title: Photoinduced One-Electron Oxidation of Benzyl Methyl Sulfides in Acetonitrile: Time-Resolved Spectroscopic Evidence for a Thionium Ion Intermediate.
abstract: The photo-oxidation of 4-methoxybenzyl methyl sulfide (1a), benzyl methyl sulfide (1b), and 4-cyanobenzyl methyl sulfide (1c) has been investigated in the presence of N-methoxy phenanthridinium hexafluorophosphate (MeOP(+)PF6(-)) under nitrogen in CH3CN. The steady-state photolysis experiments showed for the investigated sulfides exclusively the formation of the corresponding benzaldehyde as the oxidation product, reasonably due to a deprotonation of the sulfide radical cations. Photo-oxidation of 1a-1c occurs through an electron transfer process. Indeed, laser flash photolysis measurements showed an efficient formation of sulfide radical cations, detected in their dimeric form [(4-X-C6H4CH2SCH3)2(+•)] at ≈520 nm. At longer delay times, the absorption of the dimer radical cation was replaced by an absorption band assigned to the (α-thio)benzyl cation (thionium ion, λmax = 420-400 nm), formed by oxidation of the benzyl radical and not by that of the (α-thiomethyl)benzyl radical, as expected if a Cα-H bond cleavage is operative. This finding highlights a particular stability of this kind of cation never reported before, even though its involvement in one-electron oxidation mechanisms of various sulfides has already been invoked. Density functional theory calculations allowed identification of a significant charge and spin delocalization involving both the phenyl ring and the sulfur atom of the radical cations.
authors.affiliation: †Dipartimento di Ingegneria Civile ed Ambientale, ‡Dipartimento di Chimica, Biologia e Biotecnologie, and §Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, 06123 Perugia, Italy.
authors.forename: Marta
authors.initials: M
authors.lastname: Bettoni
authors.affiliation: †Dipartimento di Ingegneria Civile ed Ambientale, ‡Dipartimento di Chimica, Biologia e Biotecnologie, and §Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, 06123 Perugia, Italy.
authors.forename: Tiziana
authors.initials: T
authors.lastname: Del Giacco
authors.affiliation: †Dipartimento di Ingegneria Civile ed Ambientale, ‡Dipartimento di Chimica, Biologia e Biotecnologie, and §Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, 06123 Perugia, Italy.
authors.forename: Marina
authors.initials: M
authors.lastname: Stradiotto
authors.affiliation: †Dipartimento di Ingegneria Civile ed Ambientale, ‡Dipartimento di Chimica, Biologia e Biotecnologie, and §Centro di Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, 06123 Perugia, Italy.
authors.forename: Fausto
authors.initials: F
authors.lastname: Elisei
mesh_terms: D000097:Acetonitriles / Q000737:chemistry*; D001593:Benzyl Compounds / Q000737:chemistry*; D004583:Electrons; D007477:Ions / Q000737:chemistry; D007834:Lasers; D015394:Molecular Structure; D010084:Oxidation-Reduction; D055668:Photochemical Processes; D011789:Quantum Theory; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D013057:Spectrum Analysis; D013440:Sulfides / Q000737:chemistry*; D013997:Time Factors
chemical_list: D000097:Acetonitriles; D001593:Benzyl Compounds; D007477:Ions; D000644:Quaternary Ammonium Compounds; D013440:Sulfides; C059915:thionium; C032159:acetonitrile