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Excited-State Proton Transfer in Push-Pull N-Methyl Pyridium Reversible Super-Photoacids Ruled by Intramolecular
Alessio Cesaretti1, Carmela Bonaccorso2, Martina Alebardi1
1Department of Chemistry, Biology and Biotechnology and Center of Excellence on Innovative Nanostructured Materials (CEMIN), University of Perugia, Via Elce di Sotto 8, Perugia, 06123, Italy.
Chemistry (Weinheim an Der Bergstrasse, Germany)
|November 10, 2025
Summary
These novel super-photoacids exhibit reversible pH regulation via light. Their excited-state proton transfer (ESPT) behavior shows potential for photocatalytic applications.
Area of Science:
- Photochemistry
- Supramolecular Chemistry
- Physical Chemistry
Background:
- Photoacid behavior is crucial for light-driven chemical processes.
- Understanding excited-state proton transfer (ESPT) is key to designing novel photoresponsive materials.
Purpose of the Study:
- To investigate the photoacid behavior of three push-pull N-methyl pyridium phenols.
- To elucidate the mechanism and kinetics of excited-state proton transfer (ESPT).
- To explore the potential of these compounds as light-regulated pH modifiers and photocatalysts.
Main Methods:
- Stationary and time-resolved spectroscopic techniques (fluorimetric titrations, ultrafast transient absorption, fluorescence up-conversion).
- Quantum-mechanical calculations.
- Nanosecond laser flash photolysis.
Main Results:
- Two ortho-substituted N-methyl pyridium phenols exhibit excited-state proton transfer (ESPT) with negative pKa* values, classifying them as super-photoacids.
- Intramolecular hydrogen-bond-like interactions in ortho isomers are strengthened upon excitation, enhancing acidity.
- ESPT was observed on the picosecond timescale, and proton recombination was studied, showing reversibility.
- Para-substituted isomers did not show proton transfer.
Conclusions:
- N-methyl pyridium phenols with ortho-substituted phenol groups function as efficient super-photoacids.
- These compounds demonstrate reversible light-induced pH regulation.
- Their ability to transfer protons suggests potential applications in photocatalysis.
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