Electronic structure analysis of phthalocyanine complexes using magnetic circular dichroism and magnetic circularly polarized luminescence spectroscopy.

In the study of phthalocyanine complexes using magnetic circular dichroism (MCD) spectroscopy, the electronic structure of excited states is generally discussed based on the rigid-shift approximation, in which the band profiles for left-handed circularly polarized (lcp) and right-handed circularly polarized (rcp) light are assumed to be the same. This assumption may not necessarily be valid for cases where there are multiple initial states having different geometries. Magnetic circularly polarized luminescence (MCPL) from phthalocyanine complexes can be regarded as an example of such cases, since the two degenerate emission states are split in a magnetic field and can undergo a structural deformation. Here, we investigated an alternative approach, where the lcp and rcp components are independently determined. This method, which we refer to as the direct-separation approach, allows direct determination of the distribution of the two emission states as well as the orbital angular momentum L z $$ \left|{L}_z\right| $$ . Using this approach, L z $$ \left|{L}_z\right| $$ and the distribution were determined from MCD and MCPL spectra of a series of phthalocyanine complexes. Comparison of the two methods shows that the rigid-shift and the direct-separation approaches give practically equivalent results for the systems under study, but the latter is advantageous for systems where the former is not applicable.

Construction of high-throughput magnetic circular dichroism measurement system and its application to research on magnetic and optical properties of phthalocyanine complexes.

Magnetic circular dichroism (MCD) spectroscopy is a powerful method for evaluating the electronic structure and magnetic and optical properties of molecules. In particular, MCD measurements have been performed on phthalocyanines and porphyrins with various central metal ions, axial ligands, and substituents to elucidate their properties. It is essential to develop a robust high-throughput technique to perform these measurements comprehensively and efficiently. However, MCD spectroscopy requires very high optical quality for each component of the instrument, and even slight cell distortions can impair the baseline flatness. Consequently, when versatility and data quality are important, an optical system designed for a microplate reader is not suitable for the MCD spectrometer. Therefore, in this study, we develop a new magnetic flow-through cell and combine it with an existing CD spectrometer and autosampler to construct a high-throughput system. The effectiveness and performance of this new system are then evaluated. In addition, based on the MCD and absorption spectra of various phthalocyanine complexes, the effects of substituents and solvents on their magnetic and optical properties and the causes of these effects are discussed. The results demonstrate that this system is effective for the evaluation of the physicochemical properties of various phthalocyanine complexes.

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.

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 ΔJL 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.

Spectroscopic Investigation of Interaction between the 4f Electronic System and the Photoexcited Cyclic π System in Terbium(III) Monoporphyrinato Complex.

Electronic interaction between the total angular momentum (J) of the 4f electronic system and an angular momentum (L) of the photoexcited cyclic π system in a terbium(III) monoporphyrinato complex with a capping ligand cyclen, [Tb(TPP)(cyclen)]Cl (TPP = 5,10,15,20-tetraphenylporphyrinato; cyclen = 1,4,7,10-tetraazacyclododecane), have been investigated by varied-temperature and varied-magnetic-field magnetic circular dichroism (MCD) spectroscopy. Three MCD A-term patterns which correspond to B(0,0), Q(0,0), and Q(1,0) absorption bands were observed for the complex which was incorporated into a thin film of poly(methyl methacrylate), PMMA, as well as that in solution phase. The A-term patterns in the Soret band region and Q-band region were found to show opposite behavior with changes in temperature and magnetic field. This finding clearly indicated not only the presence of the J-L interaction but also the different directions of the J-L interaction; i.e., J and L interact ferromagnetically in the Q(0,0) and Q(1,0) states, while antiferromagnetically in the B(0,0) state. The magnitude of the interaction was determined by simulation-based fitting to experimental A1/D0 ratios, where A1 refers to the A-term intensity and D0 to the oscillator strength of the band. The first ab initio study was also carried out for further insight into the interaction. From the computational study, the relationship between the orbital angular momentum of π electronic system, L(π), and the orbital angular momentum of the 4f system, L(f), was clearly revealed. A computational approach to determine the magnitude and the direction of J-L interaction was established.

Synthesis of a Series of Heavy Lanthanide(III) Monoporphyrinato Complexes with Tetragonal Symmetry.

A series of heavy lanthanide(III) and yttrium(III) monoporphyrinato complexes formulated in [Ln(TPP)(cyclen)]Cl (Ln = Tb, Dy, Ho, Er, Tm, Yb, and Y; TPP = 5,10,15,20-tetraphenylporphyrinato), with cyclen, 1,4,7,10-tetraazacyclododecane, as a capping ligand, have been prepared in mild conditions and studied using single-crystal X-ray diffraction crystallography. The complexes exhibit an electronic absorption band (B(0,0)) in the range of 421-423 nm, showing a bathochromic shift associated with the increase of the ionic radii of the lanthanide, as well as two peaks of Q(1,0) and Q(0,0) bands between 548-586 nm. All of the complexes are isostructural, where both TPP and cyclen are coordinated to a lanthanide(III) or yttrium(III) ion giving an eight-coordinate square-antiprismatic (SAP) geometry (average skew angles are in the range of 43.01°-43.67°). The mean plane of the four nitrogen atoms of TPP (N4t) and that of the cyclen (N4c) are virtually parallel with a dihedral angle of less than 1°. The lanthanide(III) or yttrium(III) ions lie between N4t and N4c. The position of the metal ion is closer to the N4t plane, which is presumably caused by the different charges of the ligands, the size of the N4 square ligands, and the steric factor. The average Ln-N and interplanar distances (dN) decrease with decreasing lanthanide(III) ionic radii, showing the effect of lanthanide contraction. The skew angles, opening angles, and N-N distances are nearly unchanged, keeping the rigid square antiprismatic geometry throughout the series.

Tuning of the antiferromagnetic-type interaction in photo-excited single-decker porphyrin-lanthanide complexes with different crown capping ligands.

The magnetic interaction between the total angular momentum (J) of the 4f electrons in a lanthanide ion and the orbital angular momentum (L) of a porphyrin in the photo-excited states of 5,10,15,20-tetraphenylporphyrinato Dy(III) complexes capped with a crown ether with one of two different symmetries, 12-crown-4 or 1-aza-12-crown-4, was investigated by temperature- and magnetic dependent magnetic circular dichroism (VT-VH MCD). The analysis was conducted on the complexes with different non-aromatic ligands to investigate how different symmetries of the non-aromatic ligands have an impact on the electronic interaction causing an anti-parallel orientation of J and L. The magnitude of the J-L interaction was determined from simulation-based fitting to experimental ratios. While in both cases an antiferromagnetic-type interaction between J and L was identified, an asymmetric non-aromatic ligand resulted in an increased magnitude of the J-L interaction.

Magnetic interaction of photoexcited terbium-porphyrin complexes with non-aromatic ligands having different symmetries.

Interaction between the total angular momentum (J) of 4f electrons in lanthanides and the orbital angular momentum (L) of porphyrins in the photoexcited states was investigated by temperature-dependent and magnetic circular dichroism (MCD) for 5,10,15,20-tetraphenylporphyrinato (TPP) terbium(III) complexes with two different non-aromatic ligands i.e. 12-crown-4(1,4,7,10-tetraoxacyclododecane) and 1-aza-12-crown-4(1,4,7-trioxa-10-azacyclododecane). The two cases with different ligands were examined in order to understand how magnetic interaction depends on the symmetry of the non-aromatic ligands. The three absorption bands in the visible region, B(0,0), Q(1,0), and Q(0,0) bands, showed temperature-dependent MCD A term. For each band, the J-L interaction was determined from the simulation-based fitting to experimental ratios. An increase in the magnitude of the J-L interaction was observed when the second ligand was the aza-crown with a lower symmetry. Ab initio RASSCF/RASSI calculations were performed to explore the effect of the difference in the second ligand to the ligand centred excited states and the ligand-field-splitting structure on the metal-centred ground multiplet of J = 6.

Determination of ligand field splitting in lanthanide(iii) monoporphyrinato complexes.

The 4f-electronic structures of the ground multiplet states of a series of lanthanide(iii) monoporphyrinato complexes with a cyclen as the capping ligand [Ln(TPP)(cyclen)]Cl (Ln = Tb, Dy, Ho, Er, Tm, and Yb; TPP = 5,10,15,20-tetraphenylporphyrinato; cyclen = 1,4,7,10-tetraazacyclododecane) have been determined using experimental NMR and magnetic susceptibility data. Magnetic susceptibilities of the microcrystalline samples of [Ln(TPP)(cyclen)]Cl were measured in the range of 1.8-300 K. NMR signals corresponding to the protons on the porphyrin ring show marked paramagnetic shifts (lower = Tb, Dy, Ho, and Yb; higher = Er and Tm) in comparison with the diamagnetic Y congener. A set of ligand-field parameters that simultaneously reproduces the magnetic susceptibilities and the paramagnetic shifts has been determined by using a self-developed multidimensional optimization program. In addition, CASSCF calculations were carried out for further insight into their electronic structures.