A triphenylamine derivative and its Cd(ii) complex with high-contrast mechanochromic luminescence and vapochromism

Abstract

An acetonitrile solvate of a Schiff base molecule (HL) with acetonitrile (HL·2CH₃CN) and its Cd(II) complex (Cd(HL)₂Cl₂, 1) was designed and synthesized. The different conformation of HL in HL·2CH₃CN is adjusted by the hydrogen bonding O–H⋯O between HL molecules, together with N–H⋯N between HL molecules and acetonitrile molecules. Meanwhile, the conformation of HL in complex 1 is adjusted by coordination interactions between Zn ions and atoms O and N, together with hydrogen bonding N–H⋯Cl. The presence of the triphenylamine group makes HL·2CH₃CN and complex 1 loosely packed. Upon grinding, HL·2CH₃CN and complex 1 both showed high-contrast mechanochromic luminescence (MCL) change from blue to green and cyan to yellow, respectively. These changes can be eliminated by fumigation with organic vapor. The results of powder X-ray diffraction (PXRD) show that their MCL is due to the phase transformation from a crystalline state to an amorphous state. In addition, HL·2CH₃CN and complex 1 also exhibited high-contrast acidochromism upon exposure to HCl and NH₃ vapor. X-ray photoelectron spectroscopy (XPS) and PXRD studies show that the protonation of the –NH– group together with phase transformation from a crystalline state to an amorphous state is attributed to the fluorescence switching. For HL·2CH₃CN, the protonation process was accompanied by the departure of acetonitrile molecules. The emission of HL was restored to the amorphous state rather than the original crystalline state emission after further exposure to NH₃ vapor, while 1-HCl could revert to the original crystalline state emission. In addition, HL·2CH₃CN and complex 1 have been successfully used to produce writable and acid-responsive test papers.