title: Direct photoreduction and ketone-sensitized reduction of nitrospirobenzopyranindolines by aliphatic amines.
abstract: The photoreduction of 6-nitrospiro[2H-1-benzopyran-2,2'-indoline] (N1) and two derivatives (N2 and N3) by diethylamine or triethylamine (TEA) in solution was studied by pulsed and steady-state photolysis. The quantum yield of coloration of the ring-closed Sp form, due to photoinduced ring opening, decreases in acetonitrile with increasing the TEA concentration. The main reason is reaction of TEA with the triplet-excited open merocyanine form. Quenching of this triplet state by amines is rather inefficient for N1-N3; the rate constant for triplet quenching by TEA is k(6) = (2-3) × 10(6) M(-1) s(-1). The secondary transient with an absorption maximum at 420 nm is ascribed to the radical anion. This and the corresponding α-aminoethyl radical subsequently undergo slow termination reactions, yielding a relatively stable product with a maximum at 420-450 nm, which is attributed to a ring-opened dihydromerocyanine (MH(-)). The mechanisms of the two subsequent reduction reactions are discussed. Using acetone as sensitizer the same dihydroproduct was obtained with the Sp form as acceptor, indicating a reaction sequence from photogenerated radicals via a ring-opened radical to MH(-)/MH(2). The effect of TEA concentration on the direct and ketone-sensitized reduction mechanisms was analyzed. Photoreduction by amines, due to competing triplet quenching, is strongly decreased on admission of oxygen.
authors.affiliation: Max-Planck-Institut für Bioanorganische Chemie, D-45413 Mülheim an der Ruhr, Germany.
authors.forename: Helmut
authors.initials: H
authors.lastname: Görner
mesh_terms: D000097:Acetonitriles / Q000737:chemistry; D001578:Benzopyrans / Q000138:chemical synthesis*; D002627:Chemistry, Physical / Q000379:methods*; D004048:Diethylamines / Q000737:chemistry; D005021:Ethylamines / Q000737:chemistry; D007211:Indoles / Q000138:chemical synthesis*; D007659:Ketones / Q000737:chemistry*; D007700:Kinetics; D008027:Light; D010084:Oxidation-Reduction; D010100:Oxygen; D010782:Photolysis / Q000528:radiation effects; D011714:Pyrans / Q000737:chemistry*; D012996:Solutions; D012997:Solvents / Q000737:chemistry; D013057:Spectrum Analysis; D013141:Spiro Compounds / Q000737:chemistry*
chemical_list: D000097:Acetonitriles; D001578:Benzopyrans; D004048:Diethylamines; D005021:Ethylamines; D007211:Indoles; D007659:Ketones; D011714:Pyrans; D012996:Solutions; D012997:Solvents; D013141:Spiro Compounds; C548873:merocyanine; C034281:diethylamine; D010100:Oxygen; C016162:triethylamine; C032159:acetonitrile