Electrochemically Organized Isolated Fullerene-Rich Thin Films with Optical Limiting Properties

Electric field-assisted anion-π catalysis on carbon nanotubes in electrochemical microfluidic devices

The vision to control the charges migrating during reactions with external electric fields is attractive because of the promise of general catalysis, emergent properties, and programmable devices. Here, we explore this idea with anion-π catalysis, that is the stabilization of anionic transition states on aromatic surfaces. Catalyst activation by polarization of the aromatic system is most effective. This polarization is induced by electric fields. The use of electrochemical microfluidic reactors to polarize multiwalled carbon nanotubes as anion-π catalysts emerges as essential. These reactors provide access to high fields at low enough voltage to prevent electron transfer, afford meaningful effective catalyst/substrate ratios, and avoid interference from additional electrolytes. Under these conditions, the rate of pyrene-interfaced epoxide-opening ether cyclizations is linearly voltage-dependent at positive voltages and negligible at negative voltages. While electromicrofluidics have been conceived for redox chemistry, our results indicate that their use for supramolecular organocatalysis has the potential to noncovalently electrify organic synthesis in the broadest sense.

Two-photon absorption in halide perovskites and their applications

Active research on halide perovskites has given us a deep understanding of this family of materials and their potential for applications in advanced optoelectronic devices. One of the prominent outcomes is the use of perovskite materials for nonlinear optical applications. Two-photon absorption in perovskites, in particular their nanostructures, has been extensively studied and shows huge promise for many applications. However, we are still far from a thorough understanding of two-photon absorption in halide perovskites from a micro to macro perspective. Here we summarize different techniques for studying the two-photon absorption in nonlinear optical materials. We discuss the in-depth photophysics in two-photon absorption in halide perovskites. A comprehensive summary about the factors which influence two-photon absorption provides the direction to improve the two-photon absorption properties of halide perovskites. A summary of the recent applications of two-photon absorption in halide perovskites provides inspirations for engineers to utilize halide perovskites in two-photon absorption device development. This review will help readers to have a comprehensive and in-depth understanding of the research field of two-photon absorption of halide perovskites from microscopic mechanisms to applications. The article can serve as a manual and give inspiration for future researchers.

Weak Polyelectrolytes as Nanoarchitectonic Design Tools for Functional Materials: A Review of Recent Achievements

The ionization degree, charge density, and conformation of weak polyelectrolytes can be adjusted through adjusting the pH and ionic strength stimuli. Such polymers thus offer a range of reversible interactions, including electrostatic complexation, H-bonding, and hydrophobic interactions, which position weak polyelectrolytes as key nano-units for the design of dynamic systems with precise structures, compositions, and responses to stimuli. The purpose of this review article is to discuss recent examples of nanoarchitectonic systems and applications that use weak polyelectrolytes as smart components. Surface platforms (electrodeposited films, brushes), multilayers (coatings and capsules), processed polyelectrolyte complexes (gels and membranes), and pharmaceutical vectors from both synthetic or natural-type weak polyelectrolytes are discussed. Finally, the increasing significance of block copolymers with weak polyion blocks is discussed with respect to the design of nanovectors by micellization and film/membrane nanopatterning via phase separation.

Fullerene superlattices containing charge transfer complexes for an improved nonlinear optical performance

To improve the nonlinear optical (NLO) properties of fullerene C₆₀, chemical modifications are normally needed to construct a donor-π-acceptor (D-π-A) system, which requires tedious and time-consuming synthesis procedures. In addition, the conjugated structure of C₆₀ will inevitably be destroyed, which is disadvantageous for other applications. Here, we use solvent-based nanoarchitectonics to obtain highly ordered, three-dimensional (3D) C₆₀ supramolecular structures. For this purpose, a liquid-liquid interfacial precipitation (LLIP) method was employed using quinoline as the good solvent. Hollow polyhedra (HPH) and multilayer flowers (MFs) were obtained when methanol and ethanol were selected as the poor solvents, respectively. While quinoline failed to enter the HPH, it was found to be successfully intercalated with the MFs, which induced a transition of the C₆₀ organization from a pristine face-centered-cubic (fcc) phase to a hexagonal close packed (hcp) lattice. When embedded in a poly(methyl methacrylate) (PMMA) matrix, the HPH and MFs both show reverse saturable absorption (RSA) and optical limiting (OL) properties. The MFs-based film showed a third-order nonlinear absorption coefficient (β) of 1.25 × 10⁵ cm·GW^-1 and an optical limiting threshold (F_ol) of 0.00625 J·cm^-2. Comparatively, the HPH-based film exhibited a lower β value of 9.80 × 10⁴ cm GW^-1 and a higher F_ol value of 0.00834 J cm^-2. The better NLO performance of the MFs was mainly ascribed to the formation of the charge transfer complexes between quinoline and C₆₀, proven by UV-vis and electrochemical measurements. The fine tuning of the NLO properties of C₆₀ without chemical modification provides new opportunities for C₆₀ to be applied in nonlinear optics.

[2 + 1] Cycloaddition reactions of fullerene C60 based on diazo compounds

The most common variant of fullerene core functionalization is the [2 + 1] cycloaddition process. Of these, reactions leading to methanofullerenes are the most promising. They are synthesized in two main reactions: nucleophilic cyclopropanation according to the Bingel method and thermal addition of diazo compounds. This present review summarizes the material on the synthesis of monofunctionalized methanofullerenes - analogues of [60]PCBM - based on various diazo compounds. The main cyclopropanating agents for the synthesis of monosubstituted methanofullerenes, the optimal conditions and the mechanism of the [2 + 1] cycloaddition, as well as the practical application of the target products are analyzed.

Charge transfer induced excitons and nonlinear optical properties of ZnO/PEDOT:PSS nanocomposite films

In this current work, we have prepared zinc oxide (ZnO) nanorods by sol-gel method, and its composite films with a conducting polymer poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) also have been prepared by drop-casting method on the glass substrate. UV-Vis absorption and steady-state fluorescence studies revealed exciton dissociation and recombination at the interface of polymer chain and wide-bandgap semiconductor ZnO. Also, nonlinear optical properties of as-prepared nanocomposite films have been reported by employing an open aperture z-scan technique. A predominantly two-photon induced saturable absorption behavior, when excited with 532 nm, 10 ns laser pulses, appeared in nonlinear optical measurements. These results indicate that our as-synthesized composites can be useful in fabricating optical switch and saturable absorbers.

Graphene Oxide-Based Silico-Phosphate Composite Films for Optical Limiting of Ultrashort Near-Infrared Laser Pulses

The development of graphene-based materials for optical limiting functionality is an active field of research. Optical limiting for femtosecond laser pulses in the infrared-B (IR-B) (1.4-3 μm) spectral domain has been investigated to a lesser extent than that for nanosecond, picosecond and femtosecond laser pulses at wavelengths up to 1.1 μm. Novel nonlinear optical materials, glassy graphene oxide (GO)-based silico-phosphate composites, were prepared, for the first time to our knowledge, by a convenient and low cost sol-gel method, as described in the paper, using tetraethyl orthosilicate (TEOS), H3PO4 and GO/reduced GO (rGO) as precursors. The characterisation of the GO/rGO silico-phosphate composite films was performed by spectroscopy (Fourier-transform infrared (FTIR), Ultraviolet-Visible-Near Infrared (UV-VIS-NIR) and Raman) and microscopy (atomic force microscopy (AFM) and scanning electron microscope (SEM)) techniques. H3PO4 was found to reduce the rGO dispersed in the precursor's solution with the formation of vertically agglomerated rGO sheets, uniformly distributed on the substrate surface. The capability of these novel graphene oxide-based materials for the optical limiting of femtosecond laser pulses at 1550 nm wavelength was demonstrated by intensity-scan experiments. The GO or rGO presence in the film, their concentrations, the composite films glassy matrix, and the film substrate influence the optical limiting performance of these novel materials and are discussed accordingly.

Efficient Construction of Near-Infrared Absorption Donor-Acceptor Copolymers with and without Pt(II)-Incorporation toward Broadband Nonlinear Optical Materials

Organic nonlinear optical (NLO) materials have attracted immense scientific interest in various fields. Broadband NLO response extending to near-infrared (NIR) region is extremely important and remains challenging. Herein, two diketopyrrolopyrrole (DPP)-based donor-acceptor (D-A)-type π-conjugated copolymers with and without Pt(II) incorporation are rationally designed and synthesized toward broadband NLO response materials. The broad intramolecular charge transfer (ICT) absorption reaching 1000 nm due to the strong D-A interaction is well demonstrated by photophysical characterizations. The NLO properties of copolymers are studied using Z-scan technology. Owing to their extended π-conjugated D-A systems and near-infrared ICT absorption properties, both copolymers exhibit laser-induced NLO response to nanosecond as well as picosecond laser pulses upon the wavelengths of 532 and 1064 nm. Interestingly, introducing Pt(II) into the copolymer backbone can evidently improve the NLO property or unexpectedly switch the NLO response from saturable absorption to reverse saturable absorption. Meanwhile, both copolymers are successfully employed as optical limiting materials and exhibit broadband optical limiting abilities. Therefore, we present an efficient strategy toward broadband NLO materials, which may significantly facilitate the understanding of organic molecular structure-property relationship and promote their practical application.

C3-C3' and C6-C6' Oxidative Couplings of Carbazoles

9-Substituted carbazoles are widely used units in materials science, and their oxidative reactions have been utilized for the synthesis and characterization of polymers. Though the oxidative mechanism of carbazoles has been known for a few decades, structural definition has remained difficult, because their polymers are generally insoluble with incomplete characterization and unknown dependence of the electrochemical potentials. The oxidative reactions of 9-substituted carbazoles should be carefully considered under specific oxidative conditions; otherwise, structure definitions could be wrong, because the IR and NMR spectra used previously cannot quantitatively analyze 3,3'-coupling and 6,6'-coupling of carbazoles. In this review, the best understanding of the C3-C3' and C6-C6' oxidative couplings of 9-substituted carbazoles is presented, and the benefit of these oxidative reactions from the viewpoints of electrochemical synthesis, film engineering, and the synthesis and processing of polymers is highlighted.

Tuning Optical Limiting of Heterosized AuNPs and Fullerene by Countable Electrochemical Assembly

Tuning optical limiting was achieved based on the nanostructural and synergistic effects of heterosized gold nanoparticles and fullerene on electrochemical assembly. In particular, with thicknesses of 200, 1, and 10 nm, heterosized AuNP multilayers with periodical pairs of layers present a superior threshold of 0.59 J cm^-2 to monosized AuNP films with the values of 0.89-2.55 J cm^-2, which was further significantly enhanced by the introduction of C₇₀ with a significant threshold drop from 0.43 to 0.13 J cm^-2, indicating that the reverse saturable absorption of C₇₀ had a key contribution compared to the free carrier absorption of AuNPs. This paper not only demonstrates that the hybrid engineering of heterosized AuNPs into an identical film is an effective way to enhance the optical limiting but also indicates that the reverse saturable absorption of C₇₀ is superior to free carrier absorption of AuNPs in optical limiting in AuNPs and C₇₀ hybrid films.

Facile one-step fabrication of upconversion fluorescence carbon quantum dots anchored on graphene with enhanced nonlinear optical responses

A novel nanocomposite hybrid, carbon quantum dots (CQD)/graphene oxide (GO), which combines the favorable optical properties of both its components, is synthesized by a facile one-step electrochemical method. Transmission electron microscopy, Raman spectroscopy, UV-vis spectroscopy, and fluorescence studies show that the CQDs uniformly attach on the GO surface, which enables highly efficient energy transfer between CQDs and GO. The nonlinear optical and optical limiting (OL) performances are investigated by the open-aperture Z-scan technique in the nanosecond regime using a laser with a wavelength of 532 nm. The as-prepared CQD/GO composite offers a significantly improved OL performance compared with GO because of the charge/energy transfer process between the CQDs and GO. The main contributors to the enhanced OL effect in the CQD/GO hybrid are a combination of nonlinear scattering and increased nonlinear absorption resulting from efficient charge/energy transfer at the CQD/GO interface.

A novel nanocomposite hybrid, carbon quantum dots (CQD)/graphene oxide (GO), which combines the favorable optical properties of both its components, is synthesized by a facile one-step electrochemical method.

Electroreductive Coupling Layer-by-Layer Assembly

Rapid and covalent layer-by-layer (LbL) assembly is of significance for practical applications because of superior chemical and mechanical stability. The electrochemical LbL assembly via an accelerating trigger can be automated and programmable in response to electrical signals to in situ fabricate covalently layered thin films with chemical and mechanical stability. In this paper, electroreductive coupling layer-by-layer assembly is introduced as both covalent and rapid methodology for preparing layered thin films. This assembly is triggered by C-C coupling of peripheral alkynyls, which have own absorption below 300 nm and can transform to optical and electrical inert double/single or triple/single alternative bonding formations significantly without optical or electric alternations of desirable photoelectric building blocks, superior to other linkers among covalent LbL assemblies. Not limited to fabrication of optical thin films, this assembly is readily available for oxygen sensitive substrates or materials and also a powerful addition to electrooxidative coupling LbL assembly for developing the economically dynamoelectric LbL machines without moving or changing experimental gears.