Conformational effects on the magnetic ground state of diradicals coupled through extended conjugated chains. A theoretical investigation

The magnetic properties of a series of stable diradicals based on imino nit roxide (IN) and nitronyl nitroxide (NN) are investigated. Both radical frag ments are coupled through various diamagnetic conjugated organic spacers ma de of aromatic rings (4) alternating with either a triple (1(x), 3) or a do uble bond (2(x)), where x indicates the length of the molecules with respec t to the repeating unit. The optimised geometry and the corresponding elect ronic structure of the whole series (x = 0,...,10) have been determined by means of semiempirical calculations based on the NDDO approximation within the AM 1 parametrisation scheme. The singlet-triplet splitting has then bee n studied using configuration interaction within different active spaces. I t turns out that the most important parameters are: (a) of geometrical natu re (e.g, twist angles between the radical moieties and the planar conjugate d spacer); and (b) of topological nature (e.g. the radical substitution at ortho, meta, and para positions of phenyl cycles). The results of the calcu lations enable us to predict some rules in relation to the experimental obs ervations: (a) for a given value of x, the hierarchy of the exchange coupli ng within series of derivatives is such as J(R-1 = R-2 = NN) > J(R-1 = IN, R-2 = NN)> J(R-1 = R-2 = IN); (b) whatever the nature of the radical substi tuent, the singlet is the ground stare and the exchange coupling decreases nearly exponentially with the length of the spacer, i.e. as x increases. Th e exchange coupling is still efficient up to x = 8 for R-1 = R-2 = IN (expe rimentally it has been previously found efficient up to x = 5 in the para p osition). (C) 2001 Elsevier Science Ltd. All rights reserved.