Phosphorescence and delayed fluorescence properties of fluorone dyes in bio-related films

Tailoring Fluorescence-Phosphorescence Emission with a Single Nanocavity

Realizing the dual emission of fluorescence-phosphorescence in a single system is an extremely important topic in the fields of biological imaging, sensing, and information encryption. However, the phosphorescence process is usually in an inherently "dark state" at room temperature due to the involvement of spin-forbidden transition and the rapid non-radiative decay rate of the triplet state. In this work, we achieved luminescent harvesting of the dark phosphorescence processes by coupling singlet-triplet molecular emitters with a rationally designed plasmonic cavity. The achieved Purcell enhancement effect of over 1000-fold allows for overcoming the triplet forbidden transitions, enabling radiation enhancement with selectable emission wavelengths. Spectral results and theoretical simulations indicate that the fluorescence-phosphorescence peak position can be intelligently tailored in a broad range of wavelengths, from visible to near-infrared. Our study sheds new light on plasmonic tailoring of molecular emission behavior, which is crucial for advancing research on plasmon-tailored fluorescence-phosphorescence spectroscopy in optoelectronics and biomedicine.

Exponential Amplification Using Photoredox Autocatalysis

Exponential molecular amplification such as the polymerase chain reaction is a powerful tool that allows ultrasensitive biodetection. Here, we report a new exponential amplification strategy based on photoredox autocatalysis, where eosin Y, a photocatalyst, amplifies itself by activating a nonfluorescent eosin Y derivative (EYH^3-) under green light. The deactivated photocatalyst is stable and rapidly activated under low-intensity light, making the eosin Y amplification suitable for resource-limited settings. Through steady-state kinetic studies and reaction modeling, we found that EYH^3- is either oxidized to eosin Y via one-electron oxidation by triplet eosin Y and subsequent 1e^-/H⁺ transfer, or activated by singlet oxygen with the risk of degradation. By reducing the rate of the EYH^3- degradation, we successfully improved EYH^3--to-eosin Y recovery, achieving efficient autocatalytic eosin Y amplification. Additionally, to demonstrate its flexibility in output signals, we coupled the eosin Y amplification with photoinduced chromogenic polymerization, enabling sensitive visual detection of analytes. Finally, we applied the exponential amplification methods in developing bioassays for detection of biomarkers including SARS-CoV-2 nucleocapsid protein, an antigen used in the diagnosis of COVID-19.

Diversity of copper(I) complexes showing thermally activated delayed fluorescence: basic photophysical analysis

A comparison of three copper(I) compounds [1, Cu(dppb)(pz2Bph2); 2, Cu(pop)(pz2Bph2); 3, Cu(dmp)(phanephos)(+)] that show pronounced thermally activated delayed fluorescence (TADF) at ambient temperature demonstrates a wide diversity of emission behavior. In this study, we focus on compound 1. A computational density functional theory (DFT)/time-dependent DFT approach allows us to predict detailed photophysical properties, while experimental emission studies over a wide temperature range down to T = 1.5 K lead to better insight into the electronic structures even with respect to spin-orbit coupling efficiencies, radiative rates, and zero-field splitting of the triplet state. All three compounds, with emission quantum yields higher than ϕPL = 70%, are potentially well suited as emitters for organic light-emitting diodes (OLEDs) based on the singlet-harvesting mechanism. Interestingly, compound 1 is by far the most attractive one because of a very small energy separation between the lowest excited singlet S1 and triplet T1 state of ΔE(S1-T1) = 370 cm(-1) (46 meV). Such a small value has not been reported so far. It is responsible for the very short decay time of τ(TADF, 300 K) = 3.3 μs. Hence, if focused on the requirements of a short TADF decay time for reduction of the saturation effects in OLEDs, copper(I) complexes are well comparable or even slightly better than the best purely organic TADF emitters.

Spectral study of fluorone dyes adsorption on chitosan-based polyelectrolyte complexes

Polyelectrolyte complexes of the chitosan-chondroitin sulfate and chitosan-hyaluronate polycation-polyanion pairs were synthesized and characterized as potential dye adsorbents at different pH levels. Equilibrium isotherm analysis was applied to investigate the efficiency and the mechanism of the adsorption of fluorone dyes (fluorescein, eosin Y, erythrosin B) on the synthesized complexes. The inefficiency of the fluorescein adsorption was proved by two different quantitative spectroscopic methods. The adsorption isotherm for both eosin Y and erythrosin B was adequately described in terms of the Langmuir-Freundlich model. The observed room-temperature phosphorescence of the adsorbed erythrosin B was attributed to the surface inhomogeneity of the synthesized complexes. The revealed variation in the adsorption properties of fluorone dyes was related to the differences in their ionic forms as well as in their polarity and hydrophobicity.

Phloxine B as a probe for entrapment in microcrystalline cellulose

The photophysical behaviour of phloxine B adsorbed onto microcrystalline cellulose was evaluated by reflectance spectroscopy and laser induced time-resolved luminescence in the picosecond-nanosecond and microsecond-millisecond ranges. Analysis of the absorption spectral changes with concentration points to a small tendency of the dye to aggregate in the range of concentrations under study. Prompt fluorescence, phosphorescence and delayed fluorescence spectral decays were measured at room temperature and 77 K, without the need of sample degassing because cellulose protects triplet states from oxygen quenching. In all cases, spectral changes with time and lifetime distribution analysis were consistent with the dye coexisting in two different environments: dyes tightly entrapped between polymer chains in crystalline regions of cellulose showed longer fluorescence and phosphorescence lifetimes and more energetic triplet states, while dyes adsorbed in more amorphous regions of the support showed shorter lifetimes and less energetic triplet states. This behaviour is discussed in terms of the different dye-support interactions in both kinds of adsorption sites.