ESR. and, in part, ENDOR. studies are reported on the radical anions of 5,5'and 6,6'-biazulenyl (1 and 2, resp.), as well as on their 1,1',3,3'-tetradeuterioderivatives (1-d4 and 2-d4). The reduction processes of 1 and 2 leading to these radical anions (1Q and 28) and the dianions (1" and 2e) have been investigated by polarography and cyclic voltammetry. The half-wave reduction potential of 1 and the n-spin distribution in lQ are consistent with the model of two weakly interacting azulene n-systems, whereas the analogous findings for 2 and 2 8 point to a strong interaction between two such systems. This difference can be traced to the distinct inequality I c65 I 4 1 c66 I in the LCAO coefficients Csp at the centres p = 5 and 6 for the LUMO v6 of azulene.
Introduction. -The lowest unoccupied orbital (LUMO) v6 of azulene is characterized by large absolute values of the LCAO coefficients cgp at the even-numbered centres b = 2 , 4, 6 and 8) and small values at the odd-numbered ones (p= 1, 3, 5 and 7). Since the LUMO of a symmetric p,p'-biazulenyl, formed by linking the equivalent positions of two azulene moieties, can be considered as a bonding combination of two LUMO's of azulene, its energy and shape depend crucially on whether p (and p') is even or odd. This dependence should manifest itself in strongly differing properties of the corresponding radical anions such as their ease of formation and their n-spin distribution. Some years ago, the radical anion of 2,2'-biazulenyl was found to be relatively stable and to exhibit a well defined ESR. hyperfine pattern [l], whereas the 1,l'qounterpart had a low stability and the ESR. spectrum ascribed to it [l] stems probably from a secondary paramagnetic species 121.
Another pair of p,p'-biazulenyls, in which p is odd and even, are the 5,5 'and 6,6'-compounds (1 [3] and 2 [4], resp.). The present paper begins with the polarography and cyclic voltammetry of 1 and 2. Subsequently, it describes the ESR. spectra of the resulting radical anions 1 0 and 20. These studies also include the