Symmetry-Breaking Triplet Excited State Enhances Red Afterglow Enabling Ubiquitous Afterglow Readout.

Molecular vibrations are often factors that deactivate luminescence. However, if there are molecular motion elements that enhance luminescence, it may be possible to utilize molecular movement as a design guideline to enhance luminescence. Here, the authors report a large contribution of symmetry-breaking molecular motion that enhances red persistent room-temperature phosphorescence (RTP) in donor-π-donor conjugated chromophores. The deuterated form of the donor-π-donor chromophore exhibits efficient red persistent RTP with a yield of 21% and a lifetime of 1.6 s. A dynamic calculation of the phosphorescence rate constant (kp) indicates that the symmetry-breaking movement has a crucial role in selectively facilitating kp without increasing nonradiative transition from the lowest triplet excited state. Molecules exhibiting efficient red persistent RTP enable long-wavelength excitation, indicating the suitability of observing afterglow readout in a bright indoor environment with a white-light-emitting diode flashlight, greatly expanding the range of anti-counterfeiting applications that use afterglow.

Continuous Condensed Triplet Accumulation for Irradiance-Induced Anticounterfeit Afterglow.

Afterglow room-temperature emission that is independent of autofluorescence after ceasing excitation is a promising technology for state-of-the-art bioimaging and security devices. However, the low brightness of the afterglow emission is a current limitation for using such materials in a variety of applications. Herein, the continuous formation of condensed triplet excitons for brighter afterglow room-temperature phosphorescence is reported. (S)-(-)-2,2'-Bis(diphenylphosphino)-1,1'-binaphthyl ((S)-BINAP) incorporated in a crystalline host lattice showed bright green afterglow room-temperature phosphorescence under strong excitation. The small triplet-triplet absorption cross-section of (S)-BINAP in the whole range of visible wavelengths greatly suppressed the deactivation caused by Förster resonance energy transfer from excited states of (S)-BINAP to the accumulated triplet excitons of (S)-BINAP under strong continuous excitation. The steady-state concentration of the triplet excitons for (S)-BINAP reached 2.3 × 10-2  M, producing a bright afterglow. Owing to the brighter afterglow, afterglow detection using individual particles with sizes approaching the diffraction limit in aqueous conditions and irradiance-dependent anticounterfeiting can be achieved.

Förster resonance energy transfer involving the triplet state.

Triplet harvesting is important for high-efficiency optoelectronics devices, time-resolved bioimaging, sensing, and anti-counterfeiting devices. Förster resonance energy transfer (FRET) from the donor (D) to the acceptor (A) is important to efficiently harvest the triplet excitons after a variety of excitations. However, general explanations of the key factors of FRET from the singlet state (FRETS-S) via reverse intersystem crossing and FRET from the triplet state (FRETT-S) have not been reported beyond spectral overlap between emission of the D and absorption of the A. This feature article gives an overview of FRET involving the triplet state. After discussing the contribution of the radiation yield from the state of the D considering spin-forbidden factors of FRET, a variety of schemes involving triplet states, such as FRETS-Svia reverse intersystem crossing from the triplet state, dual FRETS-S and FRETT-S, and selective FRETT-S, are introduced. Representative examples, including the chemical structure and FRET for triplet harvesting, are highlighted using emerging applications in optoelectronics and afterglow imaging. Finally, recent developments of using FRET involving triplet states for high-efficiency optoelectronic devices and time-resolved bioimaging are discussed. This article provides crucial information for controlling state-of-the-art properties using FRET involving the triplet state.

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

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-pTC with tert-butylcarbazole (TC) as the donor and BPy-p3C with bulky tricarbazole (3C) as the donor unit. Both BPy-pTC and BPy-p3C exhibited prominent emission with TADF properties in solution and in the solid phase. The stronger excited-state charge transfer was obtained for BPy-p3C due to the bulkier donor, leading to a more twisted D-A geometry than that of BPy-pTC. Hence, BPy-p3C exhibited aggregation-induced enhanced emission (AIEE) in a THF/water mixture. Interestingly, the singlet-triplet energy gap (ΔE ST) was reduced for both compounds in the aggregated state as compared to toluene solution. Consequently, a faster reverse intersystem crossing rate (k RISC) 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 ΔE ST in the aggregated state as well as the relevance for fluorescence-based acid-base sensing.

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

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 (EQEmax) of 13.5% with a luminance of 17 250 cd m-2 than the conventional (30 nm) non-doped device.

UV to NIR multistate electrochromism and electrofluorochromism in dibenzophenazine-arylamine derivatives.

An intriguing case of intramolecular and intervalence charge transfer-driven multistate electrochromism and electrofluorochromism in dibenzophenazin-(phenyl)methanone and arylamine-based redox-active donor-acceptor-donor molecules was elucidated. Tunable absorption from UV to NIR and on-off switching of fluorescence in a single-component all-organic molecular material by a subtle variation of electric potentials were demonstrated.

Molecular Engineering Approaches Towards All-Organic White Light Emitting Materials.

White light emitting (WLE) materials are of increasing interest owing to their promising applications in artificial lighting, display devices, molecular sensors, and switches. In this context, organic WLE materials cater to the interest of the scientific community owing to their promising features like color purity, long-term stability, solution processability, cost-effectiveness, and low toxicity. The typical method for the generation of white light is to combine three primary (red, green, and blue) or the two complementary (e.g., yellow and blue or red and cyan) emissive units covering the whole visible spectral window (400-800 nm). The judicious choice of molecular building blocks and connecting them through either strong covalent bonds or assembling through weak noncovalent interactions are the key to achieve enhanced emission spanning the entire visible region. In the present review article, molecular engineering approaches for the development of all-organic WLE materials are analyzed in view of different photophysical processes like fluorescence resonance energy transfer (FRET), excited-state intramolecular proton transfer (ESIPT), charge transfer (CT), monomer-excimer emission, triplet-state harvesting, etc. The key aspect of tuning the molecular fluorescence under the influence of pH, heat, and host-guest interactions is also discussed. The white light emission obtained from small organic molecules to supramolecular assemblies is presented, including polymers, micelles, and also employing covalent organic frameworks. The state-of-the-art knowledge in the field of organic WLE materials, challenges, and future scope are delineated.

T and V-shaped donor-acceptor-donor molecules involving pyridoquinoxaline: large Stokes shift, environment-sensitive tunable emission and temperature-induced fluorochromism.

The intramolecular charge transfer-driven emission properties of T and V-shaped donor-acceptor-donor molecules involving a new acceptor core of pyridoquinoxaline were demonstrated. The T-shaped molecule exhibits a large Stokes shift, red emission in the solid state and remarkable viscosity and temperature-dependent tunable fluorescence including a thermally-induced single-component near white-light emission.

Tuning the Förster Resonance Energy Transfer through a Self-Assembly Approach for Efficient White-Light Emission in an Aqueous Medium.

A simple and cost-effective methodology employing environmentally benign substances for the fabrication of white-light emitting materials is important for practical applications in the field of lighting and display devices. Designing purely organic-based white-light-emitting systems with high quantum efficiency in aqueous media is an unmet challenge. With this objective, a new class of pyridoindole-based hydrophobic fluorophore 6,7,8,9-tetrapropylpyrido[1,2-a]indole-10-carbaldehye (TPIC) was introduced. A strategy of self-assembly using nonionic surfactants was employed to enhance the fluorescence of TPIC in an aqueous medium and was exploited as energy donor. The steady-state and time-resolved emission spectra analysis revealed the micelle-mediated energy transfer from TPIC to Nile red (energy acceptor) leading to tunable fluorescence along with white-light emission. The white-light emitting aqueous solution was obtained with the Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of (0.33, 0.36) and significantly high quantum yield of 37 %. Solid-state white-light emission was achieved retaining the assembly of fluorophores in the form of a gel having the high quantum efficiency of 33 % with CIE coordinates of (0.32, 0.36); close to that of pure white light. The bright white luminescence of the inscription prepared using white-light emitting gel on a solid substrate offers promising applications for full-color flat panel displays.

A Pyridoindole-Based Multifunctional Bioprobe: pH-Induced Fluorescence Switching and Specific Targeting of Lipid Droplets.

A versatile fluorescent probe, PITE, based on alkyl-substituted pyridoindole (PI) and tetraphenylethylene (TE), which exhibits facile pH-induced fluorescence switching in solution, as nanoparticles, and in the solid state, is presented. Strong fluorescence in the solid state, as well as in solution and the aggregated state, allow sensing of toxic acid vapors. Fluorescence "off-on" switching of PITE through exposure to trifluoroacetic acid and triethylamine vapor is visualized by the naked eye. A unified picture of the switchable fluorescence of PITE is obtained by comprehensive spectroscopic investigations coupled with quantum mechanical calculations. Strong fluorescence, a large Stokes shift, high photostability, and biocompatibility of PITE make it a viable probe for subcellular imaging. Extensive fluorescence microscopic studies by employing organisms including lower and higher eukaryotes reveal specific localization of PITE to lipid droplets (LDs). LDs are dynamic subcellular organelles linked to various physiological processes and human diseases. Hence, the specific detection of LDs in diverse organisms is important to biomedical research and healthcare. Isolation of LDs and subsequent colocalization studies ascertain selective targeting of LDs by the easily affordable, lipophilic bioprobe, PITE. Thus, PITE is a promising multifunctional probe for chemosensing and the selective tracking of LDs.