The effect of the structure and conformational dynamics on quenching of triplet states of porphyrins and their chemical dimers by molecular oxygen and on singlet oxygen generation

Abstract

Quantum chemical calculations are performed to analyze the factors affecting rate constant k T of quenching of the lowest triplet state by molecular oxygen and quantum yield γ Δ of singlet oxygen generation in chemical dimers of porphyrins bound by phenyl spacers at one of the meso positions (OEP) 2 Ph, (TPP) 2 , and their Zn complexes. It is established that, for both types of dimers, the triplet excitation is localized on one of the mac rocycles. The steric hindrance of macrocycles at the site of the phenyl ring of (OEP) 2 Ph, (ZnOEP) 2 Ph, and their monomeric analogues OEP Ph and ZnOEP Ph facilitates its rotation by 90° in the triplet state. The lowest triplet state energy in this (U) conformation is lower than 7800 cm -1 , which makes impossible elec tronic excitation energy transfer to molecular oxygen. The potential barrier of transformation to the U con formation is considerably lower for dimers than for monomers. Because of this, the rate of conformational transformations for dimers is higher and some of the (OEP) 2 Ph and (ZnOEP) 2 Ph molecules have time to transform into the new U conformation before diffusion collision with O 2 molecules in solution. This leads to a noticeable decrease in γ Δ in accordance with experimental data. It is shown that the behavior of k T in the series of the studied dimers, their monomeric analogues, and relative compounds corresponds to the model of dipole-dipole electronic excitation energy transfer 1 ( 3 M⋅⋅⋅ ) → 1 ( 1 M 0 ⋅⋅⋅ 1 Δ g ) in collisional complexes.