Bodipy dyes bearing oligo(ethylene glycol) groups on the meso-phenyl ring: tuneable solid-state photoluminescence and highly efficient OLEDs

Abstract

Borondipyrromethene (Bodipy) dyes with strong solid-state photoluminescence are highly desirable for their applications in OLEDs. In this work, a series of meso-(4-R–C₆H₄)-substituted Bodipy dyes (R = H for B1; R = (OCH₂CH₂)_nOCH₂CH₃, n = 0–3 for B2–B5, respectively) were prepared through an acid-catalyzed reaction. The flexible ether groups attached to the periphery of B2–B5 are expected to influence the molecular arrangement and intermolecular interactions in solid state. Crystallographic analysis of B1–B4 reveals that the meso-phenyl ring is almost orthogonal to the indacene plane. Intermolecular π–π interactions are observed in B1 but absent in B2–B4. As a consequence, the dye B1 is weakly emissive in solid state, while B2–B4 emit strongly in the red region with emission quantum yields of up to 0.33. The PMMA films doped with B2 show two separated emission peaks, and their relative intensity is concentration dependent, leading to the fluorescence color varying from greenish yellow to red as the concentrations successively increased from 5 to 80 wt%. The formation of dimers in the ground state is found to be responsible for the red emission in the condensed state. Efficient OLEDs were fabricated by doping 2 wt% B2–B4 as emitters and gave high luminance, current efficiency, and external quantum efficiency of up to 920 cd m⁻², 8.00 cd A⁻¹, and 2.15%, respectively.