 110 nm red-shift of the absorption maximum in the BPQs 1–3 (up to 583 nm). Interestingly, dyes 1–6 all exhibit red solid-state fluorescence with moderate to high fluorescence quantum yields except for PQ 4 which showed bright yellow solid-state fluorescence. X-ray structures of BPQs 2–3 showed the planar structure of quinoxaline with one pyrrole unit via the BF₂ chelation and the almost perpendicular orientation of the uncoordinated pyrrole to the NBN core plane (the dihedral angle of 70–73°). The extended π-conjugation was in good agreement with the observed red-shift of the spectra. These dyes formed well-ordered intermolecular packing structures via the intermolecular hydrogen bonding between the N atoms of quinoxaline moieties and the NH units of adjacent pyrroles. The lack of π–π stacking in their crystal packing structures may explain the interestingly intense solid-state fluorescence of these dyes.