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#Stokes shift

3 public questions tagged with this topic.

UV-VIS spectroscopy

In fluorescence, Stokes shift is caused by:

Fluorescence process visualized by Jablonski diagram begins with photon absorption promoting ground state S0 to singlet excited S1 or S2, followed by ultrafast internal conversion and vibrational relaxation to lowest vibrational level of S1. Emission originates from this level to ground S0. Energy lost non-radiatively during relaxation and solvent reorientation around excited dipole reduces emission energy relative to excitation, causing longer wavelength. This difference defined as Stokes shift arises from non-radiative dissipation as heat before photon emission. Magnitude influences choice of filter sets, sensitivity and separation of excitation and emission in fluorescence spectroscopy and microscopy.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Fluorescence and CD Spectroscopy

Emission λ shifts longer than excitation λ due to:

After photon absorption, fluorophore attains upper vibrational level of S1. Within picoseconds it undergoes internal conversion, vibrational cooling and solvent reorganization, releasing portion of energy non-radiatively as heat to surrounding bath. Emission then originates from lowest vibrational level of S1, carrying less energy than initial excitation, hence longer wavelength. This energy loss process underlies Stokes shift, essential for separating excitation and emission in microscopy filters. Light scattering is wavelength elastic, absorption error would not systematically increase wavelength, triplet transfer gives phosphorescence.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Fluorescence and CD Spectroscopy

What causes the Stokes shift?

Stokes shift refers to fluorescence emission appearing at longer wavelength and lower energy than absorption. After excitation to upper vibrational level of S1, the fluorophore undergoes rapid vibrational relaxation, internal conversion and solvent reorientation, dumping small quanta of energy as heat before photon emission. This non-radiative dissipation in the excited manifold reduces energy gap for radiative return. Intersystem crossing instead populates triplet state producing phosphorescence. Radiative loss is the emitted photon itself. Hence non-radiative energy loss during S1 lifetime explains systematic red displacement of emission spectrum.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details