Revisiting the anti-heavy-atom effect: absence of halogen-based bonding and lack of enhanced aggregation-induced emission in bromine-substituted tetraphenylethylene derivatives†

Abstract

The well-established phenomenon of fluorescence quenching due to heavy atoms has been extensively recognized within optical physics. Nevertheless, recent research has revealed an intriguing counterpoint termed as “anti-heavy-atom effect”. The introduction of halogens into aggregation-induced emission (AIE) luminescent materials leads to enhanced fluorescence emission. Herein, we synthesized a series of (ethene-1,1,2,2-tetrayltetrakis(benzene-4,1-diyl))tetrakis (phenylmethanone) (ETTP) derivatives with different positions of bromine substitutions to explore their structural-photophysical relationships. Contrary to our expectations, the brominated ETTP derivatives did not exhibit a higher fluorescence quantum yield (Φ_F) than ETTP. Single-crystal analysis and theoretical calculations revealed that the absence of bromine-based bonding resulted in inadequate suppression of non-radiative relaxation pathways. Moreover, the introduction of carbonyl groups, with their lone electron pairs, resulted in interactions with the excited-state electrons, leading to a decrease in fluorescence intensity. Furthermore, the ETTP derivatives displayed excellent mechanofluorochromic properties arising from crystalline-to-amorphous phase transformations, which could have potential applications in information storage and anti-counterfeiting. This study demonstrates that if the introduced halogen atoms in AIE materials do not participate in the bond formation, the fluorescence emission will not be enhanced. These findings provide deeper insights into traditional views and hold significant implications for future developments of novel high-efficiency AIE-active materials.