Mechanism of Fluorescence Quenching by Acylamino Twist in the Excited State for 1-(Acylamino)anthraquinones.

Nitrogen-containing anthraquinone derivatives are widely applied in vegetable fiber dyes. In this paper, the fluorescence quenching mechanism by an acylamino group twist in the excited state for the 1-(acylamino)anthraquinones (AYAAQs) derivatives in acetonitrile is investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The calculated Stokes shift is in good agreement with the experimental data. The energy profiles show that each AYAAQs derivative reveals a barrierless twist process, indicating that the involvement of acylamino group rotation in addition to proton transfer becomes as another important coordinate in the excited state relaxation pathway. The effects of electron-substituted group promote twist process compared with 1-aminoanthraquinone (AAQ). Then, the cross points are searched by the constructed linearly interpolated internal coordinate (LIIC) pathways for AYAAQs, demonstrating that the potential energy curves of the S1 and T2 states intersect each other and are in accord with the El-Sayed rules. So one can conclude that the acylamino group twist and following intersystem crossing (ISC) processes are important nonradiative inactivation channel for the S1 state of the AYAAQs derivatives, which is more prone to proton transfer process and can explain the low fluorescence efficiency. In addition, we have measured the phosphorescence spectra of AAQ, and on this basis, it can be predicted that the phosphorescence may occur for the AYAAQs derivatives.