Tuning the Mechanochromic Luminescence of BOPIM Complexes by Rational Introduction of Aromatic Substituents

Considering
the great advantages of boron 2-(2′-pyridyl)­imidazole
(BOPIM) based dyes, such as facile synthesis, high fluorescence quantum
yield, large Stokes shift, and good thermal stability, three BOPIM
complexes (BOMPIM-Ph, BOPIM-Th, and BOPIM-TTh) with different aromatic side groups have been developed.
Their crystal structures and photophysical and mechanochromic luminescence
properties were systematically studied. It was found that their solid
state emission and mechanochromic behaviors are related to the side
group. High color contrast reversible mechanochromism and piezochromism
were observed for the three analogues, while BOPIM-TTh with more bulky thienothiophene groups showed a more sensitive and
pronounced mechanochromic response. Steric and electronic effects
of the aromatic donating substitutes were proved playing significant
roles in regulating the intermolecular interactions and intramolecular
charge transfer effect. Fine-tuning the molecular structures of such
kind of BOPIM dyes allows convenient modulation of their optical properties
and mechano-responsive behaviors.