Open-shell organic molecules

In Figure 4.20(b), the encounter of a closed-shell species M with the open-shell excited species M shows that the occupation of the very same molecular orbitals leads to a net stabilizing situation two electrons sink to the lower Pb orbital, with an energy gain — 2A E, while one electron is raised by +AE to I, but the excited electron now sinks into P so that the energy balance is approximately E = — 2AE + AE — AE = — 2AE. This simple picture of excimer formation applies to closed-shell organic molecules as well as to He atoms. The excimer is indeed an electronically excited dimer which is stable only so long as its electrons follow the distribution of M. ... 

Free radicals and radical ions usually have excited states of low energy, and this can be understood from a simple orbital picture (Figure 4.89). In a closed-shell organic molecule the energy spacing between the HOMO and LUMO is quite large, but in the open-shell species the lowest excitations involve transitions of an electron between closely spaced orbitals. Many free radicals and radical ions of organic molecules absorb in the VIS or NIR, while the closed-shell precursors absorb only in the UV. 

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