Regioisomerism vs Conformation: Impact of Molecular Design on the Emission Pathway in Organic Light-Emitting Device Emitters.

Despite the design and proposal of several new structural motifs as thermally activated delayed fluorescent (TADF) emitters for organic light-emitting device (OLED) applications, the nature of their interaction with the host matrix in the emissive layer of the device and their influence on observed photophysical outputs remain unclear. To address this issue, we present, for the first time, the use of up to four regioisomers bearing a donor-acceptor-donor electronic structure based on the desymmetrized naphthalene benzimidazole scaffold, equipped with various electron-donating units and possessing distinguished conformational lability. Quantum chemical calculations allow us to identify the most favorable conformations adopted by the electron-rich groups across the entire pool of regioisomers. These conformations were then compared with conformational changes caused by the interaction of the emitter with the Zeonex and 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) matrices, and the correlation with observed photophysics was monitored by UV-vis absorption and steady-state photoluminescence spectra, combined with time-resolved spectroscopic techniques. Importantly, a CBP matrix was found to have a significant impact on the conformational change of regioisomers, leading to unique TADF emission mechanisms that encompass dual emission and inversion of the singlet-triplet excited-state energies and result in the enhancement of TADF efficiency. As a proof of concept, regioisomers with optimal donor positions were utilized to fabricate an OLED, revealing, with the best-performing dye, an external quantum emission of 11.6%, accompanied by remarkable luminance (28,000 cd/m2). These observations lay the groundwork for a better understanding of the role of the host matrix. In the long term, this new knowledge can lead to predicting the influence of the host matrix and adopting the structure of the emitter in a way that allows the development of highly efficient and efficient OLEDs.

Facile Functionalization of Ambipolar, Nitrogen-Doped PAHs toward Highly Efficient TADF OLED Emitters.

Despite promising optoelectronic features of N-doped polycyclic aromatic hydrocarbons (PAHs), their use as functional materials remains underdeveloped due to their limited post-functionalization. Facing this challenge, a novel design of N-doped PAHs with D-A-D electronic structure for thermally activated delayed fluorescence (TADF) emitters was performed. Implementing a set of auxiliary donors at the meta position of the protruding phenyl ring of quinoxaline triggers an increase in the charge-transfer property simultaneously decreasing the delayed fluorescence lifetime. This, in turn, contributes to a narrow (0.04-0.28 eV) singlet-triplet exchange energy split (ΔEST) and promotes a reverse intersystem crossing transition that is pivotal for an efficient TADF process. Boosting the electron-donating ability of our N-PAH scaffold leads to excellent photoluminescence quantum yield that was found in a solid-state matrix up to 96% (for phenoxazine-substituted derivatives, under air) with yellow or orange-red emission, depending on the specific compound. Organic light-emitting diodes (OLEDs) utilizing six, (D-A)-D, N-PAH emitters demonstrate a significant throughput with a maximum external quantum efficiency of 21.9% which is accompanied by remarkable luminance values which were found for all investigated devices in the range of 20,000-30,100 cd/m2 which is the highest reported to date for N-doped PAHs investigated in the OLED domain.

V-shaped donor-acceptor organic emitters. A new approach towards efficient TADF OLED devices.

We report the synthesis and characterization of a series of donor-acceptor TADF emitters with a new architecture, where the donor moiety and the dibenzazepine-based acceptor moiety are separated by a phenylene linker in a V-shaped spatial arrangement. Such spatial separation and electronic decoupling between the donor and the acceptor moieties leads to low singlet-triplet energy gaps and favors efficient exciton up-conversion.

Tandem rigidification and π-extension as a key tool for the development of a narrow linewidth yellow hyperfluorescent OLED system.

Hyperfluorescence (HF), a relatively new phenomenon utilizing the transfer of excitons between two luminophores, requires careful pairwise tuning of molecular energy levels and is proposed to be the crucial step towards the development of new, highly effective OLED systems. To date, barely few HF yellow emitters with desired narrowband emission but moderate external quantum efficiency (EQE < 20%) have been reported. This is because a systematic strategy embracing both Förster resonance energy transfer (FRET) and triplet to singlet (TTS) transition as complementary mechanisms for effective exciton transfer has not yet been proposed. Herein, we present a rational approach, which allows, through subtle structural modification, a pair of compounds built from the same donor and acceptor subunits, but with varied communication between these ambipolar fragments, to be obtained. The TADF-active dopant is based on a naphthalimide scaffold linked to the nitrogen of a carbazole moiety, which through the introduction of an additional bond leads not only to π-cloud enlargement, but also rigidifies and inhibits the rotation of the donor. This structural change prevents TADF, and guides bandgaps and excited state energies to simultaneously pursue FRET and TTS processes. New OLED devices utilizing the presented emitters show excellent external quantum efficiency (up to 27%) and a narrow full width at half maximum (40 nm), which is a consequence of very good alignment of energy levels. The presented design principles prove that only a minor structural modification is needed to obtain commercially applicable dyes for HF OLED devices.

Modular Nitrogen-Doped Concave Polycyclic Aromatic Hydrocarbons for High-Performance Organic Light-Emitting Diodes with Tunable Emission Mechanisms.

Although bowl-shaped N-pyrrolic polycyclic aromatic hydrocarbons (PAHs) can achieve excellent electron-donating ability, their application for optoelectronics is hampered by typically low photoluminescence quantum yields (PLQYs). To address this issue, we report the synthesis and characterization of a series of curved and fully conjugated nitrogen-doped PAHs. Through structural modifications to the electron-accepting moiety, we are able to switch the mechanism of luminescence between thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP), and to tune the overall PLQY in the range from 9 % to 86 %. As a proof of concept, we constructed solid-state organic light-emitting diode (OLED) devices, which has not been explored to date in the context of concave N-doped systems being TADF/RTP emitters. The best-performing dye, possessing a peripheral trifluoromethyl group adjacent to the phenazine acceptor, exhibits yellow to orange emission with a maximum external quantum efficiency (EQE) of 12 %, which is the highest EQE in a curved D-A embedded N-PAH to date.

Addressing a lattice of rotatable molecular dipoles with the electric field of an STM tip.

Functional molecular groups mounted on specific foot structures are ideal model systems to study intermolecular interactions, due to the possibility to separate the functionality and the adsorption mechanism. Here, we report on the rotational switching of a thioacetate group mounted on a tripodal tetraphenylmethane (TPM) derivative adsorbed in ordered islands on a Au(111) surface. Using low temperature scanning tunnelling microscopy, individual freestanding molecular groups of the lattice can be switched between two bistable orientations. The functional dependence of this rotational switching on the sample bias and tip-sample distance allows us to model the energy landscape of this molecular group as an electric dipole in the electric field of the tunnelling junction. As expected for the interaction of two dipoles, we found states of neighbouring molecules to be correlated.

Tuning Anion-Binding Properties of 22-Membered Unclosed Cryptands by Structural Modification of the Lariat Arm.

A series of 22-membered unclosed cryptands end-capped with intra-annular methyl (1), phenyl (2), p-tert-butylphenyl (3), and p-nitrophenyl (4) amide substituents (lariat arm) were synthesized to elucidate the effect of steric and electronic factors on their anion recognition behavior. The 1H NMR titrations in DMSO-d 6 with 0.5% water reveal enhanced selectivity for H2PO4 - vs Cl- and PhCO2 -. The para-attachment of the electron-withdrawing nitro group (-NO2 vs -H and -t-Bu) was found to increase anion-binding affinity, whereas the steric bulkiness of lariat arm (methyl vs aryl) has a marginal effect. DFT calculations reveal that binding of H2PO4 - is associated with minimal conformational change in the lariat arm moiety and involve four hydrogen bond acceptor and one donor sites of host.

Tuning the contact conductance of anchoring groups in single molecule junctions by molecular design.

A tetraphenylmethane tripod functionalized with three thiol moieties in the para position can serve as a supporting platform for functional molecular electronic elements. A combined experimental scanning tunneling microscopy break junction technique with theoretical approaches based on density functional theory and non-equilibrium Green's function formalism was used for detailed charge transport analysis to find configurations, geometries and charge transport pathways in the molecular junctions of single molecule oligo-1,4-phenylene conductors containing this tripodal anchoring group. The effect of molecular length (n = 1 to 4 repeating phenylene units) on the charge transport properties and junction configurations is addressed. The number of covalent attachments between the electrode and the tripodal platform changes with n affecting the contact conductance of the junction. The longest homologue n = 4 adopts an upright configuration with all three para thiolate moieties of the tripod attached to the gold electrode. The contact conductance of the tetraphenylmethane tripod substituted by thiols in the para position is higher than that substituted in the meta position. Such molecular arrangement is highly conducting and allows well-defined directional positioning of a variety of functional groups.

Six state molecular revolver mounted on a rigid platform.

The rotation of entire molecules or large moieties happens at 100 ps time scales and the transition process itself is experimentally inaccessible to scanning probe techniques. However, the reversible switching of a molecule between more than two metastable states allows to assign a rotational switching direction. Rotational switching is a phenomenon that is particularly interesting with regard to possible applications in molecular motors. In this work, single tetraphenylmethane molecules deposited on a Au(111) surface were studied in a low temperature scanning tunneling microscope (STM). These molecules comprise rotational axes mounted on a tripodal sulfur-anchored stand and with the STM tip, we were able to induce transitions between six rotational states of the molecular motif. We were able to identify critical parameters for the onset of rotational switching and to characterize the influence of the local environment. The subtle difference between fcc and hcp stacking and the rotational state of neighboring molecules clearly influence the population of the rotational states.

Probabilistic mapping of single molecule junction configurations as a tool to achieve the desired geometry of asymmetric tripodal molecules.

Four molecules containing identical tripodal anchors and p-oligophenylene molecular wires of increasing length were used to demonstrate tuning of the asymmetric molecular junction to the desired geometry by probabilistic mapping of single molecule junction configurations in a scanning tunnelling microscopy break junction experiment.

An Indirect Synthetic Approach toward Conformationally Constrained 20-Membered Unclosed Cryptands via Late-Stage Installation of Intraannular Substituents.

A new protocol for PTC-mediated O-alkylation of the intraannular position of 20-membered unclosed cryptands (UCs) is reported. In contrast to the classical, "direct" strategy, which requires functionalization of the lariat arm at the beginning of synthesis, this "indirect" approach enables the late-stage introduction of various benzylic substituents after an unfavorable macrocyclization step (11 examples, yields up to 98%). Notably, this method permits preparation of, previously inaccessible, crowded UCs bearing 1-acetylpyrene substituent and dimer joined by p-xylene linker.

Molecular Graph Paper.

We present a self-assembled template that consists of tetraphenylmethane derivatives and adopts a periodic lateral arrangement on a Au(111) surface with acetyl groups sticking out of the molecular film. By using the tip of a scanning tunneling microscope, these acetyl groups can be removed in a spatially controlled way without significantly affecting the remaining molecular assembly. The chemically modified molecules can be readily distinguished from the original ones such that information can be engraved in the molecular film. Both the modified nature of an individual molecule and the order of the molecular film are shown to persist at room temperature. The mesh size of this molecular graph paper can be tuned by varying the length of the molecular spacer so that writing and reading information on the nanoscale with variable letter sizes becomes possible.

Importance of the Anchor Group Position (Para versus Meta) in Tetraphenylmethane Tripods: Synthesis and Self-Assembly Features.

The efficient synthesis of tripodal platforms based on tetraphenylmethane with three acetyl-protected thiol groups in either meta or para positions relative to the central sp(3) carbon for deposition on Au (111) surfaces is reported. These platforms are intended to provide a vertical arrangement of the substituent in position 4 of the perpendicular phenyl ring and an electronic coupling to the gold substrate. The self-assembly features of both derivatives are analyzed on Au (111) surfaces by low-temperature ultra-high-vacuum STM, high-resolution X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and reductive voltammetric desorption studies. These experiments indicated that the meta derivative forms a well-ordered monolayer, with most of the anchoring groups bound to the surface, whereas the para derivative forms a multilayer film with physically adsorbed adlayers on the chemisorbed para monolayer. Single-molecule conductance values for both tripodal platforms are obtained through an STM break junction experiment.

Rigid multipodal platforms for metal surfaces.

In this review the recent progress in molecular platforms that form rigid and well-defined contact to a metal surface are discussed. Most of the presented examples have at least three anchoring units in order to control the spatial arrangement of the protruding molecular subunit. Another interesting feature is the lateral orientation of these foot structures which, depending on the particular application, is equally important as the spatial arrangement of the molecules. The numerous approaches towards assembling and organizing functional molecules into specific architectures on metal substrates are reviewed here. Particular attention is paid to variations of both, the core structures and the anchoring groups. Furthermore, the analytical methods enabling the investigation of individual molecules as well as monomolecular layers of ordered platform structures are summarized. The presented multipodal platforms bearing several anchoring groups form considerably more stable molecule-metal contacts than corresponding monopodal analogues and exhibit an enlarged separation of the functional molecules due to the increased footprint, as well as restrict tilting of the functional termini with respect to the metal surface. These platforms are thus ideally suited to tune important properties of the molecule-metal interface. On a single-molecule level, several of these platforms enable the control over the arrangement of the protruding rod-type molecular structures (e.g., molecular wires, switches, rotors, sensors) with respect to the surface of the substrate.