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

QM/MM Studies on Thermally Activated Delayed Fluorescence of a Dicopper Complex in the Solid State

Dicopper complexes with thermally activated delayed fluorescence (TADF) phenomena are important in enriching the arsenal of organic light-emitting diodes materials. However, the TADF mechanism is still elusive, especially in the solid state. Herein, we chose a TADF dicopper complex and investigated its geometric and electronic structures and absorption and emission spectra using DFT, TD-DFT, and QM/MM methods. On the basis of these results, we further estimate the fluorescence emission rate from the S1 state, phosphorescence emission rate from the T1 state, and forward and reverse intersystem crossing (ISC and rISC) rates between S1 and T1. The present work shows that both the S1 and the T1 states have mixed metal-to-ligand and interligand charge-transfer character. Good spatial separation between the HOMO and the LUMO makes the S1–T1 energy gap small, ca. 2.8 kcal/mol. This small energy gap leads to efficient ISC and rISC processes, whose rates are much larger than the fluorescence and phosphorescence emission rates at 300 K, therefore enabling TADF. In contrast, at 77 K, the rISC process is blocked because its rate is much smaller than the phosphorescence emission. Thus, TADF disappears at 77 K. Further analysis shows that high-frequency deformation and low-frequency torsional vibrational modes make a large contribution to the Huang–Rhys factors, but Duschinsky rotation effects are essentially negligible for both the ISC and the rISC processes

Stereoselective Excited-State Isomerization and Decay Paths in <i>cis</i>-Cyclobiazobenzene

Herein, we have employed OM2/MRCI-based full-dimensional nonadiabatic dynamics simulations to explore the photoisomerization and subsequent excited-state decay of a macrocyclic cyclobiazobenzene molecule. Two S1/S0 conical intersection structures are found to be responsible for the excited-state decay. Related to these two conical intersections, we found two stereoselective photoisomerization and excited-state decay pathways, which correspond to the clockwise and counterclockwise rotation motions with respect to the NN bond of the azo group. In both pathways, the excited-state isomerization is ultrafast and finishes within ca. 69 fs, but the clockwise isomerization channel is much more favorable than the counterclockwise one with a ratio of 74% versus 26%. Importantly, the present work demonstrates that stereoselective pathways exist not only in the photoisomerization of isolated azobenzene (AB)-like systems but also in macrocyclic systems with multiple ABs. This finding could provide useful insights for understanding and controlling the photodynamics of macrocyclic nanostructures with AB units as the main building units

Multiparameter Assessment of Foam Cell Formation Progression Using a Dual-Color Switchable Fluorescence Probe

The assessment of atherosclerosis (AS) progression has emerged as a prominent area of research. Monitoring various pathological features of foam cell (FC) formation is imperative to comprehensively assess AS progression. Herein, a simple benzospiropyran-julolidine-based probe, BSJD, with switchable dual-color imaging ability was developed. This probe can dynamically and reversibly adjust its molecular structure and fluorescent properties in different polar and pH environments. Such a polarity and pH dual-responsive characteristic makes it superior to single-responsive probes in dual-color imaging of lipid droplets (LDs) and lysosomes as well as monitoring their interaction. By simultaneously tracking various pathological features, including LD accumulation and size changes, lysosome dysfunction, and dynamically regulated lipophagy, more comprehensive information can be obtained for multiparameter assessment of FC formation progression. Using BSJD, not only the activation of lipophagy in the early stages and inhibition in the later phases during FC formation are clearly observed but also the important roles of lipophagy in regulating lipid metabolism and alleviating FC formation are demonstrated. Furthermore, BSJD is demonstrated to be capable of rapidly imaging FC plaque sites in AS mice with fast pharmacokinetics. Altogether, BSJD holds great promise as a dual-color organelle-imaging tool for investigating disease-related LD and lysosome changes and their interactions