The impact of macrocyclization: electronic structures and excited state dynamics of pillar[4]arene[1]quinone

The distinct architectures in macrocyclic molecules allow them to fulfil the specialized roles as important molecular containers or recognition sites. However, the impact of macrocyclization on the excited states of the molecular building blocks that construct the macrocycles is yet to be understood. Herein, the electronic structures and excited-state dynamics of pillar[4]arene[1]quinone (P4Q1) were compared with those of the 2,5-dimethylquinone reference molecule through steady-state and time-resolved transient absorption spectroscopic investigations and quantum chemical computations. The macrocyclic skeleton is shown to facilitate the formation of a set of low-lying intramolecular charge transfer excited states in the photoexcited P4Q1, leading to the tremendous suppression of intersystem crossing via a conical intersection mechanism and causing a significantly reduced triplet excited-state yield. Furthermore, the solvation dynamics was also explored as a function of the host-guest interactions between P4Q1 macrocycles and polar acetonitrile molecules, revealing the negligible influence of the acetonitrile guest molecule encapsulated inside the cavity. These results provide guidance to understand the excited-state dynamics of important supramolecular macrocycles and the related photochemical processes.