Deactivation Processes: Jablonski Diagram

A Jablonski diagram is a graphical representation of molecular energy levels and transitions during luminescence. It illustrates the ground singlet state (S0) and excited electronic states, including first and second singlet states (S1, S2) and the first triplet state (T1), along with their vibrational energy levels. This diagram visualizes how molecules absorb photons and return to lower energy states through various radiative and radiationless deactivation pathways.

Vibrational relaxation is a radiationless process where excited molecules lose energy by moving to lower vibrational levels within the same electronic state. This causes the fluorescence emission band to shift toward lower frequencies or longer wavelengths compared to the absorption band, a phenomenon called Stokes shift. The energy difference between absorption and emission reflects the vibrational energy lost during relaxation.

Internal conversion involves transitions between electronic states of the same multiplicity, such as singlet-to-singlet or triplet-to-triplet transitions, without photon emission. Intersystem crossing, by contrast, is a crossover between states of different multiplicity, such as singlet-to-triplet transitions. Both are radiationless processes enhanced when vibrational energy levels of the two states overlap, and intersystem crossing is particularly common in molecules containing heavy atoms like iodine or bromine.

Whether a molecule exhibits fluorescence depends on the relative rates of competing deactivation pathways. If fluorescence emission is rapid compared to radiationless processes like internal conversion or intersystem crossing, fluorescence is observed. Conversely, if a radiationless deactivation pathway has a more favorable rate constant, fluorescence is either absent or less intense. The favored route minimizes the lifetime of the excited state.

The primary radiationless deactivation processes are vibrational relaxation, internal conversion, external conversion, and intersystem crossing. Vibrational relaxation occurs within the same electronic state. Internal conversion and external conversion transfer energy between electronic states without photon emission, with external conversion transferring energy to the surrounding solvent or sample matrix. Intersystem crossing involves transitions between singlet and triplet states of different multiplicities.

Singlet states (S0, S1, S2) have paired electrons with opposite spins, while triplet states (T1) have unpaired electrons with parallel spins. Transitions between singlet and triplet states involve spin flips and occur through intersystem crossing, a radiationless process. Transitions within the same multiplicity, such as singlet-to-singlet or triplet-to-triplet, do not require spin flips and occur more readily through internal conversion or vibrational relaxation.

Intersystem crossing is most common in molecules containing heavy atoms such as iodine or bromine due to increased spin and orbital interactions. These interactions enhance the probability of transitions between electronic states of different multiplicity, such as singlet-to-triplet crossovers. The presence of heavy atoms effectively increases the rate of intersystem crossing, making it a more competitive deactivation pathway compared to fluorescence emission.