Probing the adsorption configuration of methanol at a charged air/aqueous interface using nonlinear spectroscopy.

Investigating the effects of electrolyte ions on the adsorption configuration of methanol at a charged interface is important for studying the interface structure of electrolyte solutions and the oxidation mechanism of methanol in fuel cells. This study uses sum frequency generation (SFG) and heterodyne-detected second harmonic generation (HD-SHG) to investigate the adsorption configuration of methanol at the air/aqueous interface of 0.1 M NaClO4 solution, 0.1 M HClO4 solution and pure water. The results elucidate that the ion effect in the electrolyte solution affects the interface's charged state and the methanol's adsorption conformation at the interface. The negatively charged surface of the 0.1 M NaClO4 solution and the positively charged surface of the 0.1 M HClO4 solution arise from the corresponding specific ionic effects of the electrolyte solution. The orientation angle of methyl with respect to the surface normal is 43.4° ± 0.1° at the 0.1 M NaClO4 solution surface and 21.5° ± 0.2° at the 0.1 M HClO4 solution surface. Examining these adsorption configurations in detail, we find that at the negatively charged surface the inclined orientation angle (43.4°) of methanol favors the hydroxymethyl production by breaking the C-H bond, while at the positively charged surface the upright orientation angle (21.5°) of methanol promotes the methoxy formation by breaking the O-H bond. These findings not only illuminate the intricate ion effects on small organic molecules but also contribute to a molecular-level comprehension of the oxidation mechanism of methanol at electrode interfaces.

Direct observation of long-range chirality transfer in a self-assembled supramolecular monolayer at interface in situ.

Due to the interest in the origin of life and the need to synthesize new functional materials, the study of the origin of chirality has been given significant attention. The mechanism of chirality transfer at molecular and supramolecular levels remains underexplored. Herein, we study the mechanism of chirality transfer of N, N'-bis (octadecyl)-L-/D-(anthracene-9-carboxamide)-glutamic diamide (L-/D-GAn) supramolecular chiral self-assembled at the air/water interface by chiral sum-frequency generation vibrational spectroscopy (chiral SFG) and molecular dynamics (MD) simulations. We observe long-range chirality transfer in the systems. The chirality of Cα-H is transferred first to amide groups and then transferred to the anthracene unit, through intermolecular hydrogen bonds and π-π stacking to produce an antiparallel ß-sheet-like structure, and finally it is transferred to the end of hydrophobic alkyl chains at the interface. These results are relevant for understanding the chirality origin in supramolecular systems and the rational design of supramolecular chiral materials.

In situ nonlinear optical spectroscopic study of the structural chirality in DPPC Langmuir monolayers at the air/water interface.

We conduct a molecular study on the structural chirality in Langmuir monolayers composed of dipalmitoylphosphatidylcholine (DPPC) using in situ nonlinear optical spectroscopies, including second harmonic generation (SHG) and sum frequency generation (SFG). Chiral SHG response is observed from L-DPPC monolayers at moderate surface pressures and almost vanishes at a high surface pressure. SFG spectra of L-DPPC monolayers show chiral features that can be assigned to the terminal CH3 groups and the CH2 groups attached to the chiral center atom. This means that these achiral moieties form chiral superstructures at the interface. Along with increasing surface pressure, the structural chirality of CH3 groups shows a similar trend as that of chiral SHG, but CH2 chirality increases monotonically. Furthermore, in a racemic DPPC monolayer with a moderate surface pressure, both chiral SHG and chiral SFG of CH3 groups are absent, whereas chiral SFG of CH2 groups is clearly present, indicating that L- and D-DPPC are diastereomers at the air/water interface and interfacial CH2 prefers a certain orientation regardless of the molecular handedness. A molecular mechanism is proposed to explain the origin of the structural chirality in DPPC monolayers.

Guanosine Assembly Enabled Gold Nanorods with Dual Thermo- and Photoswitchable Plasmonic Chiroptical Activity.

Noble metal nanostructures with plasmonic circular dichroism (PCD) have attracted interest, and a modulation of PCD is of great importance for their potential applications. Herein, we propose a supramolecular strategy for achieving dual thermal and photoswitchable PCD. When guanosine (G), deoxyguanosine (dG), and boric acid modified achiral gold nanorods (GNRs) were coassembled into a hydrogel, hybrid nanofibers with PCD were produced. When the hydrogel was heated, the nanofiber was disassembled and the PCD disappeared. As the hydrogel was thermally reversible, a thermo-controlled PCD could be realized. The hybrid hydrogel also showed photoswitchable PCD. When the gel was irradiated with an IR laser, the PCD disappeared. It can be restored by being placed at room temperature. Moreover, the hybrid gel was selectively responsive to the circularly polarized light (CPL). For (G/dG)-GNR hybrid assemblies, the R-CPL irradiation showed photothermal efficiency higher than that of L-CPL, which made it useful for an IR-irradiation-controlled release of drug molecules.

Hierarchically Chiral Lattice Self-Assembly Induced Circularly Polarized Luminescence.

Biomaterials in nature often exhibit hierarchical chiral structures with an intriguing mechanism involving hierarchical chirality transfer from molecular to supramolecular and the nano- or microscale level. To mimic the cross-level chirality transfer, we present here one kind of host-guest complex system built of ß-cyclodextrin (ß-CD), sodium dodecyl sulfate (SDS), and fluorescent dyes, which show multilevel chirality, including molecular chirality of ß-CD, induced supramolecular chirality of ß-CD/SDS host-guest complexes, a chiral lattice self-assembled nanosheet, mesoscopic chirality of an assembled helical tube, induced chirality of a dye-doped chiral tube. The hierarchical chirality involved a chiral lattice self-assembly process, which can be identified by small-angle X-ray scattering, optical studies, circular dichroism, and circularly polarized luminescence spectral measurements. Benefiting from the chiral lattice self-assembly, intense circularly polarized luminescence was observed from the achiral dye-doped complexes with a large dissymmetry factor up to +0.1. This work thus provides a feasible insight for developing hierarchical chiroptical materials based on the lattice self-assembly.

Amplifying Dissymmetry Factor of Upconverted Circularly Polarized Luminescence through Chirality-Induced Spin Polarization in the Photon Upconversion Process.

A general phenomenon about upconverted circularly polarized luminescence (UC-CPL) based on triplet-triplet annihilation (TTA) was realized in an ambient environment by coupling three kinds of chiral acceptors with corresponding achiral sensitizers. All of the dissymmetry factors of UC-CPL exhibited significant amplification compared with the prompt CPL of the used chiral acceptors. Chirality-induced spin polarization during the TTA-UC process was in charge of the amplified dissymmetry factor of UC-CPL. Chirality-induced spin-polarized triplet excitons will suppress the TTA efficiency because the spin-polarized electrons go against the electron exchange within triplet excitons. However, the chirality-induced spin-polarized singlet excitons resulting from the TTA process can be promoted, enabling a large dissymmetry factor of UC-CPL.

Two-Photon Absorption-Based Upconverted Circularly Polarized Luminescence Generated in Chiral Perovskite Nanocrystals.

Upconverted circularly polarized luminescence (UCPL) processes have attracted great interest, because the chiroptical properties could be expressed in different photophysical processes. In this Letter, the first example of two-photon absorption-based upconverted circularly polarized luminescence (TP-UCPL) is demonstrated. The chiral α-octylamine-modified cesium lead bromides perovskite nanocrystals exhibited TP-UCPL with a two-photon absorption cross section at 800 nm (σ2,800 nm) up to 3.68 × 104 GM and luminescence dissymmetric factor ( glum) up to 7.0 × 10-3. Depending on the molecular chirality of the capping ligands, the TP-UCPL sense can be selected and the mirror-imaged CPL is obtained. It is envisaged that this approach will afford a new viewpoint for designing UCPL processes.

(R)-Binaphthyl derivatives as chiral dopants: substituent position controlled circularly polarized luminescence in liquid crystals.

A series of nonbridged axially chiral binaphthyl derivatives were synthesized and used as chiral dopants for chiral nematic liquid crystals (N*-LCs). It was found that binaphthyl derivatives substituted at 3,3' positions or 2,2' positions of binaphthyl rings could induce N*-LCs to show opposite circularly polarized luminescence (CPL) signals, despite having the same configuration. Additionally, a CPL switch could be constructed in these N*-LCs regulated by the applied electric field.