Organic Compounds for Electronic and Optical Applications

Four series of random graft copolymers with stilbenoid side chains on a polystyrene (PS) backbone were synthesized, and their optical properties were compared with blends of a series of model compounds in PS. The graft loading (5, 15, and 25 mol %), the type of link to the polymer backbone ("ether" or "direct"), and the number of methoxy groups substituted on the stilbene moiety were systematically varied. Absorption, emission, and time-resolved photoluminescence (PL) properties are presented for the compounds in solution and as spin-coated films, and relative quantum yields have been determined. Mixed monomer/excimer emission was found to be the characteristic emission for all grafts, while the reversibility of the excimer formation seems to vary for the different grafts. In the blends the miscibility of the model compounds is quite different: the 4-methoxystilbene/PS system readily forms aggregates and crystallites, whereas the 3,4,5-trimethoxy-4'-methylstilbene/PS system forms uniform mixtures showing excimer-type emission. The PL efficiencies decreased with concentration in all cases where excimeric emission was found. For the blend containing 4-methoxystilbene, however, the PL efficiency increased with concentration.

Aerts, G.; Wuyts, C.; Dermaut, W.; Goovaerts, E.; Geise, H.J.; Blockhuys, F. Macromolecules 2004, 37, 5406-5414.

The synthesis and spectroscopic and electrochemical characterisation of 16 alkoxylated derivatives of E,E-1,4-bis(2-phenylethenyl)benzene are described. The influence of the size and position of the functional groups on the carbon backbone on the formal oxidation potential and the diffusion coefficient has been investigated. The experimental values of the formal oxidation potential are correlated with calculated values of the ionisation potential, at different levels of theory and with different methodologies.

Baeke, J.K.; Nowaczyk, J.; Moens, M.; Chen, L.J.; Dieltiens, P.; Geise, H.J.; Van Alsenoy, C.; Blockhuys, F. J. Electroanal. Chem. 2007, 599, 1-11.

A novel class of self-doping conjugated oligomers, E,E-2-(sulfoalkoxy)-5-alkoxy-1,4-bis[2-(2,4,6-trimethoxyphenyl)ethenyl]benzenes, is presented. The synthesis and spectroscopic characterisation of five such oligomers are described, and an electrochemical analysis using cyclic voltammetry is performed to determine the anodic peak potentials. A structural study is performed on six self-doping oligomers in which the structures and energies of the possible mono-molecular forms of the electrically conducting doped material are described and evaluated using Hirshfeld charges and the Quantum Theory of Atoms In Molecules.

Baeke, J.K.; De Borger, R.; Lemière, F.; Van Alsenoy, C.; Blockhuys, F. J. Phys. Org. Chem. 2009, 22, 925-932.

Using a combination of multi-frequency EPR and NMR spectroscopy and quantum-chemical calculations at the level of Density Functional Theory (DFT), the organization of self-doped PPV oligomers in their solid state is investigated. The analysis of the different spectra shows that the electrochemical procedure used to self-dope these materials produces positive radicals (polarons) in an almost quantitative way, but still magnetically isolated polarons are observed. The difference between chemical and electrochemical oxidation of the oligomers is studied in detail. Furthermore, ageing of the electrochemically oxidized oligomers may be accompanied by a stacking of the oligomers.

Ling, Y.; Kozakiewicz, P.; Blockhuys, F.; Biesemans, M.; Van Alsenoy, C.; Moons, H.; Goovaerts, E.; Willem, R.; Van Doorslaer, S. Phys. Chem. Chem. Phys. 2011, 13, 18516-18522.

Impedance and current-voltage spectral measurements have been carried out on the Au/poly(3-pentylthiophene)/Au sandwich system in which the polymer was in the form of a pressed powder. The device shows an ohmic dc resistance. The specific conductivity of the material was in the range of insulators. The investigated sandwich system displays a relationship between the specific conductivity and the polymer thickness which is characteristic for molecular-scale disordered materials. Modification of the polymer by gaseous ozone in relatively high concentration causes a significant increase of the specific conductivity. Ozone treatment leads to p-doping of the polymer resulting in a temporary increase in conductivity as well as a modification of the morphology; the latter is stable and results in a permanent increase of the sample conductivity.

Nowaczyk, J.; Blockhuys, F.; Czerwiński, W. Mater. Chem. Phys. 2012, 132, 823-831.