Excited state dynamics of homoleptic Zn(ii)dipyrrin complexes and their application in photocatalysis

We present photophysical studies on homoleptic zinc-dipyrrin complexes representing intramolecular donor-acceptor systems. Intramolecular charge transfer depends on ligand variation and intermolecular environmental influences. In particular, we focus on steady-state and transient absorption methods to compare the response of the ligand dynamics with that of the complexes after photoexcitation. For the first time, we successfully tested zinc bis(dipyrrinato) complexes as photocatalysts in two different aerobic photooxidation reactions. Early-time dynamics after 100 fs in dichloromethane and in ethanol can be assigned to charge transfer states, while the subsequent response constitutes broad-band excited state dynamics, most likely of overlapping singlet and triplet states when probed in a relatively wide spectral window from 350 to 1400 nm and up to a delay time of 1.6 ns. In addition, a strong correlation between steady-state and time-resolved experiments makes this experimental approach a versatile tool for future investigations of this class of molecules, especially with respect to photocatalysis. The results of this study underpin the need for further development for quantifying amplitudes of the excited state population of electronically excited singlet and triplet states.