ARTICLE IN PRESS JID: MOLSTR [m5G;October 30, 2020;4:8] Journal of Molecular Structure xxx (xxxx) xxx Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstr Ultrafast investigation of intramolecular proton transfer dynamics and vibration relaxation in 1,8-dihydroxyanthraquinone Chaochao Qin a , Hua Liu a , Simei Sun b,∗ , Zhongpo Zhou a,∗ , Yufang Liu a a Henan Key Laboratory of Infrared Materials & Spectrum Measures and Applications, School of Physics, Henan Normal University, Xinxiang 453007, China b College of Physics and Electronics Science, Hubei Normal University, Huangshi 435002, China a r t i c l e i n f o Article history: Received 27 April 2020 Revised 20 October 2020 Accepted 21 October 2020 Available online xxx Keywords: Excited-state intramolecular proton transfer 1,8-dihydroxyanthraquinone Transient absorption spectroscopy Quantum chemical calculations a b s t r a c t Photoinduced dynamics of excited-state intramolecular proton transfer (ESIPT) and vibration relaxation (VR) of 1,8-DHAQ in ethanol are investigated by time-resolved transient absorption spectroscopy com- bined with the quantum chemical calculations. Experimental and calculated results show dual fluores- cence is obtained from the S 1 state of the normal form of 9,10-quinone and the tautomer form of 1,10- quinone. Three characteristic bands of excited state absorption and stimulated emission are observed in transient absorption spectroscopy upon excitation with ultrafast laser pulses at 390 nm. ESIPT process happens within 96.8 fs from the normal S 1 state to the tautomer S 1 state, and the VR process is observed in the time constant of 4.1 ps. Ultimately, the tautomer S 1 state decays within 269.1 ps for a deactivation pathway of emitting fluorescence. The mechanism of ESIPT dynamics and ultrafast excited-state dynam- ics in 1,8-DHAQ in ethanol is concluded by a schematic diagram of four electronic states and vibrational energy levels. © 2020 Elsevier B.V. All rights reserved. 1. Introduction 1,8-Dihydroxyanthraquinone (1,8-DHAQ) is a kind of an- thraquinone derivatives which is often used as chromophores and chelating agents [1–3]. 1,8-DHAQ is a unique quinone due to the excited state intramolecular proton transfer (ESIPT) existing once being photo-excited into the tautomeric proton-transferred state (T ∗ ) from the normal structure (N ∗ ) [4]. ESIPT, as an issue of fun- damental importance and one of the simplest examples of in- tramolecular reactions in electronically excited molecules, has at- tracted a great deal of interest [5]. Due to the drastic structural alternation, the chromophores with ESIPT including the 1,8-DHAQ offer great versatility in a variety of applications such as fluores- cent chemo-sensors [6], fluorescence imaging [7], proton transfer lasers [8], and organic light-emitting diodes [9]. Like most ESIPT molecules, there is a preexisting intramolec- ular hydrogen bond in the ground state between the proton- donating and proton-accepting units in 1,8-DHAQ. The proton do- nating and the proton-accepting constituents undergo a reversal during a photo-excitation, causing the initially photo-excited N ∗ form at higher energy than the conjugate excited T ∗ state, and the ESIPT takes place along the hydrogen bond, converting N ∗ to T ∗ in ∗ Corresponding authors. E-mail addresses: simeisun@hbnu.edu.cn (S. Sun), zpzhou@htu.edu.cn (Z. Zhou). general. Steady-state fluorescence emission spectroscopy provides important indications of the ESIPT process [10–12]. The effect of proton transfer on the Franck-Condon factors causes the break-up of the mirror symmetry between absorption and emission spec- tra, and the light emission from the proton-transferred state occurs at longer wavelengths with a redshift concerning absorption range when the nuclear configuration of the molecule and its surround- ing medium remain close to the ground state [13]. An ultrafast time-resolved fluorescence study suggested that the ultrafast ESIPT process of 1,8-DHAQ in hexane occurs in a time scale of tens of femtoseconds (fs) to several picoseconds (ps) [10]. The ESIPT dynamics of 1,8-DHAQ were also studied by ultrafast electronic and vibrational spectroscopy [14], as well as fluorescence up-conversion measurements [1], both of which predicted the ES- IPT would be faster than 200 fs for the limit of instrument re- sponse function (IRF). From these studies, a series of non-radiation dynamics processes were also predicted at the time scale of 0.1– 50 ps. However, the process and time constants of VR and ESIPT were not given in detail, which still poses challenges for the com- plexity of its physical and chemical nature. It is not easy to detect the VR process with single wavelength method because the signal is very weak. But it can be gained by the femtosecond transient absorption (fs-TA) followed by analysis of singular value decompo- sition (SVD) and globally fitting. In this paper, the excited state dynamics, the time constant of ESIPT, and VR of 1,8-DHAQ in ethanol have been studied by https://doi.org/10.1016/j.molstruc.2020.129502 0022-2860/© 2020 Elsevier B.V. All rights reserved. Please cite this article as: C. Qin, H. Liu, S. Sun et al., Ultrafast investigation of intramolecular proton transfer dynamics and vibration relaxation in 1,8-dihydroxyanthraquinone, Journal of Molecular Structure, https://doi.org/10.1016/j.molstruc.2020.129502