1,2,3-Triazol-5-ylidene- vs. 1,2,3-triazole-based tricarbonylrhenium(I) complexes: influence of a mesoionic carbene ligand on the electronic and biological properties.

The tricarbonylrhenium complexes that incorporate a mesoionic carbene ligand represent an emerging and promising class of molecules, the solid-state optical properties of which have rarely been investigated. The aim of this comprehensive study is to compare three of these complexes with their 1,2,3-triazole-based analogues. The Hirshfeld surface analysis of the crystallographic data revealed that the triazolylidene derivatives are more prone to π-π interactions than their 1,2,3-triazole-based counterparts. The FT-IR and electrochemical data indicated a stronger electron donor effect from the organic ligand to the rhenium atom for triazolylidene derivatives, which was confirmed by DFT calculations. All compounds were phosphorescent in solution, where the 1,2,3-triazole-based complexes showed unusually strong dependence on dissolved oxygen. All compounds also emitted in the solid state, some of them exhibited marked solid-state luminescence enhancement (SLE) effect. The 1,2,3-triazole based complex Re-Phe even displayed astounding photoluminescence efficiency with quantum yield up to 0.69, and proved to be an excellent candidate for applications linked to aggregation-induced emission (AIE). Interestingly, one triazolylidene-based complex (Re-T-BOP) showed attractive antibacterial activity. This study highlights the potential of these new molecules for applications in the fields of photoluminescent and therapeutic materials, and provides the first bases for the design of efficient molecules in these research areas.

Using a diphenyl-bi-(1,2,4-triazole) tricarbonylrhenium(I) complex with intramolecular π-π stacking interaction for efficient solid-state luminescence enhancement.

Since intramolecular π-π stacking interactions can modify the geometry, crystal packing mode, or even the electronic properties of transition metal complexes, they are also likely to influence the solid-state luminescence properties. Following this concept, a new tricarbonylrhenium(I) complex (Re-BPTA) was designed, based on a simple symmetrical 5,5'-dimethyl-4,4'-diphenyl-3,3'-bi-(1,2,4-triazole) organic ligand. The complex was prepared in good yield using a three-step procedure. The crystallographic study revealed that both phenyl rings are located on the same side of the molecule, and twisted by 71° and 62°, respectively, with respect to the bi-(1,2,4-triazole) unit. They overlap significantly, although they are slipped parallel to each other to minimize the intramolecular interaction energy. The π-π stacking interaction was also revealed by 1H NMR spectroscopy, in good agreement with the results of theoretical calculations. In organic solutions, a peculiar electrochemical signature was observed compared to closely-related pyridyl-triazole (pyta)-based complexes. With regard to the optical properties, the stiffness of the Re-BPTA complex led to the stabilization of the 3MLCT state, and thus to an enhancement of the red phosphorescence emission compared to the more flexible pyta complexes. However, an increased sensitivity to quenching by oxygen appeared. In the microcrystalline phase, the Re-BPTA complex showed strong photoluminescence (PL) emission in the green-yellow wavelength range (λPL = 548 nm, ΦPL = 0.52, 〈τPL〉 = 713 ns), and thus a dramatic solid-state luminescence enhancement (SLE) effect. These attractive emission properties can be attributed to the fact that the molecule undergoes little distortion between the ground state and the triplet excited state, as well as to a favorable intermolecular arrangement that minimizes detrimental interactions in the crystal lattice. The aggregation-induced phosphorescence emission (AIPE) effect was clear, with a 7-fold increase in emission intensity at 546 nm, although the aggregates formed in aqueous medium were much less emissive than the native microcrystalline powder. In this work, the rigidity of the Re-BPTA complex is reinforced by the intramolecular π-π stacking interaction of the phenyl rings. This original concept provides a rhenium tricarbonyl compound with very good SLE properties, and could be used more widely to successfully develop this area of research.

Luminescent fac-[ReX(CO)3(phenyl-pyta)] (X = Cl, Br, I) complexes: influence of the halide ligand on the electronic properties in solution and in the solid state.

Tricarbonylrhenium(I) complexes that incorporate a chloride ligand are promising photoluminescent materials, but those incorporating a bromide or iodide ligand have received very little attention regarding their solid-state properties. In this work, three rhenium(I) complexes differing only by the nature of their halide ligand (X = Cl, Br, and I) were compared. They are based on a fac-[ReX(CO)3(N^N)] framework where the N^N bidentate ligand is a 3-(2-pyridyl)-1,2,4-triazole unit functionalized by an appended phenyl group. DFT calculations showed that the character of the lowest energy transitions progressively changes from Re → N^N ligand (MLCT) to X → N^N ligand (XLCT) when increasing the size of the halogen atom. Regarding the electrochemical behavior, the chloride and bromide complexes 1-Cl and 1-Br were similar, while the iodide complex 1-I exhibited a strikingly different electrochemical signature in oxidation. From a spectroscopic viewpoint, all three complexes emitted weak red-orange phosphorescence in dichloromethane solution. However, in the solid state, marked differences appeared. Not only was 1-Cl a good emitter of yellow light, but it had strong solid-state luminescence enhancement (SLE) properties. In comparison, 1-Br and 1-I were less emissive and they showed better mechanoresponsive luminescence (MRL) properties, probably related to a loose molecular arrangement in the crystal packing and to the opening of vibrational non-radiative deactivation pathways. This study highlights for the first time how the nature of the halide ligand in this type of complex allows fine tuning of the solid-state optical properties, for potential applications either in bio-imaging or in the field of MRL-active materials.

Proton catalysis in the redox responsivity of a mini-sized photochromic diarylethene.

A thermally irreversible dithienylethene (DTE) photochrom can be turned into a thermally reversible one in presence of Cu(II) triflate. A ring opening (DTEC closed→DTEO open) occurs through the formation of a copper-containing fast transient intermediate. Stopped-flow experiments monitored at 410 and 780 nm have allowed to show that the stoichiometry of this intermediate is DTE/Cu=1:1. At longer monitoring times (i.e., several seconds after mixing), the intermediate undergoes a slow decay while the residual DTEC closed form opens. A joint detailed kinetic and electrochemical analysis has unveiled a proton catalysis scenario in which electron transfer between DTEC and Cu(II), ligand exchange, protonation-deprotonation equilibria of the cation radicals and ring opening are embedded into two main reaction cycles. At the beginning of the reaction, Cu(II) is reduced into Cu(I) and DTE is degraded without ring opening. Then, as the reaction progresses, the triflic acid released from the Cu(II) reduction switches-on a propagation cycle during which ring opens without any more Cu(II) consumption. Cyclic voltammetry, spectro-electrochemical measurements, delayed photocoloration experiments in presence of Cu(II) and acid-base additions have confirmed the main features of the proton catalysis.