Insights from QM/MM-ONIOM calculations: the TADF phenomenon of phenanthro[9,10-d]imidazole-anthraquinone in the solid state

Combined DFT and QM/MM Modeling on Multifunctional TADF Sensitizers and Hot-Exciton Emitters via Carborane Triads for Blue Hyperfluorescent OLEDs

The scarcity of novel luminogens significantly impedes the advancement of TADF sensitizers and hot exciton emitters, attracting considerable attention for their potential to enhance energy conversion efficiencies in hyperfluorescent OLEDs. In this study, a systematic investigation is employed to design and develop multifunctional materials based on carborane cores through DFT and TD-DFT methods. In pursuit of this objective, 45 carborane triad-type molecules were systematically designed using four donors and two acceptor units. Electronic structure calculations revealed that (i) the singlet, triplet, ΔEST, and SOC values exhibit an increased trend as the carborane core shifts from ortho to meta to para, while an increase in donor strength on the core leads to a decrease in these values. (ii) Moreover, there is a decrease in reorganization energies, absorption wavelengths, ISC, and RISC rates as the carborane switches from ortho to meta to para while witnessing an increase in donor strength. The QM/MM study reveals that para carborane's restricted intramolecular motions improve its solid-state aggregation over ortho carborane and solution phases. Interestingly, carborane triads featuring P-DMB and P-BODIPY acceptor units satisfy the desired criteria for multifunctional TADF sensitizers and hot exciton emitters, respectively.

The luminescent and reaction mechanisms of a fluorescent probe for the detection of hypochlorous acid: Insights from theory

We have designed and synthesized a novel fluorescent probe BMH for detection of hypochlorous acid (HClO), which can increase dramatically the fluorescence intensity and had ultrafast response, a low detection limit and a wide pH range of application. In this paper, we further studied its fluorescence quantum yield and photoluminescence mechanism theoretically. The calculated results indicated the first excited states of BMH and BM (it was the oxidized product by HClO) were bright states with large oscillator strengths, however, due to more larger reorganization energy of BMH, the predicted internal conversion rate k_IC of BMH was four orders of magnitude larger than that of BM; moreover, owing to the effect of heavy atom from sulfur atom in BMH, the predicted intersystem crossing rate k_isc of BMH was five orders of magnitude larger than that of BM; meanwhile there was no significant difference found between both the predicted radiative rates k_r, thus the calculated fluorescence quantum yield of BMH was nearly zero and that of BM was more than 90%, the data showed the BMH had no fluorescence but its oxidated produce BM possessed strong fluorescence. In addition, the reaction mechanism of BMH transforming into BM has been investigated too, according to the potential energy profile, we found that the course of BMH converting into BM consisted of three elementary reactions. The research results revealed the solvent effect can decreased the activation energy, which was more favorable for these elementary reactions.

Fluorescent Mechanism of a Highly Selective Probe for Copper(II) Detection: A Theoretical Study

A highly selective probe for copper(II) detection based on the dansyl group was theoretically studied by means of (time-dependent) density functional theory. The calculated results indicated that the oscillator strength of the fluorescent process for the probe molecule is considerably large, but the counterpart of its copper(II) complex is nearly zero; therefore, the predicted radiative rate k_r of the probe is several orders of magnitude larger than that of its complex; however, the predicted internal conversion rate k_ic of both the probe and its complex is of the same order of magnitude. In addition, the simulated intersystem crossing rate k_isc of the complex is much greater than that of the probe due to the effect of heavy atom from the copper atom in the complex. Based on the above information, the calculated fluorescence quantum yield of the probe is 0.16% and that of the complex becomes 10^-6%, which implies that the first excited state of the probe is bright state and that of the complex is dark state. For the complex, the hole-electron pair analysis indicates that the process of S₀ → S₁ belongs to metal-to-ligand charge transfer; its density-of-state diagram visually illustrates that the highest occupied molecular orbital (HOMO) contains the ingredient of the s orbital from the copper atom, which decreases the frontier orbital energy level and the overlap integral of HOMO and LUMO.

Theoretical perspective for substitution effect on luminescent properties of through space charge transfer-based thermally activated delayed fluorescence molecules

Recently, through space charge transfer (TSCT)-based thermally activated delayed fluorescence (TADF) molecules have shown advantages in achieving high efficiencies and tunable emissions. However, the relationships between basic molecular structures and luminescent properties are unclear. Theoretical investigations to reveal the substitution effects with different numbers and positions on excited-state properties are highly desired. Herein, by taking TSCT-based TADF molecules S-CNDF-S-tCz, S-CNDF-D-tCz and T-CNDF-T-tCz as skeletons, a series of promising TADF molecules are designed by adopting ortho, meta and para substitutions with different numbers and positions. Photophysical properties of total 16 molecules are theoretically studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods in chloroform combined with polarizable continuum model. Results indicate that molecules with ortho-substitution possess small geometric changes and short Donor-Acceptor distances which are induced by the intramolecular van der Waals interactions. Decreased non-radiative consumption and increased TSCT ratio and therefore excellent performance for them can be expected. For molecules with large substitution numbers, twist structures facilitate them to realize small adiabatic energy gaps between the lowest singlet excited state (S₁) and the lowest triplet excited state (T₁), this designing strategy is consistent with the TADF dendrimers. Thus, the relationships between molecular structures and luminescent properties are revealed and promising TSCT-based TADF molecules with high efficiencies are theoretically proposed. Our investigations provide theoretical perspectives for inner mechanisms of substitution effect, which could further afford meaningful guidance to design new efficient TSCT-based TADF molecules.

First Hyperpolarizabilities of Intramolecular Charge-Transfer Architectures Based on Acenaphthene Derivatives in Gas, Solution, and Solid States

Constructing charge transfer (CT) systems and packing arrangement are common and effective methods to control the efficiency of nonlinear optical (NLO) materials. Apart from the traditional through-bond CT (TBCT) systems, through-space CT (TSCT) also leads to distinctive optical and electronic properties. Meanwhile, corresponding theoretical investigations of the aggregation effect are highly desired. In this work, some TSCT and model compounds incorporating acenaphthene as a scaffold and triphenylamine (TPA) as the donor are theoretically performed to systematically reveal the effect of both solvent and solid environments on their static first hyperpolarizabilities (β_tot) by using the polarizable continuum model (PCM) and the combined quantum mechanics and molecular mechanics (QM/MM) method. Results indicate that the dichloromethane solvent effect within the PCM approach causes an almost 2 times increase of the β_tot values. Besides, the different packing modes and intermolecular interactions have remarkable influence on the second-order NLO properties. For the case of TPA-ace-CN in the crystal state, the parallel arrangement will lead to large NLO responses (4.9 × 10^-30 esu) compared to the correspondingly isolated molecule (3.4 × 10^-30 esu). However, for the TPA-ace-TRZ compound with the TSCT architecture, selection of the molecular arrangement may make the aggregate ineffective due to the offset of the through-space dipole and charge transfer between D-A groups, which lead to the β_tot values decreasing from 15.2 × 10^-30 to 7.7 × 10^-30 esu. We believe that our calculation will serve as a guide for the exploration of more efficient NLO materials wherein the molecules are oriented in their most favorable arrangements.

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.