Two-Dimensional, Pyrazine-Based Nonlinear Optical Chromophores with Ruthenium(II) Ammine Electron Donors

Six new nonlinear optical (NLO) chromophores with pyrazinyl-pyridinium electron acceptors have been synthesized by complexing a known pro-ligand with electron donating {Ru^(II)(NH_3)_5}^(2+) or trans-{Ru^(II)(NH_3)_4(py)}^(2+) (py = pyridine) centers. These cationic complexes have been characterized as their PF_6^− salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible d → π* metal-to-ligand charge-transfer (MLCT) absorptions gain intensity on increasing the number of Ru^(II) centers from one to two, but remain at constant energy. One or two Ru^(III/II) redox processes are observed which are reversible, quasi-reversible, or irreversible, while all of the ligand-based reductions are irreversible. Molecular first hyperpolarizabilities β have been determined by using hyper-Rayleigh scattering (HRS) at 1064 nm, and depolarization studies show that the NLO responses of the symmetric species are strongly two-dimensional (2D) in character, with dominant “off-diagonal” β_(zyy) components. Stark (electroabsorption) spectroscopic measurements on the MLCT bands also allow the indirect determination of estimated static first hyperpolarizabilities β_0. Both the HRS and the Stark-derived β_0 values increase on moving from mono- to bimetallic complexes, and substantial enhancements in NLO response are achieved when compared with one-dimensional (1D) and 2D monometallic Ru^(II) ammine complexes reported previously

Polyoxometalate multi-electron-transfer catalytic systems for water splitting

The viable production of solar fuels requires a visible-light-absorbing unit, a H2O (or CO2) reduction catalyst (WRC), and a water oxidation catalyst (WOC) that work in tandem to split water or reduce CO2 with H2O rapidly, selectively, and for long periods of time. Most catalysts and photosensitizers developed to date for these triadic systems are oxidatively, thermally, and/or hydrolytically unstable. Polyoxometalates (POMs) constitute a huge class of complexes with extensively tunable properties that are oxidatively, thermally, and (over wide and adjustable pH ranges) hydrolytically stable. POMs are some of the fastest and most stable WOCs to date under optimal conditions. This Microreview updates the very active POM WOC field; it reports the application of POMs as WRCs and initial self-assembling metal oxide semiconductor–photosensitizer–POM catalyst triad photoanodes. The complexities of investigating these POM systems, including but not limited to the study of POM-hydrated metal-ion–metal-oxide speciation processes, are outlined. The achievements and challenges in POM WOC, WRC, and triad research are outlined

Syntheses and Properties of Two-Dimensional, Dicationic Nonlinear Optical Chromophores Based on Pyrazinyl Cores

Six new dicationic 2D nonlinear optical (NLO) chromophores with pyrazinyl-pyridinium electron acceptors have been synthesized by nucleophilic substitutions of 2,6-dichloropyrazine with pyridyl derivatives. These compounds have been characterized as their PF_6^− salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. Large red shifts in the intense, π → π* intramolecular charge-transfer (ICT) transitions on replacing −OMe with –Nme_2 substituents arise from the stronger π-electron donor ability of the latter. Each compound shows a number of redox processes which are largely irreversible. Single crystal X-ray structures have been determined for five salts, including two nitrates, all of which adopt centrosymmetric packing arrangements. Molecular first hyperpolarizabilities β have been determined by using femtosecond hyper-Rayleigh scattering at 880 and 800 nm, and depolarization studies show that the NLO responses of the symmetric species are strongly 2D, with dominant “off-diagonal” β_(zyy) components. Stark (electroabsorption) spectroscopic measurements on the ICT bands afford estimated static first hyperpolarizabilities β_0. The directly and indirectly derived β values are large, and the Stark-derived β_0 response for one of the new salts is several times greater than that determined for (E)-4′-(dimethylamino)-N-methyl-4-stilbazolium hexafluorophosphate. These Stark spectroscopic studies also permit quantitative comparisons with related 2D, binuclear RuII ammine complex salts

Do Study Abroad Programs Enhance the Employability of Graduates?

Using data on a large sample of recent Italian graduates, this paper investigates the extent to which participation in study abroad programs during university studies impacts subsequent employment likelihood. To address the problem of endogeneity related to participation in study abroad programs, I use a combination of fixed effects and instrumental variable estimation where the instrumental variable is exposure to international student exchange schemes. My estimates show that studying abroad has a relatively large and statistically meaningful effect on the probability of being in employment three years after graduation. This effect is mainly driven by the impact that study abroad programs have on the employment prospects of graduates from disadvantaged (but not very disadvantaged) backgrounds, though positive but imprecise effects are also found for graduates from advantaged backgrounds

Organoimido-Polyoxometalate Nonlinear Optical Chromophores: A Structural, Spectroscopic, and Computational Study

Ten organoimido polyoxometalate (POM) based chromophores have been synthesized and studied by hyperRayleigh Scattering (HRS), Stark and Resonance Raman spectroscopies and DFT calculations. HRS β0 values for chromophores with resonance electron donors are significant (up to 139 × 10-30 esu, ca. 5 × that of the DAS+ cation), but systems with no donor, or the –NO2 acceptor show no activity, in some cases despite large DFT-predicted β-values. In active systems with short (phenyl) π-bridges, β0-values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic β-values, β0/N3/2 (N = number of bridge π-electrons) thus appear to break empirical performance limits (β0/N3/2 vs λmax) for planar organic systems. However, β0-values obtained for extended systems with a diphenylacetylene bridge are comparable to or lower than that of their nitro analogue N,N-dimethyl-4-[(4-nitrophenyl)ethynyl]-aniline (DMNPEA). Resonance Raman spectroscopy confirms involvement of the POM in the electronic transitions whether donor groups are present or not, but Stark spectroscopy indicates that in their absence the transitions have little dipolar character (hence NLO inactive), consistent with DFT-calculated frontier orbitals which extend over both POM and organic group. Stark and DFT also suggest that β is enhanced in the short compounds because extension of charge transfer (CT) onto the POM increases excited state dipole moment changes. With extended π-systems this effect does not increase CT distances relative to a –NO2 acceptor, so β0-values do not exceed that of DMNPEA. Overall, our results show that: (i) the organoimido–POM unit is an efficient acceptor for 2nd order NLO, but an ineffective donor; (ii) the nature of electronic transitions in arylimido-POMs is strongly influenced by the substituents of the aryl group; and (iii) organoimido-POMs outperform organic acceptors with short π-bridges, but lose their advantage with extended π-conjugation

Fine-tuning polyoxometalate non-linear optical chromophores: a molecular electronic “Goldilocks” effect

A new aryl-imido polyoxometalate non-linear optical chromophore (POMophore) with a diphenylamino donor group attains the highest βzzz, 0 value (196 × 10−30 esu by Hyper-Rayleigh Scattering, HRS), and best transparency/non-linearity trade off yet for such materials. Stark spectroscopic and DFT investigation of this compound, plus NMe2 and carbazole analogues, show that its high performance results from a combination of strongly dipolar electronic transitions, and strong electronic communication across the π-system

Increasing p-type dye sensitised solar cell photovoltages using polyoxometalates

Lindqvist polyoxometalate (POM) additives increase VOC in p-type DSSCs by up to 140%, yielding substantial efficiency gains for poorly matched dyes and redox mediators. For better dye/electrolyte combinations, these gains are typically outweighed by losses in JSC. Charge lifetime and transient IR measurements show that this is due to retardation of both recombination and electron transfer to the mediator, and a positive shift in the NiO valence band edge. The POMs also show their own, limited sensitizing effect

Transition Metal Substitution Effects on Metal-to-Polyoxometalate Charge Transfer

A series of heterobimetallic transition metal substituted polyoxometalates (TMSPs) have been synthesized based on the CoII-centered ligand [CoIIW11O39]10-. The eight complex series, [CoII(MxOHy)W11O39](12-x-y)- (MxOHy = VIVO, CrIII(OH2), MnII(OH2), FeIII(OH2), CoII(OH2), NiII(OH2), CuII(OH2), ZnII(OH2)), of which six are reported for the first time, was synthesized starting from [CoIIIW11O39]9- and studied using spectroscopic, electrochemical, and computational techniques to evaluate the influence of substituted transition metals on the photodynamics of the metal-to-polyoxometalate charge transfer (MPCT) transition. The bimetallic complexes all show higher visible light absorption than the plenary [CoIIW12O40]6- and demonstrate the same MPCT transition as the plenary complex, but have shorter excited state lifetimes (sub-300 ps in aqueous media). The decreased lifetimes are rationalized on the basis of nonradiative relaxation due to coordinating aqua ligands, increased interaction with cations due to increased negative charge, and the energy gap law, with the strongest single factor appearing to be the charge on the anion. The most promising results are from the Cr- and Fe-substituted systems, which retain excited state lifetimes at least 50% of that of [CoIIW12O40]6- while more than tripling the absorbance at 400 nm

‘Moving in concentric circles’? The history and politics of press inquiries

In this paper, we consider the Leveson inquiry's use of a narrative device – the policy cycle – to justify the need for a break with the past. We challenge that narrative, which runs through much of the literature, and posit a more nuanced and complex account of the politics and history of press inquiries, drawing upon the political science literature. We then reflect upon the implications of our findings for the future of press regulation

Beyond solvent exclusion: i-Motif detecting capability and an alternative DNA light-switching mechanism in a ruthenium(II) polypyridyl complex

Cytosine-rich DNA can fold into secondary structures known as i-motifs. Mounting experimental evidence suggests that these non-canonical nucleic acid structures form in vivo and play biological roles. However, to date, there are no optical probes able to identify i-motif in the presence of other types of DNA. Herein, we report for the first time the interactions between the three isomers of [Ru(bqp)2]2+ with i-motif, G-quadruplex, and double-stranded DNA. Each isomer has vastly different light-switching properties: mer is “on”, trans is “off”, and cis switches from “off” to “on” in the presence of all types of DNA. Using emission lifetime measurements, we show the potential of cis to light up and identify i-motif, even when other DNA structures are present using a sequence from the promoter region of the death-associated protein (DAP). Moreover, separated cis enantiomers revealed Λ-cis to have a preference for the i-motif, whereas Δ-cis has a preference for double-helical DNA. Finally, we propose a previously unreported light-switching mechanism that originates from steric compression and electronic effects in a tight binding site, as opposed to solvent exclusion. Our work suggests that many published non-emissive Ru complexes could potentially switch on in the presence biological targets with suitable binding sites, opening up a plethora of opportunity in the detection of biological molecules