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

Abstract

For organic charge-transfer (CT) complexes, although the relative positions of 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 was proposed, and two positional isomers of intramolecular CT complexes 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 of 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 for the first time highlights the elegance of isomer engineering in deciphering the effect of D-A relative positions for CT complexes.