Perovskite versus ZnCuInS/ZnS Luminescent Nanoparticles in Wavelength-Shifting Layers for Sensor Applications.

In this work, the application of quantum dots is evaluated in order to sensitize the commercially popular Si detectors in the UV range. The wavelength-shifting properties of two types of all-inorganic halide perovskite quantum dots as well as ZnCuInS/ZnS quantum dots are determined in order to assess their potential in the effective enhancement of the sensors' detection range. In a further part of the study, the wavelength-shifting layers are formed by embedding the quantum dots in two kinds of polymers: PMMA or Cyclic Olefin Polymer. The performance of the layers is evaluated by transmission and PLE measurement. Incorporating the nanoparticles seemingly increases the transmittance in the UV range by several percent. The observed phenomenon is proportional to the quantum dots to polymer concentration, which indicates the successful conversion action of the luminescent agents.

Effect of the substituent position on the electrochemical, optical and structural properties of donor-acceptor type acridone derivatives.

Three new donor-acceptor (D-A) compounds, positional isomers of phenoxazine-substituted acridone, namely 1-phenoxazine-N-hexylacridone (o-A), 2-phenoxazine-N-hexylacridone (m-A) and 3-phenoxazine-N-hexylacridone (p-A), were synthesized. The synthesized compounds showed interesting, isomerism-dependent electrochemistry. Their oxidation was reversible and their potential (given vs. Fc/Fc+) changed from 0.21 V for o-A to 0.36 V for p-A. In contrast, their reduction was irreversible, isomerism-independent and occurred at rather low potentials (ca. -2.25 to -2.28 V). The electrochemical results led to the following values of the ionization potentials (IPs) and electron affinities (EAs): 5.03 eV and -2.14 eV, 5.15 eV and -2.20 eV, and 5.20 eV and -2.28 eV for o-A, m-A and p-A, respectively. The experimentally obtained values were in very good agreement with those predicted by DFT calculations. All three isomers readily formed single crystals suitable for their structure determination. o-A and p-A crystallized in P1[combining macron] and P21/n space groups, respectively, with one molecule per asymmetric unit, while m-A crystallized in the P21/c space group with two molecules in the asymmetric unit accompanied by disordered solvent molecules. The UV-vis spectra of the studied compounds were isomerism and solvent independent, yielding absorption maxima in the vicinity of 400 nm. Their photoluminescence spectra, in turn, strongly depended on isomerism and the used solvent showing smaller Stokes shifts for the emission bands registered in toluene as compared to the corresponding bands measured in dichloromethane. The photoluminescence quantum yields (φ) were systematically higher for toluene solutions reaching the highest value of 20% for p-A. For all three isomers studied, stationary and time-resolved spectroscopic investigations carried out in toluene at different temperatures revealed spectral features indicating a contribution of thermally activated delayed fluorescence (TADF) to the observed spectroscopic behaviour. The measured photoluminescence quantum yields (φ) were higher for solid state films of pure compounds and for their dispersions in solid matrices (zeonex) than those recorded for toluene and dichloromethane solutions of the studied phenoxazine-N-hexylacridone isomers. The obtained experimental spectroscopic and structural data were confronted with theoretical predictions based on DFT calculations.

Effect of ß-Ketoiminato Ancillary Ligand Modification on Emissive Properties of New Iridium Complexes.

A series of new bis(benzo[h]quinolinato) Ir(III) complexes with modified ß-ketoiminato ancillary ligands were synthesized, and their electrochemical, photophysical properties were determined with the support of theoretical calculations. Moreover, all the synthesized heteroleptic Ir(III) complexes were examined as dopants of the host-guest type emissive layers in solution-processed phosphorescent organic light emitting diodes (PhOLEDs) of a simple structure. As expected on the basis of voltammetry measurements as well as DFT calculations, all the compounds appeared to be green emitters. Their examination showed that alteration of ß-ketoiminato ligand structure causes frontier orbitals' energy levels to be slightly changed, while significantly affecting photoluminescence and electroluminescence efficiencies of iridium phosphors containing these ligands. It was also found that modification of ancillary ligands might enhance charge trapping on the dopant, thus increasing its efficiency, especially in electroluminescence. From among the iridium complexes studied, the compound bearing 1-naphthyl group bonded to the nitrogen atom of the ancillary ligand proved to be the most efficient emitter. The PhOLED fabricated on the basis of this dopant has reached a luminance level of 16000 cd/m2, current efficiency close to 12 cd/A, and an external quantum efficiency around 3.2%.

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.

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.

Effect of TADF Assistance on Performance Enhancement in Solution Processed Green Phosphorescent OLEDs.

Many methods have been proposed to increase the efficiency of organic electroluminescent materials applied as an emissive layer in organic light emitting diodes (OLEDs). Herein, we demonstrate enhancement of electroluminescence efficiency and operational stability solution processed OLEDs by employing thermally activated delayed fluorescence (TADF) molecules as assistant dopants in host-guest systems. The TADF assistant dopant (SpiroAC-TRZ) is used to facilitate efficient energy transfer from host material poly(N-vinylcarbazole) (PVK) to a phosphorescent Ir(III) emitter. We present the analysis of energy transfer and charge trapping-two main processes playing a crucial role in light generation in host-guest structure OLEDs. The investigation of photo-, electro- and thermoluminescence for the double-dopant layer revealed that assistant dopant does not only harvest and transfer the electrically generated excitons to phosphorescent emitter molecules but also creates exciplexes. The triplet states of formed PVK:SpiroAC-TRZ exciplexes are involved in the transport process of charge carriers and promote long-range exciton energy transfer to the emitter, improving the efficiency of electroluminescence in a single emissive layer OLED, resulting in devices with luminance exceeding 18 000 cd/m2 with a luminous efficiency of 23 cd/A and external quantum efficiency (EQE) of 7.4%.

Unusual, solvent viscosity-controlled tautomerism and photophysics: meso-alkylated porphycenes.

Stationary and time-resolved studies of 9,10,19,20-tetramethylporphycene and 9,10,19,20-tetra-n-propylporphycene in condensed phases reveal the coexistence of trans and cis tautomeric forms. Two cis configurations, cis-1 and cis-2, play a crucial role in understanding the excited-state deactivation and tautomer conversion dynamics. The trans-trans tautomerization, involving intramolecular transfer of two hydrogen atoms, is extremely rapid (k ≥ 10(13) s(-1)), both in the ground and lowest electronically excited states. The cis-1-trans conversion rate, even though the process is thermodynamically more favorable, is much slower and solvent-dependent. This is explained by the coupling of alkyl group rotation with the hydrogen motion. Excited-state deactivation is controlled by solvent viscosity: the S(1) depopulation rate decreases by more than 2 orders of magnitude when the chromophore is transferred from a low-viscosity solution to a polymer film. Such behavior confirms a model for excited state deactivation in porphycene, which postulates that a conical intersection exists along the single hydrogen transfer path leading from the trans to a high energy cis-2 tautomeric form. For this process, the tautomerization coordinate includes not only hydrogen translocation but also large-amplitude twisting of the two protonated pyrrole moieties attached to the opposite sides of the ethylene bridge.

On the origin of fluorescence quenching of pyridylindoles by hydroxylic solvents.

Three isomeric 4'-pyridyl-substituted indoles, with the substituent in positions 2, 3 and 7, reveal strong fluorescence in aprotic solvents, both polar and nonpolar, whereas the emission is strongly quenched in water and alcohol solutions. Both viscosity and alcohol acidity play a role in efficient excited state deactivation. The process becomes faster for more acidic alcohols. It can be slowed down by increasing viscosity, which indicates that the proton movement is accompanied by large amplitude motions in the hydrogen-bonded complex. Quenching is not observed upon formation of solvates in which pyridylindoles act as hydrogen bond donors. The experimental results, combined with calculations can be explained by a model which assumes excited state protonation of the pyridine nitrogen atom, followed by twisting of the pyridyl group, leading to a low-energy structure. An alternative mechanism is also considered, in which the excited state proton transfer is accompanied by electron transfer from water or alcohol into a half-filled orbital of the chromophore, which leads to a conical intersection of the S(1) and S(0) energy surfaces.

Structure and hydrogen-bond vibrations of water complexes of azaaromatic compounds: 7-(3'-pyridyl)indole.

Complexes with water have been studied in the regime of supersonic jet isolation for 7-(3'-pyridyl)indole, a bifunctional molecule possessing hydrogen bond donor and acceptor groups. Two rotameric forms, syn and anti, are possible, of which only the former is able to form cyclic hydrogen bonds with protic solvents. Infrared-induced ion depletion spectroscopy was used to obtain vibrational patterns for 1:1 and 1:2 complexes in the hydrogen bond stretching region. The analysis of the spectra, supported by DFT calculations, revealed that for both stoichiometries the dominant forms correspond to cyclic, doubly or triply hydrogen-bonded species. The frequencies of NH...O, OH...N, and OH...O stretching vibrations were compared with the literature data to assess the strength of single vs multiple hydrogen bonds. Several new assignments and reassignments were proposed.

The Effect of Acceptor Structure on Emission Color Tuning in Organic Semiconductors with D-π-A-π-D Structures.

A series of novel donor-acceptor D-π-A-π-D compounds were synthesized and characterized in order to determine the influence of different acceptor units on their properties. The introduction of acceptor moieties had a direct impact on the HOMO and LUMO energy levels. Fluorescence spectra of compounds can be changed by the choice of an appropriate acceptor and were shifted from the green to the near-infrared part of spectra. Due to observed concentration induced emission quenching, the green exciplex type host was used to evaluate the potential of synthesized molecules as emitters in organic light emitting diodes (OLEDs).

The effect of locking π-conjugation in organoboron moieties in the structures of luminescent tetracoordinate boron complexes.

A series of 8 luminescent borafluorene complexes were extensively studied both experimentally and theoretically in order to elucidate the effect of organoboron moiety rigidification on the physicochemical properties of these compounds. Due to the spiro geometry of the boron atom, borafluorene and ligand units are perpendicularly aligned, which considerably affects the flexibility of the molecule as well as its solid-state structure. Through comparative analysis with close diphenyl analogues, we show how these structural features influence the thermal, photoluminescent and charge mobility behaviour of the studied compounds. Crystal structural analysis revealed that the molecules are connected mostly through C-HO and C-Hπ interactions formed between perpendicularly aligned borafluorene and ligand moieties from neighboured molecules, serving as a complementary donor and acceptor of electron density, respectively. This also efficiently prevents molecules from engaging in unfavoured π-stacking contact. Furthermore, structural analysis suggests that borafluorene complexes possess a considerable degree of flexibility due to OBN heterocycle distortions and mutual borafluorene-ligand plane movements. The magnitude of these effects strictly depends on the ligand structure and may lead either to enhancing or lowering the quantum yield value with respect to BPh2 analogues, while the absorption and emission wavelength are slightly affected. The measured photophysical parameters for solid-state samples showed that the studied complexes are much better emitters in their crystalline states that in amorphous films. The TD-DFT and NTO calculations revealed a significant change in frontier molecular distribution, with the HOMO localized on the borafluorene moiety. However, as the HOMO-LUMO transition is geometrically not favoured, excitation occurred from HOMO-1 localized on the ligand. Finally, aggregation effects were discussed based on supramolecular arrangements in crystal structures and charge transfer rates obtained from theoretical calculations in the framework of the Marcus-Hush approximation. They suggest that borafluorene complexes are much better electron carriers with respect to non-annulated BPh2 complexes.

Realizing 20% External Quantum Efficiency in Electroluminescence with Efficient Thermally Activated Delayed Fluorescence from an Exciplex.

The investigation of nondoped exciplex blends of 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), working as the one-electron acceptor molecule, with different electron donors is reported. The emissions of these exciplexes span from the blue to orange-red regions, showing clear contribution from thermally activated delayed fluorescence (TADF) and delayed fluorescence originated from nongeminate recombination of charge carriers created by the dissociation of optically generated exciplexes. We focus our studies on the properties of TADF in these systems, covering in particular the physical meaning of the different transient components observed in their luminescence decays. Our results unravel the intricate role of reverse intersystem crossing due to spin-orbit coupling and possibly also due to hyperfine interactions and internal conversion, which affect the efficiency of the TADF mechanism. Remarkable performances are obtained in prototype organic light-emitting diodes fabricated with some of these blends. Green exciplex blends, in particular, exhibited the current efficiency of 60 cd A-1, power efficiency of 71 lm W-1, and external quantum efficiency of 20%. We believe that our results will contribute significantly to highlight the potential advantages of intermolecular exciplexes in the area of organic light-emitting diodes.