Photochemical Properties of Mono‑, Tri‑, and Penta-Cationic Antimony(V) Metalloporphyrin Derivatives on a Clay Layer Surface

Three types of mono-, tri-, and penta-cationic antimony­(V) porphyrin derivatives (Sb^VPors) were synthesized, and their photochemical properties on the anionic clay were systematically investigated. Sb^VPor derivatives are dihydroxo­(5,10,15,20-tetraphenylporphyrinato)­antimony­(V) chloride ([Sb^V(TPP)­(OH)₂]⁺Cl^–), dihydroxo­[5,10-diphenyl-15,20-di­(<i>N</i>-methyl-pyridinium-4-yl)­porphyrinato]­antimony­(V) trichloride ([Sb^V(DMPyP)­(OH)₂]³⁺3Cl^–), and dihydroxo­[5,10,15,20-tetrakis­(<i>N</i>-methyl-pyridinium-4-yl)­porphyrinato]­antimony­(V) pentachloride ([Sb^V(TMPyP)­(OH)₂]⁵⁺5Cl^–). The photochemical behaviors of three cationic Sb^VPors with and without clay were examined in aqueous solution. For all Sb^VPor, aggregation behaviors were not observed in the clay complexes even at high density adsorption conditions. The transition probabilities and fluorescence quantum yields of Sb^VPor showed a tendency to be increased by the complex formation with clay. The less cationic Sb^VPor/clay complex showed the larger fluorescence quantum yield. The more cationic Sb^VPor/clay complex showed the longer fluorescence lifetime. These effects of complex formation with clay on the photochemical properties of Sb^VPors were discussed using the molecular potential energy curves of the porphyrin ground state and excited state. It is concluded that two types of effects work in the Sb^VPor/clay system: effect i (structure resembling effect) is that the most stable structure becomes relatively similar between the ground and excited states, mainly by hydrophobic interactions between the porphyrin molecule and the clay surface, and effect ii (structure fixing effect) is that sharpened potential energy curves of clay complexes can lead to the increase of activation energy for the internal conversion from excited state to a high vibration level of ground state, mainly by electrostatic interactions between cationic porphyrin and anionic clay. Like this, the unique effects of the clay surface on the photochemical behavior of dyes were observed and the mechanisms were rationally discussed

Artificial Light-Harvesting Model in a Self-Assembly Composed of Cationic Dyes and Inorganic Nanosheet

This paper proposes an efficient artificial light-harvesting system in a host–guest assembly composed of functional dyes and inorganic nanosheet. Although we have already reported an efficient energy transfer between two types of porphyrin molecules on inorganic nanosheets (e.g., <i>J. Am. Chem. Soc.</i> <b>2011</b>, <i>133</i>, 14280), the number of photons captured by one acceptor molecule (photon-harvesting efficiency: the donor/acceptor ratio when the total energy transfer efficiency is 50% as defined in the main text) was a few. To overcome this low photon-harvesting efficiency, we designed and investigated a new nanosheet type light-harvesting system including phthalocyanine. As a result from steady-state and time-resolved fluorescence measurements, the energy transfer reaction was highly efficient even under the donor excess conditions. The efficiency was almost 100% even under the ratio of donor/acceptor = 1/1–6/1. The most advanced point of this study is the presence of energy transfer between nonadjacent donor–acceptor, and the photon-harvesting efficiency of this system progressed seven times compared to that of the previous porphyrin–porphyrin system. Additionally, the efficient utilization of visible region of sunlight (visible-light-harvesting efficiency: the percentage of visible region of sunlight (380–780 nm), in which the extinction coefficient of the light-harvesting molecules excesses 10⁴ M^–1 cm^–1) was realized in the present donor–acceptor combination. The visible-light-harvesting efficiency of the present system reached 86%. Thus, our host–guest system took a step closer to realize an artificial light-harvesting system utilizing the wide-wavelength region of sunlight with high photon-harvesting efficiency, in which a few energy acceptor molecules can harvest the excitation energies from a large number of adjacent and/or nonadjacent donor molecules efficiently

“Surface-Fixation Induced Emission” of Porphyrazine Dye by a Complexation with Inorganic Nanosheets

This paper proposes a unique phenomenon of the strong enhancement in the fluorescence quantum yield (φ_f) and the excited lifetime (τ) of tetra-cationic porphyrazine dye (Pz) upon a complexation with inorganic nanosheets. Although Pz does not strongly fluoresce in a bulk solution (φ_f = 0.01, τ = 0.1 ns), φ_f and τ increased up to 19 and 34 times by an intercalation into stacked clay nanosheets. Steady-sate and time-resolved fluorescence measurements revealed that this strong enhancement in φ_f and τ is derived from the suppression of nonradiative deactivation pathways of Pz by a complexation with clay nanosheets. We here name this phenomenon a “Surface-Fixation Induced Emission (S-FIE)”. S-FIE can be predicted easier than aggregation-induced emission (AIE) due to its clear mechanism depending on the flat solid surface, and we can thus simply design the photophysically enhanced system. Since photophysical characteristics of organic molecules directly influence the efficiency of objective reactions such as energy or electron transfers and photocatalysis, this study is beneficial to propose a novel strategy to create efficient photochemical reaction systems and photodevices

Artificial Photosynthesis Model: Photochemical Reaction System with Efficient Light-Harvesting Function on Inorganic Nanosheets

In natural photosynthesis system, its complicated photofunctions are achieved with high efficiency through precise arrangements of dye molecules in proteins. However, it is difficult to imitate such reaction systems artificially because of the complexity of the protein structures. As the way to approach this issue, we suggest the self-assembling behavior of photofunctional dyes on inorganic nanosheets. In this study, photochemical reaction system with a light-harvesting function was newly constructed on a clay nanosheet as an artificial photosynthesis system model by using a metalloporphyrin as a photocatalyst and a subporphyrin as a photoanntena. Under the condition of their co-adsorption on the nanosheet, efficient energy transfer from the subporphyrin to the metalloporphyrin of up to 98% was achieved in the case of donor/acceptor ratio of 1:1. By utilizing such dye–clay complexes, the metalloporphyrin photocatalyst could catalyze the photochemical conversion of cyclohexene by the excitation of both the subporphyrin photoantenna and itself. This light-harvesting system enabled the photocatalytic reaction to use a wider range of visible region without any energy loss because of suppression of unexpected other deactivation processes by precise arrangement of dyes in contrast to general co-adsorption systems. These results would be useful in constructing various types of artificial photosynthesis systems using self-assembling behavior

Photophysical Properties and Adsorption Behaviors of Novel Tri-Cationic Boron(III) Subporphyrin on Anionic Clay Surface

Two types of +3-charged subporphyrin derivatives with <i>m</i>- and <i>p</i>-methylpyridinium as the <i>meso</i>-aryl substituents were designed and synthesized. Their photophysical properties with and without anionic saponite clay were investigated. These cationic subporphyrins were suitably designed for adsorption on the saponite nanosheet surface with their photoactivity. Absorption and emission spectra of these subporphyrin-saponite complexes exhibited strong bathochromic shifts due to the flattening of the molecules on the nanosheet. This behavior was observed as drastic visual changes in their luminescence colors. Additionally, aggregation behaviors were not observed in the saponite complexes even at high dye loading levels for both subporphyrins. Moreover, under such condition, their fluorescence properties on the saponite surface were not only maintained but also enhanced without unexpected deactivations despite the dye molecules are densely introduced on the solid surface. These findings are beneficial for applications of the dye–clay complexes to photofunctional materials such as strongly luminescent materials, highly sensitive clay sensors and artificial photosynthesis systems

Artificial Light-Harvesting System with Energy Migration Functionality in a Cationic Dye/Inorganic Nanosheet Complex

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 light-harvesting 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, time-resolved, 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

Unique Photochemical Properties of <i>p</i>‑Substituted Cationic Triphenylbenzene Derivatives on a Clay Layer Surface

Two types of novel tricationic 1,3,5-triphenylbenzene (TPB) derivatives were synthesized. The TPB derivatives are 1,3,5-tris­(<i>N</i>,<i>N</i>,<i>N</i>-trimethylanilinium-4-yl)­benzene (TMAB) and 1,3,5-tris­[(<i>N</i>-pyridinium)­aniline-4-yl]­benzene (TPAB). The photochemical behaviors of both cationic TPBs with and without clay were examined in aqueous solution. For both TPBs, the aggregation behavior was not observed in the clay complexes even at saturated adsorption conditions. Interestingly, the fluorescence intensity of TPAB was extremely increased by the complex formation with clay compared to that without clay in a bulk aqueous solution, although the increase of fluorescence was not observed for TMAB. Time-resolved fluorescence measurement revealed that the increase of fluorescence turned out to be due to the suppression of the nonradiative deactivation process from its excited singlet state, because the molecular motion of TPAB should be restricted due to the strong fixation on the clay surface. TPAB exhibited a little self-fluorescence quenching behavior as the loadings increased, while TMAB exhibited obvious self-fluorescence quenching on the clay surface. The difference of adsorption strength of TPBs onto the clay surface is supposed to affect their photochemical properties such as the increase of fluorescence and the self-fluorescence quenching behavior in the excited singlet state. It was found that the pyridinium substituent as cationic sites is beneficial to construct efficient photochemical reaction systems using a clay complex without unexpected fluorescence quenching

“In-water” Dehydration Reaction of an Aromatic Diol on an Inorganic Surface

The effect of a synthetic saponite surface on the “in-water” dehydration reaction of diol was examined using 4-formyl-1-methylquinolinium salt (MQu+) as a substrate. The equilibrium between aldehyde (MQu+-Aldehyde) and diol (MQu+-Diol) was affected by the surrounding environment. The equilibrium behavior was observed by 1H nuclear magnetic resonance (NMR) and UV–vis absorption measurements. Although MQu+ was completely in the form of MQu+-Diol in water, the equilibrium almost shifted to the MQu+-Aldehyde side when MQu+ was adsorbed on the saponite surface in water. In addition, the MQu+-Aldehyde ratio depended on the negative charge density of saponite. The factors that determine MQu+-Aldehyde: MQu+-Diol ratio were discussed from the thermodynamic analysis of the system. These data indicate that the electrostatic interaction between the charged saponite surface and MQu+ stabilized the aldehyde side enthalpically and destabilized it entropically. The major reason for these results is considered to be the difference in adsorption stabilization between MQu+-Aldehyde and MQu+-Diol on saponite surfaces

Kinetic Analysis by Laser Flash Photolysis of Porphyrin Molecules’ Orientation Change at the Surface of Silicate Nanosheet

In a mixed solvent of water and dimethylformamide (DMF), porphyrin molecules have two types of orientation, tilted and parallel, toward a surface of silicate nanosheet. In the solvent, tilted species have lower energy. The T<i>_n</i> ← T₁ absorption of porphyrin molecules adsorbed at the surface of the nanosheet in the mixed solvent was observed at five different temperatures. The decay curve was analyzed with an equation for transient absorption difference, describing the behavior of parallel and tilted adsorbed species in the ground state and excited state to determine the rate constants for the orientation change and the radiationless deactivation. The rate constants of the orientation change increased with the temperature. The activation energy and energy gap between parallel and tilted species were estimated by analyzing the temperature dependence of the rate constants. The energy gap obtained in this kinetic study was consistent with our thermodynamically obtained value previously reported

Monolayer Modification of Spherical Amorphous Silica by Clay Nanosheets

Clay-silica nanocomposite materials (CSiN) were prepared by an electrostatic interaction between negatively charged clay nanosheets and positively charged spherical silica, which was modified with an alkyl ammonium group by silane coupling. By optimization of the preparation conditions, 84% coverage of the silica surface by the clay nanosheets was achieved. Adsorption experiments using cationic porphyrin dyes on the CSiN revealed that the clay nanosheet covers the spherical silica as a single layer and does not detach from the silica surface under aqueous conditions. In addition, it turned out that the cationic porphyrin dye did not penetrate the space between the silica surface and the clay nanosheet. Porphyrin molecules were adsorbed only at the outer surface of the clay nanosheet without molecular aggregation even under the high-density adsorption conditions. By combining spherical silica and clay nanosheets, it is possible to prepare novel hybrid materials where the surface can act as a unique adsorption field for dyes