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Fluorescence of Dyes

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Spectral Properties

The spectral properties of a dye are determined by the molecular structure. Absorption in the visible spectral range (400 - 700 nm) requires a corresponding energy gap between the ground and excited state. The striking feature of dye structures is a conjugated p-electron system.

In most cases the fluorescence spectrum is nearly a mirror image of the long-wavelength absorption band. The fluorescence is typically shifted by 25 - 40 nm to the red relative to the main absorption band. This so-called Stokes-Shift is caused mainly by reorientation of the solvent molecules surrounding the dye. The spectral properties are often used to distinguish and identify different dyes in a sample.

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