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Photochemistry of 4‐(2‐Nitrophenyl)‐1,4‐Dihydropyridines. Evidence for Electron Transfer and Formation of an Intermediate †
Author(s) -
Fasani Elisa,
Dondi Daniele,
Ricci Andrea,
Albini Angelo
Publication year - 2006
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/2005-06-01-ra-561
Subject(s) - chemistry , photochemistry , quantum yield , intramolecular force , quenching (fluorescence) , zwitterion , electron transfer , benzophenone , chromophore , moiety , singlet state , yield (engineering) , fluorescence , excited state , stereochemistry , molecule , organic chemistry , physics , materials science , quantum mechanics , nuclear physics , metallurgy
New evidence about the path followed in the photochemical reaction of 4‐(2‐nitrophenyl)‐1,4‐dihydropyridines such as the drugs nifedipine (Compound 1) and nisoldipine (Compound 2) to give the corresponding nitrosophenylpyridines has been found through determination of the steady‐state photochemical parameters and a comparison of the photoreactions in solution and in matrix at 90 K. Additional support is given by comparison with the isomeric 4‐(3‐nitrophenyl)dihydropyridine as well as with simpler derivatives, such as the corresponding 4‐methyldihydropyridine. In Compounds 1 and 2, the lowest lying singlet, localized on the dihydropyridine chromophore, is deactivated by (largely exothermic) electron transfer to the nitrobenzene moiety, as evidenced by the complete quenching of the blue fluorescence observed in analogues not containing the electron‐accepting group. Intramolecular proton transfer ensues in the 2‐nitrophenyl derivatives with a relatively medium‐independent quantum yield of ∼0.3 and leads to an aromatic zwitterion, which is detected in matrix at 90 K (photoionization of this intermediate takes place in 2‐methyltetrahydrofuran secondary). The intermediate is smoothly converted into the end product upon melting the glass. The 3‐nitrophenyl analog, for which such a path is not available, is less reactive by about three orders of magnitude at 366 nm, although the quantum yield arrives at ∼0.01 by irradiation at 254 nm in MeOH, reasonably via the nitrophenyl localized triplet.