Conjugated double helices<i>via</i>self-dimerization of α,α′-dianilinotripyrrins

Abstract

A new motif for artificial double helices was developed on the basis of α,α'-disubstituted tripyrrin. α,α'-Dibromotripyrrin <b>3</b> was prepared by gentle bromination at the pyrrolic α-positions of 5,10-diphenyltripyrrane followed by oxidation with DDQ. Nucleophilic substitution reactions of <b>3</b> with anilines proceeded efficiently to furnish a series of α,α'-dianilinotripyrrins <b>4-11</b>, 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 CDCl₃ are enthalpy-driven. Larger association constants of the dimerization are attained for 3,5-di-<i>t</i>-butylanilino- and 3,5-bis(trifluoromethyl)anilino-substituted tripyrrins (<b>7</b> and <b>8</b>) <i>via</i> additional multiple intermolecular interactions. In a nonpolar and aprotic solvent, tripyrrins (<b>9</b> and <b>10</b>) bearing bulkier 1-naphthylamino and mesitylamino groups do not dimerize but undergo unique tautomerization.