The Jablonski diagram is a powerful visual tool used in photochemistry and spectroscopy to map the journey of energy within a molecule after it absorbs light. Named for Polish physicist Aleksander Jabłoński, the diagram illustrates the electronic states of a molecule and the various pathways it can take to dissipate the absorbed energy. This schematic helps explain light-induced phenomena, particularly the difference between the rapid glow of fluorescence and the delayed light of phosphorescence. Understanding the diagram provides a framework for how molecules absorb photons and then relax back to their original state, either by emitting light or releasing heat.
The Visual Framework: Electronic States and Vibrational Levels
The Jablonski diagram uses a vertical axis to represent increasing energy levels. Electronic states are grouped horizontally based on their spin multiplicity, which describes the alignment of electron spins. The ground state, where the molecule begins, is the singlet ground state (S₀), where all electron spins are paired.
Excited states are categorized as singlet states (S₁, S₂, etc.) or triplet states (T₁, T₂, etc.). In singlet excited states, the excited electron maintains its paired spin orientation. Triplet states are characterized by two unpaired electrons with parallel spins, resulting from a spin flip. Triplet states are typically offset horizontally to denote this difference in spin multiplicity.
Each electronic state is represented by a thick horizontal line containing a ladder of closely spaced, thinner lines above it. These smaller lines represent the vibrational sub-levels, which correspond to the energy associated with the stretching and bending motions of the molecule’s atoms. This layered structure allows the diagram to represent both the electronic and vibrational components of a molecule’s total energy.
Excitation and Prompt Light Emission (Fluorescence)
The process begins with the molecule absorbing a photon of light, a transition represented by a straight, vertical arrow on the diagram. This absorption of energy is rapid, occurring on the femtosecond timescale, and instantly promotes an electron from the S₀ ground state to a higher energy level, often to an excited vibrational level of a singlet state like S₁ or S₂. Since the electronic transition occurs much faster than the molecule’s nuclei can rearrange, the transition is drawn vertically, adhering to the Franck-Condon principle.
Once in a higher excited singlet state (S₂ or above), the molecule rapidly loses excess energy through internal conversion (IC) or vibrational relaxation (VR) until it reaches the lowest vibrational level of the S₁ state. Fluorescence is the subsequent radiative transition, where the electron returns from the lowest S₁ level directly to the S₀ ground state, emitting a photon of light. This is a spin-allowed transition, meaning it is highly probable and therefore very fast, typically occurring within nanoseconds.
The emitted photon of fluorescence always has less energy and a longer wavelength than the photon that was originally absorbed. This difference is known as the Stokes shift, and it occurs because the molecule loses a small amount of energy as heat during the rapid vibrational relaxation steps before the light is emitted.
Non-Radiative Energy Loss and Spin Changes
Before light can be emitted, the excited molecule has several ways to lose its energy without generating photons, which are known as non-radiative transitions. Vibrational relaxation (VR) is the quickest of these, involving the dissipation of vibrational energy as heat to the surrounding environment. Internal conversion (IC) is a similar non-radiative process where the electron transitions from a higher singlet state (like S₂) to a lower singlet state (S₁), without changing its spin.
A significant non-radiative pathway is intersystem crossing (ISC), which involves a change in the electron’s spin state. ISC is the transition from an excited singlet state (S₁) to a triplet state (T₁), where the electron’s spin flips, moving the molecule into a state with a different spin multiplicity. This transition is crucial because it traps the electron in the T₁ state, a lower energy level that is spin-forbidden from directly returning to the S₀ ground state. Once the molecule has undergone ISC to the T₁ state, it quickly loses any excess vibrational energy through vibrational relaxation, settling at the lowest energy level of the triplet state.
Delayed Light Emission (Phosphorescence)
The triplet state (T₁) serves as a temporary energy trap because the transition back to the singlet ground state (S₀) is spin-forbidden. This means the return journey is highly improbable and occurs at a much slower rate than the direct transition seen in fluorescence. The excited molecule must wait for a rare event, a second spin flip, to allow it to return to the S₀ state.
When the molecule finally transitions from the T₁ state to the S₀ ground state, it emits a photon in a process called phosphorescence. Because the transition is slow, the excited-state lifetime is significantly longer than fluorescence, ranging from microseconds to minutes or even hours. This is why phosphorescent materials appear to glow in the dark long after the excitation source is removed.
The energy of the phosphorescence photon is also lower than the fluorescence photon, as the T₁ state is always slightly lower in energy than the S₁ state from which fluorescence occurs. The Jablonski diagram thus connects the initial absorption of light through a series of energy dissipation steps, showing how molecular structure dictates whether a molecule will quickly fluoresce, slowly phosphoresce, or simply release all of its absorbed energy as heat.