Interfacial charge-transfer transitions enable photovoltaic conversion with CO2-fixation products.

We demonstrate that organic-inorganic interfacial charge-transfer transitions enable favourable photovoltaic conversion with CO2-fixation products such as aromatic carboxylic acids, verifying a new possibility of CO2-fixation products in the development of optoelectronic conversion materials.

Linkage Dependence of Interfacial Charge-Transfer Transitions in ZnO: Carboxylate versus Sulfur Linker.

Interfacial charge-transfer transitions (ICTTs) between organic compounds and inorganic semiconductors have recently attracted much attention due to the unique features of a wide range of visible light absorption with colorless organic molecules and direct interfacial charge separation for their potential applications in photoenergy conversions and chemical sensing. As the research on ICTT has almost been limited to titanium oxide semiconductors such as TiO2, the exploration of ICTT in other inorganic semiconductors is a high-priority issue. Recently, we demonstrated that ICTT is strongly induced by chemisorption of aromatic thiols on ZnO nanoparticles via the sulfur atom. Here, we report on ICTT in ZnO nanoparticles adsorbed with benzoic acid derivatives and the linkage dependence of ICTT in ZnO. We observed ICTT bands in the visible region upon adsorption of 4-(dimethylamino)benzoic acid (4-DMABA) and 3,4-dimethoxybenzoic acid (3,4-DMOBA) on ZnO nanoparticles via the carboxylate group. Notably, the ICTT absorption intensities are about 1 order of magnitude lower than those in the ZnO surface complexes with aromatic thiol compounds. Time-dependence density functional theory (TD-DFT) calculations well reproduce the linkage dependence of ICTT. This characteristic linkage dependence of ICTT in ZnO is attributed to the difference in the valence orbital of bridging atoms. The sulfur bridging atom with the larger 3p valence orbitals gives rise to strong electronic couplings between ZnO and adsorbates for ICTT, in contrast to very weak electronic couplings via the smaller 2p valence orbitals of the oxygen bridging atoms in the carboxylate linkage. Our research reveals the important linkage dependence of ICTT in ZnO and elucidates the mechanism.

Interfacial charge-transfer transitions in SnO2 functionalized with benzoic acid derivatives.

Interfacial charge-transfer transitions (ICTTs) between organic compounds and inorganic semiconductors have recently attracted increasing attention for their potential applications in solar energy conversions and chemical sensing due to the unique functions of visible-light absorption with colourless organic molecules and direct charge separation. However, inorganic semiconductors available for ICTT are quite limited to a few kinds of metal-oxide semiconductors (TiO2, ZnO, etc.). Particularly, the exploration of ICTT in inorganic semiconductors with a lower-energy conduction band such as SnO2 is an important issue for realizing a wide range of visible-light absorption for organic adsorbates with the deep highest occupied molecular orbital (HOMO) such as benzoic acid derivatives. Here, we report the first observation of ICTT in SnO2. SnO2 nanoparticles show a broad absorption band in the visible region by chemisorption of 4-dimethylaminobenzoic acid (4-DMABA) and 4-aminobenzoic acid (4-ABA)) via the carboxylate group. The wavelength range of the ICTT band significantly changes depending on the kind of substituent group. The ionization potential measurement and density functional theory (DFT) analysis reveal that the absorption band is attributed to ICTT from the HOMO of the adsorbed benzoic acid derivatives to the conduction band of SnO2. In addition, we clarify the mechanism of ICTT in SnO2 computationally. Our research opens up a way to the fundamental research on ICTT in SnO2 and applications in solar energy conversions and chemical sensing.

Silicon-silicon π single bond.

A carbon-carbon double bond consists of a σ bond and a π bond. Recently, the concept of a π single bond, where a π bond is not accompanied by a σ bond, has been proposed in diradicals containing carbon and heteroatom radical centers. Here we report a closed-shell compound having a silicon-silicon π single bond. 1,2,2,3,4,4-Hexa-tert-butylbicyclo[1.1.0]tetrasilane has a silicon-silicon π single bond between the bridgehead silicon atoms. The X-ray crystallographic analysis shows that the silicon-silicon π single bond (2.853(1) Å) is far longer than the longest silicon-silicon bond so far reported. In spite of this unusually long bond length, the electrons of the 3p orbitals are paired, which is confirmed by measurement of electron paramagnetic resonance, and magnetic susceptibility and natural bond orbital analysis. The properties of the silicon-silicon π single bond are studied by UV/Vis and 29Si NMR spectroscopy, and theoretical calculations.

Interfacial charge-transfer transitions in ZnO induced exclusively by adsorption of aromatic thiols.

Interfacial charge-transfer transitions (ICTTs) between organic compounds and inorganic semiconductors have recently gained increasing interest as a new visible light absorption mechanism for optical biosensing via direct visualization, surface enhanced Raman scattering (SERS), and circular dichroism (CD) and also as a direct charge separation mechanism for photoenergy conversions such as photocatalytic reactions. So far, ICTTs have been observed with various organic compounds, while inorganic materials are almost limited to titanium oxides such as TiO2. Although SERS via ICTTs has been reported with several kinds of inorganic semiconductors, their ICTT bands have not been observed directly except for TiO2. From these viewpoints, the direct observation of ICTT bands in inorganic semiconductors other than TiO2 is an important issue. In this study, we demonstrate ICTTs in ZnO induced by the adsorption of aromatic thiols. ICTTs take place from the HOMO of the adsorbed thiol compounds to the conduction band of ZnO via a Ti-S linkage. Notably, ZnO selectively shows ICTTs with aromatic thiols, but almost no ICTT with oxygen-linkage-type organic compounds such as phenol. In addition, the wide-range control of ICTTs was achieved by the chemical modification of aromatic thiols. Our research not only opens up a new way for the research of ICTTs but also supports the reported ICTT-based SERS in ZnO.

Photoinduced H-aggregation of cationic dyes on metal-oxide surfaces via light activated molecular migration.

Molecular transport by use of light energy is a new photofunction for light powered molecular carriers and light induced nano-structure formation on a substrate. However, this research field has not been cultivated yet. In order to understand the basic mechanism and potentialities, experimental observations of photoactivated molecular migration and its functions in various systems are required. Here, we report visible light induced H-aggregation of cationic dyes (methylene blue and toluidine blue) on metal-oxide surfaces (TiO2 and ZnO) via photoactivated surface migration of these dyes. Our research widely expands the scope of photoinduced surface molecular migration to the conventional dyes and metal-oxide surfaces, and also demonstrates that the photoinduced molecular assembly provides a photochromic function. It is noteworthy that high aggregates, which are difficult to form in the dark, can be formed by the photoinduced surface migration of excited dyes.

Visible-light circular dichroism of colourless chiral organic compounds enabled by interfacial charge-transfer transitions.

We report the visible-light circular dichroism (CD) of colourless organic compounds based on interfacial charge-transfer (ICT) transitions with TiO2 nanoparticles. We employed three kinds of colourless chiral compounds, l-ascorbic acid, d-ascorbic acid, and l-noradrenaline. These compounds showed a broad ICT band in the visible region between 400 and 600 nm upon their chemisorption on TiO2 nanoparticles. l-Ascorbic acid and l-noradrenaline adsorbed on the TiO2 nanoparticles showed positive and negative CD signals in the visible region, respectively. d-Ascorbic acid, which is the enantiomer of l-ascorbic acid, exhibited positive CD signals in the visible region, but different g factors (Δε/ε) from those of TiO2-l-ascorbic acid, well reflecting the different chirality of the substituent group. The visible-light CD based on ICT transitions enables selective visible-light CD sensing and imaging of colourless chiral biomolecules even if coexisting with other colourless chiral compounds such as proteins and DNA. Furthermore, the molecular dependence of the g factor allows us to identify chiral molecules.

Achievement of over 1.4 V photovoltage in a dye-sensitized solar cell by the application of a silyl-anchor coumarin dye.

A dye-sensitized solar cell (DSSC) fabricated by using a novel silyl-anchor coumarin dye with alkyl-chain substitutes, a Br3-/Br- redox electrolyte solution containing water, and a Mg2+-doped anatase-TiO2 electrode with twofold surface modification by MgO and Al2O3 exhibited an open-circuit photovoltage over 1.4 V, demonstrating the possibility of DSSCs as practical photovoltaic devices.

Interfacial charge-transfer transitions and reorganization energies in sulfur-bridged TiO2-x-benzenedithiol complexes (x: o, m, p).

Surface complexes formed between TiO2 nanoparticles and enediol compounds such as 1,2-benzenediol (o-BDO) via Ti-O-C linkages show absorption of visible light due to interfacial charge-transfer (ICT) transitions. The ICT transitions take place from the π-conjugated systems to TiO2. Recently, we reported a surface complex formed between TiO2 and 1,2-benzenedithiol (o-BDT) via Ti-S-C linkages. This sulfur-bridged complex shows ICT transitions from the sulfur bridging atoms to TiO2. Interestingly, it was demonstrated that the ICT transitions in the sulfur-bridged TiO2-o-BDT complex induce photoelectric conversion more efficiently than those in the oxygen-bridged TiO2-o-BDO complex. This result suggests that carrier recombination is suppressed with the sulfur bridging atoms. In this paper, we examine ICT transitions and reorganization energies in the sulfur-bridged TiO2-x-BDT complexes (x: o, m, p) and compare them with those in the oxygen-bridged TiO2-x-BDO complexes. The estimated reorganization energies for the sulfur-bridged TiO2-x-BDT complexes (x: o, m, p) are much smaller than those for the oxygen-bridged TiO2-x-BDO ones. Based on the Marcus theory, the small reorganization energy calculated for the TiO2-o-BDT complex, which is less than half of that for the TiO2-o-BDO complex, increases the activation energy of carrier recombination. The small reorganization energy is attributed to the characteristic distribution of the highest occupied molecular orbital (HOMO) on the sulfur-bridging atoms in the TiO2-o-BDT complex, which inhibits structural changes in the benzene ring in the ICT excited state. Our work reveals the important role of the sulfur bridging atoms in the suppression of carrier recombination.

A strategy to minimize the energy offset in carrier injection from excited dyes to inorganic semiconductors for efficient dye-sensitized solar energy conversion.

Photoinduced carrier injection from dyes to inorganic semiconductors is a crucial process in various dye-sensitized solar energy conversions such as photovoltaics and photocatalysis. It has been reported that an energy offset larger than 0.2-0.3 eV (threshold value) is required for efficient electron injection from excited dyes to metal-oxide semiconductors such as titanium dioxide (TiO2). Because the energy offset directly causes loss in the potential of injected electrons, it is a crucial issue to minimize the energy offset for efficient solar energy conversions. However, a fundamental understanding of the energy offset, especially the threshold value, has not been obtained yet. In this paper, we report the origin of the threshold value of the energy offset, solving the long-standing questions of why such a large energy offset is necessary for the electron injection and which factors govern the threshold value, and suggest a strategy to minimize the threshold value. The threshold value is determined by the sum of two reorganization energies in one-electron reduction of semiconductors and typically-used donor-acceptor (D-A) dyes. In fact, the estimated values (0.21-0.31 eV) for several D-A dyes are in good agreement with the threshold value, supporting our conclusion. In addition, our results reveal that the threshold value is possible to be reduced by enlarging the π-conjugated system of the acceptor moiety in dyes and enhancing its structural rigidity. Furthermore, we extend the analysis to hole injection from excited dyes to semiconductors. In this case, the threshold value is given by the sum of two reorganization energies in one-electron oxidation of semiconductors and D-A dyes.

Interfacial charge-transfer transitions in a TiO2-benzenedithiol complex with Ti-S-C linkages.

Interfacial charge-transfer (ICT) transitions between organic materials and inorganic semiconductors are a new mechanism for light absorption at organic-semiconductor interfaces. ICT transitions cause one-step interfacial charge separation without loss of energy. This feature is potentially useful to realize efficient organic-inorganic hybrid solar cells. ICT transitions have been examined by employing titanium dioxide (TiO2) nanoparticles chemisorbed with π-conjugated molecules via Ti-O-C linkages. Here, we report ICT transitions in a TiO2 and 1,2-benzenedithiol (BDT) complex with Ti-S-C linkages. BDT adsorbs on TiO2 by the bridging bidentate coordination of the sulfur atoms to surface titanium atoms. The TiO2-BDT complex shows ICT transitions from the BDT moiety to the conduction band of TiO2 in the visible region. The ICT transitions occur by orbital overlaps between the d orbitals of the surface titanium atoms and the π orbitals of the benzene ring. Our density-functional-theory (DFT) analysis reveals that the 3p valence orbitals of the sulfur bridging atoms contribute to more than 50% of the highest occupied molecular orbital (HOMO) and the 3d-3p(sulfur)-π interaction via the Ti-S-C linkage enhances the electronic mixing between the titanium atoms and the benzene moiety as compared to the 3d-2p(oxygen)-πvia the Ti-O-C linkage. This result indicates the important role of the heavier-atom linkers for strong organic-inorganic electronic couplings.

Charge-transfer complex versus σ-complex formed between TiO2 and bis(dicyanomethylene) electron acceptors.

A novel group of organic-inorganic hybrid materials is created by the combination of titanium dioxide (TiO2) nanoparticles with bis(dicyanomethylene) (TCNX) electron acceptors. The TiO2-TCNX complex is produced by the nucleophilic addition reaction between a hydroxy group on the TiO2 surface and TCNX, with the formation of a σ-bond between them. The nucleophilic addition reaction generates a negatively-charged diamagnetic TCNX adsorbate that serves as an electron donor. The σ-bonded complex characteristically shows visible-light absorption due to interfacial charge-transfer (ICT) transitions. In this paper, we report on another kind of complex formation between TiO2 and TCNX. We have systematically studied the structures and visible-light absorption properties of the TiO2-TCNX complexes, with changing the electron affinity of TCNX. We found that TCNX acceptors with lower electron affinities form charge-transfer complexes with TiO2 without the σ-bond formation. The charge-transfer complexes show strong visible-light absorption due to interfacial electronic transitions with little charge-transfer nature, which are different from the ICT transitions in the σ-bond complexes. The charge-transfer complexes induce efficient light-to-current conversions due to the interfacial electronic transitions, revealing the high potential for applications to light-energy conversions. Furthermore, we demonstrate that the formation of the two kinds of complexes is selectively controlled by the electron affinity of TCNX.

Highly-efficient dye-sensitized solar cells with collaborative sensitization by silyl-anchor and carboxy-anchor dyes.

In dye-sensitized solar cells co-photosensitized with an alkoxysilyl-anchor dye ADEKA-1 and a carboxy-anchor organic dye LEG4, LEG4 was revealed to work collaboratively by enhancing the electron injection from the light-excited dyes to the TiO2 electrodes, and the cells exhibited a high conversion efficiency of over 14% under one sun illumination.

Extremely strong organic-metal oxide electronic coupling caused by nucleophilic addition reaction.

Electronic interactions between organic materials and inorganic semiconductors play important roles in various electronic and optoelectronic functions and also provide new functions such as optical interfacial charge-transfer (ICT) transitions having the following features. ICT transitions enable the capture of lower-energy photons than HOMO-LUMO gaps or band gaps and allow one-step charge separation without loss of energy. The hybrid material generated by the nucleophilic addition reaction between TiO2 and TCNQ exclusively exhibits strong ICT transitions. In this study, we report that strong organic-metal oxide electronic coupling is caused by the nucleophilic addition reaction, which enhances the ICT transitions. The electronic coupling between TiO2 and TCNQ occurs according to a two-step mechanism. First, the lowest unoccupied molecular orbital (LUMO (π*)) of TCNQ is elevated by the nucleophilic attack of a deprotonated hydroxy group on TiO2 to TCNQ and the electron distribution is moved toward TiO2. By this elevation and redistribution, the LUMO (π*) strongly interacts with the d(t2g) orbitals of a surface Ti atom. From avoided-crossing behavior with a large splitting energy of ca. 0.95 eV, the coupling energy was estimated to be as much as 0.5 eV in the mono-Ti model complex. This strong d-π* electronic coupling leads to strong coupling between complete ICT excited states and partial ICT excited states with a large splitting energy of ca. 0.92 eV, which considerably increases the probabilities of ICT transition. This study clarified the mechanisms of the strong organic-inorganic electronic coupling and the enhancement of ICT absorption in the TiO2-TCNQ hybrid material.

Fabrication of a high-performance dye-sensitized solar cell with 12.8% conversion efficiency using organic silyl-anchor dyes.

The co-sensitization of organic silyl-anchor dyes in dye-sensitized solar cells (DSSCs) using carbazole and coumarin dyes with organosilicon tethers for binding to titanium dioxide has been examined. We have succeeded in fabricating a high-performance DSSC with a light-to-electric energy conversion efficiency of 12.8% under one sun simulated solar irradiation.

An achievement of over 12 percent efficiency in an organic dye-sensitized solar cell.

Dye-sensitized solar cells fabricated by using a novel metal-free alkoxysilyl carbazole as a sensitizing dye and a Co(3+/2+)-complex redox electrolyte exhibited light-to-electric energy conversion efficiencies of over 12% with open-circuit photovoltages higher than 1 V by applying a hierarchical multi-capping treatment to the photoanode.

Fabrication of a dye-sensitized solar cell containing a Mg-doped TiO2 electrode and a Br3(-)/Br- redox mediator with a high open-circuit photovoltage of 1.21 V.

A dye-sensitized solar cell (DSSC) fabricated by using a Mg(2+)-doped anatase-TiO(2) electrode with an alkoxysilyl dye and a Br(3)(-)/Br(-) electrolyte solution exhibited successfully a remarkably high open-circuit photovoltage over 1.2 V, demonstrating a new possibility of DSSCs as practical photovoltaic devices.

The Valence-Detrapping Phase Transition in a Crystal of the Mixed-Valence Trinuclear Iron Cyanoacetate Complex [Fe(3)O(O(2)CCH(2)CN)(6)(H(2)O)(3)].

A mixed-valence trinuclear iron cyanoacetate complex, [Fe(3)O(O(2)CCH(2)CN)(6)(H(2)O)(3)] (1), was prepared, and the nature of the electron-detrapping phase transition was studied by a multitemperature single-crystal X-ray structure determination (296, 135, and 100 K) and calorimetry by comparison with an isostructural mixed-metal complex, [CoFe(2)O(O(2)CCH(2)CN)(6)(H(2)O)(3)] (2). The mixed-valence states at various temperatures were also determined by (57)Fe Mössbauer spectroscopy. The Mössbauer spectrum of 1 showed a valence-detrapped state at room temperature. With decreasing temperature the spectrum was abruptly transformed into a valence-trapped state around 129 K, well corresponding to the heat-capacity anomaly due to the phase transition (T(trs) = 128.2 K) observed in the calorimetry. The single-crystal X-ray structure determination revealed that 1 has an equilateral structure at 296 and 135 K, and that the structure changes into an isosceles one at 100 K due to the electron trapping. The crystal system of 1 at 296 K is rhombohedral, space group R&thremacr; with Z = 6 and a = 20.026(1) Å, c = 12.292(2) Å; at 135 K, a = 19.965(3) Å, c = 12.145(4) Å; and at 100 K, the crystal system changes into triclinic system, space group P&onemacr;, with Z = 2 and a = 12.094(2) Å, b = 12.182(3) Å, c = 12.208(3) Å, alpha = 110.04(2) degrees, beta = 108.71(2) degrees, gamma = 109.59(2) degrees. The X-ray structure determination at 100 K suggests that the electronically trapped phase of 1 at low temperature is an antiferroelectrically ordered phase, because the distorted Fe(3)O molecules, which are expected to possess a nonzero electronic dipole moment, oriented alternatively in the opposite direction with respect to the center of symmetry. On the other hand, no heat-capacity anomaly was observed in 2 between 7 and 300 K, and X-ray structure determination indicated that 2 shows no structure change when the temperature is decreased from 296 K down to 102 K. The crystal system of 2 at 296 K is rhombohedral, space group R&thremacr; with Z = 6 and a = 19.999(1) Å, c = 12.259(1) Å; at 102 K, a = 19.915(2) Å, c = 12.061(1) Å. Even at 102 K the CoFe(2)O complex still has a C(3) axis, and the three metal ion sites are crystallographically equivalent because of a static positional disorder of two Fe(III) ions and one Co(II) ion. The activation energy of intramolecular electron transfer of 1 in the high-temperature disordered phase was estimated to be 3.99 kJ mol(-)(1) from the temperature dependence of the Mössbauer spectra with the aid of the spectral simulation including the relaxation effect of intramolecular electron transfer. Finally the phase-transition mechanism of 1 was discussed in connection with the intermolecular dielectric interaction.