Functional Derivatives Of MEHPPV Using The Dithiocarbamate Precursor Route

Abstract

Emissive conjugated polymers, namely PPV, PPP, polyfluorenes, etc, have gained considerable attention in recent times, specifically because of their potential application in the fields of PLED’s, displays, FET’s and sensors. The main target of the present work is the synthesis and utilization of dithiocarbamate (DTC) precursor polymers for: (a) generation of segmented conjugated polymers of poly[2-methoxy-5-((2'-ethylhexyl)oxy)-1,4- phenylenevinylene], MEHPPV-x, for color control and the study of their thermal elimination kinetics, (b) modulating phase separation and energy transfer in MEHPPV-x blends, (c) generation of tunable two-color patterns of conjugated polymers and (d) modification of the precursor polymer backbone by grafting and the study of its fluorescence modulation in the presence of different probe molecules. In the first part of this work, various dithiocarbamate (DTC) precursor copolymers, MDP-x, containing two types of leaving groups viz. methoxy and diethyldithiocarbamate with precise control over the DTC composition, were synthesized. Thermal elimination of these precursor polymers generated segmented MEHPPV with controlled conjugation, and hence the tuning of color from green to red is achieved (figure 1). These copolymers were synthesized via a modified Wessling’s route previously developed in our laboratory.1 The advantage of the DTC precursor over the acetate and xanthate precursor routes was that the elimination can be carried out at lower temperature (160OC) for the generation of segmented MEHPPV-x.2 (Figure 1) Kinetic parameters, namely activation energy (Ea) and pre-exponential factor (A), associated with the thermal elimination process of MDP-x to MEHPPV-x were determined in solution and as well as in thin films by following the evolution of the absorption spectra during the elimination process (figure 2). It was seen that the activation energy required for the elimination process was similar in both thin film and solution, whereas the pre-exponential factor was two order magnitude higher in thin films.2 This fact holds good for all the DTC compositions investigated. The thermal degradation products, carbon disulphide and diethyl amine, were analyzed using a mass spectrometer coupled with TGA instrument, confirming the selective elimination of the DTC groups. (Figure 2) Phase separation and energy transfer characteristics of segmented MEHPPV blends containing two different distributions of conjugation lengths, namely MEHPPV-8 (LC) and MEHPPV-100 (HC), were investigated using FL, UV and confocal fluorescence microscopic studies (figure 3). The phase separation and energy transfer in blends of the HC and LC were (Figure 3) modulated by addition of selective non-solvent for HC, namely cyclohexane, to the film casting solution. Typically, the extent of energy transfer to HC from LC is reduced in the presence of high volume fraction of cyclohexane.3 A novel way to generate two-color patterned substrates of MEHPPV was developed based on the control of “molecular conjugation length” using standard photo-acid generator based photolithographic methods (figure 4). This approach relies on the use of a single precursor containing controllable amounts of two types of thermally eliminatable groups, only one of which eliminates in the presence of an acid while the other that is labile even in its absence. An important feature of this approach is that the colour of the unexposed regions can be controlled by varying the composition of the MDP-x precursor. (Figure 4) Benzyl diethyl dithiocarbamate (BDTC) is known to act as iniferter (initiator-transfer agent and terminator). MDP-x precursor polymers, which contain similar benzyl dithiocarbamate groups, were used as macro-iniferter for grafting various acrylates, viz, (Figure 5) methyl acrylate, benzoyloxy ethyl acrylate and t-butyl acrylate, on to the precursor backbone, which resulted in MEHPPV-g-polyacrylate after acid catalyzed thermal elimination of the residual methoxy groups (figure 5).4 The t-butyl acrylate-grafted precursor polymers, namely, MDP-g-PtBA on thermal elimination in presence of acid underwent simultaneous acid-catalyzed thermal elimination as well as the complete hydrolysis of the t-butyl groups, leading to the formation of water soluble MEHPPV-grafted with polyacrylic acid chains, namely MEHPPV-g-PAA (figure 6). These PAA-grafted MEHPPV’s were shown to respond to various probe molecules and their optical responses were studied using fluorescence spectroscopy. These polymers could sense methyl viologen at very low concentrations. Single-tail ammonium surfactants and non-ionic surfactant, like triton-X-100, caused a dramatic enhancement of fluorescence in solution, due to the modulation of the conjugated backbone conformation, and as a consequence the break up of intra-chain inter-chromophore excitons (figure 6). (Figure 6) Fof figures and molecular formula pl see the original thesis)