Abstract: This invention relates to a solar energy storage material comprising N,N'-bis(phenylacetyl)indigo dissolved in a polar solvent. Of the indigo derivatives capable of storing the solar energy by the photoisomerization from trans-isomer to cis-isomer of the molecule, all of the compounds obtained so far had an energy storage capacity of only 30 cal or less per 1 g of the compound and also a high thermal isomerization rate from cis form to trans form in solution. Thus, they cannot be called a solar energy storage material of practical value.In the case of the present invention, on the other hand, when N,N'-bis(phenylacetyl)indigo is dissolved in a polar solvent such as acetonitrile, it is possible to store about 60 cal of energy per 1 g of N,N'-bis(phenylacetyl)indigo, and the half-life period of isomerization from cis to trans from in acetonitrile at 17.degree. C. is 1,264 minutes, which is also a satisfactory value for practical application.

Abstract: This invention is a method for storing light energy using an indigo derivative. Among indigo derivatives capable of storing light energy by the photoisomerization of the molecule from the trans-form to the cis-form, compounds heretofore obtained have been soluble only in expensive solvents such as benzene, toluene, chloroform, carbon tetrachloride and the like and harmful substances.On the other hand, the indigo derivative of this invention has the form of a pyridinium salt, is soluble in water and/or ethanol, and permits employment of safe and inexpensive water and/or an alcohol as a solvent.The indigo derivative of this invention converted from the trans-form to the cis-form by irradiation with sunlight and storing sunlight energy liberates the aforesaid stored energy as heat on addition of a catalyst and/or a heat trigger.