Exploring quantum phenomena and vibrational control in sigma* mediated photochemistry

Non-adiabatic dynamics involving (1)pi sigma* or (1)n sigma* excited electronic states play a key role in the photochemistry of numerous heteroatom containing aromatic (bio-) molecules. In this contribution, we investigate more exotic phenomena involved in sigma* mediated dynamics, namely: (i) the role of purely quantum mechanical behavior; and (ii) manipulating non-adiabatic photochemistry through conical intersections (CIs) with 'vibration-specific control'. This is achieved by investigating S-CH3 bond fission via a (1)n sigma* potential energy surface (PES) in thioanisole (C6H5SCH3). Using a combination of time- and frequency-resolved velocity map ion imaging techniques, together with ab initio calculations, we demonstrate that excitation to the (1)pi pi* <- S-0 origin [(1)pi pi*(nu = 0)] results in S-CH3 bond fission on the (1)n sigma* PES, despite an (apparent) energetic barrier to dissociation formed by a CI between the (1)pi pi* and (1)n sigma* PESs. This process occurs by accessing 'classically forbidden' regions of the excited state potential energy landscape where the barrier to dissociation becomes negligible, aided by torsional motion of the S-CH3 group out of the plane of the phenyl ring. Control over these dynamics is attained by populating a single quantum of the S-CH3 stretch mode in the (1)pi pi* state [(1)pi pi*(nu(7a) = 1)], which mirrors the nuclear motion required to promote coupling through the (1)pi pi*/(1)n sigma* CI, resulting in a marked change in the electronic branching in the C6H5S radical products. This observation offers an elegant contribution towards a vision of 'quantum control' in photo-initiated chemical reaction dynamics.