Intermediates of the Photoinduced 2,4-Bis(4-diethylaminobenzylidene)cyclobutanone Redox Reaction in Methanol
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OCHEMISTRY
Intermediates of the Photoinduced 2,4-Bis(4Diethylaminobenzylidene)cyclobutanone Redox Reaction in Methanol V. N. Gutrova, G. V. Zakharovaa, *, M. V. Fominaa, **, S. P. Gromova, b, and A. K. Chibisova a
Photochemistry Center, “Crystallography and Photonics” Federal Scientific Research Center, Russian Academy of Sciences, Moscow, 119421 Russia b Faculty of Chemistry, Moscow, 119991 Russia *e-mail: [email protected] **e-mail: [email protected] Received July 2, 2020; revised July 2, 2020; accepted July 6, 2020
Abstract—Formation of intermediates (triplet states, radical anions, and radical cations) has been revealed by pulse laser photolysis in the photoinduced redox reaction of 2,4-bis(4-diethylaminpbenzyliden)cyclobutanone in methanol in the presence of an electron donor (ascorbic acid) and electron acceptors (methylviologen and p-nitroacetophenone). The reactions of photoinduced reduction by ascorbic acid and oxidation by methylviologen and p-nitroacetophenone occur via the triplet state with the formation of the dye radical anion and radical cation, respectively. The formation of the radical products of the photoinduced redox reaction is characterized by short-lived changes in the absorption spectra at λmax = 460 nm and a lifetime of 3 × 10−4 s for the radical anion and at λmax = 395 and 620 nm and a lifetime of 5 × 10−5 s for the radical cation. Keywords: 2,4-bis(4-diethylaminobenzylidene)cyclobutanone, pulse laser photolysis, radical anion, radical cation DOI: 10.1134/S0018143920060053
Symmetric cross-conjugated dienones (ketocyanine dyes) bearing electron-donating groups in the para-position display solvatochromic properties [1] and can be applied, for example, for determining the polarity of a medium [2, 3]. Dienones undergo intersystem crossing to a triplet state with a quantum yield of 0.25–0.36 and a lifetime of 4 × 10−7 s [4, 5] and participate in the [2 + 2]photocycloaddition reaction [6]. The intersystem crossing capability of dienones allows for the use of the triplet molecules as sensitizer of singlet oxygen in photodynamic therapy [7]. Recently, we have investigated spectral, luminescent, and spectral–kinetic properties of symmetric 2,4-dibenzylidenecyclobutanones with various electron-donating substituents in the para-position of the phenyl cycles [8]. In this paper, we present the results of revealing intermediates in the photoinduced redox reaction of diethylamino-substituted 2,4-dibenzylidenecyclobutanone in methanol by pulse laser photolysis. EXPERIMENTAL 2,4-Bis(4-diethylaminobenzylidene)cyclobutanone (DACB) was synthesized at the Photochemistry Center of the “Crystallography and Photonics” Federal
Scientific Research Center, Russian Academy of Sciences. O
Et2N
NEt2 DACB
Methanol (Merck) was used for preparation of the DACB solutions. Difference spectra of the dienone triplet–triplet (T–T) absorption and photoreaction transient radical species were measured on a pulse laser setup described elsewhere [9, 10]. The second harmonics of a Solar Nd : YAG laser (λ = 532 nm) was use
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