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 porphin derivatives on the energy of the intersystem T₁→S₀ transition was studied theoretically. Using quantum chemistry methods, the molecular conformation of 15 porphine and 8 Zn-porphine derivatives in the ground singlet S₀ and lowest triplet T₁ states was optimized, the energies of molecular orbitals were determined, and the energies of the T₁→S₀ transition were calculated. It has been established that the energy of T₁→S₀ transition decreases from 11700 down to 6200 сm^–1 with an increase in the number of NH₂-groups in C_m-positions of the macrocycle, with the energy of the T₁→S₀ transition being a linear function of the weighted sum of the inductive and resonant Hammett constants 0.2σ_I + 0.8σ_R of substituents. The ratio of the inductive and resonant contributions of the NH₂-group depends on the attachment to the macrocycle, with the contribution of resonant interactions decreasing with increasing the spacer length. It has been shown that the main reason for the bathochromic shift of T₁→S₀ transition is a significant increase in the energy of b₁-orbital, which has antinodes on C_m atoms of the macrocycle. The dependence is kept also for Zn-porphyrins with the same architecture of peripheral substitution. It has been noticed that the energy of T₁→S₀ transition differs both for NH-tautomers and for conformers that differ in the position of NH₂-groups relative to the macrocycle mean plane. The calculation results show the promise of experimental studies of aminoporphyrins for obtaining new phosphors in the IR spectral region. Based on the results obtained, the method has been proposed to predict the energy of T₁→S₀ transition for the synthesis of compounds with the required spectral-luminescent characteristics.