Davidov splitting

Weak coupling leads to minor alterations of the absorption spectrum (hypo-chromism or hyperchromism, Davidov splitting of certain vibronic bands). 

Concentration quenching is not a major problem in semiconducting polymers. Typically, the quantum efficiency for photoluminescence from thin films is comparable to that from dilute solutions. The absence of strong concentration quenching is the result of the spatial delocalization of the excited states. Because the weakly bound excitons are spread over many repeat units, the Davidov splitting that results from interchain interactions is small. As a result, the disorder that is present in films cast from solution is sufficiently large to mix the dark and emissive Davidov-split states. Thus, quantum efficiencies in excess of 60-70% can be obtained from thin solid films of luminescent semiconducting polymers. 

Figure 14 Davidov splitting 2Vij of the two degenerate excited states of two interacting identical chromophores in a compound of Cj symmetry for 0 < Oq.

The Ih symmetry of the 60-atoms molecule allows 2Ag and SH modes to be Raman active and 4T modes to be IR active. The four IR active modes are at 1430, 1185, 580 and 528 cm , respectively. The most important Raman modes are at 1469 (tangential bond alternation or pinch mode, Ag), 495 (radial breathing mode, T ) and 271 (squashing mode, Hg) cm , respectively. In the low temperatur phase degenerate modes split from a crystal field and Davidov interaction. Good reviews on the group theoretical analysis and on the line positions are given in (Dresselhaus et al., 1992 Matus and Kuzmany, 1993). 

As one might expect, weak coupling leads to minor alterations in the absorption spectrum. The vibrational envelope of the single molecule is essentially retained and differences exist in the individual vibronic bands only. In typical examples these show characteristic splitting patterns with intervals of the magnitude 2 US . This is the so-called (vibronic) Davidov... 

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