Photosynthesis is a physico-chemical process. The chemical process of photosynthesis is started in 

Antenna pigment molecules are involved only in the physical process. They absorb the light energy and transfer the excitation energy to reaction center either directly to reaction center or through various pigment molecules of light harvesting complex. It is the reaction center which is excited and expels the electron (oxidation - a chemical process) with which the chemical process of photosynthesis is started. Hence Photosystems are involved in both physical and chemical processes, but others of thylakoids are involved in only photochemical process.