Self-assembly and photo-electrochemistry of metalloporphyrin nanostructures at the interface between two immiscible liquids

Charge transfer reactions across interfaces are a fascinating physico-chemical process that have yet to be fully understood at the molecular level. Significant progress has been made in understanding charge transfer across solid-solid or solid-liquid interfaces. However, charge transfer across membrane-like interfaces, like the interfacebetween two immiscible electrolyte solutions (ITIES), remains rather unexplored.

In the present work, interfacial charge transfer reactions mediated by porphyrin interfacial nanostructures or INs are investigated. The interface used was the one formed between an aqueous phase and an organic solvent such as α, α, α-trifluorotoluene or 1,2-dichloroethane. INs of zinc (II) meso-tetrakis(4-carboxyphenyl)porphyrin (ZnTPPc) and tetrakis(4-carboxyphenyl)porphyrin (H2TPPc) were obtained selectively at the ITIES by a one-step self-assembly reaction. This reaction was observed to take place only when the pH of the aqueous phase was in the range 4.8 - 6.0. The self-assembly process yielded crystalline networks of porphyrin-INs bound together by intermolecular hydrogen bonds.

In the dark, protons located in the carboxylic groups of the porphyrin-INs can undergo an ionic exchange with cations in the organic electrolyte. The driving force for this reaction was a Galvani potential difference between the two electrolyte solutions (∆w o φ). The exchange is accompanied by a charge transfer that was followed by electrochemical techniques, and a change in the geometric arrangement of the individual porphyrins within the INs that was followed by in situ UV/vis spectroscopy. The thermodynamics and the kinetics of this reaction were successfully described by models commonly used at solid-liquid interfaces. 

Under illumination, the porphyrin-INs mediated the charge transfer between an electron donor in the organic phase and an electron acceptor in the aqueous phase. Studies using ferrocene (Fc) as electron donor and O2 as electron acceptor in the aqueous phase, showed that depending on ∆w o φ, photo-produced charge carriers Fc+or e − can be selectively transferred between the two electrolytes. The potential of zero charge of the interface and the ion transfer potential of Fc+ were identified as two energy barriers that direct the charge transfer across the ITIES. Control of ∆w o φ and the chemical nature of the ferrocenes, allowed the transfer of one of the carriers to be favoured.