Non-aromatic annulene-based aggregation-induced emission system via aromaticity reversal process

Zheng Zhao,Jacky W Y Lam,David Lee Phillips,Xuewen He,Ying Yu,Fengyan Song,Yu Xiong,Xueqian Zhao,Jing Zhang,Hua Wang,Yingjie Liu,Wei He,Ryan T K Kwok,Xiaoyan Zheng,Lili Du,Xuhui Huang,Xiuxiu Gao,Bin Xu,Yuanjing Cai,Chunli Li,Ben Zhong Tang

doi:10.1038/s41467-019-10818-5

Abstract

Aggregation-induced emission (AIE) is a photophysical phenomenon correlated closely with the excited-state intramolecular motions. Although AIE has attracted increasing attention due to the significant applications in biomedical and optoelectronics, an in-depth understanding of the excited-state intramolecular motion has yet to be fully developed. Here we found the non-aromatic annulene derivative of cyclooctatetrathiophene shows typical AIE phenomenon in spite of its rotor-free structure. The underlying mechanism is investigated through photoluminescence spectra, time-resolved absorption spectra, theoretical calculations, circular dichroism as well as by pressure-dependent fluorescent spectra etc., which indicate that the aromaticity reversal from ground state to the excited state serves as a driving force for inducing the excited-state intramolecular vibration, leading to the AIE phenomenon. Therefore, aromaticity reversal is demonstrated as a reliable strategy to develop vibrational AIE systems. This work also provides a new viewpoint to understand the excited-state intramolecular motion behavior of lumiongens.

Highlights

Aggregation-induced emission (AIE) is a photophysical phenomenon correlated closely with the excited-state intramolecular motions
Since the aromaticity reversal will result in a change of molecular conformation, the excited-state aromaticity reversal can possibly serve as a driving force to induce a large amplitude conformation flip to quench the emission and the suppression of which theoretically can light up the emission, leading to the generation of a new type of AIE system
We investigate the luminescent behavior of COTh and its derivatives by photoluminescence (PL) spectra, timeresolved absorption spectra, theoretical calculations, circular dichroism (CD), and circular polarized luminescence (CPL) spectroscopy, as well as pressure-dependent fluorescent spectra

Summary

Introduction

Aggregation-induced emission (AIE) is a photophysical phenomenon correlated closely with the excited-state intramolecular motions. Aromatic compounds generally show suppressed molecular motion via superior electron conjugation, which makes their vibrational/rotational degrees of freedom and torsional space rather limited for further tuning In this regard, the annulene system with non-aromaticity shows a large magnitude of vibration upon photoexcitation, which provides a suitable platform for studying the influence of vibrational behavior on the exciton relaxation[24,25,26]. Hada et al monitored the dynamic structural change of thiazole fused COT with liquid crystal character induced by excited-state aromaticity reversal[38] These COT derivatives show active conformation flip after photoexcitation, their luminescent properties have not been investigated, which inspired us to explore the possibility of using COT derivatives to construct vibrational AIE system and unveil the relationship among the aromaticity reversal, molecular motion, and luminescent properties. The extensive application platform of AIE materials provides a new outlet for utilizing Baird’s rule to develop luminescent functional materials

Methods

Results

Conclusion