Without light, morphologies of the resultant metal-Aβ40 aggregates were not significantly changed even with the compounds. Thick fibrils were formed by incubation of metal-free Aβ40 in the presence of all three compounds, compared to fibrillary aggregates produced in the sample of metal-free Aβ40 only. This suggests that the AQ backbone itself may interact with metal-free Aβ40 and affect its aggregation. When 9,10-AQ was incubated with metal-free Aβ40 with photoirradiation, long and thick fibrils as well as similar fibrils to those formed from Aβ40 only were detected. The conformational transformation of metal-free Aβ40 by photoexcited Pur and Alo was more noticeable than that by 9,10-AQ resulting in smaller and thinner aggregates. In the case of metal-Aβ40 species, small amorphous assemblies and thin fibrils were visualized with the illumination of Pur and Alo, but 9,10-AQ did not noticeably alter their morphologies. These amorphous aggregates are reported to be less toxic than structured assemblies. Modest or no changes found in the structures of metal-free and metal-bound Aβ40 species with photoactivated 9,10-AQ denote the importance of efficient 1 O2 production in modifying their aggregation pathways.