Time resolved luminescence measurements on polydiacetylenes : an ordered and isolated one dimensional model system

In this work time resolved luminescence measurements on the red phase of 3BCMU are presented. These polydiacetylenes form an ordered and isolated one dimensional model system. This work is divided in two parts. First we present an excitation spectrum in chapter 4. In the second part, radiative lifetimes are examined at temperatures down to 2 K (chapter 5). We present studies on excitation spectra on the red phase of 3BCMU up to an excitation energy of 850 meV above the zero phonon excitonic line. The vibronic modes of the double and triple bond of the carbon of the backbone and their combinations are identified. Compared to luminescence spectra a small anharmonicity is found. Also a relatively small Huang-Rhys factor is obtained, a parameter describing the decrease in intensity of the successive multiple of a vibronic mode. This is a result expected for such a complex system as a polymer. Furthermore, any signature of the band edge is absent in the spectrum. This result is interesting, as the theory predicts a suppression of the van Hove singularity at the band edge. Experimental results on other one-dimensional systems on this subject are rare. The radiative lifetime is a parameter which is dependent on the dimensionality of the structure. In the special case of one-dimensional systems it is predicted to be proportional to the square root of the temperature. Measurements of other one-dimensional systems show that the law is broken at relatively high temperatures (around 30 K) because of fluctuation of the confinement potential, which leads to localisation. For our sample, the radiative lifetime follows the mentioned law to a temperature down to 2 K. However, we show that in samples of less quality, mainly due to ageing, the law is not valid anymore. This ageing of the sample is caused by a higher polymerisation rate and defect in the crystalline structure.