Photochromic systems. Part 1. Structural and spectroscopic study of photochromically active products of Stobbe condensation. 2,3-Dibenzylidenesuccinic acid and its anhydride

E,E-2,3-Dibenzylidenesuccinic acid ( 2 ) and its anhydride ( 1 ) were synthesized, their crystal structures and 1 H and 13 C nuclear magnetic resonance spectra were determined, and electron charge densities of carbon atoms were calculated. These results were related to the corresponding data available for E-cinnamic acid ( 3 ) in order to evaluate the effect of structural changes in a series 3 → 2 → 1 on the molecular parameters and spectroscopic properties of the cinnamic system. In the molecule of 2 most of the steric strain is released by the rotation about the C(α)C(α′) bond giving rise to a structure consisting of two, approximately independent, cinnamic acid moieties. In 1 , severe steric strain is introduced, as demonstrated by the unusually large values of the CCC bond angles exocyclic with respect to the anhydride ring, as well as by significant deviations from the plane of the cinnamic skeleton. The geometry of 1 results in an intramolecular shielding of the aromatic hydrogen atoms due to the proximity of the two benzene rings; this shielding effect for the ortho, meta, and para hydrogen atoms correlates well with the intramolecular distances between the corresponding positions of both rings. The common linear relationship between 13 C chemical shifts and the electron charge densities on the given carbon atom has been obtained for compounds 1 , 2 , and 3 .Crystal data. Anhydride ( 1 ): space group P2 1 /c with a = 13.536(3), b = 14.391(3), c = 7.159(1) Å; β = 98.69(1)°; R w = 0.029 and R = 0.055. Succinic acid ( 2 ): space group with a = 8.881(3), b = 9.786(1), c = 11.355(3) Å; α = 85.56(1), β = 88.76(2), γ = 69.46(2)°; R w = 0.048 and R = 0.081. This compound cocrystallizes with one molecule of the solvent (acetic acid). Keywords: photochromic, cinnamic, succinic.