Pyridyl-benzimidazole derivatives decorated with phenylazo substituents and their low-spin iron(II) complexes: a study of the synthesis, structure and photoisomerization

A series of phenylazo-substituted derivatives of pyridyl-benzimidazoles L1-L5 has been synthesized via a modified Mills reaction and their structural, spectral and photoswitching properties were investigated. NMR spectroscopy and single-crystal X-ray diffraction analysis of the desired compounds confirmed their E conformation. The light-induced E & LRARR; Z (also called trans & LRARR; cis) isomerization was monitored using UV-VIS and H-1 NMR spectroscopy that revealed a reversible photochromic behavior governed by first-order kinetics. State-of-the-art meta-dynamics simulations (iMTD-GC) and ab initio calculations (CASSCF-NEVPT2 and STEOM-DLPNO-CCSD) made the analysis of the electronic structures of ligands possible, which helped to explain the bland E-to-Z conversion of L1 and pronounced switching of L2-L5 ligands. Furthermore, the computational studies identified the active wavelengths of E-to-Z conversion and allowed assessing the mechanisms of photoisomerization. Additionally, ligands L1 and L2 were used for the preparation of four ferrous coordination compounds C1-C4 in which the structural and magnetic investigations confirmed the permanent low-spin state and diamagnetic behavior in the solid state. On the other hand, dissolved compounds C1 and C2 are high-spin at room temperature and their blue light irradiation causes the decomplexation instead of the desired E-to-Z isomerization.