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

Pentacene molecules have recently been observed to form a well-ordered monolayer on the (110) surface of rutile TiO, with the molecules 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 intriguing model to study charge-transfer excitations where 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 system by means of ab initio calculations and compute its excitonic absorption spectrum. Molecular states appear in the band gap of the clean TiO surface, which enables charge-transfer excitations directly from the molecular HOMO to the TiO 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.This work was financed by the Deutsche Forschungsgemeinschaft (DFG) through the SFB 1083 project. MPL, DSP and PK also acknowledges support from Spanish MINECO (Grants No. MAT2013-46593-C6-2-P and MAT2016-78293-C6-4-R) and PK also acknowledges financial support from Fellows Guipuzcoa program from Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union.Peer Reviewe