Ruihong Duan

Unusual Aggregation‐Induced Emission of a Coumarin Derivative as a Result of the Restriction of an Intramolecular Twisting Motion

Aggregation-induced emission (AIE) is commonly observed for propeller-like luminogens with aromatic rotors and stators. Herein, we report that a coumarin derivative containing a seven-membered aliphatic ring (CD-7) but no rotors showed typical AIE characteristics, whereas its analogue with a five-membered aliphatic ring (CD-5) exhibited an opposite aggregation-caused quenching (ACQ) effect. Experimental and theoretical results revealed that a large aliphatic ring in CD-7 weakens structural rigidity and promotes out-of-plane twisting of the molecular backbone to drastically accelerate nonradiative excited-state decay, thus resulting in poor emission in solution. The restriction of twisting motion in aggregates blocks the nonradiative decay channels and enables CD-7 to fluoresce strongly. The results also show that AIE is a general phenomenon and not peculiar to propeller-like molecules. The AIE and ACQ effects can be switched readily by the modulation of molecular rigidity.

Deep‐Red to Near‐Infrared Thermally Activated Delayed Fluorescence in Organic Solid Films and Electroluminescent Devices

The design and synthesis of highly efficient deep red (DR) and near-infrared (NIR) organic emitting materials with characteristic of thermally activated delayed fluorescence (TADF) still remains a great challenge. A strategy was developed to construct TADF organic solid films with strong DR or NIR emission feature. The triphenylamine (TPA) and quinoxaline-6,7-dicarbonitrile (QCN) were employed as electron donor (D) and acceptor (A), respectively, to synthesize a TADF compound, TPA-QCN. The TPA-QCN molecule with orange-red emission in solution was employed as a dopant to prepare DR and NIR luminescent solid thin films. The high doped concentration and neat films exhibited efficient DR and NIR emissions, respectively. The highly efficient DR and NIR organic light-emitting devices (OLEDs) were fabricated by regulating TPA-QCN dopant concentration in the emitting layers.

Aromatic Imide Based Thermally Activated Delayed Fluorescence Materials for Highly Efficient Organic Light Emitting Diodes

Aromatic-imide-based thermally activated delayed fluorescence (TADF) materials with a twisted donor-acceptor-donor skeleton were efficiently synthesized and exhibited excellent thermal stability and high photoluminescence quantum yields. The small ΔEST value (<0.1 eV) along with the clear temperature-dependent delayed component of their transient photoluminescence (PL) spectra demonstrated their excellent TADF properties. Moreover, the performance of organic light-emitting diodes in which TADF materials AI-Cz and AI-TBCz were used as dopants were outstanding, with external quantum efficiencies up to 23.2 and 21.1 %, respectively.

Revealing the influence of the solvent evaporation rate and thermal annealing on the molecular packing and charge transport of DPP(TBFu)2

By means of atomistic molecular dynamics simulations, we have investigated the effect of the solvent evaporation rate and thermal annealing on the molecular packing morphology of a diketopyrrolopyrrole based organic photovoltaic donor material, DPP(TBFu)2, which displays excellent hole mobility. It is observed that slow evaporation of solvent will lead to a relatively high degree of molecular packing order while leaving many voids in the as-cast sample. Upon thermal annealing, the as-cast samples at both fast and slow evaporation rates become more compact and much more apparently at the slow evaporation rate. Interestingly, the effect of thermal annealing on molecular packing order depends on the solvent evaporation rates of the as-cast samples. Upon thermal annealing, the molecular packing order of the fast evaporated sample is enhanced with increased π–π stacks. In contrast, thermal annealing will decrease the degree of packing order for the slow evaporated sample since the orientations and conformations of the molecules at the aggregate boundaries are substantially modulated to squeeze the voids. Electrical network analyses point to the fact that the mesoscopic electrical connectivities for all the samples are quite effective and insensitive to the modifications of local molecular ordering due to the delocalized HOMO of DPP(TBFu)2 providing efficient intermolecular electronic interactions. The hole mobilities of all the fabricated samples are thus estimated to be similar and quite high. Finally, our simulations point to the fact that the modest enhancement of mobility upon thermal annealing is correlated with the increased density rather than the varied ordering of molecular packing. Our work provides an atomistic insight into the evolution of thin-film morphology of organic photovoltaic molecular materials during solution processing and thermal annealing treatments and sheds light on the correlation between the molecular structure, packing morphology and hole transport capability.

Importance of side-chain anchoring atoms on electron donor/fullerene interfaces for high-performance organic solar cells

Side-chain engineering is crucial to improve the performance of solution-processed organic solar cells. However, the correlation between side-chain structures and photovoltaic properties is still unclear. Here, we have investigated the local interface morphologies of PC71BM blended with two donors, DR3TBDT and DR3TSBDT with alkyloxy and alkylthio side chains on the BDT core, by means of atomistic molecular dynamics simulations. Compared with alkyloxy, alkylthio exhibits larger steric hindrance after changing the side-chain anchoring atom from oxygen to sulfur, leading to an obvious reduction of the PC71BM–BDT face-on orientations in which charge recombination is demonstrated to be the most severe by quantum-chemical calculations. This suggests that the performance of DR3TBDT/PC71BM solar cells is likely to be more affected by charge recombination than that of DR3TSBDT/PC71BM-based devices. For the first time, our work unravels the important role of side-chain anchoring atoms in tuning the donor/fullerene interfacial arrangements toward high-performance organic solar cells.

One-Compound-Based Highly Efficient Deep-Red to Near-Infrared Thermally Activated Delayed Fluorescence Organic Solid Films and Electroluminescent Devices

The design and synthesis of highly efficient deep red (DR) and near-infrared (NIR) organic emitting materials with characteristic of thermally activated delayed fluorescence (TADF) still remains a great challenge. In this contribution, we developed a strategy to construct TADF organic solid films with strong DR or NIR emission feature. The triphenylamine (TPA) and quinoxaline-6,7-dicarbonitrile (QCN) were respectively employed as electron donor (D) and acceptor (A) to synthesize a TADF compound, TPA-QCN. The TPA-QCN molecule with orange-red emission in solution was employed as a dopant to prepare DR and NIR luminescent solid thin films. The high doped concentration and neat films exhibited efficient DR and NIR emissions, respectively. The highly efficient DR and NIR organic light-emitting devices (OLEDs) were fabricated by regulating TPA-QCN dopant concentration in the emitting layers.

Triplet decay-induced negative temperature dependence of the transient photoluminescence decay of thermally activated delayed fluorescence emitter

The photophysical properties of three TX-based D–A isomers (TXO-PhCz1, TXO-PhCz3, and TXO-PhCz4) with a PhCz donor at different substitution positions of phenyl group on a TXO unit were investigated. The substitution position of the PhCz unit significantly impacts the photophysical properties of these isomers. TXO-PhCz1 exhibits a very weak emission in both undoped and doped films, while TXO-PhCz3 and TXO-PhCz4 exhibit a strong emission with the requisite properties for TADF emitters, including a small ΔEST and transient PL decay curves with a prompt and delayed fluorescent component. TXO-PhCz4 exhibits a much stronger orbital coupling than TXO-PhCz3 and then the phosphorescent emission causes the inverse temperature dependence of the transient PL decay, which is contrary to that of TXO-PhCz3 and other TADF emitters. TXO-PhCz4 exhibits a small ΔEST of 23 meV and a short decay time of 14 μs at room temperature, which are much smaller and shorter than those of TXO-PhCz3. Multilayer OLEDs based on TXO-PhCz4 exhibit a very low-efficiency roll-off with a maximum current efficiency of 49.2 cd A−1, a maximum power efficiency of 47.7 lm W−1, and a maximum EQE of 16.3%.

Regulation of excitation transitions by molecular design endowing full-color-tunable emissions with unexpected high quantum yields for bioimaging application

Organic fluorophores are indispensible in chemical/biological imaging. The conjugated fluorescent molecules simultaneously possessing highly tunable emission, high quantum yield in solvents of different polarities, and large Stokes shift are quite rare. Herein, we report a new category of fluorophores based on diarylated thieno[3,4-b]thiophenes efficiently synthesized by direct C–H arylation reaction. TbT-Fluors showed full-color-tunable emissions with large Stokes shifts. Intriguingly, the fluorescence quantum yields of TbT-Fluors are barely sensitive to solvent polarities, approaching 100%. Based on photophysical and theoretical investigations, we found that the enhanced oscillator strength of the S1-S0 transition and increased T2-S1 energy difference can sufficiently compensate the negative effect from the decreased energy gap and increased reorganization energy in dimethyl sulfoxide (DMSO). Bioimaging applications revealed that some TbT-Fluors can penetrate the cell membrane and are superior for imaging in terms of high photochemical stability and low cytotoxicity. Furthermore, TbT-PhF exhibits specific colocalization with mitochondria in living cells.

Boosting the electron mobilities of dimeric perylenediimides by simultaneously enhancing intermolecular and intramolecular electronic interactions

For organic solar cells using perylenediimide (PDI) derivatives as electron acceptors, bulky substitution and covalent dimerization (or multimerization) are often used to reduce the intermolecular π–π interaction to prevent PDI from forming oversized phase separation with electron donors. Here, we have investigated the influence of different dimerization and alkylation processes on the molecular packing and electron transport properties of PDI derivatives by means of molecular dynamics simulations in combination with electronic structure calculations and kinetic Monte Carlo simulations. The results suggest that like branched alkyl substitution, dimerization can effectively prevent π–π aggregation. Particularly, the π–π stacking is mostly prohibited for the bay C–C dimeric PDIs, even with linear alkyl substituents. More importantly, different from the case of bulky substitution, dimerization will hardly reduce short intermolecular contacts between PDI units. Unexpectedly, the electron mobilities of dimeric PDIs are always higher than those of the parent monomeric PDIs due to extra intramolecular electronic connections. Owing to simultaneously enhanced intermolecular and intramolecular interactions, the electron mobility of the bay C–C dimeric PDI with linear alkyl chains is increased by nearly 20 times with respect to the monomeric PDI with branched alkyl chains. Our work demonstrates that high charge mobilities along with suppressed π–π aggregation can be achieved by properly tailoring both the modes of dimerization and substitution.

CCDC 1543551: Experimental Crystal Structure Determination

Related Article: Meng Li, Yanwei Liu, Ruihong Duan, Xiaofang Wei, Yuanping Yi, Ying Wang, Chuan-Feng Chen|2017|Angew.Chem.,Int.Ed.|56|8818|doi:10.1002/anie.201704435