Revisit anti-heavy-atom effect: no halogen-based bond, no enhanced aggregation induced emission for 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 researchers have revealed an intriguing counterpoint termed as 'anti-heavy-atom effect'. The introduction of halogens into aggregation-induced emission luminescent (AIE) materials leads to enhanced fluorescence emission. Herein, we synthesize 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 expectation, these brominated ETTP derivatives do not exhibit higher fluorescence quantum yield (ΦF) compared to ETTP. Single crystal analysis and theoretical calculations unveil that without the formation of bromine based bonding leads to inadequate suppression of non-radioactive relaxation pathways. Moreover, the introduction of carbonyl groups possessing lone electron pairs interacts with the excited state electrons, leading to the decrease of fluorescence intensity. Furthermore, the ETTP derivatives display excellent mechanofluorochromic properties arising from crystalline-to-amorphous phase transformations and 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. The finding provides deeper insight into traditional views and held significant implications for future development towards novel high-efficiency AIE-active materials.