The Jablonski diagram, typically used to illustrate fluorescence in molecular spectroscopy, demonstrates the excited states of a molecule along with the radiative and non-radiative transitions that can occur between them.
Fig. 1 shows the Jablonski diagram (Jablonski, 1933), a schematic of the transition of electronic state of a molecule during the fluorescence phenomenon. The left axis shows increasing energy, where a typical fluorescent molecule has an absorbance spectrum. This spectrum shows the energy or wavelengths, where the molecule will absorb light.
In conventional fluorescence, photons are emitted at higher wavelengths than the photons that are absorbed. If the incident light is at a wavelength where the molecule will absorb the photon, the molecule is then excited from the electronic ground state to a higher excited state, denoted S2 here.
The electrons then go through internal conversion, affected by vibrational relaxation and heat loss to the environment. As shown in the figure, the final photo-emission transition can either occur through a fast singlet state (fluorescence) or through a slower triplet state (phosphorescence). In conventional photoluminescence, photons are emitted at higher wavelengths (lower energy) than the wavelength of the absorbed photons.
Two non-radiative deactivation processes compete with fluorescence: internal conversion from the lowest singlet excited to the ground state and intersystem crossing from the excited singlet state to the triplet state. This last process leads to phosphorescence.
The Jablonski diagram in figure 1 is extremely important to understand for any photoluminescence spectroscopist. When measuring a photoluminescence spectrum, we typically look at the intensity of the emission, its wavelength or energy, and the time over which the emission occurs. The latter is the photoluminescence lifetime. Any number of things can affect these PL observables, including energy transfer to and from other molecules, quenching by other molecules, temperature, pH, local polarity, aggregation or binding.
Understanding the mechanisms of these interactions can provide insight into what is being observed with a change in photoluminescence spectra and its associated observables.
Polish physicist Aleksander Jabłoński, regarded as the father of fluorescence spectroscopy, developed the Jablonski Diagram.
He devoted his life to the study of molecular absorbance and emission of light. His doctoral thesis, “On the influence of the change of wavelengths of excitation light on the fluorescence spectra,” showed that the fluorescence spectrum is independent to the wavelength of the excitation light.
Jablonski advanced our knowledge of fluorescence polarization in solutions, quenching, and his namesake diagram, explaining the spectra and kinetics of fluorescence, delayed fluorescence and phosphorescence.
The Jablonski Diagram is also known as the Perrin-Jablonski diagram in recognition of the contributions of French physicists Jean Baptist Perrin, and his son Francis Perrin
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