QM/MM studies on luminescence mechanism of dinuclear copper iodide complexes with thermally activated delayed fluorescence

Strategy for an Effective Eco-Optimized Design of Heteroleptic Cu(I) Coordination Polymers Exhibiting Thermally Activated Delayed Fluorescence

The new series of copper(I) coordination polymers [Cu(N-N)(μ-PTA)]n[PF6]n {N-N = dmbpy (1), bpy (2), ncup (3), and phen (4)} were generated by straightforward reaction in solution or through a mechanochemical route, of [Cu(MeCN)4][PF6] with 1,3,5-triaza-7-phosphaadamantane (PTA) and the corresponding polypyridines, namely, 5,5'-dimethyl-2,2'-bipyridine (dmbpy), 2,2'-bipyridine (bpy), 2,9-dimethyl-1,10-phenanthroline (ncup), and 1,10-phenanthroline (phen). The compounds were obtained as air-stable solids and fully characterized by IR, NMR spectroscopy, and elemental analyses. The molecular structures were confirmed by single-crystal X-ray diffraction analysis (for 1, 2, and 4), revealing infinite one-dimensional (1D) linear chains driven by μ-PTA N,P-linkers. All tested Cu(I) polymeric compounds show emission at room temperature, which was attributed to thermally activated delayed fluorescence (TADF). Evidence of the involvement of the excited singlet state in the emission process is presented. Comparing the photophysical properties of 1 and 2 as well as 3 and 4, of which 1 and 3 have a stiffened structure, by introducing a methyl group to one of the ligands, we demonstrate how TADF properties depend on molecular rigidity. It is shown that stiffening of the structure reduces the flattening distortion around the Cu(I) center in the 3MLCT state. As a result, the ΔE(S1-T1) energy gap becomes smaller and the fluorescence quantum yield increases without significantly extending the emission lifetime. In particular, the ΔE(S1-T1) values for complexes 1 and 3 are among the shortest reported in the scientific literature, 253 and 337 cm-1, and the TADF lifetimes are τ(300 K) = 5.7 and 4.2 μs, respectively. The fluorescence quantum yields for these complexes are measured to be ΦPL(300 K) = 70 and 80%.

Theoretical studies on thermally activated delayed fluorescence of "carbene-metal-amide" Cu and Au complexes: geometric structures, excitation characters, and mechanisms

"Carbene-metal(I)-amide" (CMA) complexes have garnered significant attention due to their remarkable properties and potential TADF applications in organic electronics. However, the atomistic working mechanism is still elusive. Herein, we chose two CMA complexes, i.e., cyclic (alkyl)(amino) carbene-copper[gold](I)-carbazole (CAAC-Cu[Au]-Cz), and employed both DFT and TD-DFT methods, in combination with radiative and nonradiative rate calculations, to investigate geometric and electronic structures of these two complexes in the ground and excited states, including orbital compositions, electronic transitions, absorption and emission spectra, and the luminescence mechanism. It is found that the coplanar or perpendicular conformations are coexistent in the ground state (S0), the lowest excited singlet state (S1), and the triplet state (T1). Both the coplanar and perpendicular S1 and T1 states have similar ligand-to-ligand charge transfer (LLCT) character between CAAC and Cz, and some charge-transfer character between metal atoms and ligands, which is beneficial to minimize the singlet-triplet energy gaps (ΔEST) and increase the spin-orbit coupling (SOC). An interesting three-state (S0, S1, T1) model involving two regions (coplanar and perpendicular) is proposed to rationalize the experimental TADF phenomena in the CMA complexes. In addition to the coplanar ones, the perpendicular S1 and T1 states also play a role in promoting the repopulation of the coplanar S1 exciton, which is a primary source for the delayed fluorescence.

Thermally activated delayed fluorescence of a Ir(III) complex: absorption and emission properties, nonradiative rates, and mechanism

One recent experimental study reported a Ir(III) complex with thermally activated delayed fluorescence (TADF) phenomenon in solution, but its luminescent mechanism is elusive. In this work, we combined density functional theory (DFT), time-dependent DFT (TDDFT) and multi-state complete active space second-order perturbation theory (MS-CASPT2) methods to investigate excited-state properties, photophysics, and emission mechanism of this Ir(III) complex. Two main absorption bands observed in experiments can be attributed to the electronic transition from the S₀ state to the S₁ and S₂ states; while, the fluorescence and phosphorescence are generated from the S₁ and T₁ states, respectively. Both the S₁ and T₁ states have clear metal-to-ligand charge transfer (MLCT) character. The present computational results reveal a three-state model including the S₀, S₁ and T₁ states to rationalize the TADF behavior. The small energy gap between the S₁ and T₁ states benefits the forward and reverse intersystem crossing (ISC and rISC) processes. At 300 K, the rISC rate is five orders of magnitude larger than the phosphorescence rate therefore enabling TADF. At 77 K, the rISC rate is sharply decreased but remains close to the phosphorescence rate; therefore, in addition to the phosphorescence, the delayed fluorescence could also contribute to the experimental emission. The estimated TADF lifetime agrees well with experiments, 9.80 vs. 6.67 μs, which further verifies this three-state model.

Thermally Activated Delayed Fluorescence of a Pyromellitic Diimide Derivative in the Film Environment Investigated by Combined QM/MM and MS-CASPT2 Methods

Arylene diimide compounds exhibit thermally activated delayed fluorescence (TADF), but its mechanism remains elusive. Herein we studied the TADF mechanism of a carbazole-substituted pyromellitic diimide derivative (CzPhPmDI) in poly(methyl methacrylate) (PMMA) film by using DFT, TD-DFT, and MS-CASPT2 methods within the QM/MM framework. We found that the TADF mechanism involves three electronic states (i.e., S₀, S₁, and T₁), but the T₂ state is not involved because its energy is higher than the S₁ state by 6.9 kcal/mol. By contrast, the T₁ state is only 3.2 kcal/mol lower than the S₁ state and such small energy difference benefits the reverse intersystem crossing (rISC) process from T₁ to S₁ thereto TADF. This point is seconded by relevant radiative and nonradiative rates calculated. At room temperature, the ISC rate from S₁ to T₁ is calculated to be 6.1 × 10⁶ s^-1, which is larger than the fluorescence emission rate, 2.2 × 10⁵ s^-1; thus, the dominant S₁ population converts to the T₁ state. However, in the T₁ state, the rISC process (1.8 × 10⁴ s^-1) becomes the most important channel because of the negligible phosphorescence emission rate (3.5 × 10^-2 s^-1). So, the T₁ population is still converted back to the S₁ state to fluoresce enabling TADF. Unfortunately, the rISC process is blocked in low temperature. Besides, we found that relevant Huang-Rhys factors have dominant contribution from low-frequency vibrational motion related to the torsional motion of functional groups. These gained insights could provide useful information for the design of organic TADF materials with excellent luminescence efficiency.

TADF: Enabling luminescent copper(i) coordination compounds for light-emitting electrochemical cells

The last decade has seen a surge of interest in the emissive behaviour of copper(i) coordination compounds, both neutral compounds that may have applications in organic light-emitting doides (OLEDs) and copper-based ionic transition metal complexes (Cu-iTMCs) with potential use in light-emitting electrochemical cells (LECs). One of the most exciting features of copper(i) coordination compounds is their possibility to exhibit thermally activated delayed fluorescence (TADF) in which the energy separation of the excited singlet (S₁) and excited triplet (T₁) states is very small, permitting intersystem crossing (ISC) and reverse intersystem crossing (RISC) to occur at room temperature without the requirement for the large spin-orbit coupling inferred by the presence of a heavy metal such as iridium. In this review, we focus mainly in Cu-iTMCs, and illustrate how the field of luminescent compounds and those exhibiting TADF has developed. Copper(i) coordination compounds that class as Cu-iTMCs include those containing four-coordinate [Cu(P^P)(N^N)]⁺ (P^P = large-bite angle bisphosphane, and N^N is typically a diimine), [Cu(P)₂(N^N)]⁺ (P = monodentate phosphane ligand), [Cu(P)(tripodal-N₃)]⁺, [Cu(P)(N^N)(N)]⁺ (N = monodentate N-donor ligand), [Cu(P^P)(N^S)]⁺ (N^S = chelating N,S-donor ligand), [Cu(P^P)(P^S)]⁺ (P^S = chelating P,S-donor ligand), [Cu(P^P)(NHC)]⁺ (NHC = N-heterocyclic carbene) coordination domains, dinuclear complexes with P^P and N^N ligands, three-coordinate [Cu(N^N)(NHC)]⁺ and two-coordinate [Cu(N)(NHC)]⁺ complexes. We pay particular attention to solid-state structural features, e.g. π-stacking interactions and other inter-ligand interactions, which may impact on photoluminescence quantum yields. Where emissive Cu-iTMCs have been tested in LECs, we detail the device architectures, and this emphasizes differences which make it difficult to compare LEC performances from different investigations.

Thermally Activated Delayed Fluorescence Mechanism of a Bicyclic "Carbene-Metal-Amide" Copper Compound: DFT/MRCI Studies and Roles of Excited-State Structure Relaxation

Herein we investigated the luminescence mechanism of one "carbene-metal-amide" copper compound with thermally activated delayed fluorescence (TADF) using density functional theory (DFT)/multireference configuration interaction, DFT, and time-dependent DFT methods with the polarizable continuum model. The experimentally observed low-energy absorption and emission peaks are assigned to the S₁ state, which exhibits clear interligand and partial ligand-to-metal charge-transfer character. Moreover, it was found that a three-state (S₀, S₁, and T₁) model is sufficient to describe the TADF mechanism, and the T₂ state should play a negligible role. The calculated S₁-T₁ energy gap of 0.10 eV and proper spin-orbit couplings facilitate the reverse intersystem crossing (rISC) from T₁ to S₁. At 298 K, the rISC rate of T₁ → S₁ (∼10⁶ s^-1) is more than 3 orders of magnitude larger than the T₁ phosphorescence rate (∼10³ s^-1), thereby enabling TADF. However, it disappears at 77 K because of a very slow rISC rate (∼10¹ s^-1). The calculated TADF rate, lifetime, and quantum yield agree very well with the experimental data. Methodologically, the present work shows that only considering excited-state information at the Franck-Condon point is insufficient for certain emitting systems and including excited-state structure relaxation is important.

First-principles identification of VI + Cuidefect cluster in cuprous iodide: origin of red light photoluminescence

Theγ-phase cuprous iodide (CuI) emerges as a promising transparent p-type semiconductor for next-generation display technology because of its wide direct band gap, intrinsic p-type conductivity, and high carrier mobility. Two main peaks are observed in its photoluminescence (PL). One is short wavelength (410-430 nm) emission, which is well attributed to the electronic transitions at Cu vacancy, whereas the other long wavelength emission (680-720 nm) has not been fully understood. In this paper, through first-principles simulations, we investigate the formation energies and emission line shapes for various defects, and discover that the intrinsic point defect clusterVI+Cui2+is the source of the long wavelength emission. Our finding is further supported by the prediction that the defect concentration decreases dramatically as the chemical condition changes from Cu-rich to I-rich, explaining the significant reduction in the red light emission if CuI is annealed in abundant I environment.

Thermally activated delayed fluorescence: A critical assessment of environmental effects on the singlet-triplet energy gap

The effective design of dyes optimized for thermally activated delayed fluorescence (TADF) requires the precise control of two tiny energies: the singlet-triplet gap, which has to be maintained within thermal energy, and the strength of spin-orbit coupling. A subtle interplay among low-energy excited states having dominant charge-transfer and local character then governs TADF efficiency, making models for environmental effects both crucial and challenging. The main message of this paper is a warning to the community of chemists, physicists, and material scientists working in the field: the adiabatic approximation implicitly imposed to the treatment of fast environmental degrees of freedom in quantum-classical and continuum solvation models leads to uncontrolled results. Several approximation schemes were proposed to mitigate the issue, but we underline that the adiabatic approximation to fast solvation is inadequate and cannot be improved; rather, it must be abandoned in favor of an antiadiabatic approach.

A new application of terahertz time-domain absorption spectra in luminescent complexes: characterization of the C-Hπ weak interactions in Cu(I) complexes

Six Cu(i) complexes, [Cu(2,3-f)(bdppmapy)]BF4 (1), [Cu(2,3-f)(bdppmapy)]ClO4 (2), [Cu(2,3-f)(bdppmapy)]CF3SO3 (3), [Cu(imidazo[4,5-f])(bdppmapy)]BF4 (4), [Cu(imidazo[4,5-f])(bdppmapy)]ClO4 (5), and [Cu(imidazo[4,5-f])(bdppmapy)]CF3SO3·MeOH (6·MeOH) (bdppmapy = N,N-bis[(diphenylphosphino)methyl]-2-pyridinamine, 2,3-f = pyrazine[2,3-f][1,10]-phenanthroline, and imidazo[4,5-f] = 1H-imidazo[4,5-f][1,10]-phenanthroline), have been synthesized to explore the effects of counteranions on their crystal structures, photophysical properties, and terahertz (THz) spectra. Time-dependent density functional theory (TD-DFT) shows that the luminescence performance of these complexes is attributed to the metal-to-ligand charge transfer (MLCT) in combination with ligand-to-ligand charge transfer (LLCT). In complexes 1-3, the characteristic peak at 1.4 THz is mainly related to the C-Hπ interaction formed by the H atom on the 4#/5# position of 2,3-f and the benzene ring from the bdppmapy on the adjacent asymmetric unit. The common C-Hπ interaction enhances the rigidity of the structure and has non-negligible influence on the photoluminescence quantum yields (PLQYs): the stronger the C-Hπ interaction is, the higher the quantum yield (QY) is. In complexes 4-6, similar absorption peaks (1.10-1.30 THz) are mainly related to the C-Hπ interactions, and strong absorption peaks (1.50-1.90 THz) are affected by the typical hydrogen bonds N-HF/O and O-HO. These results show that some weak interactions can be characterized by THz time-domain spectroscopy (THz-TDS). So, the THz spectroscopy method would make it possible to tune some of the weak interactions in complex structures to regulate the luminescence of materials.