Photochemical excitation creating radicals

Radical ions are created in solution by chemically or electrochemically induced electron transfer to or from a conjugated ir-system. Even if these ions are thermodynamically stable they are only of limited persistence since they are susceptible to reactions with electrophiles and nucleophiles or undergo other processes like dimerization or electron-transfer induced bond cleavage [9, 10]. Pairs of radical anions and radical cations can also be formed by electron transfer between neutral donors and acceptors either in the ground state or upon photochemical excitation [11, 12]. 

The photochemical reaction of the acridinium ion 8.203 in methanol is an example of a general reaction type in which radical ions and radicals are created by electron transfer to or from the excited state. As we saw earlier in the discussion about Fig. 8.10, the HOMO of an excited state is a low-energy, half-filled orbital, into which another electron can be fed from a filled orbital of suitably close energy. In this case, the lone pair in methanol transfers an electron to the HOMO of the excited state 8.204, giving the radical 8.205 and the radical cation 8.206. The latter easily loses a proton to give the radical 8.207, and the C—C coupling of this... 

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