10.3.2: The Stokes Shift

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The Stokes shift is the term used to describe the difference in the wavelength at which a molecule emits light is relative to the wavelength at which the molecule was excited. As shown in the Jablonski diagram below in Figure \(\PageIndex{1}\), the energy associated with the blue arrows indicating fluorescence is never longer (more energy) than the green arrows indicating absorption.

Figure \(\PageIndex{1}\): "File:Diagramme de Jablonski-EU.jpg" by Iratipedia is licensed under CC BY-SA 4.0

The absorption of a photon is a very fast process taking place on the femtosecond time scale or faster. Consequently, not only are the nuclei in the species of interest frozen in position during the time (the Franck-Condon Principle) are any molecules in the bath surrounding the species of interest. Thus any interaction of the bath molecules with the species of interest is largely limited to the ground electronic state. These interactions lead to the relatively small solvent dependent shifts observed in absorption spectroscopy in the UV and Vis (2 - 40 nm).

Fluorescence in condensed media takes place on the nanosecond time scale, a time scale during which solvent molecules in the bath can reorientate or relax around the molecule in the excited state. Figure \(\PageIndex{2}\): is a simple energy level diagram for illustrating the absorption, solvent reorientation, and emission processes. For most species, the excited state is much more polar than the ground state and \({\mu_e}\) is much larger than \({\mu_g}\). As shown in Figure\(\PageIndex{2}\): the polar solvent molecules can quickly reorientate their dipoles to stabilize the energy of the excited state to a much greater extent than for the ground state. Consequently, the energy difference between the excited and ground state can be much less following solvent reorientation and the Stokes shift much greater.

solvent and fl 2.jpg — Figure \(\PageIndex{2}\): A simple energy level diagram for the absorption and emission processes in the presence of a strongly interacting solvent. Lakowicz principles of fluorescence spectroscopy

Thus the Stokes shift for fluorescence is a much more sensitive reported for the local environment of the fluorophore. In Figure \(\PageIndex{3}\) the fluorescence of a lipophilic styrene fluorophore of the kind used in studies of biomembranes is shown for a series of solvents ranging from very nonpolar (cyclohexane) to very polar (butanol). Over this range of solvents, the peak in the emission intensity ranges from 420 nm to almost 600 nm and the peak in emission intensity when the fluorophore is interacting with a model DPPC membrane vesicle reveals the fluorophore is contained in volume of polar molecules, as found in the inside of the membrane based vesicle.

stokes shift and environ.jpg — Figure\(\PageIndex{3}\): Lakowicz principles of fluorescence spectroscopy