Here, a comprehensive photophysical investigation of a the emitter molecule DPTZ‐DBTO2, showing thermally activated delayed fluorescence (TADF), with near‐orthogonal electron donor (D) and acceptor (A) units is reported. It is shown that DPTZ‐DBTO2 has minimal singlet–triplet energy splitting due to its near‐rigid molecular geometry. However, the electronic coupling between the local triplet (3LE) and the charge transfer states, singlet and triplet, (1CT, 3CT), and the effect of dynamic rocking of the D–A units about the orthogonal geometry are crucial for efficient TADF to be achieved. In solvents with low polarity, the guest emissive singlet 1CT state couples directly to the near‐degenerate 3LE, efficiently harvesting the triplet states by a spin orbit coupling charge transfer mechanism (SOCT). However, in solvents with higher polarity the emissive CT state in DPTZ‐DBTO2 shifts below (the static) 3LE, leading to decreased TADF efficiencies. The relatively large energy difference between the 1CT and 3LE states and the extremely low efficiency of the 1CT to 3CT hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between 1CT and 3LE, and the (dynamic) orientation of the D–A units are thus critical elements that dictate reverse intersystem crossing processes and thus high efficiency in TADF.

A new diethylenetriamine-derived macrocycle known as L, bearing 2-methylquinoline arms and containing m-xylyl spacers, was prepared in good yield by a one-pot [2 + 2] Schiff base condensation procedure, followed by reduction with sodium borohydride. Up to now this is the first hexaazamacrocycle with appended fluorophore units. Single-crystal X-ray diffraction determination of the dinuclear zinc(II) complex of L showed that metal centers are located at about 7.20(2) Å from one another. This complex exhibits only weak fluorescence in aqueous solution, but the addition of 1 equiv of pyrophosphate (PPi) caused a 21-fold enhancement of the fluorescence intensity. The sensor response is linear up to a value of 10 μM HPPi(3-) and has a detection limit of 300 nM. The receptor behaves as a highly selective sensor for pyrophosphate as other anions, including phosphate, phenylphosphate (PhP), adenosine monophosphate (AMP), adenosine diphosphate (ADP), and adenosine triphosphate (ATP), failed to induce any fluorescence change and practically do not affect the fluorescence intensity of the sensor in the presence of HPPi(3-). Competition titrations carried out in aqueous solution at pH 7.4 [in 20 mM 3-(N-morpholino)propanesulfonic acid (MOPS) buffer] by spectrofluorometry revealed a high association constant value of 6.22 log units for binding of PPi by the dinuclear zinc(II) receptor, one of the highest reported values for colorimetric/fluorometric sensors able to work under real aqueous physiological conditions, while association constant values for binding of the other phosphorylated substrates are in the 5.51-4.03 log unit range.

A complete homologous series of fluorescent 7-nitrobenz-2-oxa-1,3-diazol-4-yl-(NBD) labelled fatty amines of varying alkyl chain lengths, NBD-Cn, inserted in 1-palmitoyl, 2-oleoyl-sn-glycero-3-phosphocholine (POPC) or N-palmitoyl sphingomyelin (SpM) bilayers, with 50 mol% and 40 mol% cholesterol (Chol), respectively, was studied using atomistic molecular dynamics simulations. For all amphiphiles in both bilayers, the NBD fluorophore locates at the interface, in a more external position than that previously observed for pure POPC bilayers. This shallower location of the NBD group agrees with the lower fluorescent quantum yield, shorter fluorescence lifetime, and higher ionisation constants (smaller pKa) determined experimentally. The more external location is also consistent with the changes measured in steady-state fluorescence anisotropy from POPC to POPC/Chol (1 : 1) vesicles. Accordingly, the equilibrium location of the NBD group within the various bilayers is mainly dictated by bilayer compositions, and is mostly unaffected by the length of the attached alkyl chain. Similarly to the behaviour observed in POPC bilayers, the longer-chained NBD-Cn amphiphiles show significant mass density near the mixed bilayers' midplanes, and the alkyl chains of the longer derivatives, mainly NBD-C16, penetrate the opposite bilayer leaflet to some extent. However, this effect is quantitatively less pronounced in these ordered bilayers than in POPC. Similarly to POPC bilayers, the effects of these amphiphiles on the structure and dynamics of the host lipid were found to be relatively mild, in comparison with acyl-chain phospholipid analogues.

The phosphorescence and thermally activated delayed fluorescence (TADF) lifetimes of C and C in two different glassy hydrocarbon polymers, one aliphatic (cyclic polyolefin) and one aromatic (polystyrene), were measured between -200 and 100 ºC. The temperature dependence of the lifetimes is equally well described by a three-state mechanism (ground state, S, and two excited states in thermal equilibrium, T and S, the lifetime of T being temperature dependent) and by a four-state mechanism (ground state, S, and three excited states in thermal equilibrium, T, T and S, all with temperature-independent lifetimes). The estimated S-T and T-T energy gaps (four-state mechanism) are in good agreement with spectroscopic measurements. These and the determined quantum yield of triplet formation, 0.997 ± 0.001, are found to be essentially independent of the polymer matrix and of the isotopic composition of the fullerene. On the other hand, the lifetimes of both T and T (four-state mechanism) are weakly dependent on the polymer matrix but strongly vary with the fullerene isotopic composition, nearly doubling when going from C to C. A parameter useful for the characterization of TADF, the on-set temperature T, is also introduced.

A complete set of criteria for the classification, design, and selection of optimal thermally activated delayed fluorescence (TADF) emitters, for both photoluminescence and electroluminescence applications, is presently unavailable. In this work, and as a contribution toward this aim, a detailed characterization of TADF photophysical kinetics is presented, contrasting TADF in photoluminescence and TADF in electroluminescence. Two different types of TADF are identified: One-way and two-way TADF. It is shown that, for a given efficiency, one-way TADF allows lower rates of reverse intersystem crossing, an aspect that may be significant with respect to stability and roll-off issues. Graphical and quantitative indicators of singlet− triplet interconversion and photophysical efficiency are obtained and applied to the photoluminescence of eosin, coronene, and fullerenes and to the electroluminescence of several TADF emitters specifically designed for organic light emitting diodes. Relations for the photoluminescence TADF onset temperature and for the electroluminescence internal quantum efficiency are also derived.