Electronic Excited States of the Cyclic 1,4-Diaza-1,3-diene (Cyclic α-Diimine) System. Photochemical Reactions of 2H-Isoimidazole, 5,6-Dihydropyrazine, and 1,4-Diazepine Derivatives

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Abstract
The five-, six-, and seven-membered cyclic 1,4-diaza-1,3-dienes have been irradiated under a variety of conditions. The six-membered ring (dihydropyrazine) opens in a 6πe valence isomerization reaction. The resulting enediimine is stable at low temperatures and has been characterized (i.r.). Irradiation of 2,3,5,5-tetramethyl-5,6-dihydropyrazine (1b) results in formation of the enediimine 2b which reacts further to give the rearranged monomer 12 and/or dimers 13 depending upon the temperature. The five-and seven-membered systems are stable to irradiation in inert solvents. However, irradiation of 2,2-dimethyl-4,5-diphenylisoimidazole (10h) in isopropyl alcohol gives the dihydroderivative 19 and in tetramethylethylene the one-to-one adduct 25.The u.v. spectrum of the six-membered ring (5,6-dihydropyrazine) is very different from that of the five- and seven-membered rings. The long wavelength maximum (ca. 350 nm) of the six-membered ring is attributed to a transition directly into an excited state common to the enediimine 2 (i.e., the excited state has a node between C-5 and -6 of the dihydropyrazine).

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