Intramolecular Magnetic Interaction in a Photogenerated Dual Angular Momentum System in a Terbium–Phthalocyaninato 1:1 Complex

Intramolecular magnetic interaction between a localized open-shell 4f-electronic system and a photoexcited macrocyclic π-conjugate system in terbium–phthalocyaninnato (Tb–Pc) 1:1 complex was investigated using variable-temperature variable-field magnetic circular dichroism (VTVH MCD) spectroscopy. The 1:1 complex [Tb­(Pc)­(cyclen)]­Cl (Pc2– = phthalocyaninato dianion, cyclen = 1,4,7,10-tetraazacyclododecane) with the capping ligand providing an exact fourfold symmetry showed a significant temperature dependence and a nonlinear field dependence in the MCD intensity of the Pc-centered highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) π–π* transition, while a diamagnetic congener [Y­(Pc)­(cyclen)]Cl showed a temperature-independent MCD with a linear-field dependence. This indicates that the (4f)8 system of the Tb ion with a total angular momentum J and the photoexcited π-system of the Pc macrocycle with an orbital angular momentum L are magnetically coupled. By numerical simulation using a model where ground doublet state |Jz ⟩ = |±6⟩ and excited quartet state |Jz , Lz ⟩ = |±6, ±|Lz |⟩ are included, the J–L interaction magnitude Δ JL and the Pc-centered orbital angular momentum |Lz |ℏ were determined to be 1.1 cm–1 and 2.0 ℏ, respectively. From ab initio restricted active space self-consistent field (RASSCF)-restricted active space state interaction (RASSI) calculations on the π–π* excited states of the Tb complex, the magnitude of the J–L interaction was estimated. The comparison between the calculations on the Y and Tb complexes revealed that the ferromagnetic-type coupling occurs between the orbital component in the J of Tb and the L on Pc, supporting the model that we employed for the analysis of the experimental data.