Microscopic Mechanism of Ultrafast Excited-State Intramolecular Proton Transfer: A 30-fs Study of 2-(2‘-Hydroxyphenyl)benzothiazole

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Abstract

A detailed analysis of the excited-state intramolecular proton transfer (ESIPT) of 2-(2‘-hydroxyphenyl)benzothiazole and the associated wave packet motion is presented. It is based on the evolution of the emission spectrum observed by UV−vis pump−probe absorption measurements with a cross correlation of 35 fs. The rise of the emission is delayed by 33 fs and reveals the time the wave packet needs to evolve along the reaction coordinate. Four decisive molecular motions and their role during the process are identified by the frequencies and phases extracted from the oscillatory signal contributions. A novel model is developed that describes the ESIPT as a ballistic wave packet motion consisting of three major components: First, the H-chelate ring contracts by in-plane bending of the whole molecule, resulting in an acceleration along the corresponding normal mode. The time scale of the motion is given by the inertia of the participating atoms. When the ON distance is sufficiently shortened, the electronic configuration changes, new bonds are formed, and a new equilibrium geometry results. The molecule is now displaced with respect to this geometry and begins to oscillate in those modes that have large projections on the displacement. The proton is shifted passively toward the nitrogen by the initial contraction of the ring and stays there because of the electronic configuration change.

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