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A new motif for artificial double helices was developed on the basis of α,α'-disubstituted tripyrrin. α,α'-Dibromotripyrrin 3 was prepared by gentle bromination at the pyrrolic α-positions of 5,10-diphenyltripyrrane followed by oxidation with DDQ. Nucleophilic substitution reactions of 3 with anilines proceeded efficiently to furnish a series of α,α'-dianilinotripyrrins 4-11 , which displayed monomeric and dimeric forms depending upon the solvent used for crystallization and the structures of the substituted anilines. Dimeric forms show double helical structures with smooth π-conjugation as indicated by their absorption spectra. van't-Hoff plot analyses revealed that the dimerizations in CDCl 3 are enthalpy-driven. Larger association constants of the dimerization are attained for 3,5-di- t -butylanilino- and 3,5-bis(trifluoromethyl)anilino-substituted tripyrrins ( 7 and 8 ) via additional multiple intermolecular interactions. In a nonpolar and aprotic solvent, tripyrrins ( 9 and 10 ) bearing bulkier 1-naphthylamino and mesitylamino groups do not dimerize but undergo unique tautomerization.

Conjugated double helices via self-dimerization of α,α′-dianilinotripyrrins