Studies on photocatalytically active materials containing structure elements of a pyridinium alkaloid from Punica granatum

Abstract

The alkaloid punicin from Punica granatum, N-(29,59-dihydroxyphenyl)pyridinium chloride, forms heterocyclic mesomeric betaines and radicals, which were examined by ESR measurements and DFT calculations, in aqueous solution. By reaction of poly(4-vinylpyridine) with p-benzoquinone under two different reaction conditions, polymeric structures with punicin constituents were prepared and characterized. The ESR signals of these polymers were calculated and discussed. The 4,49-bipyridine derivative of these polymers was prepared starting from Merrifield resin which was subsequently reacted with 4,49-bipyridine and p-benzoquinone, or, alternatively, with N-(20-50-dihydroxyphenyl)-4-(49-pyridine)pyridinium chloride. The properties of these new materials in reversible photocatalytic processes via radical species were examined. Therefore, solutions or suspensions were irradiated in water in the presence of proflavinium as a sensitizer, and EDTA as a sacrificial donor under an inert atomsphere. Oxygen (air) recovers the starting materials by formation of hydroxide ions which were determined by pH measurements.

Pyridinium salts and ylides as partial structures of photoresponsive Merrifield resins

“…4,4 0 -Bipyridinium derivatives can also be used in coupled photocatalytic electron transfers (Scheme 9). 25 In this redox cycle, radicals such as 13c are generated by irradiation in the presence of proflavinium 12 as a sensitizer in aqueous solution. The resulting radical species 13c are visually observable by their greenish blue color under an inert atmosphere.…”

“…Mechanistically, either pericyclic reactions (spiropyranes, 17 spirooxazines, 18 chromenes, 19 fulgides, 20 diarylethenes 21 ), cis/ trans-isomerisations (azo compounds, 22 anils 23 ), or electron transfer reactions (viologens) cause photochromism. Photocatalytic reversible electron transfer reactions of viologens, 24 including punicin derivatives, 25 have also been developed.…”

“…An alternative procedure described in the literature is the electron transfer from a sensitizer (S) to the cation (II). 24,25 Electron transfers from the anionic part of mesomeric betaines (''inner salts'') into the cationic part (III) are scarcely described in the literature, although they play an important role in negatively solvatochromic dyes, such as Reichardt's betaine, to determine solvent polarities. 34 This aroused our interest in studying in detail the photoresponsive properties of materials which possess mesomeric betaine partial structures.…”

Synthesis of Tetraoxygenated Terephthalates via a Dichloroquinone Route: Characterization of Cross‐Conjugated Liebermann Betaine Intermediates

“…The case of Diels ' betaine ( 16 )[25], which has also been identified as the alkaloid punicine from Punica granatum ,[26] is interesting as it could principally occur in two tautomeric forms 16a / 16b , of which 16a is a conjugated mesomeric betaine, isoconjugate to an odd alternant hydrocarbon anion (class 1), and 16b is a cross‐conjugated betaine, isoconjugate to an odd, alternant hydrocarbon monoanion (class 9). The more recently reported betaine 17 [9c] combines a 1,3‐diketoenolate anion with an imidazolium cation.…”

Redox active donor-substituted punicin derivatives

“…Moreover, punicin forms radical anions as well as semiquinones when stabilized by polymer backbones (ESR). 15 Scheme 4 the synthesis of other donor-substituted punicin derivatives and chose the (E)-3-oxoprop-1-en-1-olate group I (Fig. 1) as an anionic partial structure of the target betaines.…”