Design, Synthesis And Photophysical Properties Of Cationic Iridium(?) Complexes With Piezochromic Luminescence Behavior

Stimuli-responsive materials with specific optical properties that can be tuned and controlled by external stimuli are attracting enormous attention for their potential applications in dynamic functional materials. These materials are considered to be smart materials because their properties can be modified by external conditions, including pressure, temperature, light and electricity. In particular, the newly developed piezochromic luminescent (PCL) materials exhibiting vivid and reversible changes of luminescence in the solid state without any chemical transformations have been the object of intense research efforts driven by their potential widespread applications ranging from sensors, and data storage, to security ink for optoelectronic devices. Recently, a number of materials displaying PCL behavior have been reported, including organic and inorganic complexes, liquid crystals, polymers as well as metal-organic frameworks. However, the investigation of PCL materials is still in the initial stage, especially for metal-containing complexes. By introducing different alkyl chains into the iridium(?), it will help us to shed new light on the relationship between the structures and the PCL behavior and further develop more effective iridium(?)-based PCL material.In this paper, we synthesize two series of cationic Iridium complexes. The relationship between the structures and the PCL behavior has been discussed. The outline of these studies is as follows.1. A series of cationic iridium(?) complexes containing 2-phenyl-lH-benzimidazoletypeligands 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 oron 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. 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(?) 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.2. We have designed and synthesized a series of multifunctional cationic iridium(?) complexes with different lengths of N-alkyl chains on 2-phenyl-lH-benzimidazole-based cyclometalated ligands and phenyl-pyridine type ancillary ligands. All complexes exhibit fascinating visible piezochromic luminescence (PCL) behaviour. Most interestingly, these iridium(?)-based luminophores with longer N-alkyl chains display significant emission colour changes and unique reversible features by mechanical grinding. 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, we chose complex 4 as an efficient sensor for the sensitive and selective detection of the explosive,2,4,6-trinitrophenol (TNP).