Mechanisms of Bathochromic Band Shifts in Absorption Spectra of N-Substituted Porphine Derivatives

The role of the molecular structure in the formation of the bathochromic shift of the S₀ → S₁ transition for a family of N-substituted porphine derivatives was studied. Molecular conformations of porphine, four of its N-substituted derivatives, and two model porphines with selected fixed bond angles and lengths in the macrocycles were optimized, the energies of the molecular orbitals were determined, and electronic absorption spectra were calculated using quantum-chemistry methods. It was found that N-substitution led to significant pyramidalization of the nitrogen atom. The degree of hybridization λ² of the N atom depended on the volume of the N-substituent and reached a value of λ² = 2.729 in porphyrin H(N–CCl₃)P. The degree of hybridization λ² of the N atom was established as the factor determining the energy of the long-wavelength S₀ → S₁ transition because the conjugation along the inner C_a–N–C_a fragment of the pyrrole ring decreased upon pyramidalization of the N atom while π-conjugation via the outer C_a–C_b–C_b–C_a fragment, which led to an increase in the size of the conjugated π-system, simultaneously strengthened. The tilt of the N-substituted pyrrole ring relative to the macrocycle mean plane and the electron-donating/electron-withdrawing properties of the N-substituents did not directly affect the bathochromic shift of the S₀ → S₁ transition.