Supramolecular ordering, thermal behavior, and photophysical, electrochemical, and electroluminescent properties of alkoxy-substituted yne-containing poly(phenylene-vinylene)s

Alkoxy-substituted and defect-free yne-containing poly(phenylene-vinylene)s, 15a-d, having the general constitutional unit (-Ph-CdropC-Ph-CH=CH-Ph-CH=CH-) synthesized through the olefination reactions of the fluorophoric dialdehyde 7 [1,2-bis(4-formyl-2,5-dialkoxyphenyl)acetylene] with various 2,5-dialkoxy-p-xylylenebis(diethylphosphonates), 14a-d, are reported. A new synthetic route to 1-bromo-2,5-dialkoxybenzaldehyde (starting material for the synthesis of 7) allowing the grafting of all types of alkoxy side chains is also reported. Bulk state properties of the four polymeric materials have been analyzed using various experimental methods giving information on the thermal behavior as well as self-assembling morphologies at different size scales. Crystalline superstructures of either spherulitic or rodlike morphology, comprised of polymeric backbone layers separated by the side chains are identified to develop by nucleation and growth process for symmetrically alkoxy substituted 15a-c. Discrepancy between optical (E-g(opt)) and electrochemical (E-g(ee)) band gaps was observed to be dependent on the bulkiness (15c) and length (15b) of the grafted side chains. Octadecyloxy side chains in 15b not only lead to well structured photoluminescence and electroluminescence spectra and higher fluorescence quantum yields but also allow the design of LED-devices of the configuration ITO/PEDOT/polymer/Ca with improved parameters (eta(ext) = 2.15%, luminance = 5760 cd/m(2)) relative to the octyloxy-substituted polymer 15a (eta(ext) = 0.79%, luminance = 1406 cd/m(2)). Despite the differences in the conjugation pattern between polymers 15 and polymers 17 [(-Ph-CdropC-Ph-CdropC-Ph-CH=CH-Ph-CH=CH-)], an identical chromophore system of structure Ph-CdropC-Ph-CH=CH-Ph-CH=CH-Ph-CdropC-Ph was found to be responsible for their emissive behavior as confirmed by fluorescence kinetics measurements (tau, k(f), k(nr)) and quantum chemical calculations. The differences observed in their solid-state photophysical properties are ascribed to the combined effects of the backbone rigidity (as a function of the number of -CdropC- moieties in the repeating unit) and the nature of the side chains. Moreover less -CdropC- units within polymers 15 lead to the more than 100-fold improvement of their electroluminescent parameters compared to polymers 17.