Abstract
Owing to their attractive potential in optoelectronic application, luminescent Ru(II) complexes with diamine ligands are harvesting more and more research efforts. These literature efforts, however, are mostly mononuclear ones, with no detailed discussion on the performance comparison between mononuclear and multinuclear Ru(II) complexes. This work synthesized three diamine ligands having two or multiple chelating sites in each ligand, as well as their Ru(II) complexes. The single-crystal structure, electronic structure, and photophysical parameters of these Ru(II) complexes were analyzed and compared. It was found that multinuclear Ru(II) complexes had a pure MLCT (metal-to-ligand charge transfer)–based emissive center, showing longer emission lifetime and higher emission quantum yield, which were desired for oxygen sensing. Then, the oxygen sensing performance of these mononuclear and multinuclear Ru(II) complexes was systematically compared by doping them into polymer fibers via electrospinning method. Improved oxygen sensing performance was observed from binuclear Ru(II)-doped nanofibrous samples, compared with the sensing performance of mononuclear ones, including higher sensitivity, shorter response/recovery time, and better photostability. The causation was attributed to the fact that the emissive state of multinuclear Ru(II) complexes was MLCT-based ones and thus more sensitive to O2 quenching than monocular Ru(II) complexes whose emissive state was a mixture of MLCT and LLCT (ligand-to-ligand charge transfer). In addition, a multinuclear Ru(II) complex had multiple emissive/sensing components, so that its sensing collision probability with O2 was increased, showing better photostability and shorter response/recovery time. The novelty of this work was the linear oxygen sensing curve, which was rarely reported in the previous work.
Highlights
The research and exploration for functional systems with desired features and performance continuously push the improvement of organic and composite materials (Guan et al, 2015)
A monoclinic system is adopted by both Ru-1 and Ru-3 crystals, where each Ru(II) coordination center falls in the center of an octahedral coordination sphere formed by three bidentate diamine ligands
Ru-n complexes all adopted a traditional octahedral coordination sphere at each Ru(II) center. Their onset electronic transition was a mixture of MLCT and LLCT
Summary
The research and exploration for functional systems with desired features and performance continuously push the improvement of organic and composite materials (Guan et al, 2015). Theoretical calculation on typical [Ru(N-N)3]2+ complexes has revealed the electronic structure of MLCT (metal-to-ligand charge transfer) transition, which means that the occupied FMOs (frontier molecular orbitals) are composed of dominant metal d contribution, whereas the unoccupied ones consist of π* orbitals of N-N ligands Such MLCT-based emissive state generally has emissive lifetime at a scale of microsecond and Stokes shift at a scale of dozens of nanometers, which makes [Ru(N-N)3]2+ complexes a candidate structural component for optoelectronic materials. The porous structure of supporting host ensures gradual O2 concentration around [Ru(N-N)3]2+ probe, so that linear sensing response is observed Besides this silica-based supporting host, some research efforts have reported another type of supporting host of nanofibrous film synthesized by electrospinning (ES) method (Wang et al, 2009; Wang et al, 2010). By doping them into a supporting host of ES fibers, the influence of inter-molecular aggregation and dispersal in supporting host shall be minimized, so that the sensing performance of multinuclear Ru(II) complexes can be compared with that of mononuclear ones
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