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Jablonski diagram including vibrational levels for absorbance, non-radiative decay, and fluorescence. When a molecule absorbs a photon, the photon energy is converted and increases the molecule's internal energy level. Likewise, when an excited molecule releases energy, it can do so in the form of a photon.
Thus, triplet states generally have longer lifetimes than singlet states. These transitions are usually summarized in a state energy diagram or Jablonski diagram, the paradigm of molecular photochemistry. These excited species, either S 1 or T 1, have a half-empty low-energy orbital, and are consequently more oxidizing than the ground state.
Jablonski diagram indicating intersystem crossing (left) and internal conversion (right). Internal conversion is a transition from a higher to a lower electronic state in a molecule or atom. [1] It is sometimes called "radiationless de-excitation", because no photons are emitted.
A Jablonski diagram describing the mechanism of triplet-triplet annihilation. The energy of the first triplet excited state (T 1) is transferred to a second triplet excited state (T 1), resulting in (1) the first T 1 returning to the singlet ground state S0 and (2) the second T 1 promoting to the singlet excited state (S 1).
Jablonski diagram illustrating the electronic states accessible during photoexcitation. Note: ISC stands for Intersystem Crossing. E 0,0 is a measurement of the energy gap between the ground state and the lowest energy triplet state. This parameter is proportional to the phosphorescence wavelength and is used to compute the redox potentials of ...
Bottom: Jablonski diagram representing the aqueous quenching of [Ru(phen) 2 dppz] 2+ emission, depicting bright states and dark states. Alternatively, photoswitches may themselves be emissive and exhibit environmental control over their properties. One such example is a class of ruthenium polypyridyl coordination complexes.
The former is typically a fast process, yet some amount of the original energy is dissipated so that re-emitted light photons will have lower energy than did the absorbed excitation photons. The re-emitted photon in this case is said to be red shifted, referring to the reduced energy it carries following this loss (as the Jablonski diagram shows).
Jablonski diagram showing the redshift of the stimulated photon. This redshift allows the stimulated photon to be ignored. Diagram of the design of a STED device. The double laser design allows for excitation and stimulated emission to be used together for STED.