We focused our attention on the recently emerging group of TADF sensitizers, and found that the purely organic, blue-emitting 2,3,5,6-tetra(9H-carbazol-9-yl)benzonitrile (4CzBN), developed by the Zhang group, was able to sensitize all annihilators efficiently. TADF molecules exhibit small singlet-triplet energy splittings (DES-T, typically below 0.3 eV) which results from a high degree of intramolecular charge-transfer (CT) character in the singlet and triplet excited states. In 4CzBN this is manifested by the CT absorption band with an onset at around 430 nm (Figure ), and the covalent linkage between electron donor and acceptor units further enhances the CT character. The photophysics of organic TADF compounds has been thoroughly investigated by others, and the key processes of a conventional TADF compound are depicted in the left part of Figure . Upon excitation, the first singlet excited state can decay either non-radiatively or by prompt fluorescence. Because ISC is quite strong in these purely organic molecules TADF compounds also populate their triplet state efficiently via ISC. DES-T then dictates how fast reverse ISC (rISC) proceeds, which together with the rates for non-radiative triplet decay and phosphorescence dictates the lifetime of the triplet state. The recycling of singlet and triplet states ultimately results in thermally activated delayed fluorescence (TADF) from the singlet state, typically on the microsecond timescale. 4CzBN exhibits both prompt fluorescence and TADF in toluene. The fluorescence quantum yield (FF) and lifetime (t) of the prompt (PF) component were determined from measurements in air-saturated samples, while the delayed (DF) component was readily observed in oxygen-free samples. The total FF of 4CzBN was determined as 0.64, with FPF and FDF being 0.11 and 0.53, respectively, according well with previous studies on 4CzBN. tPF showed minor susceptibility to the presence of oxygen, decreasing from 2.34 ns in oxygen-free solution to 2.22 ns upon exposure to air. The lifetime of the delayed component, tDF, is of particular importance in TTA-UC since it corresponds to the triplet lifetime. tDF of 4CzBN was determined to be 62 µs, which is sufficiently long to promote diffusion-controlled Dexter-type triplet energy transfer (TET) upon addition of an annihilator species. This relatively long lifetime is the result of a rather large DES-T of 0.28 eV, thus impeding the rate of rISC. The ISC efficiency can be estimated as 1-FPF, yielding FISC = 0.89. The most important photophysical parameters of 4CzBN are summarized in Table . and examples include using the difference between the quenched and unquenched donor total fluorescence quantum yield or delayed component lifetime. Given that the equilibrium between the singlet and triplet state in a TADF compound is perturbed upon addition of a quencher, the methods mentioned above are riddled with assumptions that are valid only for certain compounds. To ensure that the chosen method was valid for 4CzBN we performed simulations (Figure ). The results indicate that probing the changes in tDF upon quenching of 4CzBN yield excellent agreement with the true TET efficiency, as given by Equation S1E. Note that the definition for TET efficiency used herein includes the ISC event, i.e., the maximum value for FTET = FISC (Equation S1E, for a more detailed discussion see the Supporting Information, Section 2.1).