Light-harvesting devices are of fundamental importance in solar energy conversion. By mimicking natural photosynthetic systems, artificial photosynthetic adaptations are usually based on using organic dyes and transition metal complexes as light-harvesting antennas. To this aim, donor-p-acceptor dyes are potentially used as antennas in dye sensitized solar cells (DSSCs). The advantages of organic dyes are their adjustable optical properties coupled with low production costs. Metal complexes, in particular ruthenium(II) polypyridine complexes, are widely studied because of their unique combination of chemical and physical properties. The present thesis is a theoretical investigation of the photophysical and photochemical properties of several light-harvesting antennas and photosensitisers. The first part of the thesis is devoted to study a series of donor-p-acceptor dyes, based on 4-methoxy-thiazole chromophores and ruthenium(II) polypyridine complexes with 4H-imidazole ligands. Quantum chemical and TDDFT methods have been applied to investigate photophysical properties of the dyes, special mention deserve the performed simulation of resonance Raman (RR) intensities. Based on the calculated RR spectra, protonation effects and the character of the involved excited states could be unraveled. Substitution as well as anchoring was found to be of substantial influence for the photophysical properties, such as excitation energies and excited states characters, of the ruthenium(II) complexes. To allow for applications of the dyes, as e.g. in DSSCs, knowledge of electron transfer (ET) processes occurring at the dye-semiconductor interface is necessary. Such processes can be studied by means of semi-classical Marcus theory. To this aim, a model system of a ruthenium(II) dye linked to a titanium dioxide cluster was constructed. Quantum mechanical/molecular mechanical simulations coupled with molecular dynamics have been performed in order to get the ET rate.