Solvent Polarity Modulated Excited-State Dynamics within a Pyrene-Based Covalent Organic Cage

It is still a great challenge to establish the relationship between intermolecular packing and photophysical properties in the solid state of photofunctional materials. Covalent linked multichromophoric architecture is a versatile platform to mimic the surrounding environment in the solid state. Here, a [3 + 6]-type covalent organic cage incorporating three 1,6-disubstituted pyrene units (denoted as PyTC1) was synthesized and its excited-state dynamics was systematically investigated using a transient spectroscopy technique. An H-type aggregate was formed in this cage, showing a strong intramolecular electronic coupling. As revealed by transient absorption spectroscopy, the excited-state dynamics of PyTC1 are strongly dependent on solvent polarities. In low-polarity solvents, PyTC1 undergoes intersystem crossing (ISC) to form the triplet state with a rate (3.11 ns in toluene) approximately twice that of the monomer (7.32 ns in toluene), while in high-polarity solvents, a rapid symmetry breaking charge separation (SB-CS) process (τ = 143.2 ps) occurs to generate a charge transfer state that forms an equilibrium with the singlet state, leading to the formation of delayed fluorescence (τ = 18.6 ns) from the reverse process of SB-CS. Then, the triplet state was formed from the charge transfer state via a spin-orbit charge-transfer intersystem crossing mechanism. These findings not only emphasize the importance of the solvent polarity on excited-state dynamics of organic cages but also provide molecular-level insights for the establishment of the structure-property relationship in the solid state.