This is due to severe intermolecular interactions in solid state as well as presence of charge transfer state. Piezochromism mainly arise because of different packing behavior of molecules (crystalline and amorphous). In crystalline phase, the molecules are conformationaly twisted, which not only prevent closer intermolecular interactions but also prevent extension of conjugation in the molecule. On the other hand, in amorphous phase, molecules may attain conformational planarity, which causes extention of conjugation leading to red shift. Now as donor acceptor complexes have charge transfer character, so the red shift is far more as compared to normal molecules. The same molecular planarity also induce close intermolecular interactions which can lead to the formation of excimers causing further red shift the emission spectra.
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