14 September 2008

Two parameters determines the effectiveness of a sunscreen are :
  1. The wavelength covered.
  2. The numerical size of the extinction coefficient.
Therefore, chemical with a high extinction coefficient are more efficient in absorbing the energy of the harmful UV radiation than chemicals with a lower extinction coefficient. But, if only part of the spectrum or harmful wavelength is absorbed, UV radiation may still be harmful to soft tissue.

Electronic transitions for any compound may be characterized as symmetry allowed or symmetry forbidden. This symmetry depends on the special ease of delocalization of electron in the compound. Symmetry-allowed trantitions generally have high extinction coefficient, and symmetry-forbidden transtitions have lower extinction coefficient.

The number of double bonds and the extent of resonance delocalization in a molecule are the critical factors that give a clear indications as to the maximum wavelength and a qualitative predictions of the extinction coefficient.

The more efficient the electron delocalization in a molecule, the higher its extinction coefficient. Example, Padimate-O, the two substituents on the benzene ring are in a para relation, whereas the two substituents in the homosalate are in a srerically hindered ortho relation. In ortho-disubstitued aromatic compounds, the two groups are close to one another, causing a deviation from planarity. The slightest deviation from coplarinity will significantly reduce resonance delocalizations. This results in a lower extinction coefficient for homosalate (=4300) than for padimate-O (27,300). Increased conjugation of double bond, leading to more efficient resonance delocalization, will also yield higher extinction coefficients.

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