Charge carrier

Adiabatic process

Diabatic

Physics

Electron

Photoexcitation

Chemical physics

Femtosecond

Exciton

Excited state

Atomic physics

Chemistry

Charge (physics)

Materials science

Phonon

Condensed matter physics

Acceptor

Femtochemistry

Spectroscopy

Molecular physics

Optoelectronics

Laser

Quantum mechanics

Polaron

Nature Communications

Covalent organic frameworks (COFs) have emerged as a promising light-harvesting module for artificial photosynthesis and photovoltaics. For efficient generation of free charge carriers, the donor–acceptor (D-A) conjugation has been adopted for two-dimensional (2D) COFs recently. In the 2D D-A COFs, photoexcitation would generate a polaron pair, which is a precursor to free charge carriers and has lower binding energy than an exciton. Although the character of the primary excitation species is a key factor in determining optoelectronic properties of a material, excited-state dynamics leading to the creation of a polaron pair 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 this investigation, we elucidate that the polaron pair is formed through ultrafast intra-layer hole transfer coupled with coherent vibrations of the 2D lattice, suggesting a mechanism of phonon-assisted charge transfer.

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