The development of a water oxidation catalyst has been a demanding challenge in realizing water splitting systems. The asymmetric geometry and flexible ligation of the biological Mn4CaO5 cluster are important properties for the function of photosystem II, and these properties can be applied to the design of new inorganic water oxidation catalysts. We identified a new crystal structure, Mn3(PO4)2·3H2O, that precipitates spontaneously in aqueous solution at room temperature and demonstrated its high catalytic performance under neutral conditions. The bulky phosphate polyhedron induces a less-ordered Mn geometry in Mn3(PO4)2·3H2O. Computational analysis indicated that the structural flexibility in Mn3(PO4)2·3H2O could stabilize the Jahn-Teller-distorted Mn(III) and thus facilitate Mn(II) oxidation. This study provides valuable insights into the interplay between atomic structure and catalytic activity.

Considerable attention has been focused on the synthesis of monodisperse macromolecular rods of precise length and constitution in light of their potential application as molecular-scale electronics, optical devices, sensors, and for conversion of solar energy.[1±3] Discrete molecular rods of known structure are used to position two active centers at a known distance, and the resulting assemblies are of interest as potent electronic or photonic molecular wires. Recently, the length of linear, monodisperse, p-conjugated oligomers have reached the range of approximately 10 nm. [1,4] Yet it still remains a great synthetic challenge to explore discrete, finite functional supramolecules with well-defined structures far beyond these achievements.One of the most attractive building blocks for supramolecular rods are porphyrins, since they offer a variety of desirable features such as a rigid, planar geometry, high stability, an intense electronic absorption, a strong fluorescence emission, a small HOMO ± LUMO energy gap, as well as flexible tunability of their optical and redox properties by appropriate metalation.[5] Recent efforts on the preparation of supramolecular porphyrin arrays have become increasingly focused on the realization of various molecular devices. [5,6] However, these studies are often hampered by poor solubility, difficult separations, and demanding characterizations. Therefore, high solubilities, easy separations, and reliable characterizations of the arrays are of prime importance in devising a larger molecular system. Recently we found that the Ag I -promoted meso ± mesocoupling reaction of Zn II 5,15-diarylporphyrins has several advantages; [7,8] 1) the regioselectivity of the meso ± meso coupling is quite high, 2) the porphyrin arrays have essentially the same linear rodlike shape, 3) the porphyrin arrays are highly soluble, presumably because of orthogonal conformations arising from steric hindrance around the meso ± meso linkage, 4) the separation of the coupling products is easy by recycling preparative GPC-HPLC chromatography as a result of large differences in molecular weight, and finally 5) the long coupling products still bear two free meso positions that are available for the next reaction.Here we report the synthesis and characterization of meso ± meso-coupled porphyrin oligomers up to 128-mer, which is, to the best of our knowledge, the longest (ca. 106 nm) monodisperse, rodlike molecule prepared so far. Previously we employed Zn II 3,5-di-tert-butylphenylporphyrin as a building block, but we encountered a serious solubility problem at the stage of the porphyrin 8-mer. In order to circumvent the solubility problem, we employed the more soluble Zn II 3,5-dioctyloxyphenylporphyrin Z1 (here we denote the meso ± meso-coupled Zn II porphyrin arrays as Zn where n represents the number of porphyrins; Ar 3,5-dioctyloxyphenyl). Chain elongation can be achieved quite simply by repeating the dimerization reactions from Z1 to Z2, Z2 to Z4, Z4 to Z8, Z8 to Z16, and Z16 to Z32. The yields of the di...

We have developed biocompatible, photostable, and multiplexing-compatible surface-enhanced Raman spectroscopic tagging material (SERS dots) composed of silver nanoparticle-embedded silica spheres and organic Raman labels for cellular cancer targeting in living cells. SERS dots showed linear dependency of Raman signatures on their different amounts, allowing their possibility for the quantification of targets. In addition, the antibody-conjugated SERS dots were successfully applied to the targeting of HER2 and CD10 on cellular membranes and exhibited good specificity. SERS dots demonstrate the potential for high-throughput screening of biomolecules using vibrational information.

The development of active water oxidation catalysts is critical to achieve high efficiency in overall water splitting. Recently, sub-10 nm-sized monodispersed partially oxidized manganese oxide nanoparticles were shown to exhibit not only superior catalytic performance for oxygen evolution, but also unique electrokinetics, as compared to their bulk counterparts. In the present work, the water-oxidizing mechanism of partially oxidized MnO nanoparticles was investigated using integrated in situ spectroscopic and electrokinetic analyses. We successfully demonstrated that, in contrast to previously reported manganese (Mn)-based catalysts, Mn(III) species are stably generated on the surface of MnO nanoparticles via a proton-coupled electron transfer pathway. Furthermore, we confirmed as to MnO nanoparticles that the one-electron oxidation step from Mn(II) to Mn(III) is no longer the rate-determining step for water oxidation and that Mn(IV)═O species are generated as reaction intermediates during catalysis.

The molecular design of directly meso-meso-linked porphyrin arrays as a new model of light-harvesting antenna as well as a molecular photonic wire was envisaged to bring the porphyrin units closer for rapid energy transfer. For this purpose, zinc(II) 5,15-bis(3,5-bis(octyloxy)phenyl)porphyrin (Z1) and its directly meso-meso-linked porphyrin arrays up to Z128 (Zn, n represents the number of porphyrins) were synthesized. The absorption spectra of these porphyrin arrays change in a systematic manner with an increase in the number of porphyrins; the high-energy Soret bands remain at nearly the same wavelength (413-414 nm), while the low-energy exciton split Soret bands are gradually red-shifted, resulting in a progressive increase in the exciton splitting energy. The exciton splitting is nicely correlated with the values of cos[pi/(N + 1)] according to Kasha's exciton coupling theory, providing a value of 4250 cm(-1) for the exciton coupling energy in the S(2) state. The increasing red-shifts for the Q-bands are rather modest. The fluorescence excitation anisotropy spectra of the porphyrin arrays show that the photoexcitation of the high-energy Soret bands exhibits a large angle difference between absorption and emission dipoles in contrast with the photoexcitation of the low-energy exciton split Soret and Q-bands. This result indicates that the high-energy Soret bands are characteristic of the summation of the individual monomeric transitions with its overall dipole moment deviated from the array chain direction, while the low-energy Soret bands result from the exciton splitting between the monomeric transition dipoles in line with the array chain direction. From the fluorescence quantum yields and fluorescence lifetime measurements, the radiative coherent length was estimated to be 6-8 porphyrin units in the porphyrin arrays. Ultrafast fluorescence decay measurements show that the S(2) --> S(1) internal conversion process occurs in less than 1 ps in the porphyrin arrays due to the existence of exciton split band as a ladder-type deactivation channel, while this process is relatively slow in Z1 (approximately 1.6 ps). The rate of this process seems to follow the energy gap law, which is mainly determined by the energy gap between the two Soret bands of the porphyrin arrays.