We present a computational investigation on the effect of halogen substitution on the electronic, optical, and transport properties of the nucleobase guanine. We performed all-electrons Density Functional Theory and Time-Dependent Density Functional Theory calculations with a localized Gaussian basis-set and the hybrid exchangecorrelation functional B3LYP. Results obtained with the above level of theory are in good agreement with the available experimental data for the unsubstituted molecule. We report and discuss the effect of functional substitution on several physico-chemical properties such as: electron affinities, ionization energies, fundamental energy-gap, optical absorption, exciton binding energy and molecular reorganization energies for holes and electrons.