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

Abstract

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 S and T 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 S 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.

Intramolecular Charge Transfer of 1-Aminoanthraquinone and Ultrafast Solvation Dynamics of Dimethylsulfoxide

“…Since the excited-state dynamics of AAQ in DMSO are not dependent on the excitation energy, the excited-state intramolecular proton transfer (ESIPT) across a high energy barrier in the S 1 excited state is not considered plausible [ 24 ]. In the previous transient absorption measurements of AAQ in ethanol, the intersystem crossing with a 550 ps time constant to the triplet state was observed, but the phosphorescence band centered at 770 nm was not observed [ 21 , 23 , 24 ]. As shown in Figure 2 b, the recovery of the GSB band is almost completed in a 1.2 ns delay time and the transient absorption signals of the triplet states were not observed.…”

“…Thus, the intersystem crossing to the triplet state is considered much less significant in a DMSO solution compared to the case of an ethanol solution. However, the assignment of the ICT dynamics of 150–180 fs requires further experimental evidence of the structural changes of AAQ accompanying the ICT, although a twist of the amino group has been suggested by the theoretical investigations based on TDDFT simulations [ 21 , 23 , 24 ].…”

“…The steady-state absorption and emission measurements of AAQ showed that the fluorescence of AAQ is strongly dependent on the solvent polarity and large Stokes shifts are observed in polar solvents [ 18 , 19 , 20 ]. Theoretical investigations of AAQ through TDDFT simulations suggested that the ICT process occurs with a relatively low energy barrier (Δ E a ≈ 350 cm −1 ) in acetonitrile with a twist of the amino group [ 21 ]. Additionally, the intramolecular proton transfer reaction of AAQ is estimated as implausible in the S 1 excited state due to the high energy barrier (Δ E a ≈ 3300 cm −1 ) of the proton transfer [ 22 ].…”

Lack of the ESIPT band of aromatic ortho-aminoaldehyde derivatives triggered by N–H vibration

“…10–12 The 1-(acylamino)anthraquinone (AAAQ) molecule and its derivatives as near-ideal models of NO-type systems for studying ESIPT processes have been extensively studied in the past three decades. 13–22 In 1991, Smith et al studied the excited state properties of AAAQ derivatives by static fluorescence and time-resolved emission experiments; they found two fluorescence bands, the short wavelength emission band and the long wavelength emission band. 13 They assigned the short wavelength band to the normal emission corresponding to a normal structure with the hydrogen atom bonded to the nitrogen atom, and the other band with a large Stokes shift was attributed to the ESIPT emission, in which the proton was transferred to the oxygen atom to form a tautomer/ESIPT structure.…”

“…The double-well potential in AAAQ derivatives was obtained with time-dependent density functional theory (TDDFT) calculations by Li 15 and Zhou. 16 They computed three kinds of AAAQ derivatives, 1-(chloroacetylamino)anthraquinone (CAAQ), 1-(dichloroacetylamino)anthraquinone (DCAQ) and 1-(trifluoroacetylamino)anthraquinone (TFAQ), and obtained two stable structures (normal and tautomer) in the S 1 state with a small barrier height (less than 5.0 kcal mol −1 ) between them. Furthermore, Cembran et al calculated the free energy surface of 1-(trifluoroacetylamino)naphthaquinone (TFNQ) (a simplified model of TFAQ) along the proton transfer coordinates using the high-level ab initio methods of complete active space self-consistent field (CASSCF) and complete active space perturbation theory of second order (CASPT2).…”