Identification of an Emitting Metastable State of p-Fluorophenol-Ammonia 1:2 Complex by Laser-Induced Fluorescence Spectroscopy

Abstract

We have demonstrated here, for the first time to our knowledge, the formation of an emitting metastable species upon lowest electronic excitation (S1) of a hydrogen-bonded 1:2 complex of para-fluorophenol (pFP) with ammonia (NH3), which is known to be one of the smallest reactive complexes to undergo excited state H-atom transfer (HAT) reaction to produce •NH4(NH3) radical fragment. The emission spectrum of the species is characterized to be red-shifted, broad, and structureless. From the viewpoint of energy balance, an excited state proton transfer (ESPT) is unfavorable, but according to predicted electronic structure parameters, the metastable state species could be stabilized by charge transfer (CT) interaction at the hydrogen-bonded geometry of the complex. We propose that this species could act as an intermediate to the HAT process in the excited state. The observation of such a state could be valuable to understand the complex dynamics of similar events in biologically relevant systems.