A new type of electronic interaction which couples two angular momenta, i.e. the angular momentum of a localized 4f system (J) and an orbital angular momentum generated in a cyclic π conjugated system by irradiation with a circularly-polarized light, has been identified in a lanthanide single molecule magnet.
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.
The presence of a new electronic interaction, which couples a 4 f-electronic system with a total angular momentum J and a photoexcited cyclic π-conjugated system with an orbital angular momentum L, in the bis(phthalocyaninato)dysprosium single-molecule magnet ([DyPc ] ) is reported. Two π-π* excited states in the visible spectral region of the [DyPc ] complex, which are denoted here as Q and Q , showed significantly different temperature and field dependences of the magnetic circular dichroism (MCD) A-term intensity. This phenomenon not only indicates the presence of a "J-L" interaction, but also that the interaction generates two different preferred orientations of the J-L pair, either parallel (for the Q band) or antiparallel (for the Q band), depending on the excitation energy. We have constructed a theoretical model that reproduces the temperature and field dependences, and quantitatively evaluated the J-L interaction.
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