Exploring quantum phenomena and vibrational control in σ* mediated photochemistry

Abstract

Non-adiabatic dynamics involving ¹πσ* or ¹nσ* 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 σ* 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 ¹nσ* 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 ¹ππ* ← S₀ origin [¹ππ*(ν = 0)] results in S–CH₃ bond fission on the ¹nσ* PES, despite an (apparent) energetic barrier to dissociation formed by a CI between the ¹ππ* and ¹nσ* 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 ¹ππ* state [¹ππ*(ν_7a = 1)], which mirrors the nuclear motion required to promote coupling through the ¹ππ*/¹nσ* 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.