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

Abstract

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.

2D framework materials for energy applications

“…Generally speaking, the use of 2DFMs in photovoltaics are still only scarcely researched, even though plenty of studies on 3D and layered framework materials exist, still there is a long way to go to reach the full potential of these materials. [208][209][210] Jiang and co-workers prepared the layered MOF material [Ni 3 (OH) 2 (bdc) 4 ], 211 which readily exfoliates to 4.2 nm sheets upon inclusion into a saturable absorber precursor solution. 212 The preparation process also lead to the inclusion of Ni ions into the framework nanosheets, which lead to a striking shi of the bandgap from 3.12 eV (bulk, no Ni doping) to 0.86 eV, comparable to conventional 2D materials.…”

Nitrogen-rich covalent organic frameworks with multiple carbonyls for high-performance sodium batteries

“…ovalent organic frameworks (COFs), which are a class of polymers with designable periodic skeletons and ordered nanopores, have been demonstrated potential applications widely in the fields such as catalysis, semiconductors, proton conduction, and gas capture [1][2][3][4][5][6] . Moreover, COFs with controllable pore size and redox sites can also be applied in the field of electrochemical energy storage and conversion [7][8][9] .…”

Emissive Single-Crystalline Boroxine-Linked Colloidal Covalent Organic Frameworks

“…During the course of our investigation, we observed that all of the colloidal 2D COF suspensions were noticeably more fluorescent than their boronic acid monomers at the same fluorophore concentrations (Figure 5). Motivated by the results of our previous investigations into enhanced optical characterization of 2D COF colloids, 19,43 we set out to explore the origin of enhanced nanocrystal fluorescence using EEMS. While EEMS showed that all 2D COF colloids were more emissive than their respective monomers, we chose to focus our investigation on the highly fluorescent Py-COF and DBD-COF materials.…”