Elaborating the excited state multiple proton transfer mechanism for 9H-pyrido[3,4-b]indole

In this work, we theoretically study the excited state multiple proton transfer (ESMPT) mechanism of 9H-pyrido[3,4-b]indole (NOR) coupling with acetic acid (AcOH) in DCM solvent. We analyze structural changes, infrared (IR) vibrational spectra and charge redistribution, which reveals the tendency of ESMPT process. Exploring potential energy curves, we confirm that the protonation of pyridine nitrogen firstly occurs, which opens ESMPT reaction. In the whole ESMPT process, the powerful acceptance ability of AcO− plays a vital role. Our investigation not only indicates NOR system undergoes a proton relay process in excited state and clarifies the ambiguous mechanism in previous experiment, but also paves the way for further investigating intermolecular multiple proton transfer interactions in biological and chemical science.