Long-lived Triplet State Research Articles

Protonation Weakens the Influence of Ribose on Triplet Decay of 2-Thiocytidine.

Long-lived triplet states are critical intermediates of thiobases for their applications in photodynamic therapy and as photoprobes for DNA/RNA-protein interactions, where thiobases are embedded in DNA/RNA and exist as thionucleosides. However, sugar moieties accelerate triplet decay rates, which is a common issue that must be resolved for thionucleosides. Here, we explore whether protonation of 2-thiocytosine (2tCyt) and 2-thiocytidine (2tCyd) under acidic pH can alter their triplet decays. Femtosecond spectroscopy demonstrates that 2tCytH+ and 2tCydH+ exhibit similar triplet lifetimes (3.1 ns), suggesting protonation diminishes the influence of ribose on triplet decay. Notably, the triplet lifetime of 2tCydH+ is 2-fold prolonged compared to 2tCyd. Further calculations reveal protonation effectively weakens the influence of ribose by altering electronic structures of triplet states and inhibiting the photoisomerization leading to rapid decay. These findings point to a new avenue of using protonation to modulate triplet decay dynamics of thionucleosides and develop pH-responsive strategy in photochemical applications.

Efficient Energy and Electron Transfer Photocatalysis with a Coulombic Dyad.

Photocatalysis holds great promise for changing the way value-added molecules are currently prepared. However, many photocatalytic reactions suffer from quantum yields well below 10%, hampering the transition from lab-scale reactions to large-scale or even industrial applications. Molecular dyads can be designed such that the beneficial properties of inorganic and organic chromophores are combined, resulting in milder reaction conditions and improved reaction quantum yields of photocatalytic reactions. We have developed a novel approach for obtaining the advantages of molecular dyads without the time- and resource-consuming synthesis of these tailored photocatalysts. Simply by mixing a cationic ruthenium complex with an anionic pyrene derivative in water a salt bichromophore is produced owing to electrostatic interactions. The long-lived organic triplet state is obtained by static and quantitative energy transfer from the preorganized ruthenium complex. We exploited this so-called Coulombic dyad for energy transfer catalysis with similar reactivity and even higher photostability compared to a molecular dyad and reference photosensitizers in several photooxygenations. In addition, it was shown that this system can also be used to maximize the quantum yield of photoredox reactions. This is due to an intrinsically higher cage escape quantum yield after photoinduced electron transfer for purely organic compounds compared to heavy atom-containing molecules. The combination of laboratory-scale as well as mechanistic irradiation experiments with detailed spectroscopic investigations provided deep mechanistic insights into this easy-to-use photocatalyst class.

Enhanced efficiency of interfacial charge transfer in Ag3PO4/BiPO4@CNTs photocatalyst for amoxicillin degradation: Synergistic effect of p-n heterojunction and conjugated channels

Inspired by the remarkable efficiency in separating photogenerated carriers achieved through the fabrication of conjugated polymeric photocatalysts, we have developed a novel photocatalytic system. This system utilizes carbon nanotubes coated with an Ag3PO4/BiPO4 p-n heterostructure (ABCx) for the degradation of amoxicillin in aquatic environments under visible light. The ABCx conjugated photocatalysts demonstrate excellent efficiency in photogenerated charge separation, material stability, and effective amoxicillin removal. The first-order kinetic constants for ABCx (x[‰]=2, 5, 7) show enhancements of 1.40-fold, 2.53-fold, and 2.10-fold, respectively, compared to those of Ag3PO4/BiPO4 in the degradation of amoxicillin. FT-IR and photochemical conversion studies reveal the presence of delocalized π-bonds within the conjugated system (P = CC = C···C = O), which can lead to the formation of a rapidly generated and long-lived triplet state T1 rather than the lowest singlet excited state S1 generated from either direct excitation or reverse intersystem crossing. In situ XPS analysis confirms that photogenerated charges migrate from BiPO4 to Ag3PO4 through a conjugated redox pathway. Density Functional Theory (DFT) was employed to investigate the work function, determining the Fermi levels of Ag3PO4 and BiPO4 to be -4.27 eV and -5.09 eV, respectively, and further confirming the direction of charge migration. In summary, the incorporation of conjugated π bonds via an inorganic semiconductor heterojunction enhances the efficiency of photogenerated charge separation and transport.

2,3 : 6,7-Naphthalene Bis(dicarboximide) Cyclophane: A Photofunctional Host for Ambient Delayed Fluorescence in Solution.

Harvesting triplet excitons of heavy atom-free purely organic chromophores under aerated conditions is challenging due to the quenching of long-lived triplet states by molecular oxygen and vibrational dissipation. Herein, we show a supramolecular approach of triplet harvesting via mitigating quenching pathways of a triplet harvester. Specifically, we used a host-guest system based on 2,3 : 6,7-naphthalene bis(dicarboximide)-derived cyclophane (NBICy) and carbazole derivative (EtCz). Complexation studies and single-crystal X-ray analysis showed the formation of a rigid host-guest complex (K≈104 M-1 in CCl4), resulting in triplet-exciton stabilization under aerated conditions via mitigating vibrational interference and oxygen quenching. Photophysical studies elucidate the delayed fluorescence emission from the charge-transfer state (1CT) with a quantum yield (QY) of 6-8 % under ambient conditions which increased up to 36 % in an inert atmosphere.

Long Excited-State Lifetimes in Three-Coordinate Copper(I) Complexes via Triplet-Triplet Energy Transfer to Pyrene-Decorated Isocyanides.

There has been much effort to improve excited-state lifetimes in photosensitizers based on earth-abundant first-row transition metals. Copper(I) complexes have gained significant attention in this field, and in most cases, sterically driven approaches are used to optimize their lifetimes. This study presents a series of three-coordinate copper(I) complexes (Cu1-Cu3) where the excited-state lifetime is extended by triplet-triplet energy transfer. The heteroleptic compounds feature a cyclohexyl-substituted β-diketiminate (CyNacNacMe) paired with aryl isocyanide ligands, giving the general formula Cu(CyNacNacMe)(CN-Ar) (CN-dmp = 2,6-dimethylphenyl isocyanide for Cu1; CN-pyr = 1-pyrenyl isocyanide for Cu2; CN-dmp-pyr = 2,6-dimethyl-4-(1-pyrenyl)phenyl isocyanide for Cu3). The nature, energies, and dynamics of the low-energy triplet excited states are assessed with a combination of photoluminescence measurements at room temperature and 77 K, ultrafast transient absorption (UFTA) spectroscopy, and DFT calculations. The complexes with the pyrene-decorated isocyanides (Cu2 and Cu3) exhibit extended excited-state lifetimes resulting from triplet-triplet energy transfer (TTET) between the short-lived charge-transfer excited state (3CT) and the long-lived pyrene-centered triplet state (3pyr). This TTET process is irreversible in Cu3, producing exclusively the 3pyr state, and in Cu2, the 3CT and 3pyr states are nearly isoenergetic, enabling reversible TTET and long-lived 3CT luminescence. The improved photophysical properties in Cu2 and Cu3 result in improvements in activity for both photocatalytic stilbene E/Z isomerization via triplet energy transfer and photoredox transformations involving hydrodebromination and C-O bond activation. These results illustrate that the extended excited-state lifetimes achieved through TTET result in newly conceived photosynthetically relevant earth-abundant transition metal complexes.

Nanoarchitectonics of fluorescent gold nanoclusters: A platform for image guided photodynamic therapy of hypoxic tumor

Metal nanoclusters are atomically precise materials comprising metal core of few atoms exhibiting unique photoluminescence properties, unlike their bigger counterparts. Some metal nanocluster with ligand-to-metal charge transfer, long-lived excited state and excited triplet state contribute to inherent photosensitizing (PS) property. However, the therapeutic efficacy of PDT is hindered by the insufficient oxygen supply (O2) in tumor microenvironment. In the present work, cysteine-capped gold nanocluster (AuC) are studied for their unique molecular architecture for PS efficiency. The co-existence of monodispersed and self-assembled structures contribute to the photoluminescence from the quantum confinement of electronic states and aggregation-induced emission (AIE) based PS property, respectively. In-silico model was performed to study the interaction of cysteine to gold cluster, its ground and excited-state properties and the charge transfer mechanism. The AuC as PS generates cytotoxic radicals in both Type I and Type II photodynamic pathways and the dominant radical species involved were elucidated by EPR spectroscopy. In vitro analysis in HeLa cells showed excellent biocompatibility and bioimaging properties. The intracellular ROS production and Live/Dead assay confirmed the generation of ROS in HeLa cells upon laser irradiation. The image-guided photodynamic property with synergistic Type I and Type II PDT reactions of AuC promises its potential application in cancer therapy in both hypoxic and normoxic conditions

Triplet-Mediator Ligand-Protected Metal Nanocluster Sensitizers for Photon Upconversion.

Triplet-triplet annihilation photon upconversion (TTA-UC) is attracting a great deal of attention as a viable approach to exploit unutilized wavelengths of light in solar-driven devices. Recently, ligand-protected metal nanoclusters have emerged as a compelling platform for serving as triplet sensitizers for TTA-UC. In this study, we developed an atomically precise, triplet-mediator ligand (TL)-protected metal nanocluster, Au2Cu6(S-Adm)6[P(DPA)3]2 (Au2Cu6DPA; S-Adm = 1-adamanthanethiolate, DPA = 9,10-diphenylanthracene). In Au2Cu6DPA, the excitation of the Au2Cu6 core rapidly generates a metal-to-ligand charge transfer state, followed by the formation of the long-lived triplet state (approximately 150 μs) at a DPA site in the TL. By combining Au2Cu6DPA with a DPA annihilator, we achieved a red-to-blue upconversion quantum yield (ΦUCg) of 20.7 ± 0.4% (50% max.) with a low threshold excitation intensity of 36 mW cm-2 at 640 nm. This quantum yield almost reaches the maximum limit achievable using a DPA annihilator and establishes a record-setting value, outperforming previously reported nanocrystal and nanocluster sensitizers. Furthermore, strong upconversion emission based on a pseudo-first-order TTA process was observed under 1 sun illumination, indicating that the Au2Cu6DPA sensitizer holds promise for applications in solar-energy-based systems.

Near-IR-Absorbing Bis-Donor Functionalized Aza-BODIPY Derivatives: Synthesis and Photophysical Study by Using Transient Optical Spectroscopy.

A series of near-IR absorbing 2,6-diarylated BF2-chelated aza-boron-dipyrromethenes (aza-BDPs) derivatives bearing different electron donors (benzene, naphthalene, phenanthrene, phenothiazine and carbazole) were designed and synthesized. The effect of different electron donor substitutions on the photophysical properties was studied by steady-state UV-vis absorption and fluorescence spectra, electrochemical, time-resolved nanosecond transient absorption (ns-TA) spectroscopy and theoretical computations. The UV-vis absorption spectra of AzaBDP-PTZ and AzaBDP-CAR (λabs=710 nm in toluene) showed a bathochromic absorption profile compared with the reference AzaBDP-Ph (λabs=685 nm in toluene), indicating the non-negligible electronic interaction at the ground state between donor and acceptor moieties. Moreover, the fluorescence is almost completely quenched for AzaBDP-PTZ/AzaBDP-CAR (fluorescence quantum yield, ΦF=0.2-0.7 % in toluene) as compared with the AzaBDP-Ph (ΦF=27 % in toluene). However, the apparent intersystem crossing ability of these compounds is poor, based on the singlet oxygen quantum yield (ΦΔ=0.3-1.5 %). The ns-TA spectral study showed typical Bodipy localized triplet state transient features, short-lived excited triplet state for AzaBDP-Ph (τT=53.2 μs) versus significantly long-lived triplet state for AzaBDP-CAR (τT=114 μs) was observed under deaerated experimental conditions. These triplet state lifetimes are much longer than that obtained with diiodoAzaBDP (intramolecular heavy atom effect, τT=1.5~7.2 μs). These information are useful for molecular structure design of triplet photosensitizers, for which longer triplet state lifetimes are usually desired. Theoretical computations displayed that the triplet state is mainly localized on the AzaBDP core, moreover, it was found that the HOMO/LUMO energy gap decreased after introducing donor moieties to the skeleton as compared with the reference.

Quantifying Förster Resonance Energy Transfer from Single Perovskite Quantum Dots to Organic Dyes.

Colloidal quantum dots (QDs) are promising regenerable photoredox catalysts offering broadly tunable redox potentials along with high absorption coefficients. QDs have thus far been examined for various organic transformations, water splitting, and CO2 reduction. Vast opportunities emerge from coupling QDs with other homogeneous catalysts, such as transition metal complexes or organic dyes, into hybrid nanoassemblies exploiting energy transfer (ET), leveraging a large absorption cross-section of QDs and long-lived triplet states of cocatalysts. However, a thorough understanding and further engineering of the complex operational mechanisms of hybrid nanoassemblies require simultaneously controlling the surface chemistry of the QDs and probing dynamics at sufficient spatiotemporal resolution. Here, we probe the ET from single lead halide perovskite QDs, capped by alkylphospholipid ligands, to organic dye molecules employing single-particle photoluminescence spectroscopy with single-photon resolution. We identify a Förster-type ET by spatial, temporal, and photon-photon correlations in the QD and dye emission. Discrete quenching steps in the acceptor emission reveal stochastic photobleaching events of individual organic dyes, allowing a precise quantification of the transfer efficiency, which is >70% for QD-dye complexes with strong donor-acceptor spectral overlap. Our work explores the processes occurring at the QD/molecule interface and demonstrates the feasibility of sensitizing organic photocatalysts with QDs.

Nucleic-acid-base photofunctional cocrystal for information security and antimicrobial applications

Cocrystal engineering is an efficient and simple strategy to construct functional materials, especially for the exploitation of novel and multifunctional materials. Herein, we report two kinds of nucleic-acid-base cocrystal systems that imitate the strong hydrogen bond interactions constructed in the form of complementary base pairing. The two cocrystals studied exhibit different colors of phosphorescence from their monomeric counterparts and show the feature of rare high-temperature phosphorescence. Mechanistic studies reveal that the strong hydrogen bond network stabilizes the triplet state and suppresses non-radiative transitions, resulting in phosphorescence even at 425 K. Moreover, the isolation effects of the hydrogen bond network regulate the interactions between the phosphor groups, realizing the manipulation from aggregation to single-molecule phosphorescence. Benefiting from the long-lived triplet state with a high quantum yield, the generation of reactive oxygen species by energy transfer is also available to utilize for some applications such as in photodynamic therapy and broad-spectrum microbicidal effects. In vitro experiments show that the cocrystals efficiently kill bacteria on a tooth surface and significantly help prevent dental caries. This work not only provides deep insight into the relationship of the structure-properties of cocrystal systems, but also facilitates the design of multifunctional cocrystal materials and enriches their potential applications.

Disruptive Model That Explains for the Long-Lived Triplet States Observed for 2-Thiocytosine upon UVA Radiation.

The possible role of radical species in the formation of the long-lived triplet states observed for 2-thiocytosine upon UV irradiation was theoretically investigated. It is predicted that the radical fragments arising from the homolytic rupture of the NH group of the thiobase can be yielded upon ultraviolet-A radiation. Recombination of the radicals through the most favorable singlet channel yields the lowest-lying tautomer of the 2-thiocytosine (the amino-thiol form) through a barrierless pathway. The rebounding of the radical fragments along the triplet channels that emerge from the attack of the hydrogen to the nitrogen atoms next to the C-S bond leads to stable structures for the amino-thion-N1H and amino-thion-N3H tautomers. These results allow for the rationalization of the near-unity triplet yields observed when this pure light-atom organic molecule is exposed to UV irradiation, without invoking intersystem crossings between the electronic states of different spin-multiplicities. A similar study for cytosine showed that the energy required to induce the homolytic breaking of the N-H bond of the nucleobase is not attainable under UVA radiation. This result is consistent with the experimental fact that no triplet states are observed when this molecule is exposed to that light.

Dual Functionality of 6-Methylthioguanine: Synergistic Effects Enhancing the Photolability of DNA Nucleobases.

A small chemical modification of the nucleobase structure can significantly enhance the photoactivity of DNA, which may incur DNA damage, thus holding promising applications in photochemotherapy treatment of cancers or pathogens. However, single substitution confers only limited phototoxicity to DNA. Herein, we combine femtosecond and nanosecond time-resolved spectroscopy with high-level ab initio calculations to disentangle the excited-state dynamics of 6-methylthioguanine (me6-TG) under variable wavelength UVA excitation (310-330 nm). We find that double substitution of nucleobases (thionation and methylation) boosts the photoactivity by introducing more reactive channels. Intriguingly, 1nNπ*, rather than 1nSπ*, acts as the doorway state engendering the formation of the long-lived reactive triplet state in me6-TG. The 1nNπ* induces a low spin-orbit coupling of 8.3 cm-1, which increases the intersystem crossing (ISC) time (2.91 ± 0.14 ns). Despite the slowed ISC, the triplet quantum yield (ΦT) still accounts for a large fraction (0.6 ± 0.1), consistent with the potential energy surface that favors excited-state bifurcation to 1nNπ*min (3.36 ± 0.15 ps) rather than 1ππ*min (5.05 ± 0.26 ps), such that the subsequent ISC to triplet via 1nNπ*min constitutes the main relaxation pathway in me6-TG. Although this ΦT is inferior to its single-substituted predecessor 6-thioguanine (6-TG, 0.8 ± 0.2), the effect of thionation in synergy with methylation opens a unique C-S bond cleavage pathway through crossing to a repulsive 1πσ* state, generating thiyl radicals as highly reactive intermediates that may invoke biological damage. This photodissociation channel is extremely difficult for conventional nucleobases. These findings demonstrate the synergistic effects of double functionality substitution in modulating excited-state dynamics and enhancing the photolabile character of DNA nucleobases, providing inspirations for the rational design of advanced photodynamic and photochemotherapy approaches.

Long-lived triplet state carbon nitride (urea-CNx) catalyzed metal-free photo-ATRP with oxygen acceleration

A long-lived triplet state urea-functionalized carbon nitride (urea-CNx) was successfully synthesized and applied as a recyclable and efficient photocatalyst for metal free photo-ATRP without any co-catalyst.

Photophysics and photochemistry of thermally activated delayed fluorescence emitters based on the multiple resonance effect: transient optical and electron paramagnetic resonance studies.

The photochemistry of two representative thermally activated delayed fluorescence (TADF) emitters based on the multiple resonance effect (MRE) (DABNA-1 and DtBuCzB) was studied. No significant TADF was observed in fluid solution, although the compounds have a long-lived triplet state (ca. 30 μs). We found that these planar boron molecules bind with Lewis bases, e.g., 4-dimethylaminopyridine (DMAP) or an N-heterocyclic carbene (NHC). A new blue-shifted absorption band centered at 368 nm was observed for DtBuCzB upon formation of the adduct; however, the fluorescence of the adduct is the same as that of the free DtBuCzB. We propose that photo-dissociation occurs for the DtBuCzB-DMAP adduct, which is confirmed by femtosecond transient absorption spectra, implying that fluorescence originates from DtBuCzB produced by photo-dissociation; the subsequent in situ re-binding was observed with nanosecdon transient absorption spectroscopy. No photo-dissociation was observed for the NHC adduct. Time-resolved electron paramagnetic resonance (TREPR) spectra show that the triplet states of DABNA-1 and DtBuCzB have similar zero field splitting (ZFS) parameters (D = 1450 MHz). Theoretical studies show that the slow ISC is due to small SOC and weak Herzberg-Teller coupling, although the S1/T1 energy gap is small (0.14 eV), which rationalizes the lack of TADF.

Dynamics of photoexcited 5-bromouracil and 5-bromo-2'-deoxyuridine studied by extreme ultraviolet time-resolved photoelectron spectroscopy in liquid flat jets.

The UV-induced photo-relaxation dynamics of 5-bromouracil (BrU) and 5-bromo-2'-deoxyuridine (BrUrd) in aqueous solution were investigated using femtosecond time-resolved photoelectron spectroscopy with an extreme ultraviolet (XUV) probe in a flat liquid jet. Upon excitation to the 1ππ* state by 4.66 eV UV photons, both molecules exhibited rapid relaxation into lower-lying electronic states followed by decay to the S0 ground state. By employing a 21.7 eV XUV probe pulse, we were able to differentiate the relaxation of the excited state population from the initially excited 1ππ* state to an intermediate electronic state with 100 fs. Computational results identify this intermediate as the 1πσ* excited state, accessed by a 1ππ*/1πσ* conical intersection, and the signal from this intermediate state disappears within ∼200 fs. In contrast to thymine, formation of neither the 1nπ* state nor a long-lived triplet state was observed. Although the 1πσ* state is largely repulsive, prior studies have reported a low quantum yield for dissociation, and we observe weak signals that are consistent with production of hot S0 ground state (for BrUrd) on a time scale of 1.5-2 ps. It thus appears that solvent caging effects limit the dissociation yield in solution.

Electronic relaxation pathways in thio-acridone and thio-coumarin: two heavy-atom-free photosensitizers absorbing visible light.

Heavy-atom-free photosensitizers (HAF-PSs) have emerged as a new class of photosensitizers aiming to broaden their applicability and versatility across various fields of the photodynamic therapy of cancers. The strategy involves replacing the exocyclic oxygen atoms of the carbonyl groups of established biocompatible organic fluorophores with sulfur, thereby bathochromically shifting their absorption spectra and enhancing their intersystem crossing efficiencies. Despite these advancements, the photophysical attributes and electronic relaxation mechanisms of many of these HAF-PSs remain inadequately elucidated. In this study, we investigate the excited state dynamics and photochemical properties of two promising HAF-PSs, thio-coumarin and thio-acridone. Employing a combination of steady-state and time-resolved techniques from femtoseconds to microseconds, coupled with quantum chemical calculations, we unravel the electronic relaxation mechanisms that give rise to the efficient population of long-lived and reactive triplet states in these HAF-PSs.

Supramolecular red-light-photosensitized nitric oxide release with fluorescence self-reporting within biocompatible nanocarriers.

The strict dependence of the biological effects of nitric oxide (NO) on its concentration and generation site requires this inorganic free radical to be delivered with precise spatiotemporal control. Light-activation by suitable NO photoprecursors represents an ideal approach. Developing strategies to activate NO release using long-wavelength excitation light in the therapeutic window (650-1300 nm) is challenging. In this contribution, we demonstrate that NO release by a blue-light activatable NO photodonor (NOPD) with self-fluorescence reporting can be triggered catalytically by the much more biocompatible red light exploiting a supramolecular photosensitization process. Different red-light absorbing photosensitizers (PSs) are co-entrapped with the NOPD within different biocompatible nanocarriers such as Pluronic® micelles, microemulsions and branched cyclodextrin polymers. The intra-carrier photosensitized NO release, involving the lowest, long-lived triplet state of the PS as the key intermediate and its quenching by the NOPD, is competitive with that by molecular oxygen. This allows NO to be released with good efficacy, even under aerobic conditions. Therefore, the adopted general strategy provides a valuable tool for generating NO from an already available NOPD, otherwise activatable with the poorly biocompatible blue light, without requiring any chemical modification and using sophisticated and expensive irradiation sources.

Triplet properties and intersystem crossing mechanism of PtAg28 nanocluster sensitizers achieving low threshold and efficient photon upconversion.

Ligand-protected metal nanoclusters have emerged as a promising platform for providing sensitizers for triplet-triplet annihilation upconversion (TTA-UC). Herein, we report [PtAg28(BDT)12]4- (PtAg28; BDT = 1,3-benzenedithiolate) as a sensitizer enabling TTA-UC at low excitation intensities. PtAg28 exhibits a long-lived triplet state (approximately 7 μs) generated with a 100% intersystem crossing (ISC) quantum yield. The mechanism driving this efficient ISC was unveiled with the aid of theoretical calculations. Specifically, the S1-T1 ISC reveals a small spin-orbit coupling (SOC) matrix element, attributed to their similar electron configuration. In contrast, the T2 state, which is energetically close to S1, features a hole distribution derived from the Py superatomic orbital of the icosahedral Pt@Ag12 core. This distribution enables direct SOC based on the orbital angular momentum change from the S1 state with a Pz-derived hole distribution. Consequently, the efficient ISC was rationalized by the S1 → T2 → T1 pathway. The T1 state possesses a metal core-to-surface metal charge transfer character, facilitating triplet energy transfer and conferring superior sensitization ability. Leveraging these characteristics, the combination of PtAg28 sensitizer with a 9,10-diphenylanthracene annihilator/emitter attained an extremely low UC threshold of 0.81 mW cm-2 at 532 nm excitation, along with efficient green-to-blue TTA-UC with an internal quantum yield (ΦUCg) of 12.2% (50% maximum). This results in a pseudo-first-order TTA process with strong UC emission under 1-sun conditions.

Ultrafast spectroscopy of size-dependent hole-transfer-mediated triplet energy transfer in a quantum dot-molecule complex

Triplet energy transfer (TET) from quantum dots to molecules has attracted tremendous attention in the last decade because the long-lived triplet state of molecules benefits various photophysical or photochemical processes and quantum dots (QDs), as the triplet sensitizer, provide prominent optical absorption and emission tunable through flexible size and composition control. Although several dynamical models for some certain QD-molecule complexes have been proposed, the TET mechanism in such complexes is still incomplete due to intricate energy level configuration. More specifics of TET being revealed is advantageous for its further applications. In this paper, a series of QD-molecule complexes composed of CdSe QDs with different diameters and 5-tetracene carboxylic acid (TCA) are assembled. The precise size control of CdSe QDs is realized by combining the transmission electron microscope and steady-state spectra. All CdSe-TCA complexes display expected Type-II like band alignment, which favors the hole-transfer-mediated TET confirmed by a series of spectral measurements. On the surface, ultrafast spectroscopy shows the charge dynamics, specifically the stepwise hole and electron transfer during TET, slow down with increasing size of the CdSe QDs. However, comprehensive analysis of the experimental results reveals that the hole transfer and the following electron transfer essentially depend on the energy level difference and wavefunction overlap between the QD donor and molecule acceptor rather than simply the QD size. The conclusions can facilitate our understanding of the TET mechanism in a QD-molecule complex and help optimize the performance of TET-related photoelectronic devices designed based on size control.

Efficient and stable green organic light-emitting diode utilizing an asymmetric butterfly-shaped chromophore with enhanced reverse intersystem crossing and suppressed exciton annihilation

Intermolecular electron-exchange interactions involving long-lived triplet state excitons dominate the quenching process of the films. In this work, a butterfly-shaped thermally activated delayed fluorescence (TADF) emitter 10,10′-(5-((3,5-di(9H-carbazol-9-yl)phenyl)sulfonyl)-1,3-phenylene)bis(9,9-dimethyl-9,10-dihydroacridine) (BCZ-DPS-BAD) with diphenylsulfone skeleton is constructed by modulating the rigid steric hindrance on the electron donor fragment to weaken intermolecular electron-exchange interactions. Analysis of the ground and excited state properties, crystal structure, steady-state and transient photophysical characteristics reveals that BCZ-DPS-BAD exhibits loose intermolecular arrangements with no direct facing close packing and weak π–π interactions of 4.18 Å and 4.67 Å. The triplet state excitons can swiftly return to the single exciton state in accordance with a short delayed fluorescence lifetime (2.3 μs) and a quick reverse intersystem crossing rate (1.85 × 106 s−1), obstructing the way for the triplet state to lose energy. A remarkable maximum external quantum efficiency of 21.8 %, maximum current efficiency of 56.8 cd/A and exciton utilization rate of 82 % is achieved for BCZ-DPS-BAD-based green non-doped OLED, concomitant with Commission Internationale d'Eclairage (CIE) coordinates of (0.30, 0.50). Notably, BCZ-DPS-BAD behaves stable non-doped OLED performance with efficiency roll-off of 4 % at 1000 cd m−2.