The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Molecular Design Principles for Photoactive Transition Metal Complexes: A Guide for "Photo-Motivated" Chemists

Luminescence and photochemistry involve electronically excited states that are inherently unstable and therefore spontaneously decay to electronic ground states, in most cases by nonradiative energy release that generates heat. This energy dissipation can occur on a time scale of 100 fs (∼10-13 s) and usually needs to be slowed down to at least the nanosecond (∼10-9 s) time scale for luminescence and intermolecular photochemistry to occur. This is a challenging task with many different factors to consider. An alternative emerging strategy is to target dissociative excited states that lead to metal-ligand bond homolysis on the subnanosecond time scale to access synthetically useful radicals. Based on a thorough review at the most recent advances in the field, this article aims to provide a concise guide to obtaining luminescent and photochemically useful coordination compounds with d-block elements. We hope to encourage "photo-motivated" chemists who have been reluctant to apply their synthetic and other knowledge to photophysics and photochemistry, and we intend to stimulate new approaches to the synthetic control of excited state behavior.

Red and Near-Infrared Thermally Activated Delayed Fluorescence Emitters Based on a Dibenzo[f,h]pyrido[2,3-b]quinoxaline Acceptor

Recently, there has been growing interest in deep red (DR) and near-infrared (NIR) thermally activated delayed fluorescence (TADF) emitters due to their potential use in applications in bioimaging and night-vision displays. Herein, we designed and synthesized a series of red/NIR TADF emitters, DMAC, PXZ, and DPACz, that all contain the same electron-accepting PyBP (dibenzo [f, h] pyrido [2,3-b]quinoxaline) moiety. These compounds emit at 643 nm for DMACPyBP, 722 nm for DPACz PyBP, and 743 nm for PXZPyBP in toluene solution, while their thin films singlet-triplet energy gaps (ΔEST) are <0.1 eV. DMACPyBP, with the weakest donor, has the highest ΦPL of 62.3 %, the smallest ΔEST of 0.03 eV, and a fast reverse intersystem crossing rate constant (kRISC) of 0.5×105 s-1 in a 1 wt% doped film in CBP. In contrast, PXZPyBP, containing the strongest donor, has a lower ΦPL (21.2 %), a relatively larger ΔEST (0.10 eV), and a slower kRISC (0.04×105 s-1). Thus, our work highlights the molecular design challenges involved in pushing emission into the NIR region while maintaing both TADF and high ΦPL in PyBP-based donor-acceptor emitters.

An Activatable Long-Fluorescence-Lifetime Probe for Exploring the Dual Function of StrH in Biofilm Formation and Necroptosis

Streptococcus pneumoniae infections, particularly those associated with biofilm formation, pose significant therapeutic challenges due to their increased resistance to antibiotics and immune evasion. Identifying new biomarkers is crucial for accurate diagnosis and the development of innovative treatment strategies. StrH is recognized as a key enzyme in S. pneumoniae carbohydrate metabolism, however, its specific role in biofilm formation and its interactions with the host remain poorly understood. In this study, a highly sensitive and selective turn-on long-lifetime fluorescence probe, "HBT-PXZ-St," is designed to detect StrH activity and to reveal its functions. Using fluorescence lifetime imaging microscopy (FLIM), "HBT-PXZ-St" can quantify StrH activity and image live S. pneumoniae cells, achieving a fluorescence lifetime of ≈2 µs, which effectively minimizes background short-lived fluorescence interference. Additionally, the findings suggest that StrH activity significantly contributes to biofilm development and induces necroptosis in A549 host cells via the receptor-interacting serine/threonine-protein kinase 1 (RIPK1) pathway, thereby promoting bacterial colonization and invasion. This study provides insight into StrH's dual role as both a "sword and shield" during colonization and invasion, suggesting its potential as a therapeutic target for novel treatments against S. pneumoniae infections.

Interplay of Charge Transfer and Local Triplet States in Donor-Acceptor-Based TADF Compounds

Donor-acceptor-based thermally activated delayed fluorescence (TADF) emitters have a complicated energy level scheme with multiple singlet and triplet energy levels. Among them, TADF compounds with the lowest-energy charge-transfer triplet state (type III compounds) are especially interesting due to their specific emission properties. We show that type III TADF compounds may show dual phosphorescence at low temperatures as well as weak high-energy emission at room temperature, somewhat resembling the phosphorescence spectrum of the local triplet. Such untypical behavior was demonstrated only in compounds with a low energy gap between the triplet states of different character, imposing efficient communication between them through vibronic coupling.