Ultrafast dynamics of charge transfer coupled with lattice phonons in two-dimensional covalent organic frameworks

초록

Covalent organic frameworks (COFs) have emerged as a promising light harvesting module for artificial photosynthesis and photovoltaic devices [1, 2]. For efficient generation of free charge carriers, the donor-acceptor (D-A) conjugation was employed for two-dimensional (2D) COFs recently [3, 4]. In the 2D COFs with D-A conjugation, photoexcitation generates the excited state called a polaron pair, which is a precursor to free charge carriers and has lower binding energy than an exciton. The character of the primary excitation species is a key factor in determining optoelectronic properties of a material, but excited-state dynamics leading to the creation of a polaron pair in the 2D D-A COFs have not been investigated yet. Here, we investigate the dynamics of photogenerated charge carriers in 2D D-A COFs by combining femtosecond optical spectroscopy and non-adiabatic molecular dynamics simulation. From the spectroscopic experiment and the simulation, we elucidate that the polaron pair is formed on ultrafast time scale through hole transfer between the building blocks of a 2D COF layer and this ultrafast transfer of excited carriers is coupled with coherent lattice vibrations, suggesting a mechanism of phonon-assisted charge transfer in the reticular structure of COFs. The ultrafast photophysics in 2D D-A COFs revealed in this study will help the design of efficient COF-based light harvesting module applicable to high-performance optoelectronic devices.