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

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 Cm-positions. The T1 → S0-transition energy was a linear function of the weighted sum of inductive and resonant Hammett constants 0.2σI + 0.8σR 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 Cm 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. [ABSTRACT FROM AUTHOR]