Direct photoreduction and ketone-sensitized reduction of nitrospirobenzopyranindolines by aliphatic amines

The photoreduction of 6-nitrospiro2H-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.