Intramolecular hydrogen-atom transfer dependence on electronic conjugation of curcumin and related molecular models in the ground state and (1)pi pi* excited state are computationally studied at the first-principles electronic structure level. The larger, more conjugated, systems exhibit a lower reaction barrier in the ground state but a higher barrier in the excited state. This is associated with a smaller increase in the conjugation upon excitation in the, larger systems. Our studies provide a detailed description and analysis of these energy trends as well as an insight into the physical nature of the intramolecular hydrogen-atom transfer reactions.