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The photoisomerization behaviour of a dicationic azobenzene derivative on the inorganic surface was examined. The isomerization reaction was controlled by the charged array of the inorganic surface due to the "pinning effect" because of the electrostatic interaction between anionic charged sites on the inorganic surface and cationic charged sites in dye molecules.

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We investigated a reaction involving photochemical energy transfer between a cationic xanthene derivative (Flu(D)) and a cationic porphyrin (Por(A)) with an energy migration functionality, which is crucial for efficient lightharvesting on an inorganic nanosheet. Efficient energy transfer from excited Flu(D) to Por(A) took place, and the maximum energy transfer efficiency was 99%. Even under light-harvesting conditions, Por(A) concentration was much less than Flu(D) concentration (Flu(D)/Por(A) concentration ratio = 15), and the energy transfer efficiency was still 80%. Steady-state, timeresolved, anisotropic fluorescence measurements indicate energy migration between Flu(D) molecules. This system has the functionality of a light-harvesting system using a dye and having a large overlap between its absorption and fluorescence spectra. ■ INTRODUCTIONA natural photosynthetic system is mainly composed of a lightharvesting system (LHS) and photosystems I and II (PS I and PS II, respectively). 1−8 The LHS has several important functions in the photosynthetic system, including (i) increasing the range of absorption wavelength, (ii) dissipating excess excitation energy, and (iii) concentrating absorbed photons to PS I and PS II. The LHS of purple bacteria is composed of dye molecules such as carotenoids and bacteriochlorophylls. 9−13 Carotenoids in the LHS absorb energy (450−540 nm) higher than that absorbed by bacteriochlorophylls. The excitation energy of carotenoids is transferred to bacteriochlorophylls. Carotenoids increase the utilizable sunlight energy in the LHS of purple bacteria. Approximately 50% of the energy absorbed by carotenoids dissipates through an internal conversion process, depending on the strength of the light. This reaction is one of the protective mechanisms against excess excitation energy. In purple bacteria, approximately 200 dye molecules that comprise the LHS surround the reaction centers, 1 and the dye molecules in the LHS form a circular array. The excitation energy migrates efficiently in the ring and between rings; thus, the LHS can transfer light energy efficiently and frequently to the PS I and PS II. One of the most important roles of the LHS is concentration of dilute photon flux to enable the photochemical reaction with multielectron conversion. In an artificial multielectron conversion system, electrons or photons must be frequently supplied to catalysts within the lifetime of one of their electron-oxidized or reduced species, because they become unstable and have a short lifetime. This problem is known as the photon-flux-density problem. 4 Thus, construction of artificial LHSs is crucial for the realization of an artificial photosynthetic system that resembles natural photosynthetic systems.To realize an efficient artificial LHS, regularly arranged structures of dyes for transferring excited energy smoothly are necessary. Artificial LHSs using covalently linked systems and dendrimer systems have been reported, and efficient energy transfer reactions have been achieved in such syste...

The adsorption distribution of porphyrin dye on the inorganic surface was examined by the use of Förster resonance energy transfer (FRET). Because the efficiency of FRET depends on the distance between energy donor and acceptor, FRET could be used as a spectroscopic molecular ruler to estimate the adsorption distribution of dyes on the inorganic surface. Saponite and cationic porphyrin were used as an anionic inorganic nanosheet and an adsorbing dye (an energy acceptor) on the clay surface. The edge of saponite was modified by pyrene that is an energy donor, through silane coupling. From the estimation of FRET efficiency, the adsorption distribution of cationic porphyrin is turned out to be biased to the center of the nanosheet surface. The factor to determine such adsorption distribution of cationic porphyrin was discussed based on the electrostatic interactions between anionic nanosheet and cationic porphyrin.

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Energy transfer among three dye components, namely, p-2,4,5,7-tetrakis(N-methylpyridinium-4-yl)-6-potassium-oxy-3-fluorone (Fluorone), meso-tetra(N-methyl-3-pyridyl) porphine (m-TMPyP), and meso-tetra(N-methyl-4pyridyl)porphine (p-TMPyP), was investigated for the purpose of utilizing a wide range of sunlight wavelengths for clay nanosheets. In this paper, we define a new parameter, the enhancement ratio of the excitation frequency (Γ 380−780 nm ) of the dye, in order to comprehensively represent the performance of the light-harvesting system (LHS). Γ 380−780 nm is defined as the enhancement ratio of the excitation frequency of dye that has the lowest excitation energy in the system (p-TMPyP in this system) in terms of the frequency of p-TMPyP without the use of light-harvesting dyes (Fluorone and m-TMPyP), when the visible-light region of sunlight (380−780 nm) is used for irradiation light. Γ 380−780 nm was calculated from the absorption spectra of dyes, the results of energy transfer experiments, and the sunlight spectrum (380−780 nm). We found that Γ 380−780 nm in our LHS is 2.4, which implies enhancement of the excitation frequency of p-TMPyP by 2.4 times relative to that of p-TMPyP without a light harvesting system. We believe that Γ 380−780 nm is an important standard parameter of the performance of artificial LHSs.

The adsorption orientation behavior of tetrakis(1-methylpyridinium-3-yl)porphyrin (m-TMPyP) and p-TMPyP on the clay monolayer prepared by the Langmuir Blodgett (LB) technique was investigated using the absorption and dichroic spectra obtained on a waveguide.