Structure and properties of cocrystals of phenazine and fumaric-, 2,3-dihydroxyfumaric-, and oxalic acid

Phenazine and the dicarboxylic acids fumaric-, 2,3-dihydroxyfumaric-, and o xalic 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 we ak C(sp(2))-H . . .O hydrogen bonds. Individual molecules form staples whic h are surrounded by staples of the other molecules. The angle between neigh bouring tapes varies from ca. 90 degrees in the cocrystal of phenazine and fumaric acid to ca. 70 degrees in the co-crystal of phenazine and 2,3-dihyd roxyfumaric acid, and ca. 25 degrees in the cocrystal of phenazine and oxal ic acid. The molecules assume an offset face-to-face arrangement in individ ual phenazine staples. Negligible pi -stacking is observed in the cocrystal s of phenazine with fumaric- and 2,3-dihydroxyfumaric acid. The absence of the CC double bond as spacer in oxalic acid leads to appreciable pi -overla p of phenazine molecules in the cocrystal. As a consequence, the latter coc rystal displays special properties. An irreversible light induced electron transfer generates initially singlet and triplet biradicals with the unpair ed 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 degrees C. The proposed model is supported by UV/Vis- e.s.r.-, and SQUID measuremen ts.