Recent advances of folded tetraphenylethene derivatives featuring through-space conjugation

Switchable Dual Circularly Polarized Luminescence in Through-Space Conjugated Chiral Foldamers

Organic materials with switchable dual circularly polarized luminescence (CPL) are highly desired because they can not only directly radiate tunable circularly polarized light themselves but also induce CPL for guests by providing a chiral environment in self-assembled structures or serving as the hosts for energy transfer systems. However, most organic molecules only exhibit single CPL and it remains challenging to develop organic molecules with dual CPL. Herein, novel through-space conjugated chiral foldamers are constructed by attaching two biphenyl arms to the 9,10-positions of phenanthrene, and switchable dual CPL with opposite signs at different emission wavelengths are successfully realized in the foldamers containing high-polarizability substitutes (cyano, methylthiol and methylsulfonyl). The combined experimental and computational results demonstrate that the intramolecular through-space conjugation has significant contributions to stabilizing the folded conformations. Upon photoexcitation in high-polar solvents, strong interactions between the biphenyl arms substituted with cyano, methylthio or methylsulfonyl and the polar environment induce conformation transformation for the foldamers, resulting in two transformable secondary structures of opposite chirality, accounting for the dual CPL with opposite signs. These findings highlight the important influence of the secondary structures on the chiroptical property of the foldamers and pave a new avenue towards efficient and tunable dual CPL materials.

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.

Pyrazine as a More Efficient Luminophore than Benzene for Producing Red-Shifted and Enhanced Photoluminescence

The development of organic photoluminescent (PL) materials with red-shifted and enhanced emissions is beneficial to promoting their applications. Luminescent materials based on aromatic heterocycles (e.g., pyrazine) usually have red-shifted and enhanced photoluminescence compared with phenyl-based luminescent materials. In this work, the photoluminescence behaviors of pyrazine and its derivatives (o-dichloro-, o-dicyano-, and dichlorodicyano-substituted) are compared with those of benzene and its derivatives. All compounds exhibit fluorescence emissions ranging from blue to yellow, and the fluorescence emissions of pyrazinyl compounds are more red-shifted than those of phenyl compounds. Except for the o-dicyano-substituted compound, pyrazinyl compounds exhibit stronger fluorescence emissions than corresponding phenyl compounds in both pure substances and ethanol solutions. In addition, both 5,6-dichloro-2,3-dicyanopyrazine (P4) and 4,5-dichloro-1,2-dicyanobenzene (B4) exhibit room temperature phosphorescence, and the maximum delayed emission wavelength is red-shifted from 575 nm of B4 to 637 nm of P4. The energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital of the monomers of pyrazinyl compounds are reduced by 0.07-1.37 eV compared with the monomers of phenyl compounds, which is the fundamental reason for the red-shifted emissions of the pyrazinyl compounds. Moreover, compared to B4, the smaller molecular spacing in the P4 crystal structure facilitates interlayer electron transfer and hence the formation of more extended through-space conjugation, resulting in the red-shifted emission of P4. This work proves that pyrazine is a more efficient luminophore than benzene for constructing PL compounds with longer emission wavelengths and higher quantum yields, which are important in guiding the design and preparation of organic PL materials.

Hyperbranched Polymers for Organic Semiconductors

Hyperbranched polymers (HBPs) have attracted increasing attention owing to their distinct highly branched topological structures, resulting in unique properties and wide applications in organic semiconductors (OSCs). In this Review, recent progress in functional HBPs is outlined in the field of OSCs, including organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), dye-sensitized solar cells (DSSCs), and organic field effect transistors (OFETs), among others. Prospects of HBPs-based materials in OSCs are examined. The results revealed that multi-dimensional topologies not only regulate the electron (hole) transport but also adjust the film morphology, thereby affecting the efficiency and long life of organic electronic devices. Many studies showed the usefulness of HBPs as hole transport materials but reports dealing with n-type and ambipolar materials are still lacking. In addition, the interchain covalent bond in hyperbranched polymers could mitigate the damage caused by stretching, conducive to building stable flexible stretchable devices with long-term durability and good safety under harsh environmental conditions. Overall, the flexible stretchable design may enrich the applications of HBPs in organic semiconductors and provide new ideas for guiding the future design of functional organic semiconductor materials.

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.

Through-Space Conjugated Electron Transport Materials for Improving Efficiency and Lifetime of Organic Light-Emitting Diodes

Thermally stable electron transport (ET) materials with high electron mobility and high triplet state energy level are highly desired for the fabrication of efficient and stable organic light-emitting diodes (OLEDs). Herein, a new design strategy of constructing through-space conjugated folded configuration is proposed to explore robust ET materials, opposite to the widely used planar configuration. By bonding two quinolines to the 9,10-positions of phenanthrene, two novel folded molecules with high thermal and morphological stabilities and high triplet state energy levels (>2.7 eV) are created. These folded molecules possess excellent ET ability with electron mobilities of three orders of magnitude higher than those of linear and planar counterparts. Theoretical calculation and crystallography analysis demonstrate the through-space conjugated folded configuration has not only reduced reorganization energy but also enlarged charge transfer integral at various dimensions, bringing about efficient multi-dimensional ET, independent of molecular orientation. By adopting the folded molecule as ET layers, OLEDs with no matter delayed fluorescence or phosphorescence emitters can achieve high external quantum efficiencies and long operational lifetimes simultaneously. This work paves a new avenue towards robust ET materials to improve efficiency and stability of OLEDs.

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.

H2O2-responsive polymeric micelles with a benzil moiety for efficient DOX delivery and AIE imaging

Nano drug delivery is a promising domain in biomedical theranostics and has aroused more and more attention in recent years. We report here an amphiphilic polymer TPG1, bearing a H2O2-sensitive benzil and an AIE fluorophore tetraphenylethene (TPE) unit, which is able to self-assemble into spherical nanosized micelles in aqueous solution. Doxorubicin (DOX) can be encapsulated into TPG1 micelles efficiently with the loading capability of up to 59% by weight. The benzil moiety could be cleaved via the Baeyer-Villiger type reaction in the presence of H2O2, leading to the decomposition of TPG1 micelles and release of DOX. In vitro studies indicated that DOX-loaded TPG1 micelles can be internalized by cancer cells, followed by unloading encapsulated DOX under the stimulation of H2O2. The drug release process can be monitored by the AIE fluorescence from the degradation products containing a TPE moiety. MTT assays against HeLa and HepG2 cancer cells demonstrated that DOX-loaded micelles showed good anticancer efficacy. The polymer TPG1 and the corresponding decomposed products showed great biocompatibility. Our data suggest that TPG1 has the potential to be employed for the controlled drug delivery system.

A Versatile Naphthalimide-Sulfonamide-Coated Tetraphenylethene: Aggregation-Induced Emission Behavior, Mechanochromism, and Tracking Glutathione in Living Cells

A tetraphenylethene (TPE) derivative substituted with a sulfonyl-based naphthalimide unit (TPE-Np) was designed and synthesized. Its optical properties in solution and in the solid state were investigated. Photophysical properties indicated that the target molecule, TPE-Np, possessed aggregation-induced emission (AIE) behavior, although the linkage between TPE and the naphthalimide unit was nonconjugated. Additionally, it exhibited an unexpected, highly reversible mechanochromism in the solid state, which was attributed to the change in manner of aggregation between crystalline and amorphous states. On the other hand, a solution of TPE-Np in a mixture of dimethyl sulfoxide/phosphate-buffered saline was capable of efficiently distinguishing glutathione (GSH) from cysteine and homocysteine in the presence of cetyltrimethylammonium bromide. Furthermore, the strategy of using poly(ethylene glycol)-polyethylenimine (PEG-PEI) nanogel as a carrier to cross-link TPE-Np to obtain a water-soluble PEG-PEI/TPE-Np nanoprobe greatly improved the biocompatibility, and this nanoprobe could be successfully applied in the visualization of GSH levels in living cells.

Reaction-Based Color-Convertible Fluorescent Probe for Ferroptosis Identification

Ferroptosis is an iron-mediated, caspase-independent pathway of cell death that is accompanied with the accumulations of reactive oxygen species (ROS) and oxygenases, as well as being involved in many other pathophysiological procedures. However, specific and rapid monitoring of ferroptosis in living cells or tissues has not been achieved so far. Herein, a quinoxalinone-based fluorescent probe (termed as Quinos-4, or QS-4) with a reactive aromatic thioether moiety was designed for ferroptosis identification. Upon exposing it to high levels of ROS and hemeoxygenase-1 (HO-1), which are considered as the biochemical characteristics of ferroptosis, QS-4 could be oxidized into a sulfoxide derivative (QSO-4) and its original aggregation-induced enhanced red fluorescence emission could be converted to green fluorescence emission sharply. On the basis of this unique reaction-induced color conversion, this molecular probe can be employed for identifying the occurrence of ferroptosis both in vitro and in vivo.

Endoplasmic Reticulum-Targeted Fluorescent Nanodot with Large Stokes Shift for Vesicular Transport Monitoring and Long-Term Bioimaging

Herein, a highly stable aggregation-induced emission (AIE) fluorescent nanodot assembled by an amphiphilic quinoxalinone derivative-peptide conjugate, namely Quino-1-Fmoc-RACR (also termed as Q1-PEP), which exhibits large Stokes shift and an endoplasmic reticulum (ER)-targeting capacity for bioimaging is reported. It is found that the resulting nanodot can effectively enter the ER with high fluorescent emission. As the ER is mainly involved in the transport of synthesized proteins in vesicles to the Golgi or lysosomes, the Q1-PEP nanodot with ER-targeting capacity can be used to monitor vesicular transport inside the cells. Compared to conventional fluorescent dyes with small Stokes shifts, the self-assembled fluorescent nanodot shows superior resistance to photobleaching and aggregation-induced fluorescence quenching, and elimination of the spectra overlap with autofluorescence of biosubstrate owning to their AIE-active and red fluorescence emission characteristics. All these optical properties make the fluorescent nanodot suitable for noninvasive and long-term imaging both in vitro and in vivo.

A new blue AIEgen based on tetraphenylethene with multiple potential applications in fluorine ion sensors, mechanochromism, and organic light-emitting diodes

A new blue organoboron AIEgen with multiple potential applications is synthesized and studied.

Modulating aggregation-induced emission via a non-conjugated linkage of fluorophores to tetraphenylethenes

A fluorophore (DNS or NBD) is attached to tetraphenylethene via a non-conjugated bridge, endowing the resulting aggregates with distinct emission properties.