Ultrafast Dynamics of Charge Carriers in Two-dimensional Covalent Organic Frameworks

초록

Covalent organic frameworks (COFs) have emerged as a promising light harvesting module due to their robust platform designable with various constituents. As a means of increasing photoconversion efficiency, a donor-acceptor (D-A) conjugation scheme is commonly used in organic photovoltaics and it has also been employed in the COFs. In this work, we investigate charge transfer dynamics in two-dimensional D-A COFs consisting of alternating 3,4,9,10-perylenetetracarboxylic acid diimide (PDI) and free-base porphyrin units. Using broadband femtosecond transient absorption (TA) spectroscopy, we reveal that the photogenerated charge carriers are transferred in 124 fs between PDI and free-base porphyrin in the 2D sheet. Specifically, from the NA-MD simulation, we found that hole carriers are transferred from PDI to porphyrin, due to quasi-degeneracy of valence orbitals in the PDI and porphyrin units. In addition, temporal oscillation of TA signal arising from low-frequency lattice vibrations of COFs was observed, providing the evidence of a phonon-assisted charge transfer mechnism in the reticular structure of COFs.