title: Solvent-dependent excited-state hydrogen transfer and intersystem crossing in 2-(2'-hydroxyphenyl)-benzothiazole.
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.
authors.affiliation: 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.
authors.forename: Shawkat M
authors.initials: SM
authors.lastname: Aly
authors.forename: Anwar
authors.initials: A
authors.lastname: Usman
authors.forename: Maytham
authors.initials: M
authors.lastname: AlZayer
authors.forename: Ghada A
authors.initials: GA
authors.lastname: Hamdi
authors.forename: Erkki
authors.initials: E
authors.lastname: Alarousu
authors.forename: Omar F
authors.initials: OF
authors.lastname: Mohammed