Xingxing Shen

Highly Efficient Orange and Red Phosphorescent Organic Light‐Emitting Diodes with Low Roll‐Off of Efficiency using a Novel Thermally Activated Delayed Fluorescence Material as Host

MTXSFCz with thermally activated delayed fluorescence is synthesized. Orange and red phosphorescent organic light-emitting diodes (PHOLEDs) with low efficiency roll-off exhibit external quantum efficiencies (EQE) up to 11.8% and 15.6%. The efficient upconversion from triplet to singlet of the host reduces the triplet density and thus affords a low efficiency roll-off of PHOLEDs.

Fine-Tuning of Crystal Packing and Charge Transport Properties of BDOPV Derivatives through Fluorine Substitution

Molecular packing in organic single crystals greatly influences their charge transport properties but can hardly be predicted and designed because of the complex intermolecular interactions. In this work, we have realized systematic fine-tuning of the single-crystal molecular packing of five benzodifurandione-based oligo(p-phenylenevinylene) (BDOPV)-based small molecules through incorporation of electronegative fluorine atoms on the BDOPV backbone. While these molecules all exhibit similar column stacking configurations in their single crystals, the intermolecular displacements and distances can be substantially modified by tuning of the amounts and/or the positions of the substituent fluorine atoms. Density functional theory calculations showed that the subtle differences in charge distribution or electrostatic potential induced by different fluorine substitutions play an important role in regulating the molecular packing of the BDOPV compounds. Consequently, the electronic couplings for electron transfer can vary from 71 meV in a slipped stack to 201 meV in a nearly cofacial antiparallel stack, leading to an increase in the electron mobility of the BDOPV derivatives from 2.6 to 12.6 cm(2) V(-1) s(-1). The electron mobility of the five molecules did not show a good correlation with the LUMO levels, indicating that the distinct difference in charge transport properties is a result of the molecular packing. Our work not only provides a series of high-electron-mobility organic semiconductors but also demonstrates that fluorination is an effective approach for fine-tuning of single-crystal packing modes beyond simply lowering the molecular energy levels.

A Cofacially Stacked Electron-Deficient Small Molecule with a High Electron Mobility of over 10 cm(2) V-1 s(-1) in Air

A strong, electron-deficient small molecule, F4 -BDOPV, has a lowest unoccupied molecular orbital (LUMO) level down to -4.44 eV and exhibits cofacial packing in single crystals. These features provide F4 -BDOPV with good ambient stability and large charge-transfer integrals for electrons, leading to a high electron mobility of up to 12.6 cm(2) V(-1) s(-1) in air.

Diaceno[a,e]pentalenes from homoannulations of o-alkynylaryliodides utilizing a unique Pd(OAc)2/n-Bu4NOAc catalytic combination.

A heterogeneous catalytic system, Pd(OAc)2/n-Bu4NOAc, for the efficient synthesis of diaceno[a,e]pentalenes via a tandem Pd catalytic cycle is reported. The catalytic partner n-Bu4NOAc played indispensable and versatile roles, acting as both the media for recovering active Pd(0) species and their stabilizer. A series of new diaceno[a,e]pentalenes were obtained in moderate to high yields, among which the octacyclic dianthracenopentalene was found to be highly emissive.

Developing Quinoidal Fluorophores with Unusually Strong Red/Near-Infrared Emission

Despite the dominant position of aromatic fluorophores, we report herein the design and synthesis of quinoidal fluorophores based on rarely emissive quinoidal bithiophene. Quinoidal bitheno[3,4-b]thiophene, QBTT-C6, consisting of cruciform-fused (E)-1,2-bis(5-hexylthiophen-2-yl)ethene and quinoidal bithiophene, shows a fluorescence quantum yield of 8.5%, 25-fold higher than that of the parent quinoidal QBT chromophore, but its maximum emission is at similar wavelengths. QBTT-Ars featuring intramolecular charge transfer can further shift the maximum emission into the near-infrared region. The intramolecular charge transfer is programmably enhanced by tuning the substituents on the aryl groups from the electron-withdrawing trifluoromethyl to the electron-donating methoxy groups. Unexpectedly, a positive relationship between intramolecular charge transfer and fluorescence quantum yield is observed; as a result, QBTT-FL gives an unprecedentedly high fluorescence quantum yield of up to 53.1% for quinoidal oligothiophenes. With detailed photophysical and theoretical investigations, we demonstrate that the nonradiative intersystem crossing (S1 → T2) is significantly restrained in QBTT-Ars, which can be attributed to the faster reverse intersystem crossing (T2 → S1) characteristic of a small activation energy. This work reveals the possibility for developing red/near-infrared fluorophores from the less explored quinoidal molecules because of their intrinsically narrow bandgaps.

Bismuth Interfacial Doping of Organic Small Molecules for High Performance n‐type Thermoelectric Materials

Development of chemically doped high performance n-type organic thermoelectric (TE) materials is of vital importance for flexible power generating applications. For the first time, bismuth (Bi) n-type chemical doping of organic semiconductors is described, enabling high performance TE materials. The Bi interfacial doping of thiophene-diketopyrrolopyrrole-based quinoidal (TDPPQ) molecules endows the film with a balanced electrical conductivity of 3.3 S cm(-1) and a Seebeck coefficient of 585 μV K(-1) . The newly developed TE material possesses a maximum power factor of 113 μW m(-1)  K(-2) , which is at the forefront for organic small molecule-based n-type TE materials. These studies reveal that fine-tuning of the heavy metal doping of organic semiconductors opens up a new strategy for exploring high performance organic TE materials.

Synthesis, Single Crystal, and Physical Properties of Asymmetrical Thiophene/Selenophene-Fused Twistacenes.

Two asymmetrical twistacenes, PyPT and PyPS, have been synthesized and characterized. Single crystal X-ray analyses show that both of them have twisted structures with a torsion angle of 26.65° for PyPT and 26.59° for PyPS measured between plane C5-C23-C25 and plane C13-C15-C26. The thiophene/selenophene-fused acenes emit blue fluorescence with quantum yields of 0.39 for PyPT and 0.04 for PyPS in organic solvents, whereas the all-carbon molecule HBP emits green fluorescence. Meanwhile, PyPT and PyPS show a similar reversible oxide procedure with the onset potentials of 0.73 and 0.72 V, respectively. In addition, PyPT and PyPS can self-assemble to form nanoparticles in a mixture of THF/H2O through re-precipitation method.

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.

Field-Effect Transistors: A Cofacially Stacked Electron-Deficient Small Molecule with a High Electron Mobility of over 10 cm2 V−1 s−1 in Air (Adv. Mater. 48/2015)

Compared with abundant p-type organic semiconductors with high mobilities, n-type semiconductors with mobilities over 1 cm(2) V(-1) s(-1) are rare. On page 8051, Y. Yi, J. Pei, and co-workers report a new small molecule, F4-BDOPV, with unprecedentedly high electron mobilities of up to 12.6 cm(2) V(-1) s(-1) . This is the first time that air-stable n-type organic semiconductors have shown mobilities surpassing 10 cm(2) V(-1) s(-1) , an important benchmark for practical applications.