Phenazine andmeso-1,2-Diphenyl-1,2-ethanediol - Partners in Photochromic Cocrystals

Abstract

The formation of hydrogen-bonded networks by cocrystallisation of 1,4-bisimines and 1,2-diols [1] has led to the discovery of a new type of photochromism [2]. Excitation of the charge-transfer (CT) band of a cocrystal in which suitably placed phenyl groups were substituted by a dimethylamino group as donor and a cyano group as acceptor resulted in electron transfer followed by proton migration. The stabilization of the excited CT state by proton transfer proved to be thermally reversible. Hydrogen bridges between lone-pair electrons of imines and the protons of hydroxy groups of diols are the supramolecular building blocks of the cocrystals.One of the goals of crystal engineering [3] is the attempt to influence or even predict the arrangement of molecules in a lattice in order to induce new properties in supramolecular structures, properties which are not present in the isolated components or in solution. Here, we describe an extension of the concept of forming cocrystals from 1,4-bisimines and 1,2-diols. Phenazine is considered as a 1,4-bisimine which is forced into a synperiplanar arrangement by ring formation. The nitrogen atoms provide the two lone pairs of electrons as acceptors of hydrogen atoms from the hydroxy groups of 1,2-diols. ResultsPhenazine and meso-1,2-diphenyl-1,2-ethanediol were dissolved in ethyl acetate in a 1:1 ratio, and the solvent was allowed to evaporate slowly. From this solution when it was kept in the dark crystals were obtained which showed the yellow colour of phenazine. Keywords: Crystal engineering, Electron transfer, Supramolecular chemistry, Photochromism, Phenazine Abstract. A hydrogen-bonded network in a 1:1 cocrystal of phenazine and meso-1,2-diphenyl-1,2-ethandiol places phenazine and diol in such a way that a thermally reversible photochromism is produced. The structure of the cocrystal is determined by X-ray crystallography. Thermally reversible changes in the UV/Vis and IR spectrum on irradiation, in cocrystals were analyzed by single crystal X-ray diffraction which yielded the crystal packing in the centrosymmetric space group P 1. Figure 1a shows a projection of the arrangement of phenazine and the 1,2-diol which gives insight in the packing pattern of the crystal. The two nitrogen atoms of each phenazine molecule form hydrogen bonds to neighbouring meso-1,2-diphenyl-1,2-ethanediol molecules. The O-H ... N bridges are almost linear (165°), the oxygen-nitrogen distance is 285 pm. The projection along [1 0 0] in Fig. 1a shows the centrosymmetric molecules arranged in chains, so that additional C-H ... π edge to face interactions [4] between CH bonds of phenazine and the π-electrons of the phenyl groups of meso-1,2-diphenyl-1,2-ethanediol exist, as indicated by dotted lines at one pair of molecules in Fig. 1b. One of these interactions characterized by a CH .. (centroid phenyl ring) distance of 289 pm is significant and due to the lattice symmetry occurs alternately on each side of the phenyl groups along the chains. The other interaction showing a distance of 357 pm is exp...

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