Reactions of Photoexcited Aromatic Radical Cations with Polar Solvents

Abstract

Reactions of electronically excited radical cations of various simple aromatic hydrocarbons (e.g., naphthalene, biphenyl, perylene) and amines (indole and l-tryptophan) with polar solvents (alcohols, ethers, and water) were studied. The radical cations were generated by biphotonic (248 nm) photoionization of aromatic solutes in oxygen-saturated solutions and subsequently excited by 2.3 eV photons from a Nd:YAG laser. The photobleaching is initiated by a transfer of the valence electron of the solvent to the solute radical cation (“hole injection”). In the polar liquids, a proton transfer occurs concurrently with this electron transfer and may occur in several ways. Hole injection is the general mechanism for decay of photoexcited radical cations, in both polar and nonpolar media. The efficiency of this process correlates with the ionization potential (IP) of the solute and optical absorbance of the corresponding radical cation. For high-IP solutes, the quantum yield of the photobleaching is 0.3−0.5. From scavenging of the photoexcited radical cations, it was found that while some of these states have the lifetimes below 1 ps, other have lifetimes of 10−40 ps (in acetonitrile).