Synthesis and Characterisation of Polypyridine Derivatives: Towards Regioregular PPY
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Synthesis and Characterisation of Polypyridine Derivatives: Towards Regioregular PPY Lockhart E. Horsburgh, Andrew P. Monkman, Changsheng Wang* and Martin R. Bryce* Organic Electroactive Materials Group, Department of Physics, University of Durham, South Road, Durham, DH1 3LE, United Kingdom. *Department of Chemistry, University of Durham, South Road, Durham, DH1 3LE, United Kingdom. ABSTRACT Poly(2,5-pyridinediyl) was prepared from 2-bromo-5-iodopyridine, by a method which combines organomagnesium and organonickel chemistry, and leads to the regioselective formation of poly(2,5-pyridinediyl) (rPPY). The product of the reaction was compared to conventional poly(2,5-pyridinediyl) (PPY), thus enabling us to estimate that rPPY consists of 84±6% head-to-tail linkages. Photophysical properties of rPPY were also measured, and found to be generally similar to those of PPY, although there are some significant differences.
INTRODUCTION Poly(2,5-pyridinediyl) (PPY) has been extensively studied in recent years, primarily as a result of its unusual photophysics [1], and has shown potentially useful properties when utilised in organic light-emitting devices [2]. The synthesis of PPY was first reported by Schiavon and colleagues in 1985 [3]. The conventional preparative route, first reported by Yamamoto et al [4], is a polycondensation of 2,5-dibromopyridine, utilising Ni(0) chemistry: (bis(1,5-cyclooctadiene)nickel may be used, or the Ni(0) species can be generated in situ by reducing nickel(II) chloride with zinc, in the presence of triphenylphosphine), to activate the precursor and propagate the pyridine chain (figure 1). This method is, however, not strongly regioselective for either “head to head” or “head to tail” linkages between pyridine rings. We are investigating synthetic routes to regioregular head-to-tail polypyridines; materials which we believe will shed further light on the photophysical properties of PPY and may prove to have enhanced optoelectronic properties in their own right. We report here on the synthesis of regioregular poly(2,5-pyridinediyl) (rPPY).
R
R Br
Br N R
N
N
Zn/NiCl2 PPh3, DMF
N
N R H-T
H-H
n R T-T
Figure 1. Conventional synthesis of poly(2,5-pyridinediyl) [5], showing head-to-tail, head-tohead and tail-to-tail linkages in the product. R = H for PPY. C5.29.1
EXPERIMENTAL DETAILS UV/visible spectra were recorded on a Perkin-Elmer lambda-19 spectrophotometer; photoluminescence spectra were recorded on a Fluoromax-2 fluorimeter, exciting at 370 nm; 1H and 13C nuclear magnetic resonance spectra were recorded on a Unity-300 fourier pulse NMR spectrometer (tetramethylsilane was added to deuterated formic acid solutions to standardise chemical shifts of PPY and rPPY). PPY was synthesised by a method similar to that reported previously [5]. Films of PPY and rPPY were prepared by spin-casting from formic acid solutions of the polymers. All syntheses were conducted under dry nitrogen. 2-bromo-5-iodopyridine The synthesis was derived from work in references 6 and 7. 2,5-Dibromopyridine (3.56 g
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