Organic Optoelectronic Materials Research Articles

Room-temperature phosphorescence from organic aggregates

Triplet excitons in organic molecules underscore a variety of processes and technologies as a result of their long lifetime and spin multiplicity. Organic phosphorescence, which originates from triplet excitons, has potential for the development of a new generation of organic optoelectronic materials and biomedical agents. However, organic phosphorescence is typically only observed at cryogenic temperatures and under inert conditions in solution, which severely restricts its practical applications. In the past few years, room-temperature-phosphorescent systems have been obtained based on organic aggregates. Rapid advances in molecular-structure design and aggregation-behaviour modulation have enabled substantial progress, but the mechanistic picture is still not fully understood because of the high sensitivity and complexity of triplet-exciton behaviour. This Review analyses key photophysical processes related to triplet excitons, including intersystem crossing, radiative and non-radiative decay, and quenching processes, to illustrate the intrinsic structure–property relationships and draw clear and integrated design principles. The resulting strategies for the development of efficient and persistent room-temperature-phosphorescent systems are discussed, and newly emerged applications based on these materials are highlighted. Advances in molecular-structure design and modulation of the aggregation behaviour have enabled much progress in the observation of room-temperature phosphorescence from organic aggregates. This Review analyses key photophysical processes related to triplet excitons, illustrating the intrinsic structure–property relationships and identifying strategies to design efficient and persistent room-temperature-phosphorescent systems.

撤回：縮環π拡張（APEX）反応による多環芳香族化合物の精密合成

Polycyclic aromatic compounds are recognized as highly important class of compounds in materials science due to their high utility as organic semiconductors and organic optoelectronic materials. In general, they can be obtained through the established π-extension methods from unfunctionalized aromatics, while the syntheses are often suffered from multi-step process including halogenation, coupling reaction, Diels-Alder reaction, aromatization and so on. Recently, we have established a novel synthetic concept “annulative π-extension (APEX)” which has the potential to have a tremendous impact on the fields of synthetic chemistry since it allows rapid access to fused aromatic systems from simple unfunctionalized aromatic compounds in a single step. Here we report our recent results of palladium-catalyzed APEX reactions for the synthesis of polycyclic aromatic hydrocarbons, polycyclic heteroaromatics, nanographenes and even graphene nanoribbons from readily available unfunctionalized aromatics.

Predicting optical spectra for optoelectronic polymers using coarse-grained models and recurrent neural networks

Coarse-grained modeling of conjugated polymers has become an increasingly popular route to investigate the physics of organic optoelectronic materials. While ultraviolet (UV)-vis spectroscopy remains one of the key experimental methods for the interrogation of these materials, a rigorous bridge between simulated coarse-grained structures and spectroscopy has not been established. Here, we address this challenge by developing a method that can predict spectra of conjugated polymers directly from coarse-grained representations while avoiding repetitive procedures such as ad hoc back-mapping from coarse-grained to atomistic representations followed by spectral computation using quantum chemistry. Our approach is based on a generative deep-learning model: the long-short-term memory recurrent neural network (LSTM-RNN). The latter is suggested by the apparent similarity between natural languages and the mathematical structure of perturbative expansions of, in our case, excited-state energies perturbed by conformational fluctuations. We also use this model to explore the level of sensitivity of spectra to the coarse-grained representation back-mapping protocol. Our approach presents a tool uniquely suited for improving postsimulation analysis protocols, as well as, potentially, for including spectral data as input in the refinement of coarse-grained potentials.

1]Benzothieno[3,2‐b][1]benzothiophene‐Phthalocyanine Derivatives: A Subclass of Solution‐Processable Electron‐Rich Hole Transport Materials

The [1]benzothieno[3,2-b][1]benzothiophene (BTBT) planar system was used to functionalize the phthalocyanine ring aiming at synthesizing novel electron-rich π-conjugated macrocycles. The resulting ZnPc-BTBT and ZnPc-(BTBT)4 derivatives are the first two examples of a phthalocyanine subclass having potential use as solution-processable p-type organic semiconductors. In particular, the combination of experimental characterizations and theoretical calculations suggests compatible energy level alignments with mixed halide hybrid perovskite-based devices. Furthermore, ZnPc-(BTBT)4 features a high aggregation tendency, a useful tool to design compact molecular films. When tested as hole transport materials in perovskite solar cells under 100 mA cm-2 standard AM 1.5G solar illumination, ZnPc-(BTBT)4 gave power conversion efficiencies as high as 14.13 %, irrespective of the doping process generally required to achieve high photovoltaic performances. This work is a first step toward a new phthalocyanine core engineerization to obtain robust, yet more efficient and cost-effective materials for organic electronics and optoelectronics.

Well-defined arrangement of π-electronic systems based on precise molecular design

This focus review describes several methods for aligning π-electronic systems to achieve various functionalities in both solution and the solid phase. Cyclic oligomers, acting as tethering units, can facilitate precise design of the nanoarchitecture of π-electronic systems. This approach produces three-dimensionally aligned structures of π-electronic systems in diluted solutions, where the intermolecular interactions are partially excluded. In the solid state, charged π-electronic systems exhibit nanoarchitectures based on electrostatic interactions, including the repulsive and attractive forces generated between the identically charged and differently charged species, respectively. These methods for controlling the arrangements of π-electronic systems can produce fascinating molecular systems. The performance of organic optoelectronic materials is strongly affected by the three-dimensional (3D) alignment of π-electronic systems. This focus review describes several methods for aligning π-electronic systems to achieve various functionalities in both solution and the solid phase. Cyclic oligomers, acting as tethering units, can facilitate precise design of the nanoarchitecture of π-electronic systems in diluted solutions. In the solid state, charged π-electronic systems exhibit nanoarchitectures based on electrostatic interactions. These methods for controlling the arrangements of π-electronic systems can produce fascinating molecular systems.

Molecular Phosphorescence in Polymer Matrix with Reversible Sensitivity.

Ultralong organic phosphorescence strongly depends on the formation of aggregation, while it is difficult to obtain in dilute environments on account of excessive internal and external molecular motions. Herein, ultralong single-molecule phosphorescence (USMP) at room temperature was achieved in the monomer state by coassembling biphenyl and naphthalene derivatives at low density with poly(vinyl alcohol) (PVA), where PVA provides a confined environment to stabilize the triplet state. Various factors that affect the USMP were studied, including aggregation, conformation, temperature, and moisture. In these systems, the formation of aggregates through intermolecular stacking and hydrogen bonding interactions in the film or crystal phases completely suppresses the USMP. However, the fluorescence is enhanced when coassembling these compounds at high concentration with PVA and becomes stronger in their powder state, indicating that the intersystem crossing process is blocked by the aggregation. Theoretical calculations suggest that the aggregation depresses spin-orbit coupling between the excited singlet and triplet states and enhances the nonradiative quenching process. Moreover, a relatively twisted conformation is more conducive to the occurrence of intersystem crossing than planar conformation. The USMP shows delicate and reversible sensitivity to the changes of temperature and moisture, rendering them with the applicability as smart organic optoelectronic materials.

Molecular Packing: Another Key Point for the Performance of Organic and Polymeric Optoelectronic Materials.

ConspectusOptoelectronic material properties are governed by the whole collective of organic moieties, and these aggregate states present the characteristic performance of extended assemblies with different molecular packing, not only of single molecules themselves. Thus, controlling molecular packing is an essential issue for obtaining the optimized optical and electronic properties. It is also a great challenge because of the unclear structures and complicated intermolecular interactions, including dispersion forces, electrostatic interactions and hydrogen bonding. Moreover, upon the introduction of some external force as the stimulus source, dynamic optical properties can be achieved with the transformation of molecular packing in some cases, such as the photoinduced room temperature phosphorescence (RTP) effect, mechanochromic luminescence, the thermal treatment-dependent mechanoluminescence effect, and the optimized nonlinear optical (NLO) property achieved after electric poling. Therefore, it is essential to understand the relation between characteristics of molecular packing and the resultant optoelectronic performance at the molecular level, which becomes increasingly demanding for the further development of functional materials for their applications in organic light-emitting diodes (OLEDs), chemo- and biosensors, organic solar cells, data storage, and anticounterfeiting devices.This Account gives a summary of our research on the molecular design of optoelectronic materials, with the consideration of a molecular uniting effect in different aggregated states, such as crystalline states, thin films, and nanoparticles. Through the systematical investigation of structure-packing-performance relationships, some strategies are afforded to partially control the molecular packing via the tunable size, shape, and configuration of aromatic moieties with different electronic and steric effects, together with different types of substituents as functional units to adjust the intermolecular interactions. The utilization of π-π interactions and hydrogen bonding by rational molecular design is considered as the key point to achieve the bright emission of organic materials, including the RTP and mechanoluminescence effects. Also, the dynamic optoelectronic properties are highlighted with different kinds of stimuli, including light irradiation, mechanical force, thermal treatment, and electric field, which are mainly related to the subtle molecular motions under external force and the changeable molecular packing as the metastable state. These selected examples will not only open a window for further development of organic and polymeric optoelectronic materials by the adjustable molecular packing and noncovalent interactions, but also prompt further advances for more interesting and exciting properties.

Copper-catalyzed remote C-H arylation of polycyclic aromatic hydrocarbons (PAHs).

The regioselective C–H arylation of substituted polycyclic aromatic hydrocarbons (PAHs) is a desired but challenging task. A copper-catalyzed C7–H arylation of 1-naphthamides has been developed by using aryliodonium salts as arylating reagents. This protocol does not need to use precious metal catalysts and tolerates wide variety of functional groups. Under standard conditions, the remote C–H arylation of other PAHs including phenanthrene-9-carboxamide, pyrene-1-carboxamide and fluoranthene-3-carboxamide has also accomplished, which provides an opportunity for the development of diverse organic optoelectronic materials.

Special Issue Dedicated to Prof. Chien‐Hong Cheng's 70th Birthday

It is my great pleasure to announce that the special issue dedicated to Professor Chien-Hong Cheng on the occasion of his 70th birthday has been published. The total of 12 articles was collected in this issue, thanks to all the authors for their great contributions; also thanks to Professor Shih-Ching Chuang and Jen-Chieh Hsieh as well as the regular JCCS editorial board for their assistance. Professor Cheng is a world-renowned organometallic chemist. He has been conducting interdisciplinary research for many years and he has been mainly devoted to developing the transition metal-catalyzed new organic reactions and optoelectronic materials for many decades. His contributions impressed numerous people in both scientific and optoelectronic industrial communities. He developed many interesting organic transformations in sustainable pathways catalyzed by abundant transition metals. In addition to metal catalysis, the front cover of this issue demonstrates nicely the highlights of his research in organic light-emitting diode (OLED) display. He achieved a significant breakthrough in OLED technology by preparing new diboron illuminating materials in green OLED emitting layer. Its structure breaks through the world record of external quantum efficiency of the conventional OLEDs to about 38% with an efficiency roll-offof only 0.3% at 1000 cd/m2. Moreover, the cost is lowered down to a great extent. Because of his great research achievements, Professor Cheng was awarded many accolades and honors, and invited to deliver plenty of important lectures in international conferences. Professor Cheng was elected as an academician of Academia Sinica (AS) in 2018, which is the highest honor in academia in this country. It should be noted that during his forty-year (1979~2019) research and teaching career, he has grown many technology talents. He has delivered undergraduate inorganic chemistry and graduate organometallic chemistry to chemistry major students for a long time. The photo on the top of this page represents his achievement in training qualified personnel for academia and industry.

Insights into the Control of Optoelectronic Properties in Mixed-Stacking Charge-Transfer Complexes.

Although cocrystallization has provided a promising platform to develop new organic optoelectronic materials, it is still a big challenge to purposely design and achieve specific optoelectronic properties. Herein, a series of mixed-stacking cocrystals (TMFA, TMCA, and TMTQ) were designed and synthesized, and the regulatory effects of the acceptors on the co-assembly behavior, charge-transfer nature, energy-level structures, and optoelectronic characteristics were systematically investigated. The results demonstrate that it is feasible to achieve effective charge-transport tuning and photoresponse switching by carefully regulating the intermolecular charge transfer and energy orbitals. The inherent mechanisms underlying the change in these optoelectronic behaviors were analyzed in depth and elucidated to provide clear guidelines for future development of new optoelectronic materials. In addition, due to the excellent photoresponsive characteristics of TMCA, TMCA-based phototransistors were investigated with varying light wavelength and optical power, and TMCA shows the best performance among all reported cocrystals under UV illumination.

Near‐infrared organic photoelectric materials for light‐harvesting systems: Organic photovoltaics and organic photodiodes

AbstractThe inherent advantages of organic optoelectronic materials endow light‐harvesting systems, including organic photovoltaics (OPVs) and organic photodiodes (OPDs), with multiple advantages, such as low‐cost manufacturing, light weight, flexibility, and applicability to large‐area fabrication, make them promising competitors with their inorganic counterparts. Among them, near‐infrared (NIR) organic optoelectronic materials occupy a special position and have become the subject of extensive research in both academia and industry. The introduction of NIR materials into OPVs extends the absorption spectrum range, thereby enhancing the photon‐harvesting ability of the devices, due to which they have been widely used for the construction of semitransparent solar cells with single‐junction or tandem architectures. NIR photodiodes have tremendous potential in industrial, military, and scientific applications, such as remote control of smart electronic devices, chemical/biological sensing, environmental monitoring, optical communication, and so forth. These practical and potential applications have stimulated the development of NIR photoelectric materials, which in turn has given impetus to innovation in light‐harvesting systems. In this review, we summarize the common molecular design strategies of NIR photoelectric materials and enumerate their applications in OPVs and OPDs.image

Doubly Encapsulated Perylene Diimides: Effect of Molecular Encapsulation on Photophysical Properties.

Intermolecular interactions play a fundamental role on the performance of conjugated materials in organic electronic devices, as they heavily influence their optoelectronic properties. Synthetic control over the solid state properties of organic optoelectronic materials is crucial to access real life applications. Perylene diimides (PDIs) are one of the most highly studied classes of organic fluorescent dyes. In the solid state, π-π stacking suppresses their emission, limiting their use in a variety of applications. Here, we report the synthesis of a novel PDI dye that is encapsulated by four alkylene straps. X-ray crystallography indicates that intermolecular π-π stacking is completely suppressed in the crystalline state. This is further validated by the photophysical properties of the dye in both solution and solid state and supported by theoretical calculations. However, we find that the introduction of the encapsulating "arms" results in the creation of charge-transfer states which modify the excited state properties. This article demonstrates that molecular encapsulation can be used as a powerful tool to tune intermolecular interactions and thereby gain an extra level of control over the solid state properties of organic optoelectronic materials.

Recent Progress of BODIPY Dyes With Aggregation-Induced Emission.

With the development of organic optoelectronic materials and bioimaging technology, to exploit organic luminescent materials with high luminescent efficiency in aggregation-state has become a research hotspot. BODIPYs have become one of the research objects of this kind of material because of their obvious advantages. This review focuses on the design and synthesis of AIE-type BODIPYs, the mechanism of AIE properties and their applications in recent years. Through classification, analysis, and summary, this review aims to explore the structure-activity relationship of AIE-type BODIPYs and to provide ideas for the further design and potential applications of AIE-active fluorescent materials.

Multiple σ–π Conjugated Molecules with Selectively Enhanced Electrical Performance for Efficient Solution‐Processed Blue Electrophosphorescence

AbstractSelectively and controllably regulating molecular functions of organic optoelectronic materials with high solubility for solution‐processible devices is highly desired but remains as one of the most significant challenges in material science. Here, a concise molecular design strategy is reported to achieve effective electronic communications using efficient d‐orbital participated σ–π conjugations between Si and π unit for purposely modulating the electrical properties of organic optoelectronic materials. Through a two‐step reaction in high yield, DSiDCzSi with the enhanced σ–π conjugation is facilely constructed by introducing multiple triphenylsilanes into carbazole unit. Impressively, DSiDCzSi demonstrates a largely increased d‐orbital participated extent of Si, which results in the selectively improved frontier orbital energy level, enhanced carrier injection and transportation ability, excellent solubility, and film‐forming property. Using DSiDCzSi as a host matrix, solution‐processed blue electrophosphorescence device exhibits a maximum external quantum efficiency up to 23.5%, which is among the best values of FIr6‐based blue phosphorescent organic light‐emitting diodes reported to date. This work, which reveals the vital role of d‐orbital participated σ–π conjugations in solving the inherent interference between optical and electrical properties of π‐conjugated materials, can provide an extensible and universal avenue for designing and constructing high‐performance organic optoelectronic materials for advanced device applications.

Generalized Kasha’s Model: T-Dependent Spectroscopy Reveals Short-Range Structures of 2D Excitonic Systems

Summary Excitonic organic materials owe their tunable optoelectronic properties to intricate microscopic configurations, challenging for conventional characterization methods to resolve. We generalize Kasha’s model by incorporating the temperature (T)-dependent absorption peak shift, in addition to the monomer-aggregate absorption peak shift that defines J- and H-aggregates, to characterize the microscopic structures. We show that the short-range interactions dominate in determining the direction of the T-dependent peak shift, which accounts for previously observed T-dependent blueshifting J-aggregates (BJ-aggregates) that were not explained and predicts the existence of redshifting H-aggregates (RH-aggregates). This defines four types of excitonic aggregates: RJ-, BH-, BJ-, and RH-aggregates, where the latter two unconventional aggregates are possible because of the two dimensionality of excitonic systems and cannot be understood with the original Kasha’s theory. Our conceptual framework is useful for elucidating structure-function relationships in molecular excitonic systems and is fully compatible with existing tools. Possible extensions to other spectroscopic observables and related systems are discussed.

Enhanced thermoelectric performance from self-assembled alkyl chain-linked naphthalenediimide/single walled carbon nanotubes composites

Naphthalenediimide (NDI) derivatives with high electron affinity and strong π–π stacking interactions have been frequently used as electro-active materials in the field of organic optoelectronic materials. However, the out of control feature of the aggregate states of NDI derivatives often lead to their poor performance for thermoelectric applications. Herein, taking the advantage of van der Waals forces intermolecular interactions, alkyl chain-linked NDIs with H-(J-) aggregates through self-assembly were successfully achieved. The NDI-based polymer linked with a decyl chain (NDI-10) showed J-aggregate, in which the intrachain electronic coupling was dominated and the polymer linked with a butyl chain (NDI-4) and a hexyl chain (NDI-6) reviewed H-aggregate, in which interchain coupling was dominated, the hypothesis was confirmed by results of fluorescence, SAXS, and XRD. The different H-(J-) aggregates affects carrier transport in a two-dimensional transmission channel in our polymers. Moreover, by utilizing the high electrical conductivity of single-walled carbon nanotubes (SWCNTs), we achieved high-performance thermoelectric (TE) composites containing SWCNTs and the polymers based on the alkyl chain-linked NDIs. The obtained composites displayed enhanced TE performance with a high power factor of 238 ± 21 μW m−1 K−2 at 300 K. The corresponding TE module consisting of ten p-type junctions reaches a large output voltage of 5.82 mV and an output power of 28 nW under a 30 K temperature difference. This design strategy promotes the future fabrication of high-performance TE materials and modules.

Multistep nucleation and growth mechanisms of organic crystals from amorphous solid states

Molecular self-assembly into crystallised films or wires on surfaces produces a big family of motifs exhibiting unique optoelectronic properties. However, little attention has been paid to the fundamental mechanism of molecular crystallisation. Here we report a biomimetic design of phosphonate engineered, amphiphilic organic semiconductors capable of self–assembly, which enables us to use real-time in-situ scanning probe microscopy to monitor the growth trajectories of such organic semiconducting films as they nucleate and crystallise from amorphous solid states. The single-crystal film grows through an evolutionary selection approach in a two-dimensional geometry, with five distinct steps: droplet flattening, film coalescence, spinodal decomposition, Ostwald ripening, and self-reorganised layer growth. These sophisticated processes afford ultralong high-density microwire arrays with high mobilities, thus promoting deep understanding of the mechanism as well as offering important insights into the design and development of functional high-performance organic optoelectronic materials and devices through molecular and crystal engineering.

Evoking non-bonding S-π interaction by aryl phosphine sulfide for selectively enhanced electronic property of organic semiconductors

Selective tuning of optoelectronic properties of organic semiconductors is one of the most significant challenges in organoelectronics. Herein, on the basis of aryl phosphine sulfide (APS), we show that the electronic performance can be selectively enhanced by evoking the non-bonding S-π interactions between the sulfur atom of APS and π-conjugated carbazole moiety, on account of the large scope 3p lone-pair electrons of S for effective electronic communications. It is indicated that the material properties and device performances are strongly dependent on the beingness of the successfully established intramolecular S-π interaction. Specifically, APS-based DPSSi exhibits excellent solubility and film-forming ability and enhanced carrier transportation and injection abilities at high triplet energy level; and the DPSSi-hosted blue phosphorescent light-emitting diodes (PhOLEDs) show high current efficiency up to 36.6 cd A−1, power efficiency of 30.6 lm W−1, and external quantum efficiency of 20.8%, which are among the best results of blue PhOLEDs reported so far. The semi-conjugation feature of S-π interaction in selectively modulating electronic properties would offer exciting opportunities for settling intrinsic difficulties of organic optoelectronic materials and devices, promoting significantly the development of non-bonding interaction involved organic optoelectronic molecules.

Tuning Intramolecular Conformation and Packing Mode of Host Materials through Noncovalent Interactions for High-Efficiency Blue Electrophosphorescence.

Molecular conformation plays an important role in tuning the packing modes of organic optoelectronic materials to achieve enhanced and/or balanced charge transport. Here, we introduce the noncovalent intramolecular interactions to the host materials of phosphorescent organic light-emitting diodes (PhOLEDs). Different numbers and/or positions of intramolecular CH···N noncovalent interactions were constructed by using different N-heterocycles of pyridine, pyrimidine, and pyrazine as acceptor units and carbazole as the donor unit in a donor-acceptor-donor (D-A-D) motif. Thus, designed D-A-D molecules were synthesized facilely through a one-step Ullmann reaction in high yields, showing varied intramolecular interactions to regulate the molecular conformation significantly. Impressively, owing to the quasi-parallel molecular conformation, which is beneficial for forming facile transporting channels of both holes and electrons, the newly designed host material of 9,9′-(pyridine-2,5-diyl)bis(9H-carbazole) exhibits good device performance of blue PhOLEDs with current, power, and external quantum efficiencies up to 33.0 cd A–1, 32.1 lm W–1, and 16.3%, respectively. This work highlights the significant importance of the noncovalent interactions in designing advanced organic semiconductors for high-performance optoelectronic devices.

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.