Kasha s rule

Kasha s rule and the rate constants discussed are applicable only for molecules in solution. In the gas phase, where collisions are few, transitions from vibrationally hot states and from electronic states other than the lowest state in each manifold are common. 

After a very short period of time, 10-11—10-12 sec, crossing from higher to lower excited states 36> (internal conversion) and thermal equilibration 6 ) will bring the molecule to one or another of the numerous minima in its first excited state hypersurface. If the initial excitation was into the triplet manifold, this will typically be Ti if it was into the singlet manifold, it will typically be Si (Kasha s rule 31>), exceptionally S2 if the internal conversion to Si is slow (azulene 48>68>), or Ti if intersystem crossing into the triplet manifold proceeds unusually fast and is able to compete with the relaxation to Si (particularly in the presence of heavy... 

The much larger energy difference between Si and S0 than between any successive excited states means that, generally speaking, internal conversion between Si and S0 occurs more slowly than that between excited states. Therefore, irrespective of which upper excited state is initially produced by photon absorption, rapid internal conversion and vibrational relaxation processes mean that the excited-state molecule quickly relaxes to the Si(v0) state from which fluorescence and intersystem crossing compete effectively with internal conversion from Si. This is the basis of Kasha s rule, which states that because of the very rapid rate of deactivation to the lowest vibrational level of Si (or Td, luminescence emission and chemical reaction by excited molecules will always originate from the lowest vibrational level of Si or T ... 

We saw in the last section that because of the rapid nature of vibrational relaxation and internal conversion between excited states an electronically-excited molecule will usually relax to the lowest vibrational level of the lowest excited singlet state. It is from the Si(v = 0) state that any subsequent photophysical or photochemical changes will generally occur (Kasha s rule). 

Explain the basis of Kasha s rule and give an example of where this does not apply. 

According to Kasha s rule, fluorescence from organic compounds usually originates from the lowest vibrational level of the lowest excited singlet state (Si). An exception to Kasha s rule is the hydrocarbon azulene (2) (Figure 4.5), which shows fluorescence from S2. 

When the excited triplet state is populated, rapid vibrational relaxation and possibly internal conversion may occur (if intersystem crossing takes place to an excited triplet of greater energy than Ti). Thus the excited molecule will relax to the lowest vibrational level of the Ti state, from where phosphorescence emission can occur in compliance with Kasha s rule. 

The probability of intramolecular energy transfer between two electronic states is inversely proportional to the energy gap, AE, between the two states. The value of the rate constant for radiationless transitions decreases with the size of the energy gap between the initial and final electronic states involved. This law readily provides us with a simple explanation of Kasha s rule and Vavilov s rule. 

Emission from the upper electronic excited states of polyatomic molecules, in violation of Kasha s rule which allows emission only from the lowest excited states Q), have been observed in a reasonable variety of molecules (2 11). With notable exceptions of azulene and thioketone, however, such emission is usually very weak, because the rates of nonradiative decay processes greatly exceed the rates of radiative processes when excited states other than the lowest excited states are involved. 

These discussions provide an explanation for the fact that fluorescence emission is normally observed from the zero vibrational level of the first excited state of a molecule (Kasha s rule). The photochemical behaviour of polyatomic molecules is almost always decided by the chemical properties of their first excited state. Azulenes and substituted azulenes are some important exceptions to this rule observed so far. The fluorescence from azulene originates from S2 state and is the mirror image of S2 S0 transition in absorption. It appears that in this molecule, S1 - S0 absorption energy is lost in a time less than the fluorescence lifetime, whereas certain restrictions are imposed for S2 -> S0 nonradiative transitions. In azulene, the energy gap AE, between S2 and St is large compared with that between S2 and S0. The small value of AE facilitates radiationless conversion from 5, but that from S2 cannot compete with fluorescence emission. Recently, more sensitive measurement techniques such as picosecond flash fluorimetry have led to the observation of S - - S0 fluorescence also. The emission is extremely weak. Higher energy states of some other molecules have been observed to emit very weak fluorescence. The effect is controlled by the relative rate constants of the photophysical processes. 

Fluorescence always occurs from the lowest singlet state even if the initial excitation is to higher energy state (Kasha s rule). Azulene and some of its derivatives are exceptions to this rule. Because of vibrational relaxation of initially excited vibronic state, the fluorescence spectrum may appear as a minor image of the absorption spectrum for large polyatomic molecules. The shape of the emission spectrum is independent of the exciting wavelength. 

Emission from transition metal complexes obey Kasha s rule and originate from the lowest excited state which are (i) 3(n, n ) state in [Rh (phen)3] (C104)3 in water-methanol glass (ii) d, iz ) state in [Ru (bpy)J Clj in ethanol-methanol glass, and (iii) d d) state, in solid [RhClj (phen)J Cl. Their characteristics differ in details and are given in Figure 8.16-Sometimes weak fluorescence is also observed, from Cr + complexes. K [Co(CN)t] is highly luminescent. The 4>p and 7p are temperature... 

Azobenzene photoisomerization Two states and two relaxation pathways explain the violation of Kasha s rule. 

Figure 3.29 (a) Outline of the absorption, A fluorescence, F and phosphorescence, P spectra of a rigid polyatomic molecule. X = wavelength, vertical axis = absorbance (A) or emission intensity (F, P). (b) The Stokes shift of the absorption and fluorescence spectra is defined as the difference between their maxima. When this shift is small, there is a substantial spectral overlap between absorption and emission, (c) Jablonski diagram and outline of the absorption and fluorescence spectra of azulene, an exception to Kasha s rule. The energy gap between S0 and Sj is very small, that between Sj and S2 is very large... 

Figure 1. Jablonski diagram for a four-level system depicting absorption, non-radiative (wavy arrows) and radiative processes between singlet (total spin S = 0) and triplet (total spin S = 1) states. Emissions respect Kasha s rule. IC internal conversion. ISC intersystem crossing.

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