Ultrafast Charge Transfer 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 [1, 2]. To facilitate efficient light harvesting in the artificially synthesized constructs, it is critical to achieve fast transfer of charge carriers from primary photosensitizers, thus facilitating charge separation. As a means of achieving efficient charge transfer, a donor-acceptor conjugation scheme is commonly used in organic photovoltaics and it can be also employed in the COFs. In this work, we investigate charge transfer dynamics in two-dimensional 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 from PDI to free base porphyrin in the 2D sheet. This ultrafast intra-layer charge transfer is driven by an energy gradient formed among excited states of COFs. In addition, temporal oscillation of TA signal arising from low-frequency vibrational modes of COFs was observed, providing the evidence of structural motions that assist the ultrafast charge transfer among the building blocks of COFs.