Publication date: 21st July 2025
Poly(heptazine imide) (PHI) materials are promising metal-free photocatalysts, yet the fundamental factors governing their charge generation remain insufficiently understood. In this work, we reveal that the identity of photoexcited species in PHI—either as free charges or bound excitons—is determined within the first 200 femtoseconds after excitation, with the branching between these pathways governed by both ion substitution and excitation photon energy. Spectroscopic Insights into Tunable Charge and Exciton Pathways in Poly(Heptazine Imide): Role of Ion Substitution and Photon Energy
Using ultrafast transient absorption spectroscopy, we show that substituting K⁺ with H⁺ effectively quenches the charge generation pathway, suppressing the formation of long-lived electrons associated with hot-state separation. Instead, the photoexcited population is funnelled predominantly into bound excitonic states, as evidenced by strong bleach signals arising from radiative recombination. Crucially, we demonstrate that these competing photophysical pathways originate not from a sequential exciton dissociation process, but rather from a rapid branching between hot charge-generating states and lower-energy excitonic states—well before the time resolution of our measurements. These distinct regions of the electronic manifold are selectively accessed by further tuning the excitation photon energy (355-400 nm), as evidenced by the increased bleach-to-charge signal ratio in K-PHI at lower photon energies.
These results establish a spectroscopically supported model in which ion substitution and excitation energy jointly modulate the excited-state landscape in PHI. Our findings challenge conventional assumptions about exciton-driven charge separation and highlight hot-state access as a key design principle for tuning photocatalytic performance.
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