Trifluoroacetic acid prompted unexpected visible to NIR switching of ketoenamine-substituted triphenylamines

Abstract

We report a trifluoroacetic acid (TFA)-induced absorption shift of π-extended ketoenamine-substituted triphenylamines from the blue (420–450 nm) to near-infrared (NIR) (1020–1080 nm) spectral region. This phenomenon is highly specific to TFA, while only a mild (to ∼580 nm) shift is observed in other studied acids (stronger or weaker Brønsted and Lewis acids). The unexpected NIR absorption originated from the radical cation formed by the oxidation of triphenylamine with TFA through a series of proton and electron transfer events. The formation of a radical cation is confirmed by electron paramagnetic resonance (EPR) spectroscopy.