| Abstract: | Abstract— The absorption spectrum of lycopene can be altered to show significant absorption in the 350–360 nm region in an ethanol-water mixture, thus resembling the phototropic action spectrum of Avena coleoptiles. The hypochromicity (bleaching) of the main absorption band and appearance of the new band at 350–360 nm can be attributed to exciton interactions between two stacked lycopene molecules. β-Carotene does not show anomalous bleaching under identical conditions. Thus, the apparent modification of the absorption spectra of carotenoids in ethanol-water mixtures cannot be used as an argument to resolve the action spectrum in terms of carotenoids. In addition, we have critically reviewed the spectroscopic characteristics of carotenoids. Short lifetimes of the excited singlet states and inefficient intersystem crossing of carotenoids are not compatible with the suggestion that carotenoids are the most likely candidate for the photoreceptor pigment in phototropism. |
