Kasha exceptions

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... 

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. 

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. 

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... 

At the end of the 1950 s, Crosby and Kasha reported the rather exceptional case of near-infrared luminescence of trivalent ytterbium ion in an 1 3 (Ln L) chelate occurring after intramolecular energy transfer between the organic ligand, in this case dbm (48b), and the lan-... 

The lack of many exceptions to Kasha s rule subsequently led to the erroneous inference that no other upper excited state process, be it photophysical or photochemical, intra- or intermolecular, could efficiently compete with internal conversion. However, with the increasingly common use of pulsed lasers in photochemical studies over the past 20 years, it has been possible to show that the upper excited states of many systems have a rich and varied chemistry. Up to this... 

The best-known exception to Kasha s rule is the anomalous fluorescence displayed by azulene and its derivatives (nonaltemant hydrocarbons) and some aliphatic and aromatic thioketones. 

Kasha rule Polyatomic molecular entities luminesce with appreciable yield only from the lowest excited state of a given multiplicity. There are exceptions to this rule. 

The quantum yield is the fraction of absorbed radiation that results in photoreaction. According to the Kasha-Vavilov law, the quantum yield for a photoreaction in solution in which only a single substance is the chromophore (e.g., direct photoreactions) is generally wavelength-independent, although there are some exceptions. With indirect photoreactions in natural waters, a mixture of chromophores is involved. Thus, the apparent quantum yield for an indirect photoreaction in a natural water sample usually is significantly wavelength-dependent. 

If an excited state higher than Si is produced, it will ordinarily obey KashaRule [7] and relax rapidly to Si. From there it can either fluoresce to Sq or relax non-radiatively by internal conversion (1C) to vibrationally excited Sq the excess vibrational energy is lost rapidly in solution - less so in the dilute gas phase. Kasha s Rule finds expression in Fig. 10.1 by the absence of all processes originating in S2 except internal conversion to Si. 

The selection rules for intersystem crossing (ISC) were given in Section 9.2. If a higher triplet is produced, it too will obey Kasha s Rule and relax to Ti. Since its two modes of relaxation to So, phosporescence and ISC, both involve spin-inversion, Ti will be comparatively long-lived. Except when trapped at very low temperatures, there is ample time for the three components of the triplet T , and to reach equilibrium before it phosphoresces, relaxes by ISC or reacts chemically. 

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