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 nitroxide (IN) and nitronyl nitroxide (NN) are investigated. Both radical fragments are coupled through various diamagnetic conjugated organic spacers made of aromatic rings (4) alternating with either a triple (1x, 3) or a double bond (2x), where x indicates the length of the molecules with respect to the repeating unit. The optimised geometry and the corresponding electronic structure of the whole series (x=0,…,10) have been determined by means of semiempirical calculations based on the NDDO approximation within the AM1 parametrisation scheme. The singlet–triplet splitting has then been studied using configuration interaction within different active spaces. It turns out that the most important parameters are: (a) of geometrical nature (e.g. twist angles between the radical moieties and the planar conjugated spacer); and (b) of topological nature (e.g. the radical substitution at ortho, meta, and para positions of phenyl cycles). The results of the calculations enable us to predict some rules in relation to the experimental observations: (a) for a given value of x, the hierarchy of the exchange coupling within series of derivatives is such as J(R1=R2=NN)>J(R1=IN, R2=NN)>J(R1=R2=IN); (b) whatever the nature of the radical substituent, the singlet is the ground state and the exchange coupling decreases nearly exponentially with the length of the spacer, i.e. as x increases. The exchange coupling is still efficient up to x=8 for R1=R2=IN (experimentally it has been previously found efficient up to x=5 in the para position).