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.

The transfer and adsorption reactions of ionizable drug molecules, i.e. dipyridamole (DIP), propranolol (PRO) and warfarin (WAR), at the water|1,2-dichloroehtane (DCE) interface were studied in the presence of the carboxylate- terminated generation 3.5 (G3.5) or amino-terminated generation 4 (G4) polyamidoamine (PAMAM) dendrimers. The ionic partition diagram of the ionizable drugs was determined through the voltammetric analysis of ion transfer responses. In the DIP system, the additional voltammetric responses associated with the interfacial adsorption were observed in the positive potential region. Although the spectroscopic features of the drug species in the aqueous solution were hardly affected by the addition of the dendrimers, the ion transfer currents in the DIP and PRO systems were decreased in the presence of the G3.5 PAMAM dendrimer indicating the intermolecular association between the cationic drugs and negatively charged dendrimers in the interfacial region. The interfacial mechanism of the fluorescent DIP species was investigated in detail by potential-modulated fluorescence (PMF) spectroscopy. The PMF results demonstrated that the monoprotonated form, HDIP+, was transferred across the water|DCE interface accompanied by the adsorption process. The interfacial mechanism of the DIP species was significantly modified by the dendrimer, depending on the pH condition. Under acidic conditions, the positively charged G3.5 PAMAM dendrimer adsorbed at the interface effectively prevented the coadsorption of HDIP+. At higher pHs, DIP (or HDIP+) interacted with the hydrophobic interior moiety (or negatively charged periphery) of the dendrimers.

•Flavin derivatives associate with the positively charged PAMAM dendrimer both in solution and at liquid|liquid interfaces.•Surface-active hyperbranched polymer inhibits the interfacial adsorption of the flavin derivatives.•Ion-transfer reaction of the dendrimer-flavin associate involves the interfacial adsorption process.The ion transfer and adsorption mechanism of flavin derivatives, riboflavin (RF) and flavin mononucleotide (FMN), at the polarized water|1,2-dichloroethane (DCE) interface were studied in the presence of the fourth generation (G4) amino-terminated polyamidoamine (PAMAM) dendrimer or hyperbranched bis-MPA polyester-64-hydroxyl (HBP). The flavin derivatives associated with the positively charged G4 PAMAM dendrimer both in the aqueous solution and at the water|DCE interface. Spectroelectrochemical analysis through potential-modulated fluorescence spectroscopy demonstrated that the dendrimer-bound flavin derivatives were transferred across the water|DCE interface in the positive potential region, while the interfacial adsorption of flavin derivatives in the negative potential region was effectively inhibited by the competitive adsorption of the neutral G4 HBP molecules.

Potential-dependent adsorption behavior of meso-substituted water-soluble porphyrins at the polarized water|1,2-dichloroethane (DCE) interface was studied by polarization-modulation total internal reflection fluorescence (PM-TIRF) spectroscopy. In the PM-TIRF experiments, the fluorescence signal from the interfacial region was analyzed as a function of the periodic modulation of linear-polarizations (p and s) of the incident excitation beam. The potential-dependence of PM-TIRF responses for meso-substituted porphyrins, 5,10,15,20-tetrakis(N-methylpyridyl)porphyrin (H2TMPyP4+) and 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS4–), indicated that both free base porphyrins were adsorbed with relatively lying orientations at the polarized water|DCE interface. The average orientation angles (θ) were estimated as θ = 61 ± 1° for H2TMPyP4+ and θ = 65 ± 1° for H2TPPS4– with respect to the interface normal. The wavelength-dependence of polarization-modulated fluorescence signals (PM-TIRF spectrum), which corresponds to “pure” emission spectrum of interfacial species, clearly indicated that H2TMPyP4+ and H2TPPS4– are adsorbed with a modification of the solvation at the interface. These results demonstrated a high ability of the PM-TIRF spectroscopy for the direct characterization of fluorescent species adsorbed 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) (ZnTPPS4–) was studied at the polarized water|1,2-dichloroethane (DCE) interface. The positive photocurrent arising from the photoreduction of ZnTPPS4– 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–ZnTPPS4– 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–ZnTPPS4– 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.

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 (ZnTPPS4–) 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 (H2TPPS4–) was readily protonated under acidic conditions even in the presence of the dendrimers. In addition, the J-aggregates of diprotonated species, (H4TPPS2–)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. ZnTPPS4– exhibited relatively strong interaction with the higher generation dendrimer, whereas H2TPPS4– was less effectively associated with the dendrimers.

Heterogeneous photoinduced electron transfer between a hydrophilic dye, 5,10,15,20-tetrakis(4-carboxypheny)porphyrinato zinc(II) (ZnTPPC4−), and a lipophilic quencher, ferrocene, across the polarized water|1,2-dichloroethane interface was studied in the presence of citrate-stabilized gold nanoparticles (Au-NPs). A positive photocurrent arising from the photoreduction of ZnTPPC4- was significantly enhanced by adding Au-NPs. The photocurrent enhancement was dependent on the concentration of Au-NPs, the excitation wavelength, and the polarization angle of the excitation light, respectively. The results demonstrated that Au-NPs act as effective photoreaction catalysts at the liquid|liquid interface.Highlights► Distinct enhancement of the photoreaction yield in the presence of Au-NPs. ► Au-NPs catalyzed the heterogeneous photoinduced electron transfer across the liquid|liquid interface. ► Characteristic dependence of the photocurrent on the polarization angle of the excitation beam.