Crystal and molecular structure of 2-methyl-4-(p-bromophenylazo)-imidazole (I), 1,2-dimethyl-4-(p-bromophenylazo)-imidazole (II) and 1,2,3-trimethyl-4(p-methoxyphenylazo)-imidazolium iodide (III). Effect of methyl substitution on the aromatic character of imidazole ring