Understanding Mechanochromic Luminescence on Account of Molecular Level Based on Phosphorescent Iridium(III) Complex Isomers.

Mechanochromic luminescent (MCL) materials are promising in pressure sensors, security papers, photoelectric devices and optical data recording. Although some kinds of MCL-active iridium(III) complexes with various soft substituent functional ligands (e.g., dendritic carbazole, flexible chains, and Schiff base ligands) were reported, the MCL mechanism is still not clear and mainly ascribes to the physical phase transformations from crystalline state to amorphous state in response to force stimulus at present stage, and deserves further study in order to obtain more intelligent MCL materials. Herein, two new iridium(III) complex isomers are tactfully constructed and show distinctly opposite MCL properties in spite of the same physical phase transformations happening on them. The absolutely out of the ordinary MCL mechanism has been presented on account of molecular level for the first time via the comparative study of photophysical properties based on isomers 1 and 2 with the help of crystal structure analysis, room/low temperature emission spectra, NMR, PXRD, and TD-DFT calculations. All of these results suggest that the emitting state dominated by the triplet charge transfer excited state (3CT) plays a key role in achieving mechanochromic luminescence in iridium(III) complex systems.