  We perform state-of-the-art calculations for a prototypical dye sensitized
solar cell: catechol on rutile TiO$_2$(110). Catechol is often used as an
anchoring group for larger more complex organic and inorganic dyes on TiO$_2$
and forms a type II heterojunctions on TiO$_2$(110). In particular, we compare
quasiparticle (QP) $G_0W_0$ with hybrid exchange correlation functional (HSE)
density functional theory (DFT) calculations for the catechol-rutile
TiO$_2$(110) interface. In so doing, we provide a theoretical interpretation of
ultraviolet photoemission spectroscopy (UPS) and inverse photoemission
spectroscopy (IPES) experiments for this prototypical system. Specifically, we
demonstrate that the position, presence, and intensity of peaks associated with
catechol's HOMO, intermolecular OH$-$O bonds, and interfacial hydrogen bonds to
the surface bridging O atoms (O$_{br}$H$-$C and O$_{br}$H$-$O) may be used to
fingerprint deprotonation of catechol's OH anchoring groups. Furthermore, our
results suggest deprotonation of these groups, while being nearly isoenergetic
at high coverages, may significantly increase the photovoltaic efficiency of
catechol$-$TiO$_2$(110) interfaces.
