Molecular Engineering of Mechano­chromic Materials by Programmed C–H Arylation: Making a Counterpoint in the Chromism Trend

The
development of facile methods for screening organic functional
molecules through C–H bond activation is a revolutionary trend
in materials research. The prediction of mechano­chromism as
well as mechano­chromic trends of luminogens is an appealing
yet challenging puzzle. Here, we present a strategy for the design
of mechano­chromic luminogens based on the dipole moment of donor–acceptor
molecules. For this purpose, a highly efficient route to 2,7-diaryl-[1,2,4]­triazolo­[1,5-a]­pyrimidines (2,7-diaryl-TAPs) has been established through
programmed C–H arylation, which unlocks a great opportunity
to rapidly assemble a library of fluorophores for the discovery of
mechano­chromic regularity. Molecular dipole moment can be employed
to explain and further predict the mechano­chromic trends. The
2,7-diaryl-TAPs with electron-donating groups on the 2-aryl and electron-withdrawing
groups on the 7-aryl possess a relatively small dipole moment and
exhibit a red-shifted mechano­chromism. When the two aryls are
interchanged, the resulting luminogens have a relatively large dipole
moment and display a blue-shifted mechano­chromism. Seven pairs
of isomers with opposite mechano­chromic trends are presented
as illustrative examples. The aryl-interchanged congeners with a bidirectional
emission shift are structurally similar, which provides an avenue
for understanding in-depth the mechano­chromic mechanism.

通过碳氢键活化（C–H bond activation）实现有机功能分子筛选的简便方法开发，是材料研究领域的革命性发展趋势。精准预测发光体（luminogens）的力致变色（mechanochromism）行为及其变化规律，是一项兼具吸引力与挑战性的课题。本研究提出一种基于给体-受体分子（donor–acceptor molecules）偶极矩（dipole moment）的力致变色发光体设计策略。为此，我们通过程序化碳氢键芳基化反应，建立了高效合成2,7-二芳基-[1,2,4]三唑并[1,5-a]嘧啶（2,7-diaryl-[1,2,4]triazolo[1,5-a]pyrimidines，简称2,7-diaryl-TAPs）的通用路线，该路线为快速构建荧光团文库以探究力致变色规律提供了重要契机。分子偶极矩可用于解释并进一步预测力致变色的变化趋势：当2-芳基连有给电子基团（electron-donating groups）、7-芳基连有吸电子基团（electron-withdrawing groups）时，所得2,7-diaryl-TAPs的偶极矩相对较小，表现出红移（red-shifted）型力致变色行为；若将两个芳基的取代位置互换，则得到的发光体偶极矩相对更大，呈现蓝移（blue-shifted）型力致变色现象。本文展示了7对具有相反力致变色趋势的异构体（isomers）作为示例，这类芳基互换的同系物结构相似且发射位移（emission shift）方向双向可调，为深入解析力致变色机理提供了可行途径。