Methods based on intramolecular excimer formation

In contrast to intermolecular excimer formation, this process is not translational but requires close approach of the two moieties through internal rotations during the lifetime of the excited state. Information on fluidity is thus obtained without the difficulty of possible perturbation of the diffusion process by microheterogeneity of the medium, as mentioned above for intermolecular excimer formation. 

Moreover, the efEcienqf of excimer formation does not depend on the concentration of fluorophores, so that Eq. (8.21) should be rewritten as 

As in the case of intermolecular excimer formation, it should be recalled that difficulties may arise from the possible temperature dependence of the excimer lifetime, when effects of temperature on fluidity are investigated. It is then recommended that time-resolved fluorescence experiments are performed. The relevant equations established in Chapter 4 (Eqs 4.43-4.47) must be used after replacing ki[M] by ki. 

The viscosity dependence of intramolecular excimer formation is complex. As in the case of molecular rotors (Section 8.2), most of the experimental observations can be interpreted in terms of free volume. However, compared to molecular rotors, the free volume fraction measured by intramolecular excimers is smaller. The volume swept out during the conformational change required for excimer formation is in fact larger, and consequently these probes do not respond in frozen media or polymers below the glass transition temperature. 

Using again the Doolittle equation (8.13) and assuming that the rate constant for excimer formation is given by 

Molecular mobility in polymer films and bulk polymers can also be probed by excimer formation of pyrene (Chu and Thomas, 1990). 

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