step-step-guide-for-harnessing-organic-light-emitting-diodes-solution

Step-by-Step Guide for Harnessing Organic Light Emitting Diodes by Solution Processed Device Fabrication of a TADF Emitter

The demand for energy-efficient light sources has witnessed a significant surge in the recent era, due to the increasing awareness towards environmental ...

Indian Institute of Science

We report the characterization of rotaxanes based on a carbazole-benzophenone thermally activated delayed fluorescence luminophore. We find that the mechanical bond leads to an improvement in key photophysical properties of the emitter, notably an increase in photoluminescence quantum yield and a decrease in the energy difference between singlet and triplet states, as well as fine tuning of the emission wavelength, a feat that is difficult to achieve when using covalently bound substituents. Computational simulations, supported by X-ray crystallography, suggest that this tuning of properties occurs due to weak interactions between the axle and the macrocycle that are enforced by the mechanical bond. This work highlights the benefits of using the mechanical bond to refine existing luminophores, providing a new avenue for emitter optimization that can ultimately increase the performance of these molecules.

Using the Mechanical Bond to Tune the Performance of a Thermally Activated Delayed Fluorescence Emitter*.

We report highly efficient, ultrathin non-doped green and bluish-green organic light-emitting diodes (OLEDs) using a thermally activated delayed fluorescence (TADF) emitter. The green OLED with an ultrathin (∼1 nm) EML showed a 2.6-fold higher external quantum efficiency (EQE) of 13.5% with a luminance of 17 250 cd m than the conventional (30 nm) non-doped device.

Ultrathin non-doped thermally activated delayed fluorescence emitting layer for highly efficient OLEDs.

Metal-free organic emitters with thermally activated delayed fluorescence (TADF) characteristics are emerging due to the potential applications in optoelectronic devices, time-resolved luminescence imaging, and solid-phase sensing. Herein, we synthesized two (4-bromobenzoyl)pyridine (BPy)-based donor-acceptor (D-A) compounds with varying donor size and strength: the emitter BPy-TC with -butylcarbazole (TC) as the donor and BPy-3C with bulky tricarbazole (3C) as the donor unit. Both BPy-TC and BPy-3C exhibited prominent emission with TADF properties in solution and in the solid phase. The stronger excited-state charge transfer was obtained for BPy-3C due to the bulkier donor, leading to a more twisted D-A geometry than that of BPy-TC. Hence, BPy-3C exhibited aggregation-induced enhanced emission (AIEE) in a THF/water mixture. Interestingly, the singlet-triplet energy gap (Δ ) was reduced for both compounds in the aggregated state as compared to toluene solution. Consequently, a faster reverse intersystem crossing rate ( ) was obtained in the aggregated state, facilitating photon upconversion, leading to enhanced delayed fluorescence. Further, the lone-pair electrons of the pyridinyl nitrogen atom were found to be sensitive to acidic protons. Hence, the exposure to acid and base vapors using trifluoroacetic acid (TFA) and triethylamine (TEA) led to solid-phase fluorescence switching with fatigue resistance. The current study demonstrates the role of the donor strength and size in tuning Δ  in the aggregated state as well as the relevance for fluorescence-based acid-base sensing.

Thermally activated delayed fluorescence (TADF) emitters: sensing and boosting spin-flipping by aggregation.

The design of multifunctional materials is a challenging and important objective for a wide array of multidisciplinary applications. However, a multifunctional organic emitter exhibiting simultaneous aggregation-induced emission (AIE), multi-responsive polymorphs, mechanoluminescence and electroluminescence have been scarce. In this study, two anthracene based compounds, namely 10-(4-(9H-carbazol-9-yl)phenyl)anthracene-9-carbonitrile (CzPACN) and 10-(4-(di-p-tolylamino)phenyl)anthracene-9-carbonitrile (DTPACN) was designed and synthesized with rigid and flexible donors, respectively. The CzPACN shows the bright blue emission and DTPACN shows the bright green emission in solution. We have demonstrated an effective strategy to achieve three polymorphic phases such as DTPACN-α, DTPACN-β and DTPACN-γ from DTPACN by controlling the temperature. Under mechanical stimuli, highly restricted and non-planar crystals of the structurally tuned polymorphs DTPACN-α, and DTPACN-β exhibited red shifted emission and DTPACN-γ showed blue shifted emission. Conversely, CzPACN is not showing polymorphism and is not sensitive to external stimuli. In addition, blue and green OLEDs were fabricated using CzPACN and DTPACN, respectively, as an emitter and achieved a maximum external quantum efficiency (EQE ) of 5.5% and 5.7%, respectively, for blue and green OLEDs. Further, this study suggests designing multi-responsive smart materials via a simple modification by introducing a non-planar unit with a large twist.

Design Strategy for Enabling Multifunctional Properties of Anthracene-based Emitters.

Photofunctional co-crystal engineering strategies based on donor-acceptor π-conjugated system facilitates expedient molecular packing, consistent morphology, and switchable optical properties, conferring synergic 'structure-property relationship' for optoelectronic and biological functions. In this work, a series of organic co-crystals were formulated using a twisted aromatic hydrocarbon (TAH) donor and three diverse planar acceptors, resulting in color-tunable solid and aggregated state emission via variable packing and through-space charge-transfer interactions. While, adjusting the strength of acceptors, a structural transformation into hybrid stacking modes ultimately results in color-specific polymorphs, a configurational cis-isomer with very high photoluminescence quantum yield. The cis-isomeric co-crystal exhibits triplet-harvesting thermally activated delayed fluorescence (TADF) characteristics, presenting a key discovery in hydrocarbon-based multicomponent systems. Further, 1D-microrod-shaped co-crystal acts as an efficient photon-transducing optical waveguides, and their excellent dispersibility in water endows efficient cellular internalization with bright cell imaging performances. These salient approaches may open more avenues for the design and applications of TAH based co-crystals.

Highly efficient color-tunable organic co-crystals unveiling polymorphism, isomerism, delayed fluorescence for optical waveguides and cell-imaging.

The gelation of two TADF positional isomers, 4BPy-DTC and BPy-DTC, was explored. Among these emitters, only 4BPy-DTC forms stable organogels due to the optimized intermolecular interactions. Interestingly, the morphology of the gel can be tuned from fibre to fettuccine by changing the solvents. The electrical conductivity of the self-assembly can also be increased from 8.05 × 10 S m to 6.05 × 10 S m by tuning the morphology.

Single-component TADF gels: study of the positional isomer effect on gelation and morphological effect on conductivity.

In this manuscript we present a strategy to achieve ultranarrowband circularly polarized luminescence (CPL) from multiple 1,4-azaborine-embedded helical nanographenes. The impact of number and position of boron and nitrogen atoms in the rigid core of the molecule on optical properties─including absorption and emission maxima, photoluminescence quantum yield, Stokes shift, excited singlet-triplet energy gap and full width at half-maximum (fwhm) for CPL and fluorescence─was investigated. The molecules reported here exhibits ultranarrowband fluorescence (fwhm 16-17.5 nm in toluene) and CPL (fwhm 18-19 nm in toluene). To the best of our knowledge, this is among the narrowest CPL for any organic molecule reported to date. Quantum chemical calculations, including computed CPL spectra involving vibronic contributions, provide valuable insights for future molecular design aimed at achieving narrowband CPL.

Ultra-Narrowband Circularly Polarized Luminescence from Multiple 1,4-Azaborine-Embedded Helical Nanographenes.

Hyperfluorescence, also known as thermally activated delayed fluorescence (TADF) sensitized fluorescence, is known as a next-generation efficient and innovative process for high-performance organic light-emitting diodes (OLEDs). High external quantum efficiency (EQE) and good color purity are crucial parameters for display applications. Hyperfluorescent OLEDs (HF-OLEDs) take the lead in this respect as they utilize the advantages of both TADF emitters and fluorescent dopants, realizing high EQE with color saturation and long-term stability. Hyperfluorescence is mediated through Förster resonance energy transfer (FRET) from a TADF sensitizer to the final fluorescent emitter. However, competing loss mechanisms such as Dexter energy transfer (DET) of triplet excitons and direct charge trapping on the final emitter need to be mitigated in order to achieve fluorescence emission with high efficiency. Despite tremendous progress, appropriate guidelines and fine optimization are still required to address these loss channels and to improve the device operational lifetime. This perspective aims to provide an overview of the evolution of HF-OLEDs by reviewing both molecular and device design pathways for highly efficient narrowband devices covering all colors of the visible spectrum. Existing challenges and potential solutions, such as molecules with peripheral inert substitution, multi-resonant (MR) TADF emitters as final dopants, and exciplex-sensitized HF-OLEDs, are discussed. Furthermore, the operational device lifetime is reviewed in detail before concluding with suggestions for future device development.

A perspective on next-generation hyperfluorescent organic light-emitting diodes.

Mastering the control of external stimuli-induced chemical transformations with detailed insights into the mechanistic pathway is the key for developing efficient synthetic strategies and designing functional molecular systems. Enzymes, the most potent biological catalysts, efficiently utilize their built-in electric field to catalyze and control complex chemical reactions within the active site. Herein, we have demonstrated the interfacial electric field-induced prototropic tautomerization reaction in acylhydrazone entities by creating an enzymatic-like nanopocket within the atomically sharp gold electrodes using a mechanically controlled break junction (MCBJ) technique. In addition to that, the molecular system used here contains two coupled acylhydrazone reaction centers, hence demonstrating a cooperative stepwise electric field-induced reaction realized at the single molecular level. Furthermore, the mechanistic studies revealed a proton relay-assisted tautomerization showing the importance of external factors such as solvent in such electric field-driven reactions. Finally, single-molecule charge transport and energetics calculations of different molecular species at various applied electric fields using a polarizable continuum solvent model confirm and support our experimental findings. Thus, this study demonstrates that mimicking an enzymatic pocket using a single molecular junction's interfacial electric field as a trigger for chemical reactions can open new avenues to the field of synthetic chemistry.

Electric Field-Induced Sequential Prototropic Tautomerism in Enzyme-like Nanopocket Created by Single Molecular Break Junction.

Moisture-sensitive fluorescent emitters are a class of smart materials that can change their emission behavior upon exposure to water. In this study, we have synthesized two highly fluorescent organic emitters, 4BPy-PTA and 2BPy-PTA, and showed how moisture sensitivity can be enhanced by molecular design modification. Owing to the different nitrogen atom positions in the acceptor units, the emitters show different degrees of moisture sensitivity. Upon moisture exposure, both emitters change their emission color from greenish-yellow to blue, but a larger shift was witnessed in 4BPy-PTA (81 nm) than in 2BPy-PTA (68 nm). Moisture exposure enhances the photoluminescence quantum yield (PLQY) of 4BPy-PTA from 37 to 48%, whereas it suppresses the PLQY of 2BPy-PTA from 59 to 15%. A shorter moisture sensing time, large emission color shift, and enhanced PLQY make 4BPy-PTA a better moisture-sensitive material than 2BPy-PTA. Interestingly, the emission colors of the emitters can be completely regained by heating and partially by applying mechanical force to the moisture-exposed solids. In addition, these emitters also show mechanochromic luminescence behavior with a completely reversible emission color switch between blue and green.

Role of Pyridine Nitrogen Position on the Moisture Sensitivity of Organic Emitters.

Image-guided photodynamic therapy is acknowledged as one of the most demonstrative therapeutic modalities for cancer treatment because of its high precision, non-invasiveness, and improved imaging ability. A series of purely organic photosensitizers denoted as BTMCz, BTMPTZ, and BTMPXZ, have been designed and synthesized and are found to exhibit both thermally activated delayed fluorescence and aggregation-induced emission simultaneously. Experimental and theoretical studies are combined to reveal that modulation of the donor of the photosensitizer enables distinct thermally activated delayed fluorescence via a second-order spin-orbit perturbation mechanism involving lowest singlet charge-transfer and higher-lying triplet locally excited states, respectively. Further, different donor strengths and unique aggregations (H-, J- and X-type packings) greatly influence their color-tunable up-converted luminescence and endow them with superb dispersibility in water. The confocal microscopy-based cellular uptake study confirms the successful internalization of the nano-probes, while BTMCz enables the generation of reactive oxygen species (singlet oxygen) under white-light irradiation, enabling the efficient killing of cancer cells.

Modulation of Donor in Purely Organic Triplet Harvesting AIE-TADF Photosensitizer for Image-guided Photodynamic Therapy.

The development of high-efficiency orange phosphorescent organic light-emitting diodes (PhOLEDs) with reduced efficiency roll-off offers significant benefits, enabling a more simplified method for producing white-OLEDs through the combination of sky blue and orange OLEDs, in contrast to the conventional use of three primary colors. Herein, we have designed and synthesized two hosts with 9-(4-(14H-dibenzo[a,j]xanthen-14-yl)phenyl)-9H-carbazole (XaPCz) and 10-(4-(14H-dibenzo[a,j]xanthen-14-yl)phenyl)acridin-9(10H)-one (XaPAc) derivatives. The singlet and triplet energies for XaPCz and XaPAc are S=3.65, 3.12 eV, and T=2.67, 2.70 eV, respectively. These materials are employed as hosts for orange PhOLEDs using bis(2-phenylquinoline) (acetylacetonate) iridium (III) [Ir(pq)] as an emitter. XaPCz demonstrates maximum external quantum efficiency (EQE) of 20.5 %, and XaPAc shows EQE of 22.3 % with Commission Internationale de l'Eclairage (CIE) coordinates of (0.53, 0.46) and (0.54, 0.45), respectively. In addition, the XaPAc-based device shows a lower turn-on voltage (2.5 V) and high-power efficiency of 60.0 lm/W. More importantly, both device shows reduced roll-off and retain more than 94 % of EQE at 1,000 cd/m. The XaPAc-based device maintains an EQE of 17.9 % even at 10,000 cd/m.

Highly Efficient Orange Phosphorescent Organic Light Emitting Diodes with Reduced Roll-Off Using Xanthene-Conjugated Carbazole and Acridone as Host Materials.

Self-emissive atomically precise metal nanoclusters (NCs) are emerging as promising emissive layer material for next-generation light-emitting diodes (LEDs), thanks to their solid-state luminescence, well-defined structures, photo/thermal stability, low toxicity, and unique excited-state properties. However, achieving high external quantum efficiency (EQE) in solid-state NCs remains a formidable challenge. In this study, a highly stable bimetallic gold-copper NC forming [AuCu(Sadm)(DPPEO)] stabilized with 1-adamantanethiol (HSadm) and 1,2-bis(diphenylphosphino)ethane (DPPE) as the primary and secondary ligands, respectively is reported. Single-crystal X-ray diffraction and spectroscopic analyses suggest that the as-synthesized NC contains one phosphine bound to gold and the second phosphine has oxidized to phosphine oxide (P═O). The presence of such P═O moieties in the NC facilitated C─H···O interactions along with C─H···π and H···H interactions between ligands, promoting rapid crystallization. Due to the exceptional photo/thermal stability and enhanced solid-state photoluminescence quantum yield (PLQY), [AuCu(Sadm)(DPPEO)] NC is utilized to fabricate the NC-based LED (NC-LED) via the solution-processed technique, without using any additional host materials. The fabricated NC-LED shows a maximum brightness of 1246 cd m and an EQE of 12.60% with a pure red emission ≈668 nm. This EQE value coupled with saturated pure red emission is the best among solution-processed and non-doped NC-LEDs, suggesting the enormous potential of the NCs for electro-optical devices.

Article	Year
Using the Mechanical Bond to Tune the Performance of a Thermally Activated Delayed Fluorescence Emitter*. Angewandte Chemie (International ed. in English)\| PubMed ID: 33666324	2021
Ultrathin non-doped thermally activated delayed fluorescence emitting layer for highly efficient OLEDs. Chemical communications (Cambridge, England)\| PubMed ID: 34870289	2021
Thermally activated delayed fluorescence (TADF) emitters: sensing and boosting spin-flipping by aggregation. Beilstein journal of organic chemistry\| PubMed ID: 36128432	2022
Design Strategy for Enabling Multifunctional Properties of Anthracene-based Emitters. Chemistry, an Asian journal\| PubMed ID: 37246486	2023
Highly efficient color-tunable organic co-crystals unveiling polymorphism, isomerism, delayed fluorescence for optical waveguides and cell-imaging. Nature communications\| PubMed ID: 37863932	2023
Single-component TADF gels: study of the positional isomer effect on gelation and morphological effect on conductivity. Chemical communications (Cambridge, England)\| PubMed ID: 39113581	2024
Ultra-Narrowband Circularly Polarized Luminescence from Multiple 1,4-Azaborine-Embedded Helical Nanographenes. Journal of the American Chemical Society\| PubMed ID: 39435966	2024
A perspective on next-generation hyperfluorescent organic light-emitting diodes. Chemical science\| PubMed ID: 39444559	2024
Electric Field-Induced Sequential Prototropic Tautomerism in Enzyme-like Nanopocket Created by Single Molecular Break Junction. Journal of the American Chemical Society\| PubMed ID: 39496492	2024
Role of Pyridine Nitrogen Position on the Moisture Sensitivity of Organic Emitters. ACS materials Au\| PubMed ID: 39554854	2024
Modulation of Donor in Purely Organic Triplet Harvesting AIE-TADF Photosensitizer for Image-guided Photodynamic Therapy. Small (Weinheim an der Bergstrasse, Germany)\| PubMed ID: 39780649	2025
Highly Efficient Orange Phosphorescent Organic Light Emitting Diodes with Reduced Roll-Off Using Xanthene-Conjugated Carbazole and Acridone as Host Materials. Chemistry, an Asian journal\| PubMed ID: 39988562	2025
Bimetallic Nanocluster-Based Light-Emitting Diodes With High External Quantum Efficiency and Saturated Red Emission. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 40495657	2025

Pachaiyappan Rajamalli

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