Structural indicators of electronic interaction in the 1,1',5,5'-tetramethyl-6,6'-dioxo3,3'-biverdazyl diradical

The diradical 1,1',5,5'-tetramethyl-6,6'-dioxo-3,3'-biverdazyl, 6, crystallizes in a monoclinic unit cell with a = 4.224(1), b = 17.378(2), c = 7.529(1) Angstrom, beta = 96.02(1)degrees, space group P2(1)/c (No. 14). The crystal structure of 6 shows local D-2h symmetry. ESR measurements in a frozen chloroform solution indicate that isolated 6 has a singlet ground state with a triplet excited state 760 cm(-1) (0.094 eV) above the ground state with zero-field splitting parameters D = 0.038 cm(-1) and E = 0.0016 cm(-1). Semiempirical calculations (AM-1) suggest that 6 is twisted in solution. Crystalline 6 shows a temperature-activated ESR signal with no features characteristic of an isolated triplet. Strong intermolecular pi-stacking interactions prevent the analysis of this temperature activation in terms of intermolecular and intramolecular exchange parameters.