Structure and Properties of Cocrystals of Phenazine and Fumaric-, 2,3-Dihydroxyfumaric-, and Oxalic Acid

Abstract

Phenazine and the dicarboxylic acids fumaric-, 2,3- dihydroxyfumaric-, and oxalic acid form 1 : 1 cocrystals. X-ray analysis shows that the molecules are arranged as linear tapes, mainly held together by strong O-H…︁N and weak C(sp 2)-H…︁O hydrogen bonds. Individual molecules form staples which are surrounded by staples of the other molecules. The angle between neighbouring tapes varies from ca. 90° in the cocrystal of phenazine and fumaric acid to ca. 70° in the co-crystal of phenazine and 2,3-dihydroxyfumaric acid, and ca. 25° in the cocrystal of phenazine and oxalic acid. The molecules assume an offset face-to-face arrangement in individual phenazine staples. Negligible π-stacking is observed in the cocrystals of phenazine with fumaric- and 2,3-dihydroxyfumaric acid. The absence of the CC double bond as spacer in oxalic acid leads to appreciable π-overlap of phenazine molecules in the cocrystal. As a consequence, the latter cocrystal displays special properties. An irreversible lightinduced electron transfer generates initially singlet and triplet biradicals with the unpaired electrons positioned on neighbouring phenazine molecules. Partially, the electrons are transformed to magnetically independent electrons which show strong exchange narrowing in the e.p.r. spectrum at temperatures > 0 °C. The proposed model is supported by UV/Vis-e. s.r.-, and SQUID measurements.