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The synthesis of some novel derivatives of 4-[ N,N -bis(β-hydroxyethyl)-amino]azobenzene is reported, and the effects of ortho -substituents on photoinduced trans → cis isomerism about the azo group, mass spectral behavior, and the photostability of these dyes are described. An ortho-nitro group inhibits trans → cis photoisomerism about the azo group and affords poor photostability. Those ortho-substituted dyes showing a greater tendency to undergo photoinduced trans → cis isomerism and recover quickly are also more photostable and less subject to rupture at the β-nitrogencarbon bond in the mass spectrometer. The photolysis experiments were done using acetate and triacetate film and fiber, and the mass spectrometric analyses were done by direct probe using the electron-impact technique.The effects of various electron-donating and electron-attracting groups on the color and fastness properties of azo dyes have been described in a number of papers during the past four decades [2-5, 7, 10, 12-17, 19 J. Perhaps the more interesting of these studies are those describing the anomalous effects of an orthonitro group on the photostability of such dyes [ t 0, 12-14, 17 ] . Although as a general rule, electron-withdrawing groups increase the photostability of 4-N,N dialkylaminoazobenzenes (1) when placed ortho or para to the azo linkage, an ortho-nitro group decreases photostability significantly. To explain these results, Mehta and Peters [ 12 proposed that the negatively charged oxygen atoms of an ortho-nitro group inhibit delocalization of the electrons between the azo nitrogen atoms, and this renders the ~ -N bond on the coupler side more susceptible to photolytic cleavage. On the other hand, not only does the photostability of these dyes improve when the nitro group is moved to the para-position, but workers have also found ( for a review see reference 21) that the thermal cis -trans isomerization of azobenzenes occurs by a rotational mechanism resulting from the delocalization of the electrons at the azo group to form a nitrogen-nitrogen single bond (see dye 2).Little has been reported, however, about the effects of ortho substituents on the trans -cis photoisomerism of aminoazobenzenes and the possible relationship

X-ray diffractometer data have been used to determine the crystal structure of the 2-nitro and 2-cyano derivatives of 4'-[ N,N-bis(β-hydroxyethyl)amino]azobenzene. The structures were solved by direct methods and refined by least squares to R = 0.0352 (CN) and 0.0486 (NO2) for 1294 and 2661 independent reflections, re spectively. The nitro derivative is nonplanar with a dihedral angle of 42.1° between the two phenyl rings. The effect of the nitro group on the geometry of the molecule is discussed. On the other hand, the cyano derivative possesses a planar azobenzene skeleton. The dihedral angle between the two phenyl rings is 5.5° in this case. The O(4)-atom of the nitro substituent lies very close to the N(1)-atom of the azo moiety, as had been suggested previously to account for the low lightfastness of some closely related ortho-nitro azo dyes. The C(1)—N(1),N(1)—N(2), and N(2)—C(7) bond lengths of the azobenzene skeleton are surprisingly similar for the two dyes.