Remarkable Multichannel Conductance of Novel Single-Molecule Wires Built on Through-Space Conjugated Hexaphenylbenzene

Understanding Emergent Complexity from a Single-Molecule Perspective

Molecules, with structural, scaling, and interaction diversities, are crucial for the emergence of complex behaviors. Interactions are essential prerequisites for complex systems to exhibit emergent properties that surpass the sum of individual component characteristics. Tracing the origin of complex molecular behaviors from interactions is critical to understanding ensemble emergence, and requires insights at the single-molecule level. Electrical signals from single-molecule junctions enable the observation of individual molecular behaviors, as well as intramolecular and intermolecular interactions. This technique provides a foundation for bottom-up explorations of emergent complexity. This Perspective highlights investigations of various interactions via single-molecule junctions, including intramolecular orbital and weak intermolecular interactions and interactions in chemical reactions. It also provides potential directions for future single-molecule junctions in complex system research.

Singlet Fission in a New Series of Systematically Designed Through-space Coupled Tetracene Oligomers

Singlet fission (SF) holds great promise for current photovoltaic technologies, where tetracenes, with their relatively high triplet energies, play a major role for application in silicon-based solar cells. However, the SF efficiencies in tetracene dimers are low due to the unfavorable energetics of their singlet and triplet energy levels. In the solid state, tetracene exhibits high yields of triplet formation through SF, raising great interest about the underlying mechanisms. To address this discrepancy, we designed and prepared a novel molecular system based on a hexaphenylbenzene core decorated with 2 to 6 tetracene chromophores. The spatial arrangement of tetracene units, induced by steric hindrance in the central part, dictates through-space coupling, making it a relevant model for solid-state chromophore organization. We then revealed a remarkable increase in SF quantum yield with the number of tetracenes, reaching quantitative (196 %) triplet pair formation in hexamer. We observed a short-lived correlated triplet pair and limited magnetic effects, indicating ineffective triplet dissociation in these through-space coupled systems. These findings emphasize the crucial role of the number of chromophores involved and the interchromophore arrangement for the SF efficiency. The insights gained from this study will aid designing more efficient and technology-compatible SF systems for applications in photovoltaics.

The effect of weak π-π interactions on single-molecule electron transport properties of the tetraphenylethene molecule and its derivatives: a first-principles study

Intramolecular π-π interactions are a significant research focus in fields such as chemistry, biology, and materials science. Different configurations of benzene-benzene moieties within a molecule can affect the magnitude of their π-π interactions, consequently influencing the electronic transport capabilities of the molecule. In this study, we designed three π-conjugated molecules, TPEM, TPEEM, and TEEPM, based on tetraphenylethene (TPE). These three molecules exhibit three distinct π-conjugated structures: linear cis-π-conjugation, linear trans-π-conjugation, and cross-π-conjugation. Thereinto, TPEM and TPEEM molecules share the same TPE core, with identical π-π interaction distances, while the TEEPM molecule has acetylene groups between the TPE units, thereby increasing the π-π interaction distances between the benzene moieties. Using density functional theory calculations combined with non-equilibrium Green's function (DFT+NEGF), our results reveal that the conductance order of different π-conjugated structures in TPEM and TPEEM molecules is as follows: cis > cross ≈ trans. Through analysis of transmission spectra, transmission pathways, and the innermost π orbitals, we find that in TPEM and TPEEM molecules, the cis- and cross-π-conjugated structures exhibit π-π interactions between benzene moieties and provide special through-space electron transport pathways, enhancing their electronic transport capabilities in coordination with the bonded molecular framework, whereas their trans-conjugated structures only allow electron transport along the molecular backbone. In contrast, in TEEPM molecule, due to the absence of π-π interactions, the conductance of different π-conjugated structures is primarily determined by the molecular backbone and follows the order: trans > cis > cross. These findings provide a theoretical basis for designing single-molecule electronic devices with multiple electron channels based on intramolecular π-π interactions.

Preparation and p-phenylenediamine detection mechanism of a dialdehyde cellulose and a 7-amino-4-methylcoumarin-based fluorescent probe

A novel fluorescent probe, fluorescent dialdehyde cellulose (FDAC), was prepared to detect p-phenylenediamine (PPD) in water samples conveniently and quickly. This was achieved by grafting 7-amino-4-methylcoumarin (AMC) onto dialdehyde cellulose (DAC) via an aldol-amine condensation reaction. This method is greener, more economical, and simpler than existing methods for preparing fluorescent probes. The probe was found to be more effective for PPD detection in polar solvents, with less interference from pH and other compounds present in the sample matrix. The photoluminescence of FDAC at λex/λem = 340/430 nm was statically quenched by PPD, allowing for accurate detection within the range of 10-100 μmol/L under optimal conditions, with a detection limit of 3.2 μmol/L (3 σ/s). Meanwhile, the Schiff base (-C=N- group) generated by the condensation of DAC and AMC increased the reaction activity of the fluorescent moiety and changed the AMC conjugated structure, making FDAC more susceptible to aminolysis with PPD than AMC. This study presents a promising solution for fluorescence detection of aniline compounds, with significant potential for application in fields such as environmental analysis.

Space and Bond Synergistic Conjugation Controlling Multiple-Aniline NIR-II Absorption for Photoacoustic Imaging Guided Photothermal Therapy

Currently, clinical photothermal therapy (PTT) is greatly limited by the poor tissue penetration of the excitation light sources in visible (390-780 nm) and first near-infrared (NIR-I, 780-900 nm) window. Herein, based on space and bond synergistic conjugation, a multiple-aniline organic small molecule (TPD), is synthesized for high-efficiency second near-infrared (NIR-II, 900-1700 nm) photoacoustic imaging guided PTT. With the heterogeneity of six nitrogen atoms in TPD, the lone electrons on the nitrogen atom and the π bond orbital on the benzene ring form multielectron conjugations with highly delocalized state, which endowed TPD with strong NIR-II absorption (maximum peak at 925 nm). Besides, according to the single molecular reorganization, the alkyl side chains on TPD make more free space for intramolecular motion to enhance the photothermal conversion ability. Forming TPD nanoparticles (NPs) in J-aggregation, they show a further bathochromic-shifted absorbance (maximum peak at 976 nm) as well as a high photothermal conversion efficiency (66.7%) under NIR-II laser irradiation. In vitro and in vivo experiments demonstrate that TPD NPs can effectively inhibit the growth of tumors without palpable side effects. The study provides a novel NIR-II multiple-aniline structure based on multielectron hyperconjugation, and opens a new design thought for photothermal agents.

Quasi-Free Electron States Responsible for Single-Molecule Conductance Enhancement in Stable Radical

Stable organic radicals, which possess half-filled orbitals in the vicinity of the Fermi energy, are promising candidates for electronic devices. In this Letter, using a combination of scanning-tunneling-microscopy-based break junction (STM-BJ) experiments and quantum transport theory, a stable fluorene-based radical is investigated. We demonstrate that the transport properties of a series of fluorene derivatives can be tuned by controlling the degree of localization of certain orbitals. More specifically, radical 36-FR has a delocalized half-filled orbital resulting in Breit-Wigner resonances, leading to an unprecedented conductance enhancement of 2 orders of magnitude larger than the neutral nonradical counterpart (36-FOH). In other words, conversion from a closed-shell fluorene derivative to the free radical in 36-FR opens an electron transport path which massively enhances the conductance. This new understanding of the role of radicals in single-molecule junctions opens up a novel design strategy for single-molecule-based spintronic devices.

Conductance of o-carborane-based wires with different substitution patterns

Here, we report the synthesis, structure, and single-molecule conductance of three o-carborane-based molecular wires (ortho-, meta- and para-CN) with multiple conduction channels. The effect of connectivity in target wires compared with the corresponding phenyl-centered wires was studied using the scanning tunneling microscope break junction (STM-BJ) technique and theoretical calculations. Interestingly, the three-dimensional structure in o-carborane-based wires can effectively promote the through-space transmission paths or the formation of stable molecular junctions compared to the corresponding phenyl-centered wires. Moreover, the significant conductance difference of o-carborane-based wires was due to the combination of multiple conduction channels and quantum interference. Understanding the effects of different bridging groups and anchor group substitution patterns provides guidelines for designing o-carborane-based multichannel molecular wires.

Effects of Through-Bond and Through-Space Conjugations on the Photoluminescence of Small Aromatic and Aliphatic Aldimines

Through-bond conjugation (TBC) and/or through-space conjugation (TSC) determine the photophysical properties of organic luminescent compounds. No systematic studies have been carried out to understand the transition from aromatic TBC to non-aromatic TSC on the photoluminescence of organic luminescent compounds. In this work, a series of small aromatic and aliphatic aldimines were synthesized. For the aromatic imines, surprisingly, N,1-diphenylmethanimine with the highest TBC is non-emissive, while N-benzyl-1-phenylmethanimine and N-cyclohexyl-1-phenylmethanimine emit bright fluorescence in aggregate states. The aliphatic imines are all emissive, and their maximum emission wavelength decreases while the quantum yield increases with a decrease in steric hindrance. The imines show concentration-dependent and excitation-dependent emissions. Theoretical calculations show that the TBC extents in the aromatic imines are not strong enough to induce photoluminescence in a single molecule state, while the intermolecular TSC becomes dominant for the fluorescence emissions of both aromatic and aliphatic imines in aggregate states, and the configurations and spatial conformations of the molecules in aggregate states play a key role in the formation of effective TSC. This study provides an understanding of how chemical and spatial structures affect the formation of TBC and TSC and their functions on the photoluminescence of organic luminescent materials.

Single-molecule nano-optoelectronics: insights from physics

Single-molecule optoelectronic devices promise a potential solution for miniaturization and functionalization of silicon-based microelectronic circuits in the future. For decades of its fast development, this field has made significant progress in the synthesis of optoelectronic materials, the fabrication of single-molecule devices and the realization of optoelectronic functions. On the other hand, single-molecule optoelectronic devices offer a reliable platform to investigate the intrinsic physical phenomena and regulation rules of matters at the single-molecule level. To further realize and regulate the optoelectronic functions toward practical applications, it is necessary to clarify the intrinsic physical mechanisms of single-molecule optoelectronic nanodevices. Here, we provide a timely review to survey the physical phenomena and laws involved in single-molecule optoelectronic materials and devices, including charge effects, spin effects, exciton effects, vibronic effects, structural and orbital effects. In particular, we will systematically summarize the basics of molecular optoelectronic materials, and the physical effects and manipulations of single-molecule optoelectronic nanodevices. In addition, fundamentals of single-molecule electronics, which are basic of single-molecule optoelectronics, can also be found in this review. At last, we tend to focus the discussion on the opportunities and challenges arising in the field of single-molecule optoelectronics, and propose further potential breakthroughs.

Research Progress of Intramolecular π-Stacked Small Molecules for Device Applications

Organic semiconductors can be designed and constructed in π-stacked structures instead of the conventional π-conjugated structures. Through-space interaction (TSI) occurs in π-stacked optoelectronic materials. Thus, unlike electronic coupling along the conjugated chain, the functional groups can stack closely to facilitate spatial electron communication. Using π-stacked motifs, chemists and materials scientists can find new ways for constructing materials with aggregation-induced emission (AIE), thermally activated delayed fluorescence (TADF), circularly polarized luminescence (CPL), and room-temperature phosphorescence (RTP), as well as enhanced molecular conductance. Organic optoelectronic devices based on π-stacked molecules have exhibited very promising performance, with some of them exceeding π-conjugated analogues. Recently, reports on various organic π-stacked structures have grown rapidly, prompting this review. Representative molecular scaffolds and newly developed π-stacked systems could stimulate more attention on through-space charge transfer the well-known through-bond charge transfer. Finally, the opportunities and challenges for utilizing and improving particular materials are discussed. The previous achievements and upcoming prospects may provide new insights into the theory, materials, and devices in the field of organic semiconductors.

Achieving Multiple Quantum-Interfered States via Through-Space and Through-Bond Synergistic Effect in Foldamer-Based Single-Molecule Junctions

The construction of multivalued logic circuits by multiple quantum-interfered states at the molecular level can make full use of molecular diversity and versatility, broadening the application of molecular electronics. Understanding charge transport through different conducting pathways and how they interact with each other in molecules with a secondary structure is an indispensable foundation to achieve this goal. Herein, we elucidate the synergistic effect from through-space and through-bond conducting pathways in foldamers derived from ortho-pentaphenylene by the separate modulation on these pathways. The shrinkage of central heterocycles' sizes allows foldamers to stack with larger overlap degrees, resulting in level-crossing and thus transformation from constructive quantum interference (CQI) to destructive quantum interference (DQI) in a through-space pathway. The alteration of central heterocycles' connection sites enhances through-bond conjugation, leading to amplified contribution from a through-bond pathway. The enhanced through-bond pathway destructively interferes with the through-space pathway, exerting a suppression effect on transmission. Therefore, four quantum-interfered states of through-space and through-bond combination are generated, including through-space CQI-dominated states, through-space DQI-dominated states, through-space CQI states with through-bond suppression, and through-space DQI states with through-bond suppression. These findings enable us to regulate charge transport within high-order structures via multiple conducting pathways and provide a proof of concept to construct multivalued logic circuits.

Transport Modulation Through Electronegativity Gating in Multiple Nitrogenous Circuits

Investigating the correlations of electron transport between multiple channels shows vital promises for the design of molecule-scale circuits with logic operations. To control the electron transport through multiple channels, the modulation of electronegativity shows an effective frontier orbit control method with high universality to explore the interactions between transport channels. Here, two series of compounds with a single nitrogenous conductive channel (Sg) and dual-channels (Db) are designed to explore the influence of electronegativity on electron tunneling transport. Single-molecule conductance measured via the scanning tunneling microscope break junction technique (STM-BJ) reveals that the conductance of Db series is significantly suppressed as the electronegativity of nitrogen becomes negative, while the suppression on Sg is less obvious. Theoretical calculations confirm that the effect of electronegativity extends to a dispersive range of molecular frameworks owing to the delocalized orbital distribution from the dual-channel structure, resulting in a more significant conductance suppression effect than that on the single-channel. This study provides the experimental and theoretical potentials of electronegativity gating for molecular circuits.

ortho-Terphenylene Viologens with Through-Space Conjugation for Enhanced Photocatalytic Oxidative Coupling and Hydrogen Evolution

A series of novel ortho-terphenylene viologen derivatives (o-TPV²⁺) with through-space conjugation (TSC) via the combination of ortho-terphenylene skeletons with viologen structure is reported. Their optoelectronic properties can be adjusted by N-arylation or N-alkylation reactions. Compared with other viologen derivatives, o-TPV²⁺ not only exhibits strong photoluminescence but also retards the charge recombination process and stabilizes the diradical state without forming a quinoid structure due to the special TSC effect. Based on their special redox characteristics, o-TPV²⁺ was applied to the photocatalytic oxidative coupling of benzylamine with 96% yield. In addition, pTA-o-TPV²⁺ (tethered with p-toluic acid)-modified g-C₃N₄ was used for visible-light-driven hydrogen production for the first time, exceeding 15 times the rate over unmodified g-C₃N₄.

In situ lattice tuning of quasi-single-crystal surfaces for continuous electrochemical modulation

The ability to control the atomic-level structure of a solid represents a straightforward strategy for fabricating high-performance catalysts and semiconductor materials. Herein we explore the capability of the mechanically controllable surface strain method in adjusting the surface structure of a gold film. Underpotential deposition measurements provide a quantitative and ultrasensitive approach for monitoring the evolution of surface structures. The electrochemical activities of the quasi-single-crystalline gold films are enhanced productively by controlling the surface tension, resulting in a more positive potential for copper deposition. Our method provides an effective way to tune the atom arrangement of solid surfaces with sub-angstrom precision and to achieve a reduction in power consumption, which has vast applications in electrocatalysis, molecular electronics, and materials science.

We reported a new method capable of adjusting the lattice structure of solid surfaces with sub-angstrom precision and achieved in situ and continuous control over electrochemical activity.

Recent Advances in Clusteroluminescence

Clusteroluminescence is a phenomenon whereby the aggregation or clustering of non-conjugated electron-rich units leads to the emission of light at long wavelengths. This phenomenon was first discovered in poly(amido amine) (PAMAM) dendrimers. In recent years, clusteroluminescence has attracted growing research interest and its photophysical properties and mechanism have been thoroughly studied. In this review, we first briefly introduce the development of different types of clusteroluminogens. Then we highlight recent developments in clusteroluminescence, including mechanistic studies, the disclosure of room-temperature phosphorescence, and the extension of emission to the longer-wavelength region. Lastly, we demonstrate a few applications in various fields. With advantages such as being earth-abundant, biocompatible and biodegradable, clusteroluminogens are envisioned to be commonplace in the future.

Mechanical single-molecule potentiometers with large switching factors from ortho-pentaphenylene foldamers

Molecular potentiometers that can indicate displacement-conductance relationship, and predict and control molecular conductance are of significant importance but rarely developed. Herein, single-molecule potentiometers are designed based on ortho-pentaphenylene. The ortho-pentaphenylene derivatives with anchoring groups adopt multiple folded conformers and undergo conformational interconversion in solutions. Solvent-sensitive multiple conductance originating from different conformers is recorded by scanning tunneling microscopy break junction technique. These pseudo-elastic folded molecules can be stretched and compressed by mechanical force along with a variable conductance by up to two orders of magnitude, providing an impressively higher switching factor (114) than the reported values (ca. 1~25). The multichannel conductance governed by through-space and through-bond conducting pathways is rationalized as the charge transport mechanism for the folded ortho-pentaphenylene derivatives. These findings shed light on exploring robust single-molecule potentiometers based on helical structures, and are conducive to fundamental understanding of charge transport in higher-order helical molecules.

The inspiration and challenge for through-space charge transfer architecture: from thermally activated delayed fluorescence to non-linear optical properties

Organic molecules consisting of electron donor (D) and electron acceptor (A) subunits linked by π-conjugated bridges are promising building blocks for thermally activated delayed fluorescence (TADF) and non-linear optics (NLO) materials due to their intramolecular charge transfer (CT) processes in response to external stimuli. According to the electron interaction pattern, the CT process in D-π-A architectures can be divided into two categories, through-bond/-space charge transfer (TB/TSCT). To date, research into the TADF properties of TSCT characteristic molecules has since seen significant growth. In fact, TSCT characteristic materials show great advantages in such NLO responses. In this perspective, we first briefly introduced the basic principles of NLO and TADF effects. Successively, we discuss the influence of TBCT and TSCT patterns on NLO and TADF properties, especially for TSCT characteristic. In the final part, we address the diversity and potential advantages of TSCT characteristic molecules as high-performance NLO materials. With these, it is expected that the greater structural flexibility of spatial conjugation can bring more functionality to NLO materials in the future.

Multichannel transport in conjugated polymers based on through-space conjugated naphthalene

The introduction of through-space transport channels is beneficial for improving the redox activity and stability of electropolymerized polymers.

Nanomaterials with Supramolecular Assembly Based on AIE Luminogens for Theranostic Applications

One of the major pursuits of biomedical science is to develop advanced strategies for theranostics, which is expected to be an effective approach for achieving the transition from conventional medicine to precision medicine. Supramolecular assembly can serve as a powerful tool in the development of nanotheranostics with accurate imaging of tumors and real-time monitoring of the therapeutic process upon the incorporation of aggregation-induced emission (AIE) ability. AIE luminogens (AIEgens) will not only enable fluorescence imaging but will also aid in improving the efficacy of therapies. Furthermore, the fluorescent signals and therapeutic performance of these nanomaterials can be manipulated precisely owing to the reversible and stimuli-responsive characteristics of the supramolecular systems. Inspired by rapid advances in this field, recent research conducted on nanotheranostics with the AIE effect based on supramolecular assembly is summarized. Here, three representative strategies for supramolecular nanomaterials are presented as follows: a) supramolecular self-assembly of AIEgens, b) the loading of AIEgens within nanocarriers with supramolecular assembly, and c) supramolecular macrocycle-guided assembly via host-guest interactions. Meanwhile, the diverse applications of such nanomaterials in diagnostics and therapeutics have also been discussed in detail. Finally, the challenges of this field are listed in this review.

Simultaneous Suppression of π- and σ-Transmission in π-Conjugated Molecules

Molecular dielectric materials require ostensibly conflicting requirements of high polarizability and low conductivity. As previous efforts toward molecular insulators focused on saturated molecules, it remains an open question whether π- and σ-transport can be simultaneously suppressed in conjugated systems. Here, we demonstrate that there are conjugated molecules where the σ-transmission is suppressed by destructive σ-interference, while the π-transmission can be suppressed by a localized disruption of conjugation. Using density functional theory, we study the Landauer transmission and ballistic current density, which allow us to determine how the transmission is affected by various structural changes in the molecule. We find that in para-linked oligophenyl rings the σ-transmission can be suppressed by changing the remaining hydrogens to methyl groups due to the inherent gauche-like structure of the carbon backbone within a benzene ring, similar to what was previously seen in saturated systems. At the same time, the methyl groups fulfill a dual purpose as they modulate the twist angle between neighboring phenyl rings. When neighboring rings are orthogonal to each other, the transmission through both π- and σ-systems is effectively suppressed. Alternatively, breaking conjugation in a single phenyl ring by saturating two carbons atoms with two methyl substituents on each carbon, results in suppressed π- and σ-transport independent of dihedral angle. These two strategies demonstrate that methyl-substituted oligophenyls are promising candidates for the development of molecular dielectric materials.

Through-Space Conjugation: An Effective Strategy for Stabilizing Intramolecular Charge-Transfer States

Intramolecular charge transfer (ICT) has significant impacts on organic optoelectronic materials, photochemistry, biotechnology, and so on. However, it is hard to stabilize the ICT state because of the rapid nonradiative charge recombination process, which often quenches light emission. In this work, we use new foldamers of the protonated pyridine-modified tetraphenylethene derivatives that possess through-space conjugation (TSC) characters as the models to study the impact of TSC on the ICT state. Steady and transient spectroscopies illustrate that the lifetime of the ICT state in the molecule with strong TSC can be much longer than those of molecules without TSC, giving rise to a higher fluorescence quantum yield. By combining the theoretical calculations, we demonstrate that the strong TSC can stabilize the ICT state and slow the charge recombination rate by more efficiently dispersing charges. This is a conceptually new design strategy for functional optoelectronic materials that require more stable ICT states.

Through-Space Conjugation: A Thriving Alternative for Optoelectronic Materials

Efficient electronic coupling is the key to constructing optoelectronic functional π systems. Generally, the delocalization of π electrons must comply with the framework constructed by covalent bonds (typically σ bonds), representing classic through-bond conjugation. However, through-space conjugation offers an alternative that achieves spatial electron communication with closely stacked π systems instead of covalent bonds thus enabling multidimensional energy and charge transport. Because of the ever-accelerating advances of through-space conjugation studies, researchers are inspired greatly by the beauty of through-space conjugated systems and their potential in high-tech applications. In this mini review, we introduce some representative and newly developed π systems having the through-space conjugation feature. In addition to discussing the profound impacts of through-space conjugation on the luminescence properties and charge transport, we will review some impressive findings of distinctive molecules with attractive characteristics, such as aggregation-induced emission, thermally activated delayed fluorescence, bipolar charge transport, and multichannel. These achievements may bring about new breakthroughs of theory, materials, and devices in the fields of organic electronics and molecular electronics.

Tuning Emission Wavelength of Polymorphous Crystal via Controllable Alkyl Chain Stacking and Its Vapor- and Thermo-Responsive Fluorescence

Tuning fluorescence colour of solid-state materials has become a topic of increasing interest for both fundamental mechanism study and practical applications such as sensors, optical recording and security printing. In this work, a fluorescent colour tuneable molecule BA-C16 is rationally designed and facilely synthesized by attaching flexible long alkyl chains to 2-hydroxybenzophenone azine (BA), which shows both aggregation-induced emission (AIE) and excited-state intramolecular proton transfer (ESIPT) characteristics. Compared to BA, the simple introduction of long alkyl chains in BA-C16 leads to an emission wavelength redshift from 542 to 558 nm. This strategy of extending emission wavelength is rarely reported, and is ascribed to the enlarged through-space π-conjugation between interplanar molecules in the aggregate of BA-C16. Three crystals of BA-C16 are obtained with green, yellowish green and yellow emission. According to characterization by X-ray crystallography, X-ray powder diffraction and differential scanning calorimetry, alkyl chains play an important role in inducing different stacking modes of the three crystals, which further leads to polymorph-dependent fluorescence colour. BA-C16 exhibits tuneable solid-state fluorescence upon vapor fumigation, or annealing based on a transition between a "near-monomer" crystalline state and a "dimer" crystalline state. BA-C16 is further applied for rewritable fluorescence printing tuned by vapor- and thermal-treatment.

New Aggregation-Induced Delayed Fluorescence Luminogens With Through-Space Charge Transfer for Efficient Non-doped OLEDs

In this work, two tailor-made luminogens comprising of electron donors (acridine and phenoxazine) and acceptor (triazine) bridged by the through-space conjugated hexaphenylbenzene (HPB) are synthesized and characterized. Their thermal stability, electrochemical behaviors, crystal, and electronic structures, and photophysical properties are systematically investigated. The crystal and electronic structures reveal that the peripheral phenyls in HPB are closely aligned in a propeller-like fashion, rendering efficient through-space charge transfer between donor and electron moieties. These molecules display weak fluorescence with negligible delayed component in solutions but strong fluorescence with greatly increased delayed component upon aggregate formation, namely aggregation-induced delayed fluorescence (AIDF). Their neat films exhibit high photoluminescence quantum yields (PLQY), and prominent delayed fluorescence. The non-doped organic light-emitting diodes (OLEDs) based on these new luminogens exhibit excellent performance with maximum external quantum efficiency of 12.7% and very small efficiency roll-off of 2.7% at 1,000 cd m⁻². Designing AIDF molecules with through-space charge transfer could be a promising strategy to explore robust luminescent materials for efficient non-doped OLEDs.

Single-molecule conductance investigation of BDT derivatives: an additional pattern found to induce through-space channels beyond π–π stacking

Beyond π–π stacked benzene rings, non-bonded conducting channels are also confirmed in non-strict face-to-face aligned thiophenes or phenyl-thiophene in BDT derivatives.