Anion-induced opposite mechanochromic and thermochromic emission directions of protonated hydrazones

Abstract

Smart materials with tunable multiple-color emissions have been widely investigated in the fields of bioimaging, display, and information encryption. Herein, multicolor emissive molecules with hydrazone bridged triphenylamines and pyridinium groups are reported. The protonation of the pyridine subunit by various acids leads to aggregation-induced emission with broad emission colors ranging from blue to red in multiple states and spectra of λ_PL, dichloromethane solution = 463–584 nm, λ_PL,powder = 455–620 nm, and λ_PL,crystal = 485–657 nm, respectively. Upon grinding or heating, hydrazone with CF₃COO⁻ exhibit blue-shift emissions from red to yellow due to weakened molecular packing and conformational rigidity. In contrast, hydrazone with (CF₃SO₂)₂N⁻ exhibited redshift emission from green to yellow due to the decreased electron-donating ability of the triphenylamine unit upon transformation from a rigid pyramidal shape to a planar structure. These are rare, charged organic examples exhibiting predictable mechanochromic and thermochromic strong emissions through facile anion exchanges, potentially providing new insights into the design of smart materials.