News

Check out our new paper titled 'Serendipitous formation of semiconducting semi-NINDIGO indigoid by degradation of diindolopyrrole' on J. Org. Chem. Congrats to Nathan Yee (Ph.D. student) and Afshin Dadvand (Research associate)!

Short description:
We report the serendipitous discovery and synthesis of an indigoid “Semi-Nindigo” (2) via oxidation of a diindolopyrrole (1). Reaction of 2 with BF3Et2O affords the borylated derivate (3). The electronic spectra of 2 and 3 possess intense long wave absorptions near 600 nm and 650 nm. 3 is weakly emissive in the near-infrared. Thin film OFETs fabricated with 1 and 2 both exhibited modest p-type mobilities.

Tags: Small molecule, OFET

Check out our new tutorial review article titled 'A Surface-confined single-layer covalent organic frameworks: design, synthesis and application' on Chem. Soc. Rev. Collaboration article with Jennifer M. MacLeod (Queensland University of Technology, Australia) and Federico Rosei (INRS, Canada)

Short description:
Two-dimensional (2D) nanomaterials, such as graphene and single layer covalent organic frameworks (sCOFs) are being widely studied due to their unusual structure/property relationships. sCOFs typically feature atomic thickness, intrinsic nanoscale porosity and a crystalline lattice. Compared to other organic 2D materials, sCOFs exhibit major advantages including topological designation and constitutional tunability. This review describes the state of the art of surface-confined sCOFs, emphasizing reticular design, synthesis approaches, and key challenges related to improving quality and exploring applications.

Tags: COFs, STM

Check out our new paper titled 'A Trifluoromethyl Group Modified Non-fullerene Acceptor Towards Improved Power Conversion Efficiency Over 13% in Polymer Solar Cells' on ACS Appl. Mater. Interfaces. Collaboration article with Hong Meng (Peking University Shenzhen Graduate School, China)

Short description:
Herein, we report a new molecule structure modification strategy for non-fullerene small molecule electron acceptors (NFAs) for solar cells through trifluoromethylation of end-capping groups. The synthesized trifluoromethylated acceptor ITCF3 exhibits narrower band gap, stronger light absorption, lower molecular energy levels and better electron transport property comparing to the reference NFA without trifluoromethyl group (ITIC). Bulk heterojunction solar cells based on ITCF3 combined with PM6 polymer donor exhibit a significantly improved power conversion efficiency of 13.3% comparing with ITIC-based device (8.4%). This work reveals great potential of trifluoromethylation in design of efficient photovoltaic acceptor materials.

Tags: Small pi-conjugated molecule, Photovoltaics

Congratulations to Dr. Chaoying Fu in our group for the new paper titled 'Surface-Confined Macrocyclization via Dynamic Covalent Chemistry' on ACS Nano. Collaboration article with Luca Muccioli (Università di Bologna, Italy) and Emanuele Orgiu (INRS, Canada)

Short description:
Surface-confined synthesis is a promising approach to build complex molecular nanostructures including macrocycles. However, despite the recent advances in on-surface macrocyclization under ultrahigh vacuum, selective synthesis of monodisperse and multicomponent macrocycles remains a challenge. Here, we report on an on-surface formation of [6 + 6] Schiff-base macrocycles via dynamic covalent chemistry. The macrocycles form two-dimensional crystalline domains on the micrometer scale, enabled by dynamic conversion of open-chain oligomers into well-defined ∼3.0 nm hexagonal macrocycles. We further show that by tailoring the length of the alkyl substituents, it is possible to control which of three possible products—oligomers, macrocycles, or polymers—will form at the surface. In situ scanning tunneling microscopy imaging combined with density functional theory calculations and molecular dynamics simulations unravel the synergistic effect of surface confinement and solvent in leading to preferential on-surface macrocyclization.

Tags: STM, Macrocycle, Polymer, Dynamic covalent chemistry, DFT, Molecular dynamics

Congratulations to Dr. Lakshmi Vellanki, Cheng-Hao Liu, Rajesh Rao, and all the co-authors in our group for the new paper titled 'A Two-dimensional Poly (azatriangulene) Covalent Organic Framework with Semiconducting and Paramagnetic States' on J. Am. Chem. Soc.

Short description:
The black crystalline (aza)triangulene-based covalent organic framework TANG-COF was synthesized from its trinitro-TANG precursor via a one-pot, two-step reaction involving Pd-catalyzed hydrogenation and polycondensation with an aromatic dialdehyde. High crystallinity and permanent porosity of the layered two-dimensional (2D) structure were established. The rigid, electron-rich trioxaazatriangulene (TANG) building block enables strong π-electron interactions manifested in broad absorptions across the visible and NIR regions (E_g ≈ 1.2 eV). The high HOMO energy of TANG-COF (−4.8 eV) enables facile p doping, resulting in electrical conductivity of up to 10^–2 S/cm and room-temperature paramagnetic behavior with a spin concentration of ∼10%. DFT calculations reveal dispersion of the highest occupied band both within the 2D polymer layers (0.28 eV) and along their π-stacked direction (0.95 eV).

Tags: COFs, Open-shell Radical, Conductivity, Doping

Congratulations to Cheng-Hao Liu and Ehsan Hamzehpoor for the regular front issue cover on Can. J. Chem. for 2020! The photo they took won the ChemiSTEAM contest at 102nd Canadian Chemistry Conference and Exhibition, and it was further selected for the front issue cover of Can. J. Chem. for their regular issues in 2020. Check it out here and also on Twitter.

Congratulations to Yaroslav Zems (alumni, M.Sc.) and Dr. Rizwan Niazi in our group for the new paper titled 'Star-shaped Triarylamine-based Hole-transport Materials in Perovskite Solar Cells' on Sustainable Energy & Fuels! Collaboration article with Trystan M. Watson (Swansea University, UK) and Neil Robertson (University of Edinburgh, UK).

Short description:
Two novel star-shaped triarylamine-based hole transport materials with triphenylamine (STR1), or a partially oxygen-bridged triphenylamine (STR0), as core and para-substituted triphenylamine side arms were synthesized, fully characterized and studied in perovskite solar cells. Their thermal, optical, electrochemical and charge transport properties were examined and compared in the context of their molecular structure. Due to its more planar configuration, STR0 showed a red-shifted absorption in comparison with STR1. STR0 also forms a more stable amorphous glassy state and showed higher glass transition temperature than STR1 and Spiro-OMeTAD. These HTMs were tested in perovskite solar cells using a device configuration of FTO/bl-TiO2/mp-TiO2/CH3NH3PbI3/HTM/Au showing a power conversion effiency of 13.3% for STR0 and 11.5% for STR1. The STR0-based devices showed higher fill factor and better reproducibility than spiro-OMeTAD-based cells. Without dopant additives, solar cells based on STR0 exhibited a good photocurrent density of 16.63 mA cm-2 and the efficiency improved from a starting PCE of 3.9% to 6.6% after two weeks of storage.

Tags: Perovskite, Photovoltaics, Hole-transporting material

Congratulations to Ehsan Hamzehpoor in our group for the new paper titled 'Crystal Engineering of Room Temperature Phosphorescence in Organic Solids' on Angew. Chem. Int. Ed.!

Short description:
We report a series of highly emissive azatriangulenetrione(TANGO) solids in which the luminescent properties are controlled by engineering the molecular packing by adjusting the steric size of substituents. The co-alignment of ‘phosphorogenic’ carbonyl groups within the pi-stacks results in an almost pure triplet emission in HTANGO, TCTANGO, TBTANGO and TITANGO, while their rotation by ~60 degrees in the sterically hindered tBuTANGO leads to an almost pure singlet emission. Despite strong pi-interactions, aggregation-induced quenching and triplet-triplet annihilation are avoided in HTANGO and TCTANGO which display efficient phosphorescence in the solid state. To our knowledge, HTANGO with the solid-state phosphorescence quantum yield of 42% at room temperature is the most efficient phosphor composed of the 1st/2nd raw elements only.

Tags: (Room-temperature) Phosphorescence, Crystal engineering

Congrats to Dr. Yuan Fang and co-workers in our group for the new paper titled 'Covalent organic frameworks from a monomer with reduced symmetry: polymorphism and Sierpinski triangles' ; on Chem. Commun.!

Short description:
We report on the synthesis of a covalent organic framework based on the low-symmetry 1,3-benzenediboronic acid precursor. Control over polymorph formation, either honeycomb network or Sierpinski triangles, was achieved by varying the precursor concentration for on-surface synthesis as elucidated by scanning tunneling microscopy.

Tags: COF, STM

Congrats to Ehsan Hamzehpoor and co-workers in our group and Prof. Federico Rosei's group for the new paper titled 'Temperature-induced molecular reorganization on Au(111) driven by oligomeric defects'; on Nanoscale!

Short description:
The formation of ordered molecular structures on surfaces is determined by the balance between molecule–molecule and molecule–substrate interactions. Whether the aggregation process is guided by non-covalent forces or on-surface reactions, a deeper understanding of these interactions is pivotal to formulating a priori predictions of the final structural features and the development of bottom-up fabrication protocols. Theoretical models of molecular systems corroborate the information gathered through experimental observations and help explain the thermodynamic factors that underpin on-surface phase transitions. Here, we report a scanning tunneling microscopy investigation of a tribromo-substituted heterotriangulene on the Au(111) surface, which initially forms an extended close-packed ordered structure stabilized by Br⋯Br halogen bonds when deposited at room temperature. X-ray photoelectron spectroscopy reveals that annealing the self-assembled layer induces a fraction of the molecular precursors to partially dehalogenate that in turn leads to the formation of a less stable Br⋯O non-covalent network which coexists with the short oligomers. Density functional theory (DFT) and Monte Carlo (MC) simulations illustrate how dimer moieties act as defects whose steric hindrance prevents the retention of the more stable configuration. A small number of dimers is sufficient to drive the molecular reorganization into a lower cohesive energy phase. Our study shows the importance of a combined DFT – MC approach to understand the evolution of molecular systems on substrates.

Tags: STM

Congrats to Dr. Chaoying Fu and co-workers in our group for the new paper titled 'Dynamic covalent conjugated polymer epitaxy on graphene'; on J. Mater. Chem. C!

Short description:
Hybrid heterostructures formed from ordered molecular layers on two-dimensional materials can have unique properties differing from those of their bulk phases. By employing principles of dynamic covalent chemistry, we have synthesized a series of novel conjugated polyimines that form epitaxial ordered monolayers on graphene. The interplay between molecular physisorption and dynamic polymerization at the solid–liquid interface drives the formation of longer chains at the surface with dramatically higher rates than in solution. The physico-chemical properties of such assemblies at different length scales on graphene were investigated by a combination of experimental techniques. ‘Covalent dynamic epitaxy’ was also found to modulate the properties of both substrate and dynamers such as doping and photoluminescence, respectively.

Tags: Polymers, STM

Congrats to Cheng-Hao Liu & Dr. M. Rizwan Niazi in our group for the new paper titled 'Remarkable Enhancement of of pi-Electron Donor/Acceptor Ability by DD/AA Complementary Hydrogen Bonding' on Angew. Chem. Int. Ed.!

Short description:
Supramolecular control is critical for devices of organic pi-conjugated materials, but it often lack directionality and neglect changes in electronic structures. In their Research Article, C.-H. Liu, M.R. Niazi, D.F. Perepichka present a DD/AA complementary hydrogen bonding method to directionally assemble a family of weak donor/acceptor molecules that then act as strong D/A assemblies. The dramatically altered electronic structures manifest in low bandgaps, close π-π distances, emergence of radicals, and tunable semiconducting behaviours (p-type, ambipolar, intrinsic; gate- or light-controlled).

Tags: Crystal enginnering, Charge-transfer, (Photo)transistors

Congrats to Dr. Thaksen Jadhav and co-workers for the new paper titled '2D Poly(arylene vinylene) Covalent Organic Frameworks via Aldol Condensation of Trimethyltriazine' on Angew. Chem. Int. Ed.!

Short description:
Here we report a series of 2D vinylene-linked covalent organic frameworks via aldol condensation of trimethyltriazine. The crystalline, porous, pi-conjugated COFs display numerous interesting properties such as swellability (i.e. dynamic behaviour) and tunable emission (up to 50% quantum yield).

Tags: 2D sp2 COF, Luminescence, Dynamic

Congrats to Dr. Fang Yuan and Dr. M. Rizwan Niazi in our group and Prof. Ori Gidron's group for the new paper titled 'A macrocyclic oligofuran: synthesis, solid state structure and electronic properties' on Chem. Sci!

Short description:
We report the first π-conjugated macrocyclic system with an oligofuran backbone. The calculated HOMO–LUMO gap is similar to that of the corresponding linear polymer, indicating a remarkable electron delocalization. The X-ray structure reveals a planar conformation, in contrast to the twisted conformation of macrocyclic oligothiophenes. The intermolecular π–π stacking distance is extremely small (3.17 Å), indicating very strong interactions. The macrocycle forms large π-aggregates in solution and shows a tendency toward highly ordered multilayer adsorption at the solid–liquid interface. The face-on orientation of molecules explains the higher hole mobility observed in the out-of-plane direction.

Tags: Small pi-conjugated molecule, SCLC

Congrats to Yuxuan Che, Cheng-Hao Liu and co-workers for the new paper titled 'Understanding the Photovoltaic Behavior of A–D–A Molecular Semiconductors through a Permutation of End Groups' on J. Org. Chem.!

Short description:
We report the facile synthesis of a series of benzodithiophene (BDT)- and indacenodithiophene (IDT)-based A–D–A oligomers with different end groups, where their properties are studied by optical spectroscopy, electrochemistry, and density functional theory calculations. The permutation of central and terminal units tunes the optoelectronic properties and photovoltaic device characteristics in a predictable way, aiding in the rational design of small molecule semiconducting materials. Among the three rhodanine-derived terminal groups, N-alkylthiazolonethione revealed the strongest electron-withdrawing character, resulting in the lowest band gap, the highest stability, and the best photovoltaic device performance. The crystallographic analysis of two IDT derivatives yielded a highly unusual three-dimensional packing of the conjugated backbone, which is likely responsible for the remarkable photovoltaic performance of such A–D–A semiconductors.

Tags: Small pi-conjugated molecule, Photovoltaics