Direct Irradiation of Phenol and Para-Substituted Phenols with a Laser Pulse (266 nm) in Homogeneous and Micro-heterogeneous Media. A Time-Resolved Spectroscopy Study

Abstract

Direct irradiation of para-substituted phenols under N 2 atmosphere in homogeneous (cyclohexane, acetonitrile, and methanol) and micellar (SDS) solution was investigated by means of time-resolved spectroscopy. After a laser pulse (266 nm), two transient species were formed, viz. the para-substituted phenol radical-cations and the corresponding phenoxy radicals. The radicalcations showed a broad absorption band located between 390 and 460 nm, while the phenoxy radicals showed two characteristic bands centered at 320 nm and 400−410 nm. The deprotonation rate constant of radical-cations (k H ) of 10 5 s −1 and the reaction rate constant of the phenoxy radicals (k R ) in the order of 10 9 − 10 10 M −1 •s −1 have been derived. The k H rate constants gave good linear Hammett correlation with positive slope indicating that electron-withdrawing substituents enhance the radical-cation acidity. The binding constants (K b ) of the parasubstituted phenols with the surfactant were also measured, and NOESY experiments showed that phenols were located in the hydrophobic core of the micelle. Finally, computational calculations provided the predicted absorption spectra of the transients and nice linear correlations were obtained between the theoretical and experimental energy of the lower absorption band of these species.

Photogeneration of quinone methide from adamantylphenol in an ultrafast non-adiabatic dehydration reaction

“…On the other hand, the PA band at 425 nm may tentatively also be assigned to a radical cation obtained by one or two-photon ionization of the phenolic molecule 3. However, in aqueous solvent phenolic radical cations deprotonate to phenoxyl radicals, 39 which is not supported by our measurements, since the same signal at 425 nm persists over ns time scale and can be detected by LFP over hundreds of milliseconds (vide infra). 30 Based on previously published spectra for QM 4, the PA band at 425 nm most probably results from a transition from the ground state (S 0 ) of the intermediate 4.…”

“…4 ). Since it is not quenched by O 2 , and according to the comparison with the published spectrum, 39 it is assigned to phenoxyl radical (previously not detected). A radical cation could be formed in an ionization process, which would also explain the presence of solvated electrons detected as a red positive absorption band in fs-TA.…”

Induced selectivity in the photochemistry of estrone derivatives in sustainable and micellar environment: preparative and mechanistic studies

“…Therefore, the confined environment provided by the hydrophobic core of the micelle can be considered as a micro reactor where (photo)reactions can take place, the reactivity of radical species generated can be controlled and, at the same time, can be helpful to direct the selectivity of the (photo)products. Some interesting examples of photoreaction carried out in micellar media as optimal micro reactor that can control the selectivity and yield of the photoproducts can be found in the literature [42][43][44][45] and our research group has also contributed with several examples [46][47][48][49].…”

“…Taking into account that a differential chemical shifts (Δδ) was observed in the presence of esters 1-3, the next step was to confirm qualitatively the location of the esters within the hydrophobic core of the micelle employing 2D NMR spectroscopy. The extent of co-aggregation in water between two different kinds of surfactants or the localization of guest molecules within the micelle have been often determined using NOESY experiments [46][47][48][49][73][74][75][76][77]. In addition, these experiments give satisfactory results when cross-peaks between diagnostic signals of the substrates and the surfactants, respectively, are noticed in the corresponding contour plots [46][47][48][49].…”

“…The extent of co-aggregation in water between two different kinds of surfactants or the localization of guest molecules within the micelle have been often determined using NOESY experiments [46][47][48][49][73][74][75][76][77]. In addition, these experiments give satisfactory results when cross-peaks between diagnostic signals of the substrates and the surfactants, respectively, are noticed in the corresponding contour plots [46][47][48][49].…”