Kasha's Rule: Statement, Explanation and Exceptions
Kasha's rule states that the photon emission (fluorescence or phosphorescence) occurs with appreciable yield only from the lowest excited state of a given spin multiplicity. It is named after American spectroscopist Michael Kasha, who proposed it in 1950.
Also read Jablonski Diagram and Energy Dissipation: Radiative and Non-radiative Process
Explanation
When a molecule absorbs light, it becomes excited and jumps to various higher energy levels. However, it loses energy very quickly through non-radiative processes (like vibrations and internal conversions) and fall down to the lowest excited singlet state (S1) for fluorescence or the lowest excited triplet state (T1) for phosphorescence. Light emission (fluorescence or phosphorescence) will then predominantly occur from these lowest excited states to the ground state (So).
Exceptions to Kasha's Rule (Anti-Kasha Emission)
While Kasha's rule holds good for most molecules, there are exceptions too. These exceptions arises when there are large energy gaps between excited states, slowing down the internal conversion process and allowing emission from a higher excited state to compete (Anti-Kasha Emission). Azulene is a classic example which exhibits fluorescence from the second excited singlet state (S2). In 2023, an explanation was proposed which pointed out that the S1 excited state has antiaromatic character while the S2 excited state is aromatic.
Thioketones, certain cyclic triimidazoles like benzoimidazo[1,2-a]benzoimidazo[1,2-c]benzoimidazo[1,2-e]triazine, 1,2-diphenylphenanthroimidazole (PPI) derivatives, and thiophene-based molecules also exhibit anti-Kasha emission. Benzopyrene and other molecules can also exhibit anti-Kasha emission under specific conditions.
Factors Contributing to Anti-Kasha Emission
There are several factors that contribute to the occurrence of Anti-Kasha Emission, some of them are given below-
Large Energy Gaps: A large energy gap between the S2 and S1 states can slow down the internal conversion process from S2 to S1, and allow the emission from S2.
Strong Electronic-Vibrational Coupling: Strong electronic-vibrational coupling can facilitate S2 → So emission.
S2 Emission Probability: In some cases, the radiative emission rate from S2 to So is higher than the radiative emission rate from S1 to So, leading to S2 emission.
Vavilov's Rule
The Vavilov's rule states that the quantum yield of photoluminescence (PLQY) is independent of the wavelength of exciting radiation. This is related to Kasha's rule because molecules tend to relax into the lowest excited state non-radiatively. No matter the excitation wavelength (as long as it has enough energy to excite the molecule), photon emission will always only occur from the same electronic state. As a result, the efficiency of the radiative process (PLQY) remains constant, even though the initial excitation state may vary.