A molecule of 7-azaindole and another of 3-methyl-7-azaindole form a doubly hydrogen bonded heterodimer of C, symmetry with a first electronic excited state a' where the electronic excitation is highly localized on one of the molecular halves. Also, because the electronic excitation only increases the acidity of the pyrrole nucleus and the basicity of the pyridine nucleus in one of the two hydrogen bonds, the heterodimer undergoes single rather than double proton transfer. (c) 2005 Elsevier B.V. All rights reserved.