Competition between Arene-Perfluoroarene and Charge-Transfer Interactions in Organic Light-Harvesting Systems

Molecular- and Structural-Level Organic Heterostructures for Multicolor Photon Transportation

The rational design and the fine synthesis of organic heterostructures (OHSs) are the key steps toward integrated organic optoelectronics. Herein we have demonstrated a self-assembly approach of combining a molecular-level heterostructure with a structural-level heterostructure and regulating the noncovalent intermolecular interactions for the precise construction of OHSs: a vertical type of anthracene-TCNB heterostructure and a horizontal type of benzopyrene-TCNB heterostructure. The excellent structural compatibility and the low lattice mismatch rate of ∼5.8% between single-component microplates and cocrystal microwires allow anthracene and benzopyrene molecules to grow epitaxially on the cocrystal. Significantly, integrating the multicolor emission and the distinctive dimensional-dependent photon transportation properties of low-dimensional micro/nanostructures, the multicolor optical outputs are achieved via modulating the active/passive optical waveguides in OHSs. Our work exhibits the utilization of the multilevel heterostructure strategy, which boosts the rational design of OHSs for organic photonics.

Acceptor-induced cooperative supramolecular co-assembly with emissive charge-transfer for advanced supramolecular encryption

A novel acceptor-induced cooperative supramolecular co-assembly based on a dendritic electron donor and 1,2,4,5-tetracyanobenzene acceptor has been successfully developed. The resulting co-assembly is capable of displaying emissive charge transfer properties and intriguing vapo-fluorochromic behaviors, which can be used for supramolecular encryption applications with reversible authentication.

Effect of Alkyl Groups in Pyrene Chromophore on the Mechanical Response of Pyrene-Octafluoronaphthalene Co-Crystals

Changes in the photophysical properties of pyrene (Py)-octafluoronaphthalene (OFN) co-crystals (Py⋅OFN) upon mechanical stimuli are described herein. The Py⋅OFN co-crystal showed a mechano-induced bathochromic shift in emission, and a similar tendency was observed for the 1,3,6,8-tetramethylpyrene-OFN co-crystal. These shifts are due to disruption of the microscopic molecular orientation in the co-crystal, which allows for excimer formation. In sharp contrast to the parent Py⋅OFN and methyl-substituted Py-OFN co-crystals, no mechano-induced bathochromic shift was observed when longer alkyl chains were introduced to the 1-, 3-, 6-, and 8-positions of the Py chromophore. This photophysical opposability against mechanical stimuli could be explained by the orthogonally oriented alkyl groups on the Py ring, which existed between two Py cores like pillars. This fixed OFN to maintain the face-to-face alternatively stacked structure of the co-crystal and thus prevented the formation of the Py excimer. The pillar effect demonstrated herein provides a rational design for co-crystalline systems that are photophysically stable against mechanical stresses.

Band Engineering and Majority Carrier Switching in Isostructural Donor-Acceptor Complexes DPTTA-F X TCNQ Crystals (X = 1, 2, 4)

Three isostructural donor-acceptor complexes DPTTA-F X TCNQ (X = 1, 2, 4) are investigated experimentally and theoretically. By tuning the number of F atoms in the acceptor molecules, the resulting complexes display a continuous down shift of the valence band maximum, conducting band minimum, and optical bandgap. The majority carriers convert from hole (DPTTA-F1TCNQ), balanced hole, and electron (DPTTA-F2TCNQ) to electron (DPTTA-F4TCNQ). This result shows that band engineering can be realized easily in the donor-acceptor complex systems by tuning the electron affinity of the acceptor. The bandgaps of these three complexes vary from 0.31 to 0.41 eV; this narrow bandgap feature is crucial for achieving high thermoelectric performance and the unintentional doping in DPTTA-F4TCNQ leads to the effective suppression of the bipolar cancelling effect on the Seebeck coefficient and the highest power factor.

Synthetic strategies tailoring colours in multichromophoric organic nanostructures

Mimicking nature to develop light-harvesting materials is a timely challenge. This tutorial review examines the chemical strategies to engineer and customise innovative multi-coloured architectures with specific light-absorbing and emitting properties.

Fluorescence to multi-colored phosphorescence interconversion of a novel, asterisk-shaped luminogen via multiple external stimuli

Photoluminescence from blue fluorescence to green, yellow, and orange phosphorescence can be switched via multiple stimuli on an asterisk-shaped compound.

Exploring the semiconductor properties of a charge transfer cocrystal of 1-aminopyrene and TCNQ

The n-type semiconductor nature of a 1 : 1 mixed stack charge transfer cocrystal of 1-aminopyrene and TCNQ is explored.

The Emergence of Organic Single-Crystal Electronics

Organic semiconducting single crystals are perfect for both fundamental and application-oriented research due to the advantages of free grain boundaries, few defects, and minimal traps and impurities, as well as their low-temperature processability, high flexibility, and low cost. Carrier mobilities of greater than 10 cm²  V^-1  s^-1 in some organic single crystals indicate a promising application in electronic devices. The progress made, including the molecular structures and fabrication technologies of organic single crystals, is introduced and organic single-crystal electronic devices, including field-effect transistors, phototransistors, p-n heterojunctions, and circuits, are summarized. Organic two-dimensional single crystals, cocrystals, and large single crystals, together with some potential applications, are introduced. A state-of-the-art overview of organic single-crystal electronics, with their challenges and prospects, is also provided.

Cocrystal Engineering: A Collaborative Strategy toward Functional Materials

Cocrystal engineering with a noncovalent assembly feature by simple constituent units has inspired great interest and has emerged as an efficient and versatile route to construct functional materials, especially for the fabrication of novel and multifunctional materials, due to the collaborative strategy in the distinct constituent units. Meanwhile, the precise crystal architectures of organic cocrystals, with long-range order as well as free defects, offer the opportunity to unveil the structure-property and charge-transfer-property relationships, which are beneficial to provide some general rules in rational design and choice of functional materials. In this regard, an overview of organic cocrystals in terms of assembly, containing the intermolecular interactions and growth methods, two functionality-related factors including packing structure and charge-transfer nature, and those advanced and novel functionalities, is presented. An outlook of future research directions and challenges for organic cocrystal is also provided.

Hydrogen bonded co-assembly of aromatic amino acids and bipyridines that serves as a sacrificial template in superstructure formation

Design and fabrication of superstructures are intriguing yet challenging tasks, which require delicate operations at micro/nanoscale such as template-directed seeding or etching processes. In this study, we prepared integrated one dimensional (1D) microrods from co-assembled N-terminated aromatic amino acids and bipyridines that could serve as sacrificial templates for micro-superstructure formation. Organic polar solvents were utilized for generating a co-assembly that showed selectivity to both molecular topology of building blocks and solvent environments via thermodynamic and kinetic manners. The addition of specific transition metal ions would extract bipyridines from crystalline microrods, leading to well-aligned engraved motifs along the 1D direction as well as the emergence of ordered packed nanostructures on microrod surfaces. Responsive to types of metal ions, diverse superstructures such as etched sculptures and surface-encapsulated heterojunctions of metal-bipyridine coordination polymers were constructed. This study offers a proof-of-concept exploration in the rational design of 1D crystalline micro-superstructures via non-covalent complexation towards potential applications in electrical and optical applications.

Solvation-Enhanced Intermolecular Charge Transfer Interaction in Organic Cocrystals: Enlarged C-C Surface Close Contact in Mixed Packing between PTZ and TCNB

The mixed π-π packing of the donor (D) and acceptor (A) molecules is the highlighting feature of the intermolecular interactions following charge transfer (CT) issues in organic cocrystal systems. There is an inverse relationship between the D-A interplanar distance and the intermolecular CT interaction. However, the D-A C-C surface close contact (relative areas) on the intermolecular CT interactions in organic cocrystal systems is rarely investigated. Herein, we designed and constructed a novel cocrystal and its solvate cocrystal. The structural and electrostatic potential analyses suggest that the solvation destroys the N-H···N hydrogen bond interaction between phenothiazine (PTZ) and 1,2,4,5-tetracyanobenzene (TCNB), which causes the TCNB molecules to have a 90° rotation along the normal axis of the PTZ plane. Thus, the D-A C-C surface close contact is enlarged, strengthening the intermolecular π-π stacking interactions and intermolecular CT interaction between PTZ and TCNB, which are further evidenced by the absorption and Raman spectroscopic analyses. This study provides rare evidence of the enlarged C-C surface close contact in the mixed packing between D and A that greatly contributes to the intermolecular CT interaction in a D-A cocrystal system. It also provides a deeper understanding of the role of solvation in the structure-property relationship of organic cocrystal materials.

Exciton funneling in light-harvesting organic semiconductor microcrystals for wavelength-tunable lasers

Organic solid-state lasers are essential for various photonic applications, yet current-driven lasing remains a great challenge. Charge transfer (CT) complexes formed with p-/n-type organic semiconductors show great potential in electrically pumped lasers, but it is still difficult to achieve population inversion owing to substantial nonradiative loss from delocalized CT states. Here, we demonstrate the lasing action of CT complexes based on exciton funneling in p-type organic microcrystals with n-type doping. The CT complexes with narrow bandgap were locally formed and surrounded by the hosts with high-lying energy levels, which behave as artificial light-harvesting systems. Excitation light energy captured by the hosts was delivered to the CT complexes, functioning as exciton funnels to benefit lasing actions. The lasing wavelength of such composite microcrystals was further modulated by varying the degree of CT. The results offer a comprehensive understanding of exciton funneling in light-harvesting systems for the development of high-performance organic lasing devices.

1D versus 2D cocrystals growth via microspacing in-air sublimation

Organic cocrystals possess valuable properties owing to the synergistic effect of the individual components. However, the growth of molecular cocrystals is still in its primary stage. Here we develop a microspacing in-air sublimation method to grow organic cocrystals, and furthermore to realize morphology control on them, which is essential for structure–property relations. A series of polycyclic aromatic hydrocarbon (PAH)‒1,2,4,5-tetracyanobenzene (TCNB) complexes cocrystals are grown directly on the substrate, with the morphology tunable from 1D needle-like to 2D plate-like on demand. Spatially resolved photoluminescence spectra analyses on different cocrystals display morphology dependent and anisotropic optical waveguiding properties. In situ observation and energy calculations of the crystallization processes reveal the formation mechanism being from a competition between growth kinetics-defined crystal habit and the thermodynamics driving force. This growth technique may serve the future demand for tunable morphology organic cocrystals in different functional applications.

Organic cocrystals have interesting properties owing to synergistic effects of the individual components but development of effective growth methods for molecular cocrystals is still in its primary stage. Here the authors demonstrate a sublimation method which allows growing organic cocrystals with controlled morphology.

Low-Threshold Organic Lasers Based on Single-Crystalline Microribbons of Aggregation-Induced Emission Luminogens

Solid-state lasers (SSLs) play an important role in developing optoelectronic devices, optical communication, and modern medicine fields. As compared with inorganic SSLs, the electrically pumped organic SSLs (OSSLs) still remain unrealized because of the high lasing threshold and low carrier mobility. Herein, we first demonstrate the laser action at ∼520 nm based on the self-assembled single-crystalline organic microribbons of the aggregation-induced emission (AIE) molecules of 1,4-bis(( E)-4-(1,2,2-triphenylvinyl)styryl)-2,5-dimethoxybenzene (TPDSB). Moreover, these as-prepared organic microribbons exhibit an effective optical waveguide with a low optical loss of 0.012 dB μm^-1, indicating good light confinement for laser resonator feedback. Impressively, the multiple mode and the single mode lasing are both achieved from individual organic microribbons, whose lasing threshold is as low as 653 nJ cm^-2. These "bottom-up" synthesized organic microribbons based on AIE-active molecules offer a new strategy for the realization of the ultralow threshold OSSLs, which would eventually contribute to the realization of electrically pumped OSSLs.

Reducing aggregation caused quenching effect through co-assembly of PAH chromophores and molecular barriers

The features of well-conjugated and planar aromatic structures make π-conjugated luminescent materials suffer from aggregation caused quenching (ACQ) effect when used in solid or aggregated states, which greatly impedes their applications in optoelectronic devices and biological applications. Herein, we reduce the ACQ effect by demonstrating a facile and low cost method to co-assemble polycyclic aromatic hydrocarbon (PAH) chromophores and octafluoronaphthalene together. Significantly, the solid photoluminescence quantum yield (PLQYs) for the as-resulted four micro/nanococrystals are enhanced by 254%, 235%, 474 and 582%, respectively. Protection from hydrophilic polymer chains (P123 (PEO₂₀-PPO₇₀-PEO₂₀)) endows the cocrystals with superb dispersibility in water. More importantly, profiting from the above-mentioned highly improved properties, nano-cocrystals present good biocompatibility and considerable cell imaging performance. This research provides a simple method to enhance the emission, biocompatibility and cellular permeability of common chromophores, which may open more avenues for the applications of originally non- or poor fluorescent PAHs.

Supramolecular Design of Donor-Acceptor Complexes via Heteroatom Replacement toward Structure and Electrical Transporting Property Tailoring

A feasible strategy relies on using heteroatom replacement, namely, chemical modification of an organic compound. Here we present this design concept for donor-acceptor complexes, which involves introducing nitrogen atoms to the middle ring of donor molecules to promote short contacts and reduce steric effect of the mixed framework. These nitrogen-modified complexes can possess a shorter molecular distance besides the mixed-stacking pathway, enlarged π-π interactions, or even a scarce 1:2.5 molar ratio through extra acceptor insertion. As a result, the unique 1:2 complex with nitrogen atoms on the different sides demonstrated stable electron field-effect mobility performance, whereas the binary system with no nitrogen replacement or N atoms on the identical sides displayed poor ambipolar properties. These results confirmed that heteroatom replacement was a powerful molecular design tool to fine-tune the molecular packing of organic donor-acceptor complexes and their corresponding electronic properties.

Direct evidence of solvent polarity governing the intramolecular charge and energy transfer: ultrafast relaxation dynamics of push–pull fluorene derivatives

The occurrence of intramolecular charge transfer along with energy transfer controlled by the polarity of solvent is revealed by femtosecond and nanosecond transient absorption and emission spectroscopy.

Unravelling substitution effects on charge transfer characteristics in cocrystals of pyrene based donors and 3,5-dinitrobenzoic acid

Ambipolar to p-type semiconductivity switching with the change of the ⋯DADADA⋯ to ⋯ADDADD⋯ packing arrangement in charge transfer cocrystals of pyrene based donors is discussed.

Efficient Light-Harvesting Systems with Tunable Emission through Controlled Precipitation in Confined Nanospace

Light harvesting is a key step in photosynthesis but creation of synthetic light-harvesting systems (LHSs) with high efficiencies has been challenging. When donor and acceptor dyes with aggregation-induced emission were trapped within the interior of cross-linked reverse vesicles, LHSs were obtained readily through spontaneous hydrophobically driven aggregation of the dyes in water. Aggregation in the confined nanospace was critical to the energy transfer and the light-harvesting efficiency. The efficiency of the excitation energy transfer (EET) reached 95 % at a donor/acceptor ratio of 100:1 and the energy transfer was clearly visible even at a donor/acceptor ratio of 10 000:1. Multicolor emission was achieved simply by tuning the donor/acceptor feed ratio in the preparation and the quantum yield of white light emission from the system was 0.38, the highest reported for organic materials in water to date.

Switching of Monomer Fluorescence, Charge-Transfer Fluorescence, and Room-Temperature Phosphorescence Induced by Aromatic Guest Inclusion in a Supramolecular Host

Crystal engineering of three-component crystals with guest-dependent photoluminescence switching, including (i) crystallization-induced emission enhancement, (ii) intermolecular charge-transfer emission, and (iii) room-temperature phosphorescence under ultraviolet irradiation, was demonstrated. This strategy was based on the confinement of aromatic guests in a supramolecular host (denoted as EBPDI-TPFB) composed of 5,5'-(ethyne-1,2-diyl)bis(2-pyridin-3-yl-isoindoline-1,3-dione (EBPDI) with two tris(pentafluorophenyl)borane (TPFB) molecules linked by B-N dative bonds that acted as Lewis pairs. The single-crystal X-ray structures of complexes with eight different guests were collected, revealing that the size and/or shape of the supramolecular host EBPDI-TPFB was modulated by the included guest molecules. The excellent guest inclusion ability of EBPDI-TPFB allowed systematic photoluminescence regulation of the complexes, which exhibited multicolor emissions in the crystalline state. Photoluminescence switching characteristics of the complexes were observed upon removing the guests or mechanical grinding of the crystals. These results indicated that using the host-guest chemistry of multicomponent crystals not only facilitates crystallization, but also can reveal hidden optical functions by combining molecules of interest, which should contribute to the fields of physical chemistry and materials science.

Organic Semiconductor Single Crystals for Electronics and Photonics

Organic semiconducting single crystals (OSSCs) are ideal candidates for the construction of high-performance optoelectronic devices/circuits and a great platform for fundamental research due to their long-range order, absence of grain boundaries, and extremely low defect density. Impressive improvements have recently been made in organic optoelectronics: the charge-carrier mobility is now over 10 cm² V^-1 s^-1 and the fluorescence efficiency reaches 90% for many OSSCs. Moreover, high mobility and strong emission can be integrated into a single OSSC, for example, showing a mobility of up to 34 cm² V^-1 s^-1 and a photoluminescence yield of 41.2%. These achievements are attributed to the rational design and synthesis of organic semiconductors as well as improvements in preparation technology for crystals, which accelerate the application of OSSCs in devices and circuits, such as organic field-effect transistors, organic photodetectors, organic photovoltaics, organic light-emitting diodes, organic light-emitting transistors, and even electrically pumped organic lasers. In this context, an overview of these fantastic advancements in terms of the fundamental insights into developing high-performance organic semiconductors, efficient strategies for yielding desirable high-quality OSSCs, and their applications in optoelectronic devices and circuits is presented. Finally, an overview of the development of OSSCs along with current challenges and future research directions is provided.

Fluorescence of Nonaromatic Organic Systems and Room Temperature Phosphorescence of Organic Luminogens: The Intrinsic Principle and Recent Progress

Following the development of photoluminescence systems with various compositions, some nontraditional structures, including nonaromatic organic systems as fluorophores and organic luminogens as the source of phosphorescence emission at room temperature, have attracted considerable attention for their advantages in biological and medical applications, and for the updated photophysical understandings in science. In this Review, the recent progress in understanding these organic compounds or polymers for fluorescence and phosphorescence is briefly summarized, with the aim of exploring the intrinsic principle of these novel photoluminescence systems and providing reasonable constructs for molecular design. Finally, some prospects are suggested for further development of this continually expanding area of research, with the coined concept of Molecular Uniting Set Identified Characteristic (MUSIC).

White-light emission from a single organic compound with unique self-folded conformation and multistimuli responsiveness

White-light emitting organic materials attract broad attention which are ascribed to their potential for applications in lighting devices and display media. Most reported organic white-light emitters rely on the combination of several components that emit different colors of light (red/green/blue or orange/blue), which may cause problems to stability, reproducibility and device fabrication. By contrast, white-light emission from single-molecule systems offers opportunities to overcome these disadvantages, meanwhile engendering white-light with high quality. Nevertheless, limited cases of white-light emission at the molecular scale reported principally concentrate on organic solvents. Herein, we designed and synthesized new bi-functional organic molecules with a symmetric donor-acceptor-donor (D-A-D) type structure with the aim to construct a single-molecule white-light emitting system in aqueous solution. Further experiments and calculations demonstrate the possibility of stacking between the pyridinium-naphthalene (PN) core and coumarin groups in the designed molecules, ascribed to hydrophobic effects, π-π stacking and donor-acceptor interactions, which could dramatically enhance the intramolecular charge transfer (ICT) efficiency along with remarkable charge transfer (CT) emission. Based on this, multicolor photoluminescence including white-light can be finely tuned in various modes including excitation wavelength, solvent polarity, temperature, and host-guest interactions. A white-light emitting (WLE) hydrogel was also facilely prepared through the dispersion of one of the compounds in a commercial agarose gelator. This innovative study helps enrich the strategies to construct single-molecule organic white-light emitting materials in aqueous medium using the self-folding behavior.

Molecular-Barrier-Enhanced Aromatic Fluorophores in Cocrystals with Unity Quantum Efficiency

Singlet-triplet conversion in organic light-emitting materials introduces non-emissive (dark) and long-lived triplet states, which represents a significant challenge in constraining the optical properties. There have been considerable attempts at separating singlets and triplets in long-chain polymers, scavenging triplets, and quenching triplets with heavy metals; nonetheless, such triplet-induced loss cannot be fully eliminated. Herein, a new strategy of crafting a periodic molecular barrier into the π-conjugated matrices of organic aromatic fluorophores is reported. The molecular barriers effectively block the singlet-to-triplet pathway, resulting in near-unity photoluminescence quantum efficiency (PLQE) of the organic fluorophores. The transient optical spectroscopy measurements confirm the absence of the triplet absorption. These studies provide a general approach to preventing the formation of dark triplet states in organic semiconductors and bring new opportunities for the development of advanced organic optics and photonics.

Pyrene-based metal–organic framework NU-1000 photocatalysed atom-transfer radical addition for iodoperfluoroalkylation and (Z)-selective perfluoroalkylation of olefins by visible-light irradiation

Iodoperfluoroalkylation or (Z)-selective perfluoroalkylation of olefins is mediated through energy transfer processes by using pyrene-based MOF NU-1000 under visible-light irradiation.

Molecular cocrystals: design, charge-transfer and optoelectronic functionality

This perspective article primarily focuses on the research work related to optoelectronic properties of organic charge transfer cocrystals.