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Relations between Chemical Structure and electron spectra

 Absorption of light, or more general electromagnetic irradiation, strongly depends on the chemical composition of the molecule. The two functionalities of a chemical substance that significantly influence the nature of a molecule’s absorption spectrum are functional groups and conjugated π-systems. For both elements absorption of light corresponds to a transition of an electron from an n-, σ- or π-orbital to a σ- or π-orbital. For light absorption in the UV-Visrange following electron transitions are possible: n π, n σ, σσ,ππ

n σtransitions usually occur in saturated molecules containing heteroatoms such as O, N and S. In this case, electrons of heteroatom’s lone pairs are excited to the σ-orbital which can already be observed at wavelengths λ > 200 nm. Such compounds are usually used as solvents. Common examples are water, ethanol, chloroform, short chained ethers (diethylether) and amines (methylamine).[1]

n πtransitions can be observed in unsaturated molecules comprising heteroatoms. For these transitions electrons, which are located in the heteroatom’s n-orbitals, are excited to the non-bonding orbital of a π-bond. Important examples possessing n πtransitions are aldehydes and ketones. For saturated aldehydes and ketones n πtransitions can be observed at wavelengths of λ = 270 nm to 300 nm. For unsaturated carbonyl compounds with π-bonds at α and β position such transitions occur at wavelengths of λ = 300 nm to 350 nm.[1] For σσtransitions higher excitation energies are required. For saturated hydrocarbons these are the only observed transitions. Thereby σσtransitions of C–H bondings are excited more easily than σσof C–C-bondings due to small differences in the bond energy.

These transitions occur in a wavelength region of 100 nm to 200 nm.[1] ππtransitions can be observed for every molecule that contains a π-bonding system or πelectrons (e.g. chromophores, polyenes, etc.). During this transition electrons from the π-orbital are raised to a anti-bonding π-orbital. A crucial parameter for the amount of energy required for excitation is the extent of the conjugated π-system. In order to induce ππtransitions in ethylene, possessing an isolated π-bonding, a wavelength of λ = 193 nm is needed due to a high energy difference between HOMO and LUMO. Because the energy difference between the HOMO and LUMO depends on the number of conjugated π-bondings, light with longer wavelengths can induce ππtransitions in conjugated π-systems.[1;2]

As a consequence of the tuneability of the energy difference between the HOMO and LUMO, linear or cyclic conjugated π-systems are often used as basic chromophores. Chromophoric groups absorb light in the UV-VIS range and are therefore essential for the functionality of a molecule e.g. as a dye. When the extent of a π-system exceeds a certain value, absorption of light in the visible range is observed. In addition to a chromophoric backbone, the addition of auxochromic groups to the molecule can affect the absorption spectrum of the molecule.


Figure 1: Influence of different configurations and substituents on absorption behavior of azobenzene.

 Auxochromic substituents contain heteroatoms. Examples are –OR, –NR2, –SR moieties which could operate as an +M donor. These substituents lead to a shift of the absorption maximum to higher wavelengths (bathochromic effect) and an increase of the molar extinction coefficient and with that the absorption (hyperchromic effect).[2]

Antiauxochromic groups show enhanced bathochromic and hyperchromic effects by extending the conjugated π-system. These substituentes (e.g. –NO2, –CN, –CO) operate as -M or -I ,respectively, acceptors.[2] The counterpart to auxochromic substituents are hypsochromic groups. These groups are sterically demanding and voluminous. Addition of such substituents to a chromophoric backbone leads to a twist of the molecule and with that to an interruption of the conjugated π-system. This leads to a shift of the absorption to shorter wavelengths (hypsochromic effect) and often to a decrease of the absorption band intensity (hypochromic effect).[1;2] Azobenzene is an outstanding example to highlight the dependence of the absorption on possible substituents as well as on the configuration (see Figure 1). (E)-azobenzene contains a weaker absorption band of a n π∗ transition at 450 nm and a stronger absorption band of a ππ∗ transition at 330 nm. Isomerization to (Z)-azobenzene results in a blue shift of the ππ∗ band due to twisting of the molecule. Adding of auxochromic (+M-Donors) and/or antiauxochromic substitutents leads to a red shift and higher absorption intensities (see above).

  1. BECKER, H.; BÖTTCHER, H.; DIETZ, F.; REHOREK, D.; ROEWER, G.; SCHILLER, K.; TIMPE, H.-J.: Einführung in die Photochemie, 3. bearb. aufl. edn., 1991, Deutscher Verlag der Wissenschaften GmbH, ISBN 978–3–3260–0604–8.
  2. WÖHRLE, D.; TAUSCH, M. W.; STOHRER, W.-D.: Photochemie, 1998, Wiley VCH Verlag GmbH, ISBN 3527295453.

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