Examples of Free Volume Probes

Microviscosity has been a long-standing subject in the characterization of biological materials such as membranes 54 but has also been discussed in connection with 

Various photochemical reaction mechanisms lend themselves for variation of the above parameters, and the more important ones are summarized above. In general, one can expect for the volume necessary for the reaction to occur that it decreases in the sense Excimer TICT Butterfly ESIPT mechanism. Therefore, for a given size distribution of microscopic free volume voids, the fraction of the total free volume usable for the reaction and thus amenable to probing increases in the same sense with the ESIPT mechanism being the outstanding extreme, because this reaction cannot even be stopped in a rigid matrix at very low temperature. 39  

In a totally independent approach, highly fluorescent but rigid molecules can be used and their reorientational movement probed in solution, mostly by time-resolved fluorescence anisotropy, (85) For rodlike molecules likep-oligo-phenyls, this movement corresponds to the reorientation of the long molecular axis the length of which can be easily varied. 86 The use of fluorescence probes in liquid and solid media and on surfaces has been treated recently in several reviews and books. 87  

In the following, we will concentrate on the probes involving a photochemical reaction and give some examples of how some of the mechanisms have been used in developing free volume fluorescence probes. 

---