Lowest Energy Triplet Research Articles

Blue organic light-emitting diode with a turn-on voltage of 1.47 V

Among the three primary colors, blue emission in organic light-emitting diodes (OLEDs) are highly important but very difficult to develop. OLEDs have already been commercialized; however, blue OLEDs have the problem of requiring a high applied voltage due to the high-energy of blue emission. Herein, an ultralow voltage turn-on at 1.47 V for blue emission with a peak wavelength at 462 nm (2.68 eV) is demonstrated in an OLED device with a typical blue-fluorescent emitter that is widely utilized in a commercial display. This OLED reaches 100 cd/m2, which is equivalent to the luminance of a typical commercial display, at 1.97 V. Blue emission from the OLED is achieved by the selective excitation of the low-energy triplet states at a low applied voltage by using the charge transfer (CT) state as a precursor and triplet-triplet annihilation, which forms one emissive singlet from two triplet excitons.

Unraveling the Marked Differences of the Excited-State Properties of Arylgold(III) Complexes with C∧N∧C Tridentate Ligands.

Three structurally similar gold(III) complexes with C∧N∧C tridentate ligands, [1; C∧N∧C = 2,6-diphenylpyridine], [2; C∧N∧C = 2,6-diphenylpyrazine], and [3; C∧N∧C = 2,6-diphenyltriazine], have been investigated theoretically to rationalize the marked difference in emission behaviors. The geometrical and electronic structures, spectra properties, radiative and nonradiative decay processes, as well as reverse intersystem crossing and reverse internal conversion (RIC) processes were thoroughly analyzed using density functional theory (DFT) and time-dependent DFT calculations. The computed results indicate that there is a small energy difference between the lowest-energy triplet state (T1) and the second lowest-energy triplet state (T1') of complexes 2 and 3, suggesting that the excitons in the T1 state can reach the emissive higher-energy T1' through the RIC process. In addition, the non-emissive T1 states of gold(III) complexes in solution can be ascribed to the easily accessible metal-centered (3MC) state or possibly tunneling into high-energy vibrationally excited singlet states for nonradiative decay. The low efficiency of 3 is attributed to the deactivation pathway via the 3MC state. The present study elucidates the relationship between structure and property of gold(III) complexes featuring C∧N∧C ligands and providing a comprehensive understanding of the significant differences in their luminescence behaviors.

Recent Advance in the Development of Singlet-Fission-Capable Polymeric Materials.

Singlet fission (SF) is a spin-allowed process in which a higher-energy singlet exciton is converted into two lower-energy triplet excitons via a triplet pair intermediate state. Implementing SF in photovoltaic devices holds the potential to exceed the Shockley-Queisser limit of conventional single-junction solar cells. Although great progress has been made in exploiting the underlying mechanism of SF over the past decades, the scope of materials capable of SF, particularly polymeric materials, remains poor. SF-capable polymer is one of the most potential candidates in the implementation of SF into devices due to their distinct superiorities in flexibility, solution processability and self-assembly behavior. Notably, recent advancements have demonstrated high-performance SF in isolated donor-acceptor (D-A) copolymer chains. This review provides an overview of recent progress in the development of SF-capable polymeric materials, with a significant focus on elucidating the mechanisms of SF in polymers and optimizing the design strategies for SF-capable polymers. Additionally, the paper discusses the challenges encountered in this field and presents future perspectives. It is expected that this comprehensive review will offer valuable insights into the design of novel SF-capable polymeric materials, further advancing the potential for SF implementation in photovoltaic devices.

Excited State Dynamics of Dibenzothiophene Derivatives.

Polycyclic aromatic sulfur heterocycles are environmental pollutants formed from incomplete combustion processes and crude oil spills. Their excited state dynamics are not understood. Herein, femtosecond transient absorption is combined with steady-state spectroscopy and computational methods to elucidate the relaxation mechanisms of three dibenzothiophene derivatives. The low-energy singlet and triplet states all have ππ* character in the Franck-Condon region, and two minima were located in the S1 surface. Excitation at 320 nm populates their S1 state directly, which relaxes with lifetimes ranging from 4 to 13 ps. Most of the S1 population undergoes efficient intersystem crossing to the triplet state with lifetimes ranging from 820 ± 50 to 900 ± 30 ps. The compounds exhibit negligible nonradiative internal conversion, low fluorescence yields of 1.2 to 1.6%, and triplet yields of ca. 98%. Linear interpolation of internal coordinates reveals the chemical basis for relaxing the spin-forbidden intersystem crossing in these π-aromatic systems.

Singlet O2 from Ultraviolet Excitation of the Quinoline-O2 Complex.

We report results from experiments with the quinoline-O2 complex, which was photodissociated using light near 312 nm. Photodissociation resulted in formation of the lowest excited state of oxygen, O2 a 1Δg, which we detected using resonance enhanced multiphoton ionization and velocity map ion imaging. The O2+ ion image allowed for a determination of the center-of-mass translational energy distribution, P(ET), following complex dissociation. We also report results of electronic structure calculations for the quinoline singlet ground state and lowest energy triplet state. From the CCSD/aug-cc-pVDZ//(U)MP2/cc-pVDZ calculations, we determined the lowest energy triplet state to have ππ* electronic character and to be 2.69 eV above the ground state. We also used electronic structure calculations to determine the geometry and binding energy for several quinoline-O2 complexes. The calculations indicated that the most strongly bound complex has a well depth of about 0.11 eV and places the O2 moiety above and approximately parallel to the quinoline ring system. By comparing the experimental P(ET) with the energy for the singlet ground state and the lowest energy triplet state, we concluded that the quinoline product was formed in the lowest energy triplet state. Finally, we found the experimental P(ET) to be in agreement with a Prior translational energy distribution, which suggests a statistical dissociation for the complex.

Coupled Potential Energy Surfaces Strongly Impact the Lowest-Energy Spin-Flip Transition in Six-Coordinate Nickel(II) Complexes.

Luminescent complexes of earth-abundant first-row transition metals are of renewed, broad interest due to their spectroscopic and photochemical properties as well as emerging applications. New strong-field polypyridine ligands have led to six-coordinate 3d3 chromium(III) complexes with intense spin-flip luminescence in solution at room temperature. The ground and emissive states both arise from the (t2)3 electron configuration involving the dπ levels (O point group symmetry labels). Pseudoctahedral 3d8 nickel(II) complexes with such strong ligands are a priori also promising candidates for spin-flip luminescence. In contrast, the relevant electron configurations involve the dσ orbitals and (e)2 configurations. We have prepared the known nickel(II) complexes [Ni(terpy)2]2+, [Ni(phen)3]2+, and [Ni(ddpd)2]2+ as well as the novel complexes [Ni(dgpy)2]2+ and [Ni(tpe)2]2+ forming a series with increasing ligand field strengths (terpy = 2,2':6',2″-terpyridine; phen = 1,10-phenanthroline; ddpd = N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine; dgpy = 2,6-diguanidylpyridine; tpe = 1,1,1-tris(pyrid-2-yl)ethane). The lowest-energy singlet and triplet excited states of these nickel(II) complexes are analyzed based on absorption spectra using ligand field theory and CASSCF-NEVPT2 calculations for vertical transition energies and a model based on coupled potential energy surfaces, leading to calculated absorption spectra in good agreement with the experimental data. No photoluminescence signal was observed in the wavelength ranges identified through the analyses of the absorption spectra. The models provide insight into key differences between the nickel(II) complexes and their strongly luminescent chromium(III) analogues.

Double Thionated Pyrimidine Nucleobases: Molecular Tools with Tunable Photoproperties.

Sulfur-substituted nucleobases are DNA and RNA base derivatives that exhibit extremely efficient photoinduced intersystem crossing (ISC) dynamics into the lowest-energy triplet state. The long-lived and reactive triplet states of sulfur-substituted nucleobases are crucial due to their wide range of potential applications in medicine, structural biology, and the development of organic light-emitting diodes (OLEDs) and other emerging technologies. However, a comprehensive understanding of non-negligible wavelength-dependent changes in the internal conversion (IC) and ISC events is still lacking. Here, we study the underlying mechanism using joint experimental gas-phase time-resolved photoelectron spectroscopy (TRPES) and theoretical quantum chemistry methods. We combine 2,4-dithiouracil (2,4-DTU) TRPES experimental data with computational analysis of the different photodecay processes, which are induced by increasing excitation energies along the entire linear absorption (LA) ultraviolet (UV) spectrum. Our results show how the double-thionated uracil (U), i.e., 2,4-DTU, appears as a versatile photoactivatable instrument. Multiple decay processes can be initiated with different ISC rates or triplet-state lifetimes that resemble the distinctive behavior of the singly substituted 2- or 4-thiouracil (2-TU or 4-TU). We obtained a clear partition of the LA spectrum based on the dominant photoinduced process. Our work clarifies the reasons behind the wavelength-dependent changes in the IC, ISC, and triplet-state lifetimes in doubly thionated U, becoming a biological system of utmost importance for wavelength-controlled applications. These mechanistic details and photoproperties are transferable to closely related molecular systems such as thionated thymines.

Organic solar cells and their applications Shah, Razon Oda, F. Hazuan

Enhancing the power conversion efficiency (PCE) is the ma- jor task in the development of solar cells. In 1961, Willi- am Shockley and Hans J. Queisser pointed out that the highest PCE for a single-junction solar cell is limited to 31% due to the thermalization and transmission losses. Singlet fission (SF) is an appealing carrier-multiplication approach to break the Shockley-Queisser limit, since it converts one high- energy singlet exciton into two low-energy triplet excitons. Thus, one absorbed photon generates two electron–hole pairs [1]. SF phenomenon was first observed in anthracene single crystals in 1965. It attracted attention again in 2006. In organic semiconductors, a singlet exciton (S1) forms after the ab- sorption of a photon. If the energy of this singlet exciton is greater than twice of that for triplet exciton (T1), namely E(S1) > 2E(T1), then spin-allowed fission (S1 → 2T 1) could occur. The general kinetic model for SF process is S0 is the ground state, S1 is the excited singlet state, 1(TT) is the intermediate state of a correlated triplet pair, and T1 + T1 are two independent triplet states (Figure 1(a)). The application needs materials with high fission efficiency, suitable bandgap for capturing high-energy photons, appropriate triplet energy and high chemical stability. Owing to the existence of competitive pathways (i.e. S1 to S0 relaxation in picosecond–nanosecond timescale; excimer formation in femtosecond timescale; charge transfer (CT) process in femtosecond timescale, SF needs to occur in pico- second or sub-picosecond timescale. Thus, SF requires strong intermolecular coupling. The intermolecular coupling can be realized via Van der Waals force, weak non-bonded interactions and so on. The coupling distance not only affects SF rate and triplet yield, but also influences the lifetime and diffusion length of triplet excitons [2].

Crystal structure, stability, spectroscopy, electronic structure, and ultrafast excited-state dynamics of the elusive iron(II) phthalocyanine axially coordinated with DABCO ligands

The elusive PcFe(DABCO)2 (Pc = phthalocyaninato(2-) ligand; DABCO = 1,4-diazabicyclo[2.2.2]octane) complex was prepared and characterized by UV-Vis, MCD, 1H NMR, and Mössbauer spectroscopies. The X-ray crystal structure of this complex indicates the longest Fe-N(DABCO) bond distance among all known PcFeL2 complexes with nitrogen donors as the axial ligands. The target compound is only stable in the presence of large access of the axial ligand and rapidly converts into the (PcFe)2O [Formula: see text]-oxo dimer even at a modest temperature. The electronic structure of the PcFe(DABCO)2 complex was elucidated by DFT and TDDFT methods. The DFT calculations predicted a very small singlet-triplet gap in this compound. The femtosecond transient absorption spectroscopy is indicative of extremely fast ([Formula: see text]200 fs) deactivation of the first excited state in PcFe(DABCO)2 with a lack of formation of the long-lived low-energy triplet state.

Heterobimetallic iridiumIII–europiumIII complex: the role of donor energy on sensitising the EuIII ion

The lowest triplet energy from an IrIII complex that efficiently sensitises EuIII ion in an IrIII–EuIII heterobimetallic complex was determined.

High N‐H Photoacidity of a Nitrogen‐Rich Fused Ring Heteroaromatic Scaffold

AbstractIt is shown that 4‐methyl‐7‐(4‐nitrophenyl)‐triazolo[3,2‐c]triazole exhibits a high photoacidity, unprecedented in neutral molecules in which the proton is delivered by a NH bond, which promotes acetalization of aldehydes under irradiation by a near‐UV low power LED. Time dependent transient absorption spectroscopy shows a series of stimulated emission signals attributable to photo‐tautomerization and formation of the deprotonated species, followed by broad positive absorption band which does not decay within the investigated timescale, compatible with the formation of a long living species, possibly the anion in the lowest energy triplet state. Computational analyses qualitatively confirm the strong photoacidity of the molecule in all its tautomeric forms, allow to assign transient absorption signals, and show that the first singlet excited state exhibits high spin‐orbit couplings with excited triplet states which could promote a fast singlet‐triplet transition, thus leading to a long living species able to photocatalyze chemical reactions.

Organic Phosphorescence Lasing Based on a Thermally Activated Delayed Fluorescence Emitter.

Organic phosphorescence materials provide an opportunity to use triplets for lasing. However, population inversion based on phosphorescence is hard to establish, owing to low luminescent quantum efficiency and intensive optical loss. By comparison, thermally activated delayed fluorescence emitters exhibit excellent optical gain with the aid of the reverse intersystem crossing (RISC) process. In this work, we designed a multifunctional gain material, not only serving as a thermally activated delayed fluorescence (TADF) emitter with excellent optical gain but also working as a phosphorescence source with high utilization of triplets. The lone pair of electrons in oxygen substitutions promotes a fast spin-flip and high delayed fluorescence quantum yield (ΦDF = 55%), enabling TADF amplified spontaneous emissions (ASE) of CH2Cl2 solution. Single-crystalline nanowires of H-aggregates effectively lower triplet energy levels with high phosphorescence quantum yield (ΦP = 27%), demonstrating Fabry-Perot mode phosphorescence lasing at 630 nm.

Acetonitrile Ligand Photosubstitution in Ru(II) Complexes Directly from the 3MLCT State.

The excited states of the series [Ru(tpy)(L)(CH3CN)]n+ (1-11) (n = 1, 2) containing bidentate ligands L with varying electron-donating ability were characterized through Arrhenius analysis of the temperature dependence of their excited-state lifetimes. Complexes 1-11 undergo photoinduced CH3CN dissociation upon 450 nm irradiation with ligand exchange quantum yields that increase with the energy barrier to populating a dissociative triplet ligand field (3LF) state from the lowest-energy triplet metal-to-ligand charge transfer (3MLCT) excited state. Combined with DFT calculations, the data indicate that ligand photodissociation in 1-11 occurs directly from the 3MLCT state instead of a 3LF state. This finding is in contrast to the generally accepted mechanism for ligand photodissociation in Ru(II) complexes and indicates that alternative pathways for photoinduced ligand dissociation are available. These results can widely impact design principles for applications that require ligand photodissociation, such as photochemotherapy and photocatalysis, as well as for those where photosubstitution is undesirable, such as solar energy conversion.

Cascade cyclization of alkene-tethered acylsilanes and allylic sulfones enabled by unproductive energy transfer photocatalysis

Developing photo-induced cascade cyclization of alkene-tethered acylsilanes is challenging, because acylsilanes are unstable under light irradiation. Herein, we report that the energy transfer from excited acylsilanes to a photocatalyst that possesses lower triplet energy can inhibit the undesired decomposition of acylsilanes. With neutral Eosin Y as the photocatalyst, an efficient synthesis of cyclopentanol derivatives is achieved with alkene-tethered acylsilanes and allylic sulfones. The reaction shows broad substrate scope and the synthetic potential of this transformation is highlighted by the construction of cyclopentanol derivatives which contain fused-ring or bridged-ring.

Bending Pyrenacenes to Fill Gaps in Singlet-Fission-Based Solar Cells

Singlet fission is envisaged to enhance the efficiency of single-junction solar cells beyond the current theoretical limit. Even though sensitizers that undergo singlet fission efficiently are known, characteristics like low-energy triplet state or insufficient stability restrict their use in silicon-based solar cells. Pyrenacenes have the potential to overcome these limitations, but singlet-fission processes in these materials is outcompeted by excimer formation. In this work, bent pyrenacenes with a reduced propensity to stack and thus form excimers are computationally evaluated as singlet-fission materials. The energies of the S1, T1 and T2 states were estimated in a series of bent pyrenacenes by means of time-dependent density functional theory calculations. Our results show the opposite trend observed for perylene diimides, namely, an increase in the energy of the T1 and S1 states upon bending. In addition, we show that the energy levels can be tuned on demand by manipulating the bend angle to match the energy gap of various semiconductors that can be used in single-junction solar cells, making pyrenacenes promising candidates for singlet fission.

π-Extended Ligands in Two-Coordinate Coinage Metal Complexes.

Two-coordinate carbene-MI-amide (cMa, MI = Cu, Ag, Au) complexes have emerged as highly efficient luminescent materials for use in a variety of photonic applications due to their extremely fast radiative rates through thermally activated delayed fluorescence (TADF) from an interligand charge transfer (ICT) process. A series of cMa derivatives was prepared to examine the variables that affect the radiative rate, with the goal of understanding the parameters that control the radiative TADF process in these materials. We find that blue-emissive complexes with high photoluminescence efficiencies (ΦPL > 0.95) and fast radiative rates (kr = 4 × 106 s-1) can be achieved by selectively extending the π-system of the carbene and amide ligands. Of note is the role played by the increased separation between the hole and electron in the ICT excited state. Analysis of temperature-dependent luminescence data and theoretical calculations indicate that the hole-electron separation exerts a primary effect on the energy gap between the lowest-energy singlet and triplet states (ΔEST) while keeping the radiative rate for the singlet state relatively unchanged. This interpretation provides guidelines for the design of new cMa derivatives with even faster radiative rates in addition to those with slower radiative rates and thus extended excited state lifetimes.

Excited-State Transient Chemistry of Rubrene: A Whole Story.

The ability to manipulate low-energy triplet excited states into higher-energy emissive singlet states, a process known as photon upconversion (UC), has potential applications in bioimaging, photocatalysis, and in increasing the efficiency of solar cells. However, the overall UC mechanism is complex and can involve many intermediate states, especially when semiconductors such as lead halide perovskites are used to sensitize the required triplet states. Using a combination of pulse radiolytic and electrochemical techniques, we have now explored the transient features of rubrene─a commonly employed triplet annihilator in UC systems. The rubrene triplet, radical anion, and radical cation species yield unique spectra that can serve as spectral fingerprints to distinguish between transient species formed during UC processes. Using detailed kinetic studies, we have succeeded in establishing that the rubrene triplets are susceptible to self-quenching (kquench = 3.6 × 108 M-1 s-1), and as the triplets decay, an additional transient feature is observed in the transient absorption spectra. This new feature indicates a net electron transfer process occurs to form the radical cation and anion as the triplets recombine. Taken together, this work provides a comprehensive picture of the excited state and transient features of rubrene and will be crucial for understanding the mechanism(s) of photon upconversion systems.

Trigonal Copper(I) Complexes with Cyclic (Alkyl)(amino)carbene Ligands for Single-Photon Near-IR Triplet Emission.

Molecular near-IR (NIR) triplet-state emitters are of importance for the development of new, organic-electronics-based telecommunication technologies as optical fibers operating in the corresponding spectral bands allow for data transfer over much longer distances due to the significantly lower attenuation. However, achieving such low-energy triplet excited states with good radiative rate constants is very challenging, and studies regarding the single-photon emission of organometallics in this energy range are scarce. We have prepared a series of trigonal CuI CAAC complexes bearing chelating ligands with O, N, S, and Se donor atoms and studied their photophysical properties in this context. The compounds show weak low-energy absorption in solution between 400 and 500 nm due to mixed Cu → CAAC 1MLCT/LLCT states, resulting in yellow-green to orange appearance, which we have also correlated to the 15N NMR resonances of the π-accepting carbene ligand. In the solid state, phosphorescence from dominant 3(Cu → CAAC) CT states is observed at room temperature. The emission of the complexes is bathochromically shifted in comparison to structurally related linearly coordinated copper(I) CAAC complexes due to structural reorganization in the excited state to a T-shape. For [Cu(dbm)(CAACMe)], the broad phosphorescence with outstanding λmax = 760 nm tailors out to ca. 1100 nm and leads to its proof-of-concept application as a nonclassical single-photon light source, constituting key functional units for the implementation of tap-proof data transfer.

Intrinsically-ionic donor-acceptor molecules featuring thermally-activated delayed fluorescence for high-performance host-guest blue light-emitting electrochemical cells

High-performance blue light-emitting electrochemical cells (LECs) have remained a formidable challenge. Ionic donor–acceptor molecules, especially those featuring thermally-activated delayed fluorescence (TADF), are promising hosts for developing high-performance blue LECs, but such hosts have remained undeveloped. Here, we report two intrinsically-ionic donor–acceptor molecules, 1-(3-(4-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-6-phenylpyrimidin-2-yl)phenyl)-3-ethyl-1H-imidazol-3-ium hexafluorophosphate (H1) and 1-(3-(4-(2-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)-6-phenylpyrimidin-2-yl)phenyl)-3-ethyl-1H-imidazol-3-ium hexafluorophosphate (H2), and their use as hosts for high-performance blue LECs. 3,6-di-tert-butylcabazole and imidazolium-substituted 2,4-diphenylpyrimidine are used as the donor and acceptor, respectively, which are located para (for H1) or ortho (for H2) to each other on the central phenyl linker. While H1 shows a low triplet energy (<2.6 eV), a large singlet–triplet energy gap (ΔEST) (>0.4 eV) and no TADF, H2 exhibits a high triplet energy (>2.8 eV), a small ΔEST (∼0.2 eV) and distinct TADF, due to its twisted donor–acceptor conformation. Both H1 and H2 show excellent electrochemical stability. Host-guest blue LECs using H2 as the host and a phosphorescent cationic Ir(III) complex as the guest afford sky-blue electroluminescence with high blue-color stability and peak brightness/peak external quantum efficiency/half-lifetime at 1078 cd m−2/8.6%/66 min under a constant-current driving. Upon adding LiPF6 into the active layer, a half-lifetime of ∼ 2 h is achieved at 918 cd m−2. These performances are the highest for host–guest blue LECs reported so far. The efficiencies and operational stability achieved at the high brightness are also the highest for blue LECs with inert-metal cathodes reported so far.

An Overlooked Charge‐Transfer Interaction in the Interfacial Triplet–Triplet Upconversion Process in Blue Organic Light‐Emitting Diodes

AbstractTwo low‐energy triplets can generate one singlet via triplet–triplet upconversion (TTU), and result in an exciton production yield exceeding 25% in conventional fluorescence‐based organic light‐emitting diodes (OLEDs). In most cases, since such low‐energy triplets induce no serious OLED degradation, TTU‐OLEDs are the only commercialized blue OLEDs so far. Herein, it is clarified that the charge‐transfer (CT) interaction at a hole‐transport/emitter‐layer interface is an overlooked pathway to enhance TTU yield significantly. First, a small energy offset at the interface enables the formation of a high‐energy CT exciton. Second, the lower energy triplet state originated from an anthracene moiety in the emitter layer collects the interfacial triplet CT. Third, due to the high‐density interfacial triplets formation, TTU at the interface contributes to the electroluminescence from the emitter layer or blue dopants even at low current density. This finding underlines the important role of the CT interface to exploit the full potential of TTU in pure‐blue OLEDs.