Improving the Photosensitizing Properties of Ruthenium Polypyridyl Complexes Using 4-Methyl-2,2′-bipyridine-4′-carbonitrile as an Auxiliary Ligand

Abstract

We report in this work the synthesis and spectroscopic, electrochemical, spectroelectrochemical, and photophysical characterization of a novel series of ruthenium polypyridyl complexes with 4-methyl-2,2'-bipyridine-4'-carbonitrile (Mebpy-CN) as an auxiliary ligand of general formula [Ru(bpy)3-x(Mebpy-CN)x](PF6)2 (x = 1-3) (with bpy = 2,2'-bipyridine). A significant increase in the lifetime and quantum yield of emission of the lowest (3)MLCT excited state is disclosed when going from x = 1 to x = 3, evidencing an improvement of the photosensitizing properties with respect to [Ru(bpy)3](PF6)2. Furthermore, quenching by molecular oxygen of (3)MLCT excited states of the three complexes produced singlet molecular oxygen ((1)O2) with quantum yield values higher than that of [Ru(bpy)3](2+) in CH3CN. The structure of the complex with x = 1 has been determined by X-ray diffraction. The photoconductivity of ZnO nanowires covered with this same complex is increased by an order of magnitude, pointing to its feasibility as a component of a DSSC. A new dinuclear complex with Mebpy-CN as a bridging ligand has also been prepared and characterized by physicochemical techniques. The derived mixed-valent species of formula [(bpy)2Ru(II)(Mebpy-CN)Ru(III)(NH3)5](5+) displays a considerable metal-metal electronic coupling due to the delocalization effect of a nitrile group in the 4' position of the bpy ring.

ZnO nanowire sensitization with Ru polypyridyl complexes: charge-transfer probed by spectral and relaxation photocurrent measurements

“…This is mainly because of the broad absorption band for the FeL3 complex peaked at a higher wavelength (540 nm) [45] [see Figure 3(b)], which overlaps the ZnO emission band. In addition, the complexes covering the ZnO NWs are also expected to absorb part of the excitation power at 325 nm [42], [45]. We note that, while the complexes themselves have some PL (data obtained from the pure complex films, not shown), their contributions to the observed spectra in Fig.…”

“…One was kept as a reference, while the other four were soaked with each one of the polypyridyl Ru complexes RuL1, RuL2 and RuL3 and the FeL3 complex. A detailed physicochemical characterization of the Ru sensitizers has been presented in [42], [44], [46]: they have a Ru atom coordinated to bipyridyl ligands with pendant nitrile groups that act as "anchoring units". The absorption spectra of the RuL1, L2 and L3 complexes in acetonitrile are presented in [42], and that of the FeL3 complex in [45].…”

“…The reason for the observed changes in intensity and wavelength positions is mainly the selective absorption of portions of the ZnO optical emissions in the short wavelength region by the complex layers covering the samples. Indeed, the Ru complexes present prominent absorption bands peaked at about 450 nm [42] [see Figure 3(b)], which are responsible for the suppression of the intensity of the emission around this region of the PL spectra, thus leading to the observed redshift of the PL bands positions. For the case of the ZnO NW sample soaked with FeL3, the whole PL band is strongly suppressed.…”

“…Recently [42], new sensitizers based on ruthenium polypyridyl complexes have been reported where the bpy ligands (bpy = 2,2'-bipyridine) were replaced by Mebpy-CN ligands (Mebpy-CN = 4-methyl-2,2'-bipyridine-4'-carbonitrile) coordinated to the metallic center. In these complexes, nitrile groups in the periphery of the polypyridyl ligands act as the anchoring groups preserving the integrity of the ZnO nanostructures.…”

“…Considering that the lifetimes of excited states can be increased by increasing the number of nitrile groups [43], the number of ligands was varied. The new series of ruthenium complexes, of formula [Ru(bpy) 3−x (Mebpy-CN) x ] 2+ (x =1−3) described in [42], present lifetime and quantum yield of emission of the lowest 3 MLCT excited state increasing steadily when going from x = 1 to x = 3 (RuL1, RuL2, and RuL3, respectively). These three complexes have been adsorbed in TiO 2 nanoparticulate electrodes [44], showing very good charge transfer properties and good behavior as electrode in a DSSC prototype.…”