Spectral Manifestations of Energy Modulation of the Porphine Macrocycle Electronic Orbitals Upon Rotation of an NO2 Substituent

Abstract

Peculiarities of absorption spectra formation of porphine derivatives upon attachment of an NO₂ group to the C_m-position of the tetrapyrrole macrocycle have been studied. The molecular conformation of the substituted porphine molecule was optimized using quantum chemistry methods. The energies of molecular orbitals were determined. Absorption spectra were calculated. It was found that the electronic interaction between the macrocycle and the NO₂ group is determined by the orientation of the nitro group relative to the macrocycle mean plane. It has been established that the energies of the LUMO and HOMO–1 orbitals depend significantly on the dihedral angle θ between the macrocycle mean plane and the nitro group plane, while the energies of the LUMO+1 and HOMO orbitals vary slightly. As a result, the S₀ → S₁ and S₀ → S₂ absorption bands have bathochromic shifts, the magnitudes of which are different and depend on the configurational composition of the transitions. The oscillator strength of the S₀ → S₁ transition turns out to be greatest in the coplanar conformer, which has minimal configuration interaction. The oscillator strength of the S₀ → S₁ transition increases sharply in the orthogonal conformer, in which the LUMO and LUMO+1 are quasi-degenerate. All spectral characteristics can be presented as functions of the weighted sum of cos² θ and cos²2θ, taking into account the configuration composition of the electronic transitions.