Pyrene–Oxadiazoles for Organic Light-Emitting Diodes: Triplet to Singlet Energy Transfer and Role of Hole-Injection/Hole-Blocking Materials

Bromopyrene Symphony: Synthesis and Characterisation of Isomeric Derivatives at Non-K Region and Nodal Positions for Diverse Functionalisation Strategies

Pyrene, a renowned aromatic hydrocarbon, continues to captivate researchers due to its versatile properties and potential applications across various scientific domains. Among its derivatives, bromopyrenes stand out for their significance in synthetic chemistry, materials science, and environmental studies. The strategic functionalisation of pyrene at non-K region and nodal positions is crucial for expanding its utility, allowing for diverse functionalisation strategies. Bromo-substituted precursors serve as vital intermediates in synthetic routes; however, the substitution pattern of bromoderivatives significantly impacts their subsequent functionalisation and properties, posing challenges in synthesis and purification. Understanding the distinct electronic structure of pyrene is pivotal, dictating the preferential electrophilic aromatic substitution reactions at specific positions. Despite the wealth of literature, contradictions and complexities persist in synthesising suitably substituted bromopyrenes due to the unpredictable nature of substitution reactions. Building upon historical precedents, this study provides a comprehensive overview of bromine introduction in pyrene derivatives, offering optimised synthesis conditions based on laboratory research. Specifically, the synthesis of mono-, di-, tri-, and tetrabromopyrene isomers at non-K positions (1-, 3-, 6-, 8-) and nodal positions (2-, 7-) is systematically explored. By elucidating efficient synthetic methodologies and reaction conditions, this research contributes to advancing the synthesis and functionalisation strategies of pyrene derivatives, unlocking new possibilities for their utilisation in various fields.

Phosphorescence resonance energy transfer from purely organic supramolecular assembly

Phosphorescence energy transfer systems have been applied in encryption, biomedical imaging and chemical sensing. These systems exhibit ultra-large Stokes shifts, high quantum yields and are colour-tuneable with long-wavelength afterglow fluorescence (particularly in the near-infrared) under ambient conditions. This review discusses triplet-to-singlet PRET or triplet-to-singlet-to-singlet cascaded PRET systems based on macrocyclic or assembly-confined purely organic phosphorescence introducing the critical toles of supramolecular noncovalent interactions in the process. These interactions promote intersystem crossing, restricting the motion of phosphors, minimizing non-radiative decay and organizing donor-acceptor pairs in close proximity. We discuss the applications of these systems and focus on the challenges ahead in facilitating their further development.

Photoredox/Cobalt-Catalyzed Cascade Oxidative Synthesis of 2,5-Disubstituted 1,3,4-Oxadiazoles under Oxidant-Free Conditions

An efficient oxidant-free, photoredox-mediated cascade cyclization strategy for the synthesis of 1,3,4-oxadiazoles by using an organo acridinium photocatalyst and a cobaloxime catalyst has been developed. Various acylhydrazones have been transformed into the corresponding 1,3,4-oxadiazole products in up to 96% yield, and H2 is the only byproduct. Mechanistic experiments and density functional theory (DFT) calculation studies indicate carbon-centered radicals rather than oxygen-centered radicals as π-radicals produced by the oxidation of photoexcited Mes-Acr+* along with deprotonation, which is responsible for this transformation. The practical utility of this method is highlighted by the one-pot gram-scale synthesis starting directly from commercially available aldehydes and acylhydrazides.

Synthesis, photophysical and nonlinear optical properties of push-pull tetrazoles

A 2,5-disubstituted tetrazole with p-nitrophenyl and 3-pyridyl units as acceptors (1a), and three push–pull tetrazoles with p-nitrophenyl as an acceptor and phenyl (1b), 2-(dibenzo[b,d]furan-4-yl) (1c), and 4-(N,N-diphenylamino)phenyl (1d) as donor groups, were synthesized by copper-catalyzed aerobic C–N coupling of p-nitrophenyl tetrazole with appropriately substituted aryl boronic acids. The absorption and emission spectra of 1a–c showed minimal dependence on the polarity of the solvent; however, in the case of 1d a blue shift was noted in the longest absorption band (λ₁) as the polarity increased. The fluorescence intensity of the title compounds was found to be solvent-dependent; however, no apparent correlation to solvent polarity could be established. The absorption and emission characteristics of 1a–d were also influenced by the nature of the substituent as 1d, bearing a strong electron donating 4-(N,N-diphenylamino)phenyl group, displayed a significant red shifted absorption (λ₁) as well as emission (λ_em) bands compared to other compounds. Time dependent density functional calculations (CAM-B3LYP/6-311++G**) revealed that the longest wavelength band (λ₁) is associated with an intramolecular charge transfer (ICT) from HOMO/HOMO-1/HOMO-2 → LUMO/LUMO+1 in these molecules. The first hyperpolarizability values, β_HRS, of 1a–d were measured using the solution-based hyper-Rayleigh scattering technique using a femtosecond Ti:Sapphire laser and the highest NLO activity was measured for 1d with the greatest push–pull characteristics. A strong correlation was observed between the calculated hyperpolarizability (β_tot) and experimentally measured values (β_HRS).

A 2,5-disubstituted tetrazole with a p-nitrophenyl unit as an acceptor and a 4-(N,N-diphenylamino) phenyl group as a donor exhibits strong push–pull characteristics and displays high NLO activity.

Synthesis, Characterization, Antimicrobial and Molecular Docking Study of Benzooxadiazole Derivatives

In this study, a series of new1,2,5-oxadiazole compounds derived from 4-chloro-7-nitro-benzo 1,2,5-oxadiazole was synthesized using different organic procedures. The resulting derivatives were chemically characterized and their structures were confirmed by FT-IR and NMR analysis. All the compounds were also evaluated for their antibacterial and antifungal activity against four types of pathogenic bacteria: S.aureus, S.epidermidis (as gram-negative bacteria), E.coli, Klebsiella spp. (as gram-positive bacteria) and the fungus Candida albicans using the agar well diffusion method. The synthesized oxadiazole derivatives exhibited significant antibacterial and moderate antifungal activities. Exploring the binding between the potent synthesized derivative 8 within the active site of glucosamine-6-phosphate synthase, the target enzyme for the antimicrobial agents was achieved using Autodock 4.2 package. The interaction modes of the generated conformers inside the binding pocket were found to enhance the in vitro results, and strongly recommended the new derivatives as promising antimicrobial agents.

New Low-Dimensional Hybrid Perovskitoids Based on Lead Bromide with Organic Cations from Charge-Transfer Complexes

We have obtained a series of low-dimensional hybrid perovskitoids (often referred to as perovskites) based on lead bromide. As organic cations, the derivatives of polyaromatic and conjugated molecules, such as anthracene, pyrene and (E)-stilbene, were chosen to form charge-transfer complexes with various organic acceptors for use as highly tunable components of hybrid perovskite solar cells. X-ray diffraction analysis showed these crystalline materials to be new 1D- and pseudo-layered 0D-perovskitoids with lead bromide octahedra featuring different sharing modes, such as in unusual mini-rods of four face- and edge-shared octahedra. Thanks to the low dimensionality, they can be of use in another type of optoelectronic device, photodetectors.

Cobalt(II) nitrate promoted cyclization of benzoyl hydrazone for the synthesis of 2,5-diphenyl-1,3,4-oxadiazole derivatives

A Co(NO3)2 method to promote cyclization of benzoyl hydrazone for the formation of 2,5-diphenyl-1,3,4-oxadiazoles has been developed. The reaction proceeded smoothly and was promoted by Co(NO3)2 under air at 110 °C in DCE; 16 examples of products were obtained.

Functional Pyrene-Pyridine-Integrated Hole-Transporting Materials for Solution-Processed OLEDs with Reduced Efficiency Roll-Off

A series of new functional pyridine-appended pyrene derivatives, viz., 2,6-diphenyl-4-(pyren-1-yl)pyridine (Py-03), 2,6-bis(4-methoxyphenyl)-4-(pyren-1-yl)pyridine (Py-MeO), 4-(pyren-1-yl)-2,6-di-p-tolylpyridine (Py-Me), and 2,6-bis(4-bromophenyl)-4-(pyren-1-yl)pyridine (Py-Br) were designed, developed, and studied as the hole-transporting materials (HTMs) for organic light-emitting diode (OLED) application. The crystal structures of two molecules revealed to have a large dihedral angle between the pyrene and pyridine units, indicating poor π-electronic communication between them due to ineffective orbital overlap across the pyrene-pyridine systems as the two p-orbitals of pivotal atoms are twisted at 66.80° and 68.75° angles to each other in Py-03 and Py-Me, respectively. The influence of variedly functionalized pyridine units on the electro-optical properties and device performance of the present integrated system for OLED application was investigated. All of the materials have suitable HOMO values (5.6 eV) for hole injection by closely matching the HOMOs of indium tin oxide (ITO) and the light-emitting layer. All of the synthesized molecules have suitable triplet energies, glass transition temperatures, and melting temperatures, which are highly desirable for good HTMs. The pyrene-pyridine-based devices demonstrated stable performance with low-efficiency roll-off. The device with Py-Br as HTM showed a maximum luminance of 17300 cd/m² with a maximum current efficiency of 22.4 cd/A and an EQE of 9% at 3500 cd/m² with 7% roll-off from 1000 to 10 000 cd/m². Also, the devices with Py-Me and Py-03 showed performance roll-up while moving from 1000 to 10 000 cd/m².

Synthesis, spectroscopic characterization and DNA/HSA binding studies of (phenyl/naphthyl)ethenyl-substituted 1,3,4-oxadiazolyl-1,2,4-oxadiazoles

Novel 1,3,4-oxadiazolyl-1,2,4-oxadiazole derivatives with promising photophysical and DNA/HSA-binding properties are reported.

From a 1,2-azaborinine to large BN-PAHs via electrophilic cyclization: synthesis, characterization and promising optical properties

A synthetic approach towards boron-nitrogen substituted polycyclic aromatic hydrocarbons (BN-PAHs) via electrophilic cyclization is described and it is shown that the variation of the rings' connectivity may tune the emission wavelengths effectively.

6,12-Dihydro-6,12-diboradibenzo[def,mno]chrysene: A Doubly Boron-Doped Polycyclic Aromatic Hydrocarbon for Organic Light Emitting Diodes by a One-Pot Synthesis

One-pot synthesis of a new doubly boron-doped polycyclic aromatic hydrocarbon of 6,12-dimesityl-6,12-dihydro-6,12-diboradibenzo[def,mno]chrysene (MDBDBC) was reported. MDBDBC features a rigid planar electron-deficient core structure and demonstrates good chemical and thermal stabilities. A low-lying LUMO of -3.53 eV, a low locally excited triplet energy of 1.92 eV, as well as green electroluminescence with maximum EQE of 4.9% were found for MDBDBC, suggesting its potential as an n-type unit for future organic light emitting diode applications.

Acceptor Derivatization of the 4CzIPN TADF System: Color Tuning and Introduction of Functional Groups

We demonstrate modular modifications of the widely employed emitter 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) by replacing one or both nitrile acceptors with oxadiazole groups via a tetrazole intermediate. This allows the introduction of various functional groups including halides, alkynes, alkenes, nitriles, esters, ethers and a protected amino acid while preserving the thermally activated delayed fluorescence (TADF) properties. The substituents control the emission maximum of the corresponding emitters, ranging between 472-527 nm, and show high solid-state photoluminescence quantum yields up to 85 %. The TADF emission of two compounds, 4CzCNOXDtBu and 4CzdOXDtBu, a mono- and a bis-oxadiazole substituted 4CzIPN is characterized in detail by time- and temperature-dependent photoluminescence. Solution-processed OLEDs comprising 4CzCNOXDtBu and 4CzdOXDtBu show a significant blue-shift of the emission compared to the reference 4CzIPN, with external quantum efficiencies of 16 %, 5.9 % and 17 % at 100 cd m-2, respectively.

Deep-Blue Oxadiazole-Containing Thermally Activated Delayed Fluorescence Emitters for Organic Light-Emitting Diodes

A series of four novel deep-blue to sky-blue thermally activated delayed fluorescence (TADF) emitters (2CzdOXDMe, 2CzdOXD4MeOPh, 2CzdOXDPh, and 2CzdOXD4CF₃Ph) have been synthesized and characterized. These oxadiazole-based emitters demonstrated bluer emission compared with the reference emitter 2CzPN thanks to the weaker acceptor strength of the oxadiazole moieties. The oxadiazole compounds doped in hosts (mCP and PPT) emitted from 435 to 474 nm with photoluminescence quantum yields ranging from 14-55%. The emitters possess singlet-triplet excited-state energy gaps (Δ E_ST) between 0.25 and 0.46 eV resulting in delayed components ranging from 4.8 to 25.8 ms. The OLED device with 2CzdOXD4CF₃Ph shows a maximum external quantum efficiency of 11.2% with a sky-blue emission at CIE of (0.17, 0.25), while the device with 2CzdOXD4MeOPh shows a maximum external quantum efficiency of 6.6% with a deep-blue emission at CIE of (0.15, 0.11).

1-[5-(4-Tolyl)-1,3,4-oxadiazol-2-yl]methanamine

1-[5-(4-Tolyl)-1,3,4-oxadiazol-2-yl]methanamine (3) has been successfully synthesized by reacting p-toluic hydrazide (1) and glycine (2) via the polyphosphoric acid condensation route. The course of the reaction was found to be high yielding (87%) and the title compound was spectroscopically characterized by UV-Vis, FTIR, DSC, 13C/1H-NMR, and sass spectrometric techniques.

Multifunctional Dithiadiazolyl Radicals: Fluorescence, Electroluminescence, and Photoconducting Behavior in Pyren-1'-yl-dithiadiazolyl

The pyren-1'-yl-functionalized dithiadiazolyl (DTDA) radical, C₁₆H₉CNSSN (1), is monomeric in solution and exhibits fluorescence in the deep-blue region of the visible spectrum (440 nm) upon excitation at 241 nm. The salt [1][GaCl₄] exhibits similar emission, reflecting the largely spectator nature of the radical in the fluorescence process, although the presence of the radical leads to a modest quenching of emission (Φ_F = 98% for 1⁺ and 50% for 1) through enhancement of non-radiative decay processes. Time-dependent density functional theory studies on 1 coupled with the similar emission profiles of both 1⁺ and 1 are consistent with the initial excitation being of predominantly pyrene π-π* character. Spectroscopic studies indicate stabilization of the excited state in polar media, with the fluorescence lifetime for 1 (τ = 5 ns) indicative of a short-lived excited state. Comparative studies between the energies of the frontier orbitals of pyren-1'-yl nitronyl nitroxide (2, which is not fluorescent) and 1 reveal that the energy mismatch and poor spatial overlap between the DTDA radical SOMO and the pyrene π manifold in 1 efficiently inhibit the non-radiative electron-electron exchange relaxation pathway previously described for 2. Solid-state films of both 1 and [1][GaCl₄] exhibit broad emission bands at 509 and 545 nm, respectively. Incorporation of 1 within a host matrix for OLED fabrication revealed electroluminescence, with CIE coordinates of (0.205, 0.280) corresponding to a sky-blue emission. The brightness of the device reached 1934 cd/m² at an applied voltage of 16 V. The crystal structure of 1 reveals a distorted π-stacked motif with almost regular distances between the pyrene rings but alternating long-short contacts between DTDA radicals. Solid state measurements on a thin film of 1 reveal emission occurs at shorter wavelengths (375 nm) whereas conductivity measurements on a single crystal of 1 show a photoconducting response at longer wavelength excitation (455 nm).

Direct observation of the rise of delayed fluorescence in dithienylbenzothiadiazole and its role in the excited state dynamics of a donor-acceptor-donor molecule

Dithienylbenzothiadiazole (DTBT) is used as a building block in several molecules having application in organic light emitting devices (OLED) and organic photovoltaic (OPV) devices. Delayed fluorescence (DF) is the preferred design principle employed currently in OLED research. DTBT has excellent delayed fluorescence properties which makes this moiety a potentially viable OLED material. Here, the dynamics of intersystem crossing (ISC) and reverse intersystem crossing (RISC) have been explored using fluorescence, phosphorescence, fluorescence lifetime and transient absorption measurements. Experimentally it is demonstrated that singlet and triplet states of DTBT are close lying or degenerate and after a certain time the molecules can stay in the singlet or the triplet state forming an equilibrium between the two states which hinders the identification of these two states that could be characterized by routine steady state fluorescence and phosphorescence studies. Similarly in OPV material research, DTBT is coupled with strong donors to form push-pull molecules to produce a prolonged charge separated state. In this study DTBT appended with carbazole at both ends (CDTBT) was used to study the dynamics of DTBT within a donor-acceptor-donor system. The study reveals similar kinds of ISC and RISC in CDTBT along with a competitive deactivation pathway of the singlet state and it was concluded to be through the formation of a charge separated species in CDTBT.

Synthesis and Spectroscopic Properties of Carbazole-Oxadiazoles

Four new carbazole-oxadiazole derivatives (3a-b, 6a-b) were prepared from the reaction of aromatic aldehydes and carbohydrazides which were synthesized from carbazole aldehydes namely 9-hexyl-9H-carbazole-3-carbaldehyde 1 and 4-(9H-carbazole-9-yl)benzaldehyde 4 and acid hydrazides. The structures of the new derivatives were confirmed by ¹H-NMR and FT-IR. The optical properties such as maximum absorption and emission wavelengths (λ; nm), molar extinction coefficients (ε; cm^-1 M^-1), Stoke's shifts (ΔλST; nm) and quantum yields (ϕF), of the carbazole-oxadiazole derivatives were declared in dichloromethane, toluene and tetrahydrofuran solutions.

Synthesis of novel profluorescent nitroxides as dual luminescent-paramagnetic active probes

Profluorescent nitroxides containing 2,5-diaryl-1,3,4-oxadiazoles as fluorophore units were synthesized and characterized as actively responsive probes to reducing agents.