Theoretical Modelling for the Ground State Rotamerisation and Excited State Intramolecular Proton Transfer of 2-(2’-hydroxyphenyl)oxazole, 2-(2’-hydroxyphenyl)imidazole, 2-(2’-hydroxyphenyl)thiazole and Their Benzo Analogues

Abstract

Two series of compounds, one comprising of 2-(2′-hydroxyphenyl)benzoxazole (HBO), 2-(2′-hydroxyphenyl)benzimidazole (HBI), 2-(2′-hydroxyphenyl)benzothiazole (HBT), and the other of 2-(2′-hydroxyphenyl)oxazole (HPO), 2-(2′-hydroxyphenyl)imidazole (HPI) and 2-(2′-hydroxyphenyl)thiazole (HPT) are susceptible to ground state rotamerization as well as excited state intramolecular proton transfer (ESIPT) reactions. Some of these compounds show experimental evidence of the existence of two ground state conformers. Out of these two one undergoes ESIPT reaction leading to the formation of the tautomer. The two photophysical processes, in combination, result in the production of a number of fluorescence bands each one of which corresponding to a particular species. Semiempirical AM1-SCI calculations have been performed to rationalize the photophysical behaviour of the compounds. The calculations suggest that for the first series of compounds, two rotational isomers are present in the ground state of HBO and HBI while HBT has a single conformer under similar circumstances. For the molecules of the other series existence of rotamers depends very much on the polarity of the environment. The potential energy curves (PEC) for the ESIPT process in different electronic states of the molecules have been generated theoretically. The simulated PECs reveal that for all these systems the IPT reaction is unfavourable in the ground state but feasible, both kinetically and thermodynamically, in the S 1 as well as T 1 states.

Detecting local heterogeneity and ionization ability in the head group region of different lipidic phases using modified fluorescent probes

“…In water, each hydrogen bonding site in HBO ( anti and syn tautomers) is solvated by two water molecules that are in direct contact with the OH group and the N/O heteroatom37. This particular solvation mechanism is supported by the stable planarity of the HBO backbone4142.…”

Excited state intramolecular proton transfer in amino 2-(2′-hydroxyphenyl)benzazole derivatives: Effects of the solvent and the amino group position

“…Although the solvent effect on the ESIPT mechanism has been investigated experimentally [14][15][16] and theoretically [17][18][19][20][21] in several structures, a discussion concerning some amino 2-(2 0 -hydroxyphenyl)benzazole derivatives (Scheme 1) is nowadays particularly interesting since the amino group being strong electron donor in the excited state, the fluorescence spectrum of amino derivatives are more sensitive to the solvent polarity than the absorption spectrum [22]. These dyes have been used as probe to discuss hydrophobic-hydrophilic environments [23], for instance, in systems that use polymers for drug delivery [22,24].…”

Photophysical properties of hydroxyphenyl benzazoles and their applications as fluorescent probes to study local environment in DNA, protein and lipid

“…Fluorescence of HBT in the different environments is similar to that of HBO. Both molecules are reported to be planar in their crystal structures , and by semi‐empirical and ab initio calculations . The weaker hydrogen bond in HBT resulted in a very small concentration of the anion species in aqueous neutral solution as evidenced in Fig.…”

“…In an aqueous medium, the tendency of water molecules to strongly associate with each other through intermolecular hydrogen bonds allows more than one molecule of water to form a solvent network or a solvent wire along which proton transfers can take place to and from the solute (45). This particular solvation mechanism is supported by the stable planarity of the HBO backbone (46,47). In a previous study, we found that two water molecules solvate the hydrogen-bonding site of the syn-enol tautomer in which HBO undergoes water-assisted tautomerization in the excited state to yield the syn-keto species (32).…”