Photodissociation of the photoisomer crystal (AEAPI) gives a number of conformations of the 1, 2-di(9-anthryl)ethane with different photophysical and photochemical properties. Weakly absorbed light produces two nonfluorescent conformations, with slightly different orientations in the crystal, and two fluorescent conformations which are thermally related. Both sets can be photoisomerized with different rates, at liquid helium temperature. Other, more relaxed, sandwich conformations are produced by more strongly absorbed light and possible conformational arrangements of the two chromophores have been identified by polarized absorption spectra. The crystal and molecular structure of the photoisomer has been determined from single-crystal X-ray diffraction data collected on a Picker FACS-1 diffractometer. Crystals of AEAPI are monoclinic, with a = 10.095 (2) Å,b = 12.471 (2) Å,c = 8.340 (1) Å, β, = 111.20 (1) ⊰,and Z = 2. The packing arrangement is strongly pseudocentrosymmetric, approximating space group P21/c. The structure was solved using the MULTAN programs and has been refined successfully in space group P21, with one additional (noncrystallographic) constraint, viz., that the two anthracenederived moieties in each molecule are exactly related by inversion symmetry. Agreement between observed and calculated structure factor amplitudes for the terminal scattering model is only moderate, with weighted and unweighted R factors of 0.36 and 0.08, respectively, for 1776 observed unique data. Nevertheless, bond lengths and bond angles are all close to their expected values, and a number of spectral features can be correlated with the crystal packing arrangement. © 1979, American Chemical Society. All rights reserved.
