Direct observation of intersystem crossing in a thermally activated delayed fluorescence copper complex in the solid state

Disentangling Radiative and Non-Radiative Deactivation Pathways in CuI -Based TADF Emitters

The analysis of thermally activated delayed fluorescence emitters traditionally neglects the temperature dependence of the non-radiative singlet and triplet decay rates, which, for systems with quantum yields significantly departing from unity, unavoidably leads to inaccurate determination of the relevant photophysical parameters. Here we address this issue by performing a detailed photophysical study in three solid-state CuI complexes with the general formula [Cu(dmp)(PPh3)X] (X = Cl, Br, I). Decomposition of the measured temperature-dependent lifetimes τ(T) into their radiative and non-radiative contributions demonstrates a significant temperature dependence of the non-radiative rates, which leads to large deviations of the singlet-triplet gaps ΔEST from those determined in the traditional way. Our detailed analysis further confirms that non-radiative parameters follow the energy gap law and that zero field splitting effects, which are unrecognizable in the bare τ(T) data, are governed by the heavy atom effect. Our analysis is supported by measurements of the decay of the prompt fluorescence which is dominated by kISC in the studied compounds.

Synthesis and Characterization of Copper(I) Halide Heteroleptic Complexes with Thermally Activated Delayed Fluorescence

In the global context of green chemistry and sustainable development, luminescent copper(I) halide complexes hold broad applications, attributed to their abundant resources and excellent photophysical properties. Herein, two novel copper(I) halide complexes were synthesized and systematically investigated using single-crystal X-ray diffraction, photophysical characterization, and theoretical calculations among other methods. Both solid-state complexes exhibit bright green luminescent emissions with low self-absorption rates. The photoluminescence quantum yields (PLQYs) are as high as 95% and 87%, respectively. Because their singlet-triplet energy gaps (ΔEST) are very small, both being 0.10 eV, these complexes can achieve thermally activated delayed fluorescence emission through efficient reverse intersystem crossing. Theoretical calculations revealed that their high-efficiency luminescence arises from the synergistic effects of metal-to-ligand charge transfer, halogen-to-ligand charge transfer, and ligand-to-ligand charge transfer.

Molecular Design Principles for Photoactive Transition Metal Complexes: A Guide for "Photo-Motivated" Chemists

Luminescence and photochemistry involve electronically excited states that are inherently unstable and therefore spontaneously decay to electronic ground states, in most cases by nonradiative energy release that generates heat. This energy dissipation can occur on a time scale of 100 fs (∼10-13 s) and usually needs to be slowed down to at least the nanosecond (∼10-9 s) time scale for luminescence and intermolecular photochemistry to occur. This is a challenging task with many different factors to consider. An alternative emerging strategy is to target dissociative excited states that lead to metal-ligand bond homolysis on the subnanosecond time scale to access synthetically useful radicals. Based on a thorough review at the most recent advances in the field, this article aims to provide a concise guide to obtaining luminescent and photochemically useful coordination compounds with d-block elements. We hope to encourage "photo-motivated" chemists who have been reluctant to apply their synthetic and other knowledge to photophysics and photochemistry, and we intend to stimulate new approaches to the synthetic control of excited state behavior.

Immunostimulatory Hydrogel with Synergistic Blockage of Glutamine Metabolism and Chemodynamic Therapy for Postoperative Management of Glioblastoma

Glioblastoma multiforme (GBM) is one of the most lethal malignant brain tumors in the central nervous system. Patients face many challenges after surgery, including tumor recurrence, intracranial pressure increase due to cavitation, and limitations associated with immediate postoperative oral chemotherapy. Here an injected peptide gel with in situ immunostimulatory functions is developed to coordinate the regulation of glutamine metabolism and chemodynamic therapy for overcoming these postoperative obstacles. The methodology entails crafting injectable gel scaffolds with short peptide molecules, incorporating the glutaminase inhibitor CB-839 and copper peptide self-assembled particles (Cu-His NPs) renowned for their chemodynamic therapy (CDT) efficacy. By fine-tuning glutamic acid production via metabolic pathways, this system not only heightens the therapeutic prowess of copper peptide particles in CDT but also escalates intracellular oxidative stress. This dual mechanism culminates in augmented immunogenic cell death within glioblastoma multiforme cells and improves a conducive immune microenvironment. Based on the concept of metabolic reprogramming, this treatment strategy has great potential to significantly reduce GBM tumor recurrence and prolong median survival in murine models.

Harnessing of Cooperative Cu⋅⋅⋅H Interactions for Luminescent Low-Coordinate Copper(I) Complexes towards Stable OLEDs

Although two-coordinate Cu(I) complexes are highly promising low-cost emitters for organic light-emitting diodes (OLEDs), the exposed metal center in the linear coordination geometry makes them suffer from poor stability. Herein, we describe a strategy to develop stable carbene-Cu-amide complexes through installing intramolecular noncovalent Cu⋅⋅⋅H interactions. The employment of 13H-dibenzo[a,i]carbazole (DBC) as the amide ligand leads to short Cu⋅⋅⋅H distances in addition to the Cu-N coordination bond. The resultant Cu(I) complexes exhibit yellow thermally activated delayed fluorescence with photoluminescence quantum yields of up to 86 % and radiative decay rate constants on the order of 106 s-1. Comparing with the analogues without Cu⋅⋅⋅H interactions, the pincer complexes have significantly improved stability. The vacuum-deposited OLEDs show high-performance electroluminescence with maximum external quantum efficiencies of up to 29.5 % and extremely small roll-offs of only 3.5 % at 10,000 cd m-2. Remarkably, the operational lifetimes (LT90) are up to 68 h with an initial luminance of 3000 cd m-2. This work proves a feasible design of robust low-coordinate metal complexes by leveraging secondary coordination interactions, which helps to overcome the long-standing stability problem.

Proton Tunneling Allows a Proton-Coupled Electron Transfer Process in the Cancer Cell

Proton-coupled electron transfer (PCET) is a fundamental redox process and has clear advantages in selectively activating challenging C-H bonds in many biological processes. Intrigued by this activation process, we aimed to develop a facile PCET process in cancer cells by modulating proton tunneling. This approach should lead to the design of an alternative photodynamic therapy (PDT) that depletes the mitochondrial electron transport chain (ETC), the key redox regulator in cancer cells under hypoxia. To observe this depletion process in the cancer cell, we monitored the oxidative-stress-induced depolarization of mitochondrial inner membrane potential (MMP) using fluorescence lifetime imaging microscopy (FLIM). Typically, increasing metabolic stress of cancer cells is reflected in a nontrivial change in the fluorophore's fluorescence lifetime. After 30 min of irradiation, we observed a shift in the mean lifetime value and a drastic drop in overall fluorescence signal. In addition, our PCET strategy resulted in drastic reorganization of mitochondrial morphology from tubular to vesicle-like and causing an overall depletion of intact mitochondria in the hypodermis of C. elegans. These observations confirmed that PCET promoted ROS-induced oxidative stress. Finally, we gained a clear understanding of the proton tunneling effect in the PCET process through photoluminescence experiments and DFT calculations.

Nanosecond-Lived Excimer Observation in a Crystal of a Rhodium(I) Complex via Time-Resolved X-ray Laue Diffraction

The rare observation of transient Rh···Rh excimer formation in a single crystal is reported. The estimated excited-state lifetime at 100 K is 2 ns, which makes it the shortest-lived small-molecule species caught experimentally using the laser-pump/X-ray-probe time-resolved Laue method. Upon excitation with 390 nm laser light, the intermolecular Rh···Rh distance decreases from 3.379(4) to 3.19(1) Å, and the metal-metal contact gains more bonding character. On the basis of the experimental results and theoretical modeling, the structural changes determined with 100 ps time resolution reflect principally the S0 → S1 electronic transition.

Thermally Activated Delayed Fluorescence (TADF) Materials Based on Earth-Abundant Transition Metal Complexes: Synthesis, Design and Applications

Materials exhibiting thermally activated delayed fluorescence (TADF) based on transition metal complexes are currently gathering significant attention due to their technological potential. Their application extends beyond optoelectronics, in particular organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs), and include also photocatalysis, sensing, and X-ray scintillators. From the perspective of sustainability, earth-abundant metal centers are preferred to rarer second- and third-transition series elements, thus determining a reduction in costs and toxicity but without compromising the overall performances. This review offers an overview of earth-abundant transition metal complexes exhibiting TADF and their application as photoconversion materials. Particular attention is devoted to the types of ligands employed, helping in the design of novel systems with enhanced TADF properties.

Achieving efficient and stable blue thermally activated delayed fluorescence organic light-emitting diodes based on four-coordinate fluoroboron emitters by simple substitution molecular engineering

Achieving both high efficiency and high stability in blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) is challenging for practical displays and lighting. Here, we have successfully developed a series of sky-blue to pure-blue emitting donor-acceptor (D-A) type TADF materials featuring a four-coordinated boron with 2,2'-(pyridine-2,6-diyl)diphenolate (dppy) ligands, i.e.1-8. Synergistic engineering of substituents on the phenyl bridge as well as the electronic properties and the attached positions of heteroatom N-donors not only enables fine-tuning of the emission colors, but also modulates the nature and energies of their triplet excited states that are important for the reverse intersystem crossing (RISC). Particularly for the compound with two methyl substituents on the phenyl bridge (compound 8), RISC is significantly facilitated through the vibronic coupling of the energetically close-lying triplet charge transfer (3CT) and the triplet local excited (3LE) states, when compared to analogue 7. Efficient sky-blue to pure-blue OLEDs with electroluminescence peaks (λ EL) at 460-492 nm have been obtained, in which ca. five-fold higher external quantum efficiencies (EQEs) of 18.9% have been demonstrated by 8 than that by 7. Moreover, ca. thirty times longer device operational half-lifetimes (LT50) of 9113 hours for 8 than that for 7 as well as satisfactory LT50 reaching 26 643 hours for 6 at an initial luminance of 100 cd m-2 have also been demonstrated. To the best of our knowledge, these results represent one of the best high-performance blue OLEDs based on tetracoordinated boron TADF emitters. Moreover, the design strategy presented here has provided an attractive strategy for enhancing the device performance of blue TADF-OLEDs.

Influence of the substituents of the thiol ligand on the optical properties of AuCu14

Detailed photophysical processes of two AuCu14 clusters with different substituents (-F or -C(CH3)3) of the thiol ligand were studied in this work. The electronic effect of the substituents led to structural shrinkage, thus enhancing the luminous intensity. The internal conversion (IC) and intersystem crossing (ISC) rates in the AuCu14-C(CH3)3 crystal were slower compared with the AuCu14-F crystal, which was caused by the steric effect.

Activation of TADF in Photon Upconverting Crystals of Dinuclear Cu(I)-Iodide Complexes by Ligand Engineering

This work reports that ligand engineering can modulate the triplet harvesting mechanism in iodide-bridged rhombic Cu2I2 complexes. Complex-1, with a smaller Cu-Cu distance, exhibits phosphorescence from 3(M+X)LCT and 3CC states with 66% quantum yield, whereas an increased Cu-Cu distance in complex-2 results in a switch of the emission from phosphorescence to TADF, which occurs via 1/3(M+X)LCT states with 83% quantum yield. The TADF property of complex-2 has been utilized for the fabrication of a pc-LED emitting efficient warm white light. Moreover, the high charge-transfer nature of these complexes leads to the emergence of third-harmonic generation (THG). Interestingly, complex-1 exhibits efficient third-harmonic generation with a χ(3) value of 1.15 × 10-18 m2 V-2 and LIDT value of 14.73 GW/cm2. This work aims to provide a structure-property relationship to achieve effective harvestation of triplet excitons in iodide-bridged rhombic Cu2I2 complexes and their effective utilization in OLED device fabrication and nonlinear photon upconversion processes.

High-performance near-infrared OLEDs maximized at 925 nm and 1022 nm through interfacial energy transfer

Using a transfer printing technique, we imprint a layer of a designated near-infrared fluorescent dye BTP-eC9 onto a thin layer of Pt(II) complex, both of which are capable of self-assembly. Before integration, the Pt(II) complex layer gives intense deep-red phosphorescence maximized at ~740 nm, while the BTP-eC9 layer shows fluorescence at > 900 nm. Organic light emitting diodes fabricated under the imprinted bilayer architecture harvest most of Pt(II) complex phosphorescence, which undergoes triplet-to-singlet energy transfer to the BTP-eC9 dye, resulting in high-intensity hyperfluorescence at > 900 nm. As a result, devices achieve 925 nm emission with external quantum efficiencies of 2.24% (1.94 ± 0.18%) and maximum radiance of 39.97 W sr-1 m-2. Comprehensive morphology, spectroscopy and device analyses support the mechanism of interfacial energy transfer, which also is proved successful for BTPV-eC9 dye (1022 nm), making bright and far-reaching the prospective of hyperfluorescent OLEDs in the near-infrared region.

Crown-Assisted CsCu2I3 Growth and Trap Passivation for Perovskite Light-Emitting Diodes

Copper (Cu)-based perovskites are promising for lead-free perovskite light-emitting diodes (PeLEDs). However, it remains a significant challenge to achieve high performance devices due to the nonradiative loss caused by the disordered crystallization and lack of passivation. Crown ethers are known to form host-guest complexes by the interaction between C-O-C groups and certain cations, and 18-crown-6 (18C6) with an appropriate complementary size can interact with Cs+ and Cu+ cations. Herein, we studied the interaction between CsCu2I3 and two crowns with the same cyclic size, 18C6 and dibenzo-18-crown-6 (D18C6). Particularly, D18C6 can reduce the nonradiative recombination rate of CsCu2I3 film by passivating the defects and optimizing the film morphology effectively. The room mean square (RMS) decreased from 5.06 to 2.95 nm, and the PLQY was promoted from 4.71% to 19.9%. Besides, D18C6 can also decrease the barrier of hole injection. The PeLEDs based on D18C6-modified CsCu2I3 realized noticeable improvement with a maximum luminance and EQE of 583 cd/m2 and 0.662%, respectively.

Thermally Activated Delayed Fluorescence (TADF)-active Coinage-metal Sulfide Clusters for High-resolution X-ray Imaging

The study of facile-synthesis and low-cost X-ray scintillators with high light yield, low detection limit and high X-ray imaging resolution plays a vital role in medical and industrial imaging fields. However, the optimal balance between X-ray absorption, decay lifetime and excitonic utilization efficiency of scintillators to achieve high-resolution imaging is extremely difficult due to the inherent contradiction. Here two thermally activated delayed fluorescence (TADF)-actived coinage-metal clusters M6 S6 L6 (M=Ag or Cu) were synthesized by simple solvothermal reaction, where the cooperation of heavy atom-rich character and TADF mechanism supports strong X-ray absorption and rapid luminescent collection of excitons. Excitingly, Ag6 S6 L6 (SC-Ag) displays a high photoluminescence quantum yield of 91.6 % and scintillating light yield of 17420 photons MeV-1 , as well as a low detection limit of 208.65 nGy s-1 that is 26 times lower than the medical standard (5.5 μGy s-1 ). More importantly, a high X-ray imaging resolution of 16 lp/mm based on SC-Ag screen is demonstrated. Besides, rigid core skeleton reinforced by metallophilicity endows clusters M6 S6 L6 strong resistance to humidity and radiation. This work provides a new view for the design of efficient scintillators and opens the research door for silver clusters in scintillation application.

Time-resolved radioluminescence of the Cu(I) cluster Cu4I62-. Different responses to photo, X-ray, β-ray and α-particle excitation

Time-resolved radioluminescence (TRRL) properties of the Cu(I) cluster Cu4I62- upon pulsed X-ray, β-ray or α-particle excitation are described. The longer (>2 μs) TRRL component displays exponential decay comparable to pulsed UV excitation; however, temporal behaviour at shorter times indicates that high local excited state density provides an alternative decay channel.

Electronic Tuning of Photoexcited Dynamics in Heteroleptic Cu(I) Complex Photosensitizers

Photoexcited dynamics of heteroleptic Cu(I) complexes as noble-metal-free photosensitizers are closely intertwined with the nature of their ligands. By utilizing ultrafast optical and X-ray transient absorption spectroscopies, we characterized a new set of heteroleptic Cu(I) complexes [Cu(PPh3)2(BPyR)]+ (R = CH3, H, Br to COOCH3), with an increase in the electron-withdrawing ability of the functional group (R). We found that after the transient photooxidation of Cu(I) to Cu(II), the increasing electron-withdrawing ability of R barely affects the internal conversion (IC) (e.g., Jahn-taller (JT) distortion) between singlet MLCT states. However, it does accelerate the dynamics of intersystem crossing (ISC) between singlet and triplet MLCT states and the subsequent decay from the triplet MLCT state to the ground state. The associated lifetime constants are reduced by up to 300%. Our understanding of the photoexcited dynamics in heteroleptic Cu(I) complexes through ligand electronic tuning provides valuable insight into the rational design of efficient Cu(I) complex photosensitizers.

Theoretical studies on thermally activated delayed fluorescence of "carbene-metal-amide" Cu and Au complexes: geometric structures, excitation characters, and mechanisms

"Carbene-metal(I)-amide" (CMA) complexes have garnered significant attention due to their remarkable properties and potential TADF applications in organic electronics. However, the atomistic working mechanism is still elusive. Herein, we chose two CMA complexes, i.e., cyclic (alkyl)(amino) carbene-copper[gold](I)-carbazole (CAAC-Cu[Au]-Cz), and employed both DFT and TD-DFT methods, in combination with radiative and nonradiative rate calculations, to investigate geometric and electronic structures of these two complexes in the ground and excited states, including orbital compositions, electronic transitions, absorption and emission spectra, and the luminescence mechanism. It is found that the coplanar or perpendicular conformations are coexistent in the ground state (S0), the lowest excited singlet state (S1), and the triplet state (T1). Both the coplanar and perpendicular S1 and T1 states have similar ligand-to-ligand charge transfer (LLCT) character between CAAC and Cz, and some charge-transfer character between metal atoms and ligands, which is beneficial to minimize the singlet-triplet energy gaps (ΔEST) and increase the spin-orbit coupling (SOC). An interesting three-state (S0, S1, T1) model involving two regions (coplanar and perpendicular) is proposed to rationalize the experimental TADF phenomena in the CMA complexes. In addition to the coplanar ones, the perpendicular S1 and T1 states also play a role in promoting the repopulation of the coplanar S1 exciton, which is a primary source for the delayed fluorescence.

Synthesis, structure and photoluminescence of Cu(I) complexes containing new functionalized 1,2,3-triazole ligands

The reaction of a triazole ligand, 2-(1H-1,2,3-triazol-4-yl)pyridine (L1), with 2-bromopyridine afforded three new ligands, 2,2'-(1H-1,2,3-triazole-1,4-diyl)dipyridine (L2), 2,2'-(2H-1,2,3-triazole-2,4-diyl)dipyridine (L3) and 2,2'-(1H-1,2,3-triazole-1,5-diyl)dipyridine (L4). A series of luminescent mononuclear copper(I) complexes of these ligands [Cu(Ln)(P^P)](ClO4) [n = 1, P^P = (PPh3)2 (1); n = 1, P^P = POP (2); n = 2, P^P = (PPh3)2 (3); n = 2, P^P = POP (4); n = 3, P^P = (PPh3)2 (5); n = 3, P^P = POP (6); n = 4, P^P = (PPh3)2 (9); n = 4, P^P = POP (10)] have been obtained from the reaction of Ln with [Cu(MeCN)4]ClO4 in the presence of PPh3 and POP. L3 was also found to form dinuclear compounds [Cu2(L3)(PPh3)4](ClO4)2 (7) and [Cu2(L3)(POP)2](ClO4)2 (8). All of the Cu(I) compounds have been characterized by IR, UV/vis, CV, 1H NMR, and 31P{1H} NMR. The molecular structures of 1-3, 5, and 7 have been further determined by X-ray crystallography. In CH2Cl2 solutions, these Cu(I) complexes exhibit tunable green to orange emissions (563-621 nm) upon excitation at λex = 380 nm. In the solid state, these complexes show intense emissions and it is interesting to note that 1 and 3 are blue-light emitters. Density functional theory (DFT) calculations revealed that the lowest energy electronic transition associated with these complexes predominantly originates from metal-to-ligand charge transfer transitions (MLCT).

Self-Trapped, Thermally Equilibrated Delayed Fluorescence Enables Low-Reabsorption Luminescent Solar Concentrators Based on Gold-Doped Silver Nanoclusters

Reabsorption-free luminescent solar concentrators (LSCs) are crucial ingredients for photovoltaic windows. Atomically precise metal nanoclusters (NCs) with large Stokes-shifted photoluminescence (PL) hold great promise for applications in LSCs. However, a fundamental understanding of the PL mechanism, particularly on the excited-state interaction and exciton kinetics, is still lacking. Herein, we studied the exciton-phonon coupling and singlet/triplet exciton dynamics for gold-doped silver NCs in a solid matrix. Following photoexcitation, the excitons can be self-trapped via strong exciton-phonon coupling. Subsequently, rapid thermal equilibration between the singlet and triplet states occurs due to the coexistence of small energy splitting and spin-orbit coupling. Finally, broadband delayed fluorescence with a large Stokes shift can be generated, namely, self-trapped, thermally equilibrated delayed fluorescence (ST-TEDF). Benefiting from superior ST-TEDF, we demonstrated efficient LSCs with minimized reabsorption.

Columnar Liquid Crystals of Copper(I) Complexes with Ionic Conductivity and Solid State Emission

Two neutral copper(I) halide complexes ([Cu(BTU)₂X], X = Cl, Br) were prepared by the reduction of the corresponding copper(II) halides (chloride or bromide) with a benzoylthiourea (BTU, N-(3,4-diheptyloxybenzoyl)-N'-(4-heptadecafluorooctylphenyl)thiourea) ligand in ethanol. The two copper(I) complexes show a very interesting combination of 2D supramolecular structures, liquid crystalline, emission, and 1D ionic conduction properties. Their chemical structure was ascribed based on ESI-MS, elemental analysis, IR, and NMR spectroscopies (¹H and ¹³C), while the mesomorphic behavior was analyzed through a combination of differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and powder X-ray diffraction (XRD). These new copper(I) complexes have mesomorphic properties and exhibit a hexagonal columnar mesophase over a large temperature range, more than 100 K, as evidenced by DSC studies and POM observations. The thermogravimetric analysis (TG) indicated a very good thermal stability of these samples up to the isotropization temperatures and over the whole temperature range of the liquid crystalline phase existence. Both complexes displayed a solid-state emission with quantum yields up to 8% at ambient temperature. The electrical properties of the new metallomesogens were investigated by variable temperature dielectric spectroscopy over the entire temperature range of the liquid crystalline phase. It was found that the liquid crystal phases favoured anhydrous proton conduction provided by the hydrogen-bonding networks formed by the NH…X moieties (X = halide or oxygen) of the benzoylthiourea ligand in the copper(I) complexes. A proton conductivity of 2.97 × 10^-7 S·cm^-1 was achieved at 430 K for the chloro-complex and 1.37 × 10^-6 S·cm^-1 at 440K for the related bromo-complex.

Thermally activated delayed fluorescence of a Ir(III) complex: absorption and emission properties, nonradiative rates, and mechanism

One recent experimental study reported a Ir(III) complex with thermally activated delayed fluorescence (TADF) phenomenon in solution, but its luminescent mechanism is elusive. In this work, we combined density functional theory (DFT), time-dependent DFT (TDDFT) and multi-state complete active space second-order perturbation theory (MS-CASPT2) methods to investigate excited-state properties, photophysics, and emission mechanism of this Ir(III) complex. Two main absorption bands observed in experiments can be attributed to the electronic transition from the S₀ state to the S₁ and S₂ states; while, the fluorescence and phosphorescence are generated from the S₁ and T₁ states, respectively. Both the S₁ and T₁ states have clear metal-to-ligand charge transfer (MLCT) character. The present computational results reveal a three-state model including the S₀, S₁ and T₁ states to rationalize the TADF behavior. The small energy gap between the S₁ and T₁ states benefits the forward and reverse intersystem crossing (ISC and rISC) processes. At 300 K, the rISC rate is five orders of magnitude larger than the phosphorescence rate therefore enabling TADF. At 77 K, the rISC rate is sharply decreased but remains close to the phosphorescence rate; therefore, in addition to the phosphorescence, the delayed fluorescence could also contribute to the experimental emission. The estimated TADF lifetime agrees well with experiments, 9.80 vs. 6.67 μs, which further verifies this three-state model.

Photochemical and Photophysical Dynamics of the Aqueous Ferrate(VI) Ion

Ferrate(VI) has the potential to play a key role in future water supplies. Its salts have been suggested as "green" alternatives to current advanced oxidation and disinfection methods in water treatment, especially when combined with ultraviolet light to stimulate generation of highly oxidizing Fe(V) and Fe(IV) species. However, the nature of these intermediates, the mechanisms by which they form, and their roles in downstream oxidation reactions remain unclear. Here, we use a combination of optical and X-ray transient absorption spectroscopies to study the formation, interconversion, and relaxation of several excited-state and metastable high-valent iron species following excitation of aqueous potassium ferrate(VI) by ultraviolet and visible light. Branching from the initially populated ligand-to-metal charge transfer state into independent photophysical and photochemical pathways occurs within tens of picoseconds, with the quantum yield for the generation of reactive Fe(V) species determined by relative rates of the competing intersystem crossing and reverse electron transfer processes. Relaxation of the metal-centered states then occurs within 4 ns, while the formation of metastable Fe(V) species occurs in several steps with time constants of 250 ps and 300 ns. Results here improve the mechanistic understanding of the formation and fate of Fe(V) and Fe(IV), which will accelerate the development of novel advanced oxidation processes for water treatment applications.

Effect of the dangling aromatic ring on neutral luminescent bis(phosphine) Cu(i)/Ag(i) complexes with the asymmetric pyridyl-tetrazolate ligands

A series of neutral luminescent bis(phosphine) Cu(i) complexes of pyridyl-tetrazolate ligands (L1-L3) with the general formula [CuI(L n )(P^P)] (1-6) were synthesized, which have been well characterized by IR, UV/vis, CV, 1H NMR and 31P NMR. For comparison, an Ag(i) complex [AgI(L2)(PPh3)2] (7) was also synthesized. The crystal structures of 2 and 7 have been further determined by X-ray crystallography. All these Cu(i) compounds show bright luminescence in the solid state with photoluminescence quantum yields (PLQYs) in the range of 25.8% to 85.0%. More interestingly, the Cu(i) complexes bearing an additional dangling aromatic ring on the diimine ligands exhibit enhanced luminescent performance in various solutions and their PLQYs are significantly higher than those of related Cu(i) complexes without steric protection. Compared with 1, the Cu(i) complexes with an additional dangling tetrazole moiety show a significant solvatochromic effect, which is uncommon for luminescent Cu(i) complexes. Moreover, [CuI(L2)(PPh3)2] (2) was further designed as an OLED material, which showed a high external quantum efficiency of 7.7%.

Engineering the Dynamics and Transport of Excitons, Trions, and Biexcitons in Monolayer WS2

The study of transport and diffusion dynamics of quasi-particles such as excitons, trions, and biexcitons in two-dimensional (2D) semiconductors has opened avenues for their application in high-speed excitonic and optoelectronic devices. However, long-range transport and fast diffusion of these quasi-particles have not been reported for 2D systems such as transition metal dichalcogenides (TMDCs). The reported diffusion coefficients from TMDCs are low, limiting their use in high-speed excitonic devices and other optoelectronic applications. Here, we report the highest exciton diffusion coefficient value in monolayer WS₂ achieved via engineering the radiative lifetime and diffusion lengths using static back-gate voltage and substrate engineering. Electrostatic doping is observed to modulate the radiative lifetime and in turn the diffusion coefficient of excitons by ∼three times at room temperature. By combining electrostatic doping and substrate engineering, we push the diffusion coefficient to an extremely high value of 86.5 cm²/s, which has not been reported before in TMDCs and is even higher than the values in some 1D systems. At low temperatures, we further report the control of dynamic and spatial diffusion of excitons, trions, and biexcitons from WS₂. The electrostatic control of dynamics and transport of these quasi-particles in monolayers establishes monolayer TMDCs as ideal candidates for high-speed excitonic circuits, optoelectronic, and photonic device applications.

Novel Copper Complexes as Visible Light Photoinitiators for the Synthesis of Interpenetrating Polymer Networks (IPNs)

This work is devoted to the study of two copper complexes (Cu) bearing pyridine ligands, which were synthesized, evaluated and tested as new visible light photoinitiators for the free radical photopolymerization (FRP) of acrylates functional groups in thick and thin samples upon light-emitting diodes (LED) at 405 and 455 nm irradiation. These latter wavelengths are considered to be safe to produce polymer materials. The photoinitiation abilities of these organometallic compounds were evaluated in combination with an iodonium (Iod) salt and/or amine (e.g., N-phenylglycine—NPG). Interestingly, high final conversions and high polymerization rates were obtained for both compounds using two and three-component photoinitiating systems (Cu1 (or Cu2)/Iodonium salt (Iod) (0.1%/1% w/w) and Cu1 (or Cu2)/Iod/amine (0.1%/1%/1% w/w/w)). The new proposed copper complexes were also used for direct laser write experiments involving a laser diode at 405 nm, and for the photocomposite synthesis with glass fibers using a UV-conveyor at 395 nm. To explain the obtained polymerization results, different methods and characterization techniques were used: steady-state photolysis, real-time Fourier transform infrared spectroscopy (RT-FTIR), emission spectroscopy and cyclic voltammetry.

TADF: Enabling luminescent copper(i) coordination compounds for light-emitting electrochemical cells

The last decade has seen a surge of interest in the emissive behaviour of copper(i) coordination compounds, both neutral compounds that may have applications in organic light-emitting doides (OLEDs) and copper-based ionic transition metal complexes (Cu-iTMCs) with potential use in light-emitting electrochemical cells (LECs). One of the most exciting features of copper(i) coordination compounds is their possibility to exhibit thermally activated delayed fluorescence (TADF) in which the energy separation of the excited singlet (S1) and excited triplet (T1) states is very small, permitting intersystem crossing (ISC) and reverse intersystem crossing (RISC) to occur at room temperature without the requirement for the large spin-orbit coupling inferred by the presence of a heavy metal such as iridium. In this review, we focus mainly in Cu-iTMCs, and illustrate how the field of luminescent compounds and those exhibiting TADF has developed. Copper(i) coordination compounds that class as Cu-iTMCs include those containing four-coordinate [Cu(P^P)(N^N)]+ (P^P = large-bite angle bisphosphane, and N^N is typically a diimine), [Cu(P)2(N^N)]+ (P = monodentate phosphane ligand), [Cu(P)(tripodal-N3)]+, [Cu(P)(N^N)(N)]+ (N = monodentate N-donor ligand), [Cu(P^P)(N^S)]+ (N^S = chelating N,S-donor ligand), [Cu(P^P)(P^S)]+ (P^S = chelating P,S-donor ligand), [Cu(P^P)(NHC)]+ (NHC = N-heterocyclic carbene) coordination domains, dinuclear complexes with P^P and N^N ligands, three-coordinate [Cu(N^N)(NHC)]+ and two-coordinate [Cu(N)(NHC)]+ complexes. We pay particular attention to solid-state structural features, e.g. π-stacking interactions and other inter-ligand interactions, which may impact on photoluminescence quantum yields. Where emissive Cu-iTMCs have been tested in LECs, we detail the device architectures, and this emphasizes differences which make it difficult to compare LEC performances from different investigations.

NAST: Nonadiabatic Statistical Theory Package for Predicting Kinetics of Spin-Dependent Processes

We present a nonadiabatic statistical theory (NAST) package for predicting kinetics of spin-dependent processes, such as intersystem crossings, spin-forbidden unimolecular reactions, and spin crossovers. The NAST package can calculate the probabilities and rates of transitions between the electronic states of different spin multiplicities. Both the microcanonical (energy-dependent) and canonical (temperature-dependent) rate constants can be obtained. Quantum effects, including tunneling, zero-point vibrational energy, and reaction path interference, can be accounted for. In the limit of an adiabatic unimolecular reaction proceeding on a single electronic state, NAST reduces to the traditional transition state theory. Because NAST requires molecular properties at only a few points on potential energy surfaces, it can be applied to large molecular systems, used with accurate high-level electronic structure methods, and employed to study slow nonadiabatic processes. The essential NAST input data include the nuclear Hessian at the reactant minimum, as well as the nuclear Hessians, energy gradients, and spin-orbit coupling at the minimum energy crossing point (MECP) between two states. The additional computational tools included in the NAST package can be used to extract the required input data from the output files of electronic structure packages, calculate the effective Hessian at the MECP, and fit the reaction coordinate for more advanced NAST calculations. We describe the theory, its implementation, and three examples of application to different molecular systems.

Tumor-Microenvironment-Activated Reactive Oxygen Species Amplifier for Enzymatic Cascade Cancer Starvation/Chemodynamic /Immunotherapy

At present, some progress has been made in the field of cancer theranostics based on nanocatalysts (NCs), but achieving precise theranostics in response to the specific tumor microenvironment (TME) remains a major challenge. Herein, a TME-responsive upconversion nanoparticles (UCNPs)-based smart UCNPs@Cu-Cys-GOx (UCCG) nanosystem is engineered, which combines natural enzymes and nanozymes so as to amplify reactive oxygen species (ROS) generation in situ for cancer starvation/chemodynamic/immunotherapy. One of the biggest merits of this material is that it can be preserved inert (off) in normal tissues, and only in the TME can it be specifically activated (on) through a series of enzymatic cascades to boost ROS production via a strategy of open source (H₂ O₂ self-supplying ability) and reduce expenditure (glutathione (GSH) consuming ability). More importantly, the enhanced oxidative stress by UCCG NCs reverses the immunosuppressive TME, and facilitates antitumor immune responses. Meanwhile, the starvation/chemodynamic synergistic therapy triggered by UCCG combined with PD-L1 antibody effectively inhibits the growth of primary tumors and cancer metastasis. In addition, the UCNPs in UCCG present upconversion luminescence enhancement, which can be exploited to visualize the reinforced ROS generation in real time. Collectively, this work provides an original method for the devising and exploitation of UCNPs-based catalytic immunotherapy.

Time-Resolved Spectroscopy and Electronic Structure of Mono-and Dinuclear Pyridyl-Triazole/DPEPhos-Based Cu(I) Complexes

Chemical and spectroscopic characterization of the mononuclear photosensitizers [(DPEPhos)Cu(I)(MPyrT)]0/+ (CuL, CuLH) and their dinuclear analogues (Cu2 L', Cu2 L'H2 ), backed by (TD)DFT and high-level GW-Bethe-Salpeter equation calculations, exemplifies the complex influence of charge, nuclearity and structural flexibility on UV-induced photophysical pathways. Ultrafast transient absorption and step-scan FTIR spectroscopy reveal flattening distortion in the triplet state of CuLH as controlled by charge, which also appears to have a large impact on the symmetry of the long-lived triplet states in Cu2 L' and Cu2 L'H2 . Time-resolved luminescence spectroscopy (solid state), supported by transient photodissociation spectroscopy (gas phase), confirm a lifetime of some tens of μs for the respective triplet states, as well as the energetics of thermally activated delayed luminescence, both being essential parameters for application of these materials based on earth-abundant copper in photocatalysis and luminescent devices.

Negative/Zero Thermal Quenching of Luminescence via Electronic Structural Transition in Copper-Iodide Cluster-Based Coordination Networks

Photoluminescence (PL) intensity in organic or metal-organic emitters usually suffers from thermal quenching (TQ), which severely hinders their industrial applications. The development of negative thermal quenching (NTQ) and/or zero thermal quenching (ZTQ) materials depends on a better understanding of the mechanisms underpinning TQ in luminescent solids. In this work, we investigated the temperature dependence of thermally activated delayed fluorescence (TADF) in copper(I)-organic coordination polymers (CP) ligated with an imidazole or triazole derivative over a broad temperature range. The efficient PL emission of CP1 is heavily quenched as the crystalline samples are cooled to 77 K; the PL intensity shows the NTQ effect in the region of 77-238 K followed by a ZTQ effect in the temperature range of 238-318 K. No NTQ or ZTQ effect is observed for reference coordination polymer CP2, where the 1,2,4-triazole group was used instead of the imidazole one. Our work highlights the important role of the ligand's electronic structure in optimizing photophysical properties of coordination polymer emitters and may stimulate new efforts to design luminescent materials exhibiting NTQ and ZTQ effect at higher temperature.

Molecular design of efficient yellow- to red-emissive alkynylgold(iii) complexes for the realization of thermally activated delayed fluorescence (TADF) and their applications in solution-processed organic light-emitting devices

Here, we report the design and synthesis of a new class of fused heterocyclic alkynyl ligand-containing gold(iii) complexes, which show tunable emission colors spanning from the yellow to red region in the solid state and exhibit thermally activated delayed fluorescence (TADF) properties. These complexes display high photoluminescence quantum yields of up to 0.87 and short excited-state lifetimes in sub-microsecond timescales, yielding high radiative decay rate constants on the order of up to 10⁶ s⁻¹. The observation of the drastic enhancement in the emission intensity of the complexes with insignificant change in the excited-state lifetime upon increasing the temperature from 200 to 360 K indicates an increasing radiative decay rate. The experimentally estimated energy splitting between the lowest-lying singlet excited state (S₁) and the lowest-lying triplet excited state (T₁), ΔE_S1–T1, is found to be as small as ∼0.03 eV (250 cm⁻¹), comparable to the value of ∼0.05 eV (435 cm⁻¹) obtained from computational studies. The delicate choice of the cyclometalating ligand and the fused heterocyclic ligand is deemed the key to induce TADF through the control of the energy levels of the intraligand and the ligand-to-ligand charge transfer excited states. This work represents the realization of highly emissive yellow- to red-emitting gold(iii) TADF complexes incorporated with fused heterocyclic alkynyl ligands and their applications in organic light-emitting devices.

We report the design of a new class of fused heterocyclic alkynyl ligand-containing gold(iii) complexes, which shows tunable emission colors spanning yellow to red region and exhibits thermally activated delayed fluorescence (TADF) properties.

Room-Temperature Phosphorescence and Thermally Activated Delayed Fluorescence in the Pd Complex: Mechanism and Dual Upconversion Channels

The Pd complex PdN3N exhibits an unusual dual emission of room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), but the mechanism is elusive. Herein, we employed both density functional theory (DFT) and time-dependent DFT (TD-DFT) methods to explore excited-state properties of this Pd complex, which shows that the S₀, S₁, T₁, and T₂ states are involved in the luminescence. Both the S₁ → T₁ and S₁ → T₂ intersystem crossing (ISC) processes are more efficient than the S₁ fluorescence and insensitive to temperature. However, the direct T₁ → S₁ and T₂-mediated T₁ → T₂ → S₁ reverse ISC (rISC) processes change remarkably with temperature. At 300 K, these two processes are more efficient than the T₁ phosphorescence and therefore enable TADF. Importantly, the T₁ → S₁ rISC and T₁ phosphorescence rates are comparable at 300 K, which leads to dual emissions of TADF and RTP, whereas these two channels become blocked at 100 K so that only the T₁ phosphorescence is recorded experimentally.

Near-Infrared Luminescence in Trinuclear Mixed-Metal Chalcogenolate Complexes of the Types [M2Ti(EPh)6(PPh3)2] (M = Cu, Ag; E = S, Se) and [Na(thf)3]2[Ti(SPh)6]

The optical properties of four new trinuclear chalcogenolato bridged metal complexes [Ag₂Ti(SPh)₆(PPh₃)₂], [Na(thf)₃]₂[Ti(SPh)₆], [Cu₂Ti(SePh)₆(PPh₃)₂], and [Ag₂Ti(SePh)₆(PPh₃)₂] have been investigated by absorption and photoluminescence spectroscopy as well as time-dependent density functional theory (TDDFT) calculations and compared to the results published recently for [Cu₂Ti(SPh)₆(PPh₃)₂]. All of these compounds are distinguished by efficient near-infrared luminescence at ∼880-1200 nm in the solid state at low temperatures, which remains quite intense for the copper-titanium complexes at ambient temperature with PL quantum yields of 9.5 and 4.8% at λ_PL = 1090 and 1240 nm for [Cu₂Ti(EPh)₆(PPh₃)₂], E = S, Se, respectively. According to the calculations, a peculiar feature of the lowest-energy electronic transitions in these complexes is their high localization on the metal and chalcogen atoms, with negligible contributions of the "external" ligand groups. Correspondingly, the type of atoms in the M₂TiE₆ (M = Cu, Ag, Na) core structure determines optical properties such as the absorption and emission wavelengths and PL lifetime.

P∩N Bridged Cu(I) Dimers Featuring Both TADF and Phosphorescence. From Overview towards Detailed Case Study of the Excited Singlet and Triplet States

We present an overview over eight brightly luminescent Cu(I) dimers of the type Cu₂X₂(P∩N)₃ with X = Cl, Br, I and P∩N = 2-diphenylphosphino-pyridine (Ph₂Ppy), 2-diphenylphosphino-pyrimidine (Ph₂Ppym), 1-diphenylphosphino-isoquinoline (Ph₂Piqn) including three new crystal structures (Cu₂Br₂(Ph₂Ppy)₃ 1-Br, Cu₂I₂(Ph₂Ppym)₃ 2-I and Cu₂I₂(Ph₂Piqn)₃ 3-I). However, we mainly focus on their photo-luminescence properties. All compounds exhibit combined thermally activated delayed fluorescence (TADF) and phosphorescence at ambient temperature. Emission color, decay time and quantum yield vary over large ranges. For deeper characterization, we select Cu₂I₂(Ph₂Ppy)₃, 1-I, showing a quantum yield of 81%. DFT and SOC-TDDFT calculations provide insight into the electronic structures of the singlet S₁ and triplet T₁ states. Both stem from metal+iodide-to-ligand charge transfer transitions. Evaluation of the emission decay dynamics, measured from 1.2 ≤ T ≤ 300 K, gives ∆E(S₁-T₁) = 380 cm⁻¹ (47 meV), a transition rate of k(S₁→S₀) = 2.25 × 10⁶ s⁻¹ (445 ns), T₁ zero-field splittings, transition rates from the triplet substates and spin-lattice relaxation times. We also discuss the interplay of S₁-TADF and T₁-phosphorescence. The combined emission paths shorten the overall decay time. For OLED applications, utilization of both singlet and triplet harvesting can be highly favorable for improvement of the device performance.

Controlling Thermally Activated Delayed Photoluminescence in CdSe Quantum Dots through Triplet Acceptor Surface Coverage

Quantum-dot/molecule composites (QD/mol) have demonstrated useful photochemical properties for many photonic and optoelectronic applications; however, a comprehensive understanding of these materials remains elusive. This work introduces a series of cadmium(II) selenide/1-pyrenecarboxylic acid (CdSe/PCA) nanomaterials featuring bespoke PCA surface coverage on CdSe585 (coded by the peak of the first exciton absorption band) to glean insight into the QD/mol photophysical behavior. Tailoring the energy gap between the CdSe585 first exciton band (2.1 eV) and the lowest PCA triplet level (T₁ = 2.0 eV) to be nearly isoenergetic, strong thermally activated delayed photoluminescence (TADPL) is observed resulting from reverse triplet-triplet energy transfer. The resultant average decay time constant (τ_obs) of the photoluminescence emanating from CdSe585 is deterministically controlled with surface-bound PCA_n chromophores (n = average number of adsorbed PCA molecules) by shifting the triplet excited state equilibrium from the CdSe585 to the PCA molecular triplet reservoir as a function of n.

Tumor Microenvironment-"AND" Near-Infrared Light-Activated Coordination Polymer Nanoprodrug for On-Demand CO-Sensitized Synergistic Cancer Therapy

Carbon monoxide (CO) as an emerging treatment holds great promise for inducing the apoptosis of cancer cells. Here coordination assembled strategy is first reported for synthesis of Cu(II)-flavone coordination polymer (NCu-FleCP) CO nanoprodrug that is stable in normal physiological conditions, and yet readily reduces to small size prodrug complex and releases CO on demand under glutathione (GSH) and near infrared (NIR) light. Specifically, after uptaking by cancer cells, local GSH attacked coordination bond within NCu-FleCP, resulting in the release of Cu(I) and free Fle. The CC bond of Fle is cleavage under NIR light to release CO for gas therapy, and Cu(I) reacts with local H₂ O₂ through Fenton like reaction to generate hydroxyl radicals (^• OH) for chemodynamic therapy. Detailed in vitro and in vivo experiments demonstrate that the CO prodrug system in generating a sufficient quantity of CO and ^• OH offers remarkable destructive effects against cancer cells without causing toxicity to surrounding normal tissues. The study provides a solid foundation to develop smart coordination polymer CO prodrugs with on-demand CO release, enhanced permeability and retention effect, and biodegradability for multimodal synergistic therapy.

Ultrabright Au@Cu14 nanoclusters: 71.3% phosphorescence quantum yield in non-degassed solution at room temperature

The photoluminescence of metal nanoclusters is typically low, and phosphorescence emission is rare due to ultrafast free-electron dynamics and quenching by phonons. Here, we report an electronic engineering approach to achieving very high phosphorescence (quantum yield 71.3%) from a [Au@Cu₁₄(SPh ^t Bu)₁₂(PPh(C₂H₄CN)₂)₆]⁺ nanocluster (abbreviated Au@Cu ₁₄ ) in non-degassed solution at room temperature. The structure of Au@Cu ₁₄ has a single-Au-atom kernel, which is encapsulated by a rigid Cu(I) complex cage. This core-shell structure leads to highly efficient singlet-to-triplet intersystem crossing and suppression of nonradiative energy loss. Unlike the phosphorescent organic materials and organometallic complexes-which require de-aerated conditions due to severe quenching by air (i.e., O₂)-the phosphorescence from Au@Cu ₁₄ is much less sensitive to air, which is important for lighting and biomedical applications.

Understanding the luminescence properties of Cu(i) complexes: a quantum chemical perusal

Electronic structures and excited-state properties of Cu(i) complexes with varying coordination numbers have been investigated by means of advanced quantum chemical methods. The computational protocol employs density functional-based methods for geometry optimizations and vibrational analyses including solvent effects through continuum models. Excitation energies, spin-orbit couplings and luminescence properties are evaluated using multireference configuration interaction methods. Rate constants of spin-allowed and spin-forbidden transitions have been determined according to the Fermi golden rule. The computational results for the 4-coordinate (DPEPhos)Cu(PyrTet), the 3-coordinate [IPr-Cu-Py₂]⁺, and the linear CAAC^Me₂-Cu-Cl complexes agree well with experimental absorption and emission wavelengths, intersystem crossing (ISC) time constants, and radiative lifetimes in liquid solution. Spectral shifts on the ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfer (MLCT) transitions caused by the polarity of the environment are well represented by the continuum models whereas the shifts caused by pseudo-Jahn-Teller distortions in the MLCT states are too pronounced in comparison to solid-state data. Systematic variation of the ligands in linear Cu(i) carbene complexes shows that only those complexes with S₁ and T₁ states of LLCT character possess sufficiently small singlet-triplet energy gaps ΔE_ST to enable thermally activated delayed fluorescence (TADF). Complexes whose S₁ and T₁ wavefunctions are dominated by MLCT excitations tend to emit phosphorescence instead. Unlike the situation in metal-free TADF emitters, the presence of low-lying locally excited triplet states does not promote ISC. These states rather hold the danger of trapping the excitation with nonradiative deactivation being the major deactivation channel.

Real-time observation of molecular flattening and intersystem crossing in [(DPEPhos)Cu(i)(PyrTet)] via ultrafast UV/Vis- and mid-IR spectroscopy on solution and solid samples

The primary photo-induced processes in the mononuclear, heteroleptic Cu(i) complex [(DPEPhos)Cu(PyrTet)] (1), relevant for OLED applications, were investigated in various solvents and in solid state samples via femtosecond (fs) time resolved UV/Vis and fs time resolved mid-IR transient absorption spectroscopy (TA) with MLCT excitation around 340 nm. UV/Vis fs-TA on 1 in solution reveals (i) a severe blue-shift of excited state absorption on the time scale of a few picoseconds (τ₂) that is not observed in solids, and (ii), on the time scale of several tens of picoseconds (τ₃), a process with very similar dynamics in all samples. Mid-IR fs-TA in solution indicates structural changes with τ₂. Transient absorption anisotropy experiments temporally resolve a viscosity-dependent change of the excited state transition dipole moment orientation with τ₂, as quantitatively predicted for the relaxed S₁-state via TD-DFT. The results strongly suggest flattening distortion (FD) and structural rearrangement of the PyrTet-moiety to occur on the time scale of τ₂ in liquid phase, and to be suppressed in solid phase. Moreover, intersystem crossing (ISC) is assigned to the process described by τ₃, in line with its direct observation via time-resolved luminescence spectroscopy on 1 by Bergmann et al. (Sci. Adv., 2016, 2, e1500889). Overall, the use of structure-sensitive methods and the direct comparison of different preparations of 1 (i.e. solution vs. solid state), are a unique basis for a clear assignment of spectro-temporal characteristics to fundamental deactivation processes such as FD and ISC.

Supertransport of excitons in atomically thin organic semiconductors at the 2D quantum limit

Long-range and fast transport of coherent excitons is important for the development of high-speed excitonic circuits and quantum computing applications. However, most of these coherent excitons have only been observed in some low-dimensional semiconductors when coupled with cavities, as there are large inhomogeneous broadening and dephasing effects on the transport of excitons in their native states in materials. Here, by confining coherent excitons at the 2D quantum limit, we first observed molecular aggregation-enabled 'supertransport' of excitons in atomically thin two-dimensional (2D) organic semiconductors between coherent states, with a measured high effective exciton diffusion coefficient of ~346.9 cm2/s at room temperature. This value is one to several orders of magnitude higher than the values reported for other organic molecular aggregates and low-dimensional inorganic materials. Without coupling to any optical cavities, the monolayer pentacene sample, a very clean 2D quantum system (~1.2 nm thick) with high crystallinity (J-type aggregation) and minimal interfacial states, showed superradiant emission from Frenkel excitons, which was experimentally confirmed by the temperature-dependent photoluminescence (PL) emission, highly enhanced radiative decay rate, significantly narrowed PL peak width and strongly directional in-plane emission. The coherence in monolayer pentacene samples was observed to be delocalised over ~135 molecules, which is significantly larger than the values (a few molecules) observed for other organic thin films. In addition, the supertransport of excitons in monolayer pentacene samples showed highly anisotropic behaviour. Our results pave the way for the development of future high-speed excitonic circuits, fast OLEDs, and other optoelectronic devices.

Auto-Phase-Locked Time-Resolved Luminescence Detection: Principles, Applications, and Prospects

Time-resolved luminescence measurement is a useful technique which can eliminate the background signals from scattering and short-lived autofluorescence. However, the relative instruments always require pulsed excitation sources and high-speed detectors. Moreover, the excitation and detecting shutter should be precisely synchronized by electronic phase matching circuitry, leading to expensiveness and high-complexity. To make time-resolved luminescence instruments simple and cheap, the automatic synchronization method was developed by using a mechanical chopper acted as both of the pulse generator and detection shutter. Therefore, the excitation and detection can be synchronized and locked automatically as the optical paths fixed. In this paper, we first introduced the time-resolved luminescence measurements and review the progress and current state of this field. Then, we discussed low-cost time-resolved techniques, especially chopper-based time-resolved luminescence detections. After that, we focused on auto-phase-locked method and some of its meaningful applications, such as time-gated luminescence imaging, spectrometer, and luminescence lifetime detection. Finally, we concluded with a brief outlook for auto-phase-locked time-resolved luminescence detection systems.