SOMO—►LUMO excitation

Figure 2.23b shows the expected energy diagram, including electron repulsion effects. The configurations that correspond to the HOMO->SOMO and the SOMO LUMO excitation are split into and states by first-order configuration interaction, familiar from even alternant systems. The components of the transition moment from the configurations i, o... 

In terms of the frontier-orbital approach, the two radical bands , C and D, are related to the single-electron transitions HOMO - SOMO and SOMO -> LUMO, which correspond to the two first excited configurations, the electronic occupation of HOMO, SOMO and LUMO being two, one and zero respectively (Ballester et al, 1982a). 

The states in the gap and the associated optical transitions for P" are shown in Figure 22.3a. The polaron energy states in the gap are SOMO and LUMO, respectively, separated by 2wo(P) [52]. Then three optical transitions Pf, P2, and P3 are possible [52-54]. In oligomers, the parity of the HOMO, SOMO, LUMO, and LUMO + 1 levels alternates they are g, u, g, and u, respectively. Therefore, the transition vanishes in the dipole approximation, and the polaron excitation is then characterized by the appearance of two correlated optical transitions below Eg. Even for long chains in the Hiickel approximation, transition P3 is extremely weak, and therefore the existence of two optical transitions upon doping or photogeneration indicates that polarons were created [51,54]. Unfortunately, polaron transitions have not been calculated for an infinite correlated chain. This is a possible disorder-induced relaxation of the optical selection rules that may cause ambiguity as to the number of optical transitions associated with polarons in real polymer films. 

The excited state of the carbonyl compound is the (n, it ) state where one electron is excited from the HOMO to the LUMO. The SOMO is the n-orbital on the carbonyl oxygen atom. The SOMO is the antibonding jt -orbital. 

In the simplest example, a donor and acceptor pair is activated by electronic excitation of either the donor or the acceptor. In addition to photophysical deactivation or energy transfer, two processes can proceed subsequently—the electronically excited donor donates an electron from its SOMO into the acceptor LUMO or the... 

The electron in the high-energy SOMO of the radical anion 8.220 (effectively the n orbital of the carbonyl group) is evidently transferred to the relatively low-energy LUMO of the acridone (Fig. 8.14), giving the product with the orbital occupancy of the n-it excited state, which simply emits a photon when the excited state 8.222 decays to the ground state. 

The HOMO and LUMO are often easy to identify, as in Fig. 1.5, where the TT-orbitals of s-cis-butadiene are stacked in order of increasing energy, alongside those of ethylene - its reaction partner in the prototypical [,r4+7r2]-cycloaddition. Its HOMO, i/>2, is less stable than x of ethylene by virtue of the phase discontinuity between the two central atoms, so it is assumed to be the orbital bearing the frontier electrons when the reaction takes place on the ground-state surface. On photoexcitation, one of these two electrons is raised to the less antibonding of the two unoccupied orbitals, which becomes the SOMO. In both the ground-state and excited state reactions, the frontier orbital of butadiene, HOMO or SOMO respectively, is presumed to be stabilized by interaction with... 

Photochemical-concerted electrocyclic reactions of alkenes and dienes are symmetry allowed processes. For alkene system, the HOMO (SOMO) is the excited alkene 7i orbital and the LUMO is the n of ground-state alkene. The interactions of HOMO and LUMO produce two new o bonds in the product, cyclobutane. 

Schematic diagram showing the electronic configuration of a neutral (a) and transient negative ion (TNI) (b, c). The interacting electron initially captures into the unoccupied MOs of the neutral molecule resulting in TNI formation via (a) shape resonance or (c) core-excited resonance. For a shape resonance, the electron can interact with any unoccupied MO. The SOMO was the empty LUMO before the LEE interaction. In core-excited resonance, on electron interaction an electronic transition takes place from an inner shell to the vacant MOs creating a "hole"(-i- charge) in the inner shell, shown by an arrow (c). The up and down arrows show the occupancy of the molecular orbitals (MOs) with electrons of ocand 3 spins. HOMO highest occupied molecular orbital, LUMO lowest unoccupied molecular orbital, SOMO singly occupied molecular orbital...

In the photochemical cyclobutene-butadiene inter-conversion the excitation energy is used for the ir -> ir transition. Both tt and ir are SOMO levels, denoted respectively by (SOMO)i and (SOMO)2. The relevant interactions are then between (SOMO)i and the ct-HOMO level, and between (SOMOlj and the a-LUMO level, as indicated in Fig. 4.2. The (SOMO)3 and a-HOMO, or the... 

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