Boosting purely organic room-temperature phosphorescence performance through a host-guest strategy

Intersystem crossing in a dibenzofuran-based room temperature phosphorescent luminophore investigated by non-adiabatic dynamics

The use of phosphorescent luminophores is highly beneficial in diverse high-technological and biological applications. Yet, because of the formally forbidden character of intersystem crossing, the use of heavy metals or atoms is usually necessary to achieve high quantum yields. This choice imposes serious constraints in terms of high device cost and inherent toxicity. In this contribution we resort to density functional based surface hopping non-adiabatic dynamics of a potential organic luminophore intended for room-temperature applications. We confirm that intersystem crossing is operative in a ps time-scale without requiring the activation of large-scale movements, thus confirming the suitability of the El Sayed-based strategy for the rational design of fully organic phosphorescent emitters.

Room-Temperature Persistent Phosphorescence of Aggregated Gold Nanoclusters under Molecular Crystal Confinements

We report color-tunable and solvent-processable persistent fluorescence to phosphorescence switching at room temperature by doping gold nanoclusters (AuNCs) inside molecular crystals. This provides a significant insight into the tunability of the photoluminescence property of the dopant depending on the crystal environment and compactness of confinement, with the possibility of energy transfer from crystal to aggregated AuNCs. For test cases, we have doped histidine-stabilized AuNCs (HIS-AuNCs) inside histidine (HIS-AuNCs-HIS) and isophthalic acid (HIS-AuNCs-IPA) crystals, respectively, and glutathione-stabilized AuNCs (GSH-AuNCs) inside histidine crystals (GSH-AuNCs-HIS). The maximal phosphorescence decay time recorded for crystal doped aggregated AuNCs was 9.38 ms, and the photoluminescence quantum yield value was measured as 25%. The possible energy states and potential interactions between aggregated NCs and host crystals were accounted for through density functional theory calculations and docking techniques, respectively. This finding opens new possibilities for designing and producing color-tunable persistent AuNC-based luminous crystals for multilayer information encryption, display, and biological applications.

Ir(III) Metal Emitters with Cyano-Modified Imidazo[4,5-b]pyridin-2-ylidene Chelates for Deep-Blue Organic Light-Emitting Diodes

Ir(III) carbene complexes have been explored as one of the best blue phosphors for their high performance. Herein, the authors designed and synthesized a series of blue-emitting Ir(III) phosphors (f-ct9a-c), featuring fac-coordinated cyano-imidazo[4,5-b]pyridin-2-ylidene cyclometalates. These Ir(III) complexes exhibit true-blue emission with a peak maximum spanning 448-467 nm, with high photoluminescence quantum yields of 81-88% recorded in degassed toluene. Moreover, OLED devices bearing phosphors f-ct9a and f-ct9b deliver maximum external quantum efficiencies (EQEmax) of 25.9% and 30.3%, together with Commission Internationale de L'Eclairage (CIEx,y) coordinates of (0.157, 0.225) and (0.142, 0.169), respectively. Remarkably, the f-ct9b-based device displays an incredible EQE of 29.0% at 5000 cd·m-2. The hyper-OLED device based on f-ct9b and ν-DABNA exhibits an EQEmax of 34.7% and CIEx,y coordinates of (0.122, 0.131), affirming high potentials in achieving efficient blue electroluminescence.

Carbazole-Benzonitrile-Norbornadiene Conjugates for Photothermally Reversible Ambient Phosphorescence

Organic photoswitches have attracted significant attention across various fields, such as sensing, bioimaging, photopharmacology, molecular machines, and solar energy storage. However, as a result of design complexities, achieving photothermally reversible ambient phosphorescence switching in the condensed state remains elusive. Herein, we explore the impact of norbornadiene (NBD)/quadricyclane (QC) substitution at position 5 of the benzonitrile acceptor covalently attached to the carbazole donor on photothermally reversible luminescence switching. Experimental investigations demonstrated that the CzN and TBCzN switches exhibited photothermally reversible fluorescence switching in solution. Moreover, in the condensed state, fluorescence and ambient phosphorescence switching were observed as a result of a low singlet-triplet (ΔEST) gap (CzN ⇆ CzQ, ΔESTCzN/CzQ = 0.05/0.28 eV; TBCzN ⇆ TBCzQ, ΔESTTBCzN/TBCzQ = 0.06/0.09 eV). Reversible ambient phosphorescence switching is primarily influenced by modulation of acceptor conjugation resulting from NBD ⇆ QC switching. This approach may provide important clues for the design of visible-light-absorbing photothermally reversible phosphorescent materials.

Tunable afterglow for mechanical self-monitoring 3D printing structures

Self-monitoring materials have promising applications in structural health monitoring. However, developing organic afterglow materials for self-monitoring is a highly intriguing yet challenging task. Herein, we design two organic molecules with a twisted donor-acceptor-acceptor' configuration and achieve dual-emissive afterglow with tunable lifetimes (86.1-287.7 ms) by doping into various matrices. Based on a photosensitive resin, a series of complex structures are prepared using 3D printing technology. They exhibit tunable afterglow lifetime and Young's Modulus by manipulating the photocuring time and humidity level. With sufficient photocuring or in dry conditions, a long-lived bright green afterglow without apparent deformation under external loading is realized. We demonstrate that the mechanical properties of complex 3D printing structures can be well monitored by controlling the photocuring time and humidity, and quantitively manifested by afterglow lifetimes. This work casts opportunities for constructing flexible 3D printing devices that can achieve sensing and real-time mechanical detection.

Recent Advances in Organic Molecular-to-Supramolecular Self-Assembled Room-Temperature Phosphorescent Materials for Biomedical Applications

This minireview focuses on recent advancements in organic molecular-to-supramolecular self-assembled room-temperature phosphorescent (RTP) materials and their prospective biomedical applications. RTP materials, having their unique capacity to emit long-lasting phosphorescence at ambient temperature, have piqued researchers' interest in various biological applications, including biosensing, bioimaging, drug delivery, and photodynamic therapy (PDT). These materials have several benefits, including high sensitivity, remarkable photostability, and low cytotoxicity. RTP materials' self-assembly into supramolecular structures improves their performance and broadens their uses. Researchers have built organic RTP systems with long-lasting phosphorescence by leveraging weak noncovalent interactions in aquatic conditions. These materials have demonstrated incredible promise as biosensors that enable sensitive analyte detection and as photosensitizers in PDT that target and sensitize specific cell types. The review also outlines future directions and challenges in developing and utilizing pure organic RTP materials for biological imaging purposes, providing valuable guidelines for their future design and application.

Highly Efficient Purely Organic Phosphorescence Light-Emitting Diodes Employing a Donor-Acceptor Skeleton with a Phenoxaselenine Donor

Purely organic room-temperature phosphorescence (RTP) materials generally exhibit low phosphorescence quantum yield (ϕ_P ) and long phosphorescence lifetime (τ_P ) due to the theoretically spin-forbidden triplet state. Herein, by introducing a donor-acceptor (D-A) skeleton with a phenoxaselenine donor, three nonaromatic amine donor containing compounds with high ϕ_P and short τ_P in amorphous films are developed. Besides the enhanced spin-orbit coupling (SOC) by the heavy-atom effect of selenium, the D-A skeleton which facilitates orbital angular momentum change can further boost SOC, and severe nonradiative energy dissipation is also suppressed by the rigid molecular structure. Consequently, a record-high external quantum efficiency of 19.5% are achieved for the RTP organic light-emitting diode (OLED) based on 2-(phenoxaselenin-3-yl)-4,6-diphenyl-1,3,5-triazine (PXSeDRZ). Moreover, voltage-dependent color-tunable emission and single-molecule white emission are also realized. These results shed light on the broad prospects of purely organic phosphorescence materials as highly efficient OLED emitters especially for potential charming lighting applications.

Boosting the Phosphorescence Efficiency in Doped Organic Crystals: Critical Role of Hydrogen Bonding

Host-guest doping systems with phthalimides (BI) and N-methylphthalimide (NMeBI) as the host and 1,8-naphthalimide (NI) and 4-bromo-1,8-naphthalimide (4BrNI) as the guest have been developed. The 0.2% NI/BI (molar ratio) with a strong C=O···H-N hydrogen bond exhibited a phosphorescence quantum efficiency (29.2%) higher than that of NI/NMeBI with a weak C=O···H-C hydrogen bond (10.1%). A similar trend was observed in the 4BrNI guest system. A remarkable phosphorescent efficiency of 42.1% was achieved in a 0.5% 4BrNI/BI composite, which represents the highest value in NI-based phosphors. This research indicates stronger hydrogen bonding may have a greater contribution in boosting the phosphorescence efficiency.

Sulfone-Embedded Heterocyclic Narrowband Emitters with Strengthened Molecular Rigidity and Suppressed High-Frequency Vibronic Coupling

Sulfone-embedded heterocyclics are of great interest in organic light-emitting diodes (OLEDs), however, exploring highly efficient narrowband emitters based on sulfone-embedded heterocyclics remains challenging. Herein, five emitters with different sulfur valence state and molecular rigidity, namely tP, tCPD, 2tCPD, tPD and tPT, are thoroughly analysed. With restricted twisting of flexible peripheral phenyl by strengthening molecular rigidity, molecular emission spectra can be enormously narrowed. Further, introducing the sulfone group with bending vibration in low-frequency region that suppresses high-frequency vibration, sharp narrow full-widths at half-maximum of 28 and 25 nm are achieved for 2tCPD and tPD, respectively. Maximum external quantum efficiencies of 22.0 % and 27.1 % are successfully realized for 2tCPD- and tPD-based OLED devices. These results offer a novel design strategy for constructing narrowband emitters by introducing sulfone group into a rigid molecular framework.

Blue Phosphorescence and Hyperluminescence Generated from Imidazo[4,5-b]pyridin-2-ylidene-Based Iridium(III) Phosphors

Four isomeric, homoleptic iridium(III) metal complexes bearing 5-(trifluoromethyl)imidazo[4,5-b]pyridin-2-ylidene and 6-(trifluoromethyl)imidazo[4,5-b]pyridin-2-ylidene-based cyclometalating chelates are successfully synthesized. The meridional isomers can be converted to facial isomers through acid induced isomerization. The m-isomers display a relatively broadened and red-shifted emission, while f-isomers exhibit narrowed blue emission band, together with higher photoluminescent quantum yields and reduced radiative lifetime relative to the mer-counterparts. Maximum external quantum efficiencies of 13.5% and 22.8% are achieved for the electrophosphorescent devices based on f-tpb1 and m-tpb1 as dopant emitter together with CIE coordinates of (0.15, 0.23) and (0.22, 0.45), respectively. By using f-tpb1 as the sensitizing phosphor and t-DABNA as thermally activated delayed fluorescence (TADF) terminal emitter, hyperluminescent OLEDs are successfully fabricated, giving high efficiency of 29.6%, full width at half maximum (FWHM) of 30 nm, and CIE coordinates of (0.13, 0.11), confirming the efficient Förster resonance energy transfer (FRET) process.

A "Flexible" Purely Organic Molecule Exhibiting Strong Spin-Orbital Coupling: Toward Nondoped Room-Temperature Phosphorescence OLEDs

Purely organic materials usually exhibit weak spin-orbital coupling (SOC) effect because of the lack of noble heavy metals, and the generation and direct emission from the triplet state is spin-forbidden. This would lead to slow intersystem crossing, long triplet lifetime, and low phosphorescence quantum yield. Herein, strong spin-orbital coupling between singlet and triplet was observed in a "flexible" and twist thianthrene-pyrimidine-based purely organic compound in an amorphous film state, which shows a fast intersystem crossing process and a high phosphorescence rate of 1.1 × 10³ s^-1. The heavy atom sulfur and nitrogen atoms in the molecule can provide n-π* transition character for efficient spin-orbital coupling. Moreover, the flexible molecule skeleton enables conformational change and molecular vibration in excited states, which was proved to be vital for efficient vibrational spin-orbital coupling. Benefitting from the strong SOC effect, a nondoped purely organic phosphorescence light-emitting diode was fabricated, which achieves a maximum external quantum efficiency of 7.98%, corresponding to an exciton utilization ratio exceeding 87.6%.

Regioselective Syntheses of Imidazo[4,5-b]pyrazin-2-ylidene-Based Chelates and Blue Emissive Iridium(III) Phosphors for Solution-Processed OLEDs

Six homoleptic Ir(III) complexes bearing imidazo[4,5-b]pyrazin-2-ylidene chelates were successfully designed and synthesized. Narrowband blue emission (λ_max = 466-485 nm) and broadened green emission (λ_max = 518-532 nm) in degassed toluene solution with high photoluminescent quantum yields in the range of 75-81 and 45-48% were observed for f-timpz, t2impz, and t2empz as well as m-timpz, t2impz, and t2empz, respectively. In addition, the tert-butylphenyl cyclometalate is more electron donating than N-phenyl cyclometalate and, hence, all tert-butylphenyl-substituted derivatives, that is, m- and f-t2impz and m- and f-t2empz, give more red-shifted emission in comparison to that of m- and f-timpz. Moreover, solution-processed OLED with f-t2empz (20 wt %) as the dopant gave electrophosphorescence at 474 nm with maximum external quantum efficiency (max. EQE) of 5.1%, while hyper-OLED with assistant sensitizer f-t2empz (10 wt %) and the multi-resonance thermally activated delayed fluorescence emitter BCzBN (0.5 wt %) afforded narrowband emission centered at 485 nm and max. EQE up to 17.4%, confirming the high potential of this class of Ir(III) metal phosphors.

Halide-containing organic persistent luminescent materials for environmental sensing applications

Great progress has been made in the development of various organic persistent luminescent (OPL) materials in the past few years, and increasing attention has been paid to their interesting applications in environmental sensing due to their long emission lifetimes and high sensitivity. Especially, the introduction of different halogen elements facilitates highly efficient OPL emission with distinct lifetimes and colours. In this review, we summarize the current status of the halide-containing OPL materials for environmental sensing applications. To begin with, the photophysical processes and luminescence mechanisms of OPL materials are expounded in detail to better understand the relationship among molecular structures, OPL properties, and sensing applications. Then, representative halide-containing material systems, such as small molecules, polymers, and doping systems, are summarized with their interesting applications in sensing temperature, oxygen, H₂O, UV light and organic solvents. In addition, several challenges and future research opportunities in this field are discussed. This review aims to provide some reasonable guidance on the material design of OPL sensors and their practical applications, and tries to provide a new perspective on the application direction of organic optoelectronics.

This review presents a summary of the molecular design of halide-containing organic persistent luminescent materials, and their environmental sensing applications.

Lewis Pair-Functionalized Pt(II) Complexes with Tunable Emission Color and Triplet-State Properties

Two emissive Pt(II) complexes containing dynamic "flexible" Lewis pair (FlexLP) ligands are reported. The FlexLP ligand encompasses a diphenylphosphine oxide Lewis base and a dimesitylborane Lewis acid attached to a bithiophene scaffold, which can switch between an open unbound Lewis pair and a bound P-O-B Lewis adduct depending on the hydrogen bond-donating (HBD) strength of the solvent. [Pt(FlexLP)₂] contains two FlexLP ligands, and [Pt(FlexLP)(Py)] contains one FlexLP ligand and one pyrene ligand. UV-vis absorption and fluorescence studies demonstrate that the FlexLP ligands switch between the open Lewis pair and the closed Lewis adduct in MeOH, a strong HBD solvent, and acetone, a weak HBD solvent, respectively, and exhibit tunable emission color depending on the acetone/MeOH solvent ratio. Transient absorption spectroscopy reveals a large difference in the triplet-state lifetime depending on the conformation of the FlexLP ligands for both complexes. In the closed form, the triplet-state lifetimes of the two complexes are over an order of magnitude longer compared to that of the complexes in the open conformation. Calculations of optimized geometries suggest that this difference in triplet-state lifetime is due to a difference in the thiophene-thiophene torsion angle between the two conformations.