Stabilization of an elusive tautomer by metal coordination

The solid-state isolation of the different tautomers of a chemical com­pound can be a challenging problem. In many cases, tautomers with an energy very close to the most stable one cannot be isolated (elusive tautomers). In this article, with reference to the 4-methyl-7-(pyrazin-2-yl)-2H-[1,2,4]triazolo[3,2-c][1,2,4]triazole ligand, for which the elusive 3H-tautomer has an energy only 1.4 kcal mol(−1) greater than the most stable 2H form, we show that metal com­plexation is a suc­cessful and reliable way for stabilizing the elusive tauto­mer. We have pre­pared two com­plexes of the neutral ligand with CuBr(2) and ZnBr(2), namely, aqua­bromido­bis­[4-methyl-7-(pyrazin-2-yl)-3H-[1,2,4]triazolo[3,2-c][1,2,4]triazole]copper(II) bromide trihydrate, [CuBr(C(8)H(7)N(7))(2)(H(2)O)]Br·3H(2)O, and di­bro­mido­[4-methyl-7-(pyrazin-2-yl)-2H-[1,2,4]triazolo[3,2-c][1,2,4]triazole][4-methyl-7-(pyrazin-2-yl)-3H-[1,2,4]triazolo[3,2-c][1,2,4]tri­azole]zinc(II) monohydrate, [ZnBr(2)(C(8)H(7)N(7))(2)]·H(2)O. The X-ray analysis shows that, in both cases, the elusive 3H-tautomer is present. The results of the crystallographic analysis of the two com­plexes reflect the different coordination preferences of Cu(II) and Zn(II). The copper(II) com­plex is homotautomeric as it only con­tains the elusive 3H-tautomer of the ligand. The com­plex can be described as octa­hedral with tetra­gonal distortion. Two 3H-triazolotriazole ligands are bis-chelated in the equatorial plane, while a water mol­ecule and a bromide ion in elongated axial positions com­plete the coordination environment. The zinc(II) com­plex, on the other hand, is heterotautomeric and con­tains two bromide ions and two monodentate ligand mol­ecules, one in the 2H-tautomeric form and the other in the 3H-tautomeric form, both coordinated to the metal in tetra­hedral geometry. The observation of mixed-tautomer com­plexes is unprecedented for neutral ligands. The analysis of the X-ray mol­ecular structures of the two com­plexes allows the deduction of possible rules for a rational design of mixed-tautomer com­plexes

First examples of pyran based colorants as sensitizing agents of p-Type Dye-Sensitized solar cells

Three different pyran based dyes were synthesized and tested for the first time as photosensitizers of NiO based p-type dye-sensitized solar cells (p-DSSC). The molecules feature a similar molecular structure and are based on a pyran core that is functionalized with electron acceptor groups of different strength and is symmetrically coupled to phenothiazine donor branches. Optical properties of the dyes are deeply influenced by the nature of the electron-acceptor group, so that the overall absorption of the three dyes covers the most of the visible spectrum. The properties of devices based on the NiO electrodes sensitized with the investigated dyes were evaluated under simulated solar radiation: the larger short circuit current density exceeded 1mA/cm2 and power conversion efficiency as high as 0.04% could be recorded. The performances of the fabricated p-DSSC have been compared to a reference cell sensitized with P1, a high level benchmark, which afforded a photoelectrochemical activity similar to the best example of our pyran sensitized devices (1.19 mA/cm2 and 0.049%)

Volume Holographic Optical Elements as Solar Concentrators

In this chapter, we investigate the possibility to realize a holographic solar concentrator by using a new photopolymeric material as recording medium. Therefore, two different configurations of holographic lenses (lenses with spherical and cylindrical symmetry) are described in terms of both recording process and optical response characterization. Finally, we propose the possibility to use this new photopolymer to realize holographic solar concentrator for space applications