# Electronically Excited States of Free Radicals
## 4 Excited Radical Ions  
Electronically excited radical cations and radical anions have been the subject of a number of experimental and theoretical investigations. Stilbene (Sb) radical anions and radical cations were studied by pulse radiolysis and laser flash photolysis [23]. There were four species under investigation, namely _cis_- and _trans_-Sb\({}^{\bullet(+/-)^{\star}}\). It was stated that the \(\mathrm{D_{2}}\) of these species was observed [23]. Table 1 below summarizes most of the data obtained in this work:  
Radical cations of polycyclic aromatic hydrocarbons (PAH) play an important role in different processes and in the environment--in particular, in astronomy [24]. PAH are easily ionized, producing radical cations PAH\({}^{\bullet+}\). As expected, the absorption spectra of PAH\({}^{\bullet+}\) are red-shifted towards the spectra of the parent PAH [24]. The excited states of 51 different PAH\({}^{\bullet+}\) were studied with the time-dependent density functional theory [24]. The vertical excitation energies and oscillating strengths of PAH\({}^{\bullet+}\) and even of PAH\({}^{\bullet-}\) were obtained. \(\pi\leftarrow\pi\) transitions between individual \(\pi\)-orbitals in radical ions of PAH were considered [24]. Biphotonic laser photoexcitation (\(\lambda_{\text{ex}}\) = 248 nm) of different aromatic compounds (biphenyl, naphthalene, perylene, and others) and several amines in oxygenated polar solvents have led to the corresponding radical cation R**+**[25]. R**+** have obviously red-shifted spectra compared to the parent compounds and are absorbed in the visible and NIR regions. A prompt irradiation of R**+** with a second harmonic Nd:YAG laser (\(\lambda_{\text{ex}}\) = 532 nm) led to the excited R**+**+**[25]. The studied R**+**+** had a lifetime ranging from hundreds of fs to tens of ps at room temperature. The nature of R**+**+** is not discussed [25]; it is probably the D**\({}_{1(2)}\) **states. **  
**In general, excited states of multiple radicals of radical ions in the D**\({}_{\text{n}}\) **state have very short lifetimes (see Sections** 2 **and** 3 **as a result of their low-lying excited state energies (energy gap law)** [26; 27; 28]**. **  
**A study of the dynamics of the ground state recovery of the perylene radical cation (Pe**+**), of the perylene radical anion (Pe**+**-**), and of the anthraquinone radical anion (AQ**+**-**) was shown in** [29]**. Complete or partial regeneration took place after a picosecond photoexcitation of the corresponding radical ion. In boric acid glass, the excited state lifetime of Pe**+** was 35 \(\pm\) 3 ps, while in concentrated sulfuric acid, it was smaller than 15 ps, the time resolution of the experimental setup [29]. The excited state lifetime of Pe**+**+**, Pe**+**-**, and AQ**+**-** generated by a photoinduced intermolecular electron transfer reaction in acetonitrile was shorter than 15 ps [29]. **  
**It is well known that molecules in excited states essentially change their redox properties** [1; 2]**. Not surprisingly, the same holds true for radical ions. [PERSON] et al. studied the redox properties of radical anions in excited doublet states** [26; 27]**. The radical anions of aromatic diimides have been assumed recently to be participants in different photoinduced electron transfers. The photoexcitation of these radical anions produces powerful reducing agents** [26]**. However, the properties of the** \(\pi^{*}\)**-excited D states of these radical anions remain obscure. The radical anions of three complex aromatic imides with increasingly larger** \(\pi\) **systems, namely** \(N\)**-(2,5-di-_tert_-butylphenyl)phthalimide,** \(N\)**-(2,5-di-_tert_-butylphenyl)-1,8-naphthalimide, and** \(N\)**-(2,5-di-_tert_-butylphenyl)perylene-3,4-dicarboximide, as well as the three corresponding aromatic diimides** [26]**, were produced by electrochemical reduction of the neutral molecules.