Open Access

Martin Streiter, Tillmann G. Fischer, Christian Wiebeler, Sebastian Reichert, Jörn Langenickel, Kirsten Zeitler, Carsten Deibel

• We analyze internal transition rates and the singlet–triplet energy gap of the thermally activated delayed fluorescence (TADF) molecule 3CzClIPN, which recently was introduced as an efficient photocatalyst. The distribution and origin of the non-monoexponential decays, which are commonly observed in TADF films, are revealed by an analysis of transient fluorescence with an inverse Laplace transform. A numerically robust global rate fit routine, which extracts all relevant TADF parameters by modeling the complete set of data, is introduced. To compare and verify the results, all methods are also applied to the well-known 4CzIPN. The influence of the molecular matrix is discussed by embedding low concentrations of TADF molecules in polystyrene films. Finally, quantum chemical calculations are compared to the experimental results to demonstrate that the chlorine atom increases the charge-transfer character of the relevant states, resulting in a reduction of the singlet–triplet energy gap.