A Stable Triplet-Ground-State Conjugated Diradical Based on a Diindenopyrazine Skeleton.

High-spin conjugated radicals have great potential in magnetic materials and organic spintronics. However, to obtain high-spin conjugated radicals is still quite challenging due to their poor stability. We report the successful synthesis and isolation of a stable triplet conjugated diradical, 10,12-diaryldiindeno[1,2-b:2',1'-e]pyrazine (m-DIP). With the m-xylylene analogue skeleton containing electron-deficient sp2 -nitrogen atoms, m-DIP displays significant aromatic character within its pyrazine ring and its spin density mainly delocalizes on the meta-pyrazine unit, making it a triplet ground state conjugated diradical. Our work provides an effective "spin density tuning" strategy for stable high-spin conjugated radicals.

Enhancing the n-Type Conductivity and Thermoelectric Performance of Donor-Acceptor Copolymers through Donor Engineering.

Conjugated polymers with high thermoelectric performance enable the fabrication of low-cost, large-area, low-toxicity, and highly flexible thermoelectric devices. However, compared to their p-type counterparts, n-type polymer thermoelectric materials show much lower performance, which is largely due to inefficient doping and a much lower conductivity. Herein, it is reported that the development of a donor-acceptor (D-A) polymer with enhanced n-doping efficiency through donor engineering of the polymer backbone. Both a high n-type electrical conductivity of 1.30 S cm-1 and an excellent power factor (PF) of 4.65 µW mK-2 are obtained, which are the highest reported values among D-A polymers. The results of multiple characterization techniques indicate that electron-withdrawing modification of the donor units enhances the electron affinity of the polymer and changes the polymer packing orientation, leading to substantially improved miscibility and n-doping efficiency. Unlike previous studies in which improving the polymer-dopant miscibility typically resulted in lower mobilities, the strategy maintains the mobility of the polymer. All these factors lead to prominent enhancement of three orders magnitude in both the electrical conductivity and the PF compared to those of the non-engineered polymer. The results demonstrate that proper donor engineering can enhance the n-doping efficiency, electrical conductivity, and thermoelectric performance of D-A copolymers.

Chemical Modification toward Long Spin Lifetimes in Organic Conjugated Radicals.

Organic semiconductors for spin-based devices require long spin relaxation times. Understanding their spin relaxation mechanisms is critical to organic spintronic devices and applications for quantum information processing. However, reports on the spin relaxation mechanisms of organic conjugated molecules are rare and the research methods are also limited. Herein, we study the molecular design and spin relaxation mechanisms by systematically varying the structure of a conjugated radical. We found that solid-state relaxation times of organic materials are largely different from that in solution state. We demonstrate that substitution of a lower gyromagnetic ratio nucleus (e. g. D, Cl) on the para-position of the aryl rings in the triphenylmethyl (TM) radical can significantly improve their coherence times (Tm ). Flexible thin films based on such radicals exhibit ultra-long spin-lattice relaxation times (T1 ) up to 35.6(6) µs and Tm up to 1.08(4) µs under ambient conditions, which are among the longest values in films. More importantly, using the TM radical derivative (5CM), we observed room-temperature quantum coherence and Rabi cycles in thin film for the first time, suggesting that organic conjugated radicals have great potentials for spin-based information processing.

5,5'-Diazaisoindigo: an Electron-Deficient Building Block for Donor-Acceptor Conjugated Polymers.

A novel electron-deficient building block, 5,5'-diazaisoindigo (5DNIID), was synthesized through a facile method in good yield. As a derivative of isoindigo, 5DNIID shows an ultra-low LUMO level of -3.92 eV after incorporation of two electron-deficient nitrogen atoms into the para-position of the amine groups in the isoindigo π skeleton. Modified polymerization conditions were applied to efficiently afford the donor-acceptor (D-A) conjugated polymer 5DNIID-2T, which exhibited a p-type charge transport property.

Embedding electron-deficient nitrogen atoms in polymer backbone towards high performance n-type polymer field-effect transistors.

With sp2-nitrogen atoms embedded in an isatin unit, a donor-acceptor (D-A) conjugated polymer AzaBDOPV-2T was developed with a low LUMO level down to -4.37 eV. The lowered LUMO level as well as the conformation-locked planar backbone provide AzaBDOPV-2T with electron mobilities over 3.22 cm2 V-1 s-1 tested under ambient conditions, which is among the highest in n-type polymer field-effect transistors (FETs). Our results demonstrate that embedding electron-deficient sp2-nitrogen in conjugated backbones is an effective approach to develop n-type polymer semiconductors with high performance.

Toward electron-deficient pyrene derivatives: construction of pyrene tetracarboxylic diimide containing five-membered imide rings.

Electron-deficient pyrene-1,2,6,7-tetracarboxylic diimide () and its cyano derivative () have been designed and synthesized as a new family of aromatic diimides. PyrDI has two unexpected five-membered imide rings and can form excimers facilely in the solid state. These new pyrene derivatives are promising n-type semiconductors for organic electronics.

Rational molecular engineering towards efficient non-fullerene small molecule acceptors for inverted bulk heterojunction organic solar cells.

Two non-fullerene small molecules based on fluoranthene-fused imide were developed as acceptors for solution-processed inverted organic bulk heterojunction (BHJ) solar cells, which showed good power conversion efficiency and high open-circuit voltage.

Non-fullerene acceptors containing fluoranthene-fused imides for solution-processed inverted organic solar cells.

Six fluoranthene-fused imide derivatives were employed as acceptors in solution processed inverted BHJ solar cells with P3HT as the donor. The PCEs of all devices vary from 2.14% to 2.89%. All acceptors are in their amorphous state with low electron mobility, but achieving high PCEs.

A non-fullerene small molecule as efficient electron acceptor in organic bulk heterojunction solar cells.

A novel n-type small molecule FFI-1 was synthesized as the electron acceptor to replace PCBM in solution-processed organic BHJ solar cells. Its LUMO level (around -3.5 eV) both matches the work function of the cathode and increases V(OC) of the devices, making it a promising acceptor candidate. With P3HT: FFI-1 (1:2 w/w) as active layer and LiF/Al as the cathode, the best power conversion efficiency (PCE) reaches 1.86%.