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).

Ultrafast relaxation dynamics for photoexcited PAH cations isolated in boric acid glass have been studied using femtosecond and picosecond transient grating spectroscopy. With the exception of perylene + , the recovery kinetics for the ground doublet (D 0 ) states of these radical cations are biexponential, containing a fast (< 200 fs) and a slow (3-20 ps) components. No temperature dependence or isotope effect was observed for the fast component, whereas the slow component exhibits both the H/D isotope effect (1.1-1.3) and strong temperature dependence (15 to 300 K). We suggest that the fast component is due to internal D n to D 0 conversion and the slow component is due to vibrational energy transfer (VET) from a hot D 0 state to the glass matrix. The observed rapid, efficient deactivation of the photoexcited PAH cations accounts for their remarkable photostability and have important implications for astrochemistry, as these cations are the leading candidates for the species responsible for the diffuse interstellar bands (DIB) observed throughout the Galaxy.