Intersystem-crossing and phosphorescence rates in fac-IrIII(ppy)3: A theoretical study involving multi-reference configuration interaction wavefunctions

Photocatalytic Generation of Trifluoromethyl Nitrene for Alkene Aziridination

N-Trifluoromethylated organics may be applied in drug design, agrochemical synthesis, and materials science, among other areas. Yet, despite recent advances in the synthesis of aliphatic, cyclic and heterocyclic N-trifluoromethyl compounds, no strategy based on trifluoromethyl nitrene has hitherto been explored. Here we describe the formation of triplet trifluoromethyl nitrene from azidotrifluoromethane, a stable and safe-to-use precursor, by visible light photocatalysis. The addition of CF3 N to alkenes via biradical intermediates afforded previously unknown aziridines substituted with trifluoromethyl group on the nitrogen atom. The obtained aziridines were converted into either N-trifluoromethylimidazolines, via formal [3+2] cycloaddition with nitriles, mediated by a Lewis acid, or into N-trifluoromethylaldimines, via ring opening and aryl group migration mediated by a strong Brønsted acid. Our findings open new opportunities for the development of novel classes of N-CF3 compounds with possible applications in the life sciences.

Phosphorescent Properties of Heteroleptic Ir(III) Complexes: Uncovering Their Emissive Species

In this contribution, we assess the computational machinery to calculate the phosphorescence properties of a large pool of heteroleptic [Ir(C^N)2(N^N)]+ complexes (where N^N is an ancillary ligand and C^N is a cyclometalating ligand) including their phosphorescent rates and their emission spectra. Efficient computational protocols are next proposed. Specifically, different flavors of DFT functionals were benchmarked against DLPNO-CCSD(T) for the phosphorescence energies. The transition density matrix and decomposition analysis of the emitting triplet excited state enable us to categorize the studied complexes into different cases, from predominant triplet ligand-centered (3LC) character to predominant charge-transfer (3CT) character, either of metal-to-ligand charge transfer (3MLCT), ligand-to-ligand charge transfer (3LLCT), or a combination of the two. We have also calculated the vibronically resolved phosphorescent spectra and rates. Ir(III) complexes with predominant 3CT character are characterized by less vibronically resolved bands as compared to those with predominant 3LC character. Furthermore, some of the complexes are characterized by close-lying triplet excited states so that the calculation of their phosphorescence properties poses additional challenges. In these scenarios, it is necessary to perform geometry optimizations of higher-lying triplet excited states (i.e., Tn). We demonstrate that in the latter scenarios all of the close-lying triplet species must be considered to recover the shape of the experimental emission spectra. The global analysis of computed emission energies, shape of the computed emission spectra, computed rates, etc. enable us to unambiguously pinpoint for the first time the triplet states involved in the emission process and to provide a general classification of Ir(III) complexes with regard to their phosphorescence properties.

Aligning π-Extended π-Deficient Ligands to Afford Submicrosecond Phosphorescence Radiative Decay Time of Mononuclear Ir(III) Complexes

Herein, we report a profound investigation of the photophysical properties of three mononuclear Ir(III) complexes fac-Ir(dppm)₃ (Hdppm-4,6-bis(4-(tert-butyl)phenyl)pyrimidine), Ir(dppm)₂(acac) (acac-acetylacetonate), and Ir(ppy)₂(acac) (Hppy-phenylpyridine). The heteroleptic Ir(dppm)₂(acac) is found to emit with efficiency above 80% and feature a remarkably high rate of emission. As measured under ambient temperature, Ir(dppm)₂(acac) emits with the unusually short (sub-μs) radiative decay time of τ_r = τ_em/Φ_PL = 1/k_r = 0.91 μs in degassed toluene and τ_r = 0.73 μs in a doped polystyrene film under nitrogen. Investigations at cryogenic temperatures in glassy toluene showed that the emission stems from the T₁ state and thus represents T₁ → S₀ phosphorescence with individual decay times of the T₁ substates of T_1,I = 66 μs, T_1,II = 7.3 μs, T_1,III = 0.19 μs, and energy gaps between the substates of ΔE(T_1,II-T_1,I) = 14 cm^-1 and ΔE(T_1,III-T_1,I) = 210 cm^-1. Analysis of the electronic structure of Ir(dppm)₂(acac) showed that such a high rate of phosphorescence may stem from the two dppm ligands, with extended π-conjugation system and π-deficient character due to the pyrimidine ring, being serially aligned along one axis. Such alignment, along with the quasi-symmetric character of Jahn-Teller distortions in the T₁ state, affords a large chromophore, comprising four (het)aryl rings of the two dppm ligands. This affords an exceptionally large oscillator strength of the MLCT-character singlet state spin-orbit coupled with the T₁ state and thus brings about enhancement of the phosphorescence rate. These findings reveal molecular design principles paving the way to new phosphors of enhanced emission rates.

Ultra-Long Lived Luminescent Triplet Excited States in Cyclic (Alkyl)(amino)carbene Complexes of Zn(II) Halides

The high element abundance and d10 electron configuration make ZnII -based compounds attractive candidates for the development of novel photoactive molecules. Although a large library of purely fluorescent compounds exists, emission involving triplet excited states is a rare phenomenon for zinc complexes. We have investigated the photophysical and -chemical properties of a series of dimeric and monomeric ZnII halide complexes bearing a cyclic (alkyl)(amino)carbene (cAAC) as chromophore unit. Specifically, [(cAAC)XZn(μ-X)2 ZnX(cAAC)] (X=Cl (1), Br (2), I (3)) and [ZnX2 (cAAC)(NCMe)] (X=Br (4), I (5)) were isolated and fully characterized, showing intense visible light photoluminescence under UV irradiation at 297 K and fast photo-induced transformation. At 77 K, the compounds exhibit improved stability allowing to record ultra-long lifetimes in the millisecond regime. DFT/MRCI calculations confirm that the emission stems from 3 XCT/LEcAAC states and indicate the phototransformation to be related to asymmetric distortion of the complexes by cAAC ligand rotation. This study enhances our understanding of the excited state properties for future development and application of new classes of ZnII phosphorescent complexes.

Photo- and Electroluminescent Neutral Iridium(III) Complexes Bearing Imidoylamidinate Ligands

Imidoylamidinate-based heteroleptic bis(2-phenylbenzothiazole)iridium(III) and -rhodium(III) complexes [(bt)₂M(N^∩N)] (bt = 2-phenylbenzothiazole, N^∩N = N'-(benzo[d]thiazol-2-yl)acetimidamidyl (Ir1 and Rh1), N'-(6-fluorobenzo[d]thiazol-2-yl)acetimidamidyl (Ir2), N'-(benzo[d]oxazol-2-yl)acetimidamidyl (Ir3), N'-(1-methyl-1H-benzo[d]imidazol-2-yl)acetimidamidyl (Ir4); yields 70-84%) were obtained by the reaction of the in situ-generated solvento-complex [(bt)₂M(NCMe)₂]NO₃ and benzo[d]thia/oxa/N-methylimidozol-2-amines in the presence of NaOMe. Complexes Ir1-4 exhibited intense orange photoluminescence, reaching 37% at room temperature quantum yields, being immobilized in a poly(methyl methacrylate) matrix. A photophysical study of these species in a CH₂Cl₂ solution, neat powder, and frozen (77 K) MeOC₂H₄OH-EtOH glass matrix─along with density-functional theory (DFT), ab initio methods, and spin-orbit coupling time-dependent DFT calculations─verified the effects of substitution in the imidoylamidinate ligands on the excited-state properties. Electrochemical (cyclic voltammetry and differential pulse voltammetry) and theoretical DFT studies demonstrated noninnocent behavior of the imidoylamidinate ligands in Ir1-4 and Rh1 complexes due to the significant contribution coming from these ligands in the HOMO of the complexes. The iridium(III) species exhibit a ligand (L, 2-phenylbenzothiazole)-centered (³LC), metal-to-ligand (L', imidoylamidinate) charge-transfer (³ML'CT,³MLCT) character of their emission. The imidoylamidinate-based iridium(III) species were proved to be effective as the emissive dopant in an organic light-emitting diode device, fabricated in the framework of this study.

Mechanism of Ir(ppy)3 Guest Exciton Formation with the Exciplex-Forming TCTA:TPBI Cohost within a Phosphorescent Organic Light-Emitting Diode Environment

Cohosts based on hole transporting and electron transporting materials often act as exciplexes in the form of intermolecular charge transfer complexes. Indeed, exciplex-forming cohosts have been widely developed as the host materials for efficient phosphorescent organic light-emitting diodes (OLEDs). In host-guest systems of OLEDs, the guest can be excited by two competing mechanisms, namely, excitation energy transfer (EET) and charge transfer (CT). Experimentally, it has been reported that the EET mechanism is dominant and the excitons are primarily formed in the host first and then transferred to the guest in phosphorescent OLEDs based on exciplex-forming cohosts. With this, exciplex-forming cohosts are widely employed for avoiding the formation of trapped charge carriers in the phosphorescent guest. However, theoretical studies are still lacking toward elucidating the relative importance between EET and CT processes in exciting the guest molecules in such systems. Here, we obtain the kinetics of guest excitation processes in a few trimer model systems consisting of an exciplex-forming cohost pair and a phosphorescent guest. We adopt the Förster resonance energy transfer (FRET) rate constants for the electronic transitions between excited states toward solving kinetic master equations. The input parameters for calculating the FRET rate constants are obtained from density functional theory (DFT) and time-dependent DFT. The results show that while the EET mechanism is important, the CT mechanism may still play a significant role in guest excitations. In fact, the relative importance of CT over EET depends strongly on the location of the guest molecule relative to the cohost pair. This is understandable as both the coupling for EET and the interaction energy for CT are strongly influenced by the geometric constraints. Understanding the energy transfer pathways from the exciplex state of cohost to the emissive state of guest may provide insights for improving exciplex-forming materials adopted in OLEDs.

Optical Activity of Spin-Forbidden Electronic Transitions in Metal Complexes from Time-Dependent Density Functional Theory with Spin-Orbit Coupling

The calculation of magnetic transition dipole moments and rotatory strengths was implemented at the zeroth-order regular approximation (ZORA) two-component relativistic time-dependent density functional theory (TDDFT) level. The circular dichroism of the spin-forbidden ligand-field transitions of tris(ethylenediamine)cobalt(III) computed in this way agrees very well with available measurements. Phosphorescence dissymmetry factors g lum and the corresponding lifetimes are evaluated for three N-heterocyclic-carbene-based iridium complexes, two of which contain helicene moieties, and for two platinahelicenes. The agreement with experimental data is satisfactory. The calculations reproduce the signs and order of magnitude of g lum , and the large variations of phosphorescence lifetimes among the systems. The electron spin contribution to the magnetic transition dipole moment is shown to be important in all of the computations.

Spin-Orbit Natural Transition Orbitals and Spin-Forbidden Transitions

Natural transition orbitals (NTOs) are in widespread use for visualizing and analyzing electronic transitions. The present work introduces the analysis of formally spin-forbidden transitions with the help of complex-valued spin-orbit (SO) NTOs. The analysis specifically focuses on the components in such transitions that cause their intensity to be nonzero because of SO coupling. Transition properties such as transition dipole moments are partitioned into SO-NTO hole-particle pairs, such that contributions to the intensity from specific occupied and unoccupied orbitals are obtained. The method has been implemented within the restricted active space (RAS) self-consistent field wave function theory framework, with SO coupling treated by RAS state interaction. SO-NTOs have a broad range of potential applications, which is illustrated by the T₂-S₁ state mixing in pyrazine, spin-forbidden versus spin-allowed 4f-5d transitions in the Tb³⁺ ion, and the phosphorescence of tris(2-phenylpyridine) iridium [Ir(ppy)₃].

Radical Trifluoroacetylation of Alkenes Triggered by a Visible-Light-Promoted C-O Bond Fragmentation of Trifluoroacetic Anhydride

We report a mild and operationally simple trifluoroacylation strategy of olefines, that utilizes trifluoroacetic anhydride as a low‐cost and readily available reagent. This light‐mediated process is fundamentally different from conventional methodologies and occurs through a trifluoroacyl radical mechanism promoted by a photocatalyst, which triggers a C−O bond fragmentation. Mechanistic studies (kinetic isotope effects, spectroelectrochemistry, optical spectroscopy, theoretical investigations) highlight the evidence of a fleeting CF₃CO radical under photoredox conditions. The trifluoroacyl radical can be stabilized under CO atmosphere, delivering the trifluoroacetylation product with higher chemical efficiency. Furthermore, the method can be turned into a trifluoromethylation protocol by simply changing the reaction parameters. Beyond simple alkenes, this method allows for chemo‐ and regioselective functionalization of small‐molecule drugs and common pharmacophores.

The Quest to Simulate Excited-State Dynamics of Transition Metal Complexes

This Perspective describes current computational efforts in the field of simulating photodynamics of transition metal complexes. We present the typical workflows and feature the strengths and limitations of the different contemporary approaches. From electronic structure methods suitable to describe transition metal complexes to approaches able to simulate their nuclear dynamics under the effect of light, we give particular attention to build a bridge between theory and experiment by critically discussing the different models commonly adopted in the interpretation of spectroscopic experiments and the simulation of particular observables. Thereby, we review all the studies of excited-state dynamics on transition metal complexes, both in gas phase and in solution from reduced to full dimensionality.

Reliable TDDFT Protocol Based on a Local Hybrid Functional for the Prediction of Vibronic Phosphorescence Spectra Applied to Tris(2,2'-bipyridine)-Metal Complexes

An efficient computational protocol for the prediction of vibrationally resolved phosphorescence spectra is developed and validated for five tris(2,2'-bipyridine)-metal complexes ([M(bpy)₃]ⁿ⁺, where M = Zn, Ru, Rh, Os, Ir). The outstanding feature of this protocol is the use of full linear-response time-dependent density functional theory (TDDFT) for the excited-state triplet calculation, i.e., the commonly seen strategies employing the Tamm-Dancoff approximation (TDA) or unrestricted density functional theory (DFT) calculations for the T₁ state are not needed. This is achieved by the use of a local hybrid functional (LH12ct-SsirPW92) that features a real-space dependent admixture of exact exchange governed by a local mixing function. The excellent performance of this LH for triplet excitation energies known from previous studies transfers to a remarkable mean absolute error of 0.06 eV for the phosphorescence 0-0 energies investigated herein, while the popular B3PW91 functional gives an error of 0.27 eV in TDDFT and 0.09 eV in unrestricted DFT calculations, respectively. The advantages of the local hybrid are particularly apparent for excited states with a mixed-valence character. The influence of spin-orbit coupling was found to be significant for [Os(bpy)₃]²⁺ red-shifting the 0-0 energy for phosphorescence by 0.17 eV, while the effect is negligible for the other complexes (<0.03 eV). The influence of the basis-set and integration-grid sizes is evaluated, and a computationally lighter protocol is validated that leads to drastic savings in computation time with negligible loss in accuracy.

Understanding and Controlling Intersystem Crossing in Molecules

This review article focuses on the understanding of intersystem crossing (ISC) in molecules. It addresses readers who are interested in the phenomenon of intercombination transitions between states of different electron spin multiplicities but are not familiar with relativistic quantum chemistry. Among the spin-dependent interaction terms that enable a crossover between states of different electron spin multiplicities, spin-orbit coupling (SOC) is by far the most important. If SOC is small or vanishes by symmetry, ISC can proceed by electronic spin-spin coupling (SSC) or hyperfine interaction (HFI). Although this review discusses SSC- and HFI-based ISC, the emphasis is on SOC-based ISC. In addition to laying the theoretical foundations for the understanding of ISC, the review elaborates on the qualitative rules for estimating transition probabilities. Research on the mechanisms of ISC has experienced a major revival in recent years owing to its importance in organic light-emitting diodes (OLEDs). Exemplified by challenging case studies, chemical substitution and solvent environment effects are discussed with the aim of helping the reader to understand and thereby get a handle on the factors that steer the efficiency of ISC.

Behind the scenes of spin-forbidden decay pathways in transition metal complexes

The interpretation of the ultrafast photophysics of transition metal complexes following photo-absorption is quite involved as the heavy metal center leads to a complicated and entangled singlet-triplet manifold. This opens up multiple pathways for deactivation, often with competitive rates. As a result, intersystem crossing (ISC) and phosphorescence are commonly observed in transition metal complexes. A detailed understanding of such an excited-state structure and dynamics calls for state-of-the-art experimental and theoretical methodologies. In this review, we delve into the inability of non-relativistic quantum theory to describe spin-forbidden transitions, which can be overcome by taking into account spin-orbit coupling, whose importance grows with increasing atomic number. We present the quantum chemical theory of phosphorescence and ISC together with illustrative examples. Finally, a few applications are highlighted, bridging the gap between theoretical studies and experimental applications, such as photofunctional materials.

Ultrafast excited state relaxation dynamics in a heteroleptic Ir(iii) complex, fac-Ir(ppy)2(ppz), revealed by femtosecond X-ray transient absorption spectroscopy

The experimental and calculation results demonstrate that the ³ML_ppzCT state generated by the spin-forbidden transition rapidly relaxes to ³ML_ppyCT through internal conversion process with a time constant of ∼450 fs.

Phosphorescent κ3 -(N^C^C)-Gold(III) Complexes: Synthesis, Photophysics, Computational Studies and Application to Solution-Processable OLEDs

Efficient OLED devices have been fabricated using organometallic complexes of platinum group metals. Still, the high material cost and low stability represent central challenges for their application in commercial display technologies. Based on its innate stability, gold(III) complexes are emerging as promising candidates for high-performance OLEDs. Here, a series of alkynyl-, N-heterocyclic carbene (NHC)- and aryl-gold(III) complexes stabilized by a κ³ -(N^C^C) template have been prepared and their photophysical properties have been characterized in detail. These compounds exhibit good photoluminescence quantum efficiency (η_PL ) of up to 33 %. The PL emission can be tuned from sky-blue to yellowish green colors by variations on both the ancillary ligands as well as on the pincer template. Further, solution-processable OLED devices based on some of these complexes display remarkable emissive properties (η_CE 46.6 cd.A^-1 and η_ext 14.0 %), thus showcasing the potential of these motifs for the low-cost fabrication of display and illumination technologies.

Room-Temperature Phosphorescence and Low-Energy Induced Direct Triplet Excitation of Alq3 Engineered Crystals

Crystal engineering is a practical approach for tailoring material properties. This approach has been widely studied for modulating optical and electrical properties of semiconductors. However, the properties of organic molecular crystals are difficult to control following a similar engineering route. In this Letter, we demonstrate that engineered crystals of Alq₃ and Ir(ppy)₃ complexes, which are commonly used in organic light-emitting technologies, possess intriguing functional properties. Specifically, these structures not only process efficient low-energy induced triplet excitation directly from the ground state of Alq₃ but also can show strong emission at the Alq₃ triplet energy level at room temperatures. We associate these phenomena with local deformations of the host matrix around the guest molecules, which in turn lead to a stronger host-guest triplet-triplet coupling and spin-orbital mixing.

Visible Light-Induced Homolytic Cleavage of Perfluoroalkyl Iodides Mediated by Phosphines

In an effort to explain the experimentally observed variation of the photocatalytic activity of t Bu 3 P, n Bu 3 P and (MeO) 3 P in the blue-light regime [Helmecke et al., Org. Lett. 21 (2019) 7823], we have explored the absorption characteristics of several phosphine- and phosphite-IC 4 F 9 adducts by means of relativistic density functional theory and multireference configuration interaction methods. Based on the results of these computational and complementary experimental studies, we offer an explanation for the broad tailing of the absorption of t Bu 3 P-IC 4 F 9 and (MeO) 3 P-IC 4 F 9 into the visible-light region. Larger coordinate displacements of the ground and excited singlet potential energy wells in n Bu 3 P-IC 4 F 9 , in particular with regard to the P-I-C bending angle, reduce the Franck-Condon factors and thus the absorption probability compared to t Bu 3 P-IC 4 F 9 . Spectroscopic and computational evaluation of conformationally flexible and locked phosphites suggests that the reactivity of (MeO) 3 P may be the result of oxygen lone-pair participation and concomitant broadening of absorption. The proposed mechanism for the phosphine-catalyzed homolytic C-I cleavage of perfluorobutane iodide involves S₁ ← S₀ absorption of the adduct followed by intersystem crossing to the photochemically active T 1 state.

On the photophysical properties of IrIII, PtII, and PdII (phenylpyrazole) (phenyldipyrrin) complexes

The absorption and emission characteristics of (ppz)2(dipy)IrIII, (ppz)(dipy)PtII and (ppz)(dipy)PdII, where ppz stands for phenylpyrazole and dipy for a phenyl meso-substituted dipyrrin ligand, have been investigated by means of combined density functional theory and multireference configuration interaction including scalar relativistic and spin-orbit coupling effects. These results were compared with experimental spectra. The complexes exhibit a high density of low-lying electronically excited states originating from ligand-centered (LC) and metal-to-ligand charge transfer (MLCT) states involving the dipyrrin ligand. In addition, metal-centered (MC) states are found to be low-lying in the Pd complex. In all three cases, the first strong absorption band and the phosphorescence emission band stem from LC excitations on the dipyrrin ligand with small MLCT contributions. The MLCT states show more pronounced relaxation effects than the LC states, with the consequence that the first excited state with predominant singlet multiplicity is of SMLCT/LC type in the heavier Ir and Pt complexes. Substantial spin-orbit coupling between SMLCT/LC and TLC enables fast and efficient intersystem crossing (ISC) and a high triplet quantum yield. Phosphorescence rate constants are rather small in accord with the dominant LC character of the transitions. Out-of-plane distortion promotes nonradiative decay of the excited state population via the MC states thus explaining the lower phosphorescence quantum yield of the Pt complex. The spectral properties of the Pd complex are different in many aspects. Optimization of the S1 state yields a dipyrrin intraligand charge transfer (ILCT) state with highly distorted nuclear arrangement in the butterfly conformers leading to nonradiative deactivation. In contrast, the primarily excited SLC state and the SMLCT/LC state of the twist conformer have nearly equal adiabatic excitation energies. The lack of a driving force toward the SMLCT/LC minimum, the high fluorescence rate constant of the bright SLC state and its moderately efficient ISC to the triplet manifold explain the experimentally observed dual emission of the Pd complex at room temperature.

Structure-Emission Property Relationships in Cyclometalated Pt(II) β-Diketonate Complexes

Extending the ligand π-system of phosphorescent (C^∧C*) or (C^∧N) cyclometalated platinum(II) β-diketonate complexes can lead to large and seemingly abrupt variations of the photophysical properties such as triplet quantum yields and phosphorescence lifetimes. Quantum chemical studies using methods including elements from density functional theory (DFT) and multireference configuration interaction (MRCI) as well as spin-orbit coupling (SOC) provide a rationale for these observations. In the Franck-Condon region, the first excited singlet states (S₁) of these complexes are characterized by mixed metal-to-ligand charge-transfer (MLCT) and ligand-centered (LC) excitations. With increasing extension of the effective π-system, the lowest-lying triplet state yields more and more LC character, thus leading to a decrease of the phosphorescence rate constant. The ability to undergo efficient intersystem crossing from S₁ to T₁ is not diminished as the S₁ state largely retains its character. In the N-heterocyclic carbene (NHC) complexes investigated here, at least two triplet states are found energetically below the S₁ state. Out-of-plane distortion enhances the probability for nonradiative decay of the triplet population. In the smaller compounds emitting in the violet or blue spectral region, the phosphorescent state is separated from the lowest-lying dark metal-centered (MC) triplet state by a small barrier only, explaining their experimentally observed low photoluminescence quantum yields in liquid solution. The semiempirical DFT/MRCI-R2018 Hamiltonian employed in our studies proves well-suited for investigating the absorption and emission properties of these platinum(II) complexes. Generally, good agreement is observed between our calculated data and the experimental findings.

Eliminating nonradiative decay in Cu(I) emitters: >99% quantum efficiency and microsecond lifetime

Luminescent complexes of heavy metals such as iridium, platinum, and ruthenium play an important role in photocatalysis and energy conversion applications as well as organic light-emitting diodes (OLEDs). Achieving comparable performance from more-earth-abundant copper requires overcoming the weak spin-orbit coupling of the light metal as well as limiting the high reorganization energies typical in copper(I) [Cu(I)] complexes. Here we report that two-coordinate Cu(I) complexes with redox active ligands in coplanar conformation manifest suppressed nonradiative decay, reduced structural reorganization, and sufficient orbital overlap for efficient charge transfer. We achieve photoluminescence efficiencies >99% and microsecond lifetimes, which lead to an efficient blue-emitting OLED. Photophysical analysis and simulations reveal a temperature-dependent interplay between emissive singlet and triplet charge-transfer states and amide-localized triplet states.

Strong Duschinsky Mixing Induced Breakdown of Kasha's Rule in an Organic Phosphor

We present the novel observation that Duschinsky mixings can lead to the breakdown of Kasha's rule in a white light phosphor molecule, dibenzo[ b, d]thiophen-2-yl (4-chlorophenyl)methanone. Our theoretical analyses show the energy gap between the T₁ and T₂ states (0.48 eV) is too large to allow for any significant population of the T₂ state at room temperature and instead the faster intersystem crossing (ISC) between the S₁ and T₂ states is rather due to strong Duschinsky mixing, leading to the emission from the T₂ state as well. A second-order cumulant-based method has been used for the calculation of the ISC rate, which suggests 2 orders of magnitude faster ISC rates for S₁ → T₂ compared to those for S₁ → T₁. We found that the carbonyl moiety of the S₁ and T₂ states of the molecule is significantly different with respect to bond angle and dihedral angles, engendering large displacements in selective normal modes, thus giving rise to strong Duschinsky mixing.

DFT/MRCI-R2018 study of the photophysics of the zinc(ii) tripyrrindione radical: non-Kasha emission?

The fluorescence of a radical-based emitter has been theoretically investigated after measurements had shown absorption bands to lie below the emission energy. The results of the all-multiplicity DFT/MRCI-R2018 study indicate D₃ emission.

Unraveling the Microscopic Origin of Triplet Lasing from Organic Solids

We present a heuristic mechanism for the origin of the unusual triplet lasing from (E)-3-(((4-nitrophenyl)imino)methyl)-2H-thiochroman-4-olate·BF₂.We demonstrate that whereas the moderate lifetime (1.03 μs) of the first triplet state (T₁) prohibits triplet-triplet annihilation, the relatively faster S₁ → T₁ intersystem crossing and the 10⁴ times smaller reverse intersystem crossing effectively help achieve population inversion in the T₁ state. Furthermore, the triplet lasing wavelength (675 nm) for the tetramer does not overlap with the triplet-triplet absorptions wavelength, indicating that the spin-forbidden emission cross section is very large. Additionally, the almost complete absence of a vibrational progression in the vibronic phosphorescence spectrum of the monomer plays an important role in ensuring efficient triplet-state lasing from this organic material. Our results show that controlling the triplet-state lifetimes combined with lowering of the triplet-triplet absorption in the emission region and small vibronic coupling will be the key steps when designing novel organic triplet-lasing materials.

First-principles method for calculating the rate constants of internal-conversion and intersystem-crossing transitions

A method for calculating the rate constants for internal-conversion (k_IC) and intersystem-crossing (k_ISC) processes within the adiabatic and Franck–Condon (FC) approximations is proposed.

Anomalous Phosphorescence from an Organometallic White-Light Phosphor

We report theoretical results on the possible violation of Kasha's rule in the phosphorescence process of (acetylacetonato)bis(1-methyl-2-phenylimidazole)iridium(III) and show that the anomalous emission from both the T₁ and T₂ states is key to its white-light phosphorescence. This analysis is supported by the calculated Boltzmann-averaged phosphorescence lifetime of 2.21 μs, estimated including both radiative and nonradiative processes and in excellent agreement with the experimentally reported value of 1.96 ± 0.1 μs. The T₂ state is found to be of metal-to-ligand charge-transfer character (dπ → nπ), and the d orbital contribution comes from 5d_z² and 5d_x²-y², whereas the S₁ and T₁ states both have dπ-pπ character with significant 5d_xz orbital contribution, allowing for efficient intersystem crossing from the S₁ to the T₂ state and, in turn, phosphorescence from the T₂ state. Our results open new opportunities for tailoring the phosphorescence wavelength and thus the design of molecules with improved photovoltaic properties.

Trajectory Surface-Hopping Dynamics Including Intersystem Crossing in [Ru(bpy)3]2

Surface-hopping dynamics coupled to linear response TDDFT and explicit nonadiabatic and spin-orbit couplings have been used to model the ultrafast intersystem crossing (ISC) dynamics in [Ru(bpy)₃]²⁺. Simulations using an ensemble of trajectories starting from the singlet metal-to-ligand charge transfer (¹MLCT) band show that the manifold of ³MLCT triplet states is first populated from high-lying singlet states within 26 ± 3 fs. ISC competes with an intricate internal conversion relaxation process within the singlet manifold to the lowest singlet state. Normal-mode analysis and principal component analysis, combined with further dynamical simulations where the nuclei are frozen, unequivocally demonstrate that it is not only the high density of states and the large spin-orbit couplings of the system that promote ISC. Instead, geometrical relaxation involving the nitrogen atoms is required to allow for state mixing and efficient triplet population transfer.

Theory and Calculation of the Phosphorescence Phenomenon

Phosphorescence is a phenomenon of delayed luminescence that corresponds to the radiative decay of the molecular triplet state. As a general property of molecules, phosphorescence represents a cornerstone problem of chemical physics due to the spin prohibition of the underlying triplet-singlet emission and because its analysis embraces a deep knowledge of electronic molecular structure. Phosphorescence is the simplest physical process which provides an example of spin-forbidden transformation with a characteristic spin selectivity and magnetic field dependence, being the model also for more complicated chemical reactions and for spin catalysis applications. The bridging of the spin prohibition in phosphorescence is commonly analyzed by perturbation theory, which considers the intensity borrowing from spin-allowed electronic transitions. In this review, we highlight the basic theoretical principles and computational aspects for the estimation of various phosphorescence parameters, like intensity, radiative rate constant, lifetime, polarization, zero-field splitting, and spin sublevel population. Qualitative aspects of the phosphorescence phenomenon are discussed in terms of concepts like structure-activity relationships, donor-acceptor interactions, vibronic activity, and the role of spin-orbit coupling under charge-transfer perturbations. We illustrate the theory and principles of computational phosphorescence by highlighting studies of classical examples like molecular nitrogen and oxygen, benzene, naphthalene and their azaderivatives, porphyrins, as well as by reviewing current research on systems like electrophosphorescent transition metal complexes, nucleobases, and amino acids. We furthermore discuss modern studies of phosphorescence that cover topics of applied relevance, like the design of novel photofunctional materials for organic light-emitting diodes (OLEDs), photovoltaic cells, chemical sensors, and bioimaging.

Thermally Tunable Dual Emission of the d(8)-d(8) Dimer [Pt2(μ-P2O5(BF2)2)4](4)

High-resolution fluorescence, phosphorescence, as well as related excitation spectra, and, in particular, the emission decay behavior of solid [Bu4N]4[Pt2(μ-P2O5(BF2)2)4], abbreviated Pt(pop-BF2), have been investigated over a wide temperature range, 1.3-310 K. We focus on the lowest excited states that result from dσ*pσ (5dz(2)-6pz) excitations, i.e., the singlet state S1 (of (1)A2u symmetry in D4h) and the lowest triplet T1, which splits into spin-orbit substates A1u((3)A2u) and Eu((3)A2u). After optical excitation, an unusually slow intersystem crossing (ISC) is observed. As a consequence, the compound shows efficient dual emission, consisting of blue fluorescence and green phosphorescence with an overall emission quantum yield of ∼ 100% over the investigated temperature range. Our investigation sheds light on this extraordinary dual emission behavior, which is unique for a heavy-atom transition metal compound. Direct ISC processes in Pt(pop-BF2) are largely forbidden due to spin-, symmetry-, and Franck-Condon overlap-restrictions and, therefore, the ISC time is as long as 29 ns for T < 100 K. With temperature increase, two different thermally activated pathways, albeit still relatively slow, are promoted by spin-vibronic and vibronic mechanisms, respectively. Thus, distinct temperature dependence of the ISC processes results and, as a consequence, also of the fluorescence/phosphorescence intensity ratio. The phosphorescence lifetime also is temperature-dependent, reflecting the relative population of the triplet T1 substates Eu and A1u. The highly resolved phosphorescence shows a ∼ 220 cm(-1) red shift below 10 K, attributable to zero-field splitting of 40 cm(-1) plus a promoting vibration of 180 cm(-1).

Quantitative prediction of photoluminescence quantum yields of phosphors from first principles

The first quantitative prediction of the photoluminescence quantum yields (PLQY) of a series of blue-to-green Ir(iii) complexes is presented.

Optimizing the photoluminescence quantum yields of Ir(iii) complexes is the key to their application as phosphors in organic light-emitting diodes (OLEDs). This work demonstrates for the first time that quantitative predictions of photoluminescence quantum yields (PLQY) in a series of blue-to-green Ir(iii) complexes can be derived exclusively from electronic structure calculations.