There are many known mechanisms by which a molecule can emit light, classified on the basis of the excited states that are involved in the emission process (Figure ). By far the most common for allorganic molecules is fluorescence, the spontaneous emission of light associated with the relaxation of an excited state of the same multiplicity as the ground state (Figure ). In most organic emitters, which have a closed-shell configuration in the ground state, this occurs from a singlet excited state and typically has a very short (nanosecond) emission lifetime. Additionally, because high-lying excited states rapidly relax down to the lowest excited state of the same multiplicity via internal conversion, fluorescence is generally only observed from the lowest lying singlet excited state, S1. A second emission mechanism involves radiative decay from an excited state of a different multiplicity to the ground state, typically the lowest-lying triplet excited state (T1), and is termed phosphorescence (Figure ). Phosphorescence necessitates a spin-flip, termed intersystem crossing, of the relaxing electron from the triplet excited state, where the electrons of the two singly-occupied molecular orbitals (SOMOs) have parallel spin, to the singlet ground state where they antiparallel spin. Because spin must be conserved during an electronic transition, phosphorescence is formally forbidden in a molecule with well-defined spin states, and as such, the rate of phosphorescence in organic molecules, which is inversely proportional to the energy gap between the triplet excited state and the ground state, is several orders of magnitude slower than either fluorescence or nonradiative decay from S1 or T1 and thus is typically not observed. For phosphorescence to become more likely, it is necessary for the singlet and triplet excited states to mix, which is most commonly achieved through spin-orbit coupling (SOC). SOC is dependent on the difference of the symmetry of the orbitals involved in the transitions to the singlet and triplet excited states as well as scaling rapidly with atomic number of the atoms involved in these transitions. It is for these reasons that transition metal complexes containing platinoid metals are so frequently phosphorescent. As the energy gap between S1 and T1 (ΔEST) is significantly smaller than that between S1 and S0, and large SOC in these compounds readily facilitates mixing of the excited states of different multiplicity, ISC outcompetes fluorescence, and so only triplet emission is observed. However, because of the relatively large gap between T1 and S0 and the required spin-flip, the rate of phosphorescence is slow compared to the typical rate of fluorescence, with a lifetime that typically ranges from microsecond to seconds. Phosphorescence can also occur from organic compounds, where it is known as room temperature phosphorescence (RTP) in the community. As these compounds typically do not contain any heavy atoms, they show much smaller SOC, and so RTP generally has to rely on strongly suppressing non-radiative decay channels. An alternative triplet harvesting mechanism, which does not rely on heavy metals, is thermally activated delayed fluorescence (TADF). In this process, the ΔEST of the emitter is sufficiently small to permit endothermic up-conversion of triplets to singlets via reverse-intersystem crossing (RISC). This is possible even with negligible SOC, which also makes phosphorescence a disfavoured radiative decay process. Therefore, TADF materials exhibit dual fluorescence with two distinct lifetimes: 1) prompt fluorescence from radiative decay from directly populated singlet excited states, a process that is identical to the fluorescence introduced above, and therefore has a typical lifetime of nanoseconds; and 2) delayed fluorescence from the same singlet excited state that is repopulated by ISC/RISC cycling. Because this up-conversion relies on a formally forbidden and thermodynamically uphill spin-flip process, it is slow, and thus delayed fluorescence typically occurs on a microsecond-to-millisecond timescale. As RISC is an endothermic process, the rate of TADF is strongly dependent on the temperature, exhibiting faster rates at higher temperatures and slower or absent rates at lower temperature; therefore, this effect manifests itself as an increase in the delayed emission at higher temperatures. This is opposite to the behaviour observed for phosphorescent compounds wherein at higher temperatures there is a decrease in the delayed emission contribution owing to increased non-radiative deactivation of the T1 state through molecular vibrations. To enable TADF in an emitter it is necessary to adopt a molecular design that minimizes the exchange integral between the frontier orbitals involved in the radiative transition. There are numerous design strategies used in the literature to achieve this, with multi-resonant thermally activated delayed fluorescence (MR-TADF) having received particular attention in recent years. This mechanism relies on a much shorter charge-transfer distance in the S1 → S0 transition than is found in the more traditional donoracceptor based long-range charger-transfer TADF compounds, resulting in a significantly narrower emission spectrum in the former compared to the latter. Many of the compounds that exhibit MR-TADF emission are based on a p-and/or n-doped nanographene backbone. Biluminescence, or dual emission, is the simultaneous emission from multiple excited states of the same molecules. This diverse category of emission processes encompasses many different mechanisms, including S1/S2 non-Kasha emission which can be observed in some organic molecules such as porphyrins and azulenes, and S1/T1 dual fluorescence (or TADF)/phosphorescence that is observed in several Cu(I)-based organometallic emitters. Examples of S1/T1 biluminescence in all-organic emitters are rare, and the first report of an organic S1/T1 biluminescent emitter to be rigorously characterized was only in 2013 by Reineke and co-workers, although the phenomenon itself is older. The mechanism(s) of these processes are poorly understood, but in qualitative terms they necessitate a careful balance of the rate constants of fluorescence, intersystem crossing, phosphorescence, and non-radiative decay. Additionally, many of the reported examples of S1/T1 biluminescence exhibit TADF in addition to conventional fluorescence, which further necessitates the balancing of the rate of reverse-intersystem crossing with the other photophysical processes. Full design rules by which this can be reliably achieved have not yet been established, but it can be assumed that S1/T1 biluminescence requires intermediate values of ΔEST and SOC, to obtain competitive rates from fluorescence/TADF and RTP while minimizing non-radiative decay.