Substituent effect on emission of flavonolate-boron difluoride complexes: The role of π-system for dual-state (solution and solid) emission

Abstract

In a quest for the still elusive dual-state emission (DSE), i.e., intense emissions both in solution and in the solid state, the luminescence properties of novel boron difluoride complexes with flavonolates bearing substituent(s) at the B ring (R: 4-Ph (2b); 4-OPh (2c); 4-NPh2 (2d); 2,6-Me2 (2e)) have been investigated. The emission intensity and maximum wavelength are strongly influenced by the electronic and steric factors of the substituents, which are responsible for the character of the π-conjugated system over the ligand. Complexes 2b and 2c with a rigid flat π-skeleton exhibited DSE (emission quantum yield (Φ) ≥ 0.34). Complex 2d bearing a rotatable diphenylamino group showed an aggregation-induced enhanced emission (AIEE), that is strong (Φ = 0.38) in the solid state but weak (Φ = 0.13) in CH2Cl2. Thus, 2b–2d exhibited intense solid-state emissions, although the corresponding ligands presented quenched solid-state emissions. Conversely, complex 2e, in which the B ring adopted a twisted position from the fused AC ring, that is, a twisted π-system, exhibited an aggregation-caused quenching (ACQ) resulting in an intense (Φ = 0.50) and a weak (Φ = 0.02) emission in CH2Cl2 and the solid state, respectively. This work provides important insights into the role of the π-system in the changeover of emission features in dual-state.