Chengyi Xiao

Three-Bladed Rylene Propellers with Three-Dimensional Network Assembly for Organic Electronics

Two kinds of conjugated C3-symmetric perylene dyes, namely, triperylene hexaimides (TPH) and selenium-annulated triperylene hexaimides (TPH-Se), are efficiently synthesized. Both TPH and TPH-Se have broad and strong absorption in the region 300-600 nm together with suitable LUMO levels of about -3.8 eV. Single-crystal X-ray diffraction studies show that TPH displays an extremely twisted three-bladed propeller configuration and a unique 3D network assembly in which three PBI subunits in one TPH molecule have strong π-π intermolecular interactions with PBI subunits in neighboring molecules. The integration of selenophene to TPH endows TPH-Se with a more distorted propeller configuration and a more compact 3D network assembly due to the Se···O interactions. A single-crystal transistor confirms that both TPH and TPH-Se possess good electron-transport ability. TPH and TPH-Se acceptor-based solar cells show high power conversion efficiency of 8.28% and 9.28%, respectively, which mainly results from the combined properties of broad and strong absorption ability, appropriate LUMO level, desirable aggregation, high electron mobility, and good film morphology with the polymer donor.

Asymmetric Diketopyrrolopyrrole Conjugated Polymers for Field-Effect Transistors and Polymer Solar Cells Processed from a Nonchlorinated Solvent.

Newly designed asymmetric diketopyrrolopyrrole conjugated polymers with two different aromatic substituents possess a hole mobility of 12.5 cm(2) V(-1) s(-1) in field-effect transistors and a power conversion efficiency of 6.5% in polymer solar cells, when solution processed from a nonchlorinated toluene/diphenyl ether mixed solvent.

Localization/Delocalization of Charges in Bay-Linked Perylene Bisimides

The copper-mediated Ullmann coupling of 1,7-dibromoperylene bisimides afforded structurally perfect singly-linked perylene bisimide (PBI) arrays, whilst the homo-coupling of 1,12-dibromoperylene bisimides gave doubly-linked and triply-linked diperylene bisimides. The interactions of three bay-linked diperylene bisimides that differed in their linkage (singly, doubly, and triply) were investigated in their neutral and reduced forms (mono-anion to tetra-anion). UV/Vis absorption and fluorescence spectroscopy revealed different degrees of interaction, which was explained by exciton coupling and conjugation effects. The electrochemical properties and spectroelectrochemistry also showed quite-different degrees of PBI interactions in the reduced mixed-valence species, which was apparent by the observation of CT bands. The interpretation of the experimental findings was supported by spin-restricted and -unrestricted DFT and time-dependent TD-DFT calculations with the long-range-corrected CAM-B3LYP functional. Accordingly, the degree of interaction in both the neutral and reduced forms of the bay-linked PBIs was qualitatively in the order doubly linked<singly linked << triply linked, owing to the different degrees of twisting and flexibility between the two PBIs moieties. Only triply linked diPBI showed completely delocalized wavefunctions over the entire π-system.

Synthesis and Properties of Heterocyclic Acene Diimides

A series of heterocyclic acene diimides were synthesized effectively based on the condensation of o-phenylenediamine, 1,2-benzenedithiol, and 2-aminothiophenol with 2,3,6,7-tetrabromo-1,4,5,8-naphthalene tetracarboxylic diimide. The diimides exhibit interesting optical and electrical properties with one of them showing a hole mobility up to 0.02 cm(2) V(-1) s(-1).

Cyano-substituted perylene diimides with linearly correlated LUMO levels.

A series of nonbay region cyano-substituted tetrachloroperylene diimides with tunable LUMO energy levels from -4.21 to -4.64 eV were developed. The excellent linear correlation between LUMO energy levels and the number of cyano groups indicated that the LUMO energy levels could be lowered effectively and predictably by incorporating cyano groups.

Pyridine-bridged diketopyrrolopyrrole conjugated polymers for field-effect transistors and polymer solar cells

Five wide or medium band gap diketopyrrolopyrrole (DPP)-based conjugated polymers with pyridine as bridges were developed for organic field-effect transistors (OFETs) and polymer solar cells (PSCs). By introducing copolymerized aromatic building blocks from strong electron-donating units to electron-deficient units into the conjugated backbone, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the DPP polymers were tailored to the low-lying position. Therefore, the polarity of charge transport in OFETs can be switched from p-type to n-type. The DPP polymer with a low-lying LUMO of −3.80 eV provides a hole-only mobility of 2.95 × 10−2 cm2 V−1 s−1, while an electron-only mobility of 1.24 × 10−3 cm2 V−1 s−1 is found in the DPP polymer with a LUMO of −4.22 eV. Further investigation of photovoltaic cells based on these DPP polymers shows a modest power conversion efficiency (PCE) of around 2%. Our results demonstrate that wide band gap pyridine-bridged DPP polymers have potential application in OFETs and OSCs by adjusting their energy level with alternated units on the conjugated backbone.

Thermally Sensitive Self-Assembly of Glucose-Functionalized Tetrachloro-Perylene Bisimides: From Twisted Ribbons to Microplates

Chiral supramolecular structures are becoming increasingly attractive for their specific molecular arrangements, exceptional properties, and promising applications in chiral sensing and separation. However, constructing responsive chiral supramolecular structures remains a great challenge. Here, glucose-functionalized tetrachloro-perylene bisimides (GTPBIs) with thermally sensitive self-assembly behaviors are designed and synthesized. In a methanol/water mixture, GTPBIs self-assembled into twisted ribbons and microplates at 4 and 25 °C, respectively. Furthermore, the ribbon structure was metastable and could transform into microplates when the temperature was increased from 4 to 25 °C. Transmission electron microscopy (TEM) was used to track the evolution of morphology and study the assembly mechanisms of correponding nanostructures at different time intervals. The supramolecular structures were characterized with various techniques, including circular dichroism, TEM, scanning electron microscopy, atomic force microscopy, ultraviolet-visible absorption, and fluorescence spectra. This study provides insight into controlling molecular parameters and assembly conditions to construct chiral supramolecular structures.

Polycyclic aromatic hydrocarbons with orthogonal tetraimides as n-type semiconductors

Two new polycyclic aromatic hydrocarbons with orthogonal imide groups have been synthesized and characterized. These semiconductors have strong electron affinities with an electron mobility of up to 1.75 cm2 V-1 s-1 in solution-processed single-crystalline microfibers.

Conjugated polymers with deep LUMO levels for field-effect transistors and polymer–polymer solar cells

Three thiazole-bridged DPP polymers with deep lowest unoccupied molecular orbital (LUMO) levels were designed for field-effect transistors (FETs) and polymer–polymer solar cells. By introducing thiazole–thiazole coupled segments, perfluoroalkyl side chains or strong electron-deficient naphthalenediimide units into the conjugated backbone the three thiazole-bridged DPP polymers have LUMO levels of −4.0 to −4.4 eV. The three DPP polymers exhibit optical absorption in the near-infrared region, crystallinity and an electron mobility of around 0.01 cm2 V−1 s−1 in bottom contact FETs. The polymers were applied as electron acceptors in polymer–polymer solar cells to provide PCEs of around 0.4%. The low PCEs are mainly due to low short-circuit currents (Jsc) and attributed to large phase separation. Our results demonstrate several efficient strategies to lower the energy levels of conjugated polymers in order to be used as universal acceptors for photovoltaic cells.

Laterally Expanded Rylene Diimides with Uniform Branched Side Chains for Solution-Processed Air Stable n-Channel Thin Film Transistors

Molecular packing motifs in solid states is the dominant factor affecting the n-channel organic field-effect transistors (OFETs). However, few systematic researches were performed in the different extensions of π-conjugated molecules with the uniform substitution effecting the molecular packing motifs. In this manuscript, OFET devices based on three latterally expanded rylene diimides end-functionalized with uniform 3-hexylundecyl substitution on the imide positions were systematically studied on the relationship of molecular stacking, film microstructure, and charge transport. As the π-conjugated systems expanded from doubly linked perylene diimide dimer (d-4CldiPDI, 1), triply linked perylene diimide dimer (t-4CldiPDI, 2), to hybrid array (NDI-PDI-NDI, 3), their corresponding molecular packing motifs exhibited a divide: the optimized molecular configuration became more planar and d (001) spacing distances became larger, which resulted in a larger π-π overlapping. Thus, an enhanced electron mobility was obtained. A typical n-channel field-effect characteristic was observed in thin film devices based on these molecules under ambient conditions. Especially, the hybrid system (3) with more planar and π-expanded aromatic backbone exhibited superior electron mobility approaching 0.44 cm(2) V(-1) s(-1) and on/off ratio of 10(6) after optimal annealing in this study.