Deciphering the working mechanism of aggregation-induced emission of tetraphenylethylene derivatives by ultrafast spectroscopy.

Yuanjing Cai,Xinggui Gu,Fei Qi,Kerim Samedov,Jacky W Y Lam,Herman H Y Sung,Brian O Patrick,Ian D Williams,Anjun Qin,Xiaofang Jiang,Zhiping Yan,David Lee Phillips,Lili Du,Ben Zhong Tang,Haoke Zhang

doi:10.1039/c8sc01170b

Abstract

Aggregation-induced emission (AIE) is the long-sought solution to the problem of aggregation-caused quenching that has hampered efficient application of fluorescent organic materials. An important goal on the way to fully understand the working mechanism of the AIE process was, for more than a decade, and still remains obtaining more comprehensive insights into the correlation between the ultrafast excited-state dynamics in tetraphenylethylene (TPE)-based molecules and the AIE effect in them. Here we report a number of TPE-based derivatives with varying structural rigidities and AIE properties. Using a combination of ultrafast time-resolved spectroscopy and computational studies, we observe a direct correlation between the state-dependent coupling motions and inhibited fluorescence, and prove the existence of photocyclized intermediates in them. We demonstrate that the dominant non-radiative relaxation dynamics, i.e. formation of intermediate or rotation around the elongated C[double bond, length as m-dash]C bond, is responsible for the AIE effect, which is strongly structure-dependent but not related to structural rigidity.

Highlights

In less than two decades, the seminal discovery of aggregationinduced emission (AIE)[1,2] has revolutionized the research elds of bioimaging,[3,4,5] chemo/biosensing,[6,7,8] and optoelectronic materials sciences.[9,10] Aggregation-induced emission (AIE) describes the phenomenon of molecules with low uorescence quantum yields in solution “lighting up” with enhanced uorescence emission upon aggregation
We demonstrate that the dominant non-radiative relaxation dynamics, i.e. formation of intermediate or rotation around the elongated C]C bond, is responsible for the AIE effect, which is strongly structure-dependent but not related to structural rigidity
Among several hypotheses concerning the mechanism of the AIE effect, the restriction of intramolecular rotation (RIR)[15,16,17,18,19] and restriction of intramolecular vibration (RIV)[20,21,22] subsumed under the overarching concept of restriction of intramolecular motion (RIM)[2,21] are the most frequently cited ones, with photocyclization[23,24,25] as well as E–Z isomerization[18,26,27] being invoked in a number of cases

Summary

Introduction

In less than two decades, the seminal discovery of aggregationinduced emission (AIE)[1,2] has revolutionized the research elds of bioimaging,[3,4,5] chemo/biosensing,[6,7,8] and optoelectronic materials sciences.[9,10] AIE describes the phenomenon of molecules with low uorescence quantum yields in solution “lighting up” with enhanced uorescence emission upon aggregation.

Results

Conclusion