Spectral Manifestations of Energy Modulation of the Porphine Macrocycle Electronic Orbitals upon Rotation of NO2-Substituent

Abstract

   The peculiarities of absorption spectra formation of the porphine derivatives upon attachment of NO₂ group to the Cm-position of the tetrapyrrolic macrocycle have been studied. Using quantum chemistry methods, the molecular conformation of the substituted porphine molecule was optimized, the energies of molecular orbitals were determined, and electronic 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 energy of the LUMO and HOMO–1 orbitals significantly depends on the value of the dihedral angle θ between the macrocycle mean plane and the plane of the nitro group, while the energy of the LUMO+1 and HOMO orbitals varies slightly. As a result, the S0→S₁ and S0→S₂ absorption bands have bathochromic shifts, whose magnitudes are different and depend on the configurational composition of the transitions. The oscillator strength of the S_0→S₁ transition turns out to be greatest in the coplanar conformer, which has minimal configuration interaction, and the oscillator strength of the S₀→S₂ 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 cos²θ and cos²2θ, taking into account the configuration composition of electronic transitions.