Xiangchun Li

Triazatruxene-based materials for organic electronics and optoelectronics

10,15-Dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (triazatruxene or triindole), which is an aromatic and C3 symmetric planar π-extended conjugated structure, has attracted a great deal of interest due to its exceptional solubility, high thermal stability, rich-electronic properties and easy modification. Over the past ten years, owing to the advances in the synthesis of triazatruxene derivatives, the scope of applications of this attractive building block has been extended to organic electronics, two-photon absorption, nonlinear optics, fluorescent sensors, etc. This review aims at summarizing investigations into triazatruxene-based organic electronic materials from synthesis and self-assembly to optoelectronic properties to achieve a better understanding on the structure–function relationships and to explore their extensive applications. The potential of these highly promising scaffolds in future research will also be put forth.

Synthesis and characterization of symmetric cyclooctatetraindoles: exploring the potential as electron-rich skeletons with extended π-systems.

A facile one-pot tetramerization of indolin-2-one with phosphoryl chloride was applied for the first convenient direct synthesis of C2v-symmetric cyclooctatetraindole with an 8π annulene as the center. Tetra- and octa-arylated cyclooctatetraindole derivatives functionalized with fluorescent fluorene and pyrene units were thus facilely synthesized and characterized.

Catalyst-free one-step synthesis of ortho-tetraaryl perylene diimides for efficient OPV non-fullerene acceptors

ortho-Functionalized perylene diimides (PDIs) display better electronic properties than their bay-substituted counterparts, yet their synthesis requires expensive catalysts. We report the regioselective 2,5,8,11-functionalization of PDI via the less commonly observed 1,4-addition of aryl Grignard reagents. This direct functionalization offers a simple and low-cost method to synthesize desirable ortho-substituted PDIs. Four tetraarylated PDIs are reported along with their electronic and optoelectronic properties. Preliminary organic photovoltaic devices using these molecules as non-fullerene acceptors showed good power conversion efficiencies (PCEs) up to 5%, a new record for an acceptor with a single PDI core.

Efficient amplified spontaneous emission from oligofluorene-pyrene starbursts with improved electron affinity property.

A series of monodisperse starburst molecules as optical gain media have been investigated in detail. The starburst molecules were composed of a pyrene core with four short oligofluorene arms capped by cyanophenyl moieties. The compounds exhibited low amplified spontaneous emission (ASE) thresholds of 30 nJ pulse-1 and high maximum net gain coefficient of 55 cm-1 under optically pump. Our study demonstrates that the introducing of electron-withdrawing cyanophenyl end-capper onto pyrene centered starburst molecules does not disadvantage their optical gain properties while leading to reduced LUMO level, therefore, improved electron affinity. This study suggested a promising approach for organic gain material synthesis to address the challenge of electrically pumped organic laser.

An Easy Approach to Control β-Phase Formation in PFO Films for Optimized Emission Properties

We demonstrate a novel approach to control β-phase content generated in poly(9,9-dioctylfluorene) (PFO) films. A very small amount of paraffin oil was used as the additive to the PFO solution in toluene. The β-phase fraction in the spin-coated PFO films can be modified from 0% to 20% simply by changing the volume percentage of paraffin oil in the mixed solution. Organic light emitting diodes (OLEDs) and amplified spontaneous emission (ASE) study confirmed low β-phase fraction promise better OLEDs device, while high β-phase fraction benefits ASE performance.

Effects of conjugated bridges on the photovoltaic properties of ortho-functionalized perylene diimides for non-fullerene polymer solar cells

Perylene diimide (PDI) derivatives have great potential to replace the commonly used fullerene derivatives in bulk heterojunction (BHJ) polymer solar cells (PSCs). However, PDI-based molecules have a strong tendency to aggregate in thin films, which prevents the formation of appropriate phase separation. To alleviate this problem, much effort has been devoted to developing twisted PDI dimers with functional bridges at bay positions. Unfortunately, the bay-functionalized PDI dimers exhibit relatively low electron mobilities due to the excessive twist geometry. To solve this dilemma, an efficient method is to functionalize the PDI at less sterically-encumbered ortho-positions. In this article, we synthesized two novel ortho-functionalized PDI dimers, namely PDI-2Th-PDI and PDI-ThFTh-PDI, and investigated the influence of the conjugated bridge length on the physical and photovoltaic properties of PDIs. After introducing a fluorene bridge between the thiophene units, PDI-ThFTh-PDI with the extended conjugate length possessed a relatively low-crystalline nature and therefore exhibited one order of magnitude lower electron mobility compared to PDI-2Th-PDI. As a result, the power conversion efficiency (PCE) of solar cells fabricated with PDI-2Th-PDI as the electron acceptor and PTB7-Th as the electron donor is almost twice that of the solar cell with PDI-ThFTh-PDI as the acceptor. Our findings provide important guidelines for the design of more efficient ortho-functionalized PDI acceptors.