Enhanced photo and thermal oxidative stability of the charge-transfer complexes of a conjugated polymer

Abstract

The thermal oxidative stability of a conjugated polymer (CP) is very important for its processing and exploitation, while the photoand/or thermo-oxidation of polymers is a major factor that leads to shortening the lifetime and lowering the efficiency of various devices based on them.1,2 Therefore, the elucidation of mechanisms of such a degradation and correct service life predictions are necessary for optimising CP-based devices. Poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) is now a ‘classic’ model for tackling generic problems of CPs.3 It has been reported recently4 that MEH-PPV is capable of forming intermolecular charge-transfer complexes (CTCs) with small organic acceptors such as 2,4,7-trinitrofluorene-9-one (TNF). The drop-cast films of both pristine MEH-PPV (SigmaAldrich, MW = 125000) and MEH-PPV blends with TNF (Aldrich, mp 171, > 98% purity) were prepared from chlorobenzene by varying the polymer concentration in the range 0.5–2 g dm–3 and the TNF:MEH-PPV molar ratio in the range 0.01–1. The susceptibility of neat MEH-PPV to photo oxidation and a comparison with that of MEH-PPV/TNF blends is illustrated in Figure 1. An immediate blue shift of lmax and a progressive reduction of Amax were observed in the course of irradiation; this was explained by shortening the π-electron delocalization length of the polymer. The FTIR spectra of the same films measured before and after irradiation exhibited new bands at 1279 and 1691 cm–1 unambiguously indicating5 the photo oxidation of PPV. It is evident that MEH-PPV/TNF blends exhibit a markedly improved photo-oxidative stability as compared to that of the neat polymer. Moreover, for TNF contents increasing in the range 5– 25 mol%, the blends demonstrated a clear systematic gain in their thermal-oxidative stability. It is of importance that both of the blend components have revealed a synergistic behaviour in terms of the stability enhancement (Figure 2). Comparative experiments have shown that fullerene C60 is also capable of stabilizing MEH-PPV, but in much higher concentrations. Experimental data obtained by other methods indicate spectacular non-covalent interactions between the donor and the acceptor within the same (5–25 mol%) range of TNF contents. Partial electron density transfer from the MEH-PPV polymer onto TNF molecules involved in the corresponding CTC (according to FTIR and Raman data) should, in principle, reduce the susceptibility of the polymer to oxygen. It is likely that the quenching of the triplet states excited on the macromolecules can effectively prevent triplet–triplet annihilation reactions, which are normally feasible in the presence of reactive singlet oxygen.6 The kinetic studies of both MEH-PPV/TNF blends and the pristine polymer (using a model-free Friedman analysis, nonlinear regression analysis of the kinetic parameters,7 and lifetime prediction) has indicated that four-step thermal-oxidative degradation processes are operative in the system. The experimental results suggest that the complexed TNF (or the corresponding TNF/MEH-PPV charge-transfer complex) is capable 1.0