Base-Induced Phototautomerization in 7-Hydroxy-4-(trifluoromethyl)coumarin

Abstract

Excited-state proton-transfer dynamics between 7-hydroxy-4-(trifluoromethyl)coumarin and 1-methylimidazole base in toluene were studied using ultrafast pump-probe and time-resolved emission methods. Charge-transfer excitation of the hydroxycoumarin shifts electron density from the hydroxyl group to the carbonyl, resulting in an excited state where proton transfer to the base is highly favored. In addition to its the photoacid characteristics, the shift in the hydroxycoumarin electronic distribution gives it characteristics of a photobase as well. The result is a tautomerization process occurring on the picosecond time scale in which the 1-methylimidazole base acts as a proton-transfer shuttle from the hydroxyl group to the carbonyl.

Exploring Autoionization and Photoinduced Proton-Coupled Electron Transfer Pathways of Phenol in Aqueous Solution

“…the KIE = 2.5 ± 0.7 value derived from electrochemical studies of PCET involving phenol(S 0 ) molecules by Costentin et al. 4 Several previous studies have demonstrated PCET on an excited state PES (albeit typically occurring on picosecond timescales [54][55][56] ). Photoinduced PCET thus merits discussion in the context of phenol.…”

Comparing the performance of TD‐DFT and SAC‐CI methods in the description of excited states potential energy surfaces: An excited state proton transfer reaction as case study

“…This light induced reaction, represented in Scheme 1, has been extensively studied from the experimental point of view and the photophysical signature of the species involved in this process have been fully characterized. [17] As pointed out by previous calculations, [18] the photoreaction occurs as an intramolecular charge transfer (ICT) excitation, localized on the coumarin moiety and yielding to the CouOH ½ Ã ICT 1MeId adduct, enhances the coumarin acidity. This allows the PT reaction at the ES, leading to the easy formation of the final product, the CouO 2 ½ Ã ICT 21MeIdH 1 adduct.…”

“…In this context, we selected as a test case a simple excited state proton transfer (ESPT) reaction taking place between 7‐hydroxy‐4‐(trifluorometyl)coumarin (CouOH) and 1‐methylimidazole (1MeId). This light induced reaction, represented in Scheme , has been extensively studied from the experimental point of view and the photophysical signature of the species involved in this process have been fully characterized . As pointed out by previous calculations, the photoreaction occurs as an intramolecular charge transfer (ICT) excitation, localized on the coumarin moiety and yielding to the 1MeId adduct, enhances the coumarin acidity.…”

Comparative Molecular Modelling Studies of Coumarin Derivatives as Potential Antioxidant Agents

“…In both cases, there is a preference for antioxidants from natural rather than from synthetic sources. 9 As improved antioxidant status helps to minimize the oxidative damage and thus delay or prevent pathological changes. Potential antioxidant therapy should be, therefore, included either as natural free radical scav enging antioxidant enzymes or as an agent which is capable of augmenting the activity of antioxidant enzymes.…”

“…Potential antioxidant therapy should be, therefore, included either as natural free radical scav enging antioxidant enzymes or as an agent which is capable of augmenting the activity of antioxidant enzymes. 9 Free radicals are molecules or atoms (free particles or ions) having no less than one unpaired electron; sub sequently, they are very active with an extensive variety of other molecules. They are always created and kept up in balance in biological systems through metabolic processes, furthermore they assume important roles in a variety of typical biochemical capacities, for example, cell signaling, apoptosis, gene expression, ion transport, and pathological processes.…”