Barrierless Heptazine-Driven Excited State Proton-Coupled Electron Transfer: Implications for Controlling Photochemistry of Carbon Nitrides and Aza-Arenes

To inform prospective design rules for controlling aza-arene photochemistry, we studied hydrogen-bonded complexes of 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a molecular photocatalyst chemically related to graphitic carbon nitride, with a variety of phenol derivatives. We have focused on excited state proton-coupled electron transfer (ES-PCET) reactions of heptazines because the excited state properties governing this process remain conceptually opaque compared to proton reduction reactions for these materials. We used ground-state absorption, time-resolved photoluminescence, and ab initio quantum chemical calculations to analyze TAHz reactivity toward a series of six para-substituted phenol derivatives. We determined association constants (K_A), excited-state quenching rate constants (k_Q), kinetic isotope effects, and transition-state barriers (ΔE⁺). From this data, we provide a generalizable picture of hydrogen bond formation and excited state reactivity of heptazine-based materials with hydrogen-atom donating solvents. These results provide important insights into strategies to tune charge transfer state energies and increase ES-PCET rates.