Mechanochromic Phosphorescence of Rhodium(I) Isocyanide Complexes in the NIR-II Window

Luminescent mechanochromic compounds that cover the second near-infrared (NIR-II, 950-1700 nm) window would provide an interesting type of mechanoresponsive materials and can be applicable in various fields. In this work, we report the synthesis of amino acid-functionalized tetrakis(phenylisocyano)rhodium(I) complexes and their phosphorescent mechanochromism in the entire NIR-II region. Grinding the pristine yellow polycrystalline complexes leads to the formation of green amorphous phases, and correspondingly, the emission wavelength was redshifted from 976 to 1220-1290 nm. Both of the NIR-II emission bands have large Stokes shifts and microsecond luminescence lifetimes, indicative of their phosphorescent nature. Upon successive solvent fumigation, the green amorphous powders convert back to the yellow polycrystalline complexes with an emission wavelength of 976 nm. Such reversible phosphorescence changes were found to arise from the reversible formation of trimeric aggregates via close Rh(I)···Rh(I) contacts. To the best of our knowledge, this research demonstrates unexplored examples of luminescent mechanochromic materials in the NIR-II window before or after exposure to mechanical stimulus.