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Eight stable ( Z ) -9-styrylacridines, have been photochemically synthesized and their enthalpies of thermal isomerization determined by means of thermal analysis. The enthalpy values were found t o be influenced by both electronic and steric effects. The title compounds were used as models in order t o gain insight into the energetic implications involved in the Z-.E isomerization process and its applicability to photochemical energy conversion by irradiation in the visible wavelength region. Ed Z Photoisomerization of stilbene-like compounds has long been considered as a potential chemical system for the conversion and storage of solar energy.'The isomeric pair ( Z , E ) involved in the photoequilibrium must fulful several requirements,,~~ the most important being a high enthalpy of isomerization energyTo date there are no literature reports describing ( Z ) -and (E)-1,2disubstituted ethenes with such specific properties.The results reported in this paper arose from synthesis of an E-alkene derivative, the visible light absorption of which lay in the range 300 < ;1 < 700 nm, according to the more stringent condition already found by Guilford Jones2 Furthermore, the above starting compound could be further modified through appropriate functionalities, in order to monitor the energy difference between the geometric isomers of a series of stilbene derivatives.Our attention was directed towards the (E)-styrylacridines, a class of yellow compounds the synthesis of which was first accomplished by Wilson et aL4 To account for some inconsistent melting point values, it was suggested that the synthetic procedure adopted possibly resulted in a mixture of isomeric products, as a result of adventitious light.The availability of the above styrylacridines prompted us to prepare and functionalize some of them, so as to obtain systems able to undergo highly efficient photochemical conversion by irradiation in the visible spectral range. Associated thermodynamic parameters, such as enthalpy, enthropy and activation energy, were also determined by thermal analysis, for a fuller approach to the general problem related to the conversion and storage of solar energy.

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