A Systems Approach to Resolving the Valence Electronic Structures in Carbon Nitrides

The controlled polymerization of common organic precursor molecules at high temperatures is known to produce carbon nitride oligomers and polymers with a wide range of photocatalytic activity. However, tracking the electronic structures precisely in these complex functional systems remains a significant challenge. Here, we utilize bottom-up and top-down methods of high-temperature polymerization and solution-phase exfoliation to directly reveal the frontier molecular orbitals that give rise to near- and above-band edge transitions. Aqueous dispersions of the oligomeric fractions manifest a progressive increase in a molecular exciton of mixed ππ*/nπ* character that is reproducible with multiple precursors and tractable using theory. The results offer new electronic structural insights and outline a systems approach to resolving the structure-dependent optoelectronic properties of photocatalytic carbon nitrides.