Photochromic Dendrimers for Photoswitched Solid-To-Liquid Transitions and Solar Thermal Fuels

Transient-absorption spectroscopy of dendrimers via nonadiabatic excited-state dynamics simulations

The efficiency of light-harvesting and energy transfer in multi-chromophore ensembles underpins natural photosynthesis. Dendrimers are highly branched synthetic multi-chromophoric conjugated supra-molecules that mimic these natural processes. After photoexcitation, their repeated units participate in a number of intramolecular electronic energy relaxation and redistribution pathways that ultimately funnel to a sink. Here, a model four-branched dendrimer with a pyrene core is theoretically studied using nonadiabatic molecular dynamics simulations. We evaluate excited-state photoinduced dynamics of the dendrimer, and demonstrate on-the-fly simulations of its transient absorption pump-probe (TA-PP) spectra. We show how the evolutions of the simulated TA-PP spectra monitor in real time photoinduced energy relaxation and redistribution, and provide a detailed microscopic picture of the relevant energy-transfer pathways. To the best of our knowledge, this is the first of this kind of on-the-fly atomistic simulation of TA-PP signals reported for a large molecular system.

Recent Development of Photochromic Polymer Systems: Mechanism, Materials, and Applications

Photochromic polymer is defined as a series of materials based on photochromic units in polymer chains, which produces reversible color changes under irradiation with a particular wavelength. Currently, as the research progresses, it shows increasing potential applications in various fields, such as anti-counterfeiting, information storage, super-resolution imaging, and logic gates. However, there is a paucity of published reviews on the topic of photochromic polymers. Herein, this review discusses and summarizes the research progress and prospects of such materials, mainly summarizing the basic mechanisms, classification, and applications of azobenzene, spiropyran, and diarylethene photochromic polymers. Moreover, 3-dimensional (3D) printable photochromic polymers are worthy to be summarized specifically because of its innovative approach for practical application; meanwhile, the developing 3D printing technology has shown increasing potential opportunities for better applications. Finally, the current challenges and future directions of photochromic polymer materials are summarized.

Photoluminescent Polymer Nanoparticles Based on Oxazolidine Derivatives for Authentication and Security Marking of Confidential Notes

Colloidal materials have widely been used to develop innovative anticounterfeiting nanoinks for information encryption. Latex nanoparticles based on methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) bearing hydroxyl functional groups were synthesized via semicontinuous miniemulsion polymerization. The size determination of the nanoparticles and microscopic results showed mostly spherical nanoparticles with a narrow size distribution and a mean size of about 80 nm. Two oxazolidine derivatives were physically doped at the surface of the nanoparticles to prepare photoluminescent polymer nanoparticles. Hydroxyl functional groups at the surface of the nanoparticles led to their hydrogen bonding interactions with the doped luminescent compounds. Optical analysis of the photoluminescent nanoparticles displayed different fluorescence emission and UV-vis absorbance intensities based on the amount of polar groups located at the surface of colloidal nanoparticles. Reducing the particle size to below 100 nm along with increasing the surface area can assist the decrease of the light reflectance and improvement of the latex nanoparticles' efficiency in the anticounterfeiting industry. This preparation methodology can efficiently provide remarkable photoreversible anticounterfeiting nanoinks used in different applications, such as print marking, security encoded tags, labeling, probing, and handwriting.

Electrochemical Site-Selective Alkylation of Azobenzenes with (Thio)Xanthenes

Herein, we first report an electrochemical methodology for the site-selective alkylation of azobenzenes with (thio)xanthenes in the absence of any transition metal catalyst or external oxidant. A variety of groups are compatible with this electrochemical alkylation, which furnishes the products in moderate to good yields.

Light-Responsive Hexagonal Assemblies of Triangular Azo Dyes

The rational design of small building block molecules and understanding their molecular assemblies are of fundamental importance in creating new stimuli-responsive organic architectures with desired shapes and functions. Based on the experimental results of light-induced conformational changes of four types of triangular azo dyes with different terminal functional groups, as well as absorption and fluorescence characteristics associated with their molecular assemblies, we report that aggregation-active emission enhancement (AIEE)-active compound (1) substituted with sterically crowded tert-butyl (t-Bu) groups showed approximately 35% light-induced molecular switching and had a strong tendency to assemble into highly stable hexagonal structures with AIEE characteristics. Their sizes were regulated from nanometer-scale hexagonal rods to micrometer-scale sticks depending on the concentration. This is in contrast to other triangular compounds with bromo (Br) and triphenylamine (TPA) substituents, which exhibited no photoisomerization and tended to form flexible fibrous structures. Moreover, non-contact exposure of the fluorescent hexagonal nanorods to ultraviolet (UV) light led to a dramatic hexagonal-to-amorphous structure transition. The resulting remarkable variations, such as in the contrast of microscopic images and fluorescence characteristics, were confirmed by various microscopic and spectroscopic measurements.

Synthesis and photochromism of catechol-containing symmetrical azobenzene compounds

Symmetrical azobenzene derivatives with two catechol groups, 1d-4d, were synthesized as kinds of novel compounds, and the structures were confirmed using mass spectrometry and nuclear magnetic resonance spectroscopy. These compounds could attain photostationary state rapidly in solution upon UV irradiation, and their photochromism had good reversibility. Substituents and their positions on azobenzene chromophore had obvious influence on the maximum absorption and photochromic performances of these as-synthesized compounds. Electron-donating group on ortho positions could contribute to the redshift π-π* band. The sulfonamide group that is bonded to dopamine molecules and azobenzene rings caused a negligible n-π* transition of cis isomer, resulting in photobleaching upon UV irradiation. Among the four compounds, 4d had the strongest electron-donating ortho-methoxy substituents and lower planarity; thus its absorption could decrease more significantly upon UV irradiation of the same intensity, and its cis-to-trans conversion could be up to 63%. Furthermore, owing to the presence of catechol groups, 4d showed an effective affinity and adhesion to substrate, and on the surface of substrate, a weak colour change could be observed upon UV irradiation, but the reversibility was poorer than that in solution.

Molecular Solar Thermal Systems towards Phase Change and Visible Light Photon Energy Storage

Molecular solar thermal (MOST) systems have attracted tremendous attention for solar energy conversion and storage, which can generate high-energy metastable isomers upon capturing photon energy, and release the stored energy as heat on demand during back conversion. However, the pristine molecular photoswitches are limited by low storage energy density and UV light photon energy storage. Recently, numerous pioneering works have been focused on the development of MOST systems towards phase change (PC) and visible light photon energy storage to increase their properties. On the one hand, the strategy of simultaneously capturing isomerization enthalpy and PC energy between solid and liquid can not only offer high latent heat, but also promote the development of sustainable energy systems. On the other hand, the efficient photon energy storage in the visible light range opens a tremendously fascinating avenue to fabricate MOST systems powered under natural sunlight. Here, the recent advances of MOST systems towards PC and visible light photon energy storage are systematically summarized, the most promising advantages and current challenges are analyzed, and emerging strategies and future research directions are proposed.

Supramolecular Cation-π Interaction Enhances Molecular Solar Thermal Fuel

Molecular solar thermal fuels (MOSTs), especially azobenzene-based MOSTs (Azo-MOSTs), have been considered as ideal energy-storage and conversion systems in outer or confined space because of their "closed loop" properties. However, there are two main obstacles existing in practical applications of Azo-MOSTs: the solvent-assistant charging process and the high molar extinction coefficient of chromophores, which are both closely related to the π-π stacking. Here, we report one efficient strategy to improve the energy density by introducing a supramolecular "cation-π" interaction into one phase-changeable Azo-MOST system. The energy density is increased by 24.7% (from 164.3 to 204.9 J/g) in Azo-MOST with a small loading amount of cation (2.0 mol %). Upon light triggering, the cation-π-enhanced Azo-MOST demonstrates one gravimetric energy density of about 56.9 W h/kg and a temperature increase of 8 °C in ambient conditions. Then the enhanced mechanism is revealed in both molecular and crystalline scales. This work demonstrates the huge potential of supramolecular interaction in the development of Azo-MOST systems, which could not only provide a universal method for enhancing the energy density of solar energy storage but also balance the conflicts between molecular design and the condensed state for phase-changeable materials.

Thiol-Ene Click-Inspired Late-Stage Modification of Long-Chain Polyurethane Dendrimers

The construction of well-defined polyurethane dendrimers is challenging due to the high reactivity of externally added or in situ formed isocyanates leading to the formation of side products. With a primary focus of dendrimer research being the interaction of the periphery and the core, we report the synthesis of a common polyurethane dendron, which allows for the late-stage variation of both the periphery and the core. The periphery can be varied simply by installing a clickable unit in the dendron and then attaching to the core and vice-versa. Thus, a common dendron allows for varying periphery and core in the final two steps. To accomplish this, a protecting group-free, one-pot multicomponent Curtius reaction was utilized to afford a robust and versatile AB2 type polyurethane dendron employing commercially available simple molecules: 5-hydroxyisophthalic acid, 11-bromoundecanol, and 4-penten-1-ol. Subsequent late-stage modifications of either dendrons or dendrimers via a thiol-ene click reaction gave surface-functionalized alternating aromatic-aliphatic polyurethane homodendrimers to generation-three (G3). The dendrons and the dendrimers were characterized by NMR, mass spectrometry, and FT-IR analysis. A bifunctional AB2 type dendritic monomer demonstrated this approach’s versatility that can either undergo a thiol-ene click or attachment to the core. This approach enables the incorporation of functionalities at the periphery and the core that may not withstand the dendrimer growth for the synthesis of polyurethane dendrimers and other dendritic macromolecules.

Cyclodextrin-dendrimers nanocomposites functionalized high performance liquid chromatography stationary phase for efficient separation of aromatic compounds

In this work, novel cyclodextrin-dendrimers nanocomposites functionalized high performance liquid chromatography stationary phases were developed for efficient separation of aromatic compounds. β-cyclodextrin was grafted onto the surface of silica gel matrix with poly (amidoamine) dendrimers as spacers. Scanning electron microscope, fourier transform infrared spectroscopy, element analysis and Brunner-Emmet-Teller measurement proved the successful grafting of cyclodextrin-dendrimers nanocomposites. The obtained stationary phases showed satisfactory separation effects for alkylbenzenes and benzenesulfonic acid substituents in reverse phase liquid chromatography mode. Weak hydrophilic and ion exchange interactions were also confirmed at the same time. Meanwhile, the effects of dendrimers and cyclodextrin on the chromatography performance were discussed. Separation mechanism of the stationary phases were verified by two-dimensional nuclear magnetic resonance technology, and the result implied that the proposed cyclodextrin-dendrimers nanocomposites functionalized stationary phases have significant prospects for separation and determination of more aromatic compounds in future.

Dendrimeric Structures in the Synthesis of Fine Chemicals

Dendrimers are highly branched structures with a defined shape, dimension, and molecular weight. They consist of three major components: the central core, branches, and terminal groups. In recent years, dendrimers have received great attention in medicinal chemistry, diagnostic field, science of materials, electrochemistry, and catalysis. In addition, they are largely applied for the functionalization of biocompatible semiconductors, in gene transfection processes, as well as in the preparation of nano-devices, including heterogeneous catalysts. Here, we describe recent advances in the design and application of dendrimers in catalytic organic and inorganic processes, sustainable and low environmental impact, photosensitive materials, nano-delivery systems, and antiviral agents' dendrimers.

Delocalized Excitation or Intramolecular Energy Transfer in Pyrene Core Dendrimers

Light-harvesting and then intramolecular energy transfer are the crucial steps in natural photosynthesis. Dendrimers are one of the most promising artificial light-harvesting antennas. Insight into the relationship between molecular structure and energy transfer (or delocalized excitation) in dendrimers would help in understanding and mimicking photosynthesis. Here, a series of dendrimers T1-T4 based on pyrene as a core and fluorene/carbazole as the dendrons have been studied with time-resolved fluorescence and femtosecond transient absorption spectroscopies, revealing that the large planar structure of T1 and T2 has led to strong coupling of pyrene and fluorene units, enabling delocalized excitation over the entire molecules. But for T3 and T4, the carbazole units linking the first- and second-generation branches have broken the planar structure and suppressed the π-electron delocalization, enabling the Förster resonance energy transfer. The efficient intramolecular energy transfer from peripheral branches to the core occurs within 2 ps.

Tough, Transparent, and Anti-Freezing Nanocomposite Organohydrogels with Photochromic Properties

Poor mechanical properties and freezing at low temperatures of traditional photochromic hydrogels limit their applications. Here, a novel type of photochromic nanocomposite organohydrogels (NC OGHs) by adding tungsten oxide nanoparticles was prepared by a simple one-pot method. The photochromic NC OGHs demonstrated excellent integrated properties, including high transparency, high mechanical properties, low-temperature resistance, anti-dehydration, rewrite capability, and UV blocking ability. In addition, the degree of coloration of NC OGHs could be precisely controlled by UV irradiation, and the bleaching process could be controlled by the temperature and atmosphere. Besides flexible optical information storage devices and optical filters, these photochromic NC OGHs were also used for smart windows in both room temperature and cold environments. The work provides a new insight into photochromic organohydrogels.

Arylazopyrazole-Based Dendrimer Solar Thermal Fuels: Stable Visible Light Storage and Controllable Heat Release

Solar thermal fuels offer a closed cycle and a renewable energy storage strategy by harvesting photon energy within the chemical conformations of molecules and retrieving energy by an induced release of heat. However, the majority of reports are limited to the ultraviolet light storage, which potentially interferes with the surrounding environment and reduces the material lifetime. Here, we present a novel arylazopyrazole (AAP)-containing dendrimer that not only addresses the hindrance of visible light storage for solar thermal fuels but also exhibits outstanding performances of abundant energy conversion and stable storage, which are attributed to the substantial absorbance in visible wavelengths of para-thiomethyl-substituted AAP groups and the stability of cis isomers, respectively. The energy density of the dendrimer fuel after efficiently harvesting blue light (405 nm) is as high as 0.14 MJ kg^-1 (67 kJ mol^-1), and the storage half-life of the fabricated dendrimer film can reach up to 12.9 days. Moreover, the heat release of the dendrimer film can be triggered by different stimuli (light and heat). The dendrimer film displays a 6.5 °C temperature difference between trans isomers and cis isomers during green light irradiation. Our work provides a fascinating avenue to fabricate visible light storage solar thermal fuels and unlocks the possibility of developing natural sunlight storage in the future.

Capillary Action-Inspired Nanoengineering of Spheres-on-Sphere Microspheres with Hollow Core and Hierarchical Shell

The current syntheses of spheres-on-sphere (SOS) microsphere, which possesses both hollow cavity and hierarchical structure, mainly rely on complicated routes and template removal. In this study, a one pot nanoengineering strategy inspired by the automatic transport behavior of water in plants is successfully developed to fabricate SOS microsphere in tandem with a traditional soft template method in the preparation of hollow structure. Amphiphilic siloxane oligomers generated in situ from methyltriethoxylsilane (MTES) under acidic conditions are anchored on the surface of soft template St monomer droplets, sequentially completing hydrolysis-polycondensation and forming a mesoporous polysilsesquioxane (PSQ) shell. Then, the St monomers located in cavity migrate outward under the combined action of capillary force stemming from mesoporous and osmotic pressure generating from inside-outside of the PSQ shell and polymerize on the outside of the hollow PSQ shell, in which residual siloxane oligomers further anchor on the polystyrene (PS) surface to reduce the surface energy of the system, finally resulting in the successful formation of SOS particles. To reduce thermal insulation coefficient of the material, the PS phase in SOS particles is removed to obtain the particles with multiscale hollow structure (SOS-MH), which have more hollow cavities to encapsulate more air. The presence of a much hollow structure in SOS-MH particles enables the thermal conductivity of polyacrylonitrile (PAN)/SOS-MH composite fibrous membranes (0.0307 W m^-1 K^-1) to decrease by about 40% compared to that of pure PAN fibrous films (0.0520 W m^-1 K^-1) at the same thickness of 1 mm, and the material also has moisture resistance due to the existence of a hierarchical shell.