The visual pigment of vertebrates (rhodopsin), a member of the large class of G-protein-coupled receptors, and the photosynthetic pigment bacteriorhodopsin of halobacterium salinarum, an active pump, contain retinal as prosthetic group. The replacement of the native retinal by analogue chromophores helps to understand the molecular function of the light sensitive ligands. In order to study the intermolecular interaction between chromophore and protein three retinal derivatives were synthesized and incoporated (9-desmethylretinal, 9-ethylretinal and 9-isopropylretinal) which together with the native chromophore constitute a homologous series with systematically changed size of the substituent at position 9. In addition three analogue retinals with modification at position 10 and/or 13 (10-methylretinal, 13-demethylretinal and 10-methyl-13-demethylretinal) were also synthesized and studied particulary to understand the intramolecular interaction of the chromophore of rhodopsin.