Effect of alkyl chain length on piezochromic luminescence of iridium(III)-based phosphors adopting 2-phenyl-1H-benzoimidazole type ligands

In this work, a series of luminescent cationic iridium(III) complexes containing 2-phenyl-1H-benzimidazole-type ligands modified with n-alkyl chains of various lengths have been successfully synthesized and characterized. Their photophysical and electrochemical properties have been investigated in detail. Differences in n-alkyl chain length has negligible affect on their respective complex's emission spectra or on their excited-state characteristics in solution, which is supported by density functional theory calculations and cyclic voltammetry. In the solid state, these complexes exhibit piezochromic luminescence (PCL) behaviour which is visible to the naked eye. Their emission colour can be reversibly and quickly switched by grinding–fuming or grinding–heating processes with high contrast. Moreover, the n-alkyl chain lengths can effectively control their PCL and thermodynamic properties, showing chain length dependent emission behaviours: longer alkyl chains were shown to produce more marked mechanochromism. A reproducible and reversible two-colour emission writing/erasing process was achieved by employing the iridium(III) materials as a medium. Powder X-ray diffractometry and differential scanning calorimetric studies suggest that the reversible transformation between crystalline and amorphous states upon application of external stimuli is responsible for the observed piezochromism.