Temperature-Responsive Luminescent Solar Concentrators: Tuning Energy Transfer in a Liquid Crystalline Matrix

Light-Driven Sign Inversion of Circularly Polarized Luminescence Enabled by Dichroism Modulation in Cholesteric Liquid Crystals

Stimuli-responsive circularly polarized luminescence (CPL) materials show great promise in applying information encryption and anticounterfeiting. Herein, light-driven CPL sign inversion is achieved by combining a photoresponsive achiral negative dichroic dye (KG) and a static achiral positive dichroic dye (NR) as dopants at the 0.5:0.5 weight ratio into the cholesteric liquid crystal (CLC) host. The side chains of KG undergo trans/cis isomerization after 365 nm UV light irradiation, leading to the dichroism (SF) decrease. The |glum| value of CLC doping with KG (CLC-KG) weakens from 0.67 to 0.28 in response to the order degree change. Taking advantage of its unique CPL response property, the light-driven CPL sign inversion is achieved (from -0.20/0.14 to 0.02/-0.04) by incorporating NR (0.5:0.5) into the CLC-KG with helical superstructure static. Based on the synergistic use of circular polarization and responsiveness state as cryptographic primitives, the multidimensional information encryption CLC system can be realized.

Stimuli-fluorochromic smart organic materials

Smart materials based on stimuli-fluorochromic π-conjugated solids (SFCSs) have aroused significant interest due to their versatile and exciting properties, leading to advanced applications. In this review, we highlight the recent developments in SFCS-based smart materials, expanding beyond organometallic compounds and light-responsive organic luminescent materials, with a discussion on the design strategies, exciting properties and stimuli-fluorochromic mechanisms along with their potential applications in the exciting fields of encryption, sensors, data storage, display, green printing, etc. The review comprehensively covers single-component and multi-component SFCSs as well as their stimuli-fluorochromic behaviors under external stimuli. We also provide insights into current achievements, limitations, and major challenges as well as future opportunities, aiming to inspire further investigation in this field in the near future. We expect this review to inspire more innovative research on SFCSs and their advanced applications so as to promote further development of smart materials and devices.

Light-Responsive Supramolecular Liquid-Crystalline Metallacycle for Orthogonal Multimode Photopatterning

The development of multimode photopatterning systems based on supramolecular coordination complexes (SCCs) is considerably attractive in supramolecular chemistry and materials science, because SCCs can serve as promising platforms for the incorporation of multiple functional building blocks. Herein, we report a light-responsive liquid-crystalline metallacycle that is constructed by coordination-driven self-assembly. By exploiting its fascinating liquid crystal features, bright emission properties, and facile photocyclization capability, a unique system with spatially-controlled fluorescence-resonance energy transfer (FRET) is built through the introduction of a photochromic spiropyran derivative, which led to the realization of the first example of a liquid-crystalline metallacycle for orthogonal photopatterning in three-modes, namely holography, fluorescence, and photochromism.

The Rainbow Arching over the Fluorescent Thienoviologen Mesophases

Thermofluorochromic materials exhibit tunable fluorescence emission on heating or cooling. They are highly desirable for applications ranging from temperature sensing to high-security anti-counterfeiting. Luminescent matrices based on liquid crystals are very promising, particularly those based on liquid crystals with intrinsic fluorescence. However, only a few examples have been reported, suggesting ample margins for development in the field, due to the wide range of fluorophores and supramolecular organizations to be explored. Moreover, thermofluorochromic liquid crystals can be tailored with further functionalities to afford multi-stimuli responsive materials. For the first time, herein we report the thermofluorochromism of thienoviologen liquid crystals, already known to show bulk electrochromism and electrofluorochromism. In particular, we studied their photophysics in the 25 °C-220 °C range and as a function of the length of the N-linear alkyl chains, m (9 ≤ m ≤ 12 C atoms), and the type of anion, X (X = OTs^-, OTf^-, BF₄^-, NTf₂^-). Interestingly, by changing the parameters m, X and T, their fluorescence can be finely tuned in the whole visible spectral range up to the NIR, by switching among different mesophases. Importantly, by fixing the structural parameters m and X, an interesting thermofluorochromism can be achieved for each thienoviologen in a homologous series, leading to a switch of the emitted light from red to green and from white to blue as a consequence of the temperature-induced variation in the supramolecular interactions in the self-assembled phases.

Artificial Light-Harvesting System with White-Light Emission in a Bicontinuous Ionic Medium

Artificial light-harvesting systems (ALHSs), which are closely related to Förster resonance energy transfer (FRET), are among the most attractive scientific topics during the past few decades. Specifically, binary ALHSs that are composed of a fluid donor and acceptor have a simplified composition and high number density of the donor units. However, largely due to the difficulty in obtaining a fluid donor, investigation of these systems is still quite limited, especially for the ionic systems. Herein, we report a new type of binary ALHS using an ionic naphthalimide (NPI) derivative as a donor, which shows greatly improved photoluminescence for its bicontinuous liquid structure. When blending with an acceptor such as rhodamine 6G or trans-4-[4-(dimethylamino)styryl]-methylpyridinium iodide, efficient FRET was confirmed by both experimental results and molecular dynamics simulations, with an energy transfer efficiency up to ∼90%. Tunable color, including white-light emission, was achieved by tuning the acceptor/donor ratio, opening the door for a variety of applications such as light-emitting diodes and photoluminescent inks.

Förster Resonance Energy Transfer in Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are an emerging technology to collect and channel light from a large absorption area into a smaller one. They are a complementary technology for traditional solar photovoltaics (PV), particularly suitable for application in urban or indoor environments where their custom colors and form factors, and performance under diffuse light conditions may be advantageous. Förster resonance energy transfer (FRET) has emerged as a valuable approach to overcome some of the intrinsic limitations of conventional single lumophore LSCs, such as reabsorption or reduced quantum efficiency. This review outlines the potential of FRET to boost LSC performance, using highlights from the literature to illustrate the key criteria that must be considered when designing an FRET-LSC, including both the photophysical requirements of the FRET lumophores and their interaction with the host material. Based on these criteria, a list of design guidelines intended to aid researchers when they approach the design of a new FRET-LSC system is presented. By highlighting the unanswered questions in this field, the authors aim to demonstrate the potential of FRET-LSCs for both conventional solar-harvesting and emerging LSC-inspired technologies and hope to encourage participation from a diverse researcher base to address this exciting challenge.

Dynamic Circularly Polarized Luminescence with Tunable Handedness and Intensity Enabled by Achiral Dichroic Dyes in Cholesteric Liquid Crystal Medium

Cholesteric liquid crystals (CLCs) are chiral supramolecular systems that self-assemble into a highly regular helical arrangement in a liquid crystal (LC) medium. Such an arrangement is highly beneficial for the chiral enlargement effect on circularly polarized luminescence (CPL) signals. Dichroic dyes with rod-like molecular structures can exhibit fluorescence anisotropy along both the long and short molecular axes owing to their transition dipole moment (TDM) vectors. In this work, a pair of donor-accepter (D-A) achiral dichroic dyes is prepared, namely, 3,4-ethylenedioxythiophene derivative (P1, whose TDM vector is parallel to the long axis of the molecule, i.e., F_||  > F_⊥ ) and anthraquinone derivative (N1, whose TDM vector is perpendicular to the long axis of the molecule, i.e., F_||  < F_⊥ ). CLCs can be fabricated by doping P1 or N1 together with chiral 1,1'-binaphthyl-derived inducers into SLC1717 medium. Dynamic CPL with tunable handedness and intensity is achieved by changing the N1:P1 mass ratio, and the luminescence dissymmetry factor (g_em ) value reaches |0.71|. This work describes the first observation of dynamic CPL with tunable handedness and intensity enabled by TDM regulation of achiral dichroic dyes in a CLC medium.

Construction of highly efficient carbon dots-based polymer photonic luminescent solar concentrators with sandwich structure

The enhancement of photoluminescence (PL) emission and waveguide play a key role in improving the optical efficiency of luminescent solar concentrators (LSCs). In this work, to boosting PL emission and waveguide simultaneously, one photonic crystal (PC) structure (crystalline colloid arrays (CCAs)) was introduced into carbon dots (CDs)-based polymer LSCs. A sandwich-structured CDs-based polymer photonic LSC, comprising glass/CDs-based polymer PC film/glass, was created. First, CDs-based colloidal crystal suspensions were prepared by co-assembly of monodispersed p(MMA-NIPAm) colloids and multicolor-emitting CDs in HEMA monomer induced by the evaporation-driven assembly. The obtained suspensions not only had uniform PL and structural colors, but showed enhanced PL emission. Second, the above suspensions were sandwiched between two glass sheets and finally a photonic polymer LSC with sandwiched structure (25 × 25 × 1.8 mm³) were formed via one-step photopolymerization technique. Remarkably, the optimal CDs-based polymer photonic LSCs with sandwiched structure not only had high transparence at visible range (>60%), but exhibited PL emission enhancement (at least 2 times). Furthermore, the maximum external optical efficiency (η_opt) of 5.84% could be achieved based on yellow-emitting CDs-based polymer photonic LSC. The high external optical efficiency was mainly attributed to the PL emission enhancement and good PC waveguide.

Deformable Thermo-Responsive Smart Windows Based on a Shape Memory Polymer for Adaptive Solar Modulations

Thermo-responsive smart windows that control solar transmission are expected to be the promising solution to excessive building energy consumption and overheating of solar cell devices. The two performance indices, namely, the luminous transmission (T_lum) and the solar modulation (ΔT_sol), are often intrinsically limited by conventional thermo-responsive materials, which restrict their applications in smart windows. Alternatively, constructing a deformable surface morphology of smart windows can be an effective strategy to modulate the solar transmission. Here, we report a new category of thermo-responsive smart windows with a deformable surface morphology, which can be custom designed to achieve both desirable ΔT_sol and T_lum according to the sunlight incident angles of actual applications. This design is based on a thermo-responsive shape memory polymer and an optical coating, which is termed the butterfly-wing-like smart window (BSW). The BSW reversibly transforms from a temporary shape of flat topography to a predefined original shape of tilted configuration upon heating. It is demonstrated that the BSW has a high ΔT_sol of 32.6% and an excellent T_lum(average) of 64.5%. This work provides a new design strategy and mechanism for thermo-responsive smart windows.

Thermoresponsive multicolor-emissive materials based on solid lipid nanoparticles

Despite the recent advances in the field of thermofluorochromism, the fabrication of thermoresponsive multicolor-emissive materials in a simple, low-cost and versatile manner still remains a challenge. Herein we accomplish this goal by expanding the concept of matrix-induced thermofluorochromism, where a sudden two-state variation of dyes' emission is promoted by the solid-liquid transition of a surrounding phase change material (e.g., paraffins). We demonstrate that this behavior can be transferred to the nanoscale by the synthesis of dye-loaded solid lipid nanoparticles, different types of which can then be combined into a single platform to obtain multicolor thermofluorochromism using a single type of emitter. Because of the reduced dimensions of these particles, they can be utilized to prepare transparent nanocomposites and inkjet-printed patterns showing complex thermoresponsive luminescence signals and applications ranging from smart displays to thermal sensing and high-security anti-counterfeiting.

Red and yellow emissive carbon dots integrated tandem luminescent solar concentrators with significantly improved efficiency

Luminescent solar concentrators (LSCs) can collect solar light from a large area and concentrate it on their small-area edges mounted with solar cells for efficient solar-to-electricity conversion. Thus, LSCs show huge promise for realizing building-integrated photovoltaics because of their semi-transparency and light weight. However, the low optical efficiency of LSCs becomes a great obstacle for their application in real energy conversion. Herein, yellow emissive carbon dots with a record-breaking ultrahigh quantum yield of up to 86.4% were prepared via a simple hydrothermal approach using low-cost precursors. By combining them with red emissive carbon dots (quantum yield of 17.6%), a large area (∼100 cm2) tandem LSC was fabricated. The power conversion efficiency (PCE) of the large-area carbon dot-integrated tandem LSC reaches up to 3.8%, which is among the best reported in literature for a similar lateral size of LSCs. In particular, the tandem structure based on two laminated layers is novel, and is fit for the real structural application of keeping windows warm, where two glass slides are usually used. The high-efficiency tandem LSC using eco-friendly carbon dots as fluorophores paves way for real applications of LSCs.

Photo-Oxidation-Controlled Surface Pattern with Responsive Wrinkled Topography and Fluorescence

Wrinkles and photo-oxidation reactions are widely found in soft materials, which are intimately associated with the failure of materials and structures. It is expected that the photo-oxidation process could also have a positive effect on the material and its surface. Here, we report the photo-oxidation of 2-(4-dietheylaminophenyl)-4,5-bis(4-methoxyphenyl) imidazole (DEA-TAI) into a wrinkled bilayer system to control surface wrinkle and fluorescent patterns, in which a supramolecular polymer network composed of carboxylic acid-containing copolymer (PS-BA-AA; PS=poly(styrene), BA=butyl acrylate; AA=acrylic acid) and DEA-TAI were used as the skin layer. Ultraviolet (UV) irradiation can induce photo-oxidation of the imidazole ring of DEA-TAI to weaken the intermolecular hydrogen bonding between PS-BA-AA and DEA-TAI, resulting in the release of stress in the bilayer system. The wrinkled morphology and fluorescence of the surface can be simultaneously regulated by photo-oxidation of DEA-TAI under UV light, and the resulting wrinkles are extremely sensitive to the pH value, which can be quickly and reversibly erased by NH₃ gas. Smart surfaces with specific hierarchical wrinkles and fluorescence can be achieved by selective irradiation with photomasks, which may find potential applications in smart displays and multi-code information storage.

Cooperatively assembled liquid crystals enable temperature-controlled Förster resonance energy transfer

Balancing the rigidity of a π-conjugated structure for strong emission and the flexibility of liquid crystals for self-assembly is the key to realizing highly emissive liquid crystals (HELCs). Here we show that (1) integrating organization-induced emission into dual molecular cooperatively-assembled liquid crystals, (2) amplifying mesogens, and (3) elongating the spacer linking the emitter and the mesogen create advanced materials with desired thermal–optical properties. Impressively, assembling the fluorescent acceptor Nile red into its host donor designed according to the aforementioned strategies results in a temperature-controlled Förster resonance energy transfer (FRET) system. Indeed, FRET exhibits strong S-curve dependence as temperature sweeps through the liquid crystal phase transformation. Such thermochromic materials, suitable for dynamic thermo-optical sensing and modulation, are anticipated to unlock new and smart approaches for controlling and directing light in stimuli-responsive devices.

A temperature-sensitive Förster resonance energy transfer system was constructed using a highly emissive liquid crystal co-assembled with Nile red, enabling thermo-optical modulation for controlling and directing light in stimuli-responsive devices.

Ultrastrong Red Circularly Polarized Luminescence Promoted from Chiral Transfer and Intermolecular Förster Resonance Energy Transfer in Ternary Chiral Emissive Nematic Liquid Crystals

Chiral emissive liquid crystals (N*-LCs) have been proved to greatly amplify the circularly polarized luminescence (CPL) signals due to highly regular spiral arrangement of dyes in a well-organized liquid crystals system. Normally, CPL materials with a high luminescence dissymmetry factor (g_lum) and quantum yield (QY) can meet the real application requirement. Here, four chiral aggregate-induced emission (AIE) active donors (Guests A1-A4: R-C2, R-C4, R-C6, R-C8, chiral dopant, and energy donor) and achiral AIE-active acceptors (Guest B: PBCy, CPL emitter) were doped into the commercial nematic liquid crystals E7 (N-LCs, Host) to form CPL-active ternary chiral emissive N-LCs (T-N*-LCs), respectively. This kind of T-N*-LCs could emit strong red CPL with QY = 16.56% and g_lum up to 1.51 through intermolecular energy transfer and chirality induction from the supramolecular self-assembly of T-N*-LCs. This work provides the effective strategy for the development of high g_lum CPL materials.

Smart responsive organic microlasers with multiple emission states for high-security optical encryption

Modern high-security cryptography and optical communication call for covert bit sequences with high coding capacity and efficient authentication. Stimuli-responsive lasing emissions with easily distinguishable readout are promising in the coding field as a novel cryptographic primitive, while the application is frequently restricted by the limited number of emission states. Here, we report a strategy of achieving multiple competitive lasing signals in responsive organic microspheres where a donor–acceptor pair was introduced. The competitive lasing from the donor and acceptor was reversibly switched by modulating the competition between the radiative rate of the donor and the rate of energy transfer, and the generated multiple lasing signals enabled a quaternary coding for recognizable cryptographic implementation. Data encryption and extraction were demonstrated using a 4 × 4 microlaser array, showing vast prospects in avoiding the disclosure of security information. The results offer a comprehensive understanding of excited-state dynamics in organic composite materials, which may play a major role in high-security optical recording and information encryption.

Phosphorescence Energy Transfer: Ambient Afterglow Fluorescence from Water-Processable and Purely Organic Dyes via Delayed Sensitization

Ambient afterglow luminescence from metal-free organic chromophores would provide a promising alternative to the well-explored inorganic phosphors. However, the realization of air-stable and solution-processable organic afterglow systems with long-lived triplet or singlet states remains a formidable challenge. In the present study, a delayed sensitization of the singlet state of organic dyes via phosphorescence energy transfer from organic phosphors is proposed as an alternative strategy to realize "afterglow fluorescence". This concept is demonstrated with a long-lived phosphor as the energy donor and commercially available fluorescent dyes as the energy acceptor. Triplet-to-singlet Förster-resonance energy-transfer (TS-FRET) between donor and acceptor chromophores, which are co-organized in an amorphous polymer matrix, results in tuneable yellow and red afterglow from the fluorescent acceptors. Moreover, these afterglow fluorescent hybrids are highly solution-processable and show excellent air-stability with good quantum yields.

First demonstration of the use of very large Stokes shift cycloparaphenylenes as promising organic luminophores for transparent luminescent solar concentrators

The use of [n]CPP derivatives as luminophores in LSC-devices minimises reabsorption losses.

Unusual fluorescence of o-phenylazonaphthol derivatives with aggregation-induced emission and their use in two-photon cell imaging

Unusual fluorescence of o-phenylazonaphthol derivatives with aggregated-induced emission (AIE) is reported for the first time, which can be used in two-photon cell imaging applications.

Thermoresponsive luminescence properties of polyfluorinated bistolane-type light-emitting liquid crystals

We developed and characterized four polyfluorinated bistolane derivatives. These compounds, which possess either two alkoxy substituents or an alkoxy group and a bromine atom in their two molecular terminals, were synthesized from readily available 4-alkoxy-1-ethynylbenzene with a facile three-step procedure. Their thermodynamic and photophysical properties were evaluated in detail, and they were found to display both liquid-crystalline (LC) and photoluminescence properties. Remarkably, the photoluminescence behaviors dramatically changed during the thermal phase transition between the crystal and LC phases. Thus, these polyfluorinated bistolanes may be promising candidates for thermoresponsive luminous molecules.

Reversible Surface Dual-Pattern with Simultaneously Dynamic Wrinkled Topography and Fluorescence

The reversible surface patterns with fluorescence and topography can possibly enable information recording and reading and provide an important alternative to realize the higher information security. We demonstrated a reversible dual-pattern with simultaneously responsive fluorescence and topography using an anthracene (AN) and naphthalene diimide (NDI) containing copolymer (PAN-NDI-BA) as the skin layer, in which the reversible photodimerization of AN can simultaneously control the cross-linking and CT interaction between AN and NDI. Upon irradiation with UV light and thermal treatment, the resulting pattern assumes a reversible change between smooth and wrinkled states, and its fluorescence changes reversibly from red to white to blue-green. The smart surfaces with dynamic hierarchical wrinkles and fluorescence were achieved by selective irradiation with photomasks and can be employed for potential applications in smart displays and anticounterfeiting.

Temperature-Responsive Luminescent Solar Concentrators: Tuning Energy Transfer in a Liquid Crystalline Matrix

Temperature-responsive luminescent solar concentrators (LSCs) have been fabricated in which the Förster resonance energy transfer (FRET) between a donor-acceptor pair in a liquid crystalline solvent can be tuned. At room temperatures, the perylene bisimide (PBI) acceptor is aggregated and FRET is inactive; while after heating to a temperature above the isotropic phase of the liquid crystal solvent, the acceptor PBI completely dissolves and FRET is activated. This unusual temperature control over FRET was used to design a color-tunable LSC. The device has been shown to be highly stable towards consecutive heating and cooling cycles, making it an appealing device for harvesting otherwise unused solar energy.