Electronic distribution and solvatochromism investigation of a model radical (2,2,6,6-tetramethylpiperidine N-oxyl: tempo) through TD-DFT calculations including PCM solvation

The solvatochromism of the 2,2,6,6-tetramethylpiperidine N-oxyl radical (TEMPO) visible band (n→π*) is reported. When the solvent polarity increases, a blue shift is observed that is typical for a n→π* transition. This behavior is well accounted for by time-dependent density functional theory (TD-DFT) including polarizable continuum model (PCM) solvation in non-specific solvents. However, the agreement between the experiments and the computations for hydroxylic solvents is not straightforward at all. Additionally, valuable information on the electronic/spin distributions and their solvent dependence are presented, a key point for discussing the chemical reactivity of radicals.