Electronic Excitations and Energy Transfer in PCSs

PCSs are systems of chromophores bound into a single macromolecule. Therefore, the study of processes of electronic excitation and energy transfer, as well as the investigation of the ways of deactivation of excited states, should lay a foundation for the understanding of such properties of PCSs as reactivity in photochemical transformations, photosensitizing and photoelectric activity, photoinitiated paramagnetism, etc. 

We have found for polypropynoic acid that this series of polymers reveals selective fluorescence spectra together with nonselective absorption. To account for this phenomenon, a scheme was proposed according to which PCSs are characterized by energy transfer from excited levels of some conjugation sections to the lower levels of other sections, followed by luminescence from the latter40 41,246,248,249,253.  

Fluorescence and phosphorescence spectra of poly(propynoic acid)(FPA), polyphenylene (PP), and DPAcN show that the difference of energies between the lower excited singlet and triplet states, as observed in the case of PP (583 nm) and DPAcN (528 nm), is considerably greater than that of poly(propynoic acid) (270—300 nm) which besides transitions may undergo rr - transitions. PCSs showing only 

These values equal 2.0, 1.05, and 0.5, respectively, for PP, DPAcN, and PPA. It is possible that the contribution of excited states caused by n - it transitions accounts basically for a bathochromic luminescence of some PCSs and for a shift of the maxima in the luminescence spectra of polymers of this kind when proceeding from the solution to the solid phase. PCS solutions reveal concentration-quenching accom- 

A distinctive feature of this type of polymer is that the processes of intermolecu-lar transfer of excitation energy are also hindered to a considerable degree since they become noticeable only at very high (of the order of 10 3 mol/1) concentrations of 

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