Conjugated polymers electronic spectra

Figure 9 shows the electronic absorption spectrum of a PTTB film which has undergone extensive but incomplete reaction with bromine in a non-in-situ experiment. The absorption spectrum is that expected for a one-dimensional conjugated polymer. The sharpest absorption edge is at about 1490 nm (o.83 eV) and the absorption maximum is located at 1240 nm (1.0 eV). Thus, this material has a bandgap of about 0.83 eV. Note that two small... 

Readily measurable fluorescence intensities are found for molecules having aromatic and heteroaromatic rings, in particular when annulated rings are present, and in the case of conjugated 7x-electron systems. If the polymer molecules contain such fluorescence-active subunits they can be characterized by this technique, either directly via their fluorescence spectrum or via fluorescence quenching experiments (for polymers with appropriate quencher groups). It is... 

In conjugated polymers, the stretching of the coiled polymer chain usually results in a better conjugation of the tt electrons. It is expected that the absorption Amax of the film should be red-shifted as the polymer chains are stretched further. This phenomenon is indeed observed experimentally. Shi et al.17 have observed that the absorption A,max of a spin-coated polymer film obtained from a more dilute solution is significantly red-shifted in comparison to that spun from a more concentrated solution. Fig. 6.6 shows the absorption spectra of two films spun at the same speed (8000 rpm) but at two different concentrations a thinner film (180 X) spun from a 0.3-wt% MEH-PPV solution and a thicker film (900 X) spun from a l-wt% MEH-PPV solution in cyclohexanone (CHO). For easy comparison, the spectrum of the thinner film has been normalized. It can be easily seen from Fig. 6.6 that the absorption of the polymer film spun with the more dilute solution is significantly red-shifted (Amax = 510 nm) as compared to that spun with the more concentrated solution (Amax = 496 nm). This indicates that the polymer chains in the film spun from the more dilute solution are more extended and the n electrons in the polymer backbone are more conjugated. 

If strain is invoked to describe the changes observed in the spectra, it must explain (1) the large blue shift seen in the phenylurethane LT spectra, (2) the additional blue shift with attendant lack of structure observed in the HT spectra of the same series, and (3) the usual reversibility of the LT-HT transition. Since the electronic spectrum arises only from the polymer spine, it is necessary to better define the role of the strain effects on the pi conjugation of the spine. It may prove that not all of these changes, particularly the thermochromic ones, can be attributed to strain. 

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