Cation Mixing Controls Photocatalytic Activity of Ionic Carbon Nitrides by Switching Exciton Character and Electron Localization
Abstract
Ionic carbon nitrides of the poly(heptazine imide) (PHI) type count among the most versatile polymeric photocatalysts, whereby their performance depends sensitively on the incorporated alkali metal cations.However, the mechanistic origins of this cation dependence remain largely underexplored.Here, the effects of cation mixing on exciton generation and dynamics are elucidated for a series of water-soluble PHIs comprising binary combinations of Li⁺, Na⁺, K⁺, and Cs⁺.Photocatalytic H2O2 production reveals that all investigated mixtures except KCsPHI outperform the single-cation CsPHI benchmark, whereby LiCsPHI exceeds it almost threefold.Using band-resolved analysis of ultrafast transient absorption data recorded in the absence and presence of ethanol as hole quencher, complemented by hybrid-DFT and time-dependent Hartree-Fock calculations, this activity pattern is traced back to the character of the low-energy excitons: only in LiCsPHI do interlayer charge-transfer excitons conjoin with a strong localization of the conduction-band electron density at the reactive edge carbon atoms, which suppresses geminate recombination and accelerates hole extraction by up to an order of magnitude.Notably, this switch in exciton character leaves a spectroscopic fingerprint in the band-resolved decay kinetics of the shallow-trapped carriers, whereby only in LiCsPHI does the minor excited-state absorption band outlive the main band.Moreover, the calculated oscillator strength of the low-energy exciton correlates inversely with the photocatalytic activity, i.e., brighter is not better: the most active LiCsPHI is the one hosting the darkest (least emissive, most CT-like) low-energy exciton.This work establishes cation mixing as a rational strategy for engineering the excitonic landscape of ionic carbon nitrides and provides concrete design guidelines for multi-cation PHI photocatalysts.
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BibTeX
@misc{liessem2026cation,
title = {{Cation Mixing Controls Photocatalytic Activity of Ionic Carbon Nitrides by Switching Exciton Character and Electron Localization}},
author = {Johannes Liessem and Arindam Konar and Changbin Im and Mohamed M. Elnagar and Dariusz Mitoraj and Pratibha Saini and Sarah Jasmin Finkelmeyer and Martin Presselt and Timo Jacob and Benjamin Dietzek-Ivanšić and Radim Beranek},
howpublished = {ChemRxiv},
year = {2026},
month = oct,
doi = {10.26434/chemrxiv.15010159/v1},
url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010159/v1},
note = {CC BY},
}
RIS
TY - UNPB
TI - Cation Mixing Controls Photocatalytic Activity of Ionic Carbon Nitrides by Switching Exciton Character and Electron Localization
AU - Johannes Liessem
AU - Arindam Konar
AU - Changbin Im
AU - Mohamed M. Elnagar
AU - Dariusz Mitoraj
AU - Pratibha Saini
AU - Sarah Jasmin Finkelmeyer
AU - Martin Presselt
AU - Timo Jacob
AU - Benjamin Dietzek-Ivanšić
AU - Radim Beranek
PY - 2026
DA - 2026/10/09
PB - ChemRxiv
DO - 10.26434/chemrxiv.15010159/v1
UR - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010159/v1
AB - Ionic carbon nitrides of the poly(heptazine imide) (PHI) type count among the most versatile polymeric photocatalysts, whereby their performance depends sensitively on the incorporated alkali metal cations.However, the mechanistic origins of this cation dependence remain largely underexplored.Here, the effects of cation mixing on exciton generation and dynamics are elucidated for a series of water-soluble PHIs comprising binary combinations of Li⁺, Na⁺, K⁺, and Cs⁺.Photocatalytic H2O2 production reveals that all investigated mixtures except KCsPHI outperform the single-cation CsPHI benchmark, whereby LiCsPHI exceeds it almost threefold.Using band-resolved analysis of ultrafast transient absorption data recorded in the absence and presence of ethanol as hole quencher, complemented by hybrid-DFT and time-dependent Hartree-Fock calculations, this activity pattern is traced back to the character of the low-energy excitons: only in LiCsPHI do interlayer charge-transfer excitons conjoin with a strong localization of the conduction-band electron density at the reactive edge carbon atoms, which suppresses geminate recombination and accelerates hole extraction by up to an order of magnitude.Notably, this switch in exciton character leaves a spectroscopic fingerprint in the band-resolved decay kinetics of the shallow-trapped carriers, whereby only in LiCsPHI does the minor excited-state absorption band outlive the main band.Moreover, the calculated oscillator strength of the low-energy exciton correlates inversely with the photocatalytic activity, i.e., brighter is not better: the most active LiCsPHI is the one hosting the darkest (least emissive, most CT-like) low-energy exciton.This work establishes cation mixing as a rational strategy for engineering the excitonic landscape of ionic carbon nitrides and provides concrete design guidelines for multi-cation PHI photocatalysts.
ER -