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Free keywords:
cooperative effects;
crystal engineering;
electron transfer;
hydrogen bonds;
photochromism
Abstract:
The 1:1 cocrystallization of 1,4-diaryl-1,4-bisimines (Ar–CHN–CH2-)2 4–11and substituted meso-1,2-diaryl-1,2-ethanediols 1–3 leads to supramolecular structures in which the diol is hydrogen bonded by one of its hydroxy groups to an imine nitrogen atom of a 1,4-bisimine. The second functionality in each molecule leads to the generation of ladderlike polymeric structures where each molecule of the diol is linked to two molecules of the 1,4-bisimine and vice versa. If the diol carries electron donor groups in the aromatic residue and the 1,4-bisimine correspondingly acceptor groups, then charge transfer interactions are observed. The excited CT complex which corresponds to a radical ion pair is stabilized by migration of a proton of a hydroxy group to the nitrogen atom of an imino group. This is supported by the appearance of a N–H vibration in the IR spectra. The reorganization is also accompanied by changes in the UV/Vis spectra and by the generation of paramagnetism in the crystalline material. The results represent a type of photochromism which has its origin in a light-induced cooperative electron–proton transfer. The photochromism is thermally reversible.
