Photoinduced Proton Transfer Promoted by Peripheral Subunits for Some Hantzsch Esters

Abstract

It is noted that, for a small series of 3,5-diacetyl-1,4-dihydrolutidine (DDL) derivatives and the corresponding Hantzsch esters, the presence of methyl groups at the 2,6-positions serves to extinguish fluorescence in solution but not in the solid state. Emission is weakly activated and affected by changes in solvent polarity. The latter situation arises because the optical transition involves intramolecular charge transfer. Calculations, both semiempirical and DFT, indicate that, in all cases, rotation of the carbonyl function is facile and that the dihydropyridine ring is planar. These calculations also indicate that the 2,6-methyl groups do not affect the generic structure of the molecule. It is proposed that illumination increases the molecular dipole moment and pushes electron density toward the carbonyl oxygen atom. Proton transfer can now occur from one of the methyl groups, leading to formation of a relatively low-energy, neutral intermediate, followed by a second proton transfer step that forms the enol. Reaction profiles computed for the ground-state species indicate that this route is highly favored relative to hydrogen transfer from the 4-position. The barriers for light-induced proton transfer are greatly reduced relative to the ground-state process but such large-scale structural transformations are hindered in the solid state. A rigid analogue that cannot form an enol is highly emissive in solution, supporting the conclusion that proton transfer is in competition to fluorescence in solution.

Reactions of gem-Difluorinated Phosphonium Salts Induced by Light

“…According to literature data, , Hantzsch ester (HE) can serve as a reducing agent when irradiated with visible light, while its radical cation, as well as HE itself, can behave as a source of hydrogen. Correspondingly, the mechanistic scenario for the reaction of phosphonium salts 5 with electron-deficient alkenes is shown in Scheme .…”

High Throughput Determination of Stern Volmer Quenching Constants for Common Photocatalysts and Quenchers

“…36 Some cases cases can be treated using a modified Stern-Volmer equation, namely the Lehrer equation. 37 It should be noted that the Hantzsch ester, unlike the other quenchers, also has a non-negligeable absorbance up to 400 nm and emits at 450 nm, 38 which overlaps with the absorbance and emission of a few of the selected photocatalysts, thereby creating an inner filter effect which interferes with the Stern Volmer measurement. Therefore, data for this particular quencher might not reflect the true photocatalyst quenching rate.…”

High-Throughput Determination of Stern–Volmer Quenching Constants for Common Photocatalysts and Quenchers

“… 36 Some cases can be treated using a modified Stern–Volmer equation, namely, the Lehrer equation. 37 It should be noted that the Hantzsch ester, unlike the other quenchers, also has a non-negligeable absorbance up to 400 nm and emits at 450 nm, 38 which overlaps with the absorbance and emission of a few of the selected photocatalysts, thereby creating an inner filter effect, which interferes with the Stern–Volmer measurement. Therefore, data for this particular quencher might not reflect the true photocatalyst quenching rate.…”