Rose Bengal and Non-Polar Derivatives: The Birth of Dye Sensitizers for Photooxidation+

+ This paper is dedicated with sincere friendship to Profes­ sor Günther Otto Schenck on the occasion o f his 70th anniversary. * Reprint requests to Prof. Dr. D . C. Neckers. 0340-5087/84/0400-0474/$ 01.00/0 took on significance both in photobiology and dyesensitized oxygenation. Raab, in 1900, observed that paramecia, when exposed to acridine, were killed only in the presence of light but not in the dark [2], Later, this process was termed the “photodynamic effect” [3]. In 1931 Kautsky and De Bruijn first pro­ posed that singlet oxygen was the reactive intermedi­ ate in dye-sensitized oxygenations [4] though Win­ daus and Brunken had earlier reported dye-sen­ sitized photooxygenation yielding an isolable perox­ ide [5]. Subsequently, Kautsky’s singlet oxygen was discounted in favor of a mechanism in which the sen­ sitizer was excited to a metastable state having birad­ ical character, the latter reacting with oxygen to form a labile sensitizer-oxygen complex. This complex was then suggested to transfer oxygen to the substrate giving the photooxygenation product [6, 7], The first detailed kinetic investigation of dye-sensitized oxy­ genation was reported by Schenck in 1951 [8], and this was followed by the development of a method for determining the quantum yield of triplet forma­ tion of the sensitizer [9], A procedure based on quenching all dye triplets with 0 2 and the subsequent trapping of all the singlet oxygen with a very reactive acceptor was developed by Schenck and Gollnick with the reaction rates thus obtained being indepen­ dent of acceptor concentration. Presently, it is gener­ ally accepted that singlet oxygen is the reactive inter­ mediate in dye-sensitized photooxygenation reac­ tions [10—16], A part from their action as singlet oxy­ gen sensitizers (Type II mechanism) the n, tt* triplets of the halofluorescein dyes may also interact directly with substrates (Type I mechanism) [17—23], This often leads to H-transfer or electron transfer, espe­ cially w'ith easily oxidizable (phenols, amines) or re­ ducible substrates (quinones) [20—22]. A concomi­ tant dehalogenation of the dye may take place