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

Abstract

The role of molecular structure in the formation of bathochromic band shift of S₀→S₁ transition for the family of N-substituted porphine derivatives has been studied. The molecular conformation of the porphine, four its N-substituted derivatives and two model porphine molecules with selected fixed angles and bond lengths in the macrocycle has been optimized, the energies of molecular orbitals have been determined, and electronic absorption spectra have been calculated with quantum chemistry methods. It has been found that N-substitution leads to a significant pyramidalization of the nitrogen atom, and the degree of hybridization λ² of the nitrogen atom depends on the volume of the N-substituent and reaches the value of λ² = 2.729 in the H(N-CCl₃)P porphyrin molecule. It has been established that the factor that determines the energy of the long-wavelength S₀→S₁ transition is the degree of hybridization of λ² nitrogen atom, since upon pyramidalization of the nitrogen atom the conjugation along the inner C_a–N–C_a fragment of the pyrrole ring decreases while simultaneously strengthening of π-conjugation via outer C_a–C_b–C_b–C_a fragment, which leads to increase in the size of the conjugated π-system. 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 do not directly affect the bathochromic shift of the S₀→S₁ transition.