Fig. 5

Schematic of charge carrier dynamics in photoinduced COFs. Upon photoexcitation of COFs, ultrafast hole transfer through the splitting of excitons occurs from PDI to free-base porphyrin (Fb porphyrin) unit with the time constant of 124 fs. According to the NA-MD simulation, the quasi-degenerate electronic states of HOMO-3 (or -3′) and HOMO-2 facilitate such ultrafast transfer of charge carriers. The coherent oscillations with the frequencies of 76 cm⁻¹ and 285 cm⁻¹ superimposed on the decay of TA spectra are associated with coherent lattice phonons of COFs. Especially, the change in the dihedral angle of benzene linker (yellow arrow) can be related with the motions that enhance π-interaction between PDI and porphyrin units, resulting in the ultrafast transfer of charge carriers