What causes a large Stokes shift?

Large Stokes Shift: Causes and Explanations

The Stokes shift is defined as the difference in wavelength or energy between the absorption and emission spectra of a given molecule or material. A large Stokes shift is particularly noteworthy as it can have significant implications for the efficiency and functionality of devices such as fluorescent probes, light-emitting diodes, and lasers. Below we detail the factors that contribute to a large Stokes shift.

Electronic Structure and Relaxation Processes

When a molecule absorbs a photon, an electron is excited to a higher energy level. This electronic transition typically occurs within a specific molecular orbital. After excitation, the molecule relaxes to a lower vibrational level of the excited state through non-radiative processes such as internal conversion or vibrational relaxation, resulting in a loss of energy. This relaxation alters the energy landscape of the molecule, causing the emission of a photon at a lower energy (longer wavelength) than the absorbed photon. The magnitude of the Stokes shift is related to the extent of the relaxation and rearrangement of the electron density within the molecule.

Solvent Effects

The polarity of the solvent can cause a large Stokes shift. Polar solvents can stabilize the excited state of a fluorescent molecule more so than the ground state due to the molecule's change in dipole moment upon excitation. The stabilizing interaction between the polar solvent and the excited molecule results in a lower energy of the emitted photon compared to the absorbed one, leading to a large Stokes shift.

Photon Re-absorption and Re-emission

In some cases, the emitted photons are re-absorbed and re-emitted by other molecules, in a process called re-absorption or photon recycling. If the secondary emission occurs at a longer wavelength than the primary emission, this can lead to an observed large Stokes shift.

Molecular Configuration Changes

Structural changes in the molecule during the excited state can also result in a large Stokes shift. This includes changes such as proton transfer, twisting of the molecule, or isomerization, which can greatly alter the molecular orbital energies between the ground and excited states, increasing the Stokes shift.

Vibrational Energy Levels

Difference in vibrational energy levels between the ground and excited states can also be a cause. If the molecular vibration levels are significantly different between these states, it can result in a larger energy gap being traversed upon photon absorption compared to emission, generating a significant Stokes shift.

Energy Transfer Mechanisms

Energy transfer from the initially excited molecule to another species with different emission characteristics can lead to a large Stokes shift. This transfer can occur through mechanisms like Fӧrster resonance energy transfer (FRET) or Dexter electron transfer, which can effectively increase the wavelength of the emitted light.

In summary, a large Stokes shift can be caused by a variety of factors, often relating to the electronic structure of the molecule, its interaction with the surrounding environment, and the relaxation and energy transfer processes that occur following excitation. Understanding and controlling these factors is crucial for designing materials with desired photophysical properties for applications in spectroscopy, imaging, and optoelectronics.