Understanding photoinduced ultrafast charge transfer in 2D covalent organic frameworks by using non-adiabatic ab initio molecular dynamics simulation

Abstract

Covalent organic framework (COFs) has a two- and three- dimensional porous structures consisting of covalently liked different organic molecules as a building block. Photo-generated electron-hole pairs in the COFs can be efficiently separated into each the building subunits because of different electron affinities of each of the building blocks. Due to this nature, COFs are emerging as a promising light-harvesting material in the field of artificial photosynthesis and photovoltaics. In this study, non-adiabatic ab initio molecular dynamics (NA-MD) simulation was carried out to investigate photoinduced charge transfer of a two-dimensional (2D) COF consisting of alternating 3,4,9,10-perylenetetracarboxylic acid diimide (PDI) and free-base porphyrin units. By implementing the NA-MD simulation, we found that the ultrafast (~120 fs) charge transfer within the 2D COF observed from the experiment is intralayer hole transfer from the PDI to the free-base porphyrin subunit. In addition, lattice vibrations closely related to the charge transfer were theoretically identified by performing vibration normal mode analysis