Control of the T1 → S0-Transition Energy in Porphine Derivatives Substituted by NH2 Groups

Abstract

The influence of the architecture of NH₂-peripheral substitution of porphine derivatives on the intersystem T₁ → S₀-transition energy was studied theoretically. The molecular conformations of 15 porphine derivatives and 8 Zn-porphine derivatives in the ground singlet (S₀) and lowest triplet (T₁) states were optimized, the molecular orbital energies were determined, and the energies of the T₁ → S₀ transition were calculated using quantum chemical methods. The T₁ → S₀-transition energy was found to decrease from 11,700 to 6200 cm^–1 upon increasing the number of NH₂ groups in the macrocycle C_m-positions. The T₁ → S₀-transition energy was a linear function of the weighted sum of inductive and resonant Hammett constants 0.2σ_I + 0.8σ_R of the substituents. The ratio of inductive and resonant contributions of the NH₂ groups depended on the method of attachment to the macrocycle, with the contribution of resonant interactions decreasing with increasing spacer length. The main reason for the bathochromic shift of the T₁ → S₀ transition was a significant increase in the energy of the b₁-orbital, which had antinodes on the macrocycle C_m atoms. The dependence also held for Zn-porphyrins with the same peripheral substitution architecture. The energy of the T₁ → S₀ transition was noted to differ for both NH tautomers and conformers differing in the position of NH₂ groups relative to the macrocycle mean plane. The calculations showed that experimental studies of aminoporphyrins were promising for obtaining new phosphors in the IR spectral region. A method for predicting the T₁ → S₀-transition energy for the synthesis of compounds with the required spectral and luminescent characteristics was proposed based on the results.