Excitonic Effects in Polymeric Photocatalysts

Abstract

Owing to the intrinsically low dielectric properties, robust Coulomb interactions between photoinduced electrons and holes lead to dramatically strong exciton effects in polymeric photocatalysts. Such effects endow polymeric matrixes with nontrivial photoexcitation processes determining photocatalytic energy utilization. In this Minireview, we describe recent progress in the investigation of the excitonic effect in polymeric photocatalysts. On the basis of the understanding of excitonic effects in polymeric systems, we outline the relationships between excitonic behaviors and photocatalytic performance. Advances in optimizing the excitonic effect for gaining high‐efficiency polymer‐based photocatalysis are summarized. We also discuss the challenges in the field and forecast the directions for future research.

Emerging Concepts in Carbon Nitride Organic Photocatalysis

“…At the same time, advanced time resolved spectroscopic techniques, such as TAS and TRPL, are becoming indispensable tools for understanding the underpinning mechanism of photocatalytic transformations and is the foundation for rational design of photocatalysts. [144] Based on our many-year experience of working in the field of carbon nitride photocatalysis, we believe this trend will increase in future.…”

“…Although design of carbon nitride materials remains one of the hottest topics in materials chemistry, focus gradually shifts toward application of these materials in synthesis of APIs and development of synthetic protocols for synthesis of value‐added organic compounds. At the same time, advanced time resolved spectroscopic techniques, such as TAS and TRPL, are becoming indispensable tools for understanding the underpinning mechanism of photocatalytic transformations and is the foundation for rational design of photocatalysts [144] . Based on our many‐year experience of working in the field of carbon nitride photocatalysis, we believe this trend will increase in future.…”

Hydrogen evolution by polymer photocatalysts; a possible photocatalytic cycle

“…In inorganic materials the exciton binding energy (EBE) is normally a few tens of meV or less and excitons thus spontaneously dissociate into free-charge carriers. In contrast, the S 1 exciton in polymer photocatalysts is normally still bound at room temperature as a result of the much larger EBE in organic conjugated materials, 26,27 as well the fact that excitons in polymers self-trap, i.e. localize by distorting the polymer geometry.…”

“…24,25 Our empirical observations are in line with a classical semiconductor microscopic model where the exciton formed through the absorption of light either (i) spontaneously falls apart and the free electron and hole formed in the process reduce protons/SEA and oxidise water/SED, respectively, or, more likely due to the large exciton binding energy relative to k B T in polymers, (ii) drives one of the two solution half-reactions and the remaining free-electron/hole the other. 26,27 Transient spectroscopy of polymers under hydrogen evolution conditions 25,28 indeed suggest the presence of electron polarons, i.e. self-trapped electrons, though the exact catalytic cycle and nature of the sites responsible for HER/OER based on this semiconductor-like model, and the role played by (noble) metal co-catalysts, are less clear.…”

Covalent organic frameworks in Lieb lattice for tunable photocatalytic water splitting under visible light

“…Next, light absorption by COFs generates excitons, i.e., bound electron-hole pairs that can further dissociate into free charge carriers to motivate follow-up redox reactions, in which the exciton binding energy has to be overcome. Although many researchers have realized the importance of exciton dissociation in organic photocatalysts because of weak dielectric screening and strong Coulombic attraction between electrons and holes, [40][41] it remains to be resolved how to reduce the exciton binding energy in COFs. In principle, the exciton binding energy relies on the exciton mass, static dielectric constant, and exciton radius, 42 so flat bands and localized electronic states in COFs should be avoided.…”