Extended theoretical modeling of reverse intersystem crossing for thermally activated delayed fluorescence materials.

Thermally activated delayed fluorescence (TADF) materials and multi-resonant (MR) variants are promising organic emitters that can achieve an internal electroluminescence quantum efficiency of ~100%. The reverse intersystem crossing (RISC) is key for harnessing triplet energies for fluorescence. Theoretical modeling is thus crucial to estimate its rate constant (kRISC) for material development. Here, we present a comprehensive assessment of the theory for simulating the RISC of MR-TADF molecules within a perturbative excited-state dynamics framework. Our extended rate formula reveals the importance of the concerted effects of nonadiabatic spin-vibronic coupling and vibrationally induced spin-orbital couplings in reliably determining kRISC of MR-TADF molecules. The excited singlet-triplet energy gap is another factor influencing kRISC. We present a scheme for gap estimation using experimental Arrhenius plots of kRISC. Erroneous behavior caused by approximations in Marcus theory is elucidated by testing 121 MR-TADF molecules. Our extended modeling offers in-depth descriptions of kRISC.

Theoretical Insight into the Effect of Phosphorus Oxygenation on Nonradiative Decays: Comparative Analysis of P-Bridged Stilbene Analogs.

Incorporation of the phosphorus element into a π-conjugated skeleton offers valuable prospects for adjusting the electronic structure of the resulting functional π-electron systems. Trivalent phosphorus has the potential to decrease the LUMO level through σ*-π* interaction, which is further enhanced by its oxygenation to the pentavalent P center. This study shows that utilizing our computational analysis to examine excited-state dynamics based on radiative/nonradiative rate constants and fluorescence quantum yield (ΦF) is effective for analyzing the photophysical properties of P-containing organic dyes. We theoretically investigate how the trivalent phosphanyl group and pentavalent phosphine oxide moieties affect radiative and nonradiative decay processes. We evaluate four variations of P-bridged stilbene analogs. Our analysis reveals that the primary decay pathway for photoexcited bis-phosphanyl-bridged stilbene is the intersystem crossing (ISC) to the triplet state and nonradiative. The oxidation of the phosphine moiety, however, suppresses the ISC due to the relative destabilization of the triplet states. The calculated rate constants match an increase in experimental ΦF from 0.07 to 0.98, as simulated from 0.23 to 0.94. The reduced HOMO-LUMO gap supports a red shift in the fluorescence spectra relative to the phosphine analog. The thiophene-fused variant with the nonoxidized trivalent P center exhibits intense emission with a high ΦF, 0.95. Our prediction indicates that the ISC transfer is obstructed owing to the relatively destabilized triplet state induced by the thiophene substitution. Conversely, the thiophene-fused analog with the phosphine oxide moieties triggers a high-rate internal conversion mediated by conical intersection, leading to a decreased ΦF.

5-Thiaporphyrinium cation: effect of sulphur incorporation on excited state dynamics.

We synthesised thionium-ion embedded aromatic porphyrinoids: a free-base 5-thiaporphyrinium cation and its zinc complex. The sulphur atom effectively participates in the macrocyclic π-conjugation. Fluorescence quantum yields of thiaporphyrinium cations were lower than 1% unlike oxaporphyrinium cations. Detailed photophysical analysis and DFT calculations clarified the vibrational mode regarding the out-of-plane motion of the sulphur atom induced ultrafast quenching of the excited state in comparison to the corresponding oxaporphyrinium cations.

Substituent and Solvent Effects on the Photoisomerization of Cinnamate Derivatives: An XMS-CASPT2 Study.

Cinnamate derivatives show a variety of photo-induced reactions. Among them is trans-cis photoisomerization, which may involve the nonradiative decay (NRD) process. The extended multistate complete active space second-order perturbation (XMS-CASPT2) method was used in this study as a suitable theory for treating multireference electronic nature, which was frequently manifested in the photoisomerization process. The minimum energy paths of the trans-cis photoisomerization process of cinnamate derivatives were determined, and the activation energies were estimated using the resulting intrinsic reaction coordinate (IRC) paths. Natural orbital analysis revealed that the transition state's (TS) electronic structure is zwitterionic-like, elucidating the solvent and substituent effect on the energy barrier of photoisomerization paths. Furthermore, it was found that the charge on the pyramidalized carbon atom at the TS structure was strongly correlated with the activation energy barrier for the cinnamate derivatives. Thus, it seemingly provided a physical picture of the photoisomerization of cinnamates and was a good descriptor potentially applicable to molecular design for controlling the rate constant of the photoisomerization reaction.

Donor-acceptor-acceptor-type near-infrared fluorophores that contain dithienophosphole oxide and boryl groups: effect of the boryl group on the nonradiative decay.

The use of donor-π-acceptor (D-π-A) skeletons is an effective strategy for the design of fluorophores with red-shifted emission. In particular, the use of amino and boryl moieties as the electron-donating and -accepting groups, respectively, can produce dyes that exhibit high fluorescence and solvatochromism. Herein, we introduce a dithienophosphole P-oxide scaffold as an acceptor-spacer to produce a boryl- and amino-substituted donor-acceptor-acceptor (D-A-A) π-system. The thus obtained fluorophores exhibit emission in the near-infrared (NIR) region, while maintaining high fluorescence quantum yields even in polar solvents (e.g. λ em = 704 nm and Φ F = 0.69 in CH3CN). A comparison of these compounds with their formyl- or cyano-substituted counterparts demonstrated the importance of the boryl group for generating intense emission. The differences among these electron-accepting substituents were examined in detail using theoretical calculations, which revealed the crucial role of the boryl group in lowering the nonradiative decay rate constant by decreasing the non-adiabatic coupling in the internal conversion process. The D-A-A framework was further fine-tuned to improve the photostability. One of these D-A-A dyes was successfully used in bioimaging to visualize the blood vessels of Japanese medaka larvae and mouse brain.

Investigating the Nonradiative Decay Pathway in the Excited State of Silepin Derivatives: A Study with Second-Order Multireference Perturbation Wavefunction Theory.

The fluorescence quantum yield for fluorescent organic molecules is an important molecular property, and tuning it up is desired for various applications. For the computational estimation of the fluorescence quantum yield, the theoretical prediction of the nonradiative decay rate constant has become an attractive subject of study. The rate constant of thermally activated nonradiative decay is related to the activation energy in the photoreaction; thus, the accuracy and reliability of the excited-state potential energies in the quantum chemical computation are critical. In this study, we employed a second-order multireference perturbation wavefunction theory for studying the thermally activated decay via conical intersection (CI) of 1,1-dimethyldibenzo[b,f]silepin derivatives. The correlation between the computed activation energy to reach the CI geometry in the S1 state and the experimentally determined fluorescence quantum yield implied that silepins nonradiatively decay via the CI triggered by the twisting of the central C-C bond. Geometry optimization of the transition state using multireference perturbation theory drastically reduced the estimated activation energy. Our computation gave reasonable predictions of the activation free energies of photoexcited 1,1-dimethyldibenzo[b,f]silepin. The energy profiles and geometry optimizations using proper quantum chemical methods played a critical role in reliable estimation of the rate constant and fluorescence quantum yield.

Water-Soluble Phospholo[3,2-b]phosphole-P,P'-Dioxide-Based Fluorescent Dyes with High Photostability.

We recently reported that fluorescent dye PB430, which consisted of a 2-phenyl-substituted benzophosphole P-oxide skeleton that was reinforced by a methylene bridge, showed pronounced photostability and, thus, high utility for applications in super-resolution stimulated emission depletion (STED) microscopy. Herein, we replaced the methylene bridge with another P=O group to 1) investigate the role of the bridging moieties; and 2) further modulate the fluorescence properties of this skeleton. We synthesized a series of phospholo[3,2-b]phosphole-based dyes-trans-PO-PB430, cis-PO-PB430, and trans-PO-PB460-all of which showed sufficient water solubility. Moreover, trans-PO-PB430 and trans-PO-PB460 exhibited intense green and orange fluorescence, respectively, and a high photostability that was comparable to that of PB430. In contrast, cis-PO-PB430 underwent rapid photobleaching upon continuous photoirradiation, which demonstrated the importance of steric shielding of the polycyclic skeleton by the substituents on the bridging moieties. The fluorescence properties of these dyes were insensitive to concentration, pH value, and polarity changes of the environment in solution. In addition, even in the solid state, these dyes showed strong green to orange emissions. These results demonstrate the potential utility of trans-PO-PB430 and trans-PO-PB460 as highly photostable fluorescent dyes.