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.