Excited state charge transfer coupled double proton transfer reaction of 7-azaindole derivatives in methanol: a theoretical study.

Excited state charge transfer coupled excited state double proton transfer (ESCT/ESDPT) reaction in methanol (MeOH) for 3-cyano-7-azaindole(3-CNAI), 5-cyano-7-azaindole(5-CNAI), and 3,5-dicyano-7-azaindole(3,5-CNAI) were investigated using time-dependent density functional theory (TDDFT) method for the first time. The intermolecular hydrogen bonds of 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes are demonstrated to be strengthened in the excited state and weakened in tautomer excited state, which indicates that reverse proton transfer reaction is not easy to take place. Due to the formation of intermolecular hydrogen bond, the absorption and excited state fluorescence spectra of the above three complexes are red-shifted in comparison with those of isolated molecules. The tautomer excited state fluorescence spectra that are induced by ESDPT reaction are also red-shifted relative to the excited state fluorescence for the above complexes. In addition, the sites where cyano group absorbed on 7-azaindole induces a large discrepancy of electron density distribution in excited state. Frontier molecular orbitals reflect that HOMO and LUMO orbitals of proton transfer PT-3-CNAI-MeOH, PT-5-CNAI-MeOH, and PT-3,5-CNAI-MeOH complexes are different with HOMO and LUMO orbitals of 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes, respectively.