Simultaneous modification of N-alkyl chains on cyclometalated and ancillary ligands of cationic iridium(III) complexes towards efficient piezochromic luminescence properties

In this study, we have designed and synthesized a series of multifunctional cationic iridium(III) complexes with different lengths of N-alkyl chains on 2-phenyl-1H-benzimidazole-based cyclometalated ligands and phenyl-pyridine type ancillary ligands. The photophysical properties of each complex have been investigated in detail and also ascertained by comprehensive density functional theory calculations. Despite showing negligible influence on their emission spectra and excited-state characteristics in solution, altering the N-alkyl chain length can efficiently modify their photophysical properties in the solid state. All complexes exhibit fascinating visible piezochromic luminescence (PCL) behaviour. Most interestingly, these iridium(III)-based luminophores with longer N-alkyl chains display significant emission colour changes and unique reversible features by mechanical grinding and solvent fuming. The powder X-ray diffraction (PXRD), 1H NMR and MALDI-TOF/TOF mass spectrometry data demonstrate that the phase transitions between crystalline and amorphous states are crucial to the present piezochromism. Moreover, with the merit of high quantum efficiency in aggregate states, the studied iridium(III) complex can serve as an efficient sensor for the sensitive and selective detection of the explosive, 2,4,6-trinitrophenol (TNP).