Solvent-Dependent Excited-State Hydrogen Transfer and Intersystem Crossing in 2‑(2′-Hydroxyphenyl)-Benzothiazole Shawkat M. Aly, † Anwar Usman, Maytham AlZayer, Ghada A. Hamdi, Erkki Alarousu, and Omar F. Mohammed* Solar and Photovoltaics Engineering Research Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia * S Supporting Information ABSTRACT: The excited-state intramolecular hydrogen transfer (ESIHT) of 2-(2′-hydroxyphenyl) benzothiazole (HBT) has been investigated in a series of nonpolar, polar aprotic, and polar protic solvents. A variety of state-of-the-art experimental methods were employed, including femto- and nanosecond transient absorption and fluorescence upconversion spectroscopy with broadband capabilities. We show that the dynamics and mechanism of ESIHT of the singlet excited HBT are strongly solvent-dependent. In nonpolar solvents, the data demonstrate that HBT molecules adopt a closed form stabilized by O-H···N chelated hydrogen bonds with no twisting angle, and the photoinduced H transfer occurs within 120 fs, leading to the formation of a keto tautomer. In polar solvents, owing to dipole-dipole cross talk and hydrogen bonding interactions, the H transfer process is followed by ultrafast nonradiative deactivation channels, including ultrafast internal conversion (IC) and intersystem crossing (ISC). This is likely to be driven by the twisting motion around the C-C bond between the hydroxyphenyl and thiazole moieties, facilitating the IC back to the enol ground state or to the keto triplet state. In addition, our femtosecond time-resolved fluorescence experiments indicate, for the first time, that the lifetime of the enol form in ACN is approximately 280 fs. This observation indicates that the solvent plays a crucial role in breaking the H bond and deactivating the excited state of the HBT. Interestingly, the broadband transient absorption and fluorescence up-conversion data clearly demonstrate that the intermolecular proton transfer from the excited HBT to the DMSO solvent is about 190 fs, forming the HBT anion excited state. 1. INTRODUCTION Over the past three decades, excited-state intramolecular hydrogen transfer (ESIHT) has been intensively studied to gain insight into the mechanism of the transfer process and the accompanying intramolecular rearrangements. 1-10 Despite the progress in the field of ESIHT, until now, most studies have been performed in nonpolar solvents, guaranteeing that the reaction coordinate is intact during the electronic excitation, and all excited molecules are released on the same region of the excited-state energy surface. However, extensions of the studies of ESIHT into polar solvents have been limited, partly because these experiments result in a distribution of conformations with hydrogen bonding to the solvent competing with the intramolecular hydrogen bond. More specifically, electronic excitation of such an ensemble with a broad distribution of conformations may lead to multiple starting conditions in the excited-state energy surface. Additionally, polar solvation may strongly affect the ESIHT process, resulting in different outcomes for the reaction dynamics in terms of reaction yields and reaction pathways. Monitoring the excited-state relaxations of such systems in this regime using ultrafast time-resolved vibrational, absorption, and fluorescence spectroscopies is vital to deciphering the ESIHT process in a variety of molecules in many chemical and biological systems. 8,11-15 Among the potential ESIHT molecules, HBT (2-(2′- hydroxyphenyl)-benzothiazole), 16-18 HBO (2-(2′-hydroxy- phenyl)benzoxazole), 19 10-HBQ (10-hydroxybenzo[h]- quinoline), 20 and HBA (2-hydroxy-benzaldehyde) 21 are four prototypes for single H transfer where the enol-keto tautomerization process is the photoinduced reaction pathway. Typically, right after optical excitation, redistribution of electronic charge induces skeletal deformations and changes in the acidity and basicity of the H donor and acceptor moieties, respectively, leading to a motion of the H from the donor to the acceptor on a time scale of less than 60 fs. 22-26 In particular, the ground-state HBT in nonpolar solvent is stabilized by an O-H···N intramolecular hydrogen bond between the hydroxyphenyl and benzothiazole moieties, Special Issue: Photoinduced Proton Transfer in Chemistry and Biology Symposium Received: August 30, 2014 Revised: October 15, 2014 Article pubs.acs.org/JPCB © XXXX American Chemical Society A dx.doi.org/10.1021/jp508777h | J. Phys. Chem. B XXXX, XXX, XXX-XXX