Hydroquinone absorption spectra

Hydro quinone transforms in the presence of irradiated nitrite to yield ben-zoquinone and hydroxybenzoquinone [78,79]. At the irradiation wavelength adopted in the cited works (365 nm), hydroquinone direct photolysis should be limited and benzoquinone most likely forms upon reaction between hydroquinone and hydroxyl (reactions 44 and 45 hydroquinone absorbs radiation at A, < 320 nm). Hydroxybenzoquinone is likely to be a product of benzoquinone photolysis. No nitration or nitrosation intermediates of hydroquinone were observed in the presence of nitrite under irradiation, differently from the cases of resorcinol and catechol [78,79]. The reaction between hydroquinone and nitrogen dioxide is, however, quite rapid [106,115], as confirmed by the marked inhibition of phenol nitration upon nitrite photolysis by added hydroquinone [62], The point is that the reaction between hydroquinone and NO2 mainly yields benzoquinone [62], Another interesting feature in the case of hydroquinone is the formation of the fairly stable semiquinone radical anion upon reaction between benzoquinone and depro-tonated hydroquinone. The spectrum of the resulting solution shows the typical absorption bands of the semiquinone at 308, 315, 403, and 430 nm [79]. 

Fig. 40. Wavelength dependence of the Nafion-bound Ru(bpy -sensitized anodic photocurrent at n-SnOj- a Absorption spectrum of Nafion-coated SnOj b absorption spectrum of Nafion-bound Ru(bpy) c absorption spectrum of Ru(bpy in solution d anodic photocurrent at SnOa coated with Nafion-bound Ru(bpy) in 0.1 N KCl containing 10 mM hydroquinone obtained at the applied potential of 0.2 V vs SCE photocurrent normalized with respect to the lamp spectrum...

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