Radiative Rate Research Articles

A Rigid Multiple Resonance Thermally Activated Delayed Fluorescence Core Toward Stable Electroluminescence and Lasing

AbstractThe investigation of organic light‐emitting diodes (OLEDs) and organic laser devices with thermally activated delayed fluorescence (TADF) molecules is emerging due to the potential of harnessing triplets. In this work, a boron/nitrogen multiple‐resonance TADF polycyclic framework fusing carbazole units (CzBNPh) was proposed. CzBNPh exhibited a narrowband emission (<30 nm), a unity photoluminescence quantum yield, and a fast radiative rate. Consequently, CzBNPh demonstrated a low distributed feedback (DFB) lasing threshold of 0.68 μJ cm−2. Furthermore, the stimulated emission zone of CzBNPh was effectively separated from its singlet and triplet absorption, thereby minimizing the singlet‐triplet annihilation under long‐pulsed excitation ranging from 20 μs to 2.5 ms. Significantly, the enhanced rigid molecular conformation, thermal stability, and photo‐stability resulted in improved lasing and electroluminescence stability compared to that of 5,9‐diphenyl‐5,9‐diaza‐13b‐boranaphtho[3,2,1‐de]anthracene (DABNA)‐core. These findings indicate the potential of CzBN‐core as a promising framework for achieving long‐pulsed wave and electrically‐pumped lasing in the future.

Phosphorescent Carbene-Gold-Arylacetylide Materials as Emitters for Near UV-OLEDs.

A series of carbene-gold-acetylide complexes [(BiCAAC)AuCC]n C6 H5- n (n = 1, Au1; n = 2, Au2; n = 3, Au3; BiCAAC = bicyclic(alkyl)(amino)carbene) have been synthesized in high yields. Compounds Au1-Au3 exhibit deep-blue to blue-green phosphorescence with good quantum yields up to 43% in all media. An increase of the (BiCAAC)Au moieties in gold complexes Au1-Au3 increases the extinction coefficients in the UV-vis spectra and stronger oscillator strength coefficients supported by theoretical calculations. The luminescence radiative rates decrease with an increase of the (BiCAAC)Au moieties. The time-dependent density functional theory study supports a charge-transfer nature of the phosphorescence due to the large (0.5-0.6eV) energy gap between singlet excited (S1 ) and triplet excited (T1 ) states. Transient luminescence study reveals the presence of both nonstructured UV prompt-fluorescence and vibronically resolved long-lived phosphorescence 428nm. Organic light-emitting diodes (OLED) are fabricated by physical vapor deposition with 2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF) as a host material with complex Au1. The near-UV electroluminescence is observed at 405nm with device efficiency of 1% while demonstrating OLED device lifetime LT50 up to 20min at practical brightness of 10 nits, indicating a highly promising class of materials to develop stable UV-OLEDs.

Constructing Multiresonance Thermally Activated Delayed Fluorescence Emitters for Organic LEDs: A Computational Investigation.

Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials have acquired substantial attention due to their high electroluminescence efficiency with narrow emission spectra. However, the existing MR-TADF emitters suffer from substantial efficiency roll-off due to insufficient rate constants of the reverse intersystem crossing (kRISC) process compared to the traditional TADF materials. Herein, we employ the DLPNO-STEOM-CCSD method, which is computationally less expensive than the wave function-based EOM-CCSD method, to evaluate the electronic and photophysical properties of MR-TADF materials accurately. The predicted singlet-triplet energy gap (ΔEST), one of the critical parameters governing the TADF efficiency, exhibits remarkable agreement with the experimental measurement, with a standard deviation value of 0.026 eV (obtained based on five experimentally synthesized MR-TADF systems). The proposed technique was utilized to determine the suitability of 15 newly designed MR-TADF emitters via their computed radiative and nonradiative rates, luminescence efficiencies, and exciton characteristics. Moreover, most conceived molecules exhibit blue emission with decent to strong oscillator strengths, making them potential candidates for practical light-emitting applications. The proposed computational route will undoubtedly accelerate the designing and prescreening of potential MR-TADF emitters before their expensive laboratory synthesis and characterization.

Assessing biases in atmospheric parameters for radiative effects estimation in tropical regions

Many studies highlight the importance of water vapor in tropical atmospheric radiative transfer calculations. However, limited research focuses on understanding the effects of biases in different atmospheric factors, such as pressure, temperature, water vapor, and ozone densities. These biases stem from various sources, including satellites, reanalysis data, and the standard TROPICAL atmosphere. This study explores how these biases influence estimates of two key things: atmospheric radiative forcing (ARF) and heating rate (HR) profiles over the tropical region. The study is undertaken at Gadanki, a rural background site in the tropics. It considers different situations - clear skies and those with aerosols - across different parts of the electromagnetic spectrum and throughout the seasons. Additionally, this study investigates how sensitive ARF and HR profiles are to biases in each of these atmospheric factors within specific spectral ranges. The study results show that the standard TROPICAL atmosphere often differs significantly from real-time measurements, leading to varied effects on radiation. Among the various factors, biases in water vapor density have the most substantial impact on ARF and HR, followed by biases in temperature. Moreover, longwave radiation is more sensitive to water vapor and temperature changes. This study emphasizes the importance of considering the exact state of the atmosphere, especially in tropical regions, when estimating ARF and HR. To improve estimates when real-time data isn't available, the study suggests combining data from the Microwave Limb Sounder (MLS) and the Atmospheric Infrared Sounder (AIRS), and using reanalysis data for broader assessments. It's also important to be cautious when using the TROPICAL atmosphere to analyze the radiative effects of aerosols.

Preparation of V2O5 thin films on AA6061 alloy using thermal evaporation

Spacecrafts often operate in severe environments with large exterior temperature fluctuations varying from −150 °C to +150 °C. Spacecraft internal parts work at their optimal efficiency between a range of temperatures, most likely between −10° C to 40 °C, beyond which they suffer damage. Consequently, good thermal management of spacecraft is required. The current technologies may become inadequate in the future owing to stringent requirements of micro- and nanosatellites, which entail the necessity of smaller and smarter thermal control subsystems. Traditional thermal technologies such as heaters, thermostats, heat pipes, coolers, and louvers will almost probably not be adequate to satisfy the requirements, since they are typically heavy and consume more electricity. Hence, Variable emittance coatings, which may modify the effective infrared emissivity and hence the radiative heat transfer rate, will be the inevitable potential choice for flexible or intelligent thermal management of spacecraft. Among all vanadium oxides, VO2 has been shown to exhibit a reversible phase shift at positive temperatures at 68 °C. This phase shift modifies its thermo-optical properties. Thermal evaporation was used to create V2O5 thin films on an AA6061 substrate, and subsequent reduction with N2 gas resulted in the creation of VO2. SEM, EDAX, and XRD were used to characterize the materials.

The Heat and Momentum Budgets of Titan's Middle Atmosphere

AbstractThe thermal and dynamical structure of Titan's middle atmosphere (the stratosphere and mesosphere) has been observed to evolve over seasonal timescales. Measurements from the Composite Infrared Spectrometer on the Cassini spacecraft indicated the presence of a westerly jet with the strongest winds exceeding 200 m s−1 near the autumn and spring poles. The strength of the winds varied substantially with latitude and altitude, and weakened throughout most of winter. The strong winds also served to trap short‐lived trace molecules near the winter pole, leading to a chemically enriched environment. Here, to better understand the evolution of the middle atmosphere jet, we quantify the heat and zonal momentum budgets in Titan's stratosphere and mesosphere over the course of a Titan year using a three‐dimensional general circulation model. We confirm that the dominant heating balance is between the net radiative and adiabatic heating rates, and also show that the convergence of sensible heat by the atmospheric flow is important at low stratospheric altitudes above the winter pole. We show that the polar jet is maintained by the convergence of zonal momentum by the mean meridional flow, while the low‐latitude winds are maintained by an up‐gradient transport of momentum by eddies that occur on time scales of less than one Titan day. The heat and momentum budgets we determine here will be useful in constraining the factors controlling the evolution of Titan's middle atmosphere over the coming decades.

Theoretical study on the influence of substitution position on the luminescence properties and charge transfer characteristics of thermally activated delayed fluorescent molecules

Thermally active delayed fluorescence (TADF) molecules have potentially applications in organic light-emitting diodes (OLEDs) and biomedical sensing. Although TADF emitters have witnessed a rapid development, it remains challenging to study the relationship between molecular structures and luminescence properties as well as carrier mobility transfer properties in theory. In this work, the photophysical properties and luminescence mechanisms of isomers TPA-APQDCN-C (donors at para-position) and TPA-APQDCN-Y (donors at ortho-position) were studied based on density functional theory (DFT) and thermal vibration correlation function (TVCF) method. The results showed that both TPA-APQDCN-C with para-substituted donor and TPA-APQDCN-Y with ortho-substituted donors exhibit red emission in toluene and crystal state. Furthermore, compared to ortho-substituted donors, para-substituted donors promote a redshift in emission wavelength. In addition, the fluorescence efficiencies of TPA-APQDCN-C is obviously higher than that of TPA-APQDCN-Y due to its larger radiative rate and less non-radiative decay rate. Besides, para-substitution (TPA-APQDCN-C) leads to the smaller energy gap between S1 and T1 and the larger spin–orbit coupling (SOC) constant, which is beneficial for increasing the reverse crossing intersystem (RISC) rates. In addition, the carrier mobilities are studied based on the kinetic Monte Carlo simulations. The calculations show that TPA-APQDCN-C are more beneficial for the transfer of holes compared to TPA-APQDCN-Y. This study reveals TPA-APQDCN-C with donors at para-position has a better TADF properties and hole transfer ability, which holds guiding significance for the design of TADF devices with high luminescence efficiency and rapid hole transfer.

Atomic structure calculations, and electron impact ionization cross-sections of W LXI

The present study provides a wide range of calculations of atomic structure data for the tungsten ion W LXI, that belongs to the silicon isoelectronic sequence. The fully relativistic Multi-configuration Dira-Hartree–Fock (MCDHF) method including configuration interaction (CI) is applied to the current ion. Calculated results of energy levels, oscillator strengths, and radiative rates of W LXI are presented. A set of two configuration state lists including the configurations of 3s23p2, 3s3p3, 3s23p3d, and 3p4 at different angular momenta (J) and parity (P) are considered in the configuration interaction calculations of level energies and oscillator strengths. To increase the accuracy of the results, the electronic correlations from higher orbitals, i.e., 7l orbitals are included in the calculations. Quantum electrodynamics effects such as vacuum polarization and self-energy are recalled in the CI calculations. Many more transitions in the soft x-ray region (between 20 and 200 angstroms) have been identified. The flexible atomic code package has been used to compute the electron impact ionization cross-sections of 3s23p2 3P0,1, 1D2 levels in silicon-like tungsten. The cross-sections of the transitions from the ground level in W LXI to the first levels in the 61st ionization of tungsten have been computed using FAC code. The total ionization cross-sections of the 3s23p2 at electron energy range from 5–35 keV are evaluated as well.

The Effect of Monodentate Co-Ligands on the Properties of Pt(II) Complexes Bearing a Tridentate C^N*N-Luminophore

In this study, the insertion of different monodentate co-ligands on Pt(II) complexes bearing a monoanionic C^N*N luminophore as a tridentate chelator was achieved beyond the previously reported chlorido- ([PtCl(L)]) and cyanido-decorated ([PtCN(L)]) analogues. To investigate the impact of the auxiliary ligand on the photophysical properties, we introduced a neutral carbonyl-ligand and observed a lower photoluminescence quantum yield (ΦL) than with a cyanido moiety. However, the direct substitution of the chlorido co-ligand by a NO-related derivative was not successful. Interestingly, the attempted reduction of the successfully inserted nitrito-N-ligand in [PtNO2(L)] resulted in the oxidation of the Pt(II)-center to Pt(IV), as demonstrated by X-ray diffractometry. For comparison, the trifluoroacetato Pt(II) and chlorido Pt(IV) complexes ([PtTFA(L)] and [PtCl3(L)], respectively) were also synthesized. The photophysical characterization revealed similar photoluminescence profiles for all complexes, indicating a weak effect of the co-ligand on the excited state; in fact, all complexes display emission from metal-perturbed ligand-centered states (even the Pt(IV) species). Nonetheless, longer excited state lifetimes (τav) suggest a reduced thermally-activated radiationless deactivation via metal-centered states upon exchange of the chlorido units for other monodentate entities, yet without significantly improving the overall ΦL at room temperature. The irreversible oxidation waves (measured via cyclic voltammetry) mostly stem from the Pt(II)-center; the co-ligand-related drop of these potentials correlates with the increasing σ-donating capacities of the ancillary ligand. In summary, an enhanced π-acceptor capacity does not necessarily improve the ΦL and can even impair radiative rates by compromising the perturbative participation of the metal center on the emissive triplet state; in addition, strong σ-donor abilities improve the phosphorescence efficiencies by hampering the thermal population of dissociative electronic configurations related to the participation of antibonding d*-orbitals at the metal center.

Selenoxanthen-9-one-based emitters exhibiting efficient room temperature phosphorescence for electroluminescent and oxygen sensing applications

Two selenoxanthen-9-one-based emitters (Se-2DPA and Se-2TPA) were synthesized by introducing diphenylamine and triphenylamine as electron donors. The doped films of Se-2DPA and Se-2TPA exhibited bright fluorescence-phosphorescence dual emission and comparable photoluminescence quantum yields (ΦPL) of 31.4 % and 29.6 %, respectively. Unlike the Se-2DPA with phosphorescence lifetime of 1.1 ms, Se-2TPA shows significantly extended phosphorescence lifetime up to 17.7 ms. As revealed by computational and experimental investigations, Se-2DPA maintained more effective spin orbit coupling (SOC) between T1 and S0 states and its radiative rate constant of phosphorescence is 1 order of magnitude higher than that of Se-2TPA, which resulted in a relatively shorter phosphorescence lifetime. Consequently, solution-processed organic light-emitting diodes (OLEDs) based on Se-2DPA with shorter phosphorescence lifetime achieved a maximum external quantum efficiency of 8.8 %, which is much higher than the device based on Se-2TPA (only 0.8 %). In contrast, Se-2TPA showed higher sensitivity for oxygen sensing and the Stern-Volmer quenching constant (KSV) value reached 3.04 × 10−3 ppm owing to the longer phosphorescence lifetime, which is over 3-folds higher than that of Se-2DPA (8.88 × 10−4 ppm) under the same conditions.

Experimental study of ullage height on the burning rate and heat transfer of medium-scale heptane pool fires

Ullage height (the distance between the fuel surface and the pool upper rim) is one of the most important and widely recognized factors that affect the burning rate and heat transfer of pool fires. This study investigated the effects of ullage height on the burning rate, heat feedback, and radiation fraction of medium-scale pool fires. Heptane pool fires were tested with a diameter (D) of 30 cm and the non-dimensional ullage height (h/D, h is the ullage height) being varied from 0 to 1.48. Results show that the non-dimensional burning rate tends to decrease exponentially with h/D. It has been proved that the current models used to predict the burning rate of pool fires are not applicable to those medium-scale pool fires that are affected by the ullage height. The radiative, conductive, and convective heat feedback rate overall decrease with the ullage height. The conductive heat feedback fraction weakly depends on h/D. The radiative heat feedback fraction firstly decreases then stabilizes with h/D, while the convective heat feedback fraction shows an opposite trend. The radiative fractions firstly increase then decrease with h/D. This paper presents a new correlation with considering the ullage height and the mean beam length within the pool ullage. The universality and accuracy of the proposed model were verified by comparing it with extensive experimental data, where D varied from 10 cm to 35 cm, and h/D varied from 0 to 2.0.

Striking Change in Radiative Lifetime and Asymmetry Ratio with Temperature for the Double Perovskite Cs2NaEuCl6

AbstractThe measurement of quantum yield (QY) is an important process for a phosphor. This is often accomplished by the comparison of the low‐temperature lifetime with the room temperature (RT) lifetime. It is shown that this procedure provides a considerable underestimation of QY in certain cases. Additionally, the use of the europium ion as an indicator of changes in site symmetry with temperature has severe limitations. This work reveals that a decrease in measured lifetime with increasing temperature of a system cannot always be associated with a descent in site symmetry and may be due to increasing radiative rate.

Highly Luminescent Chiral Carbon Nanohoops via Symmetry Breaking with a Triptycene Unit: Bright Circularly Polarized Luminescence and Size-Dependent Properties.

Chiral carbon nanohoops with both high fluorescence quantum yield and large luminescence dissymmetry factor are essential to the development of circularly polarized luminescence (CPL) materials. Herein, the rational design and synthesis of a series of highly fluorescent chiral carbon nanohoops TP-[8-13]CPPs via symmetry breaking with a chiral triptycene motif is reported. Theoretical calculations revealed that breaking the symmetry of nanohoops causes a unique size-dependent localization in the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular obtitals (LUMOs) as the increasing of sizes, which is sharply different from those of [n]cycloparaphenylenes. Photophysical investigations demonstrated that TP-[n]CPPs display size-dependent emissions with high fluorescence quantum yields up to 92.9% for TP-[13]CPP, which is the highest value among the reported chiral conjugated carbon nanohoops. The high fluorescence quantum yields are presumably attributed to both the unique acyclic, and radial conjugations and high radiative transition rates, which are further supported by theoretical investigations. Chiroptical studies revealed that chiral TP-[n]CPPs exhibit bright CPL with CPL brightness up to 100.5M-1cm-1 for TP-[11]CPP due to the high fluorescence quantum yield. Importantly, the investigations revealed the intrigued size-dependent properties of TP-[n]CPPs with regards to (chir)optical properties, which follow a nice linear relationship versus 1/n. Such a nice linear relationship is not observed in other reported conjugated nanohoops including CPPs.

First-Principles Calculations of Excited-State Decay Rate Constants in Organic Fluorophores.

In this Perspective, we discuss recent advances made to evaluate from first-principles the excited-state decay rate constants of organic fluorophores, focusing on the so-called static strategy. In this strategy, one essentially takes advantage of Fermi's golden rule (FGR) to evaluate rate constants at key points of the potential energy surfaces, a procedure that can be refined in a variety of ways. In this way, the radiative rate constant can be straightforwardly obtained by integrating the fluorescence line shape, itself determined from vibronic calculations. Likewise, FGR allows for a consistent calculation of the internal conversion (related to the non-adiabatic couplings) in the weak-coupling regime and intersystem crossing rates, therefore giving access to estimates of the emission yields when no complex photophysical phenomenon is at play. Beyond outlining the underlying theories, we summarize here the results of benchmarks performed for various types of rates, highlighting that both the quality of the vibronic calculations and the accuracy of the relative energies are crucial to reaching semiquantitative estimates. Finally, we illustrate the successes and challenges in determining the fluorescence quantum yields using a series of organic fluorophores.

Theoretical perspective for structural isomerism effect on photoelectric properties of organic room temperature phosphorescence molecules

Purely organic room temperature phosphorescence (RTP) materials have garnered extensive attentions in anti-counterfeiting and encryption, information display, biological imaging and organic light emitting diodes (OLEDs) due to the features of long lifetimes, low toxicity and good biocompatibility. Nevertheless, both the species and amounts of the efficient RTP molecules are far from meeting the requirements for practical applications, and the quantitative relationship between molecular structures and optoelectronic properties needs full elucidated. Herein, the photophysical properties and properties of three isomers (o-BA, m-BA and p-BA) are explored based on the first-principles calculations coupled with thermal vibration correlation function (TVCF) method and kinetic Monte Carlo simulations. The substituent effects on the molecular structures, intermolecular interactions, transition properties, Huang-Rhys factors and reorganization energies, excited state dynamics as well as transfer integrals and charge carrier mobilities are investigated in detail to clarify the structure-property relationships. Results show that o-BA achieves fast radiative rate on account of impressive transition dipole moment and oscillator strength, but equally large non-radiative decay rate leads to low RTP efficiency. Promisingly, the highest holes mobility is determined for o-BA due to the largest transfer integrals of holes originating from the strong intermolecular interactions. In addition, compared with o-BA and p-BA, m-BA could achieve fast intersystem crossing (ISC) rate with large spin orbit coupling constant and low non-radiative decay rate with small geometric structure changes. Relatively balanced holes and electrons mobilities of m-BA have been demonstrated, which contributes to the recombination of excitons and the improvement of luminescence efficiency. For p-BA, the strong intermolecular hydrogen bonding restricts the free rotation of single bonds and promotes the molecular coplanarity, which contributes to efficient emission. Furthermore, the charge mobility is calculated and the temperature dependence is investigated, the hopping mechanisms for studied isomers are illustrated. This work reveals the relationship among molecular structures, crystal packing modes, photophysical properties and charge transport properties of isomers, and provides an insight for molecular design and property prediction.

Linear Gold(I) Halide Complexes with a Diamidocarbene Ligand: Synthesis, Reactivity, and Phosphorescence.

A series of halide and pseudohalide gold complexes (DAC)Au(I)X (DAC = N,N'-diamidocarbene; X = Cl, Br, I, and SCN) were prepared in high yields. All complexes possess linear geometry around the gold atom with no aurophilic interactions between neighboring molecules. Reactivity studies for (DAC)Au(I)Cl revealed that the diamido backbone of the carbene ligand is vulnerable to nucleophilic attack by a strong base, potassium tert-butoxide, resulting in cleavage of the carbene backbone and formation of a neutral trigold cluster. Halide and pseudohalide complexes are bright phosphorescent emitters in the solid state, exhibiting photoluminescence quantum yields up to unity. Phosphorescence occurs in the range 480-520 nm with lifetimes as short as 1 μs, resulting in fast radiative rates up to 9.4 × 105 s-1 which is on par with the most efficient heavy metal emitters. Photophysical properties are explained by the intrinsic π-accepting nature of the DAC carbene and are supported by TDDFT calculations.

Enhanced performance of AlGaN-based deep-UV LED by incorporating carrier injection balanced modulation layer synergistically with polarization-regulating structures

A comparable concentration of carriers injected and transported into the active region, that is, balanced hole and electron injection, significantly affects the optoelectronic performance of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). In this study, we introduce a novel structure characterized by a carrier injection balanced modulation layer, incorporating a polarization-regulating gradient p-AlGaN in a DUV LED. We conducted a systematic examination of its impact on the carrier injection and transport processes. Theoretical simulations demonstrate the mitigation of abrupt variations in Al content at the interface between electron blocking layer/p-AlGaN and p-AlGaN/p-GaN within the valence bands. Consequently, holes are more likely to be injected into the active region rather than accumulating at these interfaces. Meanwhile, due to the reduced barrier height at the top of the valence band, the holes were efficiently transported into the quantum well and confined with comparable and balanced concentrations of electrons by suppressing overflow, thereby promoting the radiative recombination rate. Compared with the conventional DUV LED, the hole concentration and radiative recombination rate of the designed structure in the final quantum well are significantly increased to 179.8% and 232.3%, respectively. The spontaneous emission intensity achieves nearly twice at the same current injection density. Moreover, the efficiency droop is significantly suppressed when operated at a gradually increasing current density. This study presents a promising approach that can serve as a reference for achieving high-efficiency AlGaN-based DUV LEDs.

Refractive index dependence of the radiative rate of a magnetic dipole transition for a nanoparticle system

The emission spectra of Cs2NaEuCl6 nanoparticles (NPs) synthesized by the hot injection method are interpreted. Dispersion of the NPs in alkane solvents experimentally enables the magnetic dipole radiative rate in vacuum for the 5D0 - 7F1 channel to be determined as 14.7 ± 0.1 s−1, in agreement with the calculated value.

Bis-oxazoline derivatives as ancillary ligands for bis-cyclometalated iridium complexes

Organometallic iridium complexes with two cyclometalated ligands (C^N) and one bis-oxazoline derived ancillary ligand (L^X), i.e. (C^N)2Ir(L^X), are reported. The C^N ligands are 1-phenylpyrazoline (ppz), 2-(4,6-difluorophenyl)pyridine (F2ppy), 2-phenylpyridine (ppy), 1-phenylisoquinoline (piq). The box ligand is (4S-+)-phenyl-α-[(4S)-phenyloxazolidin-2-ylidene]-2-oxazoline-2-acetonitrile. The emission of these complexes span across the visible and into the near-ultraviolet region of the electromagnetic spectrum with moderate to high photoluminescence quantum yields (ΦPL = 0.45–1.0). These complexes were found to emit from a metal-ligand to ligand charge transfer (ML'LCT) state and have lifetimes (1.3–2.1 μs), radiative rates (105 s−1), and nonradiative rates (104–105 s−1) comparable to state-of-the-art iridium emitters. The (ppy)2Ir(BOX-CN) complexes were resolved into the Δ- and Λ- diastereomers using differences in their solubility and additionally characterized by x-ray crystallography, stability, and chiroptic studies. The high ΦPL of these isomers results in the best to date brightness for circularly polarized luminescence (CPL) from iridium complexes (7.0 M−1 cm−1), with dissymmetry factors of −0.57 × 10−3 and +1.9 × 10−3 for 3Δ and 3Λ, respectively. The significant difference in CPL magnitude between 3Δ and 3Λ likely arises from interligand interactions (edge-to-face arrangement versus strong π-π interaction) for the pendant phenyl ring of the BOX-CN ligand which differ for the two isomers.

When do tripdoublet states fluoresce? A theoretical study of copper(II) porphyrin.

Open-shell molecules rarely fluoresce, due to their typically faster non-radiative relaxation rates compared to closed-shell ones. Even rarer is the fluorescence from states that have two more unpaired electrons than the open-shell ground state, since they involve excitations from closed-shell orbitals to vacant-shell orbitals, which are typically higher in energy compared to excitations from or out of open-shell orbitals. States that are dominated by the former type of excitations are known as tripdoublet states when they can be described as a triplet excitation antiferromagnetically coupled to a doublet state, and their description by unrestricted single-reference methods (e.g., U-TDDFT) is notoriously inaccurate due to large spin contamination. In this work, we applied our spin-adapted TDDFT method, X-TDDFT, and the efficient and accurate static-dynamic-static second order perturbation theory (SDSPT2), to the study of the excited states as well as their relaxation pathways of copper(II) porphyrin; previous experimental works suggested that the photoluminescence of some substituted copper(II) porphyrins originate from a tripdoublet state, formed by a triplet ligand π → π* excitation antiferromagnetically coupled with the unpaired d electron. Our results demonstrated favorable agreement between the X-TDDFT, SDSPT2 and experimental excitation energies, and revealed noticeable improvements of X-TDDFT compared to U-TDDFT, not only for vertical excitation energies but also for adiabatic energy differences. These suggest that X-TDDFT is a reliable tool for the study of tripdoublet state fluorescence. Intriguingly, we showed that the aforementioned tripdoublet state is only slightly above the lowest doublet excited state and lies only slightly higher than the lowest quartet state, which suggests that the tripdoublet of copper(II) porphyrin is long-lived enough to fluoresce due to a lack of efficient non-radiative relaxation pathways; an explanation for this unusual state ordering is given. Indeed, thermal vibration correlation function (TVCF)-based calculations of internal conversion, intersystem crossing, and radiative transition rates confirm that copper(II) porphyrin emits thermally activated delayed fluorescence (TADF) and a small amount of phosphorescence at low temperature (83K), in accordance with experiment. The present contribution is concluded by a few possible approaches of designing new molecules that fluoresce from tripdoublet states.