Spin–spin interactions in polyphenyls

Abstract

An attempt has been made, with some success, to interpret the zero field splitting parameters for the triplet states of molecules such as the polyphenyls where the subunits are weakly coupled and the molecules are probably nonplanar. The method used was a semi-empirical exciton theory, applied in particular to biphenyl and 1,3,5-triphenylbenzene. I n these two cases it is found that structure information, in particular the angle between rings, obtained by this method is in good agreement with electron diffraction data.Canadian Joutnal of Chemistry. Volume 45, 1707 (1967) The early electron spin resonance (e.s.r.) studies of Hutchison and Mangum (I) and van der IVaals and de Groot (2) have stimulated considerable experimental and theoretical interest in the triplet states of organic molecules. One of the main theoretical concerns has been the correlation of the zero field splitting parameters D and E mith~molecular structure.For the excited triplet states of aromatic molecules such as benzene and naphthalene, as well as various methvlenes and nitrenes which have triplet ground states, the theoretical predictions (3-7) based on molecular orbital descriptions are in good agreement with what is observed experimentally.T h e purpose of the present study is t o investigate the use of an exciton description for the calculation of zero field splitting parameters. This type of model is-widely used in the description of the optical properties of molecular crystals (8, 9). T h e same approach has achieved some success in the description of molecules in which the subunits are weakly coupled, such as linear polyenes (10, 11) and polyphenyls (8, 12).We develop the theory initially to discuss the e.s.r. results of triplet biphenyl and triphenylbenzene. Some discussion of the a~~l i c a t i o n of the model t o molecules such A A as triphenylene is also given.IPresent address: Defence Chemical, Biological and Radiation Laboratories, Ottawa, Canada. DISCUSSIONT h e e.s.r. spectra of orbitally nondegenerate hydrocarbon triplet states can be understood (1,2) on the basis of a Zeeman interaction and a second-order tensor tern1 independent of the field. T h a t is in terms of the spin Hamiltonian T h e zero field splitting tensor G is due primarily (13-15) to the first-order effect of the dipolar spin-spin interaction. In this approximation G is an average over the wave function where and ri, is the vector joining the electrons. (11) BiphenylWe consider biphenyl to consist of two weakly interacting phenyl groups. l4o(A) and 341(A) are the wave functions representing the ground state and first excited triplet state of phenyl group A respectively.T o first order we may then write the ground and lowest triplet molecular wave function as Can. J. Chem. Downloaded from www.nrcresearchpress.com by 54.202.233.140 on 05/11/18For personal use only.