The work presented in this thesis is focused on the study of photoinduced reactions such as charge separation (CS) and excitation energy transfer (EET) in a variety of organic compounds. This covers the investigation of ultrafast processes occurring on femtosecond and picosecond timescales in simple molecular arrays, composed of an electron donor and an electron acceptor unit, as well as complex molecular architectures capable of mimicking photosynthetic functions such as EET and CS. The studies are carried out by the use of several steady-state and time-resolved UV-VIS spectroscopic techniques, mainly in liquid solutions. Part of the work is dedicated to characterize ordered molecular structures deposited on indium tin oxide (ITO), which is then used in a prototype device for solar energy conversion. The aim was to identify the transients and to determine the timescales of the relevant processes.