Bimolecular Photophysical Processes

Finally, it is worth repeating that following absorption of a photon, many processes are possible. What we observe in the lab is a result of a competition of rates for these many processes. We can enhance the probability of observing a given process by either increasing its intrinsic rate constant, or by decreasing the rate constant of a competing process. 

Exciplexes are complexes of two different molecules usually of 1 1 stoichiometry. Their fluorescence phenomena are similar to those described for excimers, but their formation is not restricted to aromatic systems. If the sum of effective rate constants of the non radiative processes is so high such that the lifetime of emission is undetectable, these molecules do not necessarily luminesce. In contrast to the excimers, which are non polar, the exciplexes are polar entities. It was shown by Beens and co-workers [13] that exciplexes from aromatic hydrocarbons and aromatic tertiary amines demonstrate the charge-transfer character of the complexes as reported by Knibbe and co-workers [14], and their dipole moments were greater than 3.3 x 10 C m (10 D). 

Birks [1] and Klessinger and Michl [2] have given a more detailed discussion of the photophysics of organic molecules. 

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We are now ready to consider more complex processes induced by the absorption of light. Before considering photochemical reactions, however, we will address the several things that can happen when a molecule in an excited electronic state encounters another molecule that is in its ground state, and related phenomena. These bimolecular photophysical processes are interesting and sometimes useful, and they provide further insights into the nature of the excited state. 

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