An ellipsometric study was carried out in order to investigate the photoinduced change of the refractive index (Deltan) of a series of diarylethenes across the entire spectral range 260-1700 nm. This technique allowed examination of the influence of the resonance conditions on the refractive index. Changing the chemical design of diarylethenes by introducing electron-donating or electron-withdrawing substituents in 5,5'-positions of thiophyene allowed derivation of some correlations between chemical structure and Deltan. A strong donor substituent directly linked to the 5,5'-position of the diarylethene moiety gave rise to an increase in the change of the refractive index. 1,2-bis(2-methyl-5-formyl-3-thienyl)perfluorocyclopentene phenyl hydrazone showed a large and stable Deltan of 0.005 at 1.5 mum and appeared to be a good candidate for the development of telecommunication devices.