A distinct excited-state proton transfer mechanism for 4-(N-Substituted-amino)-1H-pyrrolo[2,3-b]pyridines in aprotic and protic solvents.

Time-dependent density functional theory (TDDFT) method was used to study the different excited states proton transfer mechanism of DPP in cyclohexane (CHE) and Methanol (MeOH). The results indicate that the concerted mechanism and the stepwise mechanism coexist in the double proton transfer process of DPP dimer in the aprotic solvent CHE, the stepwise mechanism predominates. The stepwise mechanism can only carry out single proton transfer (DPP-SPT), the second proton cannot be transferred because it is hindered by high energy barriers. The concerted mechanism produces a double proton transfer (DPP-DPT). The potential energy surface of the DPP dimer was calculated and the double fluorescence phenomenon of DPP dimer observed by Chou et al. (P.T. Chou, Y.I. Liu, H.W. Liu, W.S. Yu, Dual Excitation behavior of double proton transfer versus Charge Transfer in 4-(N-Substituted Amino)-1H-pyrrolo[2,3-b]pyridines tuned by dielectric and hydrogen-bonding perturbation, J. Am. Chem. Soc., 123 (2001) 12119-12120) was explained. In view of the protonic solvent effect of methanol, the potential energy curve of the DPP/MeOH cluster was constructed. The fluorescence quenching process of DPP/MeOH clusters was elucidated. The proton transfer pathways of DPP/MeOH clusters are revealed in two different concerted ways (Type A: protons transfer from DPP molecules to MeOH solvent molecules; Type B: protons transfer from MeOH solvent to DPP molecules). The ESPT process of DPP molecules in the protic solvent MeOH was found to be more prone to Type B. The results can help to better understand the intermolecular hydrogen bonding mechanism of DPP molecules.