Franck–Condon Blockade and Aggregation‐Modulated Conductance in Molecular Devices Using Aggregation‐Induced Emission‐Active Molecules

Development of Stable Water-Soluble Supratomic Silver Clusters Utilizing A Polyoxoniobate-Protected Strategy: Giant Core-Shell-Type Ag8@Nb162 Fluorescent Nanocluster

Atomically precise low-nuclearity (n<10) silver nanoclusters (AgNCs) have garnered significant interest due to their size-dependent optical properties and diverse applications. However, their synthesis has remained challenging, primarily due to their inherent instability. The present study introduces a new feasible approach for clustering silver ions utilizing highly negative and redox-inert polyoxoniobates (PONbs) as all-inorganic ligands. This strategy not only enables the creation of novel Ag-PONb composite nanoclusters but also facilitates the synthesis of stable low-nuclearity AgNCs. Using this method, we have successfully synthesized a small octanuclear rhombic [Ag8]6+ AgNC stabilized by six highly negative [LiNb27O75]14- polyoxoanions. This marks the first PONb-protected superatomic AgNC, designated as {Ag8@(LiNb27O75)6} (Ag8@Nb162), with an aesthetically spherical core-shell structure. The crystalline Ag8@Nb162 is stable under ambient conditions, What's more, it is water-soluble and able to maintain its molecular cluster structure intact in water. Further, the stable small [Ag8]6+ AgNC has interesting temperature- and pH-dependent reversible fluorescence response, based on which a multiple optical encryption mode for anti-counterfeit technology was demonstrated. This work offers a promising avenue for the synthesis of fascinating and stable PONb-protected AgNCs and sheds light on the development of new-type optical functional materials.

Optical Images of Molecular Vibronic Couplings from Tip-Enhanced Fluorescence Excitation Spectroscopy

Tip-based photoemission spectroscopic techniques have now achieved subnanometer resolution that allows visualization of the chemical structure and even the ground-state vibrational modes of a single molecule. However, the ability to visualize the interplay between electronic and nuclear motions of excited states, i.e., vibronic couplings, is yet to be explored. Herein, we theoretically propose a new technique, namely, tip-enhanced fluorescence excitation (TEFE). TEFE takes advantage of the highly confined plasmonic field and thus can offer a possibility to directly visualize the vibronic effect of a single molecule in real space for arbitrary excited states in a given energy window. Numerical simulations for a single porphine molecule confirm that vibronic couplings originating from Herzberg-Teller (HT) active modes can be visually identified. TEFE further enables high-order vibrational transitions that are normally suppressed in the other plasmon-based processes. Images of the combination vibrational transitions have the same pattern as that of their parental HT active mode's fundamental transition, providing a direct protocol for measurements of the activity of Franck-Condon modes of selected excited states. These findings strongly suggest that TEFE is a powerful strategy to identify the involvement of molecular moieties in the complicated electron-nuclear interactions of the excited states at the single-molecule level.

Embedding Azobenzol-Decorated Tetraphenylethylene into the Polymer Matrix to Implement a Ternary Memory Device with High Working Temperature/Humidity

The development of new high-density memories that can work in harsh environments such as high temperature and humidity will be significant for some special occasions such as oil and geothermal industries. Herein, a facial strategy for implementing a ternary memory device with high working temperature/humidity was executed. In detail, an asymmetric aggregation-induced-emission active molecule (azobenzol-decorated tetraphenylethylene, i.e., TPE-Azo) was embedded into flexible poly(ethylene-alt-maleic anhydride) (PEM) to prepare a TPE-Azo@PEM composite, which served as an active layer to fabricate the FTO/TPE-Azo@PEM/Ag device. This device can demonstrate excellent ternary memory performances with a current ratio of 1:10^4.2:10^1.6 for "OFF", "ON1", and "ON2" states. Specially, it can exhibit good environmental endurance at high working temperature (350 °C) and humidity (RH = 90%). The ternary memory mechanism can be explained as the combination of aggregation-induced current/conductance and conformational change-induced charge transfer in the TPE-Azo molecule, which was verified by Kelvin probe force microscopy, UV-vis spectra, X-ray diffraction, and single-crystal structural analysis. This strategy can be used as a universal method for the construction of high-density multilevel memristors with good environmental tolerance.

Modular engineering of gold-silver nanocluster supermolecular structure endow strong electrochemiluminescence for ultrasensitive bioanalysis

Here, the gold-silver nanocluster supramolecular network (AuAg NCs) is synthesized by the assembly of Au nanoclusters (Au NCs) and Ag NCs via host-guest complexation between 6-aza-2-thiothymine as the stabilizer of Au NCs and L-arginine as the stabilizer of Ag NCs in solution, whose electrochemiluminescence (ECL) emission is not only exceptionally stronger than that of discrete monometallic NCs, but also more significant than that of agminated monometallic NCs. The dramatically enhanced ECL emission of self-assembled AuAg NCs originates from the synergistic effect of aggregation-induced enhancement and silver effect in gold catalysis. As a proof of concept, the self-assembled AuAg NCs is successfully applied in the ultrasensitive detection of breast cancer biomarker microRNAs-21 (miR-21), which guides a new pathway for creating high-quality nano-optical elements in chemical sensors, biological imaging, and lightemitting devices.

Electric Field Controlled Single-Molecule Optical Switch by Through-Space Charge Transfer State

Controlling the photon emission property of a single molecule is an important goal for nano-optics. We propose here a new mechanism for a single-molecule optical switch that utilizes the in situ electric field (EF) in biased metallic nanojunctions to control photon emission of molecules with through-space charge transfer (TSCT) excited states. The EF-induced Stark effect is capable of flipping the order of the bright noncharge transfer state and dark TSCT state, resulting in the anticipated switching behavior. The proposed mechanism was theoretically verified by scanning tunneling microscope-induced electroluminescence from a naphtalenediimide cyclophane molecule under experimentally accessible conditions. Simulations show that the proposed switching effect can be obtained by changing either bias polarity, which alters the polarization of the field, or tip-height, which affects the magnitude of the field. Our finding indicates that the in situ EF could play an important role in the design of optoelectronic molecular devices.

Influence of nonlocal dielectric response on the Au tip-enhanced fluorescence effect

Tip-enhanced fluorescence (TEF) with ultra-high detection sensitivity and spatial resolution has been a powerful characterization technique in the study of surface science and life science. Herein, a systematically theoretical investigation in the visible range had been performed to study TEF properties of a single molecule located inside a nanogap formed by Au tip and substrate. In the strong localized surface plasmon coupling effect, the contribution of nonlocal dielectric response to the fluorescence quantum yield as well as radiative and energy dissipated decay rates were calculated. It is found that the nonlocal dielectric effects become comparable to the radiative and energy dissipated decay rates with the increasing of the tip-molecule distance, as a result, the nonlocal dielectric effect significantly suppresses the fluorescence process. The huge excitation enhancement at the shorter tip-molecule distance can efficiently compensate the low quantum yield, leading to the great fluorescence enhancement. The results show that the maximum enhancement obtained from the calculations can reach as high as four orders of magnitude by optimizing the tip-molecule distance. These results are not only helpful to our understanding of the TEF mechanism but also valuable for its further applications.

Aggregation-Induced Electrochemiluminescence of Conjugated Pdots Containing a Trace Ir(III) Complex: Insights into Structure-Property Relationships

An approach to the design of iridium(III)-contained polytetraphenylethene Pdots that could exhibit highly efficient electrochemiluminescence (ECL) was proposed. The relationships of ECL performance between the iridium complex-embedded and end-capped aggregation-induced emission (AIE) active Pdots in aqueous media were investigated for the first time. The iridium complexes with cyclometalated ligand 6-phenylphenanthridine (pphent) were incorporated into the copolymers by either embedding (P0, P2-P5) or end-capping (P1) into the backbone via an ancillary 2,2'-bipyridine (bpy) ligand. Subsequently, the corresponding Pdots of P0-P5 encapsulated with poly(styrene-co-maleicanhydride) could be obtained by the nanoprecipitation method. Compared to Pdots0, Pdots2-Pdots5 with (pphent)₂Ir(bpy) (M1) complex embedding, as the iridium complex content increases, ECL signals were decreased in the order of Pdots0 > Pdots2 > Pdots3 > Pdots4 > Pdots5; whereas among these Pdots of P0-P5, Pdots1 with M1 complex end-capping exhibited the highest ECL efficiency (relative to a Ru(bpy)₃²⁺ system of 18.9%) and 4.7-fold enhancement of the ECL signal compared to the parent Pdots of P0, which was mainly attributed to the good film conductivity of the completely conjugated architectures, thus prompting the intramolecular electron transfer. This work opened new avenues for designing highly efficient ECL emitters.

Real-time detection of single-molecule reaction by plasmon-enhanced spectroscopy

Determining structural transformations of single molecules (SMs) is an important fundamental scientific endeavor. Optical spectroscopies are the dominant tools used to unravel the physical and chemical features of individual molecules and have substantially contributed to surface science and biotechnology. In particular, Raman spectroscopy can identify reaction intermediates and reveal underlying reaction mechanisms; however, SM Raman experiments are subject to intrinsically weak signal intensities and considerable signal attenuation within the spectral dispersion systems of the spectrometer. Here, to monitor the structural transformation of an SM on the millisecond time scale, a plasmonic nanocavity substrate has been used to enable Raman vibrational and fluorescence spectral signals to be simultaneously collected and correlated, which thus allows a detection of photo-induced bond cleavage between the xanthene and phenyl group of a single rhodamine B isothiocyanate molecule in real time. This technique provides a novel method for investigating light-matter interactions and chemical reactions at the SM level.

Theoretical assessment of vibrationally resolved C1s X-ray photoelectron spectra of simple cyclic molecules

Vibrationally-resolved K-edge XPS simulation by the full/equivalent core-hole method is enabled and assessment of the C1s spectra of cyclic molecules shows excellent/acceptable agreement with the experiment.

Organic Room-Temperature Phosphorescence with Strong Circularly Polarized Luminescence Based on Paracyclophanes

Pure organic materials with intrinsic room-temperature phosphorescence typically rely on heavy atoms or heteroatoms. Two different strategies towards constructing organic room-temperature phosphorescence (RTP) species based upon the through-space charge transfer (TSCT) unit of [2.2]paracyclophane (PCP) were demonstrated. Materials with bromine atoms, PCP-BrCz and PPCP-BrCz, exhibit RTP lifetime of around 100 ms. Modulating the PCP core with non-halogen-containing electron-withdrawing units, PCP-TNTCz and PCP-PyCNCz, successfully elongate the RTP lifetime to 313.59 and 528.00 ms, respectively, the afterglow of which is visible for several seconds under ambient conditions. The PCP-TNTCz and PCP-PyCNCz enantiomers display excellent circular polarized luminescence with dissymmetry factors as high as -1.2×10^-2 in toluene solutions, and decent RTP lifetime of around 300 ms for PCP-TNTCz enantiomers in crystalline state.

Synthesis, Characterization, and Properties of Tetraphenylethylene-Based Tetrakis-NHC Ligands and Their Metal Complexes

The development of highly emissive dinuclear Ag^I or Au^I complexes [M₂ L](PF₆ )₂ (L=2 a, 2 b; M=Ag, Au) derived from tetraphenylethylene (TPE)-based tetrabenzimidazolin-2-ylidene ligands is reported. The new complexes exhibit a remarkable fluorescence enhancement compared to their parent benzimidazolium salts. The quantum yield (Φ_F ) value for salt H₄ -2 a(PF₆ )₄ in dilute solution (c=10^-5  m) was found to be less than 1 %, whereas its metal complexes show Φ_F values up to 55 % at similar solution concentration. This observation can be attributed to the rigidification of the TPE skeleton upon metalation resulting in a restriction of the intramolecular rotation of the phenyl groups. Functionalization of the complexes [M₂ 2 b](PF₆ )₂ (M=Ag, Au) with terminal coumarin groups and subsequent photoirradiation yielded the complexes [M₂ 3 b](PF₆ )₂ (M=Ag, Au) bearing a new type of ligand with an unaffected TPE moiety.

Spiro-Functionalized Diphenylethenes: Suppression of a Reversible Photocyclization Contributes to the Aggregation-Induced Emission Effect

Many aggregation-induced emission (AIE) materials are featured by the diphenylethene (DPE) moiety which exhibits rich photophysical and photochemical activities. The understanding of these activities behind AIE is essential to guide the design of fluorescent materials with improved performance. Herein by fusing a flexible DPE with a rigid spiro scaffold, we report a class of novel deep-blue material with solid-state fluorescent quantum yield (Φ_F) up to 99.8%. Along with the AIE phenomenon, we identified a reversible photocyclization (PC) on DPE with visible chromism, which is, on the contrary, popularized in solutions but blocked by aggregation. We studied the steric and electronic effects of structural perturbation and concluded that the PC is a key process behind the RIMs (restriction of intramolecular motions) mechanism for these materials. Mitigation of the PC leads to enhanced fluorescence in solutions and loss of the AIE characteristics.