Visualizing Material Processing via Photoexcitation-Controlled Organic-Phase Aggregation-Induced Emission

Photo-controlled order-to-order host-guest self-assembly transfer for an afterglow effect with water resistance

Due to the general incompleteness of photochemical reactions, the photostationary structure in traditional photo-controlled host-guest self-assembly transfer is usually disordered or irregular. This fact readily affects the photoregulation or improvement of related material properties. Herein, a photoexcitation-induced aggregation molecule, hydroxyl hexa(thioaryl)benzene (HB), was grafted into β-cyclodextrin to form a host-guest system. Upon irradiation, the excited state conformational change of HB can drive an order-to-order phase transition of the system, enabling the transfer of the initial linear nanostructure to a photostationary worm-like nanostructure with orderliness and crystallinity capability. Along with the photoexcitation-controlled phase transition, an afterglow effect was obtained from the films prepared by doping the host-guest system into poly(vinyl alcohol). The afterglow effect had a superior water resistance, which successfully overcame the general sensitivity of doped materials with the afterglow effect to water vapor. These results are expected to provide new insights for pushing forward chemical self-assembly from the light perspective, towards materials with superior and stable properties under light treatment.

A Multifunctional Aggregation-Induced Emission Luminogen with pH-Response Detachable Connector for Lipid Droplet-Specific Imaging and Tracing

Lipid droplets (LDs) targeting probes are important for investigating the biological functions of LDs. The interplay between LDs and some other organelles can help to further understand the biological functions of these organelles. However, it is still a challenge to design functional probes that can specifically target LDs and are responsive to some other organelles. Herein, a multifunctional aggregation-induced emission luminogen (AIEgen), namely the TPA-CN, was prepared by the simple aldimine condensation reaction for lipid droplet-specific imaging and tracing. TPA-CN can be sensitively responsive to the acid environment of lysosomes due to the pH-response detachable connector in TPA-CN. With the assistance of this characteristic, it can be concluded from the fluorescence imaging and co-localization analysis results that the internalization of TPA-CN and the targeting of LDs does not involve the lysosome and the lysosomal escape process. At last, the TPA-CN was successfully used for the high-sensitivity imaging of dynamic information of LDs.

Photo-Controlled Self-Assembly of Nanoparticles: A Promising Strategy for Development of Novel Structures

Photo-controlled self-assembly of nanoparticles (NPs) is an advanced and promising approach to address a series of material issues from the molecular level to the nano/micro scale, owing to the fact that light stimulus is typically precise and rapid, and can provide contactless spatial and temporal control. The traditional photo-controlled assembly of NPs is based on photochemical processes through NPs modified by photo-responsive molecules, which are realized through the change in chemical structure under irradiation. Moreover, photoexcitation-induced assembly of NPs is another promising physical strategy, and such a strategy aims to employ molecular conformational change in the excited state (rather than the chemical structure) to drive molecular motion and assembly. The exploration and control of NP assembly through such a photo-controlled strategy can open a new paradigm for scientists to deal with "bottom-up" behaviors and develop unprecedented optoelectronic functional materials.

A Single Carbon-Dot System Enabling Multiple Stimuli Activated Room-Temperature Phosphorescence

Room-temperature phosphorescent carbon dots (RTPCDs) have attracted considerable interests due to their unique nanoluminescent characteristic with time resolution. However, it is still a formidable challenge to construct multiple stimuli-activated RTP behaviors on CDs. Since the address of this issue facilitates complex and high-regulatable phosphorescent applications, we here develop a novel strategy to achieve a multiple stimuli responsive phosphorescent activation on a single carbon-dot system (S-CDs), using persulfurated aromatic carboxylic acid as the precursor. The introduction of aromatic carbonyl groups and multiple S atoms can promote the intersystem crossing process to generate RTP characteristic of the produced CDs. Meanwhile, by introducing these functional surface groups into S-CDs, the RTP property can be activated by light, acid, and thermal stimuli in solution or in film state. In this way, multistimuli responsive and tunable RTP characteristics are realized in the single carbon-dot system. Based on this set of RTP properties, S-CDs is applied to photocontrolled imaging in living cells, anticounterfeit label, and multilevel information encryption. Our work will benefit the development of multifunctional nanomaterials together with extending their application scope.

Photoexcitation-Induced Assembly: A Bottom-Up Physical Strategy for Driving Molecular Motion and Phase Evolution

ConspectusIn the field of molecular assembly, photodriven self-assembly is a smart and crucial strategy to regulate the molecular orderliness, multiscale structure, and optoelectronic properties. Traditionally, photodriven self-assembly is based on photochemical processes, through molecular structural change induced by photoreactions. Despite great progress in the photochemical self-assembly, there still exists disadvantages (e.g., the photoconversion rate never reaches 100% with the possible side reactions). Therefore, the photoinduced nanostructure and morphology are often difficult to predict due to insufficient phase transition or defects. In contrast, the physical processes based on photoexcitation are straightforward and can fully utilize photons to avoid the drawbacks of photochemistry. The photoexcitation strategy excludes the change of molecular structure, only utilizing the molecular conformational change from the ground state to excited state. Then, the excited state conformation is employed to drive molecular movement and aggregation, further promoting the synergistic assembly or phase transition of the entire material system. The regulation and exploration of molecular assembly upon photoexcitation can open up a new paradigm to deal with the "bottom-up" behavior and develop unprecedented optoelectronic functional materials.This Account starts with a brief introduction to the problems faced by photocontrolled self-assembly and presents the photoexcitation-induced assembly (PEIA) strategy. Then, we focus on exploring PEIA strategy based on persulfurated arenes as the prototype. The molecular conformational transition of persulfurated arenes from the ground state to the excited state is conducive to the formation of intermolecular interactions, successively driving molecular motion, aggregation, and assembly. Next, we describe our progress in exploring PEIA of persulfurated arenes at the molecular level and then demonstrate that the PEIA of persulfurated arenes can synergistically drive molecular motion and phase transition in various block copolymer systems. Moreover, we provide the potential applications of PEIA in dynamic visual imaging, information encryption, and surface property regulation. Finally, an outlook on further development of PEIA is prospected.

Making multi-twisted luminophores produce persistent room-temperature phosphorescence

Multi-twisted molecules, especially those with more than four branched rotation axes, have served as superior prototypes in diverse fields like molecular machines, optical materials, sensors, and so forth. However, due to excessive non-radiative relaxation of these molecules, it remains challenging to address their persistent room-temperature phosphorescence (pRTP), which limits their further development. Herein, we develop a host-guest energy-transfer relay strategy to improve the phosphorescence lifetime of multi-twisted luminophores by over thousand-fold to realize pRTP, which can be witnessed by the naked eye after removing the excitation light source. Moreover, we employ photoexcitation-induced molecular rearrangement to further prolong the phosphorescence lifetime, which, to the best of our knowledge, is the first example of photoactivation in ordered host-guest systems. Our systems show superior humidity and oxygen resistance, enabling long-term (at least over 9-12 months) stability of the pRTP properties. By achieving pRTP of multi-twisted luminophores, this work can advance the understanding of molecular photophysical mechanisms and guide the study of more molecular systems that are difficult to achieve pRTP.

In Situ Regulation of Microphase Separation-Recognized Circularly Polarized Luminescence via Photoexcitation-Induced Molecular Aggregation

Circularly polarized luminescence (CPL) has attracted great interest owing to its extensive optical information and chiral structural dependence. However, rationally regulating solid-phase CPL signals remains difficult because of the close packing of molecules in solid-state materials and the lack of structural visualization. In this work, we proposed a microphase-separation-recognized CPL regulation strategy via coassembly of a hexathiobenzene-based luminophore and chiral block copolymer (cBCP) with in situ photocontrollability. As a consequence to the continuous increase in the luminophore-to-cBCP ratio, the CPL signal of the supramolecular system exhibited an increasing trend until a critical point. Then, further increasing the ratio stretched the helical pitch of cBCP, which led to CPL reduction. With the photoexcitation-induced molecular aggregation of the luminophore, which was implemented using in situ photoirradiation, the helical pitch was retracted along with the restoration of the CPL signal. These processes were fully recognized and monitored by the microphase-separated nanomorphological change of the coassembled system, which indicated that such a structural contrast could be an effective method for rationally regulating the supramolecular chiropticity of solid-state materials.

Cross-Linked Polyphosphazene Nanospheres Boosting Long-Lived Organic Room-Temperature Phosphorescence

Long-lived organic room-temperature phosphorescence (RTP) has sparked intense explorations, owing to the outstanding optical performance and exceptional applications. Because triplet excitons in organic RTP experience multifarious relaxation processes resulting from their high sensitivity, spin multiplicity, inevitable nonradiative decay, and external quenchers, boosting RTP performance by the modulated triplet-exciton behavior is challenging. Herein, we report that cross-linked polyphosphazene nanospheres can effectively promote long-lived organic RTP. Through molecular engineering, multiple carbonyl groups (C═O), heteroatoms (N and P), and heavy atoms (Cl) are introduced into the polyphosphazene nanospheres, largely strengthening the spin-orbit coupling constant by recalibrating the electronic configurations between singlet (S_n) and triplet (T_n) excitons. In order to further suppress nonradiative decay and avoid quenching under ambient conditions, polyphosphazene nanospheres are encapsulated with poly(vinyl alcohol) matrix, thus synchronously prompting phosphorescence lifetime (173 ms longer), phosphorescence efficiency (∼12-fold higher), afterglow duration time (more than 20 s), and afterglow absolute luminance (∼19-fold higher) as compared with the 2,3,6,7,10,11-hexahydroxytriphenylene precursor. By measuring the emission intensity of the phosphorescence, an effective probe based on the nanospheres is developed for visible, quantitative, and expeditious detection of volatile organic compounds. More significantly, the obtained films show high selectivity and robustness for anisole detection (7.1 × 10^-4 mol L^-1). This work not only demonstrates a way toward boosting the efficiency of RTP materials but also provides a new avenue to apply RTP materials in feasible detection applications.

Rigid Polymer Network-Based Autonomous Photoswitches Working in the Solid State Encoded by Room-Temperature Phosphorescence

Autonomous molecular switches with self-recoverability are of great theoretical and experimental interest since they can avoid additional chemical or energy imposition during the working process. Due to the high energy barrier, however, the solid state is generally unfavorable for materials to exhibit the autonomous switch behavior. To promote the practical usage of the autonomous molecular switch, herein, we propose a prototype of an autonomous photoswitch that can work in the solid state based on a rigid polymer network. A hexacarboxylic sodium-modified hexathiobenzene compound was employed as a photoexcitation-driven unit, which can undergo molecular aggregation upon irradiation because of the distinct conformational difference between the ground state and the photoexcited state. Then, we selected a relatively rigid polymer named poly(dimethyldiallylammonium)chloride (PDDA) to complex with the hexacarboxylic sodium-modified hexathiobenzene through electrostatic coupling. Through optimization, the photoexcitation-controlled molecular aggregation and its self-recovery can work well in the solid matrix of PDDA under rhythmical photoirradiation. This process can be easily encoded by a self-recoverable room-temperature phosphorescence, featuring an excellent performance of the autonomous switch.

Enhancing the Operability of Photoexcitation-Controlled Aggregation-Induced Emissive Molecules in the Organic Phase

Controllable aggregation-induced emission luminogens (AIEgens) by photoexcitation can be conducted within a single solvent, thus opening new opportunities for preparing and processing smart materials. However, undesired side-reactions like photooxidation that can easily occur in the organic phase remain, limiting their applications. To enhance the operability of photoexcitation-controlled AIEgens (to specifically produce a phosphorescence characteristic) in the organic phase, in this work, we employ a typical prototype, hexathiobenzene, usually as the specific phosphorescent group, and investigate a series of physical and chemical factors, such as light intensity, dissolved oxygen content, and solvent polarity, to explore ways to control the photoexcitation-controllable AIEgens against the impurities from side-reactions. An organogel strategy was also developed to minimize interference factors and improve the practical application ability. We believe that the presented results provide new insights into the further development of the photoexcitation-based functional materials and the promotion of their practical usage.