Two different types of chlorophyll c (c(1) and c(2)) and their oxidized forms (c(1)(') and c(2)(')) were isolated from a brown seaweed, Undaria pinnatifida, and their structures were determined by mass spectrometry and H-1 NMR spectroscopy. The order of conversion efficiencies in titania-based solar cells using these chlorophyll sensitizers, i.e., c(2) (4.6%) > c(1) (3.4%) > c(2)(') (2.6%) >= c(1)(') (2.5%), were explained in terms of a more extended conjugated system and the resultant higher electron densities in Chl c(2) than Chl c(1) as well as the different configurations and electron densities of the vinyl carboxyl group in Chls c(2)(') and c(1)('). (c) 2007 Elsevier B.V. All rights reserved.