Aggregation-induced emissions in the mesogenic BF2 complexes of aroylhydrazines

Abstract

Herein, we report the synthesis of two new families of borondifluoride complexes derived from aroylhydrazines and the investigation of their mesomorphic and AIE properties. The crystallographic structure of the mesogenic complex 1a-BF₂ (n = 8) was determined by X-ray analysis; it crystallizes in the P triclinic space group and the geometry of boron atoms is tetrahedral. The crystal is considered to be linear-shaped with a length of L ∼ 33.92 Å. A relatively small dihedral angle of ca. 3.30°–5.75° was observed between both neighboring phenyl groups and tetrahedral BF₂ moiety. Mesomorphic properties were crucially impacted by the structure of the terminal group. Ligands 1a–b were not mesogenic; however, both the series of 1a-BF₂ and 1b-BF₂ complexes formed SmC or/and N phases. The H-bonding interactions are attributed to the occurrence of mesomorphism in compounds 1-BF₂, which was supported by FT-IR spectroscopy. The optical property of the boron complexes is related to their molecular structures, and they emitted blue-to-green emissions at λ_max = 445–466 nm in CH₂Cl₂. Compounds 1a-BF₂ aggregated in higher water fractions (f_w), which turned on their light emission process, whereas weak PL signals were recorded at f_w < 50% because the luminogen was fully dissolved and quenched in these solvent compositions. The PL intensity was enhanced at f_w > 60% until it reached the maximum intensity at f_w ∼ 90%. Thus, compound 1a-BF₂ is AIE active.