Corroles represent a unique family of tetrapyrrolic chromophores with distinct structural, chemical and photophysical properties, which render them promising materials for numerous applications. The lack of one of the meso-carbon atoms ultimately leads to nonplanar macrocycle distortions. The aim of this review is to summarize the peculiarities of the molecular conformations of free base corroles and provide a unified approach for their analysis.
Steady-state fluorescence measurements and quantum-chemical DFT geometry optimizations are applied to extend the structure–property relationships between the free-base corrole macrocycle conformation and its basicity to the lowest excited S[Formula: see text] and T[Formula: see text] states. Direct basicity estimation in the lowest excited S[Formula: see text] state is demonstrated by means of fluorescence quantum yield measurements. The long wavelength T1 tautomer is found to retain its basicity in the S[Formula: see text] state, whereas the short wavelength T2 tautomer shows a noticeable decrease in basicity in the S[Formula: see text] state, which is related to the in-plane tilting of the pyrrole ring to be protonated. The conformational changes upon going from the ground to the lowest excited T[Formula: see text] state and the influence of the meso-aryl substitution pattern on the overall degree of distortions and tilting of the pyrrole ring to be protonated are also discussed from the point of view of macrocycle basicity.
Методами квантовой химии исследована ароматичность в нижнем триплетном Ti-состоянии NH-таутомеров свободных оснований корролов с различной архитектурой периферического заме щения. Установлено, что доминирующие контуры п-сопряжения у NH-таутомеров различаются, однако для каждого из таутомеров доминирующие контуры к-сопряжения в основном синглетном (So) и возбужденном триплетном (Ti) состояниях одинаковы. Обнаружено, что степень ароматич ности макроцикла в триплетном Ti-состоянии заметно уменьшается по сравнению с основным So-состоянием. Показано, что в триплетном Ti-состоянии макроцикл свободных оснований корро лов следует считать антиароматичным. Обсуждается взаимосвязь степени ароматичности с конформацией макроцикла и электронными эффектами периферических заместителей.К лю чевые слова: свободное основание корролов, NH-таутомеры, триплетное состояние, ж-сопряжение, ароматичность.We investigated the aromaticity in the lowest triplet Ti state o f NH-tautomers o f the free base corroles with a different architecture o f peripheral substitution using the quantum chemistry methods. It is estab lished that the dominating ж -conjugation pathways differ for NH-tautomers, but the dominating ж -conjugation pathways in the ground singlet So and excited triplet Ti states fo r each o f two tautomers are the same. It is found that the aromaticity degree o f the macrocycle in the triplet T\ state distinctly decreases compared to the ground So state. It is shown that the macrocycle o f the free base corroles in the triplet Ti state should be considered as antiaromatic. Relationships o f the aromaticity degree with the macrocycle conformation and electronic effects o f peripheral substituents are discussed.
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