Proton transfer reactions of 4′-chloro substituted 3-hydroxyflavone in solvents and aqueous micelle solutions

Abstract

Flavonol 4'-chloro,3-hydroxyflavone (Cl-3HF) has been investigated in solvents of varying polarity and hydrogen-bonding capacity as well as in aqueous micelle solutions. Quantum chemical calculations indicate that although the Cl-atom at the 4'-position of the 2-phenyl ring weakly perturbs the electron distribution of the parent 3-hydroxyflavone, the nuclear framework remains largely intact, and excited state intra-molecular proton-transfer (ESIPT) is feasible. The ESIPT process in both polar solvents and micelles was found to be fast and irreversible, with remarkably long time-constants of several tens of picoseconds. This dramatic inhibition of the ESIPT rate (which is intrinsically a sub-picosecond event) could be rationalized in terms of the emergence of complexes between the solvent and the enol form of Cl-3HF, whose dynamics is coupled to the relatively slow dynamics of inter-molecular hydrogen bonds. In the micelle solutions, spectroscopic data establish that the guest Cl-3HF molecules localized almost exclusively at the polar exterior shell, where they experienced a nearly uniform local environment similar to that in moderately polar solvents. Thus, the Cl-3HF molecules tend to avoid the non-polar core of the micelles, in spite of being strongly hydrophobic themselves. This apparently unusual observation is explained by the formation of inter-molecularly hydrogen-bonded complexes between the guest Cl-3HF and the water molecules tethered to the polar shells of the micelles.

Polarizable QM/Classical Approaches for the Modeling of Solvation Effects on UV–Vis and Fluorescence Spectra: An Integrated Strategy

“…This richness of forms and species leads, as predictable, to an extremely high sensitivity of the photophysical properties of 3HF and its analogues to the physical and chemical properties of the surrounding (micro)­environment. Despite the experimental efforts and the theoretical analyses presented so far, ,− a complete rationalization of such solvent tuning is still not available. Here, by comparing and integrating solvation models of increasing complexity, we identify a cost-effective computational strategy able to assign all the main spectral signals of 3HF to the different tautomeric and anionic species and to explain their spectral position in terms of the specific nature of the solvent such as its polarity and the hydrogen-bonding and proton-transfer capacities.…”

Effect of Water Microsolvation on the Excited-State Proton Transfer of 3-Hydroxyflavone Enclosed in γ-Cyclodextrin

“…Thus, 3HF has been used as a prototype for the ESIntraPT processes and as sensitive fluorescence probes for discovering binding sites in various bio-relevant targets such as DNA, protein, and biomembranes [31][32][33]. The photophysical properties and ESPT processes of 3HF in organic solvents have been extensively studied [34][35][36][37][38][39][40][41][42]. In non-polar solvent, only the K* emission peak of 3HF in toluene was observed at 530 nm with large Stokes shift [33] because the ESIntraPT process effectively occurs, giving only K* form.…”

Experimental and Theoretical Investigation of Excited-State Intramolecular Proton Transfer Processes of Benzothiazole Derivatives in Amino-polydimethylsiloxanes before and after Cross-Linking by CO2

“…Potential monitoring of this reaction is accomplished by following the extent and susceptibility of appropriate guest molecules as sensors for excited-state intramolecular proton transfer (ESIPT); molecules undergoing ESIPT are known to be sensitive to both the polarity and viscosity of their immediate environment. − Considering the donor site to be a hydroxyl group, electronic excitation of these molecules results in a balance within the excited state between their locally excited state (enol species, E*) and the product of the reaction, a keto form (K*). Both species can deactivate radiatively, with an emission spectrum containing a dual profile where the E* species has an emission of higher energy.…”