New rigid backbone conjugated organic polymers with large fluorescence quantum yields

Abstract

The effect on optical emission of the presence of carbon-carbon triple bond linkages in conjugated aromatic copolymers is studied and leads to fluorescence quantum yields of up to 50%, substantially higher than most conventional conjugated polymers.Since the first demonstration of organic polymer light emitting diodes (polymer LEDs),l materials research in this area has intensified. The main issues to be addressed are device efficiency and material stability. Since the light emitting state in devices appears to be an excited neutral singlet,2 the issue of efficiency is very much dependent on the quantum yield of emission of the fluorescent polymers employed. To this end, conjugated polymers with high quantum yields of fluorescence are required.One approach commonly employed is to limit the delocalisation length of the polymer by interrupting x-conjugation or synthesising short chain oligomers.3 This effectively increases the 'rigidity' of such systems and reduces relaxation from excited states through non-radiative (e.g. thermal) processes.The principal problem with this approach is that limited conjugation leads to lower charge carrier mobility. In general, the lower the carrier mobility, the greater the bias required to drive the device. The ideal material therefore possesses uninterrupted n-conjugation as well as a high quantum yield of fluorescence.To date, work has focused primarily on the paraphenylene: paraphenylene vinylene and thiophene5 families of conjugated polymers. Fluorescence quantum yields for these polymers are typically between 0.1 %6 and 30%.7 There has been little study of arylene ethynylene polymers, in which aromatic moieties are connected by carbon-carbon triple bonds.8.9We have synthesised a series of soluble paraphenylene ethynylene copolymers and measured their quantum yields in solution by comparison with values for previously characterised laser dyes. The general synthetic route to these polymers is illustrated in Scheme 1. The polymerisation method is adapted from that of Giesa.9 Polymers 2 and 4 were previously synthesised by this author; however, no comprehensive structural or optical analysis was performed. The yields for this condensation polymerisation are reasonable (50-100%)t and give molecular masses ranging from 10s to 100s of repeat units.The visible absorption and emission spectra of 3 are shown in Fig. 1.1 The spectra of the samples are all sharp in comparison to other conjugated polymers and are inhomogeneously broadened. The position of the absorption maximum depends on the nature of the co-unit. For small units such as thiophene or pyridine, the maxima are around 450 nm. If a more electron rich unit (such as anthracene) is introduced (e.g. in 4), a significant red-shift of the absorption maximum is observed. Table 1 lists the position of the absorption maxima for the copolymers studied. It is tentatively suggested that the position of the absorption maximum is related to the degree of destabilisation of the HOMO and stabilisation of the LUMO. The presence of the elect...

Low band-gap polymers incorporating benzotriazole and 5,6-dialkoxy-benzothiadiazole as solution processable electrochromic materials

“…This could perhaps be attributed to the presence of two long alkoxy chains in P4 and P5 which disrupt the planarity of the conjugated backbone. In highly rigid conjugated systems, high quantum yields have been observed [50]. The bright fluorescence of P1-P3 rendered them potential as fluorescence based sensing materials [51].…”

The Synthesis and Characterisation of Fluorescent Poly(heteroaromatic oxadiazole)s

“…10 -3 M). A standard approach using quinine sulphate in 0.1 N H 2 SO 4 as reference [21] was used to estimate the relative fluorescence quantum yields [22] of the polymers in solutions. The quantum yield of a sample in solution U s relative to a reference sample of known quantum yield U r may be related by:…”

Synthesis of some novel heteroarylene ethynylene polymers containing 1,10-phenanthroline in the polymer backbone

“…10 -12 To our knowledge, the derivatives of 1,10-phenanthroline never have been used in this synthesis of poly(arylene ethynylene)s. Compared to 2,2Ј-bipyridine, the pyridyl units of 1,10-phenanthroline are fixed in the cisoid conformation. 3,25 In this article, we describe the synthesis and optical properties of two soluble poly(arylene ethynylene)s (7) with 1,10-phenanthroline in the main chain (Scheme 1) by the use of palladium-catalyzed cross-coupling methodolo-gies. Furthermore, we investigate the influence of the rigidity of the heterocyclic moiety on the chemical and physical properties of the -conjugated polymers in comparison with the equivalent poly(heteroarylene ethynylene)s containing 2,2Ј-bipyridine in the polymer backbone.…”