Electroactive Dyes Based on 1,8-Naphthalimide with Acetylene Linkers as Promising OLED Materials - the Relationship Between Structure and Photophysical Properties.

Four A-π-D-π-A type small organic molecules with 1,8-naphthalimide motifs were successfully synthesised. The designed compounds are built of two 1,8-naphthalimide units linked via ethynyl π-linkages with selected functionalised donor motifs i. e. 2,2'-bithiophene, fluorene, phenothiazine and carbazole derivative. The synthesis based on Sonogashira cross-coupling allowed us to obtain the presented dyes with good yields. The resulting symmetrical small molecules' optical, electrochemical and thermal properties were thoroughly investigated, and their potential applicability for the OLED devices was demonstrated. In addition, the relationship between molecular structure and properties was considered by employing experimental and theoretical studies. As a result of using various donor groups, it was possible to achieve efficient electroluminescence in the range from green (DEV4) to orange-red light (DEV3) with a maximum luminance of 3 820 cd/m2 for DEV4. Upon the insertion of an acetylene linker to the designed molecules, the free rotation of D and A fragments, and hence the effective π-electron communication within the entire molecule, is possible, which was confirmed by DFT studies. The obtained dyes are characterised by high thermal stability, reversible oxidation-reduction process, satisfactory optoelectronic properties and good solubility in organic solvents, which is advisable for the application in small molecular organic light-emitting diodes (SM-OLEDs) technology.

Dibenzophenazine-Based TADF Emitters as Dual Electrochromic and Electroluminescence Materials.

Previous work has reported the synthesis of donor-acceptor-donor molecules based on dibenzophenazine acceptor group, presenting thermally activated delayed fluorescent (TADF) properties and their application in the assembly of highly efficient electroluminescent devices. Herein, we focus on the characterisation of charge carrier species through UV-Vis-NIR spectroelectrochemical and potentiostatic EPR techniques, in addition to the investigation of electropolymerisation properties of some compounds depicted in this study. The promising electrochromic features of both small molecules and conjugated polymers led to the assembly and investigation of electrochromic devices, evidencing the materials' versatility, applied in such different approaches as electrochromic windows and electroluminescent devices. Furthermore, the assembled OLEDs provided high efficiencies, with small roll-off, EQEs up to 20.5 % and luminance values up to 85 000 cd/m2 .

Organic Thermoelectric Materials as the Waste Heat Remedy.

The primary reason behind the search for novel organic materials for application in thermoelectric devices is the toxicity of inorganic substances and the difficulties associated with their processing for the production of thin, flexible layers. When Thomas Seebeck described a new phenomenon in Berlin in 1820, nobody could have predicted the future applications of the thermoelectric effect. Now, thermoelectric generators (TEGs) are used in watches, and thermoelectric coolers (TECs) are applied in cars, computers, and various laboratory equipment. Nevertheless, the future of thermoelectric materials lies in organic compounds. This paper discusses the developments made in thermoelectric materials, including small molecules, polymers, molecular junctions, and their applications as TEGs and/or TECs.

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.

Comparative study of thermally activated delayed fluorescent properties of donor-acceptor and donor-acceptor-donor architectures based on phenoxazine and dibenzo[a,j]phenazine.

A new thermally activated delayed fluorescence (TADF) compound based on a donor-acceptor (D-A) architecture (D = phenoxazine; A = dibenzo[a,j]phenazine) has been developed, and its photophysical properties were characterized. The D-A compound is applicable as an emitting material for efficient organic light-emitting diodes (OLEDs), and its external quantum efficiency (EQE) exceeds the theoretical maximum of those with prompt fluorescent emitters. Most importantly, comparative study of the D-A molecule and its D-A-D counterpart from the viewpoints of the experiments and theoretical calculations revealed the effect of the number of the electron donor on the thermally activated delayed fluorescent behavior.

The Impact of C2 Insertion into a Carbazole Donor on the Physicochemical Properties of Dibenzo[a,j]phenazine-Cored Donor-Acceptor-Donor Triads.

Novel electron donor-acceptor-donor (D-A-D) compounds comprising dibenzo[a,j]phenazine as the central acceptor core and two 7-membered diarylamines (iminodibenzyl and iminostilbene) as the donors have been designed and synthesized. Investigation of their physicochemical properties revealed the impact of C2 insertion into well-known carbazole electron donors on the properties of previously reported twisted dibenzo[a,j]phenazine-core D-A-D triads. Slight structural modification caused a drastic change in conformational preference, allowing unique photophysical behavior of dual emission derived from room-temperature phosphorescence and triplet-triplet annihilation. Furthermore, electrochemical analysis suggested sigma-dimer formation and electrochemical polymerization on the electrode. Quantum chemical calculations also rationalized the experimental results.

Thermally Activated Delayed Fluorescent Donor-Acceptor-Donor-Acceptor π-Conjugated Macrocycle for Organic Light-Emitting Diodes.

A new class of thermally activated delayed fluorescent donor-acceptor-donor-acceptor (D-A-D-A) π-conjugated macrocycle comprised of two U-shaped electron-acceptors (dibenzo[a,j]phenazine) and two electron-donors (N,N'-diphenyl-p-phenyelendiamine) has been rationally designed and successfully synthesized. The macrocyclic compound displayed polymorphs-dependent conformations and emission properties. Comparative studies on physicochemical properties of the macrocycle with a linear surrogate have revealed significant effects of the structural cyclization of the D-A-repeating unit, including more efficient thermally activated delayed fluorescence (TADF). Furthermore, an organic light-emitting diode (OLED) device fabricated with the macrocycle compound as the emitter has achieved a high external quantum efficiency (EQE) up to 11.6%, far exceeding the theoretical maximum (5%) of conventional fluorescent emitters and that with linear analogue (6.9%).

Donor-Acceptor 1,2,4,5-Tetrazines Prepared by the Buchwald-Hartwig Cross-Coupling Reaction and Their Photoluminescence Turn-On Property by Inverse Electron Demand Diels-Alder Reaction.

A facile efficient synthetic tool, Buchwald-Hartwig cross-coupling reaction, for the functionalization of 1,2,4,5-tetrazines is presented. Important factors affecting the Buchwald-Hartwig cross-coupling reaction have been optimized. Seven new donor-acceptor tetrazine molecules (TA1-TA7) were conveniently prepared in good to high yields (61-72%). They have been subsequently engaged in the inverse electron demand Diels-Alder (iEDDA) reaction with cyclooctyne. The photophysical and electrochemical properties of the new pyridazines have been studied. Some are fluorescent acting as turn-on probes. More importantly, two pyridazines (DA3 and DA6) exhibit room-temperature phosphorescence (RTP) properties.

Convenient One-Pot Synthesis of 1,2,3,4-Thiatriazoles Towards a Novel Electron Acceptor for Highly-Efficient Thermally-Activated Delayed-Fluorescence Emitters.

A novel and unexpected convenient one-pot synthesis of 1,2,3,4-thiatriazoles has been discovered while investigating the classical tetrazine "Pinner synthesis". The synthetic route starts from commercially-available nitrile derivatives and gives good to high yields (51-80 %) with no need to isolate any thioacylating agents. The crucial impact of the solvent on the outcome of the modified "Pinner synthesis" is moreover examined and discussed. Using this new synthetic route, a novel donor-acceptor thiatriazole derivative has been prepared, which exhibits prominent thermally-activated delayed fluorescence (TADF) in both solution and film. The photoluminescence quantum yield (PLQY) in methylcyclohexane (MCH) and Zeonex (a cyclo olefin polymer) in oxygen-free conditions were determined to be 76 and 99 %, respectively. This work provides an efficient and practical synthetic approach to functionalized 1,2,3,4-thiatriazole derivatives, and will noticeably facilitate the application of 1,2,3,4-thiatriazole as an electron acceptor in organic electronics.

Syntheses of Diverse Donor-Substituted Bisbenzofuro[2,3- b:3',2'- e]pyridines (BBZFPys) via Pd Catalysis, and Their Photophysical Properties.

A series of bisbenzofuro[2,3- b:3',2'- e]pyridines (BBZFPys) bearing a chlorine functionality have been efficiently synthesized through a Pd-catalyzed double oxidative intramolecular C-H/C-H coupling of monochlorinated 2,6-diaryloxypyridines. The subsequent Buchwald-Hartwig amination of the chlorinated BBZFPys allowed for the access to a new class of donor-acceptor (D-A) π-conjugated compounds that comprise BBZFPy as an electron acceptor (A) and diarylamines as a donor (D) unit. The investigation of the steady-state photophysical properties of the prepared D-A compounds revealed that they are emissive in both solution and solid states in the blue-to-green color region. The singlet-triplet energy splitting (Δ EST) was found to be much smaller than that of substituent-free BBZFPy (0.70 eV), ranging from 0.01 to 0.56 eV. The time-resolved spectroscopy revealed that the D-A compounds, comprising a bis( tert-butyl)carbazole as the D and CF3-attached BBZFPy as the A, showed delayed fluorescence (DF) in nonpolar matrix host material (Zeonex), while in a polar matrix (DPEPO), room-temperature phosphorescence (RTP) was faintly observed. Furthermore, organic light-emitting diodes (OLEDs) fabricated with the D-A compounds as a blue emitter showed a moderate external quantum efficiencies (EQEs) up to 1.5%.

Triplet Harvesting with a Simple Aromatic Carbonyl.

The efficiency of organic light-emitting diodes crucially depends on triplet harvesters. These accept energy from triplet correlated electron hole pairs and convert it into light. Here, experimental evidence is given that simple aromatic carbonyls, such as thioxanthone, could serve this purpose. In these compounds, the emissive 1 ππ* excitation may rapidly equilibrate with an upper triplet state (3 nπ*). This equilibrium may persist for nanoseconds. Population of the 3 nπ* state via energy transfer from an electron hole pair should result in fluorescence emission and thereby triplet harvesting. To demonstrate the effect, solutions of 1,4-dichlorobenzene (triplet sensitizer) and thioxanthone (harvester) were excited at 266 nm with a nanosecond laser. The emission decay reveals a 100 ns decay absent in the thioxanthone only sample. This matches predictions for an energy transfer limited by diffusion and gives clear evidence that thioxanthone can convert triplet excitations into light.

Soluble Flavanthrone Derivatives: Synthesis, Characterization, and Application to Organic Light-Emitting Diodes.

Simple modification of benzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridine-8,16-dione, an old and almost-forgotten vat dye, by reduction of its carbonyl groups and subsequent O-alkylation, yields solution-processable, electroactive, conjugated compounds of the periazaacene type, suitable for the use in organic electronics. Their electrochemically determined ionization potential and electron affinity of about 5.2 and -3.2 eV, respectively, are essentially independent of the length of the alkoxyl substituent and in good agreement with DFT calculations. The crystal structure of 8,16-dioctyloxybenzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridine (FC-8), the most promising compound, was solved. It crystallizes in space group P1‾ and forms π-stacked columns held together in the 3D structure by dispersion forces, mainly between interdigitated alkyl chains. Molecules of FC-8 have a strong tendency to self-organize in monolayers deposited on a highly oriented pyrolytic graphite surface, as observed by STM. 8,16-Dialkoxybenzo[h]benz[5,6]acridino[2,1,9,8-klmna]acridines are highly luminescent, and all have photoluminescence quantum yields of about 80 %. They show efficient electroluminescence, and can be used as guest molecules with a 4,4'-bis(N-carbazolyl)-1,1'-biphenyl host in guest/host-type organic light-emitting diodes. The best fabricated diodes showed a luminance of about 1900 cd m(-12) , a luminance efficiency of about 3 cd A(-1) , and external quantum efficiencies exceeding 0.9 %.

Star-Shaped Conjugated Molecules with Oxa- or Thiadiazole Bithiophene Side Arms.

Star-shaped conjugated molecules, consisting of a benzene central unit symmetrically trisubstituted with either oxa- or thiadiazole bithiophene groups, were synthesized as promising molecules and building blocks for application in (opto)electronics and electrochromic devices. Their optical (Eg (opt)) as well as electrochemical (Eg (electro)) band gaps depended on the type of the side arm and the number of solubilizing alkyl substituents. Oxadiazole derivatives showed Eg (opt) slightly below 3 eV and by 0.2 eV larger than those determined for thiadiazole-based compounds. The presence of alkyl substituents in the arms additionally lowered the band gap. The obtained compounds were efficient electroluminophores in guest/host-type light-emitting diodes. They also showed a strong tendency to self-organize in monolayers deposited on graphite, as evidenced by scanning tunneling microscopy. The structural studies by X-ray scattering revealed the formation of supramolecular columnar stacks in which the molecules were organized. Differences in macroscopic alignment in the specimen indicated variations in the self-assembly mechanism between the molecules. The compounds as trifunctional monomers were electrochemically polymerized to yield the corresponding polymer network. As shown by UV/Vis-NIR spectroelectrochemical studies, these networks exhibited reversible electrochromic behavior both in the oxidation and in the reduction modes.

Dibenzo[a,j]phenazine-Cored Donor-Acceptor-Donor Compounds as Green-to-Red/NIR Thermally Activated Delayed Fluorescence Organic Light Emitters.

A new family of thermally activated delayed fluorescence (TADF) emitters based on U-shaped D-A-D architecture with a novel accepting unit has been developed. All investigated compounds have small singlet-triplet energy splitting (ΔEST ) ranging from 0.02 to 0.20 eV and showed efficient TADF properties. The lowest triplet state of the acceptor unit plays the key role in the TADF mechanism. OLEDs fabricated with these TADF emitters achieved excellent efficiencies up to 16 % external quantum efficiency (EQE).

Electrochemically Induced Synthesis of Poly(2,6-carbazole).

The formation of a poly(2,6-carbazole) derivative during an electrochemical polymerization process is shown. Comparison of 3,5-bis(9-octyl-9H-carbazol-2-yl)pyridine and 3,5-bis(9-octyl-9H-carbazol-3-yl)pyridine by electrochemical and UV-Vis-NIR spectroelectrochemical measurements and DFT (density functional theory) calculation prove the formation of a poly(2,6-carbazole) derivative. Both of the compounds form stable and electroactive conjugated polymers.

An unprecedented roll-off ratio in high-performing red TADF OLED emitters featuring 2,3-indole-annulated naphthalene imide and auxiliary donors.

The capability of organic emitters to harvest triplet excitons via a thermally activated delayed fluorescence (TADF) process has opened a new era in organic optoelectronics. Nevertheless, low brightness, and consequently an insufficient roll-off ratio, constitutes a bottleneck for their practical applications in the domain of organic light-emitting diodes (OLEDs). To address this formidable challenge, we developed a new design of desymmetrized naphthalimide (NMI) featuring an annulated indole with a set of auxiliary donors on its periphery. Their perpendicular arrangement led to minimized HOMO-LUMO overlap, resulting in a low energy gap (ΔE ST = 0.05-0.015 eV) and efficient TADF emission with a photoluminescence quantum yield (PLQY) ranging from 82.8% to 95.3%. Notably, the entire set of dyes (NMI-Ind-TBCBz, NMI-Ind-DMAc, NMI-Ind-PXZ, and NMI-Ind-PTZ) was utilized to fabricate TADF OLED devices, exhibiting yellow to red electroluminescence. Among them, red-emissive NMI-Ind-PTZ, containing phenothiazine as an electron-rich component, revealed predominant performance with a maximum external quantum efficiency (EQE) of 23.6%, accompanied by a persistent luminance of 38 000 cd m-2. This results in a unique roll-off ratio (EQE10 000 = 21.6%), delineating a straightforward path for their commercial use in lighting and display technologies.