Nature of the near-IR band in the electronic absorption spectra of neutral bis(tetrapyrrole) rare earth(III) complexes: Time-dependent density functional theory calculations

The nature of the near-IR band in the electronic absorption spectra of bis(tetrapyrrole) rare earth(III) complexes Y(Pc)₂ (1), La(Pc) ₂ (2), Y(Pc)(Por) (3), Y(Pc)[Pc(α-OCH₃)₄] (4), Y(Pc)[Pc(α-OCH₃)₈] (5), and Y(Pc)[Pc(β-OCH₃)₈] (6) was studied on the basis of time-dependent density functional theory (TD-DFT) calculations. The electronic dipole moment along the z-axis in the electronic transition of the near-IR band in all the studied neutral bis(tetrapyrrole) yttrium(III) and lanthanum(III) double-deckers is well explained on the basis of the composition analysis of the orbitals involved. The electronic transition in the near-IR band causes the reversion of the orbital orientation of one tetrapyrrole ring in both homoleptic and heteroleptic bis(tetrapyrrole) rare earth complexes and induces electron transfer from the tetrapyrrole ring with lower orbital energy to the other ring in the heteroleptic bis(tetrapyrrole) rare earth(III) complexes. The near-IR band can work as an ideal characteristic absorption band to reflect theπ- π interaction between the two tetrapyrrole rings in bis(tetrapyrrole) rare earth(III) double-decker complexes because of its peculiar electronic transition nature.