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 hav ing thiophene (2,4NT, 2,5NT, 2,4IT, and 2,5IT) and 2,2'-bithienyl unit s (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. Th e 2,2'-bithienyl chromophore assumed a planar anti-conformation A dime r structure with the oxygen atom in one molecule and the nearest nitro gen atom in the other at a distance of 3.9 Angstrom stacks linearly al ong 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 dirad icals in frozen toluene solutions showed typical fine structures due t o triplet states with hyperfine splitting with nitrogen as well as the signals due to Delta m(S) = 2 transitions. The zero-field splitting c onstants were determined as \D/hc\ = 0.0071, 0.0085, 0.0066, 0.0108, a nd 0.0149 cm(-1) for 2,4NT, 2,5NT, 2,4IT, 2,5IT, and 3,3'NB, respectiv ely. Temperature dependence of the EPR triplet signal intensities in t he monothiophene derivatives suggested that the triplets would be grou nd states in the 2,4-isomers and that they are excited states lying ab ove 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 2,4 IT 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 2,5 IT 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, res pectively. In the four bithienyl derivatives, the interaction was weak with \J(intra)/k(B)) values being less than 10 K. The signs were bare ly judged to be positive for 4,5'IB and negative for 3,3'NB and 5,5'IB . Manganese(II) bis(hexafluoroacetylacetonate) formed a microcrystalli ne complex with 2,4NT which underwent the transition to a ferrimagnet at 11 K, demonstrating the potentiality of 2,4NT as a multi(monodentat e) triplet diradical coupler.