Structural Control of the Molecular Packing and Dynamics of Mechanofluorochromic Materials Based on Small Donor-Acceptor Systems with Turn-On Luminescence

A series of simple donor-acceptor molecules consisting of an oligo-oxyethylene (OOE) N-substituted diphenylamine connected to a dicyanovinyl (DCV) acceptor group by a thienyl pi-conjugating spacer are synthesized. Amorphous deep-red freshly cast thin films rapidly evolve towards an almost colorless pale beige poly-crystalline stable material from which a deep-red photoluminescent state can be switched-on by mechanical stimulation. The effects of the length of the OOE chain on the molecular packing of the material and on the dynamics of the recovery process and aggregation in solution are analyzed. Crystallographic results show that the length of the OOE chain directly controls the molecular packing from head-to-tail arrangement of the dipoles for short chain to co-facial dimers leading to a non-centrosymmetric material with nonlinear optics (NLO) activity for the longer ones. The analysis of the dynamics of the self-recovery process of thin films and of molecular aggregation in solution shows that the dynamics of both processes is directly controlled by the length of the OOE chain. Since, as confirmed by optical and electrochemical results, the OOE substituents have strictly no influence on the electronic properties of the pi-conjugated system, the observed effects are unambiguously related to changes in intermolecular interactions.