Insight into magnetically induced ring currents and photophysics of six-porphyrin nanorings

• Published in: Physical chemistry chemical physics : PCCP 2024-08, Vol.26 (34), p.22337-22345
• Main Authors: Valiulina, Lenara I, Cherepanov, Victor N, Khoroshkin, Kirill
• Format: Article
• Language: English
• Subjects: Algorithms; Bonding strength; Deactivation; Density functional theory; Excitation; Internal conversion; Oscillator strengths; Oxidation; Porphyrins; Rate constants; Ring currents; Spin-orbit interactions; Valence
• ISSN: 1463-9076; 1463-9084; 1463-9084
• DOI: 10.1039/d4cp02547d

The series of nanorings based on Zn-porphyrins and tetraoxa-isophlorins in different oxidation states ( Q = 0, 2+, 4+, 6+) have been studied studied computationally at density functional theory level (DFT) using BHandHLYP functional combined with def2-SVP basis sets. Magnetically induced ring currents of nanorings have been calculated using the GIMIC method and the Ampère-Maxwell integration scheme. Ring current calculations show that neutral nanorings sustain equal diatropic and paratropic currents of 8 nA T −1 , resulting in zero net ring current strengths. The charged nanorings sustain strong ring currents with tropicity depending on the oxidation state Q . Among the considered nanorings, the nanoring composed of 6 isophlorins c-Iso6 6+ is the most aromatic with a ring current of I GIMIC = 81.6 nA T −1 . The structure c-P6 2+ with a ring current of I GIMIC = 54.9 nA T −1 can be considered as the most aromatic among the synthesized porphyrin nanorings. Spin-orbit coupling matrix elements, oscillator strengths, and excitation energies calculated at the CAM-B3LYP/def2-SVP level of theory were used to estimate rate constants for radiative and nonradiative processes. The algorithm based on X-H approximation were used to calculate the internal conversion rates ( k IC ). The main channel for the deactivation of the excitation energy in the studied nanorings is the process of internal conversion. The deactivation of excited energy occurs due to the vibrations of certain groups of C-H bonds in the nanorings. The nanoring c-Iso6 has magnetically allowed low-lying transitions that contributes significantly to the paratropic ring current, resulting in strong local antiaromaticity in the tetraoxa-isophlorin units. The X-H algorithm is used to reveal the main accepting modes for the IC process. Replacing the H atom in acceptor bonds with appropriate substituents may reduce the IC rate constant and increase the fluorescence quantum yield of porphyrin nanorings.