Interaction between Highly-Anisotropic 4f System and Photo-Excited Cyclic π –systems in Lanthanide-Porphyrin Complexes with Varied Non-Aromatic Ligands

概要

It has been long known that 18π electrons-tetrapyroles compounds such as phtalocyanine and porphyrin acquire angular momentum denoted as L in excited states due to the π–π* electronic transition. When they form coordination compounds with trivalent lanthanides (Ln), there expected to be an electronic interaction between L and the total angular quantum number (denoted as J) of Ln. Such interaction, symbolized as ∆JL, has been investigated using variable-temperature variable-magnetic field (VT-VH) magnetic circular dichroism (MCD) by our groups as the first research group who did the investigation1-5 .

The value of ∆JL were found to be determined by performing least-square fitting of the simulated A1/D0 ratios to the experimental ratios in which A1 corresponds to the A-term signal of MCD and D0 is associated with the oscillator strength. The first studies were done for phthalocyanine complexes with Ln being Tb and Dy in Ln-phthalocyanine homoleptic double-decker [Ln(Pc)2] - , it was found that the interaction inthe Tb case was ferromagnetic while that of the Dy complex showed both ferromagnetic and antiferromagnetic interaction1,2 .

When one Pc was replaced with 1,4,7,10-tetraazacyclododecane (cyclen) as a non-aromatic ligand yielding heteroleptic [Tb(Pc)Cyclen]Cl, two bands in visible wavelength as the lowest π–π* band showed the ferromagnetic-type interaction in the Tb complex3 while antiferromagnetic in Dy case. The investigation of ∆JLwas also performed for porphyrin cases specifically tetraphenylporphyrinato (TPP) complexes with Tb and Dy, namely [Ln(TPP)Cyclen]Cl previously4,5. Contrast to Pc ligand, three bands were observed called Q(0,0) and Q(1,0) bands located in the range of visible wavelength (500-600 nm) and B(0,0) band which is detected on higher energy around 400 nm. These bands have different spectral characters: B band is sharp and intense while Q bands have relatively lower intensity. B band has total angular momentum ∆L = +1 and Q bands have ∆L = +9. All of those bands generated three A-term spectral shape in MCD spectra, where different L and ∆JLwere determined and calculated. From those reports, it was concluded that the interaction was antiferromagnetic in the Dy complex while in the Tb complex, the electronic interaction was antiferromagnetic in B(0,0) band and ferromagnetic in Q(0,0) and Q(1,0) bands. In this doctorate research, the question to be answered was set as follows: Is there any impact from the second ligands toward the Lz value and also the ∆JLin Ln-TPP in B band and in Q bands? From that question, there are some aspects to be examined. First, does variation of electronegativity of the tetradentate site (in the second ligand with similar symmetry) alter the ∆JLin the complexes? Second, to what extend does Lz depend on the symmetry as well as different atoms of the second ligand? Third, do B and Q bands of porphyrin have same tendency to perturbation by different non-aromatic ligands in heteroleptic complexes? All of those questions are explored by changing cyclen with three different ligands: 1,4,7,10-tetraoxacyclododecane or 12-crown-4 ether, 1-aza-4,7,10-trioxacyclododecane or 1-aza-12-crown-4 ether, and 2,2′-Ethylenebis(nitrilomethylidene)diphenol, N,N′-Ethylenebis(salicylimine) or salen. 12-crown-4 ether and 1-aza-12-crown-4 ether are two ligands having cyclododecane structure, similar to cyclen. This is to investigate the effect from cyclododecanes ligands with different atoms in the tetradentate binding pocket. In 12-crown-4 ether the symmetry is analogous to cyclen while in 1-aza-12-crown-4 ether, the symmetry is slightly reduced with an amine replacing one oxygen. The symmetry is further lowered when cyclododecane ring structure is cleaved by having salen ligand as the non-aromatic ligand. Besides that, the coordinating atoms in salen are more varied as well.

The complexes designed to fulfill the above mentioned purposes above are denoted herein after as [Ln(TPP)Crown]Cl, [Ln(TPP)Azacrown]Cl, and [Ln(TPP)Salen]Cl with Ln is Y, Tb, or Dy. All of them were first synthesized and analyzed in thisresearch project. There has been no report on the compounds until now.

In the case of [Y(TPP)Crown]Cl and [Y(TPP)Azacrown]Cl, measurements were performed to determine Lz. Positive A-term pattern corresponding with B(0,0), Q(0,0), and Q(1,0) bands were detected and the intensity were unchanged at lowered temperature, associated with the diagmagnetic nature of Y. In [Tb(TPP)Crown]Cl and [Tb(TPP)Azacrown]Cl, temperature-dependent positive A-term profiles, in which all bands became more intense with diminished temperature due to the ferromagnetic character of the J–L interaction, were observed. On the other hand, [Dy(TPP)Crown]Cl and [Dy(TPP)Azacrown]Cl demonstrated reversal of normal A-term spectral shape as temperature was reduced especially on B(0,0) band and this is related with antiferromagnetic nature of the J–L interaction. Furthermore, even though the Lz value is practically unchanged from cyclen, 12-crown-4 ether, and 1-aza-crown-4 ether, there is substantial differences in ∆JLvalues with different non-aromatic ligands and such variation is more apparent in Q bands than in B band.

In order to explore more how distorted symmetry alters the angular momentum and the interaction, cyclododecane-group ligands was changed with salen whose symmetry is lower than 1-aza-12-crown-4 ether. More distorted non-aromatic ligand did not change the pattern of electronic transition as positive A-term patterns were observed for B(0,0) band, Q(0,0) band, and Q(1,0) band. Furthermore, [Tb(TPP)Salen]Cl showed temperature dependence in all bands where the intensity of positive A-term increased with diminished temperature. On the other hand, in [Dy(TPP)Salen]Cl, negative A-term was observed in B(0,0) band but on the contrary, positive A-term was observed in Q bands without reversal to negative A-term. In the complexes with Y, Tb, and Dy, Lz values in B(0,0) band were similar to those with 12-crown-4 ether and 1-aza-12-crown-4 ether. Interestingly, Lz of Q(0,0) band of the salen complex is higher than in the complexes with cyclen, 12-crown-4 ether, and 1-aza-12-crown-4 ether. Thus, salen has more significant effect on the total angular momentum of porphyrin than the other non-aromatic ligands. The ∆JL of B(0,0) band of [Tb(TPP)Salen]Cl and [Dy(TPP)Salen]Cl is in between that of the other complexes containing cyclododecane compounds. Contrast to that, ∆JLin Q(0,0) band of [Tb(TPP)Salen]Cl is notably higher than the other complexes while in [Dy(TPP)Salen]Cl, ∆JL is still in the range between that of [Dy(TPP)Crown]Cl as the lowest and [Dy(TPP)Cyclen]Cl as the highest.

In conclusion, different non-aromatic ligands always affect the electronic interaction in Tb and Dy. However, such alteration is more significant in Q band than in B band without necessarily change the Lz..As well as that, compared to B(0,0) band, Q(0,0) band is easier to be perturbed by lower symmetry of the second ligand as indicated by Lzvalues of B(0,0) which remained unchanged with different ligands while Lz in Q(0,0) increased when the second ligand is significantly more distorted. Moreover, between the two lanthanides, Tb is more prone to electronic interaction alteration with different non-aromatic ligands tuning than Dy.