Light-Responsive Pyrazine-Based Systems: Probing Aromatic Diarylethene Photocyclization

Abstract

We present an investigation of the photocyclization of novel aromatic diarylethene (DAE) systems 1-3 based on pyrazine, quinoxaline, and helicene scaffolds. These prospective photoswitches were designed using DFT calculations and analyzed in solution and in the solid state by cyclic and rotating disc voltammetry, UV-Vis and transient absorption spectroscopy, as well as X-ray crystallography. Additionally, Nucleus Independent Chemical Shift (NICS) calculations were performed to investigate the influence of the aromaticity on the photocyclization ability. While pyrazine-2,3-diyl-extended DAE system 1 demonstrated photoswitching ability with short lifetimes of the cyclized form, the more aromatic quinoxaline analogue 2 did not feature any photocyclization. Further extension of these aromatic systems into helicene-DAE 3 resulted in stabilization of the cyclized form through the conserved backbone aromaticity, accompanied by

Coordination-enhanced photochromism in dysprosium dinuclear complexes with photomodulated single-molecule magnet behavior

“…The optimized conditions for this coupling consist in the use of cesium fluoride together with Pd 2 (dba) 3 , tricyclohexylphosphonium tetrafluoroborate and potassium carbonate (Figure 1). Similarly, coupling between 2,5-dimethyl-4-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)thiophene and 2,3-dichloropyrazine afforded the previously published model compound BTP2 [30]. Then, BTP1 was put to react with 2 equivalents of [M(hfac) 3 .2H 2 O] (M = Y, Dy, hfac = hexafluoroacetylacetonate) in refluxing dichloromethane to afford the dinuclear compounds 1M o (M = Y or Dy) as single crystals after slow cooling.…”

“…Synthesis. Pyrazine-based photoswitches have been previously synthesized through the condensation of a bisthiophene dione and diamines to form the central pyrazine cycle [30]. Here, ligand BTP1 was conveniently synthesized by Suzuki coupling from 5-methyl-2-(pyridin-2-yl)-4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)thiazole [41] and 2,3-dichloropyrazine in 78 % yield.…”

“…For instance, in BTP1 c , the singlet signal at 2.30 ppm (2.38 ppm in BTP1 o ) was associated to the methyl groups and the pyrazine protons were observed at 7.71 ppm (8.75 ppm in BTP1 o ), while the overall spectrum showed the preserved symmetry between the two halves of the molecule ( Figure S6). The pyrazine proton signal is more shielded in the closed form due to the aromaticity loss of this pyrazine unit upon cyclization [30]. At 223 K, no thermal evolution of the system was observed in the absence of light for 40 min.…”

“…Recently, Diederich et al demonstrated that DAE with a central pyrazine bridge like 2,3-bis(2,5-dimethylthiophen-3-yl)pyrazine are also T-photochromes with a thermal half-life of ca. one minute in acetonitrile [30]. Therefore, the pyrazine core that allows for coordination-induced tuning of photochromism, deserves to be further explored to extend this class of T-type DAEs.…”

Photochromic diarylethene ligands featuring 2-(imidazol-2-yl)pyridine coordination site and their iron(II) complexes

“…In this context, it is important to specify some design principles for such ligands. Firstly, novel ligands should feature a non-aromatic ethene bridge to increase the life-time of a photoinduced closed-ring isomer [28]. Secondly, photoactive hexatriene and metal coordination sites should be close to each other.…”