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.