# Electronically Excited States of Free Radicals
## 3 Excited Neutral Radicals
However, a long-lived absorption very similar to that presented in Figure 3a was ascribed to the anions of glavinoxyl and galvanoxylate. The formation of anions in a nonpolar solvent (Figure 3) is doubtful. The photochemistry of galvinoxyl deserves further study. Figure 3: Transient absorption spectra, i.e., a change in the optical density \(\Delta\)D vs. the wavelength, obtained under \(\mu\)s flash photolysis of stable aroxyl radicals (**a**) galvinoxyl and (**b**) indophenoxyl in hexane at room temperature [11]. The structures of the radicals are presented in Scheme 2 below. Phosphoresce \(\mathrm{D_{0}}\leftarrow\mathrm{Q_{1}}\) was observed for phosphoatheynyl radicals in solid argon [13]. The scheme of the electronic levels for this radical follows Figure 1, and the lifetime of the emission is ~0.1 s [13]. Electron spin polarization and magnetic field effects have been investigated in photoexcited organic molecules that are chemically tethered to radical fragments (nitroxyls and others) [14, 15]. With TR ESR, electron spin polarization was observed in the \(\mathrm{D_{1}}\) and \(\mathrm{Q_{1}}\) states after photoexcitation of a large stable radical [15]. [PERSON], [PERSON] et al. performed a cycle of investigations on photoexcited transient radicals such as diphenylmethyl and others [16, 17, 18]. The radicals were generated by a laser pulse; after a short delay, the second laser pulse excited the produced radicals [16, 17]. A fluorescence from \(\mathrm{D_{1}}\) was apparently observed in these experiments. The lifetime of fluorescence varied from 5 to 400 ns for polyatomic radicals in liquid nonviscous solvents at room temperature [16]. The constant \(k_{\mathrm{q}}\) of quenching an excited state by dioxygen (Section 5) obtained for several \(\mathrm{R^{\bullet}}\)[16, 17, 19] testified to the occurrence of Reactions (1) or (2) in the \(\mathrm{D_{1}}\) state; see Section 5. The lifetime of photoexcited diphenylmethyl radicals in common non-viscous solvents is 250 ns at 300 K [16, 17]. Once more, this \(\mathrm{R^{\bullet}}\) is in a \(\mathrm{D_{1}}\) state [16]. The excitation of perchlorodiphenylmethyl radical (PDM\({}^{\bullet}\)) at \(\lambda_{\mathrm{ex}}\) = 530 nm leads to the population of \(\mathrm{D_{1}}\) (lifetime \(\tau\) = 31 ns), from which emission is observed (\(\phi_{\mathrm{fl}}\) = 1.0 \(\times\) 10\({}^{-3}\)) [20]. This state is quenched by electron donors and acceptors at a rate at or close to the diffusion control and at much slower rates by dioxygen and hydrogen atom donors. The fragmentation of PDM\({}^{\bullet}\) (\(\phi_{\mathrm{dec}}\) = 0.06) occurs from a higher excited doublet (\(\mathrm{D_{n}}\), \(n\geq\) 2), producing intermediates that are trapped [20]. See [20] for details of the chemical reactions. The excitation of perchlororiphenylmethyl radical (PTM\({}^{\bullet}\)) at \(\lambda_{\mathrm{ex}}\) = 532 nm leads to an excited doublet, the \(\mathrm{D_{1}}\) state (lifetime \(\tau\) = 7 ns) [21]. The lifetime of this state is insensitive to dioxygen, electron acceptors, and hydrogen-donating solvents. Electron donors triphenylamine, \(N\),\(N\)-dimethylaniline, and thianthrene quench PTM\({}^{\bullet}\) at diffusion-controlled rates. The cyclization of PTM\({}^{\bullet}\) to form the perchloro-9-phenylfluorenyl radical occurs with a quantum efficiency of \(\phi_{\mathrm{cyc}}\) = 0.3 under \(\lambda_{\mathrm{ex}}\) = 365 nm irradiation [21]. Ns and ps laser flash photolysis allowed to observe \(\mathrm{Q_{1}}\leftarrow\mathrm{D_{1}}\) ISC of flavin radicals in DNA photolyase [22]. Thus, the electronic levels in this radical follow Figure 1; see [22] for details. The lifetime of the studied \(\mathrm{D_{1}}\) (\(\mathrm{Q_{1}}\)) is 100 ps (1 us) in an aqueous solution at room temperature [22]. Paramagnetic free radicals are efficient quenchers of the excited states of molecules [1, 2]. It was observed in experiments with stable aroxyl radicals that self-quenching occurs in solutions [11]:  
\[\mathrm{R^{\bullet}}+\mathrm{R^{\bullet}}\to 2\;\mathrm{R^{\bullet}} \tag{1}\]