Kasha's rule in photochemistry states that the quantum yield of luminescence is independent of the wavelength of exciting radiation, implying that emission (fluorescence or phosphorescence) generally occurs from the lowest excited state of a given multiplicity.
Elaboration on Kasha's Rule
Kasha's rule is a fundamental principle in photochemistry that helps explain how molecules emit light after absorbing energy. Here's a breakdown:
- Excited State: When a molecule absorbs a photon of light, it transitions to a higher energy level, known as an excited state. There are many vibrational energy levels within each electronic energy level.
- Vibrational Relaxation: Very quickly (picoseconds or less), the molecule undergoes internal conversion and vibrational relaxation within the excited state manifold. It loses energy in non-radiative ways, ultimately reaching the lowest vibrational level of the lowest excited electronic state of a given spin multiplicity.
- Emission from the Lowest Excited State: Luminescence (fluorescence or phosphorescence) then occurs from this lowest energy excited state. This means the color and intensity of the emitted light are independent of the initial excitation wavelength.
Implications
- Simplicity: Kasha's rule simplifies the analysis of photoluminescence, as it suggests that only one excited state needs to be considered for emission.
- Independence from Excitation Wavelength: Regardless of the wavelength of light used to excite the molecule, the emission spectrum will be characteristic of the lowest excited state.
- Efficiency: The quantum yield (the ratio of emitted photons to absorbed photons) is determined by the properties of the lowest excited state and the competing non-radiative processes.
Exceptions
While Kasha's rule is broadly applicable, there are exceptions:
- Azulene: This molecule is a notable exception. It exhibits fluorescence from higher excited states.
- Specific Molecular Structures: Certain molecules with specific structural features or strong coupling between electronic states may violate Kasha's rule.
- Excimer/Exciplex Formation: Emission from excimers (excited dimers) or exciplexes (excited complexes) involves different mechanisms and energy levels.
Example
Imagine a molecule absorbing blue light and transitioning to a high-energy excited state. It will rapidly lose energy through vibrational relaxation and internal conversion to reach the lowest vibrational level of its lowest excited state (S1). Then, it will emit light (fluorescence) from this S1 state. If you had instead excited the molecule with UV light to an even higher excited state, the molecule would still relax down to the same S1 state and emit the same fluorescence spectrum.
