The unsuspected influence of the pyridyl-triazole ligand isomerism upon the electronic properties of tricarbonyl rhenium complexes: an experimental and theoretical insight

Abstract

Two isomeric tricarbonyl rhenium(i) complexes, ReL1 and ReL2, that possess a 2-pyridyl-1,2,n-triazole (pyta) ligand (n = 4 and 3, respectively) connected to a 2-phenylbenzoxazole (PBO) moiety, were synthesized in good yields. The X-ray structures showed that in ReL1 the PBO moiety and the pyta ligand almost form a right angle hindering electron delocalization, while in ReL2 their nearly planar arrangement favors the electron delocalization in the whole organic ligand. Therefore, the nature of the ligand significantly influences the electron distribution in the two complexes, as indicated by the results of TD-DFT calculations. An electrochemical study highlighted that, by comparison with ReL2, the smaller HOMO-LUMO energy gap of ReL1 is in line with its lower first reduction potential. From a spectroscopic viewpoint, both complexes emitted phosphorescence in organic solvents, with distinct color and intensity. They also emitted in the solid state, but only ReL1 showed significant aggregation-induced phosphorescence emission (AIPE). This complete study sheds light on the crucial role of structural isomerism of the triazole group, which has been unsuspected for a long time although it can govern the geometry and electronic properties of rhenium complexes. It is shown for the first time that grafting a non-coordinated π-conjugated fragment on the N(4) atom of a 1,2,4-triazole group can be of high value for the design of efficient light-emitting materials based on rhenium complexes.

Optimization of aggregation-induced phosphorescence enhancement in mononuclear tricarbonyl rhenium(i) complexes: the influence of steric hindrance and isomerism

“…Considering the OSWV cathodic part, both new complexes presented a reduction process beyond −1.8 V, as did ReL1. 8 This process was most likely attributed to the reduction of the PBO fragment, confirming that the latter is not involved in the complexation process. 14 Indeed, a similar reduction process was detected for both ligands L3 and L4 (Table S14, Fig.…”

“…It can be attributed to the reduction process of the substituted triazole ring whose potential value may substantially decrease by complexation as observed in related compounds. 13,15 Remarkably, the value of this first reduction process seems to be characteristic of this family of complexes incorporating a 2-pyridyl-1,2,4-triazole ligand 8 and could be related to the bent arrangement of the pyta and PBO moieties. In CV, the careful examination of the first reduction process at different scan rates showed that this process becomes quasireversible around 1 V s −1 for ReL3, as was the case for ReL1.…”

“…In the solid state, this compound displays yellow to red emission, so it could be particularly valuable for preparing nanoparticles for bio-imaging, although the photoluminescence quantum yield is quite modest (0.065). 8 The aim of the present work is now to improve the solid-state spectroscopic properties of these complexes through the rational design of the ligands.…”

“…As a matter of fact, our previous study has revealed that ReL1, which incorporates the 3-(2-pyridyl )-1,2,4-triazole fragment, has superior emission properties compared to ReL2, built on the very popular 4-(2-pyridyl)-1,2,3-triazole fragment. 8 The appended 2-phenylbenzoxazole (PBO) moiety contributes to the conjugated electron system of these complexes, and restrictions on its molecular movements in the solid state are thought to be mainly responsible for the AIPE effect. Moreover, for the vast majority of PBO derivatives, there is little π-π stacking interaction between aromatic groups, and hence an increased photoluminescence.…”

Photophysical and Electrocatalytic Properties of Rhenium(I) Triazole-Based Complexes

“…In this regard, the copper(I) catalysed 1,3-dipolar cycloaddition of alkynes and azides to form 1,4-disubstituted-1,2,3-triazoles has become a popular transformation in the design of ligands for transition metal complexes over the past decade [5][6][7][8]. This has resulted in several reports on the preparation of 1,2,3-triazole-based rhenium(I) complexes [9][10][11][12][13][14][15][16][17][18][19][20], which have been applied for their therapeutical activity [21][22][23][24][25] and as luminescent biological imaging microscopy probes [26][27][28][29][30][31][32][33][34]. Recently, Ching et al reported a series of complexes of the form [Re(NˆN)(CO) 3 Cl], where the NˆN ligand is based on a 4-(pyrid-2-yl)-1,2,3-triazole framework [35].…”