Charge-transfer states and optical transitions at the pentacene-TiO2 interface

Pentacenemolecules have recently been observed to form awell-orderedmonolayer on the (110) surface of rutile TiO2, with themolecules adsorbed lying flat, head to tail.With the geometry favorable for direct optical excitation and given its ordered character, this interface seems to provide an intriguingmodel to study charge-transfer excitationswhere the optically excited electrons and holes reside on different sides of the organic–inorganic interface. In this work, we theoretically investigate the structural and electronic properties of this systembymeans of ab initio calculations and compute its excitonic absorption spectrum.Molecular states appear in the band gap of the cleanTiO2 surface, which enables charge-transfer excitations directly from themolecularHOMO to the TiO2 conduction band. The calculated optical spectrum shows a strong polarization dependence and displays excitonic resonances corresponding to the charge-transfer states, which could stimulate new experimental work on the optical response of this interface.