Efficient TADF relies on effective reverse intersystem crossing (rISC) from triplet to singlet manifold, and therefore the transition rate associated with this process must outcompete non-radiative decay rate of the triplets. Herein the issue of non-radiative triplet quenching in benzophenone-derived compounds frequently employed as blue TADF emitters in OLEDs is addressed and an effective way to suppress this quenching is proposed. TADF emitters designed for this study were based on phenone-derived electron-accepting units linked with multiple carbazolyl donors to deliver similar and large rISC rates (k_rISC ∼ 4 × 10⁶ s⁻¹), yet significantly different non-radiative triplet quenching rates (k^T _nr). We found that substitution of a loose phenyl moiety by a methoxy group in benzophenone suppresses k^T _nr by one order of magnitude (from 3 × 10⁶ s⁻¹ down to 0.3 × 10⁶ s⁻¹), while leaving rISC rate k_rISC almost unaffected. A 10 times enlarged ratio k_rISC/k^T _nr in 5tCzMeB as compared to that in 5tCzBP emitter suffices to retrieve the vast majority of triplet population via the rISC resulting in PL quantum yields of 0.99 and 0.58 in doped DPEPO and neat films, respectively. Sky-blue TADF OLEDs with doped and non-doped emissive layers based on 5tCzMeB demonstrated impressive maximum external quantum efficiency (EQE) values of 24.6% and 13.4%, respectively, whereas at 100 cd m⁻² the devices exhibited 16.5% and 7.7% EQEs, respectively.

Over the past decade, appreciation of the roles of G-quadruplex (G4) structures in cellular regulation and maintenance has rapidly grown, making the establishment of robust methods to visualize G4s increasingly important. Fluorescent probes are commonly used for G4 detection in vitro; however, achieving sufficient selectivity to detect G4s in a dense and structurally diverse cellular environment is challenging. The use of fluorescent probes for G4 detection is further complicated by variations of probe uptake into cells, which may affect fluorescence intensity independently of G4 abundance. In this work, we report an alternative small-molecule approach to visualize G4s that does not rely on fluorescence intensity switch-on and, thus, does not require the use of molecules with exclusive G4 binding selectivity. Specifically, we have developed a novel thiazole orange derivative, TOR-G4, that exhibits a unique fluorescence lifetime when bound to G4s compared to other structures, allowing G4 binding to be sensitively distinguished from non-G4 binding, independent of the local probe concentration. Furthermore, TOR-G4 primarily colocalizes with RNA in the cytoplasm and nucleoli of cells, making it the first lifetime-based probe validated for exploring the emerging roles of RNA G4s in cellulo.

Intracellular polarity in lipid droplets as well as other organelles may provide useful knowledge about various processes taking place in live cells. Therefore, small fluorophores capable of visualising polarity are undergoing rapid development. In this paper, we report new red-emitting polarity sensitive BODIPY probes that can distinguish between liquid-ordered and liquid-disordered phases and can internalise into lipid droplets of live cells. Our reported probes sense lipid environment not through solvatochromic shift of the fluorescence spectra but through the change in the fluorescence lifetime of their monoexponential decays. This makes them convenient for fluorescence lifetime imaging microscopy. The probes were synthesised by modifying viscosity-sensitive meso-phenyl BODIPY with electron-donating 2-thienyl moieties at the α- and β-positions, significantly red-shifting absorption and fluorescence spectra of the dyes and improving sensitivity to polarity, while suppressing viscosity dependence. Finally, a novel probe – BP OC₁₆ TP2 was suitable for sensing polarity in lipid droplets of live MCF-7 human breast cancer cells. We demonstrated that different chemotherapeutics affected lipid droplet polarity differently: cisplatin had no effect on lipid droplet polarity, whereas paclitaxel, depending on its concentration, either decreased or increased lipid droplet polarity.

Viscosity is a key characteristic of lipid membranes – it governs the passive diffusion of solutes and affects the lipid raft formation and membrane fluidity. Precise determination of viscosity values in biological systems is of great interest and viscosity-sensitive fluorescent probes offer a convenient solution for this task. In this work we present a novel membrane-targeting and water-soluble viscosity probe BODIPY-PM, which is based on one of the most frequently used probes BODIPY-C₁₀. Despite its regular use, BODIPY-C₁₀ suffers from poor integration into liquid-ordered lipid phases and lack of water solubility. Here, we investigate the photophysical characteristics of BODIPY-PM and demonstrate that solvent polarity only slightly affects the viscosity-sensing qualities of BODIPY-PM. In addition, with fluorescence lifetime imaging microscopy (FLIM), we imaged microviscosity in complex biological systems – large unilamellar vesicles (LUVs), tethered bilayer membranes (tBLMs) and live lung cancer cells. Our study showcases that BODIPY-PM preferentially stains the plasma membranes of live cells, equally well partitions into both liquid-ordered and liquid-disordered phases and reliably distinguishes lipid phase separation in tBLMs and LUVs.