Three different kinds of mixed self-assembled monolayers have been prepared to mimic photosynthetic energy and electron transfer on a gold surface. Pyrene and boron-dipyrrin were chosen as a light-harvesting model. The mixed self-assembled monolayers of pyrene (or boron-dipyrrin) and porphyrin (energy acceptor model) reveal photoinduced singlet-singlet energy transfer from the pyrene (or boron-dipyrrin) to the porphyrin on the gold surface. The boron-dipyrrin has also been combined with a reaction center model, ferrocene-porphyrin-fullerene triad, to construct integrated artificial photosynthetic assemblies on a gold electrode using mixed monolayers of the respective self-assembled unit. The mixed self-assembled monolayers on the gold electrode have established a cascade of photoinduced energy transfer and multistep electron transfer, leading to the production of photocurrent output with the highest quantum yield (50 +/- 8%, based on the adsorbed photons) ever reported for photocurrent generation at monolayer-modified metal electrodes and across artificial membranes using donor-acceptor linked molecules. The incident photon-to-current efficiency (IPCE) of the photoelectrochemical cell at 510 and 430 nm was determined as 0.6% and 1.6%, respectively. Thus, the present system provides the first example of an artificial photosynthetic system, which not only mimics light-harvesting and charge separation processes in photosynthesis but also acts as an efficient light-to-current converter in molecular devices.
Absorption and fluorescence properties of rhodamine B in submono-, mono-and multilayer systems adsorbed on fused quartz plates have been studied at 295 and 77 K. Existence of fluorescent and nonfluorescent dimers of rhodamine B at 295 K and a structural change of the aggregate geometry at low temperatures have been postulated. Rates of nonradiative decay and photoinduced electron transfer of dimers adsbrbed on organic crystals and their possible role in hole injection are discussed.
A systematic series of ITO electrodes modified chemically with self-assembled monolayers (SAMs) of porphyrins and porphyrin-fullerene dyads have been designed to provide valuable insight into the development of artificial photosynthetic devices. First the ITO and gold electrodes modified chemically with SAMs of porphyrins with a spacer of the same number of atoms were prepared to compare the effects of energy transfer (EN) quenching of the porphyrin excited singlet states by the two electrodes. Less EN quenching was observed on the ITO electrode as compared to the EN quenching on the corresponding gold electrode, leading to remarkable enhancement of the photocurrent generation (ca. 280 times) in the porphyrin SAMs on the ITO electrode in the presence of the triethanolamine (TEA) used as a sacrificial electron donor. The porphyrin (H(2)P) was then linked with C(60) which can act as an electron acceptor to construct H(2)P-C(60) SAMs on the ITO surface in the presence of hexyl viologen (HV(2+)) used as an electron carrier in a three electrode system, denoted as ITO/H(2)P-C(60)/HV(2+)/Pt. The quantum yield of the photocurrent generation of the ITO/H(2)P-C(60)/HV(2+)/Pt system (6.4%) is 30 times larger than that of the corresponding system without C(60): ITO/H(2)P-ref/HV(2+)/Pt (0.21%). Such enhancement of photocurrent generation in the porphyrin-fullerene dyad system is ascribed to an efficient photoinduced ET from the porphyrin singlet excited state to the C(60) moiety as indicated by the fluorescence lifetime measurements and also by time-resolved transient absorption studies on the ITO systems. The surface structures of H(2)P and H(2)P-C(60) SAMs on ITO (H(2)P/ITO and H(2)P-C(60)/ITO) have been observed successfully in molecular resolution with atomic force microscopy for the first time.
Recent progress in fundamental studies on multiporphyrin arrays has provided structural parameters for the molecular design of artificial light-harvesting antennae which mimic the wheel-like antenna complexes of photosynthetic purple bacteria. Covalent and noncovalent approaches have been employed for the construction of artificial light-harvesting multiporphyrin arrays. Such arrays are categorized into ring-shaped, windmill-shaped, star-shaped, and dendritic architectures. In particular, dendritic multiporphyrin arrays have been proven to be promising candidates for both providing a large absorption cross-section and enabling the vectorial transfer of energy over a long distance to a designated point. Such molecular and supramolecular systems are also expected to be potent components for molecular electronics and photonic devices.
Disulfides, with a systematic series of alkyl spacers containing porphyrins at both terminals, were prepared to investigate the effect of the spacer length on the structure and photoelectrochemical properties of self-assembled monolayers (SAMs) of the porphyrins on a gold electrode. The structure of the SAMs was studied using ultraviolet (UV)−visible absorption spectroscopy in transmission mode, cyclic voltammetry, UV−visible ellipsometry, and fluorescence spectroscopy. These measurements showed that as the length of the spacers increases, the SAMs tend to form highly ordered structures on the gold electrodes. In addition, the structures of the monolayers vary depending on the even and odd number of the methylene spacers (n). From these measurements a porphyrin dimer model is proposed in that the two porphyrins take J-aggregate-like partially stacked structures in the monolayers. Photoelectrochemical studies were carried out in argon-saturated Na2SO4 aqueous solution containing methyl viologen as an electron carrier using the modified gold working electrode, a platinum wire counter electrode, and a Ag/AgCl reference electrode. The quantum yield increases in a zigzag fashion with an increase in the spacer length up to n = 6 and then starts decreasing slightly as the chain lengths become longer. A plausible explanation for the photocurrent trend comes from the following points: (i) there are two competitive deactivation pathways for the excited singlet state of the porphyrin dimer, i.e., the quenching by the electrode via energy transfer and by the electron carrier via electron transfer, (ii) the porphyrin aggregation enhances the rate of nonradiative pathway in the excited state, and (iii) the electron transfer rate from the gold electrode to the resulting porphyrin cation radical decreases with an increase of the spacer lengths. These results will provide basic information for the construction of molecular assembly with photoactive function on surface.
For the three azabenzenes pyridine, pyrazine and pyrimidine, quantum yields for fluorescence and intersystem crossing have been measured in the vapour phase as a function of the excitation energy, ranging from the zero-point level of Sl(n, n*) to the higher vibrational levels of S2(n, ;.*I. The picosecond fluorescence lifetimes have also been measured by exciting the molecules at various wavelengths. For all three azabenzenes a particularly fast non-radiative decay channel has been found. The appearance of this new decay channel has also been demonstrated by direct measurement of the picosecond lifetime. In the lower excitation-energy region of the S l t S o transition the intersystem-crossing process is dominant, whereas above the excitation-energy threshold the non-radiative transition is governed by this decay channel. These new decay channels in azabenzenes may be related to photoisomerization processes, which are known to proceed by irradiation of U.V. light corresponding to the S 2 t S o absorption transition. The discussion is developed in relation to " channel three " in the parent hydrocarbon, benzene.* 1 Torr = 101 325/760 Pa.
Self-assembled monolayers of ferrocene−porphyrin−C60 triads on gold electrodes were prepared to mimic photosynthetic electron transfer events where efficient conversion of light to chemical energy takes place via the long-lived, charge-separated state with a high quantum yield. Adsorbed amounts of the triads on the gold electrodes, estimated from the charge of the anodic peak of the ferrocene, are comparable to those of the well-ordered porphyrin−alkanethiols and C60−alkanethiols on gold electrodes. The results, together with blocking experiments using a redox probe, indicate that the triad molecules are well-packed with an almost perpendicular orientation on the gold surface. The monolayer thickness obtained using X-ray reflectivity analysis is consistent with the structural model of the monolayer. Photoelectrochemical studies were carried out in a standard three-electrode system using the gold electrodes modified with the self-assembled monolayers of the triads. Stable cathodic photocurrents were observed in the presence of electron carriers such as oxygen and/or methyl viologen in the electrolyte when the modified gold electrodes were illuminated with a monochromic light. A photoinduced multistep electron-transfer mechanism is proposed for the photoelectrochemical cells. Thus, vectorial electron transfer or partial charge transfer occurs from the excited singlet state of the porphyrin to the C60, followed by the successive charge shift from the ferrocene to the porphyrin cation radical, to produce the ferrocene cation radical and the C60 anion radical. The C60 anion radical gives an electron to the counter electrode via the electron carriers in the electrolyte solution, whereas electron transfer takes place from the gold electrode to the ferrocene cation radical, resulting in the recovery of the initial state and the generation of the overall electron flow. The artificial photosynthetic cells show the highest quantum efficiency (20−25%) ever reported for photoinduced multistep electron transfer at monolayer-modified metal electrodes and across artificial membranes using donor−acceptor linked molecules. The result indicates clearly that C60 acts as an excellent electron acceptor as well as an electron mediator in artificial photosynthetic membranes. The molecule-based methodology will provide a new direction for the development of solar energy conversion and molecular devices.
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