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π-Conjugated Polymers (πCP's)

Unique microelectronic, optical, and photonic applications, are emerging in which π-conjugated polymers and oligomers (see Figure 1) complement, or even replace, conventional inorganic and metallic components. As a result, these materials are under intense chemical, physico-chemical, and electronic scrutiny. A few examples of polymers being studied in our laboratory are provided below.

Figure 1. Examples of Conjugated Polymers synthesized in our laboratory.

Design and Synthesis of π-Conjugated Polymers: Poly(thiophene)s and derivative thereof are potentially useful components in field-effect transistors, optical and electronic sensors, light-emitting devices, nonlinear optical materials, etc. Substitution at the 3- and/or 4-position of the thiophene ring not only confers processability to poly(thiophene)s but can also be used to modify their electro-optical properties through electronic and steric interactions. However, it is difficult to prepare poly(thiophene)s with a comprehensive list of functional groups due to either an intolerance of the functional group to the rather harsh polymerization conditions; the inhibition of monomer polymerization by the functional group; or the tedious nature of the synthetic procedures. In this synthetic effort we are investigating novel routes for the functionalization of polythiophenes and related This approach has led to a rapid synthesis and screen of conjugated polymers with a wide range of functional groups with certain knowledge that the degree of polymerization, regioregularity, and spatial modification of the backbone is quantitatively and spatially identical for each polymer.

Figure 2. Examples of functionalized polythiophenes synthesized from the same original regioregular polymer and showing differences in solid state fluorescence efficiencies.

Patterning of π-Conjugated Polymers: There is growing interest in depositing conjugated polymers in a spatially controlled fashion. There several methods being investigated in our laboratory including photolithography, soft lithography, and self assembly. Examples of patterning π-conjugated polymers are demonstrated below.

Figure 3. Patterning by thermal photolithography, soft lithography (fluorescence image), and self assembly.

Recent examples make use of Schematic illustration of the formation of a well ordered micro/nano-sized πCP features (PTHPET or PTHPEF) by solution casting, catalytic reaction, and development as illustrated below.

Figure 4. Patterning πCPs by self organization and post-reaction.

A method being developed in our laboratory makes use of the thermal lability of functionalized polymers. For example, negative tone patterns of thiophene- and fluorene-based polymer films may be obtained by simply exposing selected areas of a tetrahydropyran (THP)-bearing polymer to trace quantities of catalytic acid, whereupon thermal cleavage of the THP group results in an intractable, hydrogen-bonded material. More recently, we patterned similar polymers by exposure of a NIR absorbing dye, directly incorporated into a πCP film, to a 830 nm NIR laser beam which induces localized heating and a deprotection reaction. Direct thermal patterning of πCP films containing NIR dye offers the advantage of single step film preparation, rapid computer-to-plate direct patterning, and < 10 μm resolution imaging.

Figure 5. Patterning πCPs by direct thermal lithography, and their fluorescence images.

Electroluminescence: Conjugated polymers are a promising class of polymer for electroluminescence (EL) devices due to their ease of preparation, versatility and tunable band gap. We are involved in systematic studies of photoluminescence quantum yields (solution and solid state) of stereochemically controlled polythienyl-, polyfluorenyl-, and iridium-based conjugated polymers. We are involved in the design of host-guest polymers that promote the transfer of excitation energy from higher energy conductive matrices to lower-energy, highly luminescent, polymer-bound fluorophores and phosphors.

Figure 6. Electrophosphorescence and energy level considerations of iridium-containing conjugated polymers.

Photovoltaic devices: Solar energy conversion at a heterojunction cell is accomplished by four consecutive steps: (1) Photon absorption to create an exciton (Coulombically-bound electron-hole pair). (2) Exciton diffusion to a donor-acceptor (DA) junction. (3) Exciton dissociation (charge separation). (4) Charge carrier transport to the electrodes. A generic energy diagram is shown below. We are involved in researching bicontinuous nanoarchitectures that improve the efficiency of solar into electrical energy conversion.

Figure 7. A donor-acceptor (DA) heterojunction of a typical bilayer polymer-fullerene photovoltaic cell.

For the purpose of studying bicontinuous, donor-acceptor π-conjugated nanostructures, we post-functionalize poly(3-hexylthiophene) with TEMPO and use it to initiate the pseudo-living polymerization of chloromethylstyrene (CMS) from the thienyl backbone. Attachment of C60 by atom transfer radical addition provides the first example of a polythiophene-based main chain polymer bearing side chains of multiply grafted fullerene moieties, see figure below.

In another approach a graft copolymer is obtained by NMRP of an electron transport monomer (vinyl triazole) onto post-functionalized poly(3-hexyl)thiophene to produced phase-segregated morphologies.

Donor-Acceptor, phase phase-segregated morphologies are characterized by a wide range of techniques including spectroscopy, electrochemistry, microscopies, and surface analytic techniques, and in working PV devices.