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

The influence of the architecture of NH 2 -peripheral substitution of porphine derivatives on the intersystem T 1 → S 0 -transition energy was studied theoretically. The molecular conformations of 15 porphine derivatives and 8 Zn-porphine derivatives in the ground singlet ( S 0 ) and lowest triplet ( T 1 ) states were optimized, the molecular orbital energies were determined, and the energies of the T 1 → S 0 transition were calculated using quantum chemical methods. The T 1 → S 0 -transition energy was found to decrease from 11,700 to 6200 cm –1 upon increasing the number of NH 2 groups in the macrocycle C m -positions. The T 1 → S 0 -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 NH 2 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 T 1 → S 0 transition was a significant increase in the energy of the b 1 -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 T 1 → S 0 transition was noted to differ for both NH tautomers and conformers differing in the position of NH 2 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 T 1 → S 0 -transition energy for the synthesis of compounds with the required spectral and luminescent characteristics was proposed based on the results.