A theoretical study on the double proton transfer dynamics of the 11-propyl-6H-indolo-[2,3-b]quinoline (6HIQ)/7-azaindole (7AI) hydrogen-bonded hetero-dimer in both the ground and electronically lowest lying excited state was presented. In the ground state, the double proton transfer was concluded to undergo a concerted-asynchronous pathway. In the electronically excited state, both CIS and TD-M06-2X theory predicted that the reaction favored a stepwise process; however it could not be unambiguously determined which proton triggered the reaction. Higher-level theory (EOM-CCSD) suggested a highly asynchronous pathway without intermediates and the proton from the 6HIQ triggered the double proton transfer reaction. (C) 2009 Elsevier B. V. All rights reserved.