Photoactive organic material discovery with combinatorial supramolecular assembly

Insights into Decoupled Solar Energy Conversion and Charge Storage in a 2D Covalent Organic Framework for Solar Battery Function

Decoupling solar energy conversion and storage in a single material offers a great advantage for off-grid applications. Herein, we disclose a two-dimensional naphthalenediimide (NDI)-based covalent organic framework (COF) exhibiting remarkable solar battery performance when used as a photoanode. Light-induced radicals are stabilized within the framework for several hours, offering on-demand charge extraction for electrical energy production. Our study reveals mechanistic insights into the long-term charge stabilization using optical spectroscopy and (photo)electrochemical measurements, in conjunction with density functional theory (DFT) simulations. Among several solvents, water provides the best dielectric screening and energetically favorable proton exchange to stabilize photoinduced radicals for more than 48 h without the need for additional metal cations. This study provides fundamental insights into the optoionic charge storage mechanism in NDI-COF, while introducing a highly tunable, nanoporous material platform that surpasses related materials, such as carbon nitrides, metal-organic frameworks (MOFs), or metal oxides, in terms of charge storage capacity. This study opens new perspectives for the design of optoionic charge-storing materials and the direct storage of solar energy to overcome the intermittency of solar irradiation.

Theoretical Insights into Ultrafast Separation of Photogenerated Charges in a Push-Pull Polarized Molecular Triad

Herein, we propose a purely-organic donor-acceptor (D-A) molecular triad, with a light-absorbing polarized molecular wire (PMW) used as a central linkage, as a proof of concept for the possible future applications of the D-PMW-A arrangement in molecular photovoltaics. This work builds upon our earlier study on the PMW unit itself, which proved to be highly promising for the ultrafast photogeneration of free charge carriers. Quantum-chemical calculations performed for the D-PMW-A triad at a semi-empirical level of theory reveal a large electric dipole moment of the system, and show strong charge-transfer (CT) character of its lowest-energy excited electronic states, including theS 1 ${S_1 }$ , which favours efficient dissociation of an exciton initially formed upon the absorption of light. The confirmation for this effect was found with nonadiabatic molecular dynamics simulations, revealing an ultrafast relaxation from higher, bright excited states toS 1 ${S_1 }$ , completed on a subpicosecond timescale. The architecture of the proposed molecular triad enables its electronic coupling to the surrounding environment through chemical bonds, or noncovalent stacking interactions, which might open way for synthesis of a new class of D-PMW-A efficient molecular organic photovoltaic materials.

Tailoring the Weight of Surface and Intralayer Edge States to Control LUMO Energies

The energies of the frontier molecular orbitals determine the optoelectronic properties in organic films, which are crucial for their application, and strongly depend on the morphology and supramolecular structure. The impact of the latter two properties on the electronic energy levels relies primarily on nearest-neighbor interactions, which are difficult to study due to their nanoscale nature and heterogeneity. Here, an automated method is presented for fabricating thin films with a tailored ratio of surface to bulk sites and a controlled extension of domain edges, both of which are used to control nearest-neighbor interactions. This method uses a Langmuir-Schaefer-type rolling transfer of Langmuir layers (rtLL) to minimize flow during the deposition of rigid Langmuir layers composed of π-conjugated molecules. Using UV-vis absorption spectroscopy, atomic force microscopy, and transmission electron microscopy, it is shown that the rtLL method advances the deposition of multi-Langmuir layers and enables the production of films with defined morphology. The variation in nearest-neighbor interactions is thus achieved and the resulting systematically tuned lowest unoccupied molecular orbital (LUMO) energies (determined via square-wave voltammetry) enable the establishment of a model that functionally relates the LUMO energies to a morphological descriptor, allowing for the prediction of the range of accessible LUMO energies.

Combinatorial mixtures of organic solutes for improved liquid/liquid extraction of ions

Biological systems routinely extract and organize ions in complex yet highly ordered and active systems. Much of this function is attributed to proteins, although recent evidence indicates aggregates of lipids are also capable of molecular recognition. Here we tested the hypothesis that combinatorial mixtures of organic solutes might lead to enhanced liquid/liquid extraction. We started with liquid oleic acid as an organic phase extracting copper ions from water and added a library of additives. By using Bayesian optimization to autonomously direct the combinatorial formulation, we discovered mixtures that enhanced the extraction performance. The main additive that improved the system was octylphosphonic acid. Interestingly, the optimal mixture has a significant improvement compared to this additive alone. This suggests that the combinations of organic solutes are better than using pure components in liquid/liquid extraction. Furthermore, we found that precipitation occurs in the samples showing better extraction efficiency, which has interesting material properties and potential for new types of supramolecular biosensors.

Donor-Acceptor Hybrid Heterostructures: An Emerging Class of Photoactive Materials with Inorganic and Organic Semiconductive Components

Just as the heterojunctions in physics, donor-acceptor (D-A) heterostructures are an emerging class of photoactive materials fabricated from two semiconductive components at the molecular level. Among them, D-A hybrid heterostructures from organic and inorganic semiconductive components have attracted extensive attention in the past decades due to their combined advantages of high stability for the inorganic semiconductors and modifiability for the organic semiconductors, which are particularly beneficial to efficiently achieve photoinduced charge separation and transfer upon irradiations. In this review, by analogy with the heterojunctions in physics, a definition of the D-A heterostructures and their general design and synthetic strategies are given. Meanwhile, the D-A hybrid heterostructures are focused on and their recent advances in potential applications of photochromism, photomodulated luminescence, and photocatalysis summarized.

Recent Progress in the Design of Fused-Ring Non-Fullerene Acceptors-Relations between Molecular Structure and Optical, Electronic, and Photovoltaic Properties

Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018-2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.

Peptide-Based Supramolecular Systems Chemistry

Peptide-based supramolecular systems chemistry seeks to mimic the ability of life forms to use conserved sets of building blocks and chemical reactions to achieve a bewildering array of functions. Building on the design principles for short peptide-based nanomaterials with properties, such as self-assembly, recognition, catalysis, and actuation, are increasingly available. Peptide-based supramolecular systems chemistry is starting to address the far greater challenge of systems-level design to access complex functions that emerge when multiple reactions and interactions are coordinated and integrated. We discuss key features relevant to systems-level design, including regulating supramolecular order and disorder, development of active and adaptive systems by considering kinetic and thermodynamic design aspects and combinatorial dynamic covalent and noncovalent interactions. Finally, we discuss how structural and dynamic design concepts, including preorganization and induced fit, are critical to the ability to develop adaptive materials with adaptive and tunable photonic, electronic, and catalytic properties. Finally, we highlight examples where multiple features are combined, resulting in chemical systems and materials that display adaptive properties that cannot be achieved without this level of integration.

Free Charge Carriers in Homo-Sorted π-Stacks of Donor-Acceptor Conjugates

Inspired by the high photoconversion efficiency observed in natural light-harvesting systems, the hierarchical organization of molecular building blocks has gained impetus in the past few decades. Particularly, the molecular arrangement and packing in the active layer of organic solar cells (OSCs) have garnered significant attention due to the decisive role of the nature of donor/acceptor (D/A) heterojunctions in charge carrier generation and ultimately the power conversion efficiency. This review focuses on the recent developments in emergent optoelectronic properties exhibited by self-sorted donor-on-donor/acceptor-on-acceptor arrangement of covalently linked D-A systems, highlighting the ultrafast excited state dynamics of charge transfer and transport. Segregated organization of donors and acceptors promotes the delocalization of photoinduced charges among the stacks, engendering an enhanced charge separation lifetime and percolation pathways with ambipolar conductivity and charge carrier yield. Covalently linking donors and acceptors ensure a sufficient D-A interface and interchromophoric electronic coupling as required for faster charge separation while providing better control over their supramolecular assemblies. The design strategies to attain D-A conjugate assemblies with optimal charge carrier generation efficiency, the scope of their application compared to state-of-the-art OSCs, current challenges, and future opportunities are discussed in the review. An integrated overview of rational design approaches derived from the comprehension of underlying photoinduced processes can pave the way toward superior optoelectronic devices and bring in new possibilities to the avenue of functional supramolecular architectures.

Crystal structure and orientation of organic semiconductor thin films by microcrystal electron diffraction and grazing-incidence wide-angle X-ray scattering

We use microcrystal electron diffraction (MicroED) to determine structures of three organic semiconductors, and show that these structures can be used along with grazing-incidence wide-angle X-ray scattering (GIWAXS) to understand crystal packing and orientation in thin films. Together these complimentary techniques provide unique structural insights into organic semiconductor thin films, a class of materials whose device properties and electronic behavior are sensitively dependent on solid-state order.