Theory-Guided Discovery of Ion-Exchanged Poly(heptazine imide) Photocatalysts Using First-Principles Many-Body Perturbation Theory

Poly(heptazine imides) (PHI) show strong promise in photocatalysis, but limited control over electronic properties continues to constrain their full potential. We modulated PHI's photocatalytic activity to overcome this limitation by incorporating mono-, di-, and trivalent metal cations into its framework. We employed calculations based on many-body perturbation theory─a highly accurate approach for electronic structure calculations─which provides improved accuracy in quasiparticle energy predictions compared to conventional density functional theory, particularly for band gaps and excitonic properties, to elucidate the underlying mechanisms. The coupling between exchanged metals and the resulting optoelectronic properties is often nontrivial: Pd, Pt, and Cu¹⁺, for example, produce favorable band structures but show limited photocatalytic activity due to optically forbidden intra-atomic d-d transitions. Our analysis identified several metal-doped PHI systems with electronic structures well suited for hydrogen and oxygen evolution, CO₂ reduction, and H₂O₂ production. Guided by these theoretical insights, we synthesized a subset of <i>M</i>-PHI materials (where <i>M</i> is either K, Na, Li, Ca, Mg, or Zn) predicted to enhance photocatalytic reactivity. Photocatalytic measurements confirm substantial increases in H₂O₂ generation rates─up to 4.5-fold higher than undoped PHI─for these candidates, underscoring the effectiveness of our design strategy. These findings offer molecular-level insights into tailoring <i>M</i>-PHI interactions, paving the way for next-generation photocatalysts.