Polarons play a central role in the electronic and optical properties of organic materials, and, given their relevance for various applications, are the focus of cross-disciplinary research at the intersection of chemistry, physics, and materials science. It is known that the spectral lineshape of the mid infrared (IR) absorption band provides valuable information about the "hole" polaron delocalization lengths in organic materials. Here, we present a comprehensive theory describing the spatial coherence length of polarons in COFs, revealing a simple relationship between the oscillator strength of the infrared absorption spectrum and the polaron coherence lengths. Emphasis is placed on determining the fundamental nature and origin of the components polarized along the intra and interframework directions of different COFs, as well as the dependence of these components on the spatial distribution of disorder, framework topology, and position of the dopant counter anion in the pores. We demonstrate that a simple optical probe such as steady state absoprtion can be efficiently used to extract important information about the polaron coherence lengths, quantities which are crucial for a detailed understanding of energy and charge transport processes in these materials. Our analysis provides conclusive evidence of why iodine doped COFs have so far shown lower bulk conductivity compared to polythiophenes.

Unraveling the effect of defects, topology and chemical doping on photophysics and charge transport in covalent organic frameworks

ACS Spring 2022

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