Femtosecond Spectroscopy on a Dibenzophenazine‐Cored Macrocycle Exhibiting Thermally Activated Delayed Fluorescence

The photophysics of a thermally activated delayed fluorescence (TADF) emitting macrocycle consisting of two dibenzo[a,j]phenazine acceptor moieties bridged by two N,N,N’,N’‐tetraphenylene‐1,4‐diamine donor units was scrutinized in solution by steady‐state and time‐resolved spectroscopy. The fluorescence lifetime of the compound proved to be strongly solvent‐dependent. It ranges from 6.3 ns in cyclohexane to 34 ps in dimethyl sulfoxide. In polar solvents the fluorescence decay is predominantly due to internal conversion. In non‐polar ones radiative decay and intersystem crossing contribute. Contrary to the behaviour in polymer matrices (S. Izumi et al., J. Am. Chem. Soc. 2020, 142, 1482) the excited state decay is not predominantly due to prompt and delayed fluorescence. The solvent‐dependent behaviour is analyzed with the aid of quantum chemical computations. Using steady‐state and transient absorption spectroscopy as well as quantum chemical calculations, the depicted TADF macrocycle is investigated. It features strongly solvent‐dependent S1 lifetimes and concomitantly triplet yields. While embedded in matrix environments the TADF character is evident, in solution these properties are lost.