Publications by authors named "Jean-Luc Brédas"

245 Publications

Engineering of flat bands and Dirac bands in two-dimensional covalent organic frameworks (COFs): relationships among molecular orbital symmetry, lattice symmetry, and electronic-structure characteristics.

Mater Horiz 2021 Dec 3. Epub 2021 Dec 3.

Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721-0088, USA.

Two-dimensional covalent organic frameworks (2D-COFs), also referred to as 2D polymer networks, display unusual electronic-structure characteristics, which can significantly enrich and broaden the fields of electronics and spintronics. In this Focus article, our objective is to lay the groundwork for the conceptual description of the fundamental relationships among the COF electronic structures, the symmetries of their 2D lattices, and the frontier molecular orbitals (MOs) of their core and linker components. We focus on monolayers of hexagonal COFs and use tight-binding model analyses to highlight the critical role of the frontier-MO symmetry, in addition to lattice symmetry, in determining the nature of the electronic bands near the Fermi level. We rationalize the intriguing feature that, when the core unit has degenerate highest occupied MOs [or lowest unoccupied MOs], the COF highest valence band [or lowest conduction band] is flat but degenerate with a dispersive band at a high-symmetry point of the Brillouin zone; the consequences of having such band characteristics are briefly described. Multi-layer and bulk 2D COFs are found to maintain the salient features of the monolayer electronic structures albeit with a reduced bandgap due to the interlayer coupling. This Focus article is thus meant to provide an effective framework for the engineering of flat and Dirac bands in 2D polymer networks.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/d1mh00935dDOI Listing
December 2021

Controlled n-Doping of Naphthalene Diimide-Based Two-Dimensional Polymers.

Adv Mater 2021 Nov 30:e2101932. Epub 2021 Nov 30.

Department of Chemistry, Northwestern University, Evanston, IL, 60208, USA.

Two-dimensional polymers (2DPs) are promising as structurally well-defined, permanently porous, organic semiconductors. However, 2DPs are nearly always isolated as closed shell organic species with limited charge carriers, which leads to low bulk conductivities. Here, we enhance the bulk conductivity of two naphthalene diimide (NDI)-containing 2DP semiconductors by controllably n-doping the NDI units using cobaltocene (CoCp ). Optical and transient microwave spectroscopy reveals that both as-prepared NDI-containing 2DPs are semiconducting with sub-2 eV optical bandgaps and photoexcited charge-carrier lifetimes of tens of nanoseconds. Following reduction with CoCp , both 2DPs largely retain their periodic structures and exhibit optical and electron-spin resonance spectroscopic features consistent with the presence of NDI-radical anions. While the native NDI-based 2DPs are electronically insulating, maximum bulk conductivities of >10 S cm are achieved by substoichiometric levels of n-doping. Density functional theory calculations show that the strongest electronic couplings in these 2DPs exist in the out-of-plane (π-stacking) crystallographic directions, which indicates that cross-plane electronic transport through NDI stacks is primarily responsible for the observed electronic conductivity. Taken together, this study underlines that controlled molecular doping is a useful approach to access structurally well-defined, paramagnetic, 2DP n-type semiconductors with measurable bulk electronic conductivities of interest for electronic or spintronic devices. This article is protected by copyright. All rights reserved.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/adma.202101932DOI Listing
November 2021

Lower limits for non-radiative recombination loss in organic donor/acceptor complexes.

Mater Horiz 2021 May 26. Epub 2021 May 26.

Department of Chemistry, University of Washington, Seattle, WA, 98195-2120, USA.

Understanding the factors controlling radiative and non-radiative transition rates for charge transfer states in organic systems is important for applications ranging from organic photovoltaics (OPV) to lasers and LEDs. We explore the role of charge-transfer (CT) energetics, lifetimes, and photovoltaic properties in the limit of very slow non-radiative rates by using a model donor/acceptor system with photoluminescence dominated by thermally activated delayed fluorescence (TADF). This blend exhibits an extremely high photoluminescence quantum efficiency (PLQY = ∼22%) and comparatively long PL lifetime, while simultaneously yielding appreciable amounts of free charge generation (photocurrent external quantum efficiency EQE of 24%). In solar cells, this blend exhibits non-radiative voltage losses of only ∼0.1 V, among the lowest reported for an organic system. Notably, we find that the non-radiative decay rate, , is on the order of 10 s, approximately 4-5 orders of magnitude slower than typical OPV blends, thereby confirming that high radiative efficiency and low non-radiative voltage losses are achievable by reducing . Furthermore, despite the high radiative efficiency and already comparatively slow , we find that is nevertheless much faster than predicted by Marcus-Levich-Jortner two-state theory and we conclude that CT-local exciton (LE) hybridization is present. Our findings highlight that it is crucial to evaluate how radiative and non-radiative rates of the LE states individually influence the PLQY of charge-transfer states, rather than solely focusing on the PLQY of the LE. This conclusion will guide material selection in achieving low non-radiative voltage loss in organic solar cells and high luminescence efficiency in organic LEDs.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/d1mh00529dDOI Listing
May 2021

Resolving Atomic-Scale Interactions in Non-Fullerene Acceptor Organic Solar Cells with Solid-State NMR Spectroscopy, Crystallographic Modelling, and Molecular Dynamics Simulations.

Adv Mater 2021 Nov 24:e2105943. Epub 2021 Nov 24.

University of Lille, CNRS, Centrale Lille Institut, Univ. Artois, UMR 8181, Unité de Catalyse et Chimie du Solide, Lille, F-59000, France.

Fused-ring core non-fullerene acceptors (NFAs), designated "Y-series", have enabled high-performance organic solar cells (OSCs) achieving over 18% power conversion efficiency (PCE). Since the introduction of these NFAs, much effort has been expended to understand the reasons for their exceptional performance. While several studies have identified key optoelectronic properties that govern high PCEs, little is known about the molecular level origins of large variations in performance, spanning from 5 to 18% PCE, e.g., in the case of PM6:Y6 OSCs. Here, we introduce a combined solid-state NMR, crystallography, and molecular modelling approach to elucidate the atomic-scale interactions in Y6 crystals, thin films, and PM6:Y6 bulk heterojunction (BHJ) blends. We show the Y6 morphologies in BHJ blends are not governed by the morphology in neat films or single crystals. Notably, PM6:Y6 blends processed from different solvents self-assemble into different structures and morphologies, whereby the relative orientations of the sidechains and end groups of the Y6 molecules to their fused-ring cores play a crucial role in determining the resulting morphology and overall performance of the solar cells. The molecular-level understanding of BHJs enabled by this approach will guide the engineering of next-generation NFAs for stable and efficient OSCs. This article is protected by copyright. All rights reserved.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/adma.202105943DOI Listing
November 2021

Luminescence and Stability Enhancement of Inorganic Perovskite Nanocrystals via Selective Surface Ligand Binding.

ACS Nano 2021 Nov 1. Epub 2021 Nov 1.

Advanced Membranes and Porous Materials Center, KAUST Catalysis Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia.

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as one of the most promising light-emitting materials for optoelectronic devices with outstanding performance. However, the facile detachment of surface capping organic ligands from these NCs leads to very poor colloidal stability and durability. This is mainly due to the weak interfacial interactions between the inorganic perovskite core and ligands, high density of surface defect states, and aggregation of NCs. Here, using a combination of time-resolved laser spectroscopy and density functional theory (DFT) calculations, we explored the major impact of surface orientations and terminations for both CsPbBr and CsPbBr NCs not only on the interfacial binding affinities with organic ligands but also on surface defect formation and NC aggregation. By rationalizing that surface trap states are responsible for the decrease in photoluminescence (PL) upon fabrication and purification, we propose a powerful ligand-engineering strategy for eliminating these trap states and preventing the aggregation of CsPbBr and CsPbBr NCs. Interestingly, we find that the surface orientation and dimensionality determine the degree of interfacial interactions between the inorganic perovskite core and ligands and subsequently control the overall PL intensity and NC stability. Our results demonstrate that a treatment of as-synthesized CsPbBr NCs consisting of the addition of extra oleylammonium bromide (OAmBr) as a capping ligand, allows the CsPbBr NCs to retain their green emission with increased PL intensity and quantum yields and improves colloidal durability. On the other hand, the ultraviolet emissions of CsPbBr NCs are effectively increased upon addition of extra cesium oleate (CsOL) as the trap states induced by surface cesium ions are largely reduced by the formation of Cs-O bonds. Our work provides a robust and adequate ligand engineering approach to significantly enhance the optical behavior of perovskite NCs with different dimensionalities and various compositions and to achieve more efficient and stable light-harvesting devices.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsnano.1c06480DOI Listing
November 2021

Impact of Imine Bond Orientations on the Geometric and Electronic Structures of Imine-based Covalent Organic Frameworks.

Chem Asian J 2021 Nov 26;16(22):3781-3789. Epub 2021 Oct 26.

Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721-0008, USA.

Many efforts are currently devoted to improving the stability and crystallinity of imine-based two-dimensional (2D) covalent organic frameworks (COFs) given their wide range of potential applications. The variation in the relative orientations of the imine bonds has been found to be a critical factor that impacts the stacking of the 2D COF layers, leads to the formation of isomer structures, and influences the crystallinity of the final product. Most investigations to date have focused only on the structural properties, while the role of the imine orientations on the electronic properties has not been studied systematically. Here, we explore this effect by examining how the electronic band structures, electronic couplings, and effective masses evolve when considering four isomeric structures of an imine-linked tetraphenyl-pyrene naphthalene-diimide COF. Our results provide an understanding of the impact of the imine orientations and how they need to be controlled to realize COF inter-layer stackings that can lead to efficient cross-plane electron transport. They can be used to guide the design and synthesis of imine-based COFs for applications where charge transport needs to be optimized.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/asia.202101011DOI Listing
November 2021

Strong Suppression of Thermal Conductivity in the Presence of Long Terminal Alkyl Chains in Low-Disorder Molecular Semiconductors.

Adv Mater 2021 Sep 3;33(37):e2008708. Epub 2021 Aug 3.

Optoelectronics Group, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, UK.

While the charge transport properties of organic semiconductors have been extensively studied over the recent years, the field of organics-based thermoelectrics is still limited by a lack of experimental data on thermal transport and of understanding of the associated structure-property relationships. To fill this gap, a comprehensive experimental and theoretical investigation of the lattice thermal conductivity in polycrystalline thin films of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (Cn-DNTT-Cn with n = 0, 8) semiconductors is reported. Strikingly, thermal conductivity appears to be much more isotropic than charge transport, which is confined to the 2D molecular layers. A direct comparison between experimental measurements (3ω-Völklein method) and theoretical estimations (approach-to-equilibrium molecular dynamics (AEMD) method) indicates that the in-plane thermal conductivity is strongly reduced in the presence of the long terminal alkyl chains. This evolution can be rationalized by the strong localization of the intermolecular vibrational modes in C8-DNTT-C8 in comparison to unsubstituted DNTT cores, as evidenced by a vibrational mode analysis. Combined with the enhanced charge transport properties of alkylated DNTT systems, this opens the possibility to decouple electron and phonon transport in these materials, which provides great potential for enhancing the thermoelectric figure of merit ZT.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/adma.202008708DOI Listing
September 2021

Manipulation of hot carrier cooling dynamics in two-dimensional Dion-Jacobson hybrid perovskites via Rashba band splitting.

Nat Commun 2021 Jun 28;12(1):3995. Epub 2021 Jun 28.

Advanced Membranes and Porous Materials Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal, 23955-6900, Kingdom of Saudi Arabia.

Hot-carrier cooling processes of perovskite materials are typically described by a single parabolic band model that includes the effects of carrier-phonon scattering, hot phonon bottleneck, and Auger heating. However, little is known (if anything) about the cooling processes in which the spin-degenerate parabolic band splits into two spin-polarized bands, i.e., the Rashba band splitting effect. Here, we investigated the hot-carrier cooling processes for two slightly different compositions of two-dimensional Dion-Jacobson hybrid perovskites, namely, (3AMP)PbI and (4AMP)PbI (3AMP = 3-(aminomethyl)piperidinium; 4AMP = 4-(aminomethyl)piperidinium), using a combination of ultrafast transient absorption spectroscopy and first-principles calculations. In (4AMP)PbI, upon Rashba band splitting, the spin-dependent scattering of hot electrons is responsible for accelerating hot-carrier cooling at longer delays. Importantly, the hot-carrier cooling of (4AMP)PbI can be extended by manipulating the spin state of the hot carriers. Our findings suggest a new approach for prolonging hot-carrier cooling in hybrid perovskites, which is conducive to further improving the performance of hot-carrier-based optoelectronic and spintronic devices.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41467-021-24258-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8239041PMC
June 2021

Robust Molecular Dipole-Enabled Defect Passivation and Control of Energy-Level Alignment for High-Efficiency Perovskite Solar Cells.

Angew Chem Int Ed Engl 2021 Aug 30;60(32):17664-17670. Epub 2021 Jun 30.

School of Materials Science and Engineering Atlanta, Georgia Institute of Technology, Atlanta, GA, 30332, USA.

The ability to passivate defects and modulate the interface energy-level alignment (IEA) is key to boost the performance of perovskite solar cells (PSCs). Herein, we report a robust route that simultaneously allows defect passivation and reduced energy difference between perovskite and hole transport layer (HTL) via the judicious placement of polar chlorine-terminated silane molecules at the interface. Density functional theory (DFT) points to effective passivation of the halide vacancies on perovskite surface by the silane chlorine atoms. An integrated experimental and DFT study demonstrates that the dipole layer formed by the silane molecules decreases the perovskite work function, imparting an Ohmic character to the perovskite/HTL contact. The corresponding PSCs manifest a nearly 20 % increase in power conversion efficiency over pristine devices and a markedly enhanced device stability. As such, the use of polar molecules to passivate defects and tailor the IEA in PSCs presents a promising platform to advance the performance of PSCs.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.202105512DOI Listing
August 2021

Thermally conductive ultra-low-k dielectric layers based on two-dimensional covalent organic frameworks.

Nat Mater 2021 Aug 18;20(8):1142-1148. Epub 2021 Mar 18.

Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA, USA.

As the features of microprocessors are miniaturized, low-dielectric-constant (low-k) materials are necessary to limit electronic crosstalk, charge build-up, and signal propagation delay. However, all known low-k dielectrics exhibit low thermal conductivities, which complicate heat dissipation in high-power-density chips. Two-dimensional (2D) covalent organic frameworks (COFs) combine immense permanent porosities, which lead to low dielectric permittivities, and periodic layered structures, which grant relatively high thermal conductivities. However, conventional synthetic routes produce 2D COFs that are unsuitable for the evaluation of these properties and integration into devices. Here, we report the fabrication of high-quality COF thin films, which enable thermoreflectance and impedance spectroscopy measurements. These measurements reveal that 2D COFs have high thermal conductivities (1 W m K) with ultra-low dielectric permittivities (k = 1.6). These results show that oriented, layered 2D polymers are promising next-generation dielectric layers and that these molecularly precise materials offer tunable combinations of useful properties.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41563-021-00934-3DOI Listing
August 2021

Organic Photovoltaics: Understanding the Preaggregation of Polymer Donors in Solution and Its Morphological Impact.

J Am Chem Soc 2021 02 25;143(4):1822-1835. Epub 2021 Jan 25.

Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721-0088, United States.

In bulk-heterojunction organic solar cells, it is well established that the active-layer morphology significantly impacts device performance. However, morphology control remains very challenging. An interesting step in that direction was recently provided by the development of polymer donors that display a temperature-dependent aggregation behavior in solution; the aggregation characteristics were found indeed to play a determining role in the eventual active-layer morphology. Here, a combination of thermodynamic analyses, molecular dynamics simulations, and long-range corrected density functional theory calculations enables us (i) to establish the Flory-Huggins interaction parameter, χ, as a useful figure of merit that allows us to integrate the contributions from all inter-related molecular interactions and to describe the extent of polymer preaggregation in solution at room temperature; (ii) to correlate the χ values for various polymer solutions to the extent of polymer aggregation and the morphological characteristics of the active layers; and (iii) to assess how polymer-polymer and polymer-solvent intermolecular interactions contribute to the variations in χ values among different polymer solutions. We have chosen to examine four representative polymer donors (PBT4T-2OD, PBTff4T-2OD, PffBT4T-2OD, and PffBTff4T-2DT) in solution in chlorobenzene or dichlorobenzene. With χ as a robust bridge, our results provide an unprecedented, detailed description of the relationships among intermolecular interactions, extent of polymer solution aggregation, and morphological features of the active layers.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.0c09542DOI Listing
February 2021

Electronically Coupled 2D Polymer/MoS Heterostructures.

J Am Chem Soc 2020 12 7;142(50):21131-21139. Epub 2020 Dec 7.

Department of Physics, University of California, Berkeley, Berkeley, California 94720, United States.

Emergent quantum phenomena in electronically coupled two-dimensional heterostructures are central to next-generation optical, electronic, and quantum information applications. Tailoring electronic band gaps in coupled heterostructures would permit control of such phenomena and is the subject of significant research interest. Two-dimensional polymers (2DPs) offer a compelling route to tailored band structures through the selection of molecular constituents. However, despite the promise of synthetic flexibility and electronic design, fabrication of 2DPs that form electronically coupled 2D heterostructures remains an outstanding challenge. Here, we report the rational design and optimized synthesis of electronically coupled semiconducting 2DP/2D transition metal dichalcogenide van der Waals heterostructures, demonstrate direct exfoliation of the highly crystalline and oriented 2DP films down to a few nanometers, and present the first thickness-dependent study of 2DP/MoS heterostructures. Control over the 2DP layers reveals enhancement of the 2DP photoluminescence by two orders of magnitude in ultrathin sheets and an unexpected thickness-dependent modulation of the ultrafast excited state dynamics in the 2DP/MoS heterostructure. These results provide fundamental insight into the electronic structure of 2DPs and present a route to tune emergent quantum phenomena in 2DP hybrid van der Waals heterostructures.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.0c10151DOI Listing
December 2020

Impact of secondary donor units on the excited-state properties and thermally activated delayed fluorescence (TADF) efficiency of pentacarbazole-benzonitrile emitters.

J Chem Phys 2020 Oct;153(14):144708

Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721-0088, USA.

The performance of organic light-emitting diodes based on thermally activated delayed fluorescence emitters depends on the efficiency of reverse intersystem crossing (RISC) processes, which are promoted by a small energy gap between the lowest singlet (S) and triplet (T) excited states and large spin-orbit couplings. Recently, it was proposed that the introduction of secondary donor units into 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzBN) can significantly increase the mixing between triplet states with charge-transfer (CT) and local-excitation characteristics and consequently increase the spin-orbit couplings. Here, the results of long-range corrected density functional theory calculations show that the main impact on the RISC rates of substituting 5CzBN with secondary donors is due to a decrease in adiabatic singlet-triplet energy gaps and intramolecular reorganization energies rather than to a change in spin-orbit couplings. Our calculations underline that at least two singlet and three triplet excited states contribute to the ISC/RISC processes in 5CzBN and its derivatives. In addition, we find that in all emitters, the lowest singlet excited-state potential energy surface has a double-minimum shape.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/5.0028227DOI Listing
October 2020

Organic Neutral Radical Emitters: Impact of Chemical Substitution and Electronic-State Hybridization on the Luminescence Properties.

J Am Chem Soc 2020 Oct 30;142(41):17782-17786. Epub 2020 Sep 30.

Department of Chemistry and Biochemistry The University of Arizona, Tucson, Arizona 85721-0088, United States.

Neutral donor-acceptor (D-A) organic radicals have recently attracted a great deal of attention as promising luminescent materials due to their strong doublet emission. Here, we consider a series of emitters based on substituted triarylamine (TAA) donors and a radical-carrying perchlorotriphenylmethyl (PTM) acceptor. We evaluate, by means of quantum-chemical calculations and theoretical modeling, how chemical substitution affects the electronic structures and radiative and nonradiative decay rates. Our calculations show that the radiative decay rates are dominated in all instances by the electronic coupling between the lowest excited state, which has charge-transfer (CT) character, and the ground state. On the other hand, the nonradiative decay rates in the case of TAA-PTM radicals that have high CT energies are defined by the electronic hybridization of the CT state with local excitations (LE) on the PTM moiety; also, these nonradiative rates deviate significantly from the gap law dependence that is observed in the TAA-PTM radicals that have low CT energies. These findings underscore that hybridization of the emissive state with high-energy states can, in analogy with the intensity borrowing effect commonly invoked for radiative transitions, enhance as well the nonradiative decay rates. Our results highlight that in order to understand the emissive properties of D-A radicals, it is required that the electronic hybridization of the CT states with both the ground and the LE states be properly considered.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.0c08997DOI Listing
October 2020

Delocalization of exciton and electron wavefunction in non-fullerene acceptor molecules enables efficient organic solar cells.

Nat Commun 2020 Aug 7;11(1):3943. Epub 2020 Aug 7.

State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, School of Materials Science and Engineering, South China University of Technology, 381 Wushan Road, 510640, Guangzhou, P. R. China.

A major challenge for organic solar cell (OSC) research is how to minimize the tradeoff between voltage loss and charge generation. In early 2019, we reported a non-fullerene acceptor (named Y6) that can simultaneously achieve high external quantum efficiency and low voltage loss for OSC. Here, we use a combination of experimental and theoretical modeling to reveal the structure-property-performance relationships of this state-of-the-art OSC system. We find that the distinctive π-π molecular packing of Y6 not only exists in molecular single crystals but also in thin films. Importantly, such molecular packing leads to (i) the formation of delocalized and emissive excitons that enable small non-radiative voltage loss, and (ii) delocalization of electron wavefunctions at donor/acceptor interfaces that significantly reduces the Coulomb attraction between interfacial electron-hole pairs. These properties are critical in enabling highly efficient charge generation in OSC systems with negligible donor-acceptor energy offset.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41467-020-17867-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7414148PMC
August 2020

Electronic Structure of Multicomponent Organic Molecular Materials: Evaluation of Range-Separated Hybrid Functionals.

J Chem Theory Comput 2020 Jun 8;16(6):3712-3719. Epub 2020 May 8.

Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721-0088, United States.

Range-separated hybrid (RSH) functionals have become a tool of choice to study the and -molecular electronic states in organic materials. These functionals provide the most accurate descriptions of the electronic structure when the range-separation parameter is optimally tuned (OT). However, since the range-separation parameter is molecule dependent, this approach faces consistency issues when applied to the multicomponent systems typically found in the active layers of organic solar cells or organic light-emitting diodes (OLEDs). Here, we investigate the performance of four common RSH functionals in the description of the excited states of three molecular compounds used as components of the active layer in a hyperfluorescence OLED device. Our results indicate that the excited-state energies of the investigated molecules show a very weak dependence on the range-separation parameter value when they are evaluated by means of a screened version of RSH functionals. In this instance, the excited states of all three molecular compounds can be derived accurately and consistently with the exact same functional.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jctc.0c00138DOI Listing
June 2020

Revealing the Local Electronic Structure of a Single-Layer Covalent Organic Framework through Electronic Decoupling.

Nano Lett 2020 Feb 15;20(2):963-970. Epub 2020 Jan 15.

Department of Physics , University of California at Berkeley , Berkeley , California 94720 , United States.

Covalent organic frameworks (COFs) are molecule-based 2D and 3D materials that possess a wide range of mechanical and electronic properties. We have performed a joint experimental and theoretical study of the electronic structure of boroxine-linked COFs grown under ultrahigh vacuum conditions and characterized using scanning tunneling spectroscopy on Au(111) and hBN/Cu(111) substrates. Our results show that a single hBN layer electronically decouples the COF from the metallic substrate, thus suppressing substrate-induced broadening and revealing new features in the COF electronic local density of states (LDOS). The resulting sharpening of LDOS features allows us to experimentally determine the COF band gap, bandwidths, and the electronic hopping amplitude between adjacent COF bridge sites. These experimental parameters are consistent with the results of first-principles theoretical predictions.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.nanolett.9b03998DOI Listing
February 2020

Humidity Sensing through Reversible Isomerization of a Covalent Organic Framework.

J Am Chem Soc 2020 01 2;142(2):783-791. Epub 2020 Jan 2.

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics , Georgia Institute of Technology , Atlanta , Georgia 30332-0400 , United States.

Here we report that a covalent organic framework (COF), which contains 2,5-di(imine)-substituted 1,4-dihydroxybenzene (diiminol) moieties, undergoes color changes in the presence of solvents or solvent vapor that are rapid, passive, reversible, and easily detectable by the naked eye. A new visible absorption band appears in the presence of polar solvents, especially water, suggesting reversible conversion to another species. This reversibility is attributed to the ability of the diiminol to rapidly tautomerize to an iminol/-ketoenamine and its inability to doubly tautomerize to a diketoenamine. Density functional theory (DFT) calculations suggest similar energies for the two tautomers in the presence of water, but the diiminol is much more stable in its absence. Time-dependent DFT calculations confirm that the iminol/-ketoenamine absorbs at longer wavelength than the diiminol and indicate that this absorption has significant charge-transfer character. A colorimetric humidity sensing device constructed from an oriented thin film of the COF responded quickly to water vapor and was stable for months. These results suggest that tautomerization-induced electronic structure changes can be exploited in COF platforms to give rapid, reversible sensing in systems that exhibit long-term stability.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.9b08628DOI Listing
January 2020

Nucleation-Elongation Dynamics of Two-Dimensional Covalent Organic Frameworks.

J Am Chem Soc 2020 Jan 7;142(3):1367-1374. Epub 2020 Jan 7.

School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics (COPE) , Georgia Institute of Technology , Atlanta , Georgia 30332-0400 , United States.

Homogeneous two-dimensional (2D) polymerization is a poorly understood process in which topologically planar monomers react to form planar macromolecules, often termed 2D covalent organic frameworks (COFs). While these COFs have traditionally been limited to weakly crystalline aggregated powders, they were recently grown as micron-sized single crystals by temporally resolving the growth and nucleation processes. Here, we present a quantitative analysis of the nucleation and growth rates of 2D COFs via kinetic Monte Carlo (KMC) simulations using COF-5 as an example, which show that nucleation and growth have second-order and first-order dependences on monomer concentration, respectively. The computational results were confirmed experimentally by systematic measurements of COF nucleation and growth rates performed via in situ X-ray scattering, which validated the respective monomer concentration dependencies of the nucleation and elongation processes. A major consequence is that there exists a threshold monomer concentration below which growth dominates over nucleation. Our computational and experimental findings rationalize recent empirical observations that, in the formation of 2D COF single crystals, growth dominates over nucleation when monomers are added slowly, so as to limit their concentrations. This mechanistic understanding of the nucleation and growth processes will inform the rational control of polymerization in two dimensions and ultimately enable access to high-quality samples of designed two-dimensional polymers.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.9b10869DOI Listing
January 2020

Chemical Control over Nucleation and Anisotropic Growth of Two-Dimensional Covalent Organic Frameworks.

ACS Cent Sci 2019 Nov 11;5(11):1892-1899. Epub 2019 Nov 11.

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.

Two-dimensional covalent organic frameworks (2D COFs) are composed of structurally precise, permanently porous, layered polymer sheets. 2D COFs have traditionally been synthesized as polycrystalline aggregates with small crystalline domains. Only recently have a small number of 2D COFs been obtained as single crystals, which were prepared by a seeded growth approach via the slow introduction of monomers, which favored particle growth over nucleation. However, these procedures are slow and operationally difficult, making it desirable to develop polymerization methods that do not require the continuous addition of reactants over days or weeks. Here, we achieve the rapid growth of boronate ester-linked COFs by chemically suppressing nucleation via addition of an excess of a monofunctional competitor, 4--butylcatechol (TCAT), into the polymerization. X-ray scattering measurements show that TCAT suppresses colloid nucleation, which enables seeded growth polymerizations in the presence of high monomer concentrations. Kinetic Monte Carlo simulations reveal that TCAT limits oligomers to sizes below the critical nucleus size and that in-plane expansion is restricted compared to out-of-plane oriented attachment of oligomers. The simulations are consistent with transmission electron micrographs, which show that the particles grow predominantly in the stacking direction. This mechanistic insight into the role of the modulators in 2D polymerizations enables the size and aspect ratio of COF colloids to be controlled under operationally simple conditions. This chemically controlled growth strategy will accelerate the discovery and exploration of COF materials and their emergent properties.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acscentsci.9b00944DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6891846PMC
November 2019

Tuning Hot Carrier Cooling Dynamics by Dielectric Confinement in Two-Dimensional Hybrid Perovskite Crystals.

ACS Nano 2019 Nov 21;13(11):12621-12629. Epub 2019 Oct 21.

Division of Physical Science and Engineering , King Abdullah University of Science and Technology , Thuwal 23955-6900 , Kingdom of Saudi Arabia.

Hot carrier (HC) cooling is a critical photophysical process that significantly influences the optoelectronic performance of hybrid perovskite-based devices. The hot carrier extraction at the device interface is very challenging because of its ultrashort lifetime. Here, ultrafast transient reflectance spectroscopy measurements and time-domain calculations show how the dielectric constant of the organic spacers can control and slow the HC cooling dynamics in single-crystal 2D Ruddlesden-Popper hybrid perovskites. We find that (EA)PbI (EA = HOCHNH) that correspond to a high dielectric constant organic spacer has a longer HC cooling time compared to that of (AP)PbI (AP = HOCHNH) and (PEA)PbI (PEA = CHCHNH). The slow HC relaxation process in the former case can be ascribed to a stronger screening of the Coulomb interactions, a small nonradiative internal conversion within the conduction bands, as well as a weak electron-phonon coupling. Our findings provide a strategy to prolong the hot carrier cooling time in low-dimensional hybrid perovskite materials by using organic spacers with reduced dielectric confinement.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsnano.9b04085DOI Listing
November 2019

Critical role of intermediate electronic states for spin-flip processes in charge-transfer-type organic molecules with multiple donors and acceptors.

Nat Mater 2019 10 2;18(10):1084-1090. Epub 2019 Sep 2.

Center for Organic Photonics and Electronics Research, Kyushu University, Fukuoka, Japan.

Spin-flip in purely organic molecular systems is often described as a forbidden process; however, it is commonly observed and utilized to harvest triplet excitons in a wide variety of organic material-based applications. Although the initial and final electronic states of spin-flip between the lowest singlet and lowest triplet excited state are self-evident, the exact process and the role of intermediate states through which spin-flip occurs are still far from being comprehensively determined. Here, via experimental photo-physical investigations in solution combined with first-principles quantum-mechanical calculations, we show that efficient spin-flip in multiple donor-acceptor charge-transfer-type organic molecular systems involves the critical role of an intermediate triplet excited state that corresponds to a partial molecular structure of the system. Our proposed mechanism unifies the understanding of the intersystem crossing mechanism in a wide variety of charge-transfer-type molecular systems, opening the way to greater control over spin-flip rates.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41563-019-0465-6DOI Listing
October 2019

Short Excited-State Lifetimes Enable Photo-Oxidatively Stable Rubrene Derivatives.

J Phys Chem A 2019 Sep 26;123(35):7558-7566. Epub 2019 Aug 26.

Department of Polymer Science and Engineering, University of Massachusetts, 120 Governors Drive, Amherst, Massachusetts 01003, United States.

A series of rubrene derivatives were synthesized and the influence of the side group in enhancing photo-oxidative stability was evaluated. Photo-oxidation half-lives were determined via UV-vis absorption spectroscopy, which revealed thiophene containing derivatives to be the most stable species. The electron affinity of the compounds did not correlate with stability as previously reported in literature. Our work shows that shorter excited-state lifetimes result in increased photo-oxidative stability in these rubrene derivatives. These results confirm that faster relaxation kinetics out-compete the formation of reactive oxygen species that ultimately degrade linear oligoacenes. This report highlights the importance of using molecular design to tune excited-state lifetimes in order to generate more stable oligoacenes.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpca.9b04203DOI Listing
September 2019

High stability and luminescence efficiency in donor-acceptor neutral radicals not following the Aufbau principle.

Nat Mater 2019 Sep 22;18(9):977-984. Epub 2019 Jul 22.

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, China.

With their unusual electronic structures, organic radical molecules display luminescence properties potentially relevant to lighting applications; yet, their luminescence quantum yield and stability lag behind those of other organic emitters. Here, we designed donor-acceptor neutral radicals based on an electron-poor perchlorotriphenylmethyl or tris(2,4,6-trichlorophenyl)methyl radical moiety combined with different electron-rich groups. Experimental and quantum-chemical studies demonstrate that the molecules do not follow the Aufbau principle: the singly occupied molecular orbital is found to lie below the highest (doubly) occupied molecular orbital. These donor-acceptor radicals have a strong emission yield (up to 54%) and high photostability, with estimated half-lives reaching up to several months under pulsed ultraviolet laser irradiation. Organic light-emitting diodes based on such a radical emitter show deep-red/near-infrared emission with a maximal external quantum efficiency of 5.3%. Our results provide a simple molecular-design strategy for stable, highly luminescent radicals with non-Aufbau electronic structures.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41563-019-0433-1DOI Listing
September 2019

Intramolecular Noncovalent Interactions Facilitate Thermally Activated Delayed Fluorescence (TADF).

J Phys Chem Lett 2019 Jun 4;10(12):3260-3268. Epub 2019 Jun 4.

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics , Georgia Institute of Technology , Atlanta , Georgia 30332-0400 , United States.

In the conventional molecular design of thermally activated delayed fluorescence (TADF) organic emitters, simultaneously achieving a fast rate of reverse intersystem crossing (RISC) from the triplet to the singlet manifold and a fast rate of radiative decay is a challenging task. A number of recent experimental data, however, point to TADF emitters with intramolecular π-π interactions as a potential pathway to overcome the issue. Here, we report a comprehensive investigation of TADF emitters with intramolecular π···π or lone-pair···π noncovalent interactions. We uncover the impact of those intramolecular noncovalent interactions on the TADF properties. In particular, we find that folded geometries in TADF molecules can trigger lone-pair···π interactions, introduce a n → π* character of the relevant transitions, enhance the singlet-triplet spin-orbit coupling, and ultimately greatly facilitate the RISC process. This work provides a robust foundation for the molecular design of a novel class of highly efficient TADF emitters in which intramolecular noncovalent interactions play a critical function.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpclett.9b01220DOI Listing
June 2019

Emergence of an Antiferromagnetic Mott Insulating Phase in Hexagonal π-Conjugated Covalent Organic Frameworks.

Adv Mater 2019 Apr 7;31(17):e1900355. Epub 2019 Mar 7.

School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics (COPE), Georgia Institute of Technology, Atlanta, GA, 30332-0400, USA.

While the search for 2D organic semimetallic Dirac materials displaying, like graphene, a Dirac cone at the Fermi level remains active, attention is also being paid to the quantum phase transition from semimetal to antiferromagnet. Such a transition in graphene-like materials is predicted based on theoretical investigations of the 2D honeycomb lattice; it occurs (within a Hubbard model) when the on-site electron-electron Coulomb repulsion (U) is much larger than the nearest-neighbor inter-site electronic coupling (t). Here, monomers carrying long-lived radicals are considered and used as building blocks to design 2D hexagonal π-conjugated covalent organic frameworks (COFs). Both the nonmagnetic semimetallic phase and magnetically ordered phases are evaluated. It is found that the electronic coupling between adjacent radical centers in these COFs is more than an order of magnitude smaller than in graphene while the on-site Coulomb repulsion is reduced to a lesser extent. The resulting large U/t ratio drives these COFs into the antiferromagnetic side of the phase diagram. This work provides a first theoretical evidence of the realization of an antiferromagnetic Mott insulating phase in 2D π-conjugated COFs and allows a strategy to achieve quantum phase transitions from antiferromagnet to spin liquid and to semimetal to be outlined.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/adma.201900355DOI Listing
April 2019

Assessing the nature of the charge-transfer electronic states in organic solar cells.

Nat Commun 2018 12 13;9(1):5295. Epub 2018 Dec 13.

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia, 30332-0400, USA.

The charge-transfer electronic states appearing at the donor-acceptor interfaces in organic solar cells mediate exciton dissociation, charge generation, and charge recombination. To date, the characterization of their nature has been carried out on the basis of models that only involve the charge-transfer state and the ground state. Here, we demonstrate that it is essential to go beyond such a two-state model and to consider explicitly as well the electronic and vibrational couplings with the local absorbing state on the donor and/or acceptor. We have thus developed a three-state vibronic model that allows us: to provide a reliable description of the optical absorption features related to the charge-transfer states; to underline the erroneous interpretations stemming from the application of the semi-classical two-state model; and to rationalize how the hybridization between the local-excitation state and charge-transfer state can lead to lower non-radiative voltage losses and higher power conversion efficiencies.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41467-018-07707-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6294259PMC
December 2018

Local Electronic Structure of Molecular Heterojunctions in a Single-Layer 2D Covalent Organic Framework.

Adv Mater 2019 Jan 28;31(3):e1805941. Epub 2018 Nov 28.

Department of Physics, University of California - Berkeley, Berkeley, CA, 94720, USA.

The synthesis of a single-layer covalent organic framework (COF) with spatially modulated internal potentials provides new opportunities for manipulating the electronic structure of molecularly defined materials. Here, the fabrication and electronic characterization of COF-420: a single-layer porphyrin-based square-lattice COF containing a periodic array of oriented, type II electronic heterojunctions is reported. In contrast to previous donor-acceptor COFs, COF-420 is constructed from building blocks that yield identical cores upon reticulation, but that are bridged by electrically asymmetric linkers supporting oriented electronic dipoles. Scanning tunneling spectroscopy reveals staggered gap (type II) band alignment between adjacent molecular cores in COF-420, in agreement with first-principles calculations. Hirshfeld charge analysis indicates that dipole fields from oriented imine linkages within COF-420 are the main cause of the staggered electronic structure in this square grid of atomically-precise heterojunctions.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/adma.201805941DOI Listing
January 2019

Quasi-One-Dimensional Charge Transport Can Lead to Nonlinear Current Characteristics in Organic Field-Effect Transistors.

J Phys Chem Lett 2018 Nov 5;9(22):6550-6555. Epub 2018 Nov 5.

School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics (COPE) , Georgia Institute of Technology , Atlanta , Georgia 30332-0400 , United States.

Nonlinearity in the current characteristics of organic field-effect transistor (OFET) devices has become a serious issue for accurate evaluation of the charge-carrier mobilities in organic semiconductors. In particular, in the case of several high-mobility materials, a kink appears in the transfer curves, and this nonlinearity has been generally interpreted as the result of poor contacts. Here, we describe another possible origin for the appearance of such a kink. Extensive molecular-scale device simulations indeed demonstrate that the quasi-1D nature of charge transport often encountered in organic crystals or highly oriented polymers can lead to significant transport through the bulk and result in nonlinearity of the transfer current characteristics if the actual charge injection is away from the channel. When this is the case, the low-gate voltage regime in fact does not overestimate the charge mobility along the channel direction.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.jpclett.8b02972DOI Listing
November 2018
-->