1 research outputs found
Solvent-Dependent Excited-State Hydrogen Transfer and Intersystem Crossing in 2‑(2′-Hydroxyphenyl)-Benzothiazole
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