Indications for an intermolecular photo-induced excited-state proton transfer of p-nitrophenol in water

Nitrophenols are well-known model systems to investigate photoinduced ultrafast excited-state channel branching. For example, photoexcitation of o-nitrophenol leads to stimulated emission within the first few hundreds of femtoseconds superimposed by an intramolecular proton transfer as well as intersystem crossing and vibrational relaxation. Here, we report on photo-induced processes after excitation of the first spectroscopically bright excited state of p-nitrophenol in aqueous solution at pH values ranging from 1 to 10. After excitation at wavelengths around 317 nm, transient absorption spectra were recorded between 350 and 700 nm for delay times of up to several hundreds of picoseconds. While many of the above-mentioned relaxation pathways still occur, the studies provided also strong indications for an intermolecular proton transfer forming p-nitrophenolate. The corresponding spectral features are observable at pH 7 and below on a hundreds of picosecond timescale.