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