Direct observation of excited state conversion in solid state from a TICT-Type mechanochromic luminogen

A coplanar donor-acceptor (D-A) structure in single crystal is directly observed for TICT-Type DPA-PYZ, which corresponds to a coplanar localized excited (LE) state. The conversion between coplanar LE and twisted intramolecular charge transfer (TICT) excited state in the solid state can be achieved through the change of the D-A twist angle between ~0° and ~30° during grinding, heating and solvent fumigation processes. This twist angle change between donor and acceptor was proved by the Raman, FT-IR spectroscopy and X-ray crystallographic analysis as well as theoretical calculation. This research provides the clear evidence for the conversion between coplanar LE and TICT excited states of D-A molecules in solid state and contributes much to the deep understanding of stimulus-response behavior of fluorescent D-A molecular solids from the viewpoint of excited state control.