5.2 Jablonski diagram and state transitions
2 min read•july 24, 2024
The is a visual tool that maps out energy levels and transitions in molecules. It shows ground states, excited states, and the various ways molecules can absorb, emit, or transfer energy through processes like fluorescence and phosphorescence.
Understanding these energy pathways is crucial for grasping photophysical processes. From absorption to emission, the Jablonski diagram helps predict how molecules will behave when exposed to light, which is essential for applications in spectroscopy, photochemistry, and beyond.
Understanding the Jablonski Diagram
Components of Jablonski diagram
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- Jablonski diagram structure depicts energy levels on vertical axis and spin multiplicity on horizontal axis
- Energy states include (S0), excited singlet states (S1, S2) and excited triplet states (T1, T2)
- Vibrational levels within each electronic state shown as closely spaced horizontal lines
- Radiative transitions represented by straight arrows indicate emission of photons
- Non-radiative transitions depicted by wavy arrows show energy dissipation without photon emission
Electronic states and transitions
- Singlet states (S) feature paired electron spins in orbitals (S0, S1, S2)
- Triplet states (T) have unpaired electron spins in separate orbitals (T1, T2)
- Transitions between states include:
- Absorption: ground to jump
- : relaxation between states of same multiplicity
- Intersystem crossing: transition between states of different multiplicity
- Fluorescence: excited singlet to ground state emission
- Phosphorescence: excited triplet to ground state emission
Photophysical Processes
Processes in photophysics
- Absorption occurs when molecule gains energy from incident photon, transitioning from ground to excited state in femtoseconds
- Fluorescence involves from excited singlet to ground state, emitting photon with typical nanosecond lifetime
- Phosphorescence consists of radiative transition from excited triplet to ground state, emitting photon with longer lifetime (milliseconds to seconds)
- Intersystem crossing entails non-radiative transition between states of different multiplicity, usually from excited singlet to triplet state, involving spin flip of an electron
Applications of Jablonski diagram
- measures difference between absorption and emission wavelengths, caused by vibrational relaxation and solvent effects
- calculates ratio of emitted to absorbed photons, affected by competing non-radiative processes
- Fluorescence quenching decreases fluorescence intensity due to external factors (collisions with other molecules, energy transfer)
- Energy transfer processes include Förster resonance energy transfer (FRET) and Dexter electron transfer
- Photochemical reactions initiate from excited states, leading to isomerization, dissociation, or other chemical changes (photosynthesis, vision)
- Predicting dominant relaxation pathways based on molecular structure and environment considers , energy gaps, and selection rules
Key Terms to Review (11)
Absorption Spectrum: An absorption spectrum is a spectrum that displays the wavelengths of light absorbed by a substance, providing insight into its electronic structure and the energy levels of its electrons. This concept connects to various principles such as how different substances absorb specific wavelengths due to their unique electronic configurations, the nature of electronic transitions when photons are absorbed, and the principles of absorption spectroscopy used to quantify concentrations in solutions.
Excited State: An excited state refers to a higher energy state of an atom or molecule that occurs when it absorbs energy, causing one or more electrons to move to a higher energy level. This state is crucial for understanding processes like absorption and emission of light, as well as various electronic transitions within molecules.
Ground state: The ground state refers to the lowest energy level of an atom or molecule, where its electrons are in their most stable configuration. In this state, the system possesses the minimum amount of energy necessary to maintain its structure, making it crucial for understanding how atoms and molecules interact with light and other forms of energy.
Internal Conversion: Internal conversion is a non-radiative process in which an excited molecule transitions to a lower electronic state without emitting a photon. This process is crucial for understanding how excited states relax back to their ground states, influencing various phenomena like fluorescence and phosphorescence, and impacting excited state dynamics in photochemistry.
Jablonski diagram: A Jablonski diagram is a graphical representation that illustrates the electronic states of a molecule and the transitions between these states, particularly during the absorption and emission of light. It is crucial for understanding processes like fluorescence and phosphorescence, as it highlights the different energy levels and the various pathways for relaxation from excited states to ground states.
Quantum Yield: Quantum yield is a measure of the efficiency of a photochemical process, defined as the ratio of the number of events (like the formation of a product) to the number of photons absorbed. This concept is crucial in understanding how light interacts with matter, as it helps quantify how effectively light energy is converted into chemical energy or emitted as light, linking absorption and emission phenomena.
Radiative Transition: A radiative transition refers to the process by which an excited electron in an atom or molecule returns to a lower energy state, emitting a photon in the form of electromagnetic radiation. This phenomenon is essential for understanding how substances absorb and emit light, playing a key role in various spectroscopic techniques and interactions. Radiative transitions can lead to observable effects such as fluorescence and phosphorescence, highlighting the relationship between energy levels and light emission.
Singlet to triplet transition: A singlet to triplet transition is a type of electronic transition in which an excited electron in a molecule moves from a singlet state, where all electrons are paired, to a triplet state, where two electrons have parallel spins. This transition is significant because it involves a change in the spin multiplicity of the molecule, which impacts its photochemical behavior and reactivity. Understanding this transition is key to interpreting the Jablonski diagram and the various energy states involved in light absorption and emission.
Spin-orbit coupling: Spin-orbit coupling is an interaction between the intrinsic spin of particles and their orbital motion around a nucleus, which significantly influences the electronic structure and energy levels of atoms and molecules. This effect is crucial for understanding the selection rules, transition probabilities, and state transitions in photochemical processes. By altering the allowed transitions between electronic states, spin-orbit coupling affects internal conversion and intersystem crossing, impacting how molecules absorb and emit light.
Stokes Shift: Stokes Shift refers to the difference in wavelength (or energy) between the positions of the peak of the absorption spectrum and the peak of the emission spectrum of a substance. This phenomenon occurs due to the loss of energy in non-radiative processes and has significant implications in understanding molecular transitions, fluorescence, and phosphorescence.
Vibronic Coupling: Vibronic coupling refers to the interaction between electronic states and vibrational modes in a molecule, leading to mixing of electronic and vibrational wave functions. This coupling plays a crucial role in understanding how molecules transition between different energy states, particularly when considering selection rules, transition probabilities, and the behavior of excited states during processes like internal conversion and intersystem crossing.