Mode-selective photoisomerization in 5-hydroxytropolone. I. Experiment

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Abstract
Laser‐induced fluorescence excitation, dispersed fluorescence, and population labeling spectra of the S0–S1 transition of 5‐hydroxytropolone (5‐HOTrOH) have been recorded in a supersonic free jet. In the ground state, the two in‐plane orientations of the 5‐OH substituent produce two isomers, syn and anti relative to the 2‐OH. Population labeling is used to identify transitions in the spectrum originating from the syn and anti zero‐point levels. In the excited electronic state where 2‐OH H‐atom tunneling is expected to be more facile, the single vibronic level dispersed fluorescence spectra identify excited state levels with widely varying syn–anti character. Many of the levels in the low‐energy region of the spectrum are nearly pure syn or anti in character, thereby showing little coupling to the isomerization reaction coordinate. Other levels are strongly syn–anti mixed via Fermi resonance interactions. A few levels exhibit strong syn–anti mixing despite being relatively isolated from other levels. These levels have strong syn–anti coupling matrix elements which reflect a strong coupling to the reaction coordinate. Analysis of the dispersed emission identifies a ‘‘promoter’’ vibrational mode W, with syn (anti) ground‐state frequency 336 (337) cm−1 which appears to play a key role in coupling syn and anti levels. The accessibility of excited state levels of mixed character makes it possible to efficiently and reversibly isomerize 5‐hydroxytropolone in a highly mode‐specific fashion.