The temperature dependences of photoluminescence (PL) decay rates and the PL spectrum of a thermally activated delayed-fluorescence emitter, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), are investigated. It is found that not only the lowest singlet ($S_1$) and triplet ($T_1$) excited states but also an additional triplet excited state ($T_n$) lying between $S_1$ and $T_1$ play an important role in the exciton decay process, particularly around 100 K. At around this temperature, some of the triplet excitons are thermally activated into $T_n$ but not up to $S_1$, and they then decay into the ground state ($S_0$) with phosphorescence emission. Therefore, two kinds of phosphorescence, originating from $T_n$ and $T_1$, are observed. The temperature dependence of the PL decay rates of 4CzIPN can be explained by a four-level model consisting of $S_1$, $T_1$, $T_n$, and $S_0$, and its energy gaps between $T_n$ and $T_1$ and between $S_1$ and $T_1$ are determined to be $45\pm 5\mathrm{meV}$ and $135\pm 10\mathrm{meV}$, respectively.

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• Electronic structure of atoms & molecules
• Electronic transitions
• Organic LEDs
• Phosphorescence
• Time-resolved photoluminescence