HOMO-LUMO transitions spectroscopy

Redox potential data frequently correlate with parameters obtained by other spectroscopic measurements. The correlation of E° potentials with gas-phase ionization potentials has already been briefly discussed. Electronic transitions observed by UV-visible spectroscopy involve the promotion of an electron from one orbital to another and this can be viewed as an intramolecular redox reaction. If the promotion involves the displacement of an electron from the HOMO to the LUMO, then the redox potentials for the reduction of the compound, °REd, and for its oxidation, °ox, are of importance. For a closely related series of compounds, trends in oxidation and reduction potentials can be related to shifts in the absorption frequency, v. If the structural perturbation causes the HOMO and the LUMO to rise or fall in energy in tandem, then (E°RED — E°ox) will remain constant in such cases the HOMO—LUMO frequency (energy) will be essentially independent of the structural perturbation. Where there is a differential influence of the perturbation on the HOMO and the LUMO, then ( °red E°ox) will vary as will the energy of the electronic transition. In such cases a linear correlation of °red or E°0x may result. In the limit the energy of the HOMO, or more usually the LUMO, will be unaffected by structural perturbation where the acceptor orbital is pinned, direct linear correlation of E°Gx with v should be apparent. With E°ox and v in a common energy unit, the plot E°0x versus v should have a slope close to one.33-36... 

Optical absorption spectroscopy of the neutral molecules, therefore, provides an even better measure of the HOMO-LUMO gap AE for the undisturbed system [13]. The deep color often observed for the corresponding radical ions results from new long-wavelength absorptions due to transitions from the singly occupied molecular orbital (SOMO) to the LUMO (anion) or from the HOMO to the SOMO (cations), respectively (see Scheme 1). 

Photoelectron, UV and NMR spectroscopy, as well as MNDO calculations all predict a reduced HOMO-LUMO gap for alkenes with captodative substitution, and an enhanced reactivity of the p carbon [43]. This explains their radicophilic behavior and their high reactivity in cycloaddition processes, the diradicaloid transition states of which are stabilized when the mechanism is asynchronous cf. Sec. 3.3.7). 

UV-vis spectroscopy has been established as a versatile and powerftil tool for the characterization of conjugated polymers. It is a tool that is used to quantitatively determine the extent of ir-orhital overlap in conjugated systems [120]. The extent of the conjugation is directly reflected in the maximum absorption for such systems. This is the manifestation of the tt-tt transition (HOMO/LUMO or bandgap). Consequently, UV-vis can be used to determine information regarding the conformational state and structure of conjugated polymers [120]. 

The electronic spectroscopy is in agreement with the electronic structure found from theoretical calculations. The lowest energy transition is found to be n-n, as predicted, and there is a second n n transition a little higher in energy than the HOMO-LUMO gap. Experimentally there is no direct evidence for any n-7t transitions but these could be hidden under the more intense n-n features. The conjugation length as measured by the band-gap transition is relatively short, as predicted from the calculations. The electronic spectra also suggest that the bandwidths of the electronic states are quite narrow, which is also in accordance with theory. 

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