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.