Excited-State Intramolecular Hydrogen Transfer (ESIHT) of 1,8- Dihydroxy-9,10-anthraquinone (DHAQ) Characterized by Ultrafast Electronic and Vibrational Spectroscopy and Computational Modeling Omar F. Mohammed,* ,† Dequan Xiao,* ,‡ Victor S. Batista,* ,§ and Erik T. J. Nibbering* ,∥ † Solar and Photovoltaics Engineering Research Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia ‡ Department of Chemistry and Chemical Engineering, University of New Haven, 300 Boston Post Road, West Haven, Connecticut 06516, United States § Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States ∥ Max Born Institut fü r Nichtlineare Optik und Kurzzeitspektroskopie, Max Born Strasse 2A, D-12489 Berlin, Germany * S Supporting Information ABSTRACT: We combine ultrafast electronic and vibrational spectroscopy and computational modeling to investigate the photoinduced excited-state intramolecular hydrogen-transfer dynamics in 1,8-dihydroxy-9,10-anthraquinone (DHAQ) in tetrachloroethene, acetonitrile, dimethyl sulfoxide, and meth- anol. We analyze the electronic excited states of DHAQ with various possible hydrogen-bonding schemes and provide a general description of the electronic excited-state dynamics based on a systematic analysis of femtosecond UV/vis and UV/IR pump−probe spectroscopic data. Upon photoabsorp- tion at 400 nm, the S 2 electronic excited state is initially populated, followed by a rapid equilibration within 150 fs through population transfer to the S 1 state where DHAQ exhibits ESIHT dynamics. In this equilibration process, the excited-state population is distributed between the 9,10-quinone (S 2 ) and 1,10-quinone (S 1 ) states while undergoing vibrational energy redistribution, vibrational cooling, and solvation dynamics on the 0.1−50 ps time scale. Transient UV/vis pump−probe data in methanol also suggest additional relaxation dynamics on the subnanosecond time scale, which we tentatively ascribe to hydrogen bond dynamics of DHAQ with the protic solvent, affecting the equilibrium population dynamics within the S 2 and S 1 electronic excited states. Ultimately, the two excited singlet states decay with a solvent-dependent time constant ranging from 139 to 210 ps. The concomitant electronic ground-state recovery is, however, only partial because a large fraction of the population relaxes to the first triplet state. From the similarity of the time scales involved, we conjecture that the solvent plays a crucial role in breaking the intramolecular hydrogen bond of DHAQ during the S 2 /S 1 relaxation to either the ground or triplet state. ■ INTRODUCTION Photoinduced excited-state intramolecular hydrogen transfer (ESIHT) is one of the archetypical elementary chemical reactions, lending itself as a versatile vehicle for ultrafast spectroscopic studies. In these ESIHT reactions, a proton is transferred from a donating to an accepting group being part of the same molecular system upon a change in electronic charge distribution in response to an electronic excitation, making ESIHT a special type of proton-coupled electron-transfer reaction. 1 In the pioneering work by Weller on derivatives of salicylic acid, strongly red-shifted fluorescence spectra had been ascribed to originate from a profound rearrangement of the molecular structure resulting from a proton transfer along a pre-existing hydrogen bond after electronic excitation. 2,3 Numerous cases have since then been studied with steady- state UV/vis absorption and emission spectroscopy, ranging from hydroxy fl avones, salicylaldehydes, to 2-(2 ′ - hydroxyphenyl)benzothiazole, including derivatives and related molecules. 4−7 By emission of a fluorescent photon from the ESIHT product state that is energetically much more stable than the reactant state reached after initial photoexcitation, these molecular systems typically show strongly red-shifted fluorescence emission spectra. This is indicative of an ultrafast ESIHT reaction along a hydrogen-transfer coordinate occurring Received: February 14, 2014 Revised: March 31, 2014 Published: March 31, 2014 Article pubs.acs.org/JPCA © 2014 American Chemical Society 3090 dx.doi.org/10.1021/jp501612f | J. Phys. Chem. A 2014, 118, 3090−3099