COLLOIDAL CDS-INDUCED PHOTOCATALYTIC REACTION OF 2-METHYL-INDOLE - MECHANISTIC ANALYSIS OF OXIDATION OF INDOLES

2-Methylindole (2-MI) is adsorbed on the surface of colloidal CdS part icles with an adsorption intensity of 0.6 x 10(3) dm(3) mol(-1). A new emission band at 530 nm is produced by forming an exciplex between ex cited CdS and 2-MI and the red emission due to CdS is simultaneously q uenched. The emission maxima of green bands for different indoles incr ease in the order indole < tryptophan < 2-MI < 3-MI and are observed a t 508, 520, 530 and 540 nm, respectively. The shift in emission maxima is related to the oxidation potential of these substrates. The irradi ation of an aerated reaction mixture containing CdS and 2-MI with visi ble light induces the oxidation of adsorbed 2-MI by photogenerated hol es to produce 2-methyl-3-indolinone and 2-acetamidobenzaldehyde. The l atter product is formed due to oxidative C-C bond cleavage of the pyrr ole ring. The reactivity of trapped holes towards the adsorbed 2-MI is evidenced by a decrease in the lifetime of the red emission of CdS in the presence of 2-MI. In this reaction the possibility of the partici pation of singlet oxygen is ruled out. A general mechanism of CdS-indu ced oxidation of indoles is discussed. (C) 1998 John Wiley & Sons, Ltd .