# Electronically Excited States of Free Radicals
## 2 Doublets and Quartets  
Organic free radicals (R*) are species with an \"open shell\". They have at least one SOMO. The spin state of R* is \(s=1/2\), and such states are called doublets (D). Radicals with three one-electron orbitals exist in a quartet electronic state (Q), which has \(s=3/2\). Free radicals are important intermediates in many photoinduced reactions. There are cases when radicals absorb light and produce excited D\({}_{1}\) or Q\({}_{1}\) states; see below. Obviously, the chemical reactivity of the photoexcited radicals (molecules) is different from that in D\({}_{0}\)- (S\({}_{0}\)-) states [4].  
Synthesized stable organic \(\pi\)-radicals have been recently reported, which are especially interesting for applications in organic light-emitting devices (OLED) due to their possibility of providing a quantum yield of luminescence of 1.0 [5]. This is one of the modern reasons for studying photoexcited free radicals.  
It is logical to assume an analog of the Jablonsky scheme (Figure 1) for free radicals. SOC mixes D\({}_{1}\) and Q\({}_{2}\); see Figure 1. All other processes (luminescence, internal conversion, and vibrational relaxation) take place in photoexcited molecules and radicals. Luminescence from the D\({}_{1}\) (Q\({}_{1}\)) state is named in the literature as fluorescence (phosphorescence), apparently due to an analogy with such processes in molecules.  
Stable free radicals (R*) are usually colored, whereas the parent compounds (RH) are colorless. That means that D\({}_{1}\) lies close in energy to D\({}_{0}\), namely 180-300 kJ/mol above D\({}_{0}\) (Figure 1). For example, colorless 2,4,6-tri-_tert_-butyl phenol has a longwave absorption maximum at \(\lambda=280\) nm (370 kJ/mol), whereas the corresponding phenoxyl radical (blue axoxyl) has an analogous value \(\lambda=626\) nm (190 kJ/mol). A similar bathochromic shift of the absorption maximum holds between a parent compound (say, a ketone, a quinone) and its radical R* (a ketyl radical, a semiquinone) formed by hydrogen abstraction.  
Q\({}_{1}\) may have a higher energy than D\({}_{1}\); see Figure 2.  
The formation of Q\({}_{1}\) requires three unpaired electrons with parallel spins, which may increase the energy of the excited R* compared to that of D\({}_{1}\) (The formation of T\({}_{1}\) in molecules requires only two unpaired electrons with parallel spins. The dominant role of the Pauli effect leads to a lower energy of T\({}_{1}\) compared to S\({}_{1}\) in the majority of cases [1, 2]).  
Figure 2: Possible positions of the excited states of a radical where the Q\({}_{1}\) state has a higher energy than the D\({}_{1}\) state. IC stands for internal conversion.  
Thus, in a number of cases, Q\({}_{1}\) can be populated only after primary excitation into D\({}_{2}\) or D\({}_{3}\); see Figure 2.  
Fast internal conversion (IC) from a higher D\({}_{2(3)}\) to a lower D\({}_{1}\) should lead to a low probability of the population in Q\({}_{1}\). This assumption allowed the authors of [2] to draw conclusions about the insignificant role of Q\({}_{1}\) in the photochemistry of radicals. However, there have been many reports of the experimental detection of Q\({}_{1}\) (Section 3 below). The observation of a relatively long-lived R\({}^{\bullet\bullet}\) leads to the assumption that R\({}^{\bullet\bullet}\) is in a Q\({}_{1}\) state; see Section 3 below.