Weaving host matrices with intermolecular hydrogen bonds for high-efficiency white thermally activated delayed fluorescence

A thermally activated delayed fluorescence (TADF) white organic light-emitting diode (WOLED) holds great promise for low-cost, large-scale lighting applications. Nevertheless, manipulating exciton allocation in a white TADF single layer is still a challenge. Herein, we demonstrate that the exciton kinetic process of dually doped white TADF films is strongly dependent on the grid regularity of the host matrix. Intermolecular hydrogen bonds (IHBs) are used to weave the matrices of two host molecules DPEQPO and DPSQPO featuring four phosphine oxide (PO) groups and different IHB orientations. The DPSQPO matrix forms regular grids to uniformly disperse and separate dopants, while DPEQPO exhibits chaotic IHBs, in turn inducing a heterogeneous dopant distribution. As a consequence, in both photoluminescence and electroluminescence processes, in contrast to DPEQPO hosted systems with comparable singlet Förster resonance energy transfer and triplet Dexter energy transfer, DPSQPO provides a FRET-predominant exciton allocation between blue and yellow dopants, which markedly suppresses triplet quenching and improves the white color purity, resulting in a state-of-the-art external quantum efficiency up to 24.2% of its single-emissive-layer pure-white TADF diode, in contrast to 16.0% for DPEQPO based analogs. These results indicate the significance of host engineering for exciton kinetics and suggest the feasibility of host grid design for developing high-performance TADF lighting.

A thermally activated delayed fluorescence (TADF) white organic light-emitting diode (WOLED) holds great promise for low-cost, large-scale lighting applications.     

Singlet fission in a hexacene dimer: energetics dictate dynamics

Singlet fission (SF) is an exciton multiplication process with the potential to raise the efficiency limit of single junction solar cells from 33% to up to 45%. Most chromophores generally undergo SF as solid-state crystals. However, when such molecules are covalently coupled, the dimers can be used as model systems to study fundamental photophysical dynamics where a singlet exciton splits into two triplet excitons within individual molecules. Here we report the synthesis and photophysical characterization of singlet fission of a hexacene dimer. Comparing the hexacene dimer to analogous tetracene and pentacene dimers reveals that excess exoergicity slows down singlet fission, similar to what is observed in molecular crystals. Conversely, the lower triplet energy of hexacene results in an increase in the rate of triplet pair recombination, following the energy gap law for radiationless transitions. These results point to design rules for singlet fission chromophores: the energy gap between singlet and triplet pair should be minimal, and the gap between triplet pair and ground state should be large.

We report the synthesis and photophysical characterization of highly exoergic singlet fission in a hexacene dimer revealing exciton dynamics that follow the energy gap law.     

Cluster Light-Emitting Diodes Containing Copper Iodine Cube with 100 % Exciton Utilization Using Host-Cluster Synergy

Clusters combine the advantages of organic molecules and inorganic nanomaterials, which are promising alternatives for optoelectronic applications. Nonetheless, recently emerged cluster light-emitting diodes require further excited state optimization of cluster emitters, especially to reduce population of the cluster-centered triplet quenching state (³CC). Here we report that redox-active ligands enhance reverse intersystem crossing (RISC) of Cu₄I₄ cluster for triplet-to-singlet conversion, and thermally activated delayed fluorescence (TADF) host can provide an external RISC channel. It indicates that the complementarity between TADF host and cluster in RISC transitions gives rise to 100 % triplet conversion efficiency and complete singlet exciton convergence, rendering 100-fold increased singlet radiation rate constant and tenfold decreased triplet non-radiation rate constant. We achieve a photoluminescence quantum yield of 99 % and a record external quantum efficiency of 29.4 %.     

Using fluorene to lock electronically active moieties in thermally activated delayed fluorescence emitters for high-performance non-doped organic light-emitting diodes with suppressed roll-off

Thermally activated delayed fluorescence (TADF) emitters with aggregation-induced emission (AIE) features are hot candidates for non-doped organic light-emitting diodes (OLEDs), as they are highly emissive in solid states upon photoexcitation. Nevertheless, not every AIE-TADF emitter in the past had guaranteed decent efficiencies in non-doped devices, indicating that the AIE character alone does not necessarily afford ideal non-doped TADF emitters. As intermolecular electron-exchange interaction that involves long-lived triplet excitons plays a dominant role in the whole quenching process of TADF, we anticipate that it is the main reason for the different electroluminescence performances of AIE-TADF emitters. Therefore, in this work, we designed two TADF emitters SPBP-DPAC and SPBP-SPAC by modifying a reported less successful emitter BP-DPAC with extra fluorenes to increase intermolecular distances and attenuate this electron-exchange interaction. With the fluorene lock as steric hindrance, SPBP-DPAC and SPBP-SPAC exhibit significantly higher exciton utilization in non-doped films due to the suppressed concentration quenching. The non-doped OLEDs based on SPBP-DPAC and SPBP-SPAC show an excellent maximum external quantum efficiency (EQE) of 22.8% and 21.3% respectively, and what''s even more promising is that ignorable roll-offs at practical brightness (e.g., 1000 and 5000 cd m⁻²) were realized. These results reveal that locking the phenyl rings as steric hindrance can not only enhance the molecular rigidity, but also cause immediate relief of concentration quenching, and result in significant performance improvement under non-doped conditions. Our approach proposes a feasible molecular modification strategy for AIE-TADF emitters, potentially increasing their applicability in OLEDs.

Two TADF emitters were developed by modifying a reported less successful emitter BP-DPAC with fluorene to suppress concentration quenching. Their non-doped OLEDs displayed excellent EQEs of 22.8% and 21.3% with well-suppressed roll-off.     

Manipulating exciton dynamics of thermally activated delayed fluorescence materials for tuning two-photon nanotheranostics

Rational manipulation of energy utilization from excited-state radiation of theranostic agents with a donor–acceptor structure is relatively unexplored. Herein, we present an effective strategy to tune the exciton dynamics of radiative excited state decay for augmenting two-photon nanotheranostics. As a proof of concept, two thermally activated delayed fluorescence (TADF) molecules with different electron-donating segments are engineered, which possess donor–acceptor structures and strong emissions in the deep-red region with aggregation-induced emission characteristics. Molecular simulations demonstrate that change of the electron-donating sections could effectively regulate the singlet–triplet energy gap and oscillator strength, which promises efficient energy flow. A two-photon laser with great permeability is used to excite TADF NPs to perform as theranostic agents with singlet oxygen generation and fluorescence imaging. These unique performances enable the proposed TADF emitters to exhibit tailored balances between two-photon singlet oxygen generation and fluorescence emission. This result demonstrates that TADF emitters can be rationally designed as superior candidates for nanotheranostic agents by the custom controlling exciton dynamics.

Exciton dynamics can be manipulated rationally in the design of TADF materials for nanotheranostics. Regulating the ΔE_ST and f promises efficient energy flow for tailoring balances between singlet oxygen generation and fluorescence emission.     

Persistent organic room temperature phosphorescence: what is the role of molecular dimers?

Molecular dimers have been frequently found to play an important role in room temperature phosphorescence (RTP), but its inherent working mechanism has remained unclear. Herein a series of unique characteristics, including singlet excimer emission and thermally activated delayed fluorescence, were successfully integrated into a new RTP luminogen of CS-2COOCH₃ to clearly reveal the excited-state process of RTP and the special role of molecular dimers in persistent RTP emission.

The first purely organic room temperature phosphorescence (RTP) luminogen, with singlet excimer emission and thermally activated delayed fluorescence (TADF) effect, was successfully developed.      

Recent progress in thermally activated delayed fluorescence emitters for nondoped organic light-emitting diodes

Nondoped organic light-emitting diodes (OLEDs) have drawn immense attention due to their merits of process simplicity, reduced fabrication cost, etc. To realize high-performance nondoped OLEDs, all electrogenerated excitons should be fully utilized. The thermally activated delayed fluorescence (TADF) mechanism can theoretically realize 100% internal quantum efficiency (IQE) through an effective upconversion process from nonradiative triplet excitons to radiative singlet ones. Nevertheless, exciton quenching, especially related to triplet excitons, is generally very serious in TADF-based nondoped OLEDs, significantly hindering the pace of development. Enormous efforts have been devoted to alleviating the annoying exciton quenching process, and a number of TADF materials for highly efficient nondoped devices have been reported. In this review, we mainly discuss the mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for nondoped devices. We further classify their molecular structures depending on the functional A groups and offer an outlook on their future prospects. It is anticipated that this review can entice researchers to recognize the importance of TADF-based nondoped OLEDs and provide a possible guide for their future development.

The mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for high-performance nondoped OLEDs are summarized. Their molecular structures depending on the functional A groups are further classified.     

High-efficiency hyperfluorescent white light-emitting diodes based on high-concentration-doped TADF sensitizer matrices via spatial and energy gap effects

Despite the success of monochromatic hyperfluorescent (HF) organic light-emitting diodes (OLEDs), high-efficiency HF white OLEDs (WOLEDs) are still a big challenge. Herein, we demonstrate HF WOLEDs with state-of-the-art efficiencies, featuring a quasi-bilayer emissive layer (EML) composed of an ultrathin (0.1 nm) blue fluorescence (FL) emitter (TBPe) layer and a layer of thermally activated delayed fluorescence (TADF) sensitizer matrix heavily doped with a yellow FL emitter (TBRb, 3%). Based on an asymmetric high-energy-gap TADF sensitizer host (PhCzSPOTz), such an “ultrathin blue emitting layer (UTBL)” strategy endowed the HF WOLEDs with a record power efficiency of ∼80 lm W⁻¹, approaching the level of fluorescent tubes. Transient photoluminescence (PL) and electroluminescence (EL) kinetics demonstrate that the spatial separation of TBPe from the TADF sensitizer and TBRb, and the large energy gap between the latter two effectively suppress triplet leakage, in addition to suppressing triplet diffusion in the PhCzSPOTz matrix with anisotropic intermolecular interactions. These results provide a new insight into the exciton allocation process in HF white light-emitting systems.

A thermally activated delayed fluorescence host was developed to realize high-efficiency fluorescence white organic light-emitting diodes (WOLED) through spatial and energy gap effects.     

Long-lived triplet charge-separated state in naphthalenediimide based donor–acceptor systems

1,4,5,8-Naphthalenediimides (NDIs) are widely used motifs to design multichromophoric architectures due to their ease of functionalisation, their high oxidative power and the stability of their radical anion. The NDI building block can be incorporated in supramolecular systems by either core or imide functionalization. We report on the charge-transfer dynamics of a series of electron donor–acceptor dyads consisting of a NDI chromophore with one or two donors linked at the axial, imide position. Photo-population of the core-centred π–π* state is followed by ultrafast electron transfer from the electron donor to the NDI. Due to a solvent dependent singlet–triplet equilibrium inherent to the NDI core, both singlet and triplet charge-separated states are populated. We demonstrate that long-lived charge separation in the triplet state can be achieved by controlling the mutual orientation of the donor–acceptor sub-units. By extending this study to a supramolecular NDI-based cage, we also show that the triplet charge-separation yield can be increased by tuning the environment.

Ultrafast electron transfer from singlet and triplet excited states in equilibrium results in the population of both singlet and triplet charge-separated states.     

Electron spin polarization generated by transport of singlet and quintet multiexcitons to spin-correlated triplet pairs during singlet fissions

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit of efficiency of organic solar cells. Transport of spin-entanglement in the triplet–triplet pair state via one singlet exciton is a promising phenomenon for several energy conversion applications including quantum information science. However, direct observation of electron spin polarization by transport of entangled spin-states has not been presented. In this study, time-resolved electron paramagnetic resonance has been utilized to observe the transportation of singlet and quintet characters generating correlated triplet–triplet (T + T) exciton-pair states by probing the electron spin polarization (ESP) generated in thin films of 6,13-bis(triisopropylsilylethynyl)pentacene. We have clearly demonstrated that the ESP detected at the resonance field positions of individual triplet excitons is dependent on the morphology and on the detection delay time after laser flash to cause SF. ESP was clearly explained by quantum superposition of singlet–triplet–quintet wavefunctions via picosecond triplet-exciton dissociation as the electron spin polarization transfer from strongly exchange-coupled singlet and quintet TT states to weakly-coupled spin-correlated triplet pair states. Although the coherent superposition of spin eigenstates was not directly detected, the present interpretation of the spin correlation of the separated T + T exciton pair may pave new avenues not only for elucidating the vibronic role in the de-coupling between two excitons but also for scalable quantum information processing using quick T + T dissociation via one-photon excitation.

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit of efficiency of organic solar cells.     

Conversion between triplet pair states is controlled by molecular coupling in pentadithiophene thin films

In singlet fission (SF) the initially formed correlated triplet pair state, ¹(TT), may evolve toward independent triplet excitons or higher spin states of the (TT) species. The latter result is often considered undesirable from a light harvesting perspective but may be attractive for quantum information sciences (QIS) applications, as the final exciton pair can be spin-entangled and magnetically active with relatively long room temperature decoherence times. In this study we use ultrafast transient absorption (TA) and time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy to monitor SF and triplet pair evolution in a series of alkyl silyl-functionalized pentadithiophene (PDT) thin films designed with systematically varying pairwise and long-range molecular interactions between PDT chromophores. The lifetime of the (TT) species varies from 40 ns to 1.5 μs, the latter of which is associated with extremely weak intermolecular coupling, sharp optical spectroscopic features, and complex TR-EPR spectra that are composed of a mixture of triplet and quintet-like features. On the other hand, more tightly coupled films produce broader transient optical spectra but simpler TR-EPR spectra consistent with significant population in ⁵(TT)₀. These distinctions are rationalized through the role of exciton diffusion and predictions of TT state mixing with low exchange coupling J versus pure spin substate population with larger J. The connection between population evolution using electronic and spin spectroscopies enables assignments that provide a more detailed picture of triplet pair evolution than previously presented and provides critical guidance for designing molecular QIS systems based on light-induced spin coherence.

Pentadithiophene derivatives produce triplet pairs efficiently with secondary spin state evolution that depends on their unique intermolecular juxtapositions.     

Bidirectional triplet exciton transfer between silicon nanocrystals and perylene

Hybrid materials comprised of inorganic quantum dots functionalized with small-molecule organic chromophores have emerged as promising materials for reshaping light''s energy content. Quantum dots in these structures can serve as light harvesting antennas that absorb photons and pass their energy to molecules bound to their surface in the form of spin-triplet excitons. Energy passed in this manner can fuel upconversion schemes that use triplet fusion to convert infrared light into visible emission. Likewise, triplet excitons passed in the opposite direction, from molecules to quantum dots, can enable solar cells that use singlet fission to circumvent the Shockley–Queisser limit. Silicon QDs represent a key target for these hybrid materials due to silicon''s biocompatibility and preeminence within the solar energy market. However, while triplet transfer from silicon QDs to molecules has been observed, no reports to date have shown evidence of energy moving in the reverse direction. Here, we address this gap by creating silicon QDs functionalized with perylene chromophores that exhibit bidirectional triplet exciton transfer. Using transient absorption, we find triplet transfer from silicon to perylene takes place over 4.2 μs while energy transfer in the reverse direction occurs two orders of magnitude faster, on a 22 ns timescale. To demonstrate this system''s utility, we use it to create a photon upconversion system that generates blue emission at 475 nm using photons with wavelengths as long as 730 nm. Our work shows formation of covalent linkages between silicon and organic molecules can provide sufficient electronic coupling to allow efficient bidirectional triplet exchange, enabling new technologies for photon conversion.

We demonstrate that silicon quantum dots can exchange spin triplet excitons with molecules covalently attached to their surface. Such hybrid materials can enable systems that upconvert incoherent far-red light into the visible spectral range.

Hybrid materials comprised of inorganic quantum dots (QDs) interfaced with small-molecule organic chromophores have emerged as a promising platform for materials that convert near-infrared radiation into the visible spectral range.^1–3 In these structures, QDs act as light-harvesting antennas, absorbing long-wavelength photons and passing their energy to organic molecules bound to their surface in the form of spin-triplet excitons. These excitons can then be transferred into a surrounding medium, typically a solution or thin film, where pairs of them can fuse to form a bright spin-singlet state that can emit a short-wavelength photon.^4–8 Due to the long lifetime of molecular triplet excitons, which can range from several microseconds to milliseconds, these materials can operate at low photon flux, enabling their integration into light-harvesting systems that operate under solar flux^9,10 and limiting heat dissipation during their use in biological applications, such as phototherapy,^11,12 live-cell imaging,^13,14 and optogenetics.¹⁵ These hybrid materials can also be used to study interfacial energy transfer processes fundamental to the operation of solar cells that use triplet fusion''s inverse process, singlet fission, to enhance their performance.^9,16–21 The simplest design for a cell of this type is one that interfaces a singlet fission material directly in line with a back-contacted semiconductor solar cell.^22–24 In these structures, the singlet fission material acts as a light sensitizer that captures high-energy photons and uses their energy to generate pairs of triplet excitons that can be passed to the semiconductor to produce photocurrent. As molecules can be readily attached to QDs via a variety of chemical tethers, these materials allow detailed study of how the structure of the organic:inorganic interface impacts the ability of triplet excitons to move from one material to the other.For both triplet fusion-based light upconversion and singlet fission-based light harvesting, silicon represents a key material of interest. While several upconversion systems have been derived using QDs containing toxic elements, such as Cd^5,7,25 or Pb,^6,8,26,27 Si QDs are nontoxic, making them attractive for biological applications.²⁸ Silicon also dominates the solar energy market, accounting for ∼90% of solar power production,^29,30 making Si:organic interfaces that readily transmit triplet excitons a key design target for singlet fission-based solar cells.^18,19,22 Previously, we have shown triplet exciton transfer from Si QDs to surface-bound anthracene molecules can power a photon upconversion system that operates with 7% efficiency.³¹ However, the inverse energy transfer process that is key for singlet fission devices, triplet exciton transfer from surface-bound molecules to Si, was not observed in our prior work.In this report, we address triplet exciton transfer from molecules to Si by demonstrating a hybrid Si QD:perylene system wherein photoexcitation of the Si QD establishes a spin-triplet exciton population that exists in a dynamic equilibrium between the QD and perylene molecules bound to its surface. While such exciton cycling has been reported for other QD:molecule systems,^32–34 our work represents the first observation of this behavior in Si QD based systems. Using nanosecond transient absorption spectroscopy, we find triplet exciton transfer from Si to perylene takes place on a 4.2 μs timescale while energy transfer in the reverse direction occurs more than two orders of magnitude faster, on a 22 ns timescale. We attribute this difference in energy transfer rates to differences in the exciton density of states between perylene molecules and Si QDs. To demonstrate the utility of triplet excitons produced by this system for photon conversion applications, we have constructed a photon upconversion system by interfacing perylene-functionalized Si QDs with a complementary perylene-based triplet fusion annihilator. We find this system performs well, upconverting radiation with a wavelength as long as 730 nm into blue light centered near 475 nm. Under 532 nm illumination, the system upconverts light with an efficiency of 1.5% under incident light fluxes as low as 80 mW cm⁻². This performance is comparable to that recently demonstrated using the same perylene annihilator coupled with a Pd-porphyrin light absorber.³⁵ Our work demonstrates that the introduction of short, chemical linkers between molecules and Si can enable triplet exciton exchange between these materials for the design of new systems for both photon upconversion and light harvesting.     

Vibronic effects accelerate the intersystem crossing processes of the through-space charge transfer states in the triptycene bridged acridine–triazine donor–acceptor molecule TpAT-tFFO

Quantum chemical studies employing combined density functional and multireference configuration interaction methods suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO. Three of them, a pair of singlet and triplet charge transfer (CT) states (S₁ and T₁) and a locally excited (LE) triplet state (T₃), can be associated with the (Me → N) conformer, the other two CT-type states (S₂ and T₂) form the lowest excited singlet and triplet states of the (Me → Ph) conformer. The two conformers, which differ in essence by the shearing angle of the face-to-face aligned donor and acceptor moieties, are easily interconverted in the electronic ground state whereas the reorganization energy is substantial in the excited singlet state, thus explaining the two experimentally observed time constants of prompt fluorescence emission. Forward and reverse intersystem crossing between the singlet and triplet CT states is mediated by vibronic spin–orbit interactions involving the LE T₃ state. Low-frequency vibrational modes altering the distance and alignment of the donor and acceptor π-systems tune the S₁ and T₃ states (likewise S₂ and T₃) into and out of resonance. The enhancement of intersystem crossing due to the interplay of vibronic and spin–orbit coupling is considered a general feature of organic through-space charge-transfer thermally activated delayed fluorescence emitters.

DFT/MRCI quantum chemical studies suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO.     

Activating intramolecular singlet exciton fission by altering π-bridge flexibility in perylene diimide trimers for organic solar cells

In this study, two analogous perylene diimide (PDI) trimers, whose structures show rotatable single bond π-bridge connection (twisted) vs. rigid/fused π-bridge connection (planar), were synthesized and investigated. We show via time resolved spectroscopic measurements how the π-bridge connections in A–π–D–π–A–π–D–π–A multichromophoric PDI systems strongly affect the triplet yield and triplet formation rate. In the planar compound, with stronger intramolecular charge transfer (ICT) character, triplet formation occurs via conventional intersystem crossing. However, clear evidence of efficient and fast intramolecular singlet exciton fission (iSEF) is observed in the twisted trimer compound with weaker ICT character. Multiexciton triplet generation and separation occur in the twisted (flexible-bridged) PDI trimer, where weak coupling among the units is observed as a result of the degenerate double triplet and quintet states, obtained by quantum chemical calculations. The high triplet yield and fast iSEF observed in the twisted compound are due not only to enthalpic viability but also to the significant entropic gain allowed by its trimeric structure. Our results represent a significant step forward in structure–property understanding, and may direct the design of new efficient iSEF materials.

We show via time resolved spectroscopy that triplet formation proceeds via intersystem crossing in a rigid-bridged perylene diimide trimer and via efficient and fast intramolecular singlet exciton fission in the analogous flexible-bridged trimer.     

Thermally Activated Delayed Fluorescent Polymers

The development of organic light emitting diodes (OLEDs) based on fluorescent materials has made a great progress in improving light emitting efficiency and full range colors. But it still encounters the low singlet excitons generation ratio of 25% in device. As a solution to this problem, thermally activated delayed fluorescent (TADF) materials can convert the triplet excitons to the singlet ones, thus achieve theoretically 100% exciton utilization efficiency. Up to now, the small TADF molecules have achieved great breakthrough in realizing high external quantum efficiency and full color range including blue, green, and red. While the OLED devices based on macromolecules possess the inherent advantages of simplicity and lower cost in the rapid deposition of large areas at room temperature, especially on large flexible substrates, it is still relatively difficult to realize TADF effect in macromolecules, although several reports have partially confirmed them promising candidates for practical applications. This review summarizes the recent progress in the field of TADF polymers and their device performances in OLEDs, and also gives some outlooks for the further exploration in this field at the end of this paper. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 575–584     

An imidazoacridine-based TADF material as an effective organic photosensitizer for visible-light-promoted [2 + 2] cycloaddition

Energy transfer (EnT) is a fundamental activation process in visible-light-promoted photocycloaddition reactions. This work describes the performance of imidazoacridine-based TADF materials for visible-light mediated triplet–triplet EnT photocatalysis. The TADF material ACR-IMAC has been discovered as an inexpensive, high-performance organic alternative to the commonly used metal-based photosensitizers for visible-light EnT photocatalysis. The efficiency of ACR-IMAC as a photosensitizer is comparable with Ir-based photosensitizers in both intra- and intermolecular [2 + 2] cycloadditions. ACR-IMAC mediated both dearomative and non-dearomative [2 + 2] cycloadditions in good yields, with high regio- and diastereocontrol. Cyclobutane-containing bi- tri- and tetracylic scaffolds were successfully prepared, with broad tolerance toward functional groups relevant to drug discovery campaigns. Fluorescence quenching experiments, time-correlated single-photon counting, and transient absorption spectroscopy were also conducted to provide insight into the reaction and evidence for an EnT mechanism.

This work describes the performance of imidazoacridine-based TADF materials for visible-light mediated triplet–triplet EnT photocatalysis.     

Electrogenerated Chemiluminescence of Donor–Acceptor Molecules with Thermally Activated Delayed Fluorescence

The electrochemistry and electrogenerated chemiluminescence (ECL) of four kinds of electron donor–acceptor molecules exhibiting thermally activated delayed fluorescence (TADF) is presented. TADF molecules can harvest light energy from the lowest triplet state by spin up‐conversion to the lowest singlet state because of small energy gap between these states. Intense green to red ECL is emitted from the TADF molecules by applying a square‐wave voltage. Remarkably, it is shown that the efficiency of ECL from one of the TADF molecule could reach about 50 %, which is comparable to its photoluminescence quantum yield.     

Thermally activated delayed fluorescence as a cycling process between excited singlet and triplet states: application to the fullerenes

In efficient thermally activated delayed fluorescence (TADF) the excited chromophore alternates randomly between the singlet and triplet manifolds a large number of times before emission occurs. In this work, the average number of cycles n is obtained and is shown to have a simple experimental meaning: n+1 is the intensification factor of the prompt fluorescence intensity, owing to the occurrence of TADF. A new method of data analysis for the determination of the quantum yield of triplet formation, combining steady-state and time-resolved data in a single plot, is also presented. Application of the theoretical results to the TADF of [70]fullerenes shows a general good agreement between different methods of fluorescence analysis and allows the determination of several photophysical parameters.     

Singlet exciton trapping and heterofission in tetracene doped anthracene crystals

In tetracene doped anthracene, the magnetic field modulation of prompt tetracene fluorescence following excitation into the anthracene singlet manifold has been measured as a function of the magnetic field orientation and optical excitation energy. The results show that this modulation with low energy excitation is caused by singlet heterofission into one anthracene triplet exciton and one tetracene triplet. With higher excitation energies this modulation is due to both the singlet heterofission and also singlet homofission into a pair of anthracene triplet excitons. Heterofission occurs mainly from anthracene molecules next to a tetracene and competes with the singlet trapping. From the singlet trapping rate and from the magnetic modulation of tetracene prompt fluorescence the heterofission rate is estimated as ≈10^?11s^?1.     

Exceptionally fast radiative decay of a dinuclear platinum complex through thermally activated delayed fluorescence

A novel dinuclear platinum(ii) complex featuring a ditopic, bis-tetradentate ligand has been prepared. The ligand offers each metal ion a planar O^N^C^N coordination environment, with the two metal ions bound to the nitrogen atoms of a bridging pyrimidine unit. The complex is brightly luminescent in the red region of the spectrum with a photoluminescence quantum yield of 83% in deoxygenated methylcyclohexane solution at ambient temperature, and shows a remarkably short excited state lifetime of 2.1 μs. These properties are the result of an unusually high radiative rate constant of around 4 × 10⁵ s⁻¹, a value which is comparable to that of the very best performing Ir(iii) complexes. This unusual behaviour is the result of efficient thermally activated reverse intersystem crossing, promoted by a small singlet–triplet energy difference of only 69 ± 3 meV. The complex was incorporated into solution-processed OLEDs achieving EQE_max = 7.4%. We believe this to be the first fully evidenced report of a Pt(ii) complex showing thermally activated delayed fluorescence (TADF) at room temperature, and indeed of a Pt(ii)-based delayed fluorescence emitter to be incorporated into an OLED.

Efficient thermally activated delayed fluorescence (TADF) in a brightly luminescent diplatinum(ii) complex results in significant enhancement of the radiative decay rate.