The doubly hydrogen-bonded homodimer of 11-propyl-6H-indolo[2,3-b]quinoline (11P6HIQ) has been reported elsewhere as a paradigm of the excited-state double-proton-transfer process through a concerted mechanism. Herein, a simpler molecular structure, 6H-indolo[2,3-b]quinoline (6HIQ), has been proposed as a counter example of the previous one. While the 11-propyl derivative seemingly forms doubly hydrogen-bonded homodimers, the 6HIQ molecule generate dimers similar to the card-pack dimers reported for the 1-azacarbazole molecule. The 11P6HIQ molecule has an emission centered at 560 nm (reported elsewhere), but in this work 6HIQ only exhibits emission at ca. 400 nm under the same experimental conditions. The stationary-state spectroscopy and quantum chemistry calculations demonstrate that the 6HIQ molecule exhibits aggregation on lowering the temperature from 298 to 123 K; however, this aggregate species is one other than the doubly hydrogen-bonded homodimer, and therefore, no double proton transfer can be undergone.