Sammanfattning: This thesis describes work that could help development of new photochemical reactions and light-absorbing materials. Focus is on the chemical concept "aromaticity" which is a proven conceptual tool in developing thermal chemical reactions. It is here shown that aromaticity is also valuable for photochemistry. The influence of aromaticity is discussed in terms of structure, reactivity and properties. With regard to structure, it is found that photoexcited molecules change their structure to attain aromatic stabilization (planarize, allow through-space conjugation) or avoid antiaromatic destabilization (pucker). As for reactivity, it is found that stabilization/destabilization of reactants decrease/increase photoreactivity, in accordance with the Bell-Evans-Polanyi relationship. Two photoreactions based on excited state antiaromatic destabilization of the substrates are reported. Finally, with respect to properties, it is shown that excited state energies can be tuned by considering aromatic effects of both the electronic ground state and the electronically excited states. The fundamental research presented in this thesis forms a foundation for the development of new photochemical reactions and design of compounds for new organic electronic materials.