EXCHANGE INTERACTIONS BETWEEN 2 NITRONYL-NITROXIDE OR IMINYL-NITROXIDE RADICALS ATTACHED TO THIOPHENE AND 2,2'-BITHIENYL RINGS

Eight bis(nitronyl nitroxide) and bis(iminyl nitroxide) diradicals having thiophene (2,4NT, 2,5NT, 2,4IT, and 2,5IT) and 2,2'-bithienyl units (4,4'NB, 3,3'NB, 4,5'IB, and 5,5'IB) as couplers were prepared. Both 2,5NT and 4,4'NB crystallized in monoclinic space group P2(1)/n with a = 12.430(2) Angstrom, b = 13.968(4) Angstrom, c = 12.470(2) Angstrom, beta = 107.26(1)degrees, and Z = 4 and with a = 10.766(7) Angstrom, b = 10.186(3) Angstrom, c = 11.625(4) Angstrom, beta = 1199(3)degrees, and Z = 2. The dihedral angles between the imidazoline and thiophene rings were only 6 and 10 degrees in 2,5NT and 21 degrees in 4,4'NB. The 2,2'-bithienyl chromophore assumed a planar anti-conformation A dimer structure with the oxygen atom in one molecule and the nearest nitrogen atom in the other at a distance of 3.9 Angstrom stacks linearly along the b axis in 2,5NT. Each nitronyl nitroxide group at both ends of the 4,4'NB molecule is in close contact with that of the neighboring molecule to make a one-dimensional chain. EPR spectra of all the diradicals in frozen toluene solutions showed typical fine structures due to triplet states with hyperfine splitting with nitrogen as well as the signals due to Delta m(S) = 2 transitions. The zero-field splitting constants were determined as \D/hc\ = 0.0071, 0.0085, 0.0066, 0.0108, and 0.0149 cm(-1) for 2,4NT, 2,5NT, 2,4IT, 2,5IT, and 3,3'NB, respectively. Temperature dependence of the EPR triplet signal intensities in the monothiophene derivatives suggested that the triplets would be ground states in the 2,4-isomers and that they are excited states lying above singlet ground states by Delta E(ST) (= 2J) = -218 k(B) and -52 k(B) K in 2,5NT and 2,5IT, respectively. The couplings for 2,4NT and 2,4IT were determined to be J(intra)/k(B) = 40 and 16 K (H = -2J(intra)S(i)S(2) for an S-T model), respectively, by SQUID measurements. Similar data were analyzed in terms of a linear four-spin model (H = 2J(intra)(S1S2 + S3S4) - 2J(inter)S(2)S(3)) for 2,5NT and an S-T model for 2,5IT to give J(intra)/K-B = 114.6, J(inter)/k(B) = -34, and theta/S (S + 1) = -0.8 K and J(intra)/K-B = -29.7 and theta/S(S + 1) = -2.5 K, respectively. In the four bithienyl derivatives, the interaction was weak with \J(intra)/k(B)) values being less than 10 K. The signs were barely judged to be positive for 4,5'IB and negative for 3,3'NB and 5,5'IB. Manganese(II) bis(hexafluoroacetylacetonate) formed a microcrystalline complex with 2,4NT which underwent the transition to a ferrimagnet at 11 K, demonstrating the potentiality of 2,4NT as a multi(monodentate) triplet diradical coupler.