Anodic Photocurrent Generation by Porphyrin-Terminated Helical Peptide Monolayers on Gold

Photocurrent generation of porphyrin-terminated helical peptide self-assembled monolayers (SAMs) was studied in an aqueous solution. The anodic photocurrent was prevailing, but the cathodic photocurrent was observed with applying negative bias voltage on the working electrode. The bias dependence of the photocurrent was explained successfully by theoretical calculation with taking into account the redox potential shift by J-aggregate of porpyrins, the helix dipole, and photoenergy migration in the SAM. The dark current was insignificant even at the forward bias voltage

O₂‑Triggered Directional Switching of Photocurrent in Self-Assembled Monolayer Composed of Porphyrin- and Fullerene-Terminated Helical Peptides on Gold

Directional switching of photocurrent generation in response to oxygen is attained with the self-assembled monolayer (SAM) composed of porphyrin- and fullerene-terminated helical peptides. The anodic photocurrent of the porphyrin SAM under argon gas is successfully switched over to the cathodic photocurrent in the presence of oxygen gas only in the copresence of the fullerene-terminated helical peptide. The first-principle calculations explain that the cathodic photocurrent is promoted as a result of suppression of the anodic photocurrent due to the small electron coupling between the lowest unoccupied molecular orbitals of fullerene and the amide moieties of electron mediating helix peptides

Temperature-Induced Phase Separation in Molecular Assembly of Nanotubes Comprising Amphiphilic Polypeptoid with Poly(<i>N</i>‑ethyl glycine) in Water by a Hydrophilic-Region-Driven-Type Mechanism

Two kinds of amphiphilic polypeptoids having different types of hydrophilic polypeptoids, poly­(sarcosine)-<i>b</i>-(l-Leu-Aib)₆ (ML12) and poly­(<i>N</i>-ethyl glycine)-<i>b</i>-(l-Leu-Aib)₆ (EL12), were self-assembled via two paths to phase-separated nanotubes. One path was via sticking ML12 nanotubes with EL12 nanotubes and the other was a preparation from a mixture of ML12 and EL12 in solution. In either case, nanotubes showed temperature-induced phase separation along the long axis, which was observed by two methods of labeling one phase with gold nanoparticles and fluorescence resonance energy transfer between the components. The phase separation was ascribed to aggregation of poly­(<i>N</i>-ethyl glycine) blocks over the cloud point temperature. The addition of 5% trifluoroethanol was needed for the phase separation because the tight association of the helices in the hydrophobic region should be loosened to allow lateral diffusion of the components to be separated. The phase separation in molecular assemblies in water based on the hydrophilic-region-driven-type mechanism therefore requires sophisticated balances of association forces exerting among the hydrophilic and hydrophobic regions of the amphiphilic polypeptoids

Prevailing Photocurrent Generation of D−π–A Type Oligo(phenyleneethynylene) in Contact with Gold over Dexter-Type Energy-Transfer Quenching

Photocurrent generation is observed with D−π–A type oligo­(phenylene­ethynylene) (OPE) physically contacting gold substrate. The OPE is conjugated with helical peptides, which helps the OPE moiety orient vertically on gold surface. This configuration makes the Dexter energy transfer difficult to occur even though one end of the D−π–A type OPE physically contacts gold. The anodic photocurrent continuously increases with increment of applying bias voltage from −0.3 to 0.5 V. The first principle calculations reveal that the increase in photocurrent generation is attributed partly to the change in the electron distributions of HOMO and LUMO of the D−π–A type OPE to be more localized with applying the positive potential

Phase-Separated Molecular Assembly of a Nanotube Composed of Amphiphilic Polypeptides Having a Helical Hydrophobic Block

Amphiphilic block polypeptides of poly­(sarcosine)-<i>b</i>-(l- or d-Leu-Aib)₆ (SL12OMe or SD12OMe) and poly­(sarcosine)-<i>b</i>-(l-Leu-Aib)₇ (SL14OMe) were reported to self-assemble into a nanotube morphology. Herein, we tried to construct a phase-separated nanotube by sticking two different kinds of nanotubes. SD12OMe nanotubes were found to stick to SL14OMe nanotubes with a heat treatment at 50 °C, but the sticking yield was limited. The amphiphilic polypeptides were functionalized by replacement of methyl ester with aromatic groups of <i>N</i>-ethylcarbazole (SL12Ecz) and naphthalimide (SD12NpiTEG), but they lost the ability to form homogeneous nanotubes. A fraction of the functionalized amphiphilic polypeptides mixing in the nanotube-forming amphiphilic polypeptides, a mixture of SL12OMe and SL12Ecz (9:1) as well as a mixture of SD12OMe and SD12NpiTEG (9:1), allowed nanotube formation. These two kinds of nanotubes partly stuck together with a heat treatment at 15 °C to maintain a segregated state of two kinds of aromatic groups along the nanotube, resulting in the formation of a phase-separated nanotube