Identification of a Weak Interaction between the Spin-Only 4f-Electronic System of Trivalent Gd Ion and a Photoexcited Cyclic π Electronic System.

An electronic interaction between the spin-only 4f-electronic system and the photoexcited π cyclic system has been identified in the anionic bis(phthalocyaninato)gadolinium(III) complex [PcGd] by using variable-temperature variable-magnetic-field magnetic circular dichroism (VTVH MCD). Two positive MCD -term patterns, corresponding to the Q and Q absorption bands, were observed to increase in intensity as the temperature decreased, indicating a ferromagnetic-type interaction between spin angular momentum of the 4f system and orbital angular momentum of the photoexcited π system. A theoretical model incorporating the - interaction constant was fitted to the VTVH MCD spectra, confirming the presence of a weak interaction, whose intensity was determined as = 0.27 cm for Q and = 0.02 cm for Q. Ab initio calculations using the RASSCF and RASSI methods revealed that the inclusion of the excited state calculations with the spin multiplicity (2 + 1) of 6 was crucial for assigning Δ and values. This underscores the essential role of the P term in accurately modeling the - interaction in [PcGd].