Hirohisa Nagatani

Encapsulation of anilinonaphthalenesulfonates in carboxylate-terminated PAMAM dendrimer at the polarized water|1,2-dichloroethane interface.

Molecular encapsulation of water-soluble anionic fluorescent dye molecules, 8-anilino-1-naphthalenesulfonic acid (ANS), and its bimolecular derivative (bis-ANS), in the generation 3.5 polyamidoamine (G3.5 PAMAM) dendrimer was investigated in the bulk aqueous phase and at the polarized water|1,2-dichloroethane interface. ANS(-) was electrostatically incorporated in the dendrimer, and the fluorescence enhancement with a blue shift of the emission maximum was observed at pH values <6, where the interior of the dendrimer was positively charged. The fluorescence enhancement of ANS was maximized around pH 3 and then decreased under more acidic conditions. The potential dependences of the molecular encapsulation and the interfacial mechanism were studied in detail by means of potential modulated fluorescence (PMF) spectroscopy. Under acidic conditions, the dendrimer incorporated ANS(-) at the positively polarized interface as well as in the aqueous phase. ANS(-) was released from the dendrimer at the intrinsic transfer potential and independently transferred across the interface. Bis-ANS exhibited relatively strong interaction with the dendrimer over a wide pH range (1 < pH < 8), and a negative shift of the transfer potential was observed under the corresponding pH condition. The PMF analysis clearly demonstrated that the interfacial mechanism of the dendrimer involves transfer and adsorption processes depending on the pH condition and the Galvani potential difference.

Spectroelectrochemical characterization of dendrimer-porphyrin associates at polarized liquid|liquid interfaces

Molecular encapsulation of anionic porphyrins in NH2-terminated polyamidoamine (PAMAM) dendrimers and the interfacial behavior of the dendrimer-porphyrin associates were studied at the polarized water|1,2-dichloroethane (DCE) interface. Formation of the ion associates was significantly dependent on the pH condition and on generation of dendrimers. 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS(4-)) associated with the positively charged fourth-generation (G4) PAMAM dendrimer was highly stabilized in acidic aqueous solution without protolytic demetalation in a wide range of pH values (pH > 2). In contrast to the zinc(II) complex, the free base porphyrin (H2TPPS(4-)) was readily protonated under acidic conditions even in the presence of the dendrimers. In addition, the J-aggregates of diprotonated species, (H4TPPS(2-))n, were preferably formed on the dendrimer. The interfacial mechanism of the dendrimer-porphyrin associates was analyzed in detail by potential-modulated fluorescence (PMF) spectroscopy. PMF results indicated that the dendrimers incorporating porphyrin molecules were transferred across the positively polarized water|DCE interface via adsorption step, whereas the transfer responses of the porphyrin ions released from the dendrimers were observed at negatively polarized conditions. A negative shift of the transfer potential of porphyrin ions compared to the intrinsic transfer potential was apparently observed for each ion association system. The ion association stability between the dendrimer and the porphyrin molecules could be estimated from a negative shift of the transfer potential. ZnTPPS(4-) exhibited relatively strong interaction with the higher generation dendrimer, whereas H2TPPS(4-) was less effectively associated with the dendrimers.

Interfacial Self‐Assembly of Water‐Soluble Cationic Porphyrins for the Reduction of Oxygen to Water

Meet at the border: Assembly of the water-soluble cobalt tetrakis(N-methylpyridinium-4-yl)porphyrin [CoTMPyP](4+) at soft interfaces is enhanced and stabilized by its interfacial interaction with the lipophilic anion (C(6)F(5))(4)B(-). The supramolecular structure thus formed provides excellent catalytic activity in the four-electron reduction of oxygen.

Polarized total-reflection x-ray absorption fine structure for self-assembled monolayer of zinc porphyrin at air–water interface

A new x-ray absorption fine structure (XAFS) method is applied to the air–solution interface. Under the total reflection condition, the XAFS method becomes surface sensitive, enabling the self-assembled monolayer of a metal complex at the solution surface to analyze in situ. Furthermore the introduction of two types of linearly polarized x rays allows us to determine the selective orientation of a planar metal complex at the surface. We obtain spectra for a square planar complex, 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrinato zinc(II) (ZnTPPC) adsorbed on the aqueous solution surface forming a monolayer with two polarized x rays and compared them with spectra of the same compound in a bulk solution and in solid powder. The polarized XAFS spectra exhibit different features leading to the unambiguous conclusion that the plane of the ZnTPPC molecule is oriented parallel to the air–water interface and that there is no coordination to the axial sites of the zinc atom.

Photoinduced Electron Transfer of PAMAM Dendrimer–Zinc(II) Porphyrin Associates at Polarized Liquid|Liquid Interfaces

The heterogeneous photoinduced electron-transfer reaction of the ion associates between NH2-terminated polyamidoamine (PAMAM) dendrimers and 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato zinc(II) (ZnTPPS(4-)) was studied at the polarized water|1,2-dichloroethane (DCE) interface. The positive photocurrent arising from the photoreduction of ZnTPPS(4-) by a lipophilic quencher, decamethylferrocene, in the interfacial region was significantly enhanced by the ion association with the PAMAM dendrimers. The photocurrent response of the dendrimer-ZnTPPS(4-) associates was dependent on the pH condition and on the generation of dendrimer. A few cationic additives such as polyallylamine and n-octyltrimethyammonium were also examined as alternatives to the PAMAM dendrimer, but the magnitude of the photocurrent enhancement was rather small. The high photoreactivity of the dendrimer-ZnTPPS(4-) associates was interpreted mainly as a result of the high interfacial concentration of photoreactive porphyrin units associated stably with the dendrimer which was preferably adsorbed at the polarized water|DCE interface. The photochemical data observed in the second and fourth generation PAMAM dendrimer systems demonstrated that the higher generation dendrimer which can incorporate a porphyrin molecule more completely in the interior is less efficient for the photocurrent enhancement at the interface. These results indicated that the photoreactivity of ionic reactant at a polarized liquid|liquid interface can readily be modified via ion association with the charged dendrimer.

Potential-modulated fluorescence spectroscopy of the membrane potential-sensitive dye di-4-ANEPPS at the 1,2-dichloroethane/water interface

AbstractThe spectrofluorometric behavior of a membrane potential-sensitive dye, 1-(3-sulfonatopropyl)-4-[β-{2-(di-N-butylamino)-6-naphthyl}vinyl]pyridinium betaine (di-4-ANEPPS), at the polarized 1,2-dichloroethane/water interface was studied by means of potential-modulated fluorescence (PMF) spectroscopy. The results, combined with those from cyclic and alternating current voltammetry, clearly suggested that the dye adsorbed at the interface underwent a reorientation with increasing the interfacial potential, giving a well-developed PMF response as well as a voltammetric response. In addition to the PMF response, another PMF response was observed by addition of dilauroyl phosphatidylcholine (DLPC). This additional response was well explained in terms of a reorientation of di-4-ANEPPS at the interface, which would be induced by the potential-dependent desorption of DLPC from the interface. Thus, the present study supported the reorientation/solvatochromic mechanism for the membrane potential-sensitive dye rather than the electrochromic mechanism. FigureReaction model for the potential-induced reorientation of di-4-ANEPPS at the O/W interface in the presence of DLPC in the DCE phase.

Communication: Coordination structure of bromide ions associated with hexyltrimethylammonium cations at liquid/liquid interfaces under potentiostatic control as studied by total-reflection X-ray absorption fine structure

Total-reflection X-ray absorption fine structure (TR-XAFS) technique was applied for the first time to an interface between two immiscible electrolyte solutions under potentiostatic control. The hydration structure of bromide ions was investigated at polarized 2-octanone/water interfaces. TR-XAFS spectra at Br K-edge measured in the presence of hexyltrimethylammonium bromide (C6TAB) were slightly modified depending on the Galvani potential difference (Δ(o)(w)φ). The extended X-ray absorption fine structure analysis exposed hydration structure changes of bromide ions at the polarized interface. The coordination structure of bromide ions at the interface could be analyzed as compared with bromide ions dissolved in aqueous solution and Br(-)-exchanged resin having quaternary ammonium groups. The results indicated that bromide ions were associated with C6TA(+) at the polarized interface. The relative contribution of ion association form of bromide ions with quaternary ammonium groups was enhanced at a potential close to the ion transfer of C6TA(+), where the interfacial concentration of C6TA(+) is increased as a function of Δ(o)(w)φ.

Potential-modulated fluorescence spectroscopy of zwitterionic and dicationic membrane-potential-sensitive dyes at the 1,2-dichloroethane/water interface.

AbstractThe previously introduced technique of potential-modulated fluorescence (PMF) spectroscopy was used to study the potential-induced fluorescence change of some different dyes at the polarized 1,2-dichloroethane (DCE)/water (W) interface. A zwitterionic dye (POLARIC 488PPS) showed a PMF response similar to that for the previously studied dye (di-4-ANEPPS) with the same ionic state, and the PMF response was likewise explained by the potential-dependent reorientation of the dye at the DCE/W interface. Though a monocationic dye (POLARIC 488PM) showed no distinct PMF signal, a dicationic dye (di-2-ANEPEQ) showed two relatively weak but detectable PMF signals at lower and higher potential. It has thus been found that the ionic state of a potential-sensitive dye strongly influences the potential-induced reorientation of the dye at the interface and consequently its PMF response. These results support the reorientation/solvatochromic mechanism proposed for “slow” dyes but do not necessarily exclude the electrochromic mechanism proposed for “fast” dyes. PMF spectroscopy would provide useful information on the design of slow dyes for the measurement of the resting potential of cell membranes FigPotential-dependent reorientation of a zwitterionic membrane-potential-sensitive dye (POLARIC 488PPS) at the oil/water interface and its potential-modulated fluorescence signal (inset)

X-ray absorption fine structure of bromide ions attracted by cationic surfactants at the heptane-water interface

The total-reflection X-ray spectroscopic technique was applied to an interfacial species at the liquid-liquid interface. The XAFS spectrum of bromide ions at the heptane-water interface was successfully obtained in the fluorescence mode. A slight change in the spectra was observed in the presence of the cationic surfactants, dimethyldilaurylammonium and stearyltrimethylammonium ions. This suggested that the hydrated structure of bromide ions attracted by the cationic surfactants at the heptane-water interface is different from neat interfaces or bulk aqueous phases.

Potential-Induced Aggregation of Anionic Porphyrins at Liquid|Liquid Interfaces

The adsorption and self-aggregation of anionic porphyrins were studied at the polarized water|1,2-dichloroethane (DCE) interface by polarization-modulation total internal reflection fluorescence (PM-TIRF) spectroscopy. 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrin diacid (H4TPPS2-) and protoporphyrin IX (H2PP2-) exhibited high surface activities at the interface. The selective excitation of interfacial species in PM-TIRF measurements elucidated the potential-induced aggregation mechanism of the porphyrins. The J-aggregates of H4TPPS2- were reversibly formed only at the water|DCE interface by applying appropriate potentials even when the porphyrins exist as monomers in the aqueous and organic solutions. In the H2PP2- system, the slow aggregation process was found in the negative potential region. The spectral characteristics and the signal phase of PM-TIRF indicated that the H2PP2- monomers were adsorbed with relatively standing orientation and that the long axis of the J-aggregates was nearly in plane of the interface. H2PP2- was also investigated at the biomimetic phospholipid-adsorbed water|DCE interface. The competitive adsorption of neutral glycerophospholipids effectively inhibited the potential-dependent adsorption and interfacial aggregation processes of H2PP2-. The results demonstrated that the aggregation state of the charged species can reversibly be controlled at liquid|liquid interfaces as a function of externally applied potential.