Circular dichroism of chiral 1,8,15,22-tetra(alkoxyl)phthalocyaninato lead and yttrium complexes: time-dependent density functional theory calculations

The circular dichroism (CD) spectra of (1,8,15,22- tetraalkoxylphthalocyaninato)lead complexes Pb[Pc(α-OCH₃) ₄] (1), Pb[Pc(α-OC₂H₅)₄] (2), and Pb[Pc(α-OC₅H₁₁)₄] (3) (OC ₅H₁₁ = 3-pentyloxy) together with the monoanion of heteroleptic (phthalocyaninato)(1,8,15,22-tetramethoxylphthalocyaninato)yttrium double-decker {Y(Pc)[Pc(α-OCH₃)₄]}^- (4) were investigated by time-dependent density functional theory calculations. The calculation results reveal that 1 and 2 show similar CD spectra, which however are quite different from that of 3 despite the very similar electronic absorption spectra for 1-3 due to their similar electronic structure. The difference in the calculated structural parameter of the phthalocyanine macrocyclic ligand for 1-3 is negligible. In contrast, the orientation of alkoxyl substituents relative to the isoindole segments on which the substituents are attached for 1 and 2 is almost the same but significantly different from that for 3. These results suggest that the relative orientation of alkoxyl substituents determines the CD spectrum of chiral phthalocyaninatolead complexes with C₄ molecular symmetry. Further support comes from the change in the calculated CD spectrum of 1 depending on the orientation angle of the methoxyl groups. The calculation result on the CD spectrum of the yttrium double-decker compound 4 indicates the transfer of chiral information from the (1,8,15,22-tetramethoxylphthalocyaninato)yttrium fraction to the unsubstituted phthalocyanine fraction and the in turn significant contribution of the unsubstituted phthalocyanine ring to the CD spectrum of the double-decker molecule. The present work, clarifying the structural factor influencing the CD spectroscopic properties of chiral phthalocyaninatometal complexes with nonplanar molecular structure, will be helpful for clearly understanding the relationship between the chiroptical property and molecular structure.