ABSTRACT Electrochemical switch‐on of second‐order non‐linear optical (NLO) responses ( β ) has been demonstrated in a polyoxometalate (POM) based chromophore (POMophore) for the first time. Reduction of the POM in a POM‐imidoaryl‐NO 2 derivative invokes an up to 15‐fold off/on switchable increase in intensity of scattered frequency‐doubled light, and the highest “on” state β yet reported for a redox‐cyclable POMophore. Computational and spectroscopic studies show that in the oxidized state, directionally opposed charge transfer (CT) transitions to POM and ─NO 2 result in a low net β . Upon reduction, the POM becomes a much weaker CT acceptor, resulting in more dipolar imido‐aryl to nitro CT, and thus enhanced NLO response.

A new dinuclear cobalt complex selectively catalyses 4e- reduction of O2 to water in methanol containing acetic acid. Its TOF of 0.031 s-1 and overpotential of 690 mV outperform the few previous Co N,O-chelate based catalysts for the 4e- ORR. Replacing acetic acid with NH4PF6 as proton source induces a complete and unprecedented switch to the H2O2 producing 2e- pathway. Mechanistic studies suggest a peroxo intermediate for both pathways, with acetate coordination/decoordination determining the destination of a key electron transfer.

Three linear, dipolar arylimido-polyoxometalate (POM) and one 2-dimensional bis-functionalized arylimido-polyoxometalate charge transfer chromophore, with diphenylacetylene bridges, have been synthesized and studied by spectroelectrochemistry, hyper-Rayleigh scattering (HRS), and DFT/TD-DFT calculations. The linear systems show that with julolidinyl (Jd) and -NTol2 donor groups, the alkyne bridge yields high second-order nonlinear optical (NLO) coefficients β (Jd, β0,zzz = 318 × 10-30 esu; -NTol2, β0,zzz = 222 × 10-30 esu), indeed the Jd compound gives the highest NLO activity of any organoimido-POM to date with minimal decrease in transparency. The bis-functionalized 2D (C2v) POM derivative showed increased activity over its monofunctionalized analogue with no decrease in transparency, although the NLO response was only minimally two dimensional. Spectroelectrochemistry and TD-DFT calculations showed switchable linear optical responses for the monofunctionalized derivatives due to the weakened charge transfer character of the electronic transitions in the reduced state, while TD-DFT also indicated potential for switched NLO responses. These have been demonstrated by electrochemistry-HRS for the Jd compound, but cyclability is limited by relatively poor stability in the reduced state. IR and CV studies for these sterically protected arylimido polyoxometalates indicate that decomposition proceeds via a breakdown of the {Mo6} cluster in the reduced state, rather than simple solvolysis of the Mo≡N bond.

New phenyl and stilbene-bridged polyoxometalate (POM) charge-transfer chromophores with diphenylamino donor groups produce, respectively, the highest intrinsic and absolute quadratic hyperpolarisabilities measured for such species.

We present the first study of arylimido-polyoxometalate nonlinear optical (NLO) chromophores with two-dimensional (2D) structures, and a comparison with one-dimensional analogues, through the synthesis of a family of arylimido-hexamolybdate derivatives where one or two polyoxometalate (POM) acceptors are connected to a tolyl-amino donor through phenyl bridges. Electronic absorption spectra and TD-DFT calculations reveal significant red-shifts in ligand-to-polyoxometalate charge transfer (LPCT) absorption bands for the 2D species compared to linear, dipolar analogues, consistent with the involvement of a larger conjugated (bridge + POM) system in the transitions. Electrochemical measurements indicate reversible, one-electron processes for the POM acceptors with class II mixed valence behavior observed where the POMs are connected to the same aryl ring and electronic isolation of the acceptors when they are on separate rings. Molecular first hyperpolarizabilities beta have been determined using hyper-Rayleigh scattering at 1064 and 1200 nm: for the most active compound, the HRS measurements and depolarization studies reveal a strongly 2D, off-diagonal response (beta 0,zzz = 190 x 10-30 esu; beta 0,zyy = -56.5 x 10-30 esu), consistent with the wide A-D-A angle and TD-DFT computed electronic transitions, which show both phenyl bridges and POMs equally involved in the acceptor orbitals.

Three new thiophenyl bridged triarylamine-donor based dyes with pyridyl anchoring groups and indolium acceptors have been synthesized and studied as sensitizers for the p-type dye-sensitized solar cell (p-DSSC). Compared to known dicyano and pyridinium dyes with the same triarylamine cores, these new sensitizers have broadened and red-shifted UV-vis absorption spectra, with high extinction coefficients and absorption extending beyond 700 nm. TD-DFT calculations on the indolium family indicate that these dyes combine strong donor acceptor communication, with a high degree of charge separation in the excited state - an ideal combination for sensitizer dyes. The phenylpyridyl anchored bis-indolium gives the highest power conversion efficiency (0.097%) through a short circuit photocurrent (J(SC)) of 3.04 mA cm(-2), open circuit voltage (V-OC) of 97 mV and fill factor of 36%. This exceeds the performance of the P1 reference dye in this study, and represents the best performance so far for a cationic, pyridine anchored p-DSSC dye.

Electrochemically switched 2nd order non-linear optical responses have been demonstrated for the first time in polyoxometalates (POMs), with an arylimido-derivative showing a leading combination of high on/off contrast (94 %), high visible transparency, and cyclability. Spectro-electrochemical and TD-DFT studies indicate that the switch-off results from weakened charge transfer (CT) character of the electronic transitions in the reduced state. This represents the first study of an imido-POM reduced state, and demonstrates the potential of POM hybrids as electrochemically activated molecular switches.

A family comprising seven arylimido-polyoxometalate (POM) hybrid chromophores (three of which are new), with linear dipolar, C2v and linear centrosymmetric geometries have been synthesised and studied by electronic absorption spectroscopy, electrochemistry, Z-scans (two photon absorption, TPA) and computation (DFT/TD-DFT). These reveal that POM acceptor units are an effective basis for TPA materials: the centrosymmetric bis-POM chromophores produce significant cross sections (δ up to 82 GM) from a single aryl bridge, a similar performance to larger dipolar π-systems combining carbazole or diphenylamino donors with the imido-POM acceptor. DFT/TD-DFT calculations indicate strong communication between POM and organic components is responsible for the linear and non-linear optical behaviour of these compounds, while electrochemical measurements reveal class II mixed valence behaviour resulting from an interplay of through-bond and through-space effects.

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.

: Lindqvist-type polyoxometalates (POMs) derivatized with pyrrole (Py) via an aryl − imido linkage [Mo 6 O 18 NPhPy] 2 − ( 1 ) and [Mo 6 O 18 NPhCCPhPy] 2 − ( 2 ) undergo coelectropolymerization with pyrrole, producing the fi rst electropolymer fi lms with covalently attached POM “ molecular metal oxides ” . X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray (EDX) elemental analyses indicate that the loadings of POM achieved are far higher than in conventional, noncovalent inclusion fi lms, and covalent attachment prevents loss of POM on initial reduction cycles. Cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge − discharge measurements together indicate that the POMs enhance the speci fi c capacitance (up to 5 × ) and decrease the charge-transfer resistance of the fi lms by both modifying the behavior of the polypyrrole (PPy) fi lm and introducing a substantial additional faradaic contribution through the POM redox processes. Increasing the length of the POM − PPy linker improves both capacitance and stability, with PPy-2 retaining 95% of its initial capacitance over 1200 cycles.

A family of six (five new) thiophenyl bridged triarylamine-donor based dyes with pyridine anchoring groups have been synthesized and studied as sensitizers for the p-type dye-sensitized solar cell (p-DSSC). They comprise bis-dicyano acceptor systems with a single pyridyl binder incorporated directly into the triarylamine (1), or separated by a phenyl group (2); a mono-dicyano with two phenyl pyridine binders (3); and respective homologues 4 to 6 with pyridinium acceptors. In all cases, compared to their dicyano counterparts, the pyridinium based dyes have higher extinction coefficients and smaller HOMO-LUMO gaps that give broader spectrum absorption. Thus, despite lower dye uptake, devices based on pyridiniums 4 and 6 have identical power conversion efficiencies (eta) to the equivalent dicyano systems 1 and 3. However, the best performing device (eta = 0.06%) is based on the known bis-acceptor dicyano system 2, as the large size and double positive charge of 5 leads to a substantial disadvantage in loading on NiO. Absorbed-photon-to-current efficiencies for 5 are competitive with or higher than those of 2, implying a better per dye performance consistent with the absorption profile, and DFT calculations suggesting better charge separation. Thus, pyridiniums may provide a new, and easily accessible high performance acceptor for p-DSSC dyes, but are likely better paired with anionic binding groups such as carboxylates.

Two new cobalt bis-iminopyridines, [Co(DDP)(H2O)2](NO3)2 (1, DDP = cis-[1,3-bis(2-pyridinylenamine)] cyclohexane) and [Co(cis-DDOP)(NO3)](NO3) (2, cis-DDOP = cis-3,5-bis[(2-Pyridinyleneamin]-trans-hydroxycyclohexane) electrocatalyse the 4-proton, 4-electron reduction of acetonitrile to ethylamine. For 1, this reduction occurs in preference to reduction of protons to H2. A coordinating hydroxyl proton relay in 2 reduces the yield of ethylamine and biases the catalytic system back towards H2.

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.

Ten organoimido polyoxometalate (POM)-based chromophores have been synthesized and studied by hyper-Rayleigh scattering (HRS), Stark and Resonance Raman spectroscopies, and density functional theory (DFT) calculations. HRS β0 values for chromophores with resonance electron donors are significant (up to 139 × 10-30 esu, ∼5 times greater than 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 (where N is the 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 the 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 the extension of charge transfer (CT) onto the POM increases changes in the excited-state dipole moment. 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 second-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.

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.

We show that polyoxometalates (POMs) are an excellent redox-active acceptor on which to base high performance 2(nd) order non-linear optical (NLO) chromophores. This is demonstrated through three new organoimido-Lindqvist derivatives with HRS β0-values exceeding those of any dipolar organic system with comparable donor, π-system and absorption profile. Thus, organoimido POMs may provide a new generation of high performance, high transparency, and potentially redox-switchable NLO materials.

A series of hetero-bimetallic transition metal-substituted polyoxometalates (TMSPs) were synthesized based on the Co(II)-centered ligand [Co(II)W11O39](10-). The eight complex series, [Co(II)(M(x)OHy)W11O39]((12-x-y)-) (M(x)OHy = V(IV)O, Cr(III)(OH2), Mn(II)(OH2), Fe(III)(OH2), Co(II)(OH2), Ni(II)(OH2), Cu(II)(OH2), Zn(II)(OH2)), of which six are reported for the first time, was synthesized starting from [Co(III)W11O39](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 [Co(II)W12O40](6-) and demonstrate the same MPCT transition as the plenary complex, but they 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 [Co(II)W12O40](6-) while more than tripling the absorbance at 400 nm.

Improved sensitizer design dramatically enhances visible light-driven water oxidation from dye-sensitized TiO2 photoanodes treated with polyoxometalate water oxidation catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10−.

Hostguest photofunctional materials have received much attention recently due to their potential applications in light emitting diodes, polarized emission, and other optoelectronic fields. In this work, we report the encapsulation of a photoactive ruthenium-based complex (4,4'-diphosphonate-2,2'-bipyridine) into the biphenyl-based metalorganic framework (MOF) as a hostguest material toward potential photofunctional applications. The resulting material (denoted as Ru@MOF) presents different two-color blue/red luminescences at the crystal interior and exterior as detected by three-dimensional confocal fluorescence microscopy. Additionally, up-conversion emission and an enhanced photoluminescence lifetime relative to the pristine Ru-based complex can also be observed in this Ru@MOF system. Upon attaching on the rutile TiO2 nanoarray, the Ru@MOF also exhibits alternated photoelectrochemical properties relative to the pristine complex. Moreover, a density functional theoretical calculation was performed on the Ru@MOF structure to provide understanding of the hostguest interactions. Based on the combination of experimental and theoretical studies on the Ru@MOF system, the aim of this work is to deeply investigate how the hostguest materials can present different photofunctionalities and optoelectronic properties compared with those of the individual components, and to give detailed information on the potential hostguest energy/electronic transfer between the MOF and the complex.