1. F. Gutmann and L.E. Lyons, “Organic Semiconductors”, Wiley, New York (1967); L.E. Lyons, J. Chem. Soc. 5001 (1957)
2. St.G. Christov, Ann. d. Physik (Leipzig), VII-12, 20 (1963);
3. Croatica Chim. Acta, 44, 67 (1972)
4. M.A. Slifkin, “The Purines - Theory and Experiment”, The Jerusalem Symposia on Quantum Chemistry and Biochemistry, IVth, Proc. Jerusalem 1972, pp 392 ff; see also ref. 12
5. 3. For a review, cf J. Schrieber and W. John, Phys. Stat. Solidi, (b)-78, 199 (1976)