Ultrafast Electronic Energy Transfer in an orthogonal molecular dyad

Understanding electronic energy transfer (EET) is an important ingredient in the development of artificial photosynthetic systems and photovoltaic technologies. Although EET is at the heart of these applications and crucially influences their light-harvesting efficiency, the nature of EET over short distances for covalently bound donor and acceptor units is often not well understood. Here we investigate EET in an orthogonal molecular dyad (BODT4) in which simple models fail to explain the very origin of EET. Based on nonadiabatic ab initio molecular dynamics calculations and fluorescence depolarization experiments we gain detailed microscopic insights into the ultrafast electro-vibrational dynamics following photoexcitation. Our analysis offers molecular-level insights into these processes and reveals that it takes place on timescales ≲ 100 fs and occurs through an intermediate charge-transfer state.