The Behaviour of Excited Molecules — Chemical Processes

Exciplexesn6,nl) can be formed if the excitation energy B - B is higher than the one for A - A in (1.8). Such an excited complex is associative in the excited state only, the corresponding ground state complex between A and B being dissociative (Fig. 9). Such exciplexes are important intermediates in e.g. cycloaddition reactions as precursors of diradicals 118) which are themselves precursors of the cyclized photoproducts. 

Many different examples of preparative useful photochemical reactions are going to be discussed in the following chapters. Nevertheless the mechanistic possibilities for an excited molecule to undergo chemical reactions are rather limited. One way of classifying photochemical reactions consists in 

For a molecule AB the primary monomolecular photochemical processes which can occur are  

This isomerization can lead to a molecule with different constitution, configuration or conformation, or to a valence isomer. 

In the presence of a proper second molecule bimolecular photochemical processes occur. Obviously such reactions can also occur in an intramolecular fashion in bifunctional molecules containing both reactive centres. These reactions are a) hydrogen abstraction by the excited molecule if the second molecule (or reactive centre) is a hydrogen donor RH (1.14) b) photodimerisation (1.15) c) photoaddition or photocycloaddition (1.16) d) electron transfer (1.17), if no bonding takes place between the reactants (or reactive centres).  

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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. 

ABSTRACT. This paper represents recent results on the reaction dynamics of the M + RX MX + R (M = alkali, X = halogen and R = radical group) family obtained from crossed molecular beam studies. The selectivity of the translational excitation of the reactants as weU as of the chemical nature of the M, R and X group is outlined. A comparison of these reactive processes with photofragmentation and electron attachment studies of the same RX molecule is also presented revealing important similarities. This common behaviour seems to indicate ttie same overall selectivity as a result of the similar main dynamics associated with the same R-X bond rupture. 

The molecule in its excited state cannot only be generated by light absorption, but also the excitation energy can be obtained in some chemical, biochemical, electrochemical processes, or by conversion of yet another kind of energy, eg ultrasonic or mechanical. The excited-state behaviour is, however, independent of its origin. 

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