Reaction-Environment-Dependent Photoaddition Reactions of N-Phenyl Amino Acid Esters Possessing a Silyl Group with Fullerene C60: Selective Formation of Aminomethyl-1,2-dihydrofullerenes vs Fulleropyrrolidines.

The current study investigates SET-promoted photoaddition reactions of the silyl-group-containing N-phenylglycinates and N-phenylalaninates, N-((trimethylsilyl)methyl)-N-phenyl-substituted glycinates and alaninates, respectively, with fullerene C60 to explore how the types of amino acid esters (AAEs) and molecular oxygen affect the photoaddition reaction efficiencies and chemoselectivity of in situ formed radical cations of AAEs. The results showed that under deoxygenated (N2-purged) conditions, photoreactions of N-phenylglycinates with C60 produced aminomethyl-1,2-dihydrofullerenes through the addition of α-amino radicals arising by sequential SET and desilylation processes from initially formed secondary anilines to C60. In oxygenated conditions, photoreactions of N-phenylglycinates with C60, albeit less efficient, took place to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of azomethine ylides to C60 assisted by in situ formed 1O2. The same types of photoproducts were observed with N-phenylalaninates, though the reactions were less efficient. The use of methylene blue (MB) as a photosensitizer in the photoreactions under oxygenated conditions was especially effective in enhancing the efficiency of fulleropyrrolidine formation. These results demonstrate that photoaddition reactions of silyl-tether-containing N-phenyl AAEs with C60 can be governed by the reaction conditions and the presence or absence of a photosensitizer employed.

Systematic radical species control by electron push-pull substitution in the perylene-based D-π-A compounds.

Organic radical materials have been mainly reported on the stabilization of radical species because of their high energy and reactivity, while design strategies for controlling radical species beyond stabilization have remained challenging. Here, we report the electronic push-pull control spanning the neutral to the radical state of a series of perylene-based donor-π-acceptors (D-π-A). By introducing electron-withdrawing and -donating R groups to the donor of D-π-A, the observed intramolecular interactions controllable at the HOMO level led to the exploration of radical species. D-π-A with redox-active sites was transformed to (D-π-A)˙+ and (D-π-A)˙- in response to an external electrical stimulus under stabilization by perylene, resulting in new absorption peaks. In particular, the increasing absorption peaks of (D-π-A)˙+ showed a spectral shift and intensity change according to the R group, unlike those of (D-π-A)˙-. These experimental results support that the DFT/TD-DFT data suggests the radical cationic SOMO level variability. As a result, we provide a strategy for controlling the systematic radical species using the electron push-pull effect.

Singlet fission dynamics modulated by molecular configuration in covalently linked pyrene dimers, Anti- and Syn-1,2-di(pyrenyl)benzene.

Covalently linked dimers (CLDs) and their structural isomers have attracted much attention as potential materials for improving power conversion efficiencies through singlet fission (SF). Here, we designed and synthesized two covalently ortho-linked pyrene (Py) dimers, anti- and syn-1,2-di(pyrenyl)benzene (Anti-DPyB and Syn-DPyB, respectively), and investigated the effect of molecular configuration on SF dynamics using steady-state and time-resolved spectroscopies. Both Anti-DPyB and Syn-DPyB, which have different Py-stacking configurations, form excimers, which then relax to the correlated triplet pair ((T1T1)) state, indicating the occurrence of SF. Unlike previous studies where the excimer formation inhibited an SF process, the (T1T1)'s of Anti-DPyB and Syn-DPyB are formed through the excimer state. The dissociation of (T1T1)'s to 2T1 in Anti-DPyB is more favorable than in Syn-DPyB. Our results showcase that the molecular configuration of a CLD plays an important role in SF dynamics.

Silyl Tether-Assisted Photooxygenation of Electron-Deficient Enaminoesters: Direct Access to Oxamate Formation.

Photooxygenation reactions of electron-deficient enaminoesters bearing an oxophilic silyl tether at the α-position of the nitrogen atom using methylene blue (MB) were explored to develop a mild and efficient photochemical strategy for oxidative C-C double bond cleavage reactions via singlet oxygen (1O2). Photochemically generated 1O2, through energy transfer from the triplet excited state of MB (3MB*) to molecular oxygen (3O2), was added across a C-C double bond moiety of enaminoesters to form perepoxides, which rearranged to form dioxetane intermediates. The cycloreversion of the formed dioxetane via both C-C and O-O bond cleavage processes led to the formation of oxamates. Importantly, contrary to alkyl group tether-substituted electron-deficient enaminoesters that typically disfavor photooxygenation, the silyl tether-substituted analogues undergo this photochemical transformation efficiently with the assistance of a silyl tether, which facilitates formation of the perepoxide. The observations in this study provide useful information about photosensitized oxygenation reactions of unsaturated C-C bonds, and, moreover, this photochemical strategy can be utilized as a mild and feasible method for the preparation of diversely functionalized carbonyl compounds including oxamates.

Influence of picolinate ancillary ligands on unique photophysical properties of Ir(ppz)2(LX).

Homoleptic fac-Ir(ppz)3 (ppz = phenylpyrazole) and a series of heteroleptic Ir(ppz)2(LX) complexes consisting of picolinic acid (pic), 3-hydroxypicolinic acid (picOH), and isoquinolinecarboxylic acid (iq) as ancillary ligands (LX) were synthesised to investigate the influence of the ancillary ligands on the photophysical properties of the complexes. Generally, the role of the ancillary ligand is considered insignificant compared to that of the main ligand. Ir(ppz)3 showed deep-blue emission with a vibronic structure at 77 K, whereas Ir(ppz)2(LX) showed a broad and red-shifted emission. Theoretical calculations of the molecular orbitals and energy levels were performed using density functional theory to understand the effect of the ancillary ligands on the emission changes. The 3MLCTppz state was calculated to be higher than the 3MLCTLX state. Therefore, interligand energy transfer (ILET) between the main and ancillary ligands can occur exothermically in the triplet state. The dynamics of the ILET process were monitored directly using a femtosecond time-resolved transient absorption (TA) spectroscopic technique. The 3MLCTppz state was generated upon excitation at 290 nm, and the intensity of the TA band related to the 3MLCTppz state decreased as the time delay increased. Concurrently, the TA band related to the 3MLCTLX state intensified. On the other hand, no further changes in the TA spectra were observed upon direct excitation of the 3MLCTLX state at 420 nm. In contrast with other Ir(ppz)2(LX) complexes, Ir(ppz)2(picOH) produced long-lived TA species, attributed to excited-state intramolecular proton transfer of the picOH ligand in the excited singlet state.

Solvent-modulated proton-coupled electron transfer in an iridium complex with an ESIPT ligand.

Proton-coupled electron transfer (PCET), an essential process in nature with a well-known example of photosynthesis, has recently been employed in metal complexes to improve the energy conversion efficiency; however, a profound understanding of the mechanism of PCET in metal complexes is still lacking. In this study, we synthesized cyclometalated Ir complexes strategically designed to exploit the excited-state intramolecular proton transfer (ESIPT) of the ancillary ligand and studied their photoinduced PCET in both aprotic and protic solvent environments using femtosecond transient absorption spectroscopy and density functional theory (DFT) and time-dependent DFT calculations. The data reveal solvent-modulated PCET, where charge transfer follows proton transfer in an aprotic solvent and the temporal order of charge transfer and proton transfer is reversed in a protic solvent. In the former case, ESIPT from the enol form to the keto form, which precedes the charge transfer from Ir to the ESIPT ligand, improves the efficiency of metal-to-ligand charge transfer. This finding demonstrates the potential to control the PCET reaction in the desired direction and the efficiency of charge transfer by simply perturbing the external hydrogen-bonding network with the solvent.

Visible-Light-Induced Selective C-C Bond Cleavage Reactions of Dimeric ß-O-4 and ß-1 Lignin Model Substrates Utilizing Amine-Functionalized Fullerene.

Finding a selective and efficient fragmentation process under ambient conditions is pivotal for the generation of fuels and chemical feedstocks from lignoceullosic biomass. In the present study, visible-light and amine-functionalized fullerene-based photocatalyst-promoted photodegradation reactions of dimeric ß-O-4 and ß-1 lignin model compounds, containing varying numbers of methoxy substituents on the arene ring, were explored to find and develop mild, eco-friendly photochemical techniques for efficient delignification. The results showed that, in contrast to well-known organic photoredox catalysts, amine-functionalized fullerene photocatalyst promoted photochemical reactions of lignin model compounds could lead to more efficient lignin fragmentation reactions through a pathway involving a selective Cα-Cß bond cleavage process, and in addition, Cα-hydroxyl moiety in lignin model compounds played a significant role in the success of the Cα-Cß bond cleavage reaction of lignin model substrates.

meta-Terphenyl linked donor-π-acceptor dyads: intramolecular charge transfer controlled by electron acceptor group tuning.

A series of meta-terphenyl linked donor-π-acceptor (D-π-A) dyads were prepared to understand the electronic effects of a meta-terphenyl linker according to the electron-accepting ability change. The energy band gaps of the dyads were controlled by tuning the accepting ability, which resulted in emission colors ranging from blue-green to red. In the Lippert-Mataga plots, intramolecular charge transfer (ICT) behavior was observed, which showed gradually increased ICT characteristics as the accepting ability was increased. On the other hand, in the absorption spectra, a red shift of the ICT transition was observed differently from the electron-accepting ability tendency. Thus, the experimental results show that the ICT is determined by steric hindrance rather than the acceptor ability in the ground state due to the lack of π-conjugation of the terphenyl linker by the electron node in the meta-position, whereas ICT in the excited state is controlled by electron-accepting ability.

Electron Push-Pull Effects on Intramolecular Charge Transfer in Perylene-Based Donor-Acceptor Compounds.

A series of asymmetric donor-acceptor (D-A) perylene-based compounds, 3-(N,N-bis(4'-(R)-phenyl)amino)perylene (Peri-DPA(R)), were successfully prepared to explore their intramolecular charge transfer (ICT) properties. To induce ICT between the donor and acceptor, diphenylamine (DPA) derivatives (electron donor units) with the same functional groups (R = CN, F, H, Me, or OMe) at both para positions were linked to the C-3 position of perylene to produce five Peri-DPA derivatives. A steady-state spectroscopy study on Peri-DPA(R)s exhibited a progressively regulated ICT trend consistent with the substituent effect as it progressed from the electron-withdrawing group to the electron-donating group. In particular, a comparative study using a D-A-D (donor-acceptor-donor) system demonstrated that not only the electron push-pull substituent effect but also subunit combinations influence photophysical and electrochemical properties. The different ICT characters observed in Lippert-Mataga plots of D-A(CN) and D-A-D(CN) (CN-substituted D-A and D-A-D) led to the investigation on whether ICT emission of two systems with differences in subunit combinations is of the same type or of a different type. The femtosecond transient absorption (fs-TA) spectroscopic results provided direct evidence of ICT origin and confirmed that D-A(CN) and D-A-D(CN) exhibited the same transition mix of ICT (from donor to acceptor) and reverse ICT (rICT, from arylamine to CN unit). Density functional theory (DFT)/TD-DFT calculations support the presence of ICT for all five compounds, and the experimental observations of rICT presented only for CN-substituted compounds.

Charge transfer induced by electronic state mixing in a symmetric X-Y-X-type multi-chromophore system.

Charge transfer (CT) from electron donor (D) to acceptor (A) plays an important role in photoelectric or electrochemical devices and is a useful concept for a molecule with D and A well distinguishable. Here, we report our finding that even in a molecule with D and A not resolvable, CT can be induced by electronic state mixing (ESM) in a symmetric multi-chromophore system (MCS), namely 1,4-di(1-pyrenyl)benzene (Py-Benz-Py). Unlike Py and Py-Benz, Py-Benz-Py exhibits unique photophysical properties attributable to the reduction of the energy gap between two electronic states induced by ESM. The ESM for Py-Benz-Py is due to the extended π-conjugation owing to the further introduction of Py into Py-Benz, and consequently leads to the favorable intramolecular CT, followed by the planarization due to the twisting motion between Py and phenyl moieties. Time-resolved spectroscopic data demonstrate that the twisting process of the Py moiety in acetonitrile occurs with two unequal time constants, suggesting the localized CT state and the asynchronous twisting dynamics of two Py moieties unlike the delocalized CT state in nonpolar and low-polarity solvents leading to the synchronous twisting of two Py moieties. This means that the symmetry-breaking CT in MCSs can induce an asynchronous twisting motion. The results reported here support that a molecule without CT can be turned into another molecule with CT induced by ESM and demonstrate that the excited-state relaxation dynamics can be regulated through the ESM induced by introducing the substituents or changing the environmental factors such as solvent polarities.

Control of Chemoselectivity of SET-Promoted Photoaddition Reactions of Fullerene C60 with α-Trimethylsilyl Group-Containing N-Alkylglycinates Yielding Aminomethyl-1,2-dihydrofullerenes or Fulleropyrrolidines.

Knowledge about factors that govern chemoselectivity is pivotal to the design of reactions that are utilized to produce complex organic substances. In the current study, single-electron transfer (SET)-promoted photoaddition reactions of fullerene C60 with both trimethylsilyl and various alkyl group-containing glycinates and ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates were explored to evaluate how the nature of N-alkyl substituents of glycinate substrates and reaction conditions govern the chemoselectivity of reaction pathways followed. The results showed that photoreactions of C60 with glycinates, performed in deoxygenated conditions, produced aminomethyl-1,2-dihydrofullerenes efficiently through a pathway involving the addition of α-amino radical intermediates that are generated by sequential SET-solvent-assisted desilylation of glycinate substrates to C60. Under oxygenated conditions, photoreactions of glycinate substrates, except N-benzyl-substituted analogues, did not take place efficiently owing to quenching of 3C60* by oxygen. Interestingly, N-benzyl-substituted glycinates did react under these conditions to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of in situ formed azomethine ylides to C60. The ylide intermediates were formed by regioselective H-atom transfer from glycinates by singlet oxygen. Furthermore, methylene blue (MB)-photosensitized reactions of C60 with glycinates under oxygenated conditions took place efficiently to produce fulleropyrrolidines independent of the nature of N-alkyl substituents of glycinates.

Inter-Ligand Energy Transfer Process in an Ir-Complex with Expanding π-Conjugated Ligand.

The inter-ligand energy transfer (ILET) process in heteroleptic iridium complex, [Ir(dfppy)2 (bpy-Im2 )]+ , where dfppy=2-(2,4-difluorophenyl)pyridine and bpy-Im2 =4,4'-bis(1,2-diphenyl-1H-benzo[d]imidazole)-2,2',-bipyridine, was investigated using a femtosecond transient absorption (fs-TA) spectroscopic technique. The photophysical properties of [Ir(dfppy)2 (bpy-Im2 )]+ with significantly expanding π-conjugated ligand are compared to those of [Ir(dfppy)2 (bpy)]+ (bpy=2,2'-bipyridine) and a free bpy-Im2 ligand. The emission spectrum of [Ir(dfppy)2 (bpy-Im2 )]+ shows no shift upon changing the solvent polarity, whereas the free ligand bpy-Im2 showed bathochromic fluorescence shifts with increasing solvent polarity, which is attributed to intramolecular charge transfer (ICT). The unique photophysical properties of [Ir(dfppy)2 (bpy-Im2 )]+ are due to the fast ILET process from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 , resulting in the phosphorescence emission originating from 3 MLCT/3 LCbpy-Im2 . On the other hand, the TA bands of bpy-Im2 are observed at 540 and 480 nm, corresponding to the singlet and triplet manifolds, respectively. In contrast, the TA spectrum of [Ir(dfppy)2 (bpy-Im2 )]+ showes broad bands centered at 420 and 600 nm, attributed to the transitions from 3 MLCTdfppy and 3 MLCT/3 LCbpy-Im2 , respectively. Time-resolved spectroscopic results confirm the efficient ILET dynamics from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 in [Ir(dfppy)2 (bpy-Im2 )]+ . From the relaxation times determined by singular value decomposition analysis and simple sequential kinetic model, we infer that the ILET process from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 occurs with a time constant of ca. 4 ps. The presented results in this study show that the introduction of an expanding π-conjugated ligand can lead to the efficient ILET dynamics for improving the OLED performance.

Terphenyl backbone-based donor-π-acceptor dyads: geometric isomer effects on intramolecular charge transfer.

The molecular geometry effects of ortho, meta, and para-terphenyl based donor-π-acceptor (D-π-A) dyads on intramolecular charge transfer (ICT) were studied to investigate structure-ICT relationships. Terphenyl based D-π-A dyads were prepared by two-step palladium catalyzed, Suzuki-Miyaura coupling reactions, in which triphenylamine (TPA) was used as the electron donor and 1,2-diphenyl-benzimidazole (IMI) as the electron acceptor. The photophysical and electrochemical properties of terphenyl backbone-based ortho (O), meta (M), and para (P) dyads were compared. In steady state absorption spectra, a red-shift of CT band was observed in the order O < M < P, which was attributed to terphenyl isomer conjugation effects and agreed well with density functional theory (DFT) based calculations. In particular, the emission spectra of the three terphenyl D-π-A dyads produced showed similar emission maxima at ∼475 nm and a bathochromic shift property was observed in order to increase the solvent polarity, indicating the ICT process. From Lippert-Mataga plots, excited-state dipole moment changes (Δµ) were estimated to be 31.5 Debye (D) for O, 62.9 D for M, and 51.5 D for P. For M isomer, a large Δµ and the markedly reduced quantum yield was shown, as well as photo-induced electron transfer (PET) was expected in the excited state, but no radical species were observed by femtosecond transient absorption (TA) measurements. Based on experimental results, we conclude that all three terphenyl based D-π-A dyads, including non-conjugated ortho- and meta-terphenyl dyads, exhibit partial charge transfer rather than unit-electron transfer.

Electron Push-Pull Effects in 3,9-Bis(p-(R)-diphenylamino)perylene and Constraint on Emission Color Tuning.

A series of perylene-based donor-acceptor-donor (D-A-D) compounds, 3,9-bis(p-(R)-diphenylamino)perylene (R: CN (2a), F (2b), H (2c), Me (2d), and OMe (2e)), was synthesized using 3,9-dibromoperylene with p-(R)-diphenylamine, and the intramolecular charge transfer (ICT) on the D-A-D system with regard to the electron push-pull substituent effect was investigated. By introducing various p-(R)-diphenylamine derivatives with electron-donating or electron-withdrawing R groups, the energy band gaps of the D-A-D compounds were systematically controlled and the emission colors were efficiently tuned from green to red. As expected, the steady state emission spectra of all D-A-D compounds were observed, as well as the emission color controlled, depending on the Hammett substituent constants (σp). In the Lippert-Mataga plots, a different charge-transfer character was observed depending on the electron push-pull substitution, which showed gradually increased ICT characters from the electron-withdrawing to donating substitution. However, exceptionally, the strong electron-withdrawing group of CN-substituted 2a did not correlate with the other R group compounds. From the experimental data and density functional theory calculations, we assume that there is a constraint on emission color tuning to generate higher energy of blue emission in the D-A-D molecular system, due to the reverse charge-transfer property caused by the strong electron-withdrawing group.

Photoaddition reactions of N-benzylglycinates containing α-trimethylsilyl group with dimethyl acetylenedicarboxylate: competitive formation of pyrroles vs. ß-enamino esters.

A study was conducted to gain insight into the preparative potential of photosensitized reactions of acyclic N-benzylglycinates containing an α-trimethylsilyl group with dimethyl acetylenedicarboxylate (DMAD). The photosensitizers employed in the reactions include 9,10-dicyanoanthracene (DCA), 1,4-dicyanonaphthalene (DCN), rose bengal (RB) and fullerene C60. The results show that photoirradiation of oxygenated solutions containing the photosensitizers, glycinates and dimethyl acetylenedicarboxylate leads to competitive formation of pyrroles and ß-enamino-esters. The distributions of pyrrole and ß-enamino-ester products formed in these reactions are highly influenced by the electronic nature of the phenyl ring substituent on the benzylglycinates and the photosensitizer used. These photoaddition reactions take place via mechanistic pathways involving competitive formation of azomethine ylides and secondary amines, generated by a mechanistic routes involving initial SET from the benzylglycinates to photosensitizers.

Chromophore-Catalyst Assembly for Water Oxidation Prepared by Atomic Layer Deposition.

Visible-light-driven water splitting was investigated in a dye sensitized photoelectrosynthesis cell (DSPEC) based on a photoanode with a phosphonic acid-derivatized donor-π-acceptor (D-π-A) organic chromophore, 1, and the water oxidation catalyst [Ru(bda)(4-O(CH2)3P(O3H2)2-pyr)2], 2, (pyr = pyridine; bda = 2,2'-bipyridine-6,6'-dicarboxylate). The photoanode was prepared by using a layering strategy beginning with the organic dye anchored to an FTO|core/shell electrode, atomic layer deposition (ALD) of a thin layer (<1 nm) of TiO2, and catalyst binding through phosphonate linkage to the TiO2 layer. Device performance was evaluated by photocurrent measurements for core/shell photoanodes, with either SnO2 or nanoITO core materials, in acetate-buffered, aqueous solutions at pH 4.6 or 5.7. The absolute magnitudes of photocurrent changes with the core material, TiO2 spacer layer thickness, or pH, observed photocurrents were 2.5-fold higher in the presence of catalyst. The results of transient absorption measurements and DFT calculations show that electron injection by the photoexcited organic dye is ultrafast promoted by electronic interactions enabled by orientation of the dye's molecular orbitals on the electrode surface. Rapid injection is followed by recombination with the oxidized dye which is 95% complete by 1.5 ns. Although chromophore decomposition limits the efficiency of the DSPEC devices toward O2 production, the flexibility of the strategy presented here offers a new approach to photoanode design.

Fluoropolymer-Stabilized Chromophore-Catalyst Assemblies in Aqueous Buffer Solutions for Water-Oxidation Catalysis.

Here, the application of the fluorinated polymer [Dupont AF, a copolymer of 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole and tetrafluoroethylene] is described in stabilizing phosphonate-derivatized molecular assemblies on oxide electrodes. In the procedure, the polymer was dip-coated onto the surfaces of oxide electrodes with pre-bound, phosphonate-derivatized chromophores and assemblies, including assemblies for water oxidation. The results of the experiments showed a high degree of stabilization by the added polymer and a demonstration of its use in stabilizing surface-bound assemblies for water-oxidation catalysis.

All-in-One Derivatized Tandem p+n-Silicon-SnO2/TiO2 Water Splitting Photoelectrochemical Cell.

Mesoporous metal oxide film electrodes consisting of derivatized 5.5 µm thick SnO2 films with an outer 4.3 nm shell of TiO2 added by atomic layer deposition (ALD) have been investigated to explore unbiased water splitting on p, n, and p+n type silicon substrates. Modified electrodes were derivatized by addition of the water oxidation catalyst, [Ru(bda)(4-O(CH2)3PO3H2)-pyr)2], 1, (pyr = pyridine; bda = 2,2'-bipyridine-6,6'-dicarboxylate), and chromophore, [Ru(4,4'-PO3H2-bpy) (bpy)2]2+, RuP2+, (bpy = 2,2'-bipyridine), which form 2:1 RuP2+/1 assemblies on the surface. At pH 5.7 in 0.1 M acetate buffer, these electrodes with a fluorine-doped tin oxide (FTO) back contact under ∼1 sun illumination (100 mW/cm2; white light source) perform efficient water oxidation with a photocurrent of 1.5 mA/cm2 with an 88% Faradaic efficiency (FE) for O2 production at an applied bias of 600 mV versus RHE ( ACS Energy Lett. , 2016 , 1 , 231 - 236 ). The SnO2/TiO2-chromophore-catalyst assembly was integrated with the Si electrodes by a thin layer of titanium followed by an amorphous TiO2 (Ti/a-TiO2) coating as an interconnect. In the integrated electrode, p+n-Si-Ti/a-TiO2-SnO2/TiO2|-2RuP2+/1, the p+n-Si junction provided about 350 mV in added potential to the half cell. In photolysis experiments at pH 5.7 in 0.1 M acetate buffer, bias-free photocurrents approaching 100 µA/cm2 were obtained for water splitting, 2H2O → 2H2 + O2. The FE for water oxidation was 79% with a hydrogen efficiency of ∼100% at the Pt cathode.

A Dye-Sensitized Photoelectrochemical Tandem Cell for Light Driven Hydrogen Production from Water.

Tandem junction photoelectrochemical water-splitting devices, whereby two light absorbing electrodes targeting separate portions of the solar spectrum generate the voltage required to convert water to oxygen and hydrogen, enable much higher possible efficiencies than single absorber systems. We report here on the development of a tandem system consisting of a dye-sensitized photoelectrochemical cell (DSPEC) wired in series with a dye-sensitized solar cell (DSC). The DSPEC photoanode incorporates a tris(bipyridine)ruthenium(II)-type chromophore and molecular ruthenium based water oxidation catalyst. The DSPEC was tested with two more-red absorbing DSC variations, one utilizing N719 dye with an I3-/I- redox mediator solution and the other D35 dye with a tris(bipyridine)cobalt ([Co(bpy)3]3+/2+) based mediator. The tandem configuration consisting of the DSPEC and D35/[Co(bpy)3]3+/2+ based DSC gave the best overall performance and demonstrated the production of H2 from H2O with the only energy input from simulated solar illumination.