Balance of the Deactivation Channels of the First Excited Singlet State of Phenols:  Effect of Alkyl Substitution, Sterical Hindrance, and Solvent Polarity

On the basis of laser flash photolysis with detection by picosecond and nanosecond emission as well as absorption spectroscopy, a quantitative description is given of all the deactivation channels of the first excited singlet state of phenols ArOH(S1) such as fluorescence, intersystem crossing to the triplet system (ISC), chemical dissociation into radicals, and radiationless internal conversion (IC). For this purpose, various phenols with different alkyl substitution patterns (mono to 3-fold substitution with methyl and /or tert-butyl groups) were studied in solvents of increasing polarity: cyclohexane, n-butyl chloride, tetrahydrofuran, ethanol, methanol, acetonitrile, and water. The fluorescence lifetimes of the phenols were found to range from a few tens of picoseconds up to a few nanoseconds, correlating with fluorescence quantum yields between 10-1 and 10-3, at room temperature. With a probability less than 0.1, the photodissociation of ArOH(S1) was found to be nearly unaffected by changing the type of solvent. As a result of the ISC the triplet yields amount to 0.2−0.3, with smaller values in nonpolar and the larger ones in polar media. As a very marked exception, sterically hindered phenols (2,6-di-tert-butyl substituted) behave quite differently: they exhibit extremely short living fluorescences, show no ISC, and, radiationless, are only deactivated by internal conversion. This complete physical energy dissipation makes the sterically hindered phenols ideal light quenchers.