First Principles Optical Absorption Spectra of Organic Molecules Adsorbed on Titania Nanoparticles

We present results from first principles computations on passivated rutile TiO$_2$ nanoparticles in both free-standing and dye-sensitized configurations to investigate the size dependence of their optical absorption spectra. The computations are performed using time-dependent density functional theory (TDDFT) as well as GW-Bethe-Salpeter-Equation (GWBSE) methods and compared with each other. We interpret the first principles spectra for free-standing TiO$_2$ nanoparticles within the framework of the classical Mie-Gans theory using the bulk dielectric function of TiO$_2$. We investigate the effects of the titania support on the absorption spectra of a particular set of perylene-diimide (PDI) derived dye molecules, namely brominated PDI (Br$_2$C$_{24}$H$_8$N$_2$O$_4$) and its glycine and aspartine derivatives.