Solvent-dependent excited-state hydrogen transfer and intersystem crossing in 2-(2’-hydroxyphenyl)-benzothiazole

Solvent-dependent excited-state hydrogen transfer and intersystem crossing in 2-(2’-hydroxyphenyl)-benzothiazole

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
J. Phys. Chem. B, 119 (6), pp 2596–2603, (2015)
Shawkat M. Aly, Anwar Usman, Maytham AlZayer, Ghada A. Hamdi, Erkki Alarousu, and Omar F. Mohammed
Hydrogen Transfer, Intersystem Crossing, 2-(2′-hydroxyphenyl)-benzothiazole
2015
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 non-polar 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 non-radiative 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 keto triplet state. In addition, our femtosecond time-resolved fluorescence experiments indicate, for the first time, that the lifetime of the enol form (which does not undergo hydrogen transfer) in ACN is approximately 280 fs. This observation indicates that the solvent plays a crucial role in breaking the H bond and de-activating the excited state of the HBT. Interestingly, the broadband fluorescence up-conversion data clearly demonstrate that the intermolecular proton transfer from the excited HBT to the DMSO solvent is 190 fs, forming the HBT anion excited state.



DOI: 10.1021/jp508777h