Direct and sensitized photoprocesses of bis-benzimidazole dyes and the effects of surfactants and DNA

The photoprocesses of two bis-benzimidazole dyes, Hoechst 33258 (1) and an analog, where the phenolic group in p-position is replaced by an ethoxy gro up, Hoechst 33342 (2), were studied. For 1 and 2 in aqueous solution the qu antum yield of fluorescence is strongly pH dependent; it decreases from a m aximum value of Phi (f) = 0.4 at pH 5 to Phi (f) = 0.02 at pH 8, The effect s of absorption and fluorescence, induced by sodium dodecyl sulfate surfact ants below and above the critical micelle concentration and by double-stran ded DNA, are interpreted by assuming that in bulk aqueous solution the dyes are essentially present as monomers, The strong enhancement of Phi (f), wh en the dye is bound to double-stranded DNA or solubilized in micelles, is s uggested to be due to different environments at the benzimidazole rings. A quinoid intermediate with absorption maximum at 380 nm is formed for 1 at n eutral pH using lambda (exc) = 248 or 308 nm, N-centered radicals of 1 or 2 in aqueous solution were observed by laser flash photolysis after electron ejection using wavelengths of 193 or 248 nm (mono and biphotonic, respecti vely). The precursor radical cation escaped observation but is transformed into the above radicals by deprotonation. Electron transfer from 1 in aqueo us solution to triplet acetone takes place, and subsequent deprotonation is proposed to yield N-centered radicals. In addition, energy transfer from a cetone to 1 is suggested, leading to T-T absorption with the maximum at 700 nm, The photoprocesses are discussed and the results compared with those k nown from pulse radiolysis.