Substituents effect on the methanol-assisted excited-state intermolecular proton transfer of 7-Aminoquinoline: A theoretical study

Abstract Studies have shown that changing the substituted groups applied to the amine can effectively affect the strength of the excited state hydrogen bond. However, it is not common to study the mechanism of how to effectively control the hydrogen bond strength of the excited state through the substituted group. Here, we have taken 7-Aminoquinoline and its various derivatives, the TDDFT method is used to study the influence of its R substitution on the hydrogen bond strength of the excited state and the mechanism of solvent-assisted excited-state multiple proton transfer (ESMPT) reaction. We have utilized bond length, IR vibrational spectra, atomic polar tensors (APT) charge analysis and potential energy curves to depict the relationship between hydrogen bond strength and substituted groups in excited state. We have modulated deliberately the number of methanol molecules involved in the ESMPT processes based on the site-specificity of hydrogen-bonding interactions, the potential energy curves indicate that the proton transfer process is more likely to happen in 7AQs·(MeOH)3 tetramer than in 7AQs·(MeOH)2 trimer, which is conflict with previous experimental conclusion. Meanwhile, the transition state structure during the ESMPT reaction indicates that the process is concerted. Our study demonstrates well the fluorescence behavior of 7AQs molecules regulated by different R substituents and extends the boundaries of site-specific hydrogen-bonding interactions research slightly.