  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. Distribution and origin
of the non-monoexponential decays, which are commonly observed in TADF films,
are revealed by 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.
