Recent advances in low-power-threshold nonlinear photochromic materials

Mechanism of charge accumulation in potassium poly(heptazine imide)

Dark photocatalysis is the ability of a photocatalyst to accumulate photocarriers during light irradiation and consume them for redox reactions under dark conditions. This phenomenon of photocatalysts storing photocarriers is known as charge accumulation. Dark photocatalysts can mitigate fluctuations in photocatalytic reaction efficiency in response to fluctuating solar irradiation. Potassium poly(heptazine imide) (K-PHI) has attracted considerable attention due to its high photocatalytic efficiency and ability to undergo dark photocatalysis. However, the detailed mechanism of charge accumulation in K-PHI remains unclear because photochromism, potassium ion desorption, and charge accumulation occur simultaneously triggered by light irradiation, limiting the comprehensive understanding of this mechanism. To elucidate the charge accumulation mechanism in K-PHI, highly oriented K-PHI thin films were prepared. Then, their fundamental physical properties and optical response of their electrical properties were investigated. We succeeded in separately observing photochromism, potassium ion desorption, and charge accumulation induced by light irradiation on K-PHI and proposed a comprehensive model to explain these phenomena. This study not only provides insights into the unique physical phenomena exhibited by K-PHI but also contributes to the development of solar energy-storage materials in the future.

Relaxation dynamics of higher excited states of perylene-substituted perylene bisimide derivatives

Stepwise two-photon absorption processes have received considerable attention, especially in photocatalysis, due to their relatively lower power threshold, characteristic spatial selectivity, amplification of chemical reactions, and so on. Meanwhile, studies on the relaxation dynamics of higher excited states in condensed systems have been limited for several molecular systems due to the short-lived nature of these states. In this study, we synthesized perylene-substituted perylene bisimide (PBI) and its derivate as model compounds and investigated their excited-state dynamics, including higher excited states, using pump-repump-probe spectroscopy. We revealed that these molecules form charge-transfer (CT) states instantaneously after the excitation, regardless of whether it is the perylene moiety or the PBI moiety that is excited. The lifetime of the CT state was shorter when the distance between the donor (perylene) and the acceptor (PBI) was shorter. Moreover, we also revealed that a higher-lying CT state generated by the stepwise excitation of the CT state using a 740-nm pulse induced Stark effect to the neighboring perylene moiety. The Stark effect not only gives more detailed information about the CT state, but also presents the possibility of new photofunctions, such as instantaneous modulation of the electronic state to achieve optimal electronic properties. These insights contribute to understanding advanced photochemical reactions and would be important for exploring photocatalytic reactions involving higher excited states.

Reversible Light-Triggered Stretching of Small-Molecule Photochromic Organic Nanoparticles

Functional organic nanomaterials are nowadays largely spread in the field of nanomedicine. In situ modulation of their morphology is thus expected to considerably impact their interactions with the surroundings. In this context, photoswitchable nanoparticles that are manufactured, amenable to extensive disassembling upon illumination in the visible, and reversible reshaping under UV exposure. Such reversibility turns to be strongly impaired for photochromic nanoparticles in close contact with a substrate. In situ atomic force microscopy investigations at the nanoscale actually reveal progressive disintegration of the organic nanoparticles under successive UV-vis cycles of irradiation, in the absence of intrinsic elastic forces. These results point out the dramatic interactions exerted by surfaces on the cohesion of non-covalently bonded organic nanoparticles. They invite to harness such systems, often used as biomarkers, to also serve as photoactivatable drug delivery nanocarriers.

Multiphoton-Driven Photocatalytic Defluorination of Persistent Perfluoroalkyl Substances and Polymers by Visible Light

Perfluoroalkyl substances (PFASs) and fluorinated polymers (FPs) have been extensively utilized in various industries, whereas their extremely high stability poses environmental persistence and difficulty in waste treatment. Current decomposition approaches of PFASs and FPs typically require harsh conditions such as heating over 400 °C. Thus, there is a pressing need to develop a new technique capable of decomposing them under mild conditions. Here, we demonstrated that perfluorooctanesulfonate (PFOS), known as a "persistent chemical," and Nafion, a widely utilized sulfonated FP for ion-exchange membranes, can be efficiently decomposed into fluorine ions under ambient conditions via the irradiation of visible LED light onto semiconductor nanocrystals (NCs). PFOS was completely defluorinated within 8-h irradiation of 405-nm LED light, and the turnover number of the C-F bond dissociation per NC was 17200. Furthermore, 81 % defluorination of Nafion was achieved for 24-h light irradiation, demonstrating the efficient photocatalytic properties under visible light. We revealed that this decomposition is driven by cooperative mechanisms involving light-induced ligand displacements and Auger-induced electron injections via hydrated electrons and higher excited states. This study not only demonstrates the feasibility of efficiently breaking down various PFASs and FPs under mild conditions but also paves the way for advancing toward a sustainable fluorine-recycling society.

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.

A Nonlinear Photochromic Reaction Based on Sensitizer-Free Triplet-Triplet Annihilation in a Perylene-Substituted Rhodamine Spirolactam

Nonlinear photochromic reactions that work with weak incoherent light are important for molecular operations with high spatial resolution and multiple photofunctions based on single molecules. However, nonlinear photochromic compounds generally require complex molecular design, restricting accessibility in various fields. Herein, we report nonlinear photochromic properties in a perylene-substituted rhodamine spirolactam derivative (Rh-Pe), which is synthesized from rhodamine B in facile procedures. Direct excitation of Rh-Pe produces the triplet excited state via the charge-transfer (CT) state. The triplet excited state causes triplet-triplet annihilation to bring the generation of the intensely colored ring-open form with nonlinear behavior. Furthermore, green- and red-light-induced photochromism was achieved in Rh-Pe using triplet sensitizers, although Rh-Pe can be directly excited only by ultraviolet and blue light. Our findings are expected to contribute to the development of photofunctional materials showing nonlinear behavior and low-energy light responsivity.

Symmetry-Breaking Charge Separation Mediated Triplet Population in a Perylenediimide Trimer at the Single-Molecule Level

Herein, we demonstrate triplet excited-state population in a conformationally rigid perylenediimide trimer (PDI-T) via intramolecular symmetry-breaking charge separation (SB-CS) at the single-molecule level. The single-molecule fluorescence intensity trajectories of PDI-T in nonpolar polystyrene matrix (ε = 2.60) exhibit prolonged fluorescence with infrequent dark states, representing the triplet and/or the charge transfer states. In contrast, in a poly(vinyl alcohol) matrix (ε = 7.80), erratic blinking dynamics resulting in low photon counts were observed, corroborating the feasibility of charge separation in a polar environment. In agreement with the single-molecule measurements, transient absorption spectroscopy of PDI-T reveals ultrafast SB-CS (τCS < 5 ps) in polar tetrahydrofuran (ε = 7.58) and acetone (ε = 20.70), with the population of the triplet excited-state through charge recombination. The current investigation shows the utility of rigid and weakly coupled molecular constructs in controlling triplet generation and SB-CS for potential applications in optoelectronic devices.

Photochromism and single-component white light emission from a metalloviologen complex based on 1,5-naphthyridine

Metalloviologens, as emerging electron-transfer photochromic compounds, have shown intriguing properties such as radiochromism, photochromism and photoconductance. However, only a limited number of them have been reported so far. Exploration of new metalloviologens is strongly desired. Herein, we report a new solvothermally synthesized metalloviologen complex [CdCl2(ND)2]n (1, ND = 1,5-naphthalenes) that exhibits photochromic and intrinsic white light emission properties. Density functional theory calculation results reveal that the photochromism could be assigned to photoinduced electron transfer from chlorine atoms to ND molecules. The photoinduced charge-separated states are heat/air stable, attributed to the delocalization of ND and strong intermolecular π-π interactions. Besides, complex 1 consistently emits intrinsic white light when excited with 340-370 nm UV light, achieving high color rendering index (CRI) values (82.54-94.04). By adjusting the excitation wavelength, both "warm" and "cold" white light emission can be produced, making it suitable for the application of a white light emitting diode (WLED). Thus, this work demonstrates that the ND-based metalloviologen is not only helpful in producing photochromism, but also beneficial for creating white-light emission.

Immobilization of rare-earth doped aluminate nanoparticles encapsulated with silica into polylactic acid-based color-tunable smart plastic window

An inorganic/organic nanocomposite was used to develop an afterglow and color-tunable smart window. A combination of polylactic acid (PLA) plastic waste as an environmentally-friendly hosting agent, and lanthanide-activated strontium aluminum oxide nanoparticles (SAON) encapsulated with silica nanoparticles (SAON@Silica) as a photoluminescent efficient agent resulted in a smart organic/inorganic nanocomposite. In order to prepare SAON-encapsulated silica nanoparticles (SAON@Silica), the SAON nanoparticles were coated with silica using the heterogeneous precipitation method. By using transmission electron microscopy (TEM), SAON showed a diameter range of 5-12 nm, while the SAON@Silica nanoparticles showed a diameter range of 50-100 nm. In order to ensure the development of a colorless plastic film, a homogeneous dispersion of the phosphorescent Phosphor@Silica nanoparticles throughout the plastic bulk was confirmed. CIE Lab coordinates and luminescence spectra were used to study the color shift characteristics. Under visible light conditions, the plastic films were transparent. The photoluminescent films emitted green light at 525 nm when excited at 375 nm. The hydrophobicity and ultraviolet protection were enhanced without altering the fundamental physico-mechanical performance of the plastic sheet. The current color-tunable plastic can be used in many potential applications, such as warning signs, anti-counterfeiting barcodes, smart windows, and protective apparel.

Double-Model Decay Strategy Integrating Persistent Photogenic Radicaloids with Dynamic Circularly Polarized Doublet Radiance and Triplet Afterglow

Organic phosphors integrating circularly polarized persistent luminescence (CPPL) across the visible range are widespread for applications in optical information encryption, bioimaging, and 3D display, but the pursuit of color-tunable CPPL in single-component organics remains a formidable task. Herein, via in situ photoimplanting radical ion pairing into axial chiral crystals, we present and elucidate an unprecedented double-module decay strategy to achieve a colorful CPPL through a combination of stable triplet emission from neutral diphosphine and doublet radiance from photogenic radicals in an exclusive crystalline framework. Owing to the photoactivation-dependent doublet radiance component and an inherent triplet phosphorescence in the asymmetric environment, the CPL vision can be regulated by altering the photoactivation and observation time window, allowing colorful glow tuning from blue and orange to delayed green emission. Mechanism studies clearly reveal that this asymmetric electron migration environment and hybrid n-π* and π-π* instincts are responsible for the afterglow and radical radiance at ambient conditions. Moreover, we demonstrate the applications of colorful CPPL for displays and encryption via manipulation of both excitation and observation times.

Broadband nonlinear modulation of incoherent light using a transparent optoelectronic neuron array

Nonlinear optical processing of ambient natural light is highly desired for computational imaging and sensing. Strong optical nonlinear response under weak broadband incoherent light is essential for this purpose. By merging 2D transparent phototransistors (TPTs) with liquid crystal (LC) modulators, we create an optoelectronic neuron array that allows self-amplitude modulation of spatially incoherent light, achieving a large nonlinear contrast over a broad spectrum at orders-of-magnitude lower intensity than achievable in most optical nonlinear materials. We fabricated a 10,000-pixel array of optoelectronic neurons, and experimentally demonstrated an intelligent imaging system that instantly attenuates intense glares while retaining the weaker-intensity objects captured by a cellphone camera. This intelligent glare-reduction is important for various imaging applications, including autonomous driving, machine vision, and security cameras. The rapid nonlinear processing of incoherent broadband light might also find applications in optical computing, where nonlinear activation functions for ambient light conditions are highly sought.

Revisiting Peri-Aryloxyquinones: From a Forgotten Photochromic System to a Promising Tool for Emerging Applications

Emerging applications of photochromic compounds demand new molecular designs that can be inspired by some long-known yet currently forgotten classes of photoswitches. In the present review, we remind the community about Peri-AryloxyQuinones (PAQs) and their unique photoswitching behavior originally discovered more than 50 years ago. At the heart of this phenomenon is the light-induced migration of an aromatic moiety (arylotropy) in peri-aryloxy-substituted quinones resulting in ana-quinones. PAQs feature absorbance of both isomers in the visible spectral region, photochromism in the amorphous and crystalline state, and thermal stability of the photogenerated ana-isomer. Particularly noticeable is the high sensitivity of the ana-isomer towards nucleophiles in solution. In addition to the mechanism of molecular photochromism and the underlaying structure-switch relationships, we analyze potential applications and prospects of aryloxyquinones in optically switchable materials and devices. Due to their ability to efficiently photoswitch in the solid state, PAQs are indeed attractive candidates for such materials and devices, including electronics (optically controllable circuits, switches, transistors, memories, and displays), porous crystalline materials, crystalline actuators, photoactivated sensors, and many more. This review is intended to serve as a guide for researchers who wish to use photoswitchable PAQs in the development of new photocontrollable materials, devices, and processes.

Unraveling Steric Effects on Ultrafast Bond-Dissociation Processes of Photochromic Radical Complexes

Photochromic reactions of the phenoxyl-imidazolyl radical complex (PIC), which is one of the rate-tunable fast T-type photoswitches, dramatically change by the introduction of bulky substituents around the photochromic units. While these substituents are expected to affect the initial bond dissociation processes, they have not been elucidated yet. Here, we revealed the ultrafast bond dissociation processes of PIC derivatives with different bulky substituents by subpicosecond to nanosecond transient absorption spectroscopy. We revealed that the bulky substituents around the photochromic units decelerate the bond dissociation processes, whereas they largely accelerate the thermal back reactions of the photogenerated open-ring isomer. Moreover, we found clear correlations between the formation kinetics of the open-ring isomer and molecular structural changes. The initial bond-dissociation process dictates the products and the efficiency of photochromic reactions. Therefore, revealing these processes is important not only for fundamental photochemistry but also for optimizing photochromic properties for advanced functional materials.

Recyclable soft photonic crystal film with overall improved circularly polarized luminescence

Existing circularly polarized luminescence materials can hardly satisfy the requirements of both large luminescence dissymmetry factor and high luminescent quantum yield, which hinders their practical applications. Here, we present a soft photonic crystal film embedded with chiral nanopores that possesses excellent circularly polarized luminescence performance with a high luminescence dissymmetry factor as well as a large luminescent quantum yield when loaded with various luminescent dyes. Benefitting from the retention of chiral nanopores imprinted from a chiral liquid crystal arrangement, the chiral soft photonic crystal film can not only endow dyes with chiral properties, but also effectively avoid severe aggregation of guest dye molecules. More importantly, the soft photonic crystal film can be recycled many times by loading and eluting guest dye molecules while retaining good stability as well as circularly polarized luminescence performance, enabling various applications, including smart windows, multi-color circularly polarized luminescence and anticounterfeiting.

Lamp vs Laser: A Visible Light Photoinitiator That Promotes Radical Polymerization at Low Intensities and Cationic Polymerization at High Intensities

A visible light absorbing anthraquinone derivative 1-tosyloxy-2-methoxy-9,10-anthraquinone (QT) mediates both cationic and radical polymerizations depending on the intensity of visible light used. A previous study showed that this initiator generates para-toluenesulfonic acid through a stepwise, two-photon excitation mechanism. Thus, under high-intensity irradiation, QT generates acid in sufficient quantities to catalyze the cationic ring-opening polymerization of lactones. However, under low-intensity (lamp) conditions, the two-photon process is negligible, and QT photooxidizes DMSO, generating methyl radicals which initiate the RAFT polymerization of acrylates. This dual capability was utilized to switch between radical and cationic polymerizations to synthesize a copolymer using a one-pot procedure.

Photochromic dinuclear iridium(III) complexes having phenoxyl-imidazolyl radical complex derivatives

We demonstrate that the phenoxyl-imidazolyl radical complex (PIC), which is a rate-tunable fast photoswitch, can be used as a ligand that directly coordinates with iridium (III) ions. The iridium complexes show the characteristic photochromic reactions originating from the PIC moiety, whereas the behaviour of transient species is substantially different from that of the PIC.

Dynamics of the energy transfer involved in a diarylethene-perylenebisimide dyad: comparison between the molecule and the nanoparticle level

Photochromic materials are widely used to achieve fluorescence photoswitching. Understanding the energy transfer processes occurring in these systems would be an advantage for their use and better optimization of their properties. In this scope, we studied a diarylethene-perylenebisimide (DAE-PBI) dyad that presents a bright red emission and a large ON-OFF contrast, both in solution and in an aqueous suspension of nanoparticles (NPs). Using ultrafast transient absorption spectroscopy, the excited state dynamics was characterized for this dyad in THF solution and compared to its behavior in NPs state. An efficient energy transfer process between the PBI fluorophore and the DAE photochromic unit in its closed form was demonstrated, occurring in a few hundreds of femtoseconds.

Bridged-Imidazole Dimer Exhibiting Three-State Negative Photochromism with a Single Photochromic Unit

Photochromic molecules that can exhibit multiple states of photochromism in a single photochromic unit are considered more attractive than traditional bistable photochromic molecules because they can offer more versatility and control in photoresponsive systems. We have synthesized a negative photochromic 1-(1-naphthyl)pyrenyl-bridged imidazole dimer (NPy-ImD) that has three different isomers: a colorless isomer, 6MR, a blue-colored isomer, 5MR-B, and a red-colored isomer, 5MR-R. NPy-ImD can interconvert between these isomers via a short-lived transient biradical, BR, upon photoirradiation. 5MR-R is the most stable isomer, and the energy levels of 6MR, 5MR-B, and BR are relatively close to each other. The colored isomers 5MR-R and 5MR-B are photochemically isomerized to 6MR via the short-lived BR upon irradiation with blue light and red light, respectively. The absorption bands of 5MR-R and 5MR-B are well separated by more than 150 nm, with a small overlap, which means they can be selectively excited with different light sources, visible light for 5MR-R and NIR light for 5MR-B. The colorless isomer 6MR is formed from the short-lived BR through a kinetically controlled reaction. 6MR and 5MR-B can then be converted to the more stable isomer 5MR-R through a thermodynamically controlled reaction, which is facilitated by the thermally accessible intermediate, BR. Notably, 5MR-R photoisomerizes to 6MR when irradiated with CW-UV light, whereas it photoisomerizes to 5MR-B by a two-photon process when irradiated with nanosecond UV laser pulses.

Blue-to-UVB Upconversion, Solvent Sensitization and Challenging Bond Activation Enabled by a Benzene-Based Annihilator

Several energy-demanding photoreactions require harsh UV light from inefficient light sources. The conversion of low-energy visible light to high-energy singlet states via triplet-triplet annihilation upconversion (TTA-UC) could offer a solution for driving such reactions under mild conditions. We present the first annihilator with an emission maximum in the UVB region that, combined with an organic sensitizer, is suitable for blue-to-UVB upconversion. The annihilator singlet was successfully employed as an energy donor in subsequent FRET activations of aliphatic carbonyls. This hitherto unreported UC-FRET reaction sequence was directly monitored using laser spectroscopy and applied to mechanistic irradiation experiments demonstrating the feasibility of Norrish chemistry. Our results provide clear evidence for a novel blue light-driven substrate or solvent activation strategy, which is important in the context of developing more sustainable light-to-chemical energy conversion systems.

Photochromism of colloidal ZnO nanocrystal powders under ambient conditions

Zinc oxide (ZnO) nanocrystals (NCs) exhibit photochromic reactions under specific conditions upon ultraviolet light irradiation. Since the color is originated from the excited electrons at the conduction band of ZnO NCs, the photoinduced absorption is observed only in the solution with hole acceptors under inert conditions. ZnO is earth-abundant and less toxic than many other substances, and has been widely used in various industrial fields. If the photochromic reaction of ZnO can be observed consistently under ambient conditions, the material may pave the way for large-scale photochromic applications such as in pigments, windows, and building materials in addition to conventional photochromic applications. In this study, we synthesize hydrophilic ZnO NCs and observe the solid-state photochromic reactions in the visible to mid-infrared regions even in humid-air conditions. We reveal that the coloration of powders of ZnO NCs under ambient conditions originates mainly from two factors: (1) charge separation induced by hole trapping by water molecules adsorbed on the surface of NCs, and (2) deceleration of the reactions involving the electrons in the conduction band of ZnO NCs with molecular oxygen and the adsorbed water molecules.

Solvent dielectric delimited nitro–nitrito photorearrangement in a perylenediimide derivative

The discovery of vibrant excited-state dynamics and distinctive photochemistry has established nitrated polycyclic aromatic hydrocarbons as an exhilarating class of organic compounds. Herein, we report the atypical photorearrangement of nitro-perylenediimide (NO₂-PDI) to nitrito-perylenediimide (ONO-PDI), triggered by visible-light excitation and giving rise to linkage isomers in the polar aprotic solvent acetonitrile. ONO-PDI has been isolated and unambiguously characterized using standard spectroscopic, spectrometric, and elemental composition techniques. Although nitritoaromatic compounds are conventionally considered to be crucial intermediates in the photodissociation of nitroaromatics, experimental evidence for this has not been observed heretofore. Ultrafast transient absorption spectroscopy combined with computational investigations revealed the prominence of a conformationally relaxed singlet excited-state (S^CR₁) of NO₂-PDI in the photoisomerization pathway. Theoretical transition state (TS) analysis indicated the presence of a six-membered cyclic TS, which is pivotal in connecting the S^CR₁ state to the photoproduct state. This article addresses prevailing knowledge gaps in the field of organic linkage isomers and provides a comprehensive understanding of the unprecedented photoisomerization mechanism operating in the case of NO₂-PDI.

The unprecedented photorearrangement of nitro-perylenediimide (NO₂-PDI) to nitrito-perylenediimide (ONO-PDI) is shown to occur through a cyclic six-membered transition state triggered by visible-light excitation.