Organization of supramolecular assemblies of fullerene, porphyrin and fluorescein dye derivatives on TiO2 nanoparticles for light energy conversion

A Facile One-Pot Synthesis of Biomimetic Photocatalyst Zn(II)-Porphyrin-Sensitized 3D TiO2 Hollow Nanoboxes and Synergistically Enhanced Visible-Light Degradation

A serials of biomimetic photocatalyst zinc(II) meso-tetra(4-carboxyphenyl)porphyrinato (ZnTCP)-sensitized 3D hierarchical TiO2 hollow nanoboxes (TiO2-HNBs) assembled by six ordered nanosheets with dominant {001} facets exposure (ZnTCP@TiO2-HNBs) have been successfully synthesized by a facile one-pot solvothermal method via a topological transformation process with TiOF2 as template. Infrared spectra (IR), UV-vis spectroscopy, and X-ray photoelectron spectroscopy (XPS) confirmed that ZnTCP played a decisive role in constructing 3D hollow nanoboxes through the formation of ester bond combined to TiO2-HNBs, which also provided a transferring photo excited electrons bridge to sensitize TiO2-HNBs for enhancing visible-light response. Due to the superior sensitization and biomimetic activity of ZnTCP, the photodegradation rate of rhodamine B (RhB) of as-prepared ZnTCP@TiO2-HNBs with ZnTCP/TiOF2 mass ratio of 2% (T-2p) improves 3.6 times compared to that of TiO2-HNBs with a degradation yield of 99% for 2 h under simulated sunlight irradiation (> 420 nm). The enhanced photodegradation ability was attributed to synergistic visible photocatalytic mechanism of biomimetic catalyst, which can not only produce hydroxyl radical (•OH) and superoxide radical (•O2-) coming from the excitation process of ZnTCP sensitized TiO2-HNBs, but also generate singlet oxygen (1O2) that was only provided by biomimetic enzyme porphyrins. Furthermore, the photocatalyst showed good recycling stability and dispersibility after five rounds, ascribed to ZnTCP strong chemical bonding to the support TiO2-HNBs. By means of electrochemical cyclic voltammetry analysis, the effect of central zinc ions and parent porphyrin rings on the redox property of biomimetic catalyst was studied.

NbCl5 -Promoted Synthesis of Fluorescein Dye Derivatives: Spectroscopic and Spectrometric Characterization and Their Application in Dye-Sensitized Solar Cells

Fluorescein has several applications owing to its properties, such as high molar absorptivity, high fluorescence quantum yield, high photostability, and wavelengths of absorption and emission in the visible region. The syntheses of fluorescein derivatives between phenol and anhydride derivatives by using NbCl₅ as a Lewis acid is described. The products have high yields and short reaction times are observed. The study of the UV/Vis and fluorescence spectra of these derivatives and their application in dye-sensitized solar cells is also described.

Fabrication and analysis of dye-sensitized solar cells (DSSCs) using porphyrin dyes with catechol anchoring groups

Zinc(ii) porphyrins with catechol anchoring groups used in dye-sensitized solar cells for the first time.

Supramolecular photovoltaic cells utilizing inclusion complexes composed of Li+@C60 and cyclic porphyrin dimer

We have newly constructed supramolecular photovoltaic cells using inclusion complexes of lithium-ion-encapsulated [60]fullerene ( Li +@ C 60) and cyclic porphyrin dimers (M-CPDPy, M = H 4 and Ni 2). First, supramolecular inclusion complexes of Li +@ C 60 and M-CPDPy were prepared in MeCN/PhCN (3/1, v/v) by rapid injection method. The molecular aggregates with spherical nanoparticles demonstrated a broad absorption property in the visible region. The macroscopic structures were also estimated to be ca. 200 nm in diameter by transmission electron microscope (TEM) and dynamic light scattering (DLS) measurements. The photoelectrochemical solar cells composed of these assemblies on nanostructured SnO 2 electrode were fabricated by electrophoretic deposition method. The photoelectrochemical behavior of the nanostructured SnO 2 film of supramolecular nanoassemblies of Li +@ C 60 and M-CPDPy is significantly higher than those of the single component films ( Li +@ C 60 or M-CPDPy) and supramolecular inclusion complexes of pristine C 60 and M-CPDPy.

Photo- and electro-functional self-assembled architectures of porphyrins

Recent developments in supramolecular strategies have enabled us to construct novel well-defined assemblies of dye molecules. These fundamental researches of such organic materials also entail the synthetic and photophysical processes of molecular aggregates at the nano- and micro-meter scale, since their optical properties significantly differ from those of monomeric species. One of the promising candidates for such functional molecules is a porphyrin dye, which acts as an electron donor as well as a sensitizer. In this perspective, the focus is on the recent advances in the construction of optically and electronically functionalized molecular architectures of porphyrins for light energy conversion and electronics. First, porphyrin aggregates with morphologies such as cube, rod and fiber, which are prepared by three different supramolecular techniques, are reported. Then, we discuss composite molecular nanoarchitectures of porphyrins and carbon nanotubes such as single-wall carbon nanotubes (SWCNTs), stacked-cup carbon nanotubes (SCCNTs) and carbon nanohorns (CNHs). Finally, the structural and photophysical properties of the composite assemblies of porphyrins and graphenes including polycyclic aromatic hydrocarbons (PAH) are presented.

Porphyrin-Based Supramolecular Nanoarchitectures for Solar Energy Conversion

Photofunctional molecular architectures with well-defined shapes and sizes are of great interest because of various applications such as photovoltaics, photocatalysis, and electronics. Porphyrins are promising building blocks for organized nanoscale superstructures, which perform many of the essential light-harvesting and photoinduced electron/energy transfer reaction. In this Perspective, we present the recent advances in supramolecular architectures of porphyrins for solar energy conversion. First, we state preparation and light energy conversion properties of porphyrin (donor: D) and fullerene (acceptor: A)-based composite spherical nanoassemblies. The interfacial control of D/A molecules based on our supramolecular strategy successfully demonstrates the drastic enhancement of light energy conversion properties as compared to the corresponding nonorganized systems. Then, bar-shaped structures composed of two different D and A molecules with separated inside and outside layers are discussed. This unusual rod formation shows a possibility for a novel zeolite-like photoreaction cavity with efficient visible light absorption. Finally, photophysical and phoelectrochemical properties of supramolecular composites between porphyrins and carbon naotubes/graphenes are briefly described.

Graphene-embedded 3D TiO2 inverse opal electrodes for highly efficient dye-sensitized solar cells: morphological characteristics and photocurrent enhancement

We demonstrated the preparation of graphene-embedded 3D inverse opal electrodes for use in DSSCs. The graphene was incorporated locally into the top layers of the inverse opal structures and was embedded into the TiO2 matrix via post-treatment of the TiO2 precursors. DSSCs comprising the bare and 1-5 wt% graphene-incorporated TiO2 inverse opal electrodes were compared. We observed that the local arrangement of graphene sheets effectively enhanced electron transport without significantly reducing light harvesting by the dye molecules. A high efficiency of 7.5% was achieved in DSSCs prepared with the 3 wt% graphene-incorporated TiO2 inverse opal electrodes, constituting a 50% increase over the efficiencies of DSSCs prepared without graphene. The increase in efficiency was mainly attributed to an increase in J(SC), as determined by the photovoltaic parameters and the electrochemical impedance spectroscopy analysis.

Porphyrin hexamer with a triphenylene core unit: Spectroscopy, electrochemistry and controllable supramolecular formation

A series of free-base and zinc porphyrin hexamers, where six porphyrin units are linked to a triphenylene core through amide or ester linkage, was designed and synthesized in an effort to study their spectroscopic and electrochemical properties, and aggregate formation. The distances between triphenylene core and porphyrin moieties are tuned by changing the lengths of spacer alkyl chains. The absorption spectral studies indicates that the porphyrin units in a hexamer behave like monomeric porphyrins, while a strong fluorescence quenching of triphenylene core was observed suggesting the deactivation of the excited state of triphenylene. The redox potentials of porphyrin units in a hexamer were estimated by differential pulse voltammetry (DPV). These values are very similar to those of the corresponding monomer. Consequently, absorption and electrochemical data demonstrate that six porphyrin units of (H2PAC5)6TPh behave like monomers independently in the ground state. The hexamers undergo supramolecular aggregation spontaneously in different fashions based on the solvent systems as well as spacer lengths. Highly ordered and aligned monolayer-based patterns are formed when hexamers are allowed to aggregate from toluene solution onto carbon-coated copper films. In contrast, hexamers undergo aggregation to nanoparticles in toluene/acetonitrile mixed solvent system and the particle sizes increase with increasing the spacer alkyl chain lengths.

Assembly of individual TiO2-C60/porphyrin hybrid nanoparticles for enhancement of photoconversion efficiency

Rational organization of porphyrin and C60 on the electrode surface in photovoltaic structures is essential to yield high quantum efficiency. In the present work, individual TiO2 nanoparticles were modified by introducing C60 and porphyrin units on the surface, and then electrophoretically deposited on an ITO/SnO2 electrode. The morphology of the photoactive layer on the electrode was significantly different from that of the layer produced as a result of separate deposition of C60 and porphyrin. The maximum incident photon to current efficiency of the resulting electrode approached 88% at 410 nm, which is the highest value among molecule-based photovoltaic cells reported to date. This indicates that molecular assembly of the C60 and porphyrin units on the individual nanoparticles through strong chemical attachment is a key factor in improving effective electron transfer between the photoactive units and the electrodes.

Supramolecular nanoarchitectures for light energy conversion

Recent developments in synthetic and supramolecular techniques have made it possible to control precisely, organize and arrange molecules at the nanometre level. Such synthetic and supramolecular strategies enable us to construct photofunctional molecular architectures for light energy conversion, such as photovoltaics. In photovoltaic cells, processes such as light-harvesting, charge separation for carrier generation, and carrier transport are generally required. Therefore, the construction of supramolecular assemblies based on these three processes is interesting and promising for the future development of photovoltaics. In this perspective, the focus is on the recent developments of supramolecular systems for light energy conversion, which are mainly composed of porphyrin dyes and nanocarbon materials, such as fullerenes and carbon nanotubes. The specific topics are as follows: (i) preparation, photodynamics, and photoelectrochemistry of self-assembled porphyrin nanoparticles prepared by simple blend, (ii) highly organized supramolecular nanoassemblies of porphyrins and fullerenes using gold nanoparticles, dendritic and polypeptide structures, (iii) the supramolecular formation and photoelectrochemical property of carbon nanotubes, and (iv) supramolecular photofunctional nanorods of porphyrins.

Zinc(II) tetraarylporphyrins anchored to TiO2, ZnO, and ZrO2 nanoparticle films through rigid-rod linkers

A series of six Zn(II) tetraphenylporphyrins (ZnTPP), with a phenyl (P) or oligophenyleneethynylene (OPE = (PE) n ) rigid-rod bridge varying in length (9-30 A) and terminated with an isophthalic acid (Ipa) anchoring unit, were prepared as model dyes for the study of sensitization processes on metal oxide semiconductor nanoparticle surfaces (MO(n) = TiO(2), ZnO, and insulating ZrO(2)). The dyes were designed such that the electronic properties of the central porphyrin chromophore remained consistent throughout the series, with the rigid-rod anchoring unit allowing each porphyrin unit to be located at a fixed distance from the metal oxide nanoparticle surface. Electronic communication between the porphyrin and the rigid-rod unit was not desired. Rigid-rod porphyrins ZnTPP-Ipa, ZnTPP-P-Ipa, ZnTPP-PE-Ipa, ZnTPP-(PE)(2)-Ipa, ZnTPP-(PE)(3)-Ipa, and ZnTMP-Ipa (with mesityl substituents on the porphyrin ring) were synthesized using combinations of mixed aldehyde condensations and Pd-catalyzed cross-coupling reactions. Their properties, in solution and bound, were compared with that of Zn(II) 5,10,15,20-tetra(4-carboxyphenyl)porphyrin ( p-ZnTCPP) as the reference compound. Solution UV-vis and steady-state fluorescence spectra for all six rigid-rod-Ipa porphyrins were almost identical to each other and to that of p-ZnTCPP. Cyclic voltammetry and differential pulse voltammetry scans of the methyl ester derivatives of the six rigid-rod-Ipa porphyrins, recorded in dichloromethane/electrolyte, exhibited redox behavior typical of ZnTPP porphyrins, with the first oxidation in the range +0.99 to 1.09 V vs NHE. All six rigid-rod-Ipa porphyrins and p-ZnTCPP were bound to metal oxide (MO(n) = TiO(2), ZnO, and insulating ZrO(2)) nanoparticle films. The Fourier transform infrared attenuated total reflectance spectra of all compounds bound to MO n films showed a broad band at 1553-1560 cm(-1) assigned to the v(CO(2)(-)) asymmetric stretching mode. Splitting of the Soret band into two bands at 411 and 423 nm in the UV-vis spectra of the bound compounds, and broadening and convergence of both fluorescence emission bands in the fluorescence spectra of the porphyrins bound to insulating ZrO(2) were also observed. Such changes were less evident for ZnTMP-Ipa, which has mesityl substituents on the porphyrin ring to prevent aggregation. Steady-state fluorescence emission of rigid-rod-Ipa porphyrins bound to TiO(2) and ZnO through the longest bridges (>14 A) showed residual fluorescence emission, while fluorescence quenching was observed for the shortest compounds.

Photochemical conversion of solar energy

Energy is the most important issue of the 21st century. About 85% of our energy comes from fossil fuels, a finite resource unevenly distributed beneath the Earth's surface. Reserves of fossil fuels are progressively decreasing, and their continued use produces harmful effects such as pollution that threatens human health and greenhouse gases associated with global warming. Prompt global action to solve the energy crisis is therefore needed. To pursue such an action, we are urged to save energy and to use energy in more efficient ways, but we are also forced to find alternative energy sources, the most convenient of which is solar energy for several reasons. The sun continuously provides the Earth with a huge amount of energy, fairly distributed all over the world. Its enormous potential as a clean, abundant, and economical energy source, however, cannot be exploited unless it is converted into useful forms of energy. This Review starts with a brief description of the mechanism at the basis of the natural photosynthesis and, then, reports the results obtained so far in the field of photochemical conversion of solar energy. The "grand challenge" for chemists is to find a convenient means for artificial conversion of solar energy into fuels. If chemists succeed to create an artificial photosynthetic process, "... life and civilization will continue as long as the sun shines!", as the Italian scientist Giacomo Ciamician forecast almost one hundred years ago.

Tetrachelate Porphyrin Chromophores for Metal Oxide Semiconductor Sensitization: Effect of the Spacer Length and Anchoring Group Position

Four Zn(II)-tetra(carboxyphenyl)porphyrins in solution and bound to metal oxide (TiO2, ZnO, and ZrO2) nanoparticle films were studied to determine the effect of the spacer length and anchoring group position (para or meta) on their binding geometry and photoelectrochemical and photophysical properties. The properties of three types of anchoring groups (COOH and COONHEt3) for four Zn(II)-porphyrins (Zn(II)-5,10,15,20-tetra(4-carboxyphenyl)porphyrin (p-ZnTCPP), Zn(II)-5,10,15,20-tetra(3-carboxyphenyl)porphyrin (m-ZnTCPP), Zn(II)-5,10,15,20-tetra(3-(4-carboxyphenyl)phenyl)porphyrin (m-ZnTCP2P), and Zn(II)-5,10,15,20-tetra(3-ethynyl(4-carboxyphenyl)phenyl)porphyrin (m-ZnTC(PEP)P)) were compared. In m-ZnTCPP, m-ZnTCP2P, and m-ZnTC(PEP)P the four anchoring groups are in the meta position on the meso-phenyl rings of the porphyrin macrocycle, thus favoring a planar binding mode to the metal oxide surfaces. The three meta-substituted porphyrin salts have rigid spacer units of increasing length (phenyl (P), biphenyl (P2), and diphenylethynyl (PEP)) between the porphyrin ring and the carboxy anchoring groups, thus raising the macrocycle from the metal oxide surface. All porphyrins studied here, when bound to TiO2 and ZnO, exhibited quenching of the fluorescence emission, consistent with electron injection into the conduction band of the semiconductor. Steady-state UV-vis and fluorescence studies of p-ZnTCPP on insulating ZrO2 showed evidence of aggregation and exciton coupling. This was not observed in any of the meta-substituted porphyrins. The photoelectrochemical properties (IPCE, Voc, and Isc) of the porphyrins bound to TiO2 films in solar cells have been measured and rationalized with respect to the sensitizer binding geometry and distance from the surface.

[Development and biological applications of various bioimaging probes]

Fluorescence imaging is the most powerful technique currently available for continuous observation of dynamic intracellular processes in living cells. However, only a very limited range of biomolecules can be visualized because of the lack of flexible design strategies for fluorescence probes. In our laboratory, it was elucidated that fluorescein which has been widely employed as a core of fluorescence probes could be understood as a directly linked electron donor/fluorophore acceptor system. Fluorescence properties of fluorescein derivatives could be easily anticipated and modulated by controlling the rate of photoinduced electron transfer (PeT) from the donor moiety to the xanthene fluorophore. Further, we found that the opposite direction of PeT from the singlet excited fluorophore to the electron acceptor moiety could be occurred. More than twenty probes for imaging of nitric oxide, beta-galactosidase, highly reactive oxygen species, zinc ion et al. have been developed according to precise and rational design strategies based on PeT mechanism.