WAVELENGTH EFFECTS ON THE PHOTOPROCESSES OF INDOLE AND DERIVATIVES IN SOLUTION

Abstract

Abstract— The two main primary photoprocesses (electron ejection and H-atom release) for indole, 5-methoxyindole and N-methylindole in various polar and nonpolar solvents were studied as a function of the excitation energy and were correlated with the corresponding fluorescence quantum yields. In hydrocarbon solvents, N–H bond cleavage is the main primary photoprocess from the 1Bb band of the substrates with the exception of N-methylindole. In alcohols, both processes are of negligible importance. Hydrated electrons (e−aq) are ejected from the relaxed singlet states of all three compounds in aqueous solutions with a similar yield for excitation at 280 and 254 nm (1La and 1Lb states). The yield increases when the excitation is into the 1Bb band. The quantum yields of the two primary processes from the higher excited states are generally lower than the fraction of molecules not converting to the fluorescent state. This is explained by an efficient back reaction in competition with a thermally activated radical release from an intermediate state or radical pair formed from the S2 (1Bb) state. The non-occurrence of a photoionization energy threshold is discussed.