Structure–property relationships in chalcones with extended π-conjugation: ICT perturbation and red fluorescence

Abstract

4-Dimethylamino-2′-hydroxychalcone (DHC) derivatives are known to exhibit distinctive photophysical properties for various applications, including highly fluorescent crystals and molecular imaging. In a continuous effort to establish the structure–property correlation of this class of materials, a series of methoxy-substituted chalcone derivatives with extended π-conjugation have been synthesized to understand the effect of methoxy substituents on the intramolecular charge transfer (ICT) along the backbone. These derivatives displayed low fluorescence in protic solvents (e.g. ϕ_fl ≈ 0.003–0.009 in H₂O), but high fluorescence in a polar aprotic solvent (ϕ_fl ≈ 0.2–0.35 in DMSO), with emission maxima up to 658 nm. Fluorescence lifetime measurements revealed that the major emissive species could be attributed to locally excited (LE) and ICT states, while their emissive composition was heavily dependent on the methoxy-substitution pattern. The proposed predominant ICT mechanism was confirmed by low temperature fluorescence spectroscopy and supported by TDDFT calculations. Single crystal X-ray crystallography provided structural evidence for the chalcone derives, showing that the methoxy substituents could induce different tortional angles between the aromatic A-ring and the electron donor–acceptor backbone. The degree of the torsional angles was consistent with the number of the methoxy substituents present on the A-ring. These chalcone crystals were also highly fluorescent, with red-shifted emission (up to 703 nm) and quantum yield as high as Φ_fl = 0.363. This study explores a new class of chalcones that exhibit high fluorescence in both solution and solid states, and emission in the desirable red region.