3-cyano-6-hydroxycoumarin (CHCM) is a photoacid, i.e., a molecule that acts as a strong acid if electronically excited by a photon. Photoacids provide photo-control of local acidity, which can be used for photolithography, photoswitching, and determining drug activity. However, transient absorption spectra of CHCM indicate that a significant fraction of electronically excited molecules experience rapid (less than 1 ps) de-excitation by radiationless decay to the anionic ground state, which limits potential applicability of CHCM as a photoacid (Solntsev, Poizat, private communications). In order to identify the cause of this radiationless decay, ab-initioquantum chemical calculations have been performed. Gas phase calculations show a state crossing between the first excited state and a spectroscopically dark state along the proton transfer coordinate, implying that radiationless decay may occur due to these state crossings. However, since experimental data were obtained in solution phase, modeling solvent effects is essential to cement the proposed mechanism of radiationless decay. Two types of solvation methods, the Polarizable Continuum Model (PCM) and the Effective Fragment Potential (EFP) method were used. Interestingly, unlike gas phase calculations, both solvation models failed to show the conical intersection between the bright and dark states. On the other hand, it was found that back-protonation might explain the radiationless decay in CHCM. This observation opens up a new unstudied pathway for non-adiabatic dynamics in CHCM and other photoacids.

Investigating ultrafast radiationless decay of a photoacid: Simulating 3-cyano-6-hydroxycoumarin with implicit and explicit ab initio solvent models

248th National Meeting (2014)

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