Delocalized models, molecular absorption

Figures 6 and 7 show absorption and electroabsorption spectra of [ (NH3)5Ru 2(/A-pyz)]5+ and [ (NH3)5Ru 2(M,4 -bpy)]5+, respectively. The change in AA as a function of x is uniform for the bands, which indicates that the molecular properties that give rise to AA are identically oriented with respect to the transition dipole moment. The electroabsorption spectra in the near-IR region (MMCT bands) give the greatest differences between complexes when analyzed with Eq. (31) and these are shown in Fig. 8. For the Creutz-Taube ion (Fig. 8A), the spectrum does not satisfactorily reduce to a sum of derivatives but nevertheless shows that AA(p) line shape to be modeled primarily by a negative zeroth derivative (Ax) term, especially at energies below 6500 cm-1. The fit in this case yields a value for Ap. = 0.7 0.1 D, which when compared with the maximum permanent electric dipole moment ( A/u max = 32.7 D, assuming a metal-to-metal distance) is strong evidence for a delocalized ground state. Contrast this result with the analysis of the electroabsorption spectrum of [ (NH3)5Ru 2(ja-4,4 -bpy)]5+ shown in Fig. 8B.

Deviations from OGM were recognized early on spectroscopic properties of molecular crystals Davydov shifts and splittings of absorption bands in molecular crystals are clear deviations from OGM and were rationalized based on the excitonic model (EM) [10, 14, 15, 16, 17]. This same model proved extremely successful to describe the complex and technologically relevant spectroscopy of molecular aggregates, i.e. of clusters of molecules that spontaneously self-assemble in solution or in condensed phases [IS]. Much as it occurs in molecular crystals, due to intermolecular electrostatic interactions the local bound electron-hole pair created upon photoexcitation travels in the lattice and the corresponding wave function describes an extended delocalized object called an exciton. We explicitly remark that the Frenkel picture of the exciton, as a bound electron-hole pair, both residing on the same molecule, survives, or better is the basis for the excitonic picture. The delocalization of the exciton refers to the fact that the relevant wave function describes a Frenkel exciton (a bound e-h pair) that travels in the lattice, and this is of course possible even when electrons and/or holes are, separately, totally localized. In other terms, the EM describes localized charges, but delocalized excitations. 

Use this expression to determine a reasonable empirical estimate of the parameter )3 for the series consisting of ethene, butadiene, hexatriene, and octatetraene given that light-induced absorptions from the HOMO to the LUMO occur at 61 500,46 080,39 750, and 32 900 cm h respectively, (b) Calculate the delocalization energy of octatetraene (see Exercise 10.36). (c) In the context of this Huckel model, the % molecular orbitals are written as linear combinations of the carbon 2p orbitals. The coefficient of the jth atomic orbital in the fcth molecular orbital is given by... 

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