Jablonski Diagram
Jablonski diagrams describe the electronic energy states of molecules, transitions between them, and their associated light-emitting phenomena.
A Jablonski diagram is a powerful tool for visualizing the possible transitions that can occur after a molecule has been photoexcited. The energy levels of a molecule are shown by the horizontal black lines. The naming of these electronic states is based on the spin angular momentum configuration of each state. Singlet states (having a total spin angular momentum of zero, where electron spins are paired) are denoted by S, and triplet states (having a total spin angular momentum of one, where electron spins are unpaired/parallel) are denoted by T.
- S0 is the singlet ground state of the molecule
- S1 is the first excited singlet state
- T1 is the first excited triplet state
A singlet excited state always possesses higher energy than its corresponding triplet excited state due to electron exchange energy factors. Accordingly, the energy sequence is:
$E_{S_1} > E_{T_1}$
$E_{S_2} > E_{T_2}$
$E_{S_3} > E_{T_3}$
and so on...
Upon absorption of a light photon, an electron within the absorbing molecule may jump from $S_0$ to the $S_1$, $S_2$, or $S_3$ singlet excited states, depending upon the energy of the photon absorbed. For each singlet excited state ($S_1$, $S_2$, $S_3$), there is a corresponding lower-energy triplet excited state ($T_1$, $T_2$, $T_3$). The molecule, whether in a singlet or triplet excited state, is considered "activated."
The activated molecule eventually returns to its ground state by dissipating its excess energy through radiative transitions, non-radiative transitions, or photochemical reactions.
1. Radiative Transitions
These transitions involve the return of the activated molecule from an excited state ($S_1$ or $T_1$) back to the stable ground state ($S_0$), accompanied directly by the emission of radiation.
- Fluorescence: Spectroscopically, the transition from the $S_1$ state down to the $S_0$ state is a spin-allowed transition and occurs extremely quickly, in about $10^{-8}$ seconds. The immediate emission of light during this transition is called fluorescence.
- Phosphorescence: The transition from the $T_1$ state down to the $S_0$ state is highly delayed because it is a quantum mechanically forbidden transition. This transition requires a spin inversion to take place. The delayed emission of light from this pathway is called phosphorescence. The lifetime of phosphorescence is much longer ($10^{-3}$ seconds up to several hours).
Both fluorescence and phosphorescence radiations feature lower frequencies (and longer wavelengths) than the original exciting light. This is because a portion of the absorbed photon energy is initially dissipated as heat during non-radiative steps.
2. Non-Radiative Transitions
These transitions involve the dropping of an activated molecule from higher excited states ($S_3$, $S_2$, or $T_3$, $T_2$) down to the first excited states ($S_1$ or $T_1$) without emitting any photons. Instead, the excess energy is dissipated as heat through molecular collisions:
- Internal Conversion (IC): This is a radiationless transition between states of the same spin multiplicity (e.g., from $S_2 \rightarrow S_1$). It is highly efficient and happens in less than $10^{-11}$ seconds.
- Intersystem Crossing (ISC): This process involves radiationless transitions between states of different spin multiplicities (e.g., from $S_1 \rightarrow T_1$). Although spectroscopically forbidden, ISC occurs efficiently in many molecules, particularly in the presence of heavy atoms or specific impurities.
3. Photochemical Chemical Reactions
The activated molecule may also lose its energy by undergoing a chemical transformation. Because a molecule in the singlet excited state ($S_1$) returns to the ground state incredibly fast, it often has little opportunity to participate in complex chemical reactions.
Conversely, a molecule in the triplet excited state ($T_1$) features a much longer lifetime. This provides ample time for the activated molecule to collide with other reactants and undergo photochemical transformations. Consequently, most organic photochemical reactions proceed via the triplet excited state intermediate.
Read alsoSinglet and Triplte States
Test Your Understanding: Jablonski Diagrams
1. Which of the following statements correctly identifies the relative energy relationship between corresponding singlet and triplet excited states in a molecule?
- (A) Singlet excited states always have lower energy than triplet excited states ($E_{S_1} < E_{T_1}$).
- (B) Singlet excited states always have higher energy than triplet excited states ($E_{S_1} > E_{T_1}$).
- (C) Singlet and triplet states possess identical energy values ($E_{S_1} = E_{T_1}$).
- (D) Triplet states only exist at lower energies than the ground singlet state ($E_{T_1} < E_{S_0}$).
View Answer
Explanation: Due to Hund's rule of maximum multiplicity and electron exchange energy, electrons with parallel spins (triplet state) experience less electrostatic repulsion and are more stable, meaning triplet states sit at a lower energy level than their corresponding paired singlet states.
2. What is the fundamental difference between Internal Conversion (IC) and Intersystem Crossing (ISC) on a Jablonski Diagram?
- (A) IC emits visible light, while ISC produces invisible infrared rays.
- (B) IC is a radiative transition, whereas ISC is a purely chemical reaction step.
- (C) IC occurs between states of the same spin multiplicity, while ISC occurs between states of different spin multiplicities.
- (D) IC requires hours to complete, whereas ISC occurs instantaneously.
View Answer
Explanation: Both are non-radiative transitions. However, Internal Conversion (IC) happens within the same spin system (e.g., $S_2 \rightarrow S_1$), while Intersystem Crossing (ISC) involves a spin flip between different multiplicities (e.g., $S_1 \rightarrow T_1$).
3. Photochemical reactions are most likely to occur from which excited state configuration within the Jablonski matrix, and why?
- (A) The S1 singlet state, because its short lifetime prevents external molecular collisions.
- (B) The S0 ground state, because it contains the maximum concentration of parallel electrons.
- (C) The T1 triplet state, because its longer excited-state lifetime provides sufficient time for chemical processes to occur.
- (D) Higher states like S3, because they bypass all internal conversion pathways entirely.
View Answer
Explanation: Because transitions from $T_1 \rightarrow S_0$ are spin-forbidden, the triplet state has a long lifetime ($10^{-3}$ seconds or more). This provides a large window of opportunity for the activated molecule to encounter other molecules and undergo chemical changes before dropping back to the ground state.