Photocurrent generation from porphyrin/fullerene complexes assembled in a tethered lipid bilayer

Electrochemical Impedance Spectroscopy as a Convenient Tool to Characterize Tethered Bilayer Membranes

In this paper, we describe the application of electrochemical impedance spectroscopy (EIS) to characterize process of formation and properties of solid-supported tethered bilayer membranes on solid conducting substrates. Along with the description of experimental procedures to prepare substrates and self-assembly of phospholipid bilayers onto gold-coated glass slides, we describe the detailed protocols of EIS measurements. We demonstrate the utility of EIS in the evaluation of the properties of both molecular anchor layers used to immobilize tBLMs as well as characterization of tBLMs. We show that the EIS methodology extends the applicability of this technique well beyond the mere evaluation of electric parameters. Specifically, we demonstrate how by using EIS one may evaluate both density and size of water-filled defects (ion-channels) in tBLMs, to determine the structural mode (homogeneous, heterogeneous, or clustered) of distribution of defects in tBLMs. Our methodology can be applied in both basic protein membrane interaction studies, as well as in the development of precision biosensoric systems with tBLMs as a sensing element.

Proton Removal Kinetics That Govern the Hydrogen Peroxide Oxidation Activity of Heterogeneous Bioinorganic Platforms

Precise regulation of proton-coupled electron-transfer (PCET) rates holds the key to simultaneously optimizing the turnover frequency and product selectivity of redox reactions that are central to the realization of renewable energy schemes in a sustainable future. In this work, a self-assembled monolayer (SAM) of a Ru complex electrografted onto a glassy carbon (GC) electrode was prepared as a heterogeneous electrocatalytic interface to facilitate the hydrogen peroxide (H₂O₂) oxidation half-cell reaction of a direct hydrogen peroxide/hydrogen peroxide fuel cell. A functional lipid membrane embedded with catalytic amounts of proton carriers was appended on top of the Ru SAM to construct a hybrid bilayer membrane (HBM) platform that can modulate the thermodynamics and kinetics of proton- and electron-transfer steps independently. The performances of the as-prepared Ru SAMs and HBMs toward H₂O₂ oxidation were investigated using electrochemical means, kinetic isotope effect (KIE) studies, and Tafel analyses. Proton carriers featuring borate, phosphate, and nitrile headgroups were found to dictate the transmembrane proton removal rate, thereby controlling the H₂O₂ oxidation activity. The first significance of this work was the expansion of HBM platforms to GC substrates to overcome the limited redox potential window on gold thiol systems, thereby enabling electrochemical investigations of anodic reactions at the SAM-lipid interface. The second highlight of this work was demonstrating for the first time that deprotonation kinetics can be taken advantage of to enhance the electrocatalytic oxidation performance of a metal complex anchored at the SAM-lipid interface of a HBM platform. When the knowledge gaps regarding how PCET steps govern redox pathways are closed, the advances achieved using our unique bioinorganic platform are envisioned to accelerate the understanding and optimization of electrocatalytic processes involving proton- and electron- transfer steps that are fundamental to the development of high-performance energy devices.

Electron transfer in oriented donor-acceptor dyads, intralayer charge migration, and formation of interlayer charge separated states in multi-layered Langmuir-Schäfer films

Photoinduced intra- and interlayer electron transfer (ET) of doubly bridged donor-acceptor molecule, porphyrin-fullerene dyad (PF), was studied in single- and multi-layered Langmuir-Schäfer (LS) films and in LS films, where PF and an efficient electron donating polymer polyhexyltiophene (PHT) formed a bilayer PHT/PF and multi-layered PHT/PF structures. The ET through layers were investigated by a method, which measures the photovoltaic (PV) response proportional to the number of charge-separated (CS) states and to the CS distance between the electrons and holes formed in pulsed photo-excitation. Primary conclusions were, that ET starts as formations of CS dyads (P⁺F^-) in single-layers, continues as long-range intra-layer charge migrations following interlayer CS between two adjacent monolayers. Quantitative conclusions were, that the interlayer ET efficiency is 100% in the bi-layered PF structure (2PF), where two CS dyads in adjacent layers forms CS complexes (P⁺F/PF^-) and that the probability to form longer or higher order of CS complexes follows an expression of a convergent geometric series, with a converting factor of 2/3. In the PHT/PF bilayer structure the ET efficiency was one order of magnitude higher, than that for the 2PF structure due to the ET from the CS dyads to ground state electron donor PHT, with an acceptor density, much higher than that of (P⁺F^-).

Mimicking Photosynthesis with Electrode-Supported Lipid Nanoassemblies

The grand scale, ultimate efficiency, and sustainability of natural photosynthesis have inspired generations of researchers in biomimetic light energy utilization. As an essential and ubiquitous component in all photosynthetic machinery, lipids and their assemblies have long been recognized as powerful molecular scaffolds in building artificial photosynthetic systems. Model lipid bilayers, such as black lipid membranes and liposomes (vesicles), have been extensively used to host natural as well as synthetic photo- and redox-active species, thereby enabling key photosynthetic processes, such as energy transfer and photoinduced electron transfer, to be examined in well-defined, natural-like membrane settings. Despite their long history, these lipid models remain highly relevant and still enjoy wide practice today. In this Account, we share with the reader our recent effort of introducing electrode-supported lipid nanoassemblies as new lipid models into photosynthesis biomimicking. This line of research builds off several solid-supported lipid bilayer architectures established relatively recently by workers in membrane biophysics and reveals important new features that match and sometimes exceed what earlier lipid models are capable of offering. Here, our eight-year exploration unfolds in three sections: (1) New photosynthetic mimics based on solid-supported lipid bilayers. This systematic effort has brought three solid-supported bilayers into artificial photosynthesis research: lipid bilayers supported on indium tin oxide electrodes, hybrid bilayers, and tethered lipid bilayers formed on gold. Quantitative on-electrode deposition of various photo- and redox-active agents, including fullerene, Ru(bpy)₃²⁺, and porphyrin, is realized via liposomal hosts. Vectorial electron transfer across single lipid-bilayer leaflets is achieved between electron donor/acceptor directionally organized therein, taking advantage of multiple incorporation sites offered by these bilayers as well as their sequential formation on electrodes. Supported on electrodes, these bilayers uniformly afford reliable photocurrent generation and modular system design. (2) Gold-supported hybrid bilayers as a powerful model platform for probing biomembrane-associated photoelectrochemical processes. These hybrid nanostructures consist of one alkanethiol (or substituted alkanethiol) and one lipid monolayer, whose chemical identity and makeup can be separately controlled and modified. Such precise molecular organization and flexible formation, in turn, enable a series of physicochemical parameters key to photosynthetic processes to be explicitly examined and cross-compared. A few such examples, based on donor/acceptor distance and loading, interfacial dipole, and redox level, are included here to illustrate the usefulness and versatility of this system. (3) Mimicking photosynthesis with supercomplexed lipid nanoassemblies. This research effort was motivated to address the low light absorption suffered by single-bilayer based photosynthetic mimics and has yielded a new lipid-based approach to mimicking Nature's way of organizing multiple photosynthetic subunits. Rhodamine and fullerene assembled within these lipid supercomplexes display robust electronic communication. The remarkable possibility of using lipid matrix to further improve photoconversion efficiency is revealed by cholesterol, whose addition triggers exciton formation that promotes faster energy and electron transfer in these lipid nanoassemblies.

Mimicking Photosynthesis with Supercomplexed Lipid Nanoassemblies: Design, Performance, and Enhancement Role of Cholesterol

We report here a new approach to mimicking photosynthesis that relies on supercomplexed lipid nanoassemblies to organize small organic species for coordinated light harvesting, energy/electron transfer, and photo-to-electrochemical energy conversion. Specifically, we demonstrate efficient photoinduced electron transfer (PeT) between rhodamine and fullerene assembled together via electrostatically bound liposome and lipid bilayer hosts. The remarkable impact of the lipid matrix on the photoconversion efficiency is further revealed by cholesterol, whose addition is found to modify the distribution and organization of the coassembled rhodamine dyes and thus their photodynamics. This significantly expedites the energy transfer (ET) among rhodamine dyes, as well as the PeT between rhodamines and fullerenes. A respectable 14% photon-to-electron conversion efficiency was achieved for this supercomplexed system containing 5% rhodamines, 5% fullerenes, and 30% cholesterol. The morphology, photodynamics, and photoelectrochemical behavior of these lipid supercomplexes were thoroughly characterized using atomic force microscopy (AFM), fluorescence microscopy, steady-state and time-resolved fluorescence spectroscopy, and transient absorption (TA) and photoaction spectroscopy. A detailed discussion on enhancement mechanisms of cholesterol in this lipid-complexed photosynthesis-mimicking system is provided at the end.

Surface-Bound Ruthenium Diimine Organometallic Complexes: Excited-State Properties

Ruthenium complexes of the general formula [Ru(CO)(H)(L₂)(L′₂)][PF₆] (L₂ = trans-2PPh₃, L′ = η²-4,4′-dicarboxybipyridine (1); L₂ =trans-2Ph₂PCH₂CH₂COOH, L′₂ = bipyridine (2); L₂ = Ph₂PCHCHPPh₂, L′ = η²-5-amino-1,10-phenanthroline (3); L₂ = trans-2PPh₃, L′₂ = η²-4-carboxaldehyde-4′-methylbipyridine (4)) have been shown to have longer emission lifetimes and higher quantum yields in solution compared with more symmetrical molecules such as [Ru(bpy)₃][Cl]₂. Compound 4 is obtained as a mixture with the corresponding acetal, 4′. These less symmetrical complexes have been covalently immobilized on the surface of silica polyamine composites, and their photophysical properties have been studied. The surface-bound complexes have been characterized by solid-state CPMAS ¹³C, ³¹P, and ²⁹Si NMR, UV–vis, and FT-IR spectroscopies. Excited-state lifetime studies revealed that, in general, the lifetimes of the immobilized complexes are 1.4 to 8 times longer than in solution and are dependent on particle size (300–500 μm versus 10–20 nm average diameter silica gels), polymer structure (linear poly(allylamine) versus branched poly(ethylenimine)), and the type of surface tether. One exception to this trend is the previously reported complex [Ru(bpy)₂(5-amino-1,10-phenanthroline)][PF₆]₂ (5), where only a slight increase in lifetime is observed. Only minor changes in emission wavelength are observed for all the complexes. This opens up the possibility for enhanced heterogeneous electron transfer in photocatalytic reactions.

Effects of oriented surface dipole on photoconversion efficiency in an alkane/lipid-hybrid-bilayer-based photovoltaic model system

When a phospholipid monolayer containing a zinc-coordinated porphyrin species formed atop a self-assembled monolayer of heptadecafluoro-1-decanethiol (CF3(CF2)7(CH2)2SH) is subjected to photoelectrochemical current generation, a significant modulation effect is observed. Compared with devices that contain similar photoactive lipid monolayers but formed on 1-dodecanethiol SAMs, these fluorinated hybrid bilayers produce a >60% increase in cathodic currents and a similar decrease in anodic currents. Photovoltages recorded from these hybrid bilayers are found to vary in the same fashion. The modulation of photovoltaic responses in these hybrid-bilayer-based devices is explained by the opposite surface dipoles associated with the thiols employed in this study, which in one case (fluorothiol) increase and in another (alkanethiol) decrease the work function of the underlying gold substrates. A similar trend of photovoltage/photocurrent modulation is also observed if fullerene is used as the photoagent in these devices. Our results reveal the intricacy of orientated surface dipole in influencing the photovoltaic processes, and its subtle interplay with other factors related to the photoagents, such as their location and orientation within the organic matrix.

Molecular photovoltaic system based on fullerenes and carotenoids co-assembled in lipid/alkanethiol hybrid bilayers

A hybrid molecular photovoltaic system, based on fullerene C(60) and lutein (a natural photosynthetic carotenoid pigment) that are assembled in a phospholipid/alkanethiol bilayer matrix, is described here. The assembly and photoconversion behaviors of such a system were studied by UV-vis spectroscopy, cyclic voltammetry, impedance spectroscopy, photoelectrochemical action spectroscopy, and photocurrent generation. While lutein itself is inefficient in generating photocurrent, it can strongly modulate photocurrents produced by fullerenes when coassembled in the lipid bilayer matrix presumably via photoinduced electron transfer. Our results thus provide a successful example of combining both synthetic and natural photoactive components in building molecular photovoltaic systems.

Electrochemical impedance spectroscopy of tethered bilayer membranes

The electrochemical impedance spectra (EIS) of tethered bilayer membranes (tBLMs) were analyzed, and the analytical solution for the spectral response of membranes containing natural or artificially introduced defects was derived. The analysis carried out in this work shows that the EIS features of an individual membrane defect cannot be modeled by conventional electrical elements. The primary reason for this is the complex nature of impedance of the submembrane ionic reservoir separating the phospholipid layer and the solid support. We demonstrate that its EIS response, in the case of radially symmetric defects, is described by the Hankel functions of a complex variable. Therefore, neither the impedance of the submembrane reservoir nor the total impedance of tBLMs can be modeled using the conventional elements of the equivalent electrical circuits of interfaces. There are, however, some limiting cases in which the complexity of the EIS response of the submembrane space reduces. In the high frequency limit, the EIS response of a submembrane space that surrounds the defect transforms into a response of a constant phase element (CPE) with the exponent (α) value of 0.5. The onset of this transformation is, beside other parameters, dependent on the defect size. Large-sized defects push the frequency limit lower, therefore, the EIS spectra exhibiting CPE behavior with α ≈ 0.5, can serve as a diagnostic criterion for the presence of such defects. In the low frequency limit, the response is dependent on the density of the defects, and it transforms into the capacitive impedance if the area occupied by a defect is finite. The higher the defect density, the higher the frequency edge at which the onset of the capacitive behavior is observed. Consequently, the presented analysis provides practical tools to evaluate the defect density in tBLMs, which could be utilized in tBLM-based biosensor applications. Alternatively, if the parameters of the defects, e.g., ion channels, such as the diameter and the conductance are known, the EIS data analysis provides a possibility to estimate other physical parameters of the system, such as thickness of the submembrane reservoir and its conductance. Finally, current analysis demonstrates a possibility to discriminate between the situations, in which the membrane defects are evenly distributed or clustered on the surface of tBLMs. Such sensitivity of EIS could be used for elucidation of the mechanisms of interaction between the proteins and the membranes.

Monomeric fullerenes in lipid membranes: effects of molecular shape and polarity

We report a combined theoretical and experimental study on the single-molecule interaction of fullerenes with phospholipid membranes. We studied pristine C(60) (1) and two N-substituted fulleropyrrolidines (2 and 3), one of which (3) bore a paramagnetic nitroxide group. Theoretical predictions of fullerene distribution and permeability across lipid bilayers were combined with electron paramagnetic resonance (EPR) experiments in aligned DMPC/DHPC bicelles containing the paramagnetic fulleropyrrolidine 3 or either one of the diamagnetic fullerenes together with spin-labeled lipids. We found that, at low concentrations, fullerenes are present in the bilayer as single molecules. Their preferred location in the membrane is only slightly influenced by the derivatization: all derivatives were confined just below the hydrophilic/hydrophobic interface, because of the key role played by dispersion interactions between the highly polarizable fullerene cage and the hydrocarbon chains, which are especially tight within this region. However, the deviation from spherical shape is sufficient to induce a preferential orientation of 2 and 3 in the membrane. We predict that monomeric fullerenes spontaneously penetrate the bilayer, in agreement with the results of molecular dynamics simulations, but we point out the limits of the currently used permeability model when applied to hydrophobic solutes.