Benzene, absorption spectrum transition moments

Although the valence ji-ji excitation spectra of benzene derivatives have been extensively studied over the past 65 years both experimentally and theoretically, much less is known about that of phenol, apart from its lowest excited state. In general, absorption and fluorescence spectroscopy of a benzene ring can be used to detect its presence in a larger compound and to probe its environment. While the relative constancy of the valence jt-n excitation spectrum allows a qualitative identification of spectral bands by a correspondence with those in free benzene, detailed quantitative differences could indicate the nature of substituents, ligands or medium. Key information on substituted benzene includes the excitation energies, transition moments and their direction, and electrostatic... 

Figure 1.2.10 Polyene-type spectrum of sorbaldehyde showing only one broad band for the 7t-system and—aromatic-type spectrum of benzene. An intense absorption band at low wavelength (high energy) is followed by several less intense ( forbidden ) absorptions at longer wavelengths. Broadening of abso tion bands usually either indicates more flexible or distorted or aggregated chromophore. Such a broadening of the bands leads to smaller extinction coefficients, whereas the area of the absorption bands remains about constant for a given chromophore. This area is related to the so-called transition moment, which can be approximated by the product of extinction coefficient and linewidth.

In the case of the dicyanodiphenyltriafulvene (VI), the dipole moment of the compound in the ground state is 7.9D (dioxane solution, 30°C) [14]. Application of the Lippert-Mataga approach, with the assumption of a cavity radius of 4 A, leads to a value of ID for the dipole moment in the excited state. This is of course a very crude estimation. Too much reliance should not be placed on the quantitative meaning of this result [31]. In any event, the blue shift in the absorption spectrum and the red shift in the fluorescence spectrum on going to more polar solvents or from cyclohexane to benzene, support the substantive decline of the dipole moment in the transition from the ground state to the first excited state. 

Another noticeable difference between the spectrum of 2-phenylfuran and that of the isolated monomers is the intensity around 7 eV. While the absorption cross section of benzene is very high in this region (2.5 A molecule" ), the absorption cross section of 2-phenylfuran is only about 1/3 of this value. This reduction in the absorption intensity is a direct effect of the delocalization of the orbitals over the whole molecule, reducing the transition moments. 

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