Academician Tang benzhong from Hong Kong University of science and technology: aromaticity reversal driven intramolecular vibration for the construction of vibration limited aggregation induced luminescence system

Source: Tang benzhong's research group has been focusing on the construction of advanced optical functional materials by regulating the movement of molecules in the excited state In recent years, academician Tang benzhong of Hong Kong University of science and technology has made a series of representative research results in this field Recently, based on the previous work on the regulation of excited state molecular motion (nat.commin 2019, 10, 768; adv funct Mater 2018, 28, 1705609; chem SCI 2018, 9, 6118; angel Chem Int ed 2018, 57, 7997), a new strategy to design vibrational limited aggregation induced luminescence system by aromaticity reversal is proposed (Fig 1) Relevant achievements were published in nat.commin (NAT Commin 2019, 10, 2952) and pushed in the chemical community of Nature Series in the form of Blog: https://chemistrycommunity.nature.com/users/264496-zheng-zhao/posts/50781-non-atomic-annulene-based-aggregation-induced-emission-system-via-atomic-reverse-process Fig 1 a new strategy of designing vibration limited aggregation induced luminescence system by aromaticity reversal Aggregation induced emission (AIE) refers to a kind of phenomenon that the luminescent molecules do not emit light in the dissolution state but enhance fluorescence in the aggregation state The mechanism behind this phenomenon is: in the excited state, the monodisperse form of molecules in the solution makes it possible to dissipate the excited state energy through the non radiative relaxation pathway through the active intramolecular movement; in the aggregated state, the intramolecular movement is limited, and the energy is released in the form of luminescence through the radiative transition pathway Among them, intramolecular motion includes rotation and vibration Since academician Tang benzhong put forward the concept of AIE in 2001, AIE has been widely used in the field of photoelectric devices and biochemical sensors Scientists from more than 1500 research units in more than 80 countries and regions around the world have participated in the relevant research in the field of AIE However, up to now, AIE molecular design is mainly achieved by introducing rotatable primitives into π system How to realize the AIE phenomenon by regulating the vibration in the molecule has become a challenge in the field of AIE research Figure 2 Theory of aromaticity reversal aromaticity is an important concept in organic chemistry, which determines the thermodynamic stable configuration of molecules in the ground state In theory, the change of aromaticity will cause the change of molecular configuration In 1972, Barid et al Predicted that the aromaticity of the ground state of the rotalenes is opposite to that of the excited state of the trilinear state through the molecular perturbation theory, and thus proposed the theory of aromaticity reversal (Fig 2) The theory has been proved to be reasonable and expanded by scientists through experiments According to Barid's theory, nonaromatic cyclooctylene will become aromatic after excitation, which will lead to the transformation of its configuration from nonplanar to planar The molecular vibration involved in this is consistent with the core design concept of AIE molecule, molecular motion control Therefore, academician Tang benzhong's team envisions whether this process can be used to realize the design of vibration limited AIE molecules? Fig 3 optical properties of cyclooctylthiophene and its derivatives In response to this question, they designed and synthesized a series of cyclooctylthiophene derivatives, cyclooctylthiophene system, with cyclooctylene as the core framework, and studied their optical properties As shown in Fig 3, although cyclooctylthiophene and its derivatives do not contain typical rotatable radicals, they all show typical AIE properties These results preliminarily confirm that it is a feasible strategy to design AIE molecules by using the molecular motion induced by aromaticity reversal In order to further clarify the real physical process behind it, they conducted in-depth research on its optical physical process through theoretical calculation and experimental means First of all, they found that the non radiative transition of molecules in solution state was positively correlated with the vibration of molecular skeleton, which indicated that the vibration in molecules was the main reason for the non radiative decay of molecules in solution Further spectroscopic experiments show that the chiral signal of the molecule disappears gradually under the condition of illumination, which indicates that under the condition of illumination, the molecule changes from one chiral configuration to another, that is, the configuration has been reversed In the solid state, the light can not make the chiral signal disappear The results show that in solution state, the aromaticity of the molecule is changed after excitation, which leads to the reversal of its configuration and the racemization of its chirality At the same time, the fluorescence of the molecule is quenched due to the movement of the molecule However, in the solid state, due to the restriction of molecular movement, the energy barrier of configuration inversion is increased, the molecular chirality is maintained, and the fluorescence signal can also be expressed normally (Fig 4) Fig 4 spectral experiment to further verify whether the change of excited state aromaticity results in the movement behavior of molecules, they studied the change of ground state and excited state aromaticity by means of calculation, and calculated the bond length of transition state under the ground state and excited state of molecules respectively (Fig 5) The results show that the length of single bond and double bond in excited state is more consistent than that in ground state, which is consistent with its aromaticity In addition, nuclear independent chemical shifts (NICS) and magnetic shielding currents (acid) are commonly used to characterize aromaticity The results show that the NICs of the compounds is positive in the ground state and negative in the excited state, and the current direction of acid is counter clockwise in the ground state and clockwise in the excited state, which is consistent with the aromaticity after excitation Fig 5 research on the change of ground state and excited state aromaticity The significance of this work is: from the perspective of materials, the author proposes a universal strategy for the design of vibration limited AIE materials, which is expected to promote the development of vibration limited AIE material system and the exploration of its new application In terms of mechanism, the author breaks the traditional AIE Because of the limitation of molecular design, it is the first time to think about the excited state behavior of AIE from the perspective of aromaticity, which is helpful to understand the luminescence mechanism behind AIE The first authors of this work are Dr Zhao Zheng of Hong Kong University of science and technology, Dr Zheng Xiaoyan of Beijing University of technology and Dr Du Lili of Hong Kong University The corresponding authors are academician Tang benzhong of Hong Kong University of science and technology, Professor Wang Hua of Henan University and Professor David Phillips of Hong Kong University Dr Zheng Xiaoyan provided theoretical calculation support for this work, and the ultrafast spectrum experiment and analysis were assisted by Dr Du Lili and Professor Phillips Professor Wang Hua gave some help and suggestions for the synthesis and characterization of some molecules The research is supported by NSFC, ITC, RGC and other projects in Hong Kong For more details, please see the full text link: https:// Review of previous reports: academician Tang benzhong of Hong Kong University of science and technology research group: self-report photosensitizer for in situ monitoring apoptosis process