Circular polarization of luminescence: biochemical and biophysical applications

The circular polarization of the luminescence of a chromophore is one of the manifestations of its chirality. As such, CPL has the potential of probing molecular conformation, which has a tight relationship to molecular chirality. CPL has characteristic features that make it specifically useful as a tool for the investigation of conformational problems under the proper circumstances: it is related to the molecular conformation in the electronically excited state; it is specific to the luminescent chromophores when different kinds are present in the system studied; the number of electronic transitions involved are relatively few in number, often one per chromophore, thus simplifying the interpretation of the spectra; forbidden transitions are amenable to study by CPL; and CPL permits the study of the optical activity of oriented systems by simple means. The systems tackled by CPL range from small to giant molecules, which illustrates its wide applicability. Naturally, like any other research tool, CPL has limitations as to the questions to which it can be addressed and the systems that can benefit from its services (e.g. they should not be photosensitive; they should, of course, be luminescent; and they should yield measurable signals). However, for the proper questions and suitable systems it has been found to be of tremendous help.