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.

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C70 (like other fullerenes) is known to have a very weak fluorescence (ΦF ≅ 5 × 10-4), owing to the high efficiency of triplet formation. In this work we show that, under appropriate conditions, the fluorescence quantum yield increases by one or two orders of magnitude (up to an estimated maximum value ΦF = 0.08), through the mechanism of delayed thermal fluorescence. We also report a new estimate of the singlet−triplet gap (26 ± 2 kJ mol-1), obtained from the temperature dependence of the delayed thermal fluorescence. This value is shown to be in agreement with the phosphorescence spectrum recorded in the same temperature range. An accurate value of the quantum yield of triplet formation (0.994 ± 0.001) is obtained from a new method of data analysis.

Fluorescence and phosphorescence are two forms of photoluminescence used in modern research and in practical applications. The early observations of these phenomena, before the emergence of quantum theory, highlight the investigation into the mechanism of light emission. In contrast to incandescence, photoluminescence does not require high temperatures and does not usually produce noticeable heat. Such a “cold light” was the object of an interesting controversy in the 19th century: does it fit into thermodynamics? The early applications, such as the fluorescent tube, fluorescence analysis, and fluorescent tracers, are described.