Polymorph-Dependent Multicolor-Switchable Mechanofluorochromism of 1‑(9-Anthryl)vinyl-3-(3-pyridyl)vinylbenzene: Molecular Conformation versus Intermolecular Interaction

The
mechanism of most mechanofluorochromic (MFC) materials is related
to a noncovalent change in aggregate packing. It is still a challenge
to clarify the respective effects of molecular conformation and intermolecular
interaction on the MFC performance. Herein, a new (9-anthryl)­vinyl
derivative APYB with a polymorph-dependent multicolor-switchable
MFC property is reported. APYB has only one torsion unit
in the molecule configuration and could form four different crystal
phases (APYB-a, APYB-b, APYB-c, and APYB-d). Furthermore, all polymorphs are MFC-active.
Therefore, it provides a rare and direct case to correlate the aggregate
packing with MFC performance. The solid-state emission could be finely
tuned from 465 to 500 nm by virtue of polymorphism. Crystal structure
analyses and optical studies reveal that an increasing twist angle
could result in a blue shift in the fluorescence spectrum and a high-contrast
MFC performance. In contrast, strong intermolecular interactions could
lead to high emission efficiency and amorphization difficulty during
the mechanical grinding. A tricolor-switchable MFC phenomenon was
observed for APYB-a, while both APYB-b and APYB-d display a reversible bicolor-changing MFC behavior.
As for APYB-c, mechanical grinding only causes a minor
change in the emission spectrum. However, two competitive reverse
transformation routes were found when treating the ground sample with
organic solvent vapor. XRD analyses show that the MFC mechanism for APYB-a, APYB-c, and APYB-d is a
crystal-to-amorphous transformation, while only partial amorphization
was observed in the MFC behavior of APYB-b. The acidochromism
effects of APYB-a and APYB-b were also investigated.
They also display multicolor changes due to the polymorphic nature
of APYB. Interestingly, in the deprotonation process,
NH3 vapor could induce a different transformation route
and color change from NEt3.