Key indicators: single-crystal X-ray study; T = 173 K; mean (C-C) = 0.002 Å; R factor = 0.042; wR factor = 0.125; data-to-parameter ratio = 15.8.The title Schiff base, C 19 H 20 N 2 O, exists in its zwitterionic form, as the phenolic H atom has been transferred to the iminio group; the resulting iminium and hydroxy groups are linked by an intramolecular N-HÁ Á ÁO hydrogen bond. Two zwitterions lie across a centre of inversion and interact through two N indole -HÁ Á ÁO hydrogen bonds to form a dimer. Related literatureSee Rodriguez et al. (1987) for the crystal structure of neutral 2-[2-(1H-indol-3-yl)ethyliminomethy]phenol (1H-indole-3ethylenesalicylaldimine) and Zarza et al. (1988) for the crystal structure of zwitterionic 2-[2-(1H-indol-3-yl)ethyliminomethyl]-6-methoxyphenol. organic compounds o3458 # 2007 International Union of Crystallography Comment Some Schiff bases that are formed from a primary amine and a substituted salicylaldehyde (or 2-hydroxyacetophenone) exist in a zwitterionic form as the phenolic hydrogen is transferred to the imino group. Among the (indol-3-yl)ethyl-2amine derivatives that have been synthesized, the crystal structures of only two have been reported: 2-[2-(1H-Indol-3yl)ethyliminomethy]phenol (1H-indole-3-ethylenesalicylaldimine exists as a neutral compound (Rodriguez et al.,1987) whereas 2-[2-(1H-indol-3-yl)ethylminomethyl]-6-methoxyphenol is zwitterionic (Zarza et al., 1988). The title compound, (I), (Fig. 1) exists in the latter form; two zwitterions are linked by an N indole -H···O hydrogen bond (Table 1) into a dimer.Experimental 2-(Indol-3-yl)ethylamine (3.2 g, 19.9 mmol) and 2-hydroxy-5-methylacetophenone (3.0 g, 19.9 mmol) were heated in ethanol (50 ml) for 2 h. The solution was filtered; slow evaporation of the solvent yielded faint yellow blocks of (I). RefinementAll hydrogen atoms were located in difference Fourier maps, and were refined with distance restraints C-H = 1.00±0.01 Å and N-H = 0.88±0.01 Å. Their U iso values were freely refined.