Luminescence conjugated chains

In the following section, step by step a qualitative picture is formed describing the impact of intcrmolecular interactions oil the absorption and luminescence of organic conjugated chains. The present calculations do not distinguish between dimers and aggregates (for which the wavefunctions of adjacent chains interact in the ground state, due to, for instance, solid-state effects) and excimers (where overlap occurs only upon photoexcitation) [29]. 

PPV oligomers present optical absorption and luminescence spectra whose peaks are redshifted with increasing chain length (Fig. 7). The principal absorption peak position presents an approximately linear dependence with 1/m, where m is the number of carbon atoms in the shortest path between the ends of the conjugated chain [80]. The absorption and luminescence spectra of PPV oligomers in solution show a blueshift when compared to those in... 

The optical properties of polymer conjugated hydrocarbons are of increasing importance due to the possibility to induce an electrically stimulated luminescence. Because the fluorescence is depended from the conjugation chain length the information of absorption dependence from the chain length is necessary. 

For copolymers of structure I, for both types of side-chains, there is a striking similarity with the optical properties of the corresponding models the absorption and photoluminescence maxima of the polymers arc only 0.08-0.09 eV red-shifted relative to those of the models, as shown in Figure 16-9 (left) for the octyloxy-substituted compounds. The small shift can be readily explained by the fact that in the copolymers the chromophorcs are actually substituted by silylene units, which have a weakly electron-donating character. The shifts between absorption and luminescence maxima are exactly the same for polymers and models and the width of the emission bands is almost identical. The quantum yields are only slightly reduced in the polymers. These results confirm that the active chro-mophores are the PPV-type blocks and that the silylene unit is an efficient re-conjugation interrupter. 

A.P. Monkman, L.-O. Palsson, R.W.T. Higgins, C. Wang, M.R. Bryce, A.S. Batsanov, and J.A.K. Howard, Protonation and subsequent intramolecular hydrogen bonding as a method to control chain structure and tune luminescence in heteroatomic conjugated polymers, J. Am. Chem. Soc., 124 6049-6055, 2002. 

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