Side Chain Effects in Hybrid PPV/PPE Polymers

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Abstract

The Horner−Wadsworth−Emmons olefination reaction of luminophoric dialdehydes 1 and 2 and bisphosphonates 3 provide high-molecular-weight and thermostable PPV/PPE hybrid polymers 4 and 5 of well-defined general constitutional structure −(CHCH−Ph−CHCH−Ph−C⋮C−Ph−C⋮C−Ph−)n, which was confirmed by NMR, infrared and elemental analysis. Soluble and good film-forming materials were obtained after attaching long linear alkoxy, e.g., dodecyloxy, octadecyloxy, or branched alkoxy side chains, e.g., 2-ethylhexyloxy, on the conjugated backbone. Thermotropic and lyotropic liquid crystalline behavior was observed with polarized optical microscopy. The presence of triple bonds along the PPV backbone increases the electron affinity of these polymers, which is reflected by the comparatively (with MEH−PPV) higher oxidation potential of 1.30 V vs Ag/AgCl. Polymers 4 and 5 are good photoconducting and highly luminescent materials. While almost identical photophysical behaviors for all polymers of type 4 (λmax,abs = 450 nm, λmax,em = 490 nm) or 5 (λmax,abs = 470 nm, λmax,em = 553 nm) were obtained in dilute chloroform solution, resulting in fluorescence quantum yields between 70 and 80% of the yellowish-green emission, the solid-state properties (color, thermal behavior, photoconductivity, absorption and emission spectra, and photoluminescence quantum yields) are dependent on the size, geometry, number, and location of the grafted alkoxy side groups. Exchanging for example the position of the side chains from 4ac (R2 = O(CH2)17CH3, R4 = O(CH2)7CH3) to 4ca (R2 = O(CH2) 7CH3, R4 = O(CH2)17CH3) leads not only to a change in color of the material from orange to yellow but also a dramatic change in the photophysical behavior. In general, octadecyloxy side chains in position R4 are necessary to obtain narrow and structured emission curves, small Stokes shifts, less excimer formation, and higher fluorescence quantum yields (30−40%). The fully substituted, deep orange-red polymer 5a (R2 = R4 = O(CH2)17CH3), for example, behaves as if its conjugated backbone was dissolved in a hydrocarbon solvent. This is confirmed by its very high photocurrent of 1.1 × 10-9 A (which is at least 2 orders of magnitude higher than that of all the other polymers), detected at the lowest threshold voltage of 10 V, and its highest φfl value of 54%. It can be assumed from these facts that photoconductivity is more an intramolecular phenomenon than an intermolecular one. Strong π−π interchain interactions not only lead to fluorescence quenching through excimer formation but also have a negative effect on photoconductivity.

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