Charge transfer (CT) mechanochromism: dramatic CT absorption change of crystalline π-conjugated oligomers containing TCNQ upon mechanical grinding.

New π-conjugated oligomers with high crystallinity were prepared from the simple solvothermal reaction of squaric acid and diaminopyrenes. The oligomers were bonded at the 1,3- and 1,6-positions of the pyrene units, oligo(sq-alt-1,3py) and oligo(sq-alt-1,6py), which greatly affected their planar configuration and resulting mechanochromic properties. Oligomers containing a charge transfer (CT) complex were selectively synthesized in one step. Upon mechanical grinding in the solid state, the color changed from orange to deep metallic green.