Isomeric intramolecular charge-transfer complexes: the effect of relative positions of the donor and acceptor on photophysical and mechanochromic properties

Abstract

For organic charge-transfer (CT) complexes, although the relative positions of the electron-donor (D) and electron-acceptor (A) play a crucial role in governing their optical and electronic properties, a comprehensive understanding of the specific structure–property relationship remains elusive, largely due to the scarcity of suitable model CT systems. To address this challenge, herein, a conceptually new strategy of isomer engineering of intramolecular CT complexes is proposed, and two positional isomers cis-MDF and trans-MDF were designed and synthesized accordingly, by decorating a distinctive platform of methylene-bridged difluorene with a triphenylamine unit and a 4-formylphenyl group. cis-MDF and trans-MDF have been revealed to exhibit distinct photophysical properties including absorption characteristics, polarity-dependent fluorescence behaviour and Stokes shifts, as well as diverse mechanochromic performances. Furthermore, detailed structural information for both isomers has also been provided via single crystal X-ray diffraction analysis. Based on systematic comparisons of the structure and property differences between the two isomers, mechanistic insights into how the D–A relative positions lead to varied photophysical and mechanochromic properties are presented. This work highlights the elegance of isomer engineering in elucidating the effect of D–A relative positions for CT complexes.