Synthesis, characterisation and properties of highly luminescent and liquid-crystalline alternating PPE/PPV-copolymers

Abstract

The combination of Heck–Cassar–Sonogashira–Hagihara reaction, column chromatography, leading to the synthesis of luminophoric dialdehydes 5, and the subsequent Horner–Wadsworth–Emmons polycondensation reactions of 5 and the bisphosphonates 3 enables the synthesis of a new type of defect-free π-conjugated polymers with well defined general structure –(Ar–C≡C–Ar–C≡C–Ar–CH=CH–Ar–CH=CH–) n , 7. The chemical structure was confirmed by NMR, infrared and UV/Vis spectroscopy. High molecular weight (Mw up to 500 000 g/mol), thermostable, soluble and transparent film forming materials were obtained. The new compounds show thermotropic and lyotropic mesomorphic behavior on polarized optical microscopy. The incorporation of triple bonds into the PPV backbone increases the oxidation and the reduction potentials, thus making these polymers potentially good electron transporting materials if used in light-emissive-diodes devices. The polymers are photoconductive and show very good photoluminescent properties in solution as well as in solid state. Identical absorption (λmax,abs = 445 nm) and emission (λmax,em = 490 nm) behaviors were observed for all polymers in solution (CHCl3), resulting to a fluorescence quantum yield of 70%, but the photophysical behavior in solid state was side groups-dependent. The orange-coloured polymer 7ac (R2 = (CH2)7CH3; λmax,abs = 465 nm; λmax,em = 602 nm) is characterized with a very large Stokes shift (137 nm) and lower fluorescence quantum yield (19%), whereas the yellow-coloured polymers 7aa, 7bb and 7ab, consisting of side groups equal or longer than O(CH2)11CH3(λmax,abs = 480 nm, λmax,em = 508 nm) were characterized with smaller Stokes shift (around 30 nm) and comparatively higher fluorescence quantum yields (34 to 44%); as a result of hindered interchain π -π interactions, and consequently limited formation of excimer complexes, which are regarded as an important deactivation channel of the excitons.