Antiparallel Coupling between a 4f System and a Photoexcited Cyclic π System in a Dysprosium(III) Monoporphyrinato Complex

Magnetic
coupling resulted by an orbital angular momentum L of
a photoexcited cyclic π-conjugated system and total
angular momentum J of a localized 4f electronic system
in [Dy­(TPP)­(cyclen)]Cl (TPP = 5,10,15,20-tetraphenylporphyrinato;
cyclen = 1,4,7,10-tetraazacyclododecane) has been identified using
magnetic circular dichroism (MCD). The MCD A-term patterns of the
compound show a negative increment in the intensity by decreasing
temperature and nonproportionality to the magnetic field. This phenomenon
is experimentally observed for the first time in porphyrin-based lanthanide
complexes and indicates the presence of the J–L interaction, with the preferred orientation being antiparallel.
A theoretical model in which J and L are
assumed to couple antiferromagnetically yielded a quantitative agreement
to the experimental data of varied-temperature and varied-magnetic-field
MCD spectral measurements. The ab initio calculations were performed,
revealing the relationship between two orbital angular momenta, i.e., L(π) and L(f), which result from the delocalized
π system and the localized 4f system, respectively.