# Electronically Excited States of Free Radicals
## 4 Excited Radical Ions
The radical anions of these imides and diimides demonstrated intense visible and weaker NIR absorption bands corresponding to their D**\({}_{\text{n}}\) **- D**\({}_{\text{0}}\) **transitions [26]. The excited states of the radical anions were generated by fs excitation (**\(\lambda_{\text{ex}}\) = 840 nm) into these absorption bands. Excitation of the first two radical anions mentioned above led to their decomposition, whereas excitation of the third imide and three dimides yielded transient spectra of their D**\({}_{\text{n}}\) **- D**\({}_{\text{1}}\) **transitions. The lifetimes of the observed D**\({}_{\text{1}}\) **states of the radical anions were all less than 600 ps and increased as the D**\({}_{\text{0}}\)**-D**\({}_{\text{1}}\) **energy gap increased [26]. These results imposed constraints on the use of these excited radical anions as electron donors in electron transfer systems targeted at molecular electronics and solar energy utilization** [26]**. **  
**Three photoexcited cyanoanthracenes radical anions, including the radical-anion of 9,10-discyanoanthracene, were produced by photoinduced electron transfer in liquid using femtosecond spectroscopy** [30]**. All three excited radical ions had 3-5 ps lifetimes due to \begin{table}
\begin{tabular}{c c c c c c} \hline
**Radical-Ion** & \(\tau\)**, ns** & \(k_{\text{ic}}\times 10^{-8}\)**, s\({}^{-1}\)** & \(k_{\text{i}}\times 10^{-9}\)**, s\({}^{-1}\)** & \(\phi_{\text{i}}\) & \(E\)**, kJ/mol** \\ \hline _trans_-Sb**+** & \(2.5\pm 1.0\) & \(4.0\pm 1.6\) & \(0\) & \(0\) & \(202\) \\ \hline _cis_-Sb**+**-** & \(1.5\pm 0.4\) & \(5.6\pm 1.7\) & \(1.0\pm 0.3\) & \(0.14\pm 0.05\) & \(189\) \\ \hline _trans_-Sb**+** & \(0.24\pm 0.05\) & \(41\pm 11\) & \(0\) & \(0\) & \(223\) \\ \hline _cis_-Sb**+** & \(0.12\pm 0.03\) & \(29\pm 7\) & \(4.1\pm 1.0\) & \(0.49\pm 0.12\) & \(210\) \\ \hline \end{tabular} \({}^{\dagger}\) In DMF at room temperature. See [23] for additional details.  
\end{table} 
Table 1: Properties of stilbene radical anions and radical cations in the D**\({}_{\text{2}}\) **state: lifetime (\(\tau\)), rate constants of internal conversion (**\(k_{\text{ic}}\)**) and isomerization (**\(k_{\text{i}}\)**), quantum yield of isomerization (\(\phi_{\text{i}}\)), and excitation energies (\(E\))** [13]. efficient non-radiative deactivation to the ground state. The excited state of these radical anions was D\({}_{1}\). It is suggested that a short lifetime of D\({}_{1}\) does not preclude photochemical applications of cyanoanthracene radical anions [30]. It was observed that the 10-phenyl-10\(H\)-phenothiazazine radical cation (PTZ\({}^{\bullet+}\)) had a manifold of excited D states accessible using visible NIR light [27]. PTZ\({}^{\bullet+}\) could serve as super-photooxidants with excited state potentials in excess of +2.1 V vs. SCE [27]. The photoexcitation of PTZ\({}^{\bullet+}\) in acetonitrile with \(\lambda_{\text{ex}}\) = 517 nm laser pulse populated a D\({}_{\text{n}}\) excited D state that decayed first to the unrelaxed lowest electronic excited state, D\({}_{1}\)\({}^{\prime}\) (\(\tau\) < 0.3 ps), followed by relaxation to D\({}_{1}\) (\(\tau\) = 10.9 \(\pm\) 0.4 ps) [27]. D\({}_{1}\) finally decayed to D\({}_{0}\) (\(\tau\) = 32.3 \(\pm\) 0.8 ps) [27]. To probe the oxidative power of PTZ\({}^{\bullet+}\) D\({}_{\text{n}}\) states, PTZ\({}^{\bullet+}\) was covalently linked to each of three hole acceptors: perylene, 9,10-diphenylanthracene, and 10-phenyl-9-anthracenecarbonitrile, which have oxidation potentials of 1.04, 1.27, and 1.6 V vs. SCE, respectively [27]. In all three cases, photoexcitation wavelength-dependent ultrafast hole transfer occurred from the D\({}_{\text{n}}\), D\({}_{1}\)\({}^{\prime}\), or D\({}_{1}\) of PTZ\({}^{\bullet+}\) to the three acceptors. The high oxidative power of the D\({}_{\text{n}}\) state of PTZ\({}^{\bullet+}\) will enable applications using this chromophore as a superoxidant for energy-demanding reactions [27].