Nanoscale doping of polymeric semiconductors with confined electrochemical ion implantation

Nanoresolved doping of polymeric semiconductors can overcome scaling limitations to create highly integrated flexible electronics, but remains a fundamental challenge due to isotropic diffusion of the dopants. Here we report a general methodology for achieving nanoscale ion-implantation-like electrochemical doping of polymeric semiconductors. This approach involves confining counterion electromigration within a glassy electrolyte composed of room-temperature ionic liquids and high-glass-transition-temperature insulating polymers. By precisely adjusting the electrolyte glass transition temperature (Tg) and the operating temperature (T), we create a highly localized electric field distribution and achieve anisotropic ion migration that is nearly vertical to the nanotip electrodes. The confined doping produces an excellent resolution of 56 nm with a lateral-extended doping length down to as little as 9.3 nm. We reveal a universal exponential dependence of the doping resolution on the temperature difference (Tg - T) that can be used to depict the doping resolution for almost infinite polymeric semiconductors. Moreover, we demonstrate its implications in a range of polymer electronic devices, including a 200% performance-enhanced organic transistor and a lateral p-n diode with seamless junction widths of <100 nm. Combined with a further demonstration in the scalability of the nanoscale doping, this concept may open up new opportunities for polymer-based nanoelectronics.

Elucidating the Non-Covalent Interactions that Trigger Interdigitation in Lead-Halide Layered Hybrid Perovskites

Two-dimensional (2D) hybrid organic-inorganic perovskites constitute a versatile class of materials applied to a variety of optoelectronic devices. These materials are composed of alternating layers of inorganic lead halide octahedra and organic ammonium cations. Most perovskite research studies so far have focused on organic sublattices based on phenethylammonium and alkylammonium cations, which are packed by van der Waals cohesive forces. Here, we report a more complex organic sublattice containing benzotriazole-based ammonium cations packed through interdigitated π-π stacking and hydrogen bonding. Single crystals and thin films of four perovskite derivatives are studied in depth with optical spectroscopy and X-ray diffraction, supported by density-functional theory calculations. We quantify the lattice stabilization of interdigitation, dipole-dipole interactions, and inter- as well as intramolecular hydrogen bonding. Furthermore, we investigate the driving force behind interdigitation by defining a steric occupancy factor σ and tuning the composition of the organic and inorganic sublattice. We relate the phenomenon of interdigitation to the available lattice space and to weakened hydrogen bonding to the inorganic octahedra. Finally, we find that the stabilizing interactions in the organic sublattice slightly improve the thermal stability of the perovskite. This work sheds light on the design rules and structure-property relationships of 2D layered hybrid perovskites.

Polar Polymorphism: A New Intermediate Structure toward the Thin-Film Phase in Asymmetric Benzothieno[3,2-b][1]-benzothiophene Derivatives

Understanding structure and polymorphism is relevant for any organic device optimization, and it is of particular relevance in 7-decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) since high carrier mobility in Ph-BTBT-10 thin films has been linked to the structural transformation from the metastable thin-film phase to the thermodynamically stable bilayer structure via thermal annealing. We combine here a systematic nanoscale morphological analysis with local Kelvin probe force microcopy (KPFM) that demonstrates the formation of a polar polymorph in thin films as an intermediate structure for thicknesses lower than 20 nm. The polar structure develops with thickness a variable amount of structural defects in the form of individual flipped molecules (point defects) or sizable polar domains, and evolves toward the reported nonpolar thin-film phase. The direct experimental evidence is supported by electronic structure density functional theory calculations. The structure of the film has dramatic effects on the electronic properties, leading to a decrease in the film work function (by up to 1 eV) and a considerable broadening of the occupied molecular orbitals, attributed to electrostatic disorder. From an advanced characterization point of view, KPFM stands out as a valuable tool for evaluating electrostatic disorder and the conceivable emergence of polar polymorphs in organic thin films. The emergence of polar assemblies introduces a critical consideration for other asymmetric BTBT derivatives, which may be pivotal to understanding the structure-property relationships in organic field-effect transistors (OFETs). A precise determination of any polar assemblies close to the dielectric interface is critical for the judicious design and upgrading of high-performance OFETs.

Integrating Benzoxazine-PDMS 3D Networks with Carbon Nanotubes for flexible Pressure Sensors

Shapeable and flexible pressure sensors with superior mechanical and electrical properties are of major interest as they can be employed in a wide range of applications. In this regard, elastomer-based composites incorporating carbon nanomaterials in the insulating matrix embody an appealing solution for designing flexible pressure sensors with specific properties. In this study, PDMS chains of different molecular weight were successfully functionalized with benzoxazine moieties in order to thermally cure them without adding a second component, nor a catalyst or an initiator. These precursors were then blended with 1 weight percent of multi-walled carbon nanotubes (CNTs) using an ultrasound probe, which induced a transition from a liquid-like to a gel-like behavior as CNTs generate an interconnected network within the matrix. After curing, the resulting nanocomposites exhibit mechanical and electrical properties making them highly promising materials for pressure-sensing applications.

Spin relaxation of electron and hole polarons in ambipolar conjugated polymers

The charge-transport properties of conjugated polymers have been studied extensively for opto-electronic device applications. Some polymer semiconductors not only support the ambipolar transport of electrons and holes, but do so with comparable carrier mobilities. This opens the possibility of gaining deeper insight into the charge-transport physics of these complex materials via comparison between electron and hole dynamics while keeping other factors, such as polymer microstructure, equal. Here, we use field-induced electron spin resonance spectroscopy to compare the spin relaxation behavior of electron and hole polarons in three ambipolar conjugated polymers. Our experiments show unique relaxation regimes as a function of temperature for electrons and holes, whereby at lower temperatures electrons relax slower than holes, but at higher temperatures, in the so-called spin-shuttling regime, the trend is reversed. On the basis of theoretical simulations, we attribute this to differences in the delocalization of electron and hole wavefunctions and show that spin relaxation in the spin shuttling regimes provides a sensitive probe of the intimate coupling between charge and structural dynamics.

Molecularly induced order promotes charge separation through delocalized charge-transfer states at donor-acceptor heterojunctions

The energetic landscape at the interface between electron donating and accepting molecular materials favors efficient conversion of intermolecular charge-transfer (CT) states into free charge carriers (FCC) in high-performance organic solar cells. Here, we elucidate how interfacial energetics, charge generation and radiative recombination are affected by molecular arrangement. We experimentally determine the CT dissociation properties of a series of model, small molecule donor-acceptor blends, where the used acceptors (B2PYMPM, B3PYMPM and B4PYMPM) differ only in the nitrogen position of their lateral pyridine rings. We find that the formation of an ordered, face-on molecular packing in B4PYMPM is beneficial to efficient, field-independent charge separation, leading to fill factors above 70% in photovoltaic devices. This is rationalized by a comprehensive computational protocol showing that, compared to the more amorphous and isotropically oriented B2PYMPM, the higher structural order of B4PYMPM molecules leads to more delocalized CT states. Furthermore, we find no correlation between the quantum efficiency of FCC radiative recombination and the bound or unbound nature of the CT states. This work highlights the importance of structural ordering at donor-acceptor interfaces for efficient FCC generation and shows that less bound CT states do not preclude efficient radiative recombination.

Interlayer Charge Transport in 2D Lead Halide Perovskites from First Principles

We report on the implementation of a versatile projection-operator diabatization approach to calculate electronic coupling integrals in layered periodic systems. The approach is applied to model charge transport across the saturated organic spacers in two-dimensional (2D) lead halide perovskites. The calculations yield out-of-plane charge transfer rates that decay exponentially with the increasing length of the alkyl chain, range from a few nanoseconds to milliseconds, and are supportive of a hopping mechanism. Most importantly, we show that the charge carriers strongly couple to distortions of the Pb-I framework and that accounting for the associated nonlocal dynamic disorder increases the thermally averaged interlayer rates by a few orders of magnitude compared to the frozen-ion 0 K-optimized structure. Our formalism provides the first comprehensive insight into the role of the organic spacer cation on vertical transport in 2D lead halide perovskites and can be readily extended to functional π-conjugated spacers, where we expect the improved energy alignment with the inorganic layout to speed up the charge transfer between the semiconducting layers.

Poly(benzimidazobenzophenanthroline)-Ladder-Type Two-Dimensional Conjugated Covalent Organic Framework for Fast Proton Storage

Electrochemical proton storage plays an essential role in designing next-generation high-rate energy storage devices, e.g., aqueous batteries. Two-dimensional conjugated covalent organic frameworks (2D c-COFs) are promising electrode materials, but their competitive proton and metal-ion insertion mechanisms remain elusive, and proton storage in COFs is rarely explored. Here, we report a perinone-based poly(benzimidazobenzophenanthroline) (BBL)-ladder-type 2D c-COF for fast proton storage in both a mild aqueous Zn-ion electrolyte and strong acid. We unveil that the discharged C-O- groups exhibit largely reduced basicity due to the considerable π-delocalization in perinone, thus affording the 2D c-COF a unique affinity for protons with fast kinetics. As a consequence, the 2D c-COF electrode presents an outstanding rate capability of up to 200 A g-1 (over 2500 C), surpassing the state-of-the-art conjugated polymers, COFs, and metal-organic frameworks. Our work reports the first example of pure proton storage among COFs and highlights the great potential of BBL-ladder-type 2D conjugated polymers in future energy devices.

Controlling Charge Transport in 2D Conductive MOFs─The Role of Nitrogen-Rich Ligands and Chemical Functionality

Two-dimensional electrically conducting metal-organic frameworks (2D-e-MOFs) have emerged as a class of highly promising functional materials for a wide range of applications. However, despite the significant recent advances in 2D-e-MOFs, developing systems that can be postsynthetically chemically functionalized, while also allowing fine-tuning of the transport properties, remains challenging. Herein, we report two isostructural 2D-e-MOFs: Ni3(HITAT)2 and Ni3(HITBim)2 based on two new 3-fold symmetric ligands: 2,3,7,8,12,13-hexaaminotriazatruxene (HATAT) and 2,3,8,9,14,15-hexaaminotribenzimidazole (HATBim), respectively, with reactive sites for postfunctionalization. Ni3(HITAT)2 and Ni3(HITBim)2 exhibit temperature-activated charge transport, with bulk conductivity values of 44 and 0.5 mS cm-1, respectively. Density functional theory analysis attributes the difference to disparities in the electron density distribution within the parent ligands: nitrogen-rich HATBim exhibits localized electron density and a notably lower lowest unoccupied molecular orbital (LUMO) energy relative to HATAT. Precise amounts of methanesulfonyl groups are covalently bonded to the N-H indole moiety within the Ni3(HITAT)2 framework, modulating the electrical conductivity by a factor of ∼20. These results provide a blueprint for the design of porous functional materials with tunable chemical functionality and electrical response.

Transiently delocalized states enhance hole mobility in organic molecular semiconductors

Evidence shows that charge carriers in organic semiconductors self-localize because of dynamic disorder. Nevertheless, some organic semiconductors feature reduced mobility at increasing temperature, a hallmark for delocalized band transport. Here we present the temperature-dependent mobility in two record-mobility organic semiconductors: dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]-thiophene (DNTT) and its alkylated derivative, C8-DNTT-C8. By combining terahertz photoconductivity measurements with atomistic non-adiabatic molecular dynamics simulations, we show that while both crystals display a power-law decrease of the mobility (μ) with temperature (T) following μ ∝ T -n, the exponent n differs substantially. Modelling reveals that the differences between the two chemically similar semiconductors can be traced to the delocalization of the different states that are thermally accessible by charge carriers, which in turn depends on their specific electronic band structure. The emerging picture is that of holes surfing on a dynamic manifold of vibrationally dressed extended states with a temperature-dependent mobility that provides a sensitive fingerprint for the underlying density of states.

Isomer Discrimination via Defect Engineering in Monolayer MoS2

The all-surface nature of two-dimensional (2D) materials renders them highly sensitive to environmental changes, enabling the on-demand tailoring of their physical properties. Transition metal dichalcogenides, such as 2H molybdenum disulfide (MoS2), can be used as a sensory material capable of discriminating molecules possessing a similar structure with a high sensitivity. Among them, the identification of isomers represents an unexplored and challenging case. Here, we demonstrate that chemical functionalization of defect-engineered monolayer MoS2 enables isomer discrimination via a field-effect transistor readout. A multiscale characterization comprising X-ray photoelectron spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, and electrical measurement corroborated by theoretical calculations revealed that monolayer MoS2 exhibits exceptional sensitivity to the differences in the dipolar nature of molecules arising from their chemical structure such as the one in difluorobenzenethiol isomers, allowing their precise recognition. Our findings underscore the potential of 2D materials for molecular discrimination purposes, in particular for the identification of complex isomers.

Carbon Vacancies Steer the Activity in Dual Ni Carbon Nitride Photocatalysis

The manipulation of carbon nitride (CN) structures is one main avenue to enhance the activity of CN-based photocatalysts. Increasing the efficiency of photocatalytic heterogeneous materials is a critical step toward the realistic implementation of sustainable schemes for organic synthesis. However, limited knowledge of the structure/activity relationship in relation to subtle structural variations prevents a fully rational design of new photocatalytic materials, limiting practical applications. Here, the CN structure is engineered by means of a microwave treatment, and the structure of the material is shaped around its suitable functionality for Ni dual photocatalysis, with a resulting boosting of the reaction efficiency toward many CX (X = N, S, O) couplings. The combination of advanced characterization techniques and first-principle simulations reveals that this enhanced reactivity is due to the formation of carbon vacancies that evolve into triazole and imine N species able to suitably bind Ni complexes and harness highly efficient dual catalysis. The cost-effective microwave treatment proposed here appears as a versatile and sustainable approach to the design of CN-based photocatalysts for a wide range of industrially relevant organic synthetic reactions.

Enantiopure Dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophenes: Reaching High Magnetoresistance Effect in OFETs

Chiral molecules are known to behave as spin filters due to the chiral induced spin selectivity (CISS) effect. Chirality can be implemented in molecular semiconductors in order to study the role of the CISS effect in charge transport and to find new materials for spintronic applications. In this study, the design and synthesis of a new class of enantiopure chiral organic semiconductors based on the well-known dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophene (DNTT) core functionalized with chiral alkyl side chains is presented. When introduced in an organic field-effect transistor (OFET) with magnetic contacts, the two enantiomers, (R)-DNTT and (S)-DNTT, show an opposite behavior with respect to the relative direction of the magnetization of the contacts, oriented by an external magnetic field. Each enantiomer displays an unexpectedly high magnetoresistance over one preferred orientation of the spin current injected from the magnetic contacts. The result is the first reported OFET in which the current can be switched on and off upon inversion of the direction of the applied external magnetic field. This work contributes to the general understanding of the CISS effect and opens new avenues for the introduction of organic materials in spintronic devices.

Curved graphene nanoribbons derived from tetrahydropyrene-based polyphenylenes via one-pot K-region oxidation and Scholl cyclization

Precise synthesis of graphene nanoribbons (GNRs) is of great interest to chemists and materials scientists because of their unique opto-electronic properties and potential applications in carbon-based nanoelectronics and spintronics. In addition to the tunable edge structure and width, introducing curvature in GNRs is a powerful structural feature for their chemi-physical property modification. Here, we report an efficient solution synthesis of the first pyrene-based GNR (PyGNR) with curved geometry via one-pot K-region oxidation and Scholl cyclization of its corresponding well-soluble tetrahydropyrene-based polyphenylene precursor. The efficient A2B2-type Suzuki polymerization and subsequent Scholl reaction furnishes up to ∼35 nm long curved GNRs bearing cove- and armchair-edges. The construction of model compound 1, as a cutout of PyGNR, from a tetrahydropyrene-based oligophenylene precursor proves the concept and efficiency of the one-pot K-region oxidation and Scholl cyclization, which is clearly revealed by single crystal X-ray diffraction analysis. The structure and optical properties of PyGNR are investigated by Raman, FT-IR, solid-state NMR, STM and UV-Vis analysis with the support of DFT calculations. PyGNR exhibits a narrow optical bandgap of ∼1.4 eV derived from a Tauc plot, qualifying as a low-bandgap GNR. Moreover, THz spectroscopy on PyGNR estimates its macroscopic charge mobility μ as ∼3.6 cm2 V-1 s-1, outperforming several other curved GNRs reported via conventional Scholl reaction.

Interplay of structure and photophysics of individualized rod-shaped graphene quantum dots with up to 132 sp² carbon atoms

Nanographene materials are promising building blocks for the growing field of low-dimensional materials for optics, electronics and biophotonics applications. In particular, bottom-up synthesized 0D graphene quantum dots show great potential as single quantum emitters. To fully exploit their exciting properties, the graphene quantum dots must be of high purity; the key parameter for efficient purification being the solubility of the starting materials. Here, we report the synthesis of a family of highly soluble and easily processable rod-shaped graphene quantum dots with fluorescence quantum yields up to 94%. This is uncommon for a red emission. The high solubility is directly related to the design of the structure, allowing for an accurate description of the photophysical properties of the graphene quantum dots both in solution and at the single molecule level. These photophysical properties were fully predicted by quantum-chemical calculations.

Reversible spin-optical interface in luminescent organic radicals

Molecules present a versatile platform for quantum information science1,2 and are candidates for sensing and computation applications3,4. Robust spin-optical interfaces are key to harnessing the quantum resources of materials5. To date, carbon-based candidates have been non-luminescent6,7, which prevents optical readout via emission. Here we report organic molecules showing both efficient luminescence and near-unity generation yield of excited states with spin multiplicity S > 1. This was achieved by designing an energy resonance between emissive doublet and triplet levels, here on covalently coupled tris(2,4,6-trichlorophenyl) methyl-carbazole radicals and anthracene. We observed that the doublet photoexcitation delocalized onto the linked acene within a few picoseconds and subsequently evolved to a pure high-spin state (quartet for monoradical, quintet for biradical) of mixed radical-triplet character near 1.8 eV. These high-spin states are coherently addressable with microwaves even at 295 K, with optical readout enabled by reverse intersystem crossing to emissive states. Furthermore, for the biradical, on return to the ground state the previously uncorrelated radical spins either side of the anthracene shows strong spin correlation. Our approach simultaneously supports a high efficiency of initialization, spin manipulations and light-based readout at room temperature. The integration of luminescence and high-spin states creates an organic materials platform for emerging quantum technologies.

Judicious Heteroatom Doping Produces High-Performance Deep-Blue/Near-UV Multiresonant Thermally Activated Delayed Fluorescence OLEDs

Two multiresonant thermally activated delayed fluorescence (MR-TADF) emitters are presented and it is shown how further borylation of a deep-blue MR-TADF emitter, DIDOBNA-N, both blueshifts and narrows the emission producing a new near-UV MR-TADF emitter, MesB-DIDOBNA-N, are shown. DIDOBNA-N emits bright blue light (ΦPL = 444 nm, FWHM = 64 nm, ΦPL = 81%, τd = 23 ms, 1.5 wt% in TSPO1). The deep-blue organic light-emitting diode (OLED) based on this twisted MR-TADF compound shows a very high maximum external quantum efficiency (EQEmax ) of 15.3% for a device with CIEy of 0.073. The fused planar MR-TADF emitter, MesB-DIDOBNA-N shows efficient and narrowband near-UV emission (λPL = 402 nm, FWHM = 19 nm, ΦPL = 74.7%, τd = 133 ms, 1.5 wt% in TSPO1). The best OLED with MesB-DIDOBNA-N, doped in a co-host, shows the highest efficiency reported for a near-UV OLED at 16.2%. With a CIEy coordinate of 0.049, this device also shows the bluest EL reported for a MR-TADF OLED to date.

Exceptionally high charge mobility in phthalocyanine-based poly(benzimidazobenzophenanthroline)-ladder-type two-dimensional conjugated polymers

Two-dimensional conjugated polymers (2DCPs), composed of multiple strands of linear conjugated polymers with extended in-plane π-conjugation, are emerging crystalline semiconducting polymers for organic (opto)electronics. They are represented by two-dimensional π-conjugated covalent organic frameworks, which typically suffer from poor π-conjugation and thus low charge carrier mobilities. Here we overcome this limitation by demonstrating two semiconducting phthalocyanine-based poly(benzimidazobenzophenanthroline)-ladder-type 2DCPs (2DCP-MPc, with M = Cu or Ni), which are constructed from octaaminophthalocyaninato metal(II) and naphthalenetetracarboxylic dianhydride by polycondensation under solvothermal conditions. The 2DCP-MPcs exhibit optical bandgaps of ~1.3 eV with highly delocalized π-electrons. Density functional theory calculations unveil strongly dispersive energy bands with small electron-hole reduced effective masses of ~0.15m0 for the layer-stacked 2DCP-MPcs. Terahertz spectroscopy reveals the band transport of Drude-type free carriers in 2DCP-MPcs with exceptionally high sum mobility of electrons and holes of ~970 cm2 V-1 s-1 at room temperature, surpassing that of the reported linear conjugated polymers and 2DCPs. This work highlights the critical role of effective conjugation in enhancing the charge transport properties of 2DCPs and the great potential of high-mobility 2DCPs for future (opto)electronics.

Vacuum-Deposited Donors for Low-Voltage-Loss Nonfullerene Organic Solar Cells

The advent of nonfullerene acceptors (NFAs) enabled records of organic photovoltaics (OPVs) exceeding 19% power conversion efficiency in the laboratory. However, high-efficiency NFAs have so far only been realized in solution-processed blends. Due to its proven track record in upscaled industrial production, vacuum thermal evaporation (VTE) is of prime interest for real-world OPV commercialization. Here, we combine the benchmark solution-processed NFA Y6 with three different evaporated donors in a bilayer (planar heterojunction) architecture. We find that voltage losses decrease by hundreds of millivolts when VTE donors are paired with the NFA instead of the fullerene C₆₀, the current standard acceptor in VTE OPVs. By showing that evaporated small-molecule donors behave much like solution-processed donor polymers in terms of voltage loss when combined with NFAs, we highlight the immense potential for evaporable NFAs and the urgent need to direct synthesis efforts toward making smaller, evaporable compounds.

Benchmarking DFT Functionals for Excited-State Calculations of Donor-Acceptor TADF Emitters: Insights on the Key Parameters Determining Reverse Inter-System Crossing

The importance of intermediate triplet states and the nature of excited states has gained interest in recent years for the thermally activated delayed fluorescence (TADF) mechanism. It is widely accepted that simple conversion between charge transfer (CT) triplet and singlet excited states is too crude, and a more complex route involving higher-lying locally excited triplet excited states has to be invoked to witness the magnitude of the rate of reverse inter-system crossing (RISC) rates. The increased complexity has challenged the reliability of computational methods to accurately predict the relative energy between excited states as well as their nature. Here, we compare the results of widely used density functional theory (DFT) functionals, CAM-B3LYP, LC-ωPBE, LC-ω*PBE, LC-ω*HPBE, B3LYP, PBE0, and M06-2X, against a wavefunction-based reference method, Spin-Component Scaling second-order approximate Coupled Cluster (SCS-CC2), in 14 known TADF emitters possessing a diversity of chemical structures. Overall, the use of the Tamm-Dancoff Approximation (TDA) together with CAM-B3LYP, M06-2X, and the two ω-tuned range-separated functionals LC-ω*PBE and LC-ω*HPBE demonstrated the best agreement with SCS-CC2 calculations in predicting the absolute energy of the singlet S₁, and triplet T₁ and T₂ excited states and their energy differences. However, consistently across the series and irrespective of the functional or the use of TDA, the nature of T₁ and T₂ is not as accurately captured as compared to S₁. We also investigated the impact of the optimization of S₁ and T₁ excited states on ΔE_ST and the nature of these states for three different functionals (PBE0, CAM-B3LYP, and M06-2X). We observed large changes in ΔE_ST using CAM-B3LYP and PBE0 functionals associated with a large stabilization of T₁ with CAM-B3LYP and a large stabilization of S₁ with PBE0, while ΔE_ST is much less affected considering the M06-2X functional. The nature of the S₁ state barely evolves after geometry optimization essentially because this state is CT by nature for the three functionals tested. However, the prediction of the T₁ nature is more problematic since these functionals for some compounds interpret the nature of T₁ very differently. SCS-CC2 calculations on top of the TDA-DFT optimized geometries lead to a large variation in terms of ΔE_ST and the excited-state nature depending on the chosen functionals, further stressing the large dependence of the excited-state features on the excited-state geometries. The presented work highlights that despite good agreement of energies, the description of the exact nature of the triplet states should be undertaken with caution.

Unveiling Charge-Transport Mechanisms in Electronic Devices Based on Defect-Engineered MoS2 Covalent Networks

Device performance of solution-processed 2D semiconductors in printed electronics has been limited so far by structural defects and high interflake junction resistance. Covalently interconnected networks of transition metal dichalcogenides potentially represent an efficient strategy to overcome both limitations simultaneously. Yet, the charge-transport properties in such systems have not been systematically researched. Here, the charge-transport mechanisms of printed devices based on covalent MoS₂ networks are unveiled via multiscale analysis, comparing the effects of aromatic versus aliphatic dithiolated linkers. Temperature-dependent electrical measurements reveal hopping as the dominant transport mechanism: aliphatic systems lead to 3D variable range hopping, unlike the nearest neighbor hopping observed for aromatic linkers. The novel analysis based on percolation theory attributes the superior performance of devices functionalized with π-conjugated molecules to the improved interflake electronic connectivity and formation of additional percolation paths, as further corroborated by density functional calculations. Valuable guidelines for harnessing the charge-transport properties in MoS₂ devices based on covalent networks are provided.

Electronic and Charge Transport Properties in Bridged versus Unbridged Nanohoops: The Role of the Nanohoop Size

In the field of p-conjugated nanohoops, the size of the macrocycle has a huge impact on its structural characteristics, which in turn affect its electronic properties. In this work, we report the first experimental investigations linking the size of a nanohoop to its charge transport properties, a key property in organic electronics. We describe the synthesis and the study of the first example of cyclocarbazole possessing five constituting building units, namely [5]-cyclo-N-butyl-2,7-carbazole [5]C-Bu-Cbz. By comparison with a shorter analogue, [4]-cyclo-N-butyl-2,7-carbazole [4]C-Bu-Cbz, we detail the photophysical, electrochemical, morphological and charge transport properties, highlighting the key role played by the hoop size. Particularly, we show that the saturated field effect mobility of [5]C-Bu-Cbz is four times higher than that of its smaller analogue [4]C-Bu-Cbz (4.22 × 10-5vs 1.04 × 10-5 cm².V-1.s-1). However, the study of the other OFET characteristics (threshold voltage VTH and subthreshold slope SS) suggest that a small nanohoop is beneficial for a good organization of the molecules in thin films whereas a large one increases the density of structural defects, and hence of traps for the charge carriers. The present findings are of interest for the further development of nanohoops in electronics.

An Oligomer Approach for Blue Thermally Activated Delayed Fluorescent Emitters Based on Twisted Donor-Acceptor Units

The development of efficient blue donor-acceptor thermally activated delayed fluorescence (TADF) emitters remains a challenge. To enhance the efficiency of TADF-related processes of the emitter, we targeted a molecular design that would introduce a large number of intermediate triplet states between the lowest energy excited triplet (T₁) and singlet (S₁) excited states. Here, we introduce an oligomer approach using repetitive donor-acceptor units to gradually increase the number of quasi-degenerate states. In our design, benzonitrile (BN) moieties were selected as acceptors that are connected together via the amine donors, acting as bridges to adjacent BN acceptors. To preserve the photoluminescence emission wavelength across the series, we employed a design based on an ortho substitution pattern of the donors about the BN acceptor that induces a highly twisted conformation of the emitters, limiting the conjugation. Via a systematic photophysical study, we show that increasing the oligomer size allows for enhancement of the intersystem crossing and reverse intersystem crossing rates. We attribute the increasing intersystem crossing rate to the increasing number of intermediate triplet states along the series, confirmed by the time-dependent density functional theory. Overall, we report an approach to enhance the efficiency of TADF-related processes without changing the blue photoluminescence color.

Reversible Electrical Control of Interfacial Charge Flow across van der Waals Interfaces

Bond-free integration of two-dimensional (2D) materials yields van der Waals (vdW) heterostructures with exotic optical and electronic properties. Manipulating the splitting and recombination of photogenerated electron-hole pairs across the vdW interface is essential for optoelectronic applications. Previous studies have unveiled the critical role of defects in trapping photogenerated charge carriers to modulate the photoconductive gain for photodetection. However, the nature and role of defects in tuning interfacial charge carrier dynamics have remained elusive. Here, we investigate the nonequilibrium charge dynamics at the graphene-WS₂ vdW interface under electrochemical gating by operando optical-pump terahertz-probe spectroscopy. We report full control over charge separation states and thus photogating field direction by electrically tuning the defect occupancy. Our results show that electron occupancy of the two in-gap states, presumably originating from sulfur vacancies, can account for the observed rich interfacial charge transfer dynamics and electrically tunable photogating fields, providing microscopic insights for optimizing optoelectronic devices.

Mechanistic View on the Order-Disorder Phase Transition in Amphidynamic Crystals

We combine temperature-dependent low-frequency Raman measurements and first-principles calculations to obtain a mechanistic understanding of the order-disorder phase transition of 2,7-di-tert-butylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene (ditBu-BTBT) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) semiconducting amphidynamic crystals. We identify the lattice normal modes associated with the phase transition by following the position and width of the Raman peaks with temperature and identifying peaks that exhibit nonlinear dependence toward the phase transition temperature. Our findings are interpreted according to the "hardcore mode" model previously used to describe order-disorder phase transitions in inorganic and hybrid crystals with a Brownian sublattice. Within the framework of this model, ditBu-BTBT exhibits an ideal behavior where only one lattice mode is associated with the phase transition. TIPS-pentacene deviates strongly from the model due to strong interactions between lattice modes. We discuss the origin of the different behaviors and suggest side-chain engineering as a tool to control polymorphism in amphidynamic crystals.

Persistent Ion Accumulation at Interfaces Improves the Performance of Perovskite Solar Cells

The mixed ionic-electronic nature of lead halide perovskites makes their performance in solar cells complex in nature. Ion migration is often associated with negative impacts-such as hysteresis or device degradation-leading to significant efforts to suppress ionic movement in perovskite solar cells. In this work, we demonstrate that ion trapping at the perovskite/electron transport layer interface induces band bending, thus increasing the built-in potential and open-circuit voltage of the device. Quantum chemical calculations reveal that iodine interstitials are stabilized at that interface, effectively trapping them at a remarkably high density of ∼10²¹ cm^-3 which causes the band bending. Despite the presence of this high density of ionic defects, the electronic structure calculations show no sub-band-gap states (electronic traps) are formed due to a pronounced perovskite lattice reorganization. Our work demonstrates that ionic traps can have a positive impact on device performance of perovskite solar cells.

Dielectric control of reverse intersystem crossing in thermally activated delayed fluorescence emitters

Thermally activated delayed fluorescence enables organic semiconductors with charge transfer-type excitons to convert dark triplet states into bright singlets via reverse intersystem crossing. However, thus far, the contribution from the dielectric environment has received insufficient attention. Here we study the role of the dielectric environment in a range of thermally activated delayed fluorescence materials with varying changes in dipole moment upon optical excitation. In dipolar emitters, we observe how environmental reorganization after excitation triggers the full charge transfer exciton formation, minimizing the singlet-triplet energy gap, with the emergence of two (reactant-inactive) modes acting as a vibrational fingerprint of the charge transfer product. In contrast, the dielectric environment plays a smaller role in less dipolar materials. The analysis of energy-time trajectories and their free-energy functions reveals that the dielectric environment substantially reduces the activation energy for reverse intersystem crossing in dipolar thermally activated delayed fluorescence emitters, increasing the reverse intersystem crossing rate by three orders of magnitude versus the isolated molecule.

A New Frontier in Exciton Transport: Transient Delocalization

Efficient exciton transport is crucial to the application of organic semiconductors (OSCs) in light-harvesting devices. While the physics of exciton transport in highly disordered media is well-explored, the description of transport in structurally and energetically ordered OSCs is less established, despite such materials being favorable for devices. In this Perspective we describe and highlight recent research pointing toward a highly efficient exciton transport mechanism which occurs in ordered OSCs, transient delocalization. Here, exciton-phonon couplings play a critical role in allowing localized exciton states to temporarily access higher-energy delocalized states whereupon they move large distances. The mechanism shows great promise for facilitating long-range exciton transport and may allow for improved device efficiencies and new device architectures. However, many fundamental questions on transient delocalization remain to be answered. These questions and suggested next steps are summarized.

Modeling of Multiresonant Thermally Activated Delayed Fluorescence Emitters─Properly Accounting for Electron Correlation Is Key!

With the surge of interest in multiresonant thermally activated delayed fluorescent (MR-TADF) materials, it is important that there exist computational methods to accurately model their excited states. Here, building on our previous work, we demonstrate how the spin-component scaling second-order approximate coupled-cluster (SCS-CC2), a wavefunction-based method, is robust at predicting the ΔE_ST (i.e., the energy difference between the lowest singlet S₁ and triplet T₁ excited states) of a large number of MR-TADF materials, with a mean average deviation (MAD) of 0.04 eV compared to experimental data. Time-dependent density functional theory calculations with the most common DFT functionals as well as the consideration of the Tamm-Dancoff approximation (TDA) consistently predict a much larger ΔE_ST as a result of a poorer account of Coulomb correlation as compared to SCS-CC2. Very interestingly, the use of a metric to assess the importance of higher order excitations in the SCS-CC2 wavefunctions shows that Coulomb correlation effects are substantially larger in the lowest singlet compared to the corresponding triplet and need to be accounted for a balanced description of the relevant electronic excited states. This is further highlighted with coupled cluster singles-only calculations, which predict very different S₁ energies as compared to SCS-CC2 while T₁ energies remain similar, leading to very large ΔE_ST, in complete disagreement with the experiments. We compared our SCS-CC2/cc-pVDZ with other wavefunction approaches, namely, CC2/cc-pVDZ and SOS-CC2/cc-pVDZ leading to similar performances. Using SCS-CC2, we investigate the excited-state properties of MR-TADF emitters showcasing large ΔE_T2T1 for the majority of emitters, while π-electron extension emerges as the best strategy to minimize ΔE_ST. We also employed SCS-CC2 to evaluate donor-acceptor systems that contain a MR-TADF moiety acting as the acceptor and show that the broad emission observed for some of these compounds arises from the solvent-promoted stabilization of a higher-lying charge-transfer singlet state (S₂). This work highlights the importance of using wavefunction methods in relation to MR-TADF emitter design and associated photophysics.

Chemical Modifications Suppress Anharmonic Effects in the Lattice Dynamics of Organic Semiconductors

The lattice dynamics of organic semiconductors has a significant role in determining their electronic and mechanical properties. A common technique to control these macroscopic properties is to chemically modify the molecular structure. These modifications are known to change the molecular packing, but their effect on the lattice dynamics is relatively unexplored. Therefore, we investigate how chemical modifications to a core [1]benzothieno[3,2-b]benzothiophene (BTBT) semiconducting crystal affect the evolution of the crystal structural dynamics with temperature. Our study combines temperature-dependent polarization-orientation (PO) low-frequency Raman measurements with first-principles calculations and single-crystal X-ray diffraction measurements. We show that chemical modifications can indeed suppress specific expressions of vibrational anharmonicity in the lattice dynamics. Specifically, we detect in BTBT a gradual change in the PO Raman response with temperature, indicating a unique anharmonic expression. This anharmonic expression is suppressed in all examined chemically modified crystals (ditBu-BTBT and diC8-BTBT, diPh-BTBT, and DNTT). In addition, we observe solid–solid phase transitions in the alkyl-modified BTBTs. Our findings indicate that π-conjugated chemical modifications are the most effective in suppressing these anharmonic effects.

Solution Synthesis and Characterization of a Long and Curved Graphene Nanoribbon with Hybrid Cove-Armchair-Gulf Edge Structures

Curved graphene nanoribbons (GNRs) with hybrid edge structures have recently attracted increasing attention due to their unique band structures and electronic properties as a result of their nonplanar conformation. This work reports the solution synthesis of a long and curved multi-edged GNR (cMGNR) with unprecedented cove-armchair-gulf edge structures. The synthesis involves an efficient A₂ B₂ -type Diels-Alder polymerization between a diethynyl-substituted prefused bichrysene monomer (3b) and a dicyclopenta[e,l]pyrene-5,11-dione derivative (6) followed by FeCl₃ -mediated Scholl oxidative cyclodehydrogenation of the obtained polyarylenes (P1). Model compounds 1a and 1b are first synthesized to examine the suitability and efficiency of the corresponding polymers for the Scholl reaction. The successful formation of cMGNR from polymer P1 bearing prefused bichrysene units is confirmed by FTIR, Raman, and solid-state NMR analyses. The cove-edge structure of the cMGNR imparts the ribbon with a unique nonplanar conformation as revealed by density functional theory (DFT) simulation, which effectively enhances its dispersibility in solution. The cMGNR has a narrow optical bandgap of 1.61 eV, as estimated from the UV-vis absorption spectrum, which is among the family of low-bandgap solution-synthesized GNRs. Moreover, the cMGNR exhibits a carrier mobility of ≈2 cm² V^-1 s^-1 inferred from contact-free terahertz spectroscopy.

Dinaphthotetrathienoacenes: Synthesis, Characterization, and Applications in Organic Field-Effect Transistors

The charge transport of crystalline organic semiconductors is limited by dynamic disorder that tends to localize charges. It is the main hurdle to overcome in order to significantly increase charge carrier mobility. An innovative design that combines a chemical structure based on sulfur-rich thienoacene with a solid-state herringbone (HB) packing is proposed and the synthesis, physicochemical characterization, and charge transport properties of two new thienoacenes bearing a central tetrathienyl core fused with two external naphthyl rings: naphtho[2,3-b]thieno-[2''',3''':4'',5'']thieno[2″,3″:4',5']thieno[3',2'-b]naphtho[2,3-b]thiophene (DN4T) and naphtho[1,2-b]thieno-[2''',3''':4'',5'']thieno[2'',3'':4',5']thieno[3',2'-b]naphtho[1,2-b]thiophene are presented. Both compounds crystallize with a HB pattern structure and present transfer integrals ranging from 33 to 99 meV (for the former) within the HB plane of charge transport. Molecular dynamics simulations point toward an efficient resilience of the transfer integrals to the intermolecular sliding motion commonly responsible for strong variations of the electronic coupling in the crystal. Best device performances are reached with DN4T with hole mobility up to μ = 2.1 cm² V^-1 s^-1 in polycrystalline organic field effect transistors, showing the effectiveness of the electronic coupling enabled by the new aromatic core. These promising results pave the way to the design of high-performing materials based on this new thienoacene, notably through the introduction of alkyl side-chains.

High-Efficiency Ion-Exchange Doping of Conducting Polymers

Molecular doping-the use of redox-active small molecules as dopants for organic semiconductors-has seen a surge in research interest driven by emerging applications in sensing, bioelectronics, and thermoelectrics. However, molecular doping carries with it several intrinsic problems stemming directly from the redox-active character of these materials. A recent breakthrough was a doping technique based on ion-exchange, which separates the redox and charge compensation steps of the doping process. Here, the equilibrium and kinetics of ion exchange doping in a model system, poly(2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophene) (PBTTT) doped with FeCl₃ and an ionic liquid, is studied, reaching conductivities in excess of 1000 S cm^-1 and ion exchange efficiencies above 99%. Several factors that enable such high performance, including the choice of acetonitrile as the doping solvent, which largely eliminates electrolyte association effects and dramatically increases the doping strength of FeCl₃ , are demonstrated. In this high ion exchange efficiency regime, a simple connection between electrochemical doping and ion exchange is illustrated, and it is shown that the performance and stability of highly doped PBTTT is ultimately limited by intrinsically poor stability at high redox potential.

Excited-State Modulation in Donor-Substituted Multiresonant Thermally Activated Delayed Fluorescence Emitters

Strategies to tune the emission of multiresonant thermally activated delayed fluorescence (MR-TADF) emitters remain rare. Here, we explore the effect of donor substitution about a MR-TADF core on the emission energy and the nature of the excited state. We decorate different numbers and types of electron-donors about a central MR-TADF core, DiKTa. Depending on the identity and number of donor groups, the excited state either remains short-range charge transfer (SRCT) and thus characteristic of an MR-TADF emitter or becomes a long-range charge transfer (LRCT) that is typically observed in donor-acceptor TADF emitters. The impact is that in three examples that emit from a SRCT state, Cz-DiKTa, Cz-Ph-DiKTa, and 3Cz-DiKTa, the emission remains narrow, while in four examples that emit via a LRCT state, TMCz-DiKTa, DMAC-DiKTa, 3TMCz-DiKTa, and 3DMAC-DiKTa, the emission broadens significantly. Through this strategy, the organic light-emitting diodes fabricated with the three MR-TADF emitters show maximum electroluminescence emission wavelengths, λ_EL, of 511, 492, and 547 nm with moderate full width at half-maxima (fwhm) of 62, 61, and 54 nm, respectively. Importantly, each of these devices show high maximum external quantum efficiencies (EQE_max) of 24.4, 23.0, and 24.4%, which are among the highest reported with ketone-based MR-TADF emitters. OLEDs with D-A type emitters, DMAC-DiKTa and TMCz-DiKTa, also show high efficiencies, with EQE_max of 23.8 and 20.2%, but accompanied by broad emission at λ_EL of 549 and 527 nm, respectively. Notably, the DMAC-DiKTa-based OLED shows very small efficiency roll-off, and its EQE remains 18.5% at 1000 cd m^-2. Therefore, this work demonstrates that manipulating the nature and numbers of donor groups decorating a central MR-TADF core is a promising strategy for both red-shifting the emission and improving the performance of the OLEDs.

Vibronic fingerprints in the luminescence of graphene quantum dots at cryogenic temperature

Atomically precise graphene quantum dots synthesized by bottom-up chemistry are promising versatile single emitters with potential applications for quantum photonic technologies. Toward a better understanding and control of graphene quantum dot (GQD) optical properties, we report on single-molecule spectroscopy at cryogenic temperature. We investigate the effect of temperature on the GQDs’ spectral linewidth and vibronic replica, which we interpret building on density functional theory calculations. Finally, we highlight that the vibronic signatures are specific to the GQD geometry and can be used as a fingerprint for identification purposes.

Vibronic effect and influence of aggregation on the photophysics of graphene quantum dots

Graphene quantum dots, atomically precise nanopieces of graphene, are promising nano-objects with potential applications in various domains such as photovoltaics, quantum light emitters and bio-imaging. Despite their interesting prospects, precise reports on their photophysical properties remain scarce. Here, we report on a study of the photophysics of C₉₆H₂₄(C₁₂H₂₅) graphene quantum dots. A combination of optical studies down to the single molecule level with advanced molecular modelling demonstrates the importance of coupling to vibrations in the emission process. Optical fingerprints for H-like aggregates are identified. Our combined experimental-theoretical investigations provide a comprehensive description of the light absorption and emission properties of nanographenes, which not only represents an essential step towards precise control of sample production but also paves the way for new exciting physics focused on twisted graphenoids.

Diindolocarbazole - achieving multiresonant thermally activated delayed fluorescence without the need for acceptor units

In this work we present a new multi-resonance thermally activated delayed fluorescence (MR-TADF) emitter paradigm, demonstrating that the structure need not require the presence of acceptor atoms. Based on an in silico design, the compound DiICzMes4 possesses a red-shifted emission, enhanced photoluminescence quantum yield, and smaller singlet-triplet energy gap, ΔE_ST, than the parent indolocarbazole that induces MR-TADF properties. Coupled cluster calculations accurately predict the magnitude of the ΔE_ST when the optimized singlet and triplet geometries are used. Slow yet optically detectable reverse intersystem crossing contributes to low efficiency in organic light-emitting diodes using DiICzMes4 as the emitter. However, when used as a terminal emitter in combination with a TADF assistant dopant within a hyperfluorescence device architecture, maximum external quantum efficiencies of up to 16.5% were achieved at CIE (0.15, 0.11). This represents one of the bluest hyperfluorescent devices reported to date. Simultaneously, recognising that MR-TADF emitters do not require acceptor atoms reveals an unexplored frontier in materials design, where yet greater performance may yet be discovered.

Thermally activated intra-chain charge transport in high charge-carrier mobility copolymers

Disordered or even seemingly amorphous, donor–acceptor type, conjugated copolymers with high charge-carrier mobility have emerged as a new class of functional materials, where transport along the conjugated backbone is key. Here, we report on non-adiabatic molecular dynamics simulations of charge-carrier transport along chains of poly (indacenodithiophene-co-benzothiadiazole), within a model Hamiltonian parameterized against first-principles calculations. We predict thermally activated charge transport associated with a slightly twisted ground-state conformation, on par with experimental results. Our results also demonstrate that the energy mismatch between the hole on the donor vs the acceptor units of the copolymer drives localization of the charge carriers and limits the intra-chain charge-carrier mobility. We predict that room-temperature mobility values in excess of 10 cm2 V−1 s−1 can be achieved through proper chemical tuning of the component monomer units.

Structural and Dynamic Disorder, Not Ionic Trapping, Controls Charge Transport in Highly Doped Conducting Polymers

Doped organic semiconductors are critical to emerging device applications, including thermoelectrics, bioelectronics, and neuromorphic computing devices. It is commonly assumed that low conductivities in these materials result primarily from charge trapping by the Coulomb potentials of the dopant counterions. Here, we present a combined experimental and theoretical study rebutting this belief. Using a newly developed doping technique based on ion exchange, we prepare highly doped films with several counterions of varying size and shape and characterize their carrier density, electrical conductivity, and paracrystalline disorder. In this uniquely large data set composed of several classes of high-mobility conjugated polymers, each doped with at least five different ions, we find electrical conductivity to be strongly correlated with paracrystalline disorder but poorly correlated with ionic size, suggesting that Coulomb traps do not limit transport. A general model for interacting electrons in highly doped polymers is proposed and carefully parametrized against atomistic calculations, enabling the calculation of electrical conductivity within the framework of transient localization theory. Theoretical calculations are in excellent agreement with experimental data, providing insights into the disorder-limited nature of charge transport and suggesting new strategies to further improve conductivities.

Revealing Weak Dimensional Confinement Effects in Excitonic Silver/Bismuth Double Perovskites

Lead-free perovskites are attracting increasing interest as nontoxic materials for advanced optoelectronic applications. Here, we report on a family of silver/bismuth bromide double perovskites with lower dimensionality obtained by incorporating phenethylammonium (PEA) as an organic spacer, leading to the realization of two-dimensional double perovskites in the form of (PEA)₄AgBiBr₈ (n = 1) and the first reported (PEA)₂CsAgBiBr₇ (n = 2). In contrast to the situation prevailing in lead halide perovskites, we find a rather weak influence of electronic and dielectric confinement on the photophysics of the lead-free double perovskites, with both the 3D Cs₂AgBiBr₆ and the 2D n = 1 and n = 2 materials being dominated by strong excitonic effects. The large measured Stokes shift is explained by the inherent soft character of the double-perovskite lattices, rather than by the often-invoked band to band indirect recombination. We discuss the implications of these results for the use of double perovskites in light-emitting applications.

Enhancing Thermally Activated Delayed Fluorescence by Fine-Tuning the Dendron Donor Strength

Thermally activated delayed fluorescence (TADF) relies on a small energy gap between the emissive singlet and the nonemissive triplet state, obtained by reducing the wave function overlap between donor and acceptor moieties. Efficient emission, however, requires maintaining a good oscillator strength, which is itself based on sufficient overlap of the wave functions between donor and acceptor moieties. We demonstrate an approach to subtly fine-tune the required wave function overlap by employing donor dendrons of changing functionality. We use a carbazolyl-phthalonitrile based donor-acceptor core (2CzPN) as a reference emitter and progressively localize the hole density through substitution at the 3,6-positions of the carbazole donors (Cz) with further carbazole, (4-tert-butylphenyl)amine (^tBuDPA), and phenoxazine (PXZ). Using detailed photoluminescence studies, complemented with density functional theory (DFT) calculations, we show that this approach permits a gradual decrease of the singlet-triplet gap, ΔE_ST, from 300 to around 10 meV in toluene, yet we also demonstrate why a small ΔE_ST alone is not enough. While sufficient oscillator strength is maintained with the Cz- and ^tBuDPA-based donor dendrons, this is not the case for the PXZ-based donor dendron, where the wave function overlap is reduced too strongly. Overall, we find the donor dendron extension approach allows successful fine-tuning of the emitter photoluminescence properties.

Two Boron Atoms versus One: High-performance Deep-blue Multi-resonance Thermally Activated Delayed Fluorescence Emitters

Two new deep-blue narrowband multi-resonant emitters, 1B-DTACrs and 2B-DTACrs, one of which showing thermally activated delayed fluorescence (TADF), based on boron, nitrogen, and oxygen doped nanographenes were designed and synthesized...

Multi-resonant thermally activated delayed fluorescence emitters based on tetracoordinate boron-containing PAHs: colour tuning based on the nature of chelates

Multi-resonant thermally activated delayed fluorescence (MR-TADF) materials have attracted considerable attention recently. The molecular design frequently incorporates cycloboration. However, to the best of our knowledge MR-TADF compounds containing nitrogen chelated to boron are still unknown. Reported herein is a new class of tetracoordinate boron-containing MR-TADF emitters bearing C^N^C- and N^N^N-chelating ligands. We demonstrate that the replacement of the B–C covalent bond in the C^N^C-chelating ligand by the B–N covalent bond affords an isomer, which dramatically influences the optoelectronic properties of the molecule. The resulting N^N^N-chelating compounds show bathochromically shifted absorption and emission spectra relative to C^N^C-chelating compounds. The incorporation of a tert-butylcarbazole group at the 4-position of the pyridine significantly enhances both the thermal stability and the reverse intersystem crossing rate, yet has a negligible effect on emission properties. Consequently, high-performance hyperfluorescent organic light-emitting diodes (HF-OLEDs) that utilize these molecules as green and yellow-green emitters show a maximum external quantum efficiency (η_ext) of 11.5% and 25.1%, and a suppressed efficiency roll-off with an η_ext of 10.2% and 18.7% at a luminance of 1000 cd m⁻², respectively.

A new class of tetra-coordinate boron-containing MR-TADF emitters and their corresponding high-performance hyperfluorescent organic light-emitting diodes have been successfully achieved.

Radical defects modulate the photocatalytic response in 2D-graphitic carbon nitride

Graphitic carbon nitride (gCN) is an important heterogeneous metal-free catalytic material. Thermally induced post-synthetic modifications, such as amorphization and/or reduction, were recently used to enhance the photocatalytic response of these materials for certain classes of organic transformations, with structural defects possibly playing an important role. The knowledge of how these surface modifications modulate the photocatalytic response of gCN is therefore not only interesting from a fundamental point of view, but also necessary for the development and/or tuning of metal-free gCN systems with superior photo-catalytic properties. Herein, employing density functional theory calculations and combining both the periodic and molecular approaches, in conjunction with experimental EPR measurements, we demonstrate that different structural defects on the gCN surface generate distinctive radical defect states localized within the electronic bandgap, with only those correlated with amorphous and reduced gCN structures being photo-active. To this end, we (i) model defective gCN surfaces containing radical defect states; (ii) assess the interactions of these defects with the radical precursors involved in the photo-driven alkylation of electron-rich aromatic compounds (namely perfluoroalkyl iodides); and (iii) describe the photo-chemical processes triggering the initial step of that reaction at the gCN surface. We provide a coherent structure/photo-catalytic property relationship on defective gCN surfaces, elaborating how only specific defect types act as binding sites for the perfluoroalkyl iodide reagent and can favor a photo-induced charge transfer from the gCN surface to the molecule, thus triggering the perfluoroalkylation reaction.

The nature of radical defects governs the photocatalytic activity of graphitic carbon nitride.

Cove-Edged Graphene Nanoribbons with Incorporation of Periodic Zigzag-Edge Segments

Structurally precision graphene nanoribbons (GNRs) are promising candidates for next-generation nanoelectronics due to their intriguing and tunable electronic structures. GNRs with hybrid edge structures often confer them unique geometries associated with exotic physicochemical properties. Herein, a novel type of cove-edged GNRs with periodic short zigzag-edge segments is demonstrated. The bandgap of this GNR family can be tuned using an interplay between the length of the zigzag segments and the distance of two adjacent cove units along the opposite edges, which can be converted from semiconducting to nearly metallic. A family member with periodic cove-zigzag edges based on N = 6 zigzag-edged GNR, namely 6-CZGNR-(2,1), is successfully synthesized in solution through the Scholl reaction of a unique snakelike polymer precursor (10) that is achieved by the Yamamoto coupling of a structurally flexible S-shaped phenanthrene-based monomer (1). The efficiency of cyclodehydrogenation of polymer 10 toward 6-CZGNR-(2,1) is validated by FT-IR, Raman, and UV-vis spectroscopies, as well as by the study of two representative model compounds (2 and 3). Remarkably, the resultant 6-CZGNR-(2,1) exhibits an extended and broad absorption in the near-infrared region with a record narrow optical bandgap of 0.99 eV among the reported solution-synthesized GNRs. Moreover, 6-CZGNR-(2,1) exhibits a high macroscopic carrier mobility of ∼20 cm² V^-1 s^-1 determined by terahertz spectroscopy, primarily due to the intrinsically small effective mass (m*_e = m*_h = 0.17 m₀), rendering this GNR a promising candidate for nanoelectronics.

Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors

Engineering a low singlet-triplet energy gap (ΔEST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 × 105 cm-1) and a relatively large ΔEST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 μs), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of ≥1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low ΔEST in organic DF emitters.