The Impact of Axial Ligation on the Excited State Dynamics of Cobalt(II) Phthalocyanine

Abstract

In this work, we report the effect of axial ligation on the excited state dynamics of Cobalt phthalocyanines (CoPc) using the combination of X-ray absorption spectroscopy (XAS), transient absorption (TA) spectroscopy, and density functional theory (DFT) calculation. The formation of six coordinated Co center was unambiguously confirmed by XAS when pyridine was used as the solvent, which leads to the prominent blue shift of its Q-band transition with respect to that in weak-ligating solvent (DMF). This is further supported by DFT calculation, which predicted that axial coordination by two pyridine molecules leads to blue shift of Q band. More interestingly, using TA spectroscopy, we observed the formation of a new intermediated state, which is likely due to the further axial ligation of pyridine to CoPc upon photoexcitation, resulting in the quenching of the CoPc excited state and shorter lifetime.