Supercooled Liquid β‑Diketones with Mechanoresponsive Emission

Shear-induced
crystallization of dyes in the amorphous state is
an effective strategy for generating higher energy emission after
mechanical perturbationa rare phenomenon in mechanoresponsive
materials. Recently, we reported that a β-diketone with a 3,4,5-trimethoxy-substituted
phenyl ring formed a stable supercooled liquid (SCL) phase after melting
and cooling in air. To tune the lifetime of β-diketones in the
SCL phase, a series of dyes with 3,4,5-trimethoxy-substituted phenyl
rings were synthesized. Derivatives with naphthyl and phenyl rings
were prepared in order to modulate crystallization through arene interactions.
Additionally, dyes were substituted with alkoxy chains of varying
length to promote crystallization through increased van der Waals
interactions. Video screening in conjunction with differential scanning
calorimetry and X-ray diffraction studies indicated that naphthyl-substituted
derivatives exhibited increased melted state lifetimes and that increasing
the alkoxy chain length can induce crystallization. Analysis of molecular
packing of single crystals of PH, PC1, PC3, and PC5 revealed that
the central para-substituted methoxy group of the trimethoxy-substituted
ring was forced out of the molecular plane because of steric interactions
with neighboring methoxy groups. The stabilities of the SCLs were
generally correlated with the torsion angles of the para methoxy groups,
where derivatives with smaller angles exhibited faster rates of crystallization.
Mechanical perturbation of the SCL phases resulted in shear-induced
crystallization of PH, PC1, PC3, and NC6 derivatives. In some cases,
traditional mechanochromic luminescence with a crystalline-to-amorphous
phase transition was also observed, which indicates that some trimethoxy-substituted
β-diketones exhibit more than one type of mechanoresponsive
luminescence.