| dc.description.abstract | Electroluminescent polymers are a novel class of conjugated polymers, which have promising potential for use in various optoelectronic devices like all-plastic displays, solar cells, lasers, etc. The species responsible for both EL and photoluminescence (PL) in these systems have been shown to be the same, viz., a singlet exciton that resides on a chromophore of finite length and bandgap. Thus, the HOMO-LUMO gap of the chromophores comprising the polymer governs the color of light emitted and, to some extent, the efficiency of the light emission. Hence, generation of conjugated polymers with varying extents of conjugation has been of interest during the last decade with a view of varying the colors from the LEDs and also to achieve improved emission yield.
One of the first reports for precise control of conjugation in PPV systems was developed in our group, wherein selective elimination of a precursor that contained two types of eliminatable groups, viz. acetate and methoxy groups, was affected: the acetate being more thermally labile than the methoxy group. This resulted in the generation of 2,5-dimethoxy-PPV with varying extents of conjugation, in which the wavelengths of absorption and emission were found to red-shift with an increase in conjugation.
The objectives of the current investigations were to extend and improve this approach for the synthesis of soluble PPV derivatives with a similar control over the extent of conjugation and investigate their solution photophysical behavior. The first chapter of this thesis provides a brief introduction to conjugated polymers, with emphasis on PPV-based polymers, different synthetic methodologies for their preparation, previous attempts to obtain PPV with controlled conjugation length, and also a description of the various types of electroluminescent devices that have been studied based on these systems.
The second and third chapters deal with the synthesis and characterization of the acetate-methoxy precursors and xanthate-methoxy precursors of a soluble PPV derivative - poly[2-methoxy-5-((2'-ethylhexyl)oxy)-1,4-phenylenevinylene] (MEHPPV) having different mole ratios of acetate/methoxy and xanthate/methoxy, respectively. Subsequent thermal elimination in solution, during which only the acetate or xanthate groups undergo selective elimination leaving behind the methoxy groups intact, yielded MEHPPV-x with varying conjugation length (x being the mole % of eliminated units). The precursors were prepared by a modified Wessling's precursor route, in which the polyelectrolyte precursor was substituted with methoxy and acetate (or xanthate) groups by using varying ratios of methanol and sodium acetate (or potassium ethyl xanthate) as the nucleophiles.
Synthetic scheme for the preparation of MEHPPV-x
In the case of acetate, the substitution by acetate and methoxy groups was shown to occur in a competitive manner, whereas in the xanthate, the substitution by xanthate and methoxy groups was shown to be almost sequential. Thus, a more precise control of the composition of the precursor copolymers was readily achieved in the xanthate case. Furthermore, precursors with essentially 100% xanthate were readily prepared, which was not possible using sodium acetate, in which case a maximum of only 85% acetate was achieved. This is due to unavoidable premature substitution by methanol that was used as the polymerization solvent also. This meant that small amounts of non-conjugated defects (<15%) could not be incorporated using the acetate approach. In addition, the optimum elimination temperature for the xanthate was lower than that for the acetate (180 °C compared to 220 °C), which could potentially improve the selectivity.
The kinetics of elimination of the acetate and xanthate precursors in solution were studied and in solid state using UV-visible absorption spectroscopy, in which the area under the absorption curves (? > 315 nm) were taken as a measure of the extent of elimination (Chapter 4). The rate constants were seen to be significantly higher in the case of xanthate compared to acetate precursors, while the activation energies in both cases were comparable (30-35 Kcal/mole), suggesting that the rate differences arise from the pre-exponential term. In the case of unsubstituted PPV, this approach for control of conjugation length did not work well, the reasons for which were investigated in detail and are presented toward the end of this chapter. One of the problems was the nucleophilic attack by chloride counter-ion, leading to the formation of the chloro-precursor. Detailed *H NMR studies of this equilibration process in the case of the xylylenebis-(sulfonium chloride) monomer were carried out using deuterated methanol and DMSO-d6 as the solvent.
In the fifth chapter, the complete photophysical studies of MEHPPV-x in dilute solution are presented. Both the UV-visible absorption and emission spectra show a red-shift with increase in conjugation, in solution as well as in the solid state. The color of emission was fine-tuned from 460-560 nm in solution and 500-580 nm in the solid state. Fine structure was observed in the absorption and emission spectra for the lower conjugated polymers, which has been attributed to discrete absorption and emission arising from chromophores of varying conjugation length. It was clear that, with an increase in conjugation, the contributions from the higher conjugation length segments increased at the cost of the smaller ones.
Assuming a statistical distribution of conjugation length, the probability of finding various chromophores of different lengths was calculated and their effective fractional populations were estimated. Utilizing these fractional populations and the actual recorded absorption spectra of well-defined oligomers of dipropyloxy-PPV's (OPPVs), the expected absorption spectra for the polymers were reconstructed. Comparison of the observed and reconstructed absorption spectra of the various MEHPPV-x polymers confirmed that the acetate substitution-elimination had indeed occurred in a statistically random manner. In an effort to understand energy migration in isolated single polymer chain, a similar reconstruction was carried out using quantum-yield normalized fluorescence spectra of the OPPVs.
Comparison of the observed and reconstructed spectra clearly showed that the emission from the shorter oligomeric chromophores was depleted while emission from their longer homologues was enhanced - the extent of this mismatch increased with increase in the extent of conjugation of the polymer. This was attributed to energy transfer from the excited state of a short chromophore to that of a longer one (of lower energy) prior to emission. Since the density of chromophores increases with increasing the extent of conjugation in the polymer, the probability of energy transfer and therefore its extent increases. This was the first demonstration of the occurrence of such energy transfer in a single isolated conjugated polymer chain in solution.
Partially conjugated polymers such as MEHPPV-x could thus serve as excellent models to understand conformational transitions, using simple fluorescence spectroscopy as a tool. In chapter 6, investigations to follow such conformational transitions, as the solvent-nonsolvent composition is varied, are described. Three different solvent-nonsolvent pairs, namely dichloromethane-methanol, dichloromethane-cyclohexane, and toluene-n-butanol were investigated. In the first pair, as the composition of methanol increases, a decrease in the overall fluorescence emission yield accompanied by a red-shift in the emission spectra is observed. The inflection point of the typical S-shaped plots that describe the emission area variation versus solvent composition was taken as the chain collapse point.
With increase in the extent of conjugation (x) of MEHPPV-x, there was a decrease in the amount of methanol required to cause the chain collapse. This observation is consistent with the expectation that the hydrodynamic volume of the polymer chain decreases with increasing nonsolvent, and therefore there is an increased probability of energy transfer leading to a depletion of emission yield. It is also apparent that with increasing density of chromophores along the polymer backbone, the hydrodynamic volume at which efficient energy transfer (via say a Förster-type mechanism) will occur will increase. This explains the variation in collapse point with extent of conjugation. A particularly interesting observation was seen in the case of lightly conjugated polymers (MEHPPV-30 and lower): a small but significant increase in the emission yields with addition of non-solvent was first observed prior to the sudden drop. This again is a reflection of the higher fluorescence quantum yields reported for OPPVs of intermediate length when compared to the simple stilbene derivative. Thus, energy transfer from shortest stilbene chromophores to the next immediate higher homologues results in an increase in the overall emission yield. Similar conclusions were drawn using the other two non-solvents, except that in the case of cyclohexane a blue shift in the emission maxima was noticed at low nonsolvent compositions prior to the red-shift and depletion of emission yield. This behavior was attributed to the expected solvatochromism when a non-solvent of lower dielectric constant is used.
In chapter 7, some of the preliminary investigations on light-emitting devices (LED) fabricated using MEHPPV-x have been described. The EL and PL spectra were found to be almost identical in all cases. With decrease in conjugation, the turn-on voltage increased from 1.1 to 2.6 V and the EL efficiency decreased, except for MEHPPV-55, which had higher efficiency than MEHPPV-70 and MEHPPV-85. This may have to do with the higher intrinsic emission yield of chromophores of intermediate length, although more work to confirm this hypothesis is essential. | |
