Differently substituted benzonitriles for non-doped OLEDs

DG acknowledges to the ERDF PostDoc grant No. 1.1.1.2/VIAA/1/16/177 . This research was funded by the European Regional Development Fund according to the supported activity ‘Research Projects Implemented by World-class Researcher Groups’ under Measure No. 01.2.2-LMT-K-718 .Towards highly efficient doping-free organic light-emitting diodes, five benzonitrile-based emitters with different substitution pattern were designed exploiting symmetrical donor-acceptor-donor and asymmetrical donor-acceptor-donor* structures. As it was predicted by theoretical calculations, different thermally activated delayed fluorescence of different energy with relatively high absolute quantum yields (11–42%) was detected for non-doped films of the studied compounds. The smallest singlet-triplet energy splitting of 0.05 eV thus the most efficient TADF was estimated for the film of compound with the asymmetrical donor-acceptor-donor* structure containing carbazole and acridan donor moieties. Thermogravimetry revealed sublimation of the materials with the onset temperatures in the range of 350–383 °C. Glass transition temperatures of the molecular materials were in the range of 82–94 °C. Tuning of hole injection properties of these compounds in solid-state was demonstrated. Their ionization potential was in range from 5.8 to 6.0 eV. Strong effect of different substitutions was observed on hole mobilities of the layers of compounds. They were found to be in the wide range from 3 × 10−7 cm2V−1s−1 to 1 × 10−4 cm2V−1s−1. Electron mobility values of the compounds were found to be comparable and ranged from 1.5 × 10−4 cm2V−1s−1 to 3 × 10−4 cm2V−1s−1 at electric field of 6.9 × 105 Vcm−1. Reflecting effect of substitution pattern of benzonitrile on electroluminescent properties of OLEDs, maximum external quantum efficiencies in the range from 1.6 to 5% as well as maximum brightness in the wide range from 1200 to 22600 cd/m2 were observed for the devices based on the doping-free light-emitting layerEuropean Regional Development Fund 1.1.1.2/VIAA/1/16/177; European Regional Development Fund 01.2.2-LMT-K-718; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²https://www.sciencedirect.com/science/article/pii/S014372081931238

Differently substituted benzonitriles for non-doped OLEDs

DG acknowledges to the ERDF PostDoc grant No. 1.1.1.2/VIAA/1/16/177 . This research was funded by the European Regional Development Fund according to the supported activity ‘Research Projects Implemented by World-class Researcher Groups’ under Measure No. 01.2.2-LMT-K-718 .Towards highly efficient doping-free organic light-emitting diodes, five benzonitrile-based emitters with different substitution pattern were designed exploiting symmetrical donor-acceptor-donor and asymmetrical donor-acceptor-donor* structures. As it was predicted by theoretical calculations, different thermally activated delayed fluorescence of different energy with relatively high absolute quantum yields (11–42%) was detected for non-doped films of the studied compounds. The smallest singlet-triplet energy splitting of 0.05 eV thus the most efficient TADF was estimated for the film of compound with the asymmetrical donor-acceptor-donor* structure containing carbazole and acridan donor moieties. Thermogravimetry revealed sublimation of the materials with the onset temperatures in the range of 350–383 °C. Glass transition temperatures of the molecular materials were in the range of 82–94 °C. Tuning of hole injection properties of these compounds in solid-state was demonstrated. Their ionization potential was in range from 5.8 to 6.0 eV. Strong effect of different substitutions was observed on hole mobilities of the layers of compounds. They were found to be in the wide range from 3 × 10−7 cm2V−1s−1 to 1 × 10−4 cm2V−1s−1. Electron mobility values of the compounds were found to be comparable and ranged from 1.5 × 10−4 cm2V−1s−1 to 3 × 10−4 cm2V−1s−1 at electric field of 6.9 × 105 Vcm−1. Reflecting effect of substitution pattern of benzonitrile on electroluminescent properties of OLEDs, maximum external quantum efficiencies in the range from 1.6 to 5% as well as maximum brightness in the wide range from 1200 to 22600 cd/m2 were observed for the devices based on the doping-free light-emitting layerEuropean Regional Development Fund 1.1.1.2/VIAA/1/16/177; European Regional Development Fund 01.2.2-LMT-K-718; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²https://www.sciencedirect.com/science/article/pii/S014372081931238

Methoxycarbazolyl-disubstituted dibenzofuranes as holes- and electrons-transporting hosts for phosphorescent and TADF-based OLEDs

This research was funded by European Social Fund (Project No 09.3.3-LMT-K-712-02-0105). DG acknowledges to the ERDF PostDoc grant No. 1.1.1.2/VIAA/1/16/177.In the search of universal host materials for organic light emitting diodes a new series of bipolar host materials containing methoxy-substituted carbazoles as the electron-donating and dibenzofuran as an electron-accepting units were designed and synthesized. Different linking topologies and number of methoxy groups attached to carbazolyl moiety were used to understand the impact of the strength of the donor moiety on the thermal, optical, photophysical, electrochemical and electroluminescent properties. The synthesized compounds exhibited relatively high thermal stability with 5% weight loss temperatures exceeding 378 °C and formed molecular glasses with high glass-transition temperatures ranging from 120 to 148 °C. High triplet energy values of 2.86–2.96 eV were estimated for dilute THF solutions at 77K. Hole and electron drift mobilities estimated using time-of-flight technique in solid layers approached 10−4 cm2V−1s−1 at high electric fields exceeding 3.6 × 105 V cm−1. The synthesized methoxy-carbazole based compounds were tested as hosts in electrophosphorescent and TADF organic light-emitting diodes reaching luminance of 53000 cd m−2 and external quantum efficiency of 12.5%, in the best case.European Social Fund 09.3.3-LMT-K-712-02-0105; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²https://www.sciencedirect.com/science/article/pii/S014372081931162

Nanoparticle-doped electrospun fiber random lasers with spatially extended light modes

Complex assemblies of light-emitting polymer nanofibers with molecular materials exhibiting optical gain can lead to important advance to amorphous photonics and to random laser science and devices. In disordered mats of nanofibers, multiple scattering and waveguiding might interplay to determine localization or spreading of optical modes as well as correlation effects. Here we study electrospun fibers embedding a lasing fluorene-carbazole-fluorene molecule and doped with titania nanoparticles, which exhibit random lasing with sub-nm spectral width and threshold of about 9 mJ cm^-2 for the absorbed excitation fluence. We focus on the spatial and spectral behavior of optical modes in the disordered and non-woven networks, finding evidence for the presence of modes with very large spatial extent, up to the 100 micrometer-scale. These findings suggest emission coupling into integrated nanofiber transmission channels as effective mechanism for enhancing spectral selectivity in random lasers and correlations of light modes in the complex and disordered material.Comment: 22 pages, 6 figure

Effect of donor substituents on thermally activated delayed fluorescence of diphenylsulfone derivatives

This research was funded by the European Social Fund according to the activity ‘Improvement of researchers’ qualification by implementing world-class R&D projects’ of Measure No. 09.3.3-LMT-K-712. DG acknowledges to the ERDF PostDoc project No. 1.1.1.2/VIAA/1/16/177;Diphenylsulfones substituted by acridan, carbazole, phenothiazine and phenoxazine moieties were synthesized and characterized by thermal analysis, UV-, steady-state and time-resolved luminescent spectrometries, cyclic voltametry. Quantum chemical calculations on the molecular level were performed to interpret photophysical properties of the derivatives. Structural parameters, electronic properties, HOMO-LUMO gaps, molecular orbital densities, ionization potentials, reorganization energies were determined. The lowest excitation energies and the wavelengths of absorption maxima were also estimated using the time-dependent density functional theory. All the compounds were found to be capabale to form glasses with glass transition temperatures ranging from 82° to 91°C. They exhibited high thermal stabilities, with 5% weight loss temperatures exceeding 385 °C. Strong solvatochromism arising from the intramolecular charge transfer in the excited state was evidenced by bathochromic shifts of emission maxima with increasing solvent polarity. The compounds containing acridan and phenoxazine moieties showed relatively high photoluminescence quantum yield (up to 35%) in the non-doped solid state, long delayed fluorescence lifetime (in µs range) and small singlet-triplet energy splitting (ΔEST) that is attributed to thermally activated delayed fluorescence. These compounds were tested as emissive species for the fabrication of OLEDs. The sky-blue and green devices showed maximum brightness of 3200 and 12300 cd/m2 and maximum external quantum efficiency of 6.3% and 6.9%, respectively.European Social Fund 09.3.3-LMT-K-712; European Regional Development Fund 1.1.1.2/VIAA/1/16/177; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Diverse regimes of mode intensity correlation in nanofiber random lasers through nanoparticle doping

Random lasers are based on disordered materials with optical gain. These devices can exhibit either intensity or resonant feedback, relying on diffusive or interference behaviour of light, respectively, which leads to either coupling or independent operation of lasing modes. We study for the first time these regimes in complex, solid-state nanostructured materials. The number of lasing modes and their intensity correlation features are found to be tailorable in random lasers made of light-emitting, electrospun polymer fibers upon nanoparticle doping. By material engineering, directional waveguiding along the length of fibers is found to be relevant to enhance mode correlation in both intensity feedback and resonant feedback random lasing. The here reported findings can be used to establish new design rules for tuning the emission of nano-lasers and correlation properties by means of the compositional and morphological properties of complex nanostructured materials.Comment: 30 pages, 10 figure

Multifunctional derivatives of dimethoxy-substituted triphenylamine containing different acceptor moieties

This project has received funding from the Research Council of Lithuania (LMTLT), Agreement No. [S-LZ-19-2]. This research was funded by the Région Centre, the Tunisian ministry of research, University of Monastir and the French ministry of Higher Education and Research. J. Bouclé would like to thank the Sigma-Lim LabEx environment for financial supports, and the PLATINOM facility at XLIM laboratory regarding device fabrication and characterizations. DG acknowledges the Lithuanian Academy of Sciences for the financial support.Two compounds, having dimethoxy-substituted triphenylamino groups and fragments of cyanoacrylic acid or rhodanine-3-acetic acid were prepared and characterized. Their optical, photophysical, thermal, electrochemical, photoelectrical and nonlinear optical properties were investigated. Both derivatives showed ability of glass-formation with glass transition temperatures of 83 and 61 °C. They showed comparable ionization potential values of ca. 5.20 eV. The compounds showed Kerr and two photon absorption effects. The dye containing a rhodanine-3-acetic acid fragment, namely 2-((E)-5-(4-(diphenylamino)benzylidene)-tetrahydro-4-oxo-2-thioxothiophen-3-yl)acetic acid, exhibited a promising power conversion efficiency of 2.09% in dye-sensitized solar cells using the spiro-OMeTAD as hole transporting compound.Research Council of Lithuania Agreement No. [S-LZ-19-2]; Région Centre; Tunisian ministry of research; University of Monastir; French ministry of Higher Education and Research; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

An ambipolar BODIPY derivative for a white exciplex OLED and cholesteric liquid crystal laser toward multifunctional devices

A new interface engineering method is demonstrated for the preparation of an efficient white organic light-emitting diode (WOLED) by embedding an ultrathin layer of the novel ambipolar red emissive compound 4,4-difluoro-2,6-di(4-hexylthiopen-2-yl)-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indacene (bThBODIPY) in the exciplex formation region. The compound shows a hole and electron mobility of 3.3 × 10–4 and 2 × 10–4 cm2 V–1 s–1, respectively, at electric fields higher than 5.3 × 105 V cm–1. The resulting WOLED exhibited a maximum luminance of 6579 cd m–2 with CIE 1931 color coordinates (0.39; 0.35). The bThBODIPY dye is also demonstrated to be an effective laser dye for a cholesteric liquid crystal (ChLC) laser. New construction of the ChLC laser, by which a flat capillary with an optically isotropic dye solution is sandwiched between two dye-free ChLC cells, provides photonic lasing at a wavelength well matched with that of a dye-doped planar ChLC cell

Spin- and Voltage-dependent emission from Intra- and Intermolecular TADF OLEDs

Organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) utilize molecular systems with a small energy splitting between singlet and triplet states. This can either be realized in intramolecular charge transfer states of molecules with near-orthogonal donor and acceptor moieties or in intermolecular exciplex states formed between a suitable combination of individual donor and acceptor materials. Here, we investigate 4,4'-(9H,9'H-[3,3'-bicarbazole]-9,9'-diyl)bis(3-(trifluoromethyl) benzonitrile) (pCNBCzoCF3), which shows intramolecular TADF but can also form exciplex states in combination with 4,4',4''-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA). Orange emitting exciplex-based OLEDs additionally generate a sky-blue emission from the intramolecular emitter with an intensity that can be voltage-controlled. We apply electroluminescence detected magnetic resonance (ELDMR) to study the thermally activated spin-dependent triplet to singlet up-conversion in operating devices. Thereby, we can investigate intermediate excited states involved in OLED operation and derive the corresponding activation energy for both, intra- and intermolecular based TADF. Furthermore, we give a lower estimate for the extent of the triplet wavefunction to be >1.2 nm. Photoluminescence detected magnetic resonance (PLDMR) reveals the population of molecular triplets in optically excited thin films. Overall, our findings allow us to draw a comprehensive picture of the spin-dependent emission from intra- and intermolecular TADF OLEDs.Comment: 9 pages, 5 figure