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No description available.

Conjugated polymers comprise some of the most promising materials for new technologies such as organic field effect transistors, solar light harvesting technology and sensing devices. In spite of tremendous research initiatives in materials chemistry, the potential to optimize device performance and develop new technologies is remarkable. Understanding relationships between the structure of conjugated polymers and their electronic properties is critical to improving device performance. The design and synthesis of new materials which self-organize into ordered nanostructures creates opportunities to establish relationships between electronic properties and morphology or molecular packing. This thesis details our progress in the development of synthetic routes which provide access to new classes of conjugated polymers that contain dissimilar side chains that segregate or dissimilar conjugated blocks which phase separate, and summarizes our initial attempts to characterize these materials. Poly(1,4-phenylene ethynylene)s (PPEs) have been used in a variety of organic electronic applications, most notably as fluorescent sensors. Using traditional synthetic methods, asymmetrically disubstituted PPEs have irregular placement of side chains on the conjugated backbone. Herein, we establish the first synthetic route to an asymmetrically substituted regioregular PPEs. The initial PPEs in this study have different lengths of alkoxy side chains, and both regioregular and regiorandom analogs are synthesized and characterized for comparison. The design of amphiphilic structures provides additional opportunities for side chains to influence the molecular packing and electronic properties of conjugated polymers. A new class of regioregular, amphiphilic PPEs has been prepared bearing alkoxy and semifluoroalkoxy side chains, which have a tendency to phase separate. Fully conjugated block copolymers can provide access to interesting new morphologies as a result of phase separation of the conjugated blocks. In particular, donor-acceptor block copolymers that phase separate into electron rich and electron poor domains may be advantageous in organic electronic devices such as bulk heterojunction solar cells, of which the performance relies on precise control of the interface between electron donating and accepting materials. The availability of donor-acceptor block copolymers is limited, largely due to the challenges associated with synthesizing these materials. In this thesis, two new synthetic routes to donor-acceptor block copolymers are established. These methods both utilize the catalyst transfer condensation polymerization, which proceeds by a chain growth mechanism. The first example entails the synthesis of a monofunctionalized, telechelic poly(3-alkylthiophene) which can be coupled to electron accepting polymers in a subsequent reaction. The other method describes the first example of a one-pot synthesis of a donor-acceptor diblock copolymer. The methods of synthesis are described, and characterization of the block copolymers is reported.

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This thesis is concerned with the photophysical and nonlinear optical responses, and applications of a set of conjugated polymers and small molecules in the visible and near-IR spectral regions. Poly(phenylene ethynylene) PPE polymers were substituted with conjugated side-arms in a cruciform fashion to determine the impacts of electronic coupling on the one-photon (1PA), two-photon (2PA), and excited state absorption (ESA) properties of the co-polymer system. The cruciform-like PPEs showed significant changes in their nonlinear and phophysical behavior relative to their linear models, including shifts and splittings of the 1PA bands due to moderate mixing of the lowest singlet excited states, an increase in the 2PA cross section (δ) values, and an increase in the yield of triplet excited-state species. The cruciform-like PPE polymers exhibited effective optical pulse suppression of femtosecond and nanosecond laser pulses over a broad spectral range of ~200 nm in the visible and near-IR. The suppression capability of the cruciform-like PPEs exceeded the best reported value for alkyl-substituted PPE polymers. The spectroscopic effects due to conjugation length, structural configuration, and intramolecular charge transfer (ICT) are discussed for a family of bent donor-acceptor-donor (D-A-D) -type conjugated oligomers, which incorporate electron-rich triarylamine donors and electron-deficient triarylborane acceptor units into its conjugated structure. These organoborane oligomers are highly fluorescent and exhibit strong 2PA in the visible region with δ values as large as 1410 GM, as well as overlapping ESA bands attributed to singlet-singlet and triplet-triplet absorption. Saturation of the molar absorptivity, ε, and δ was observed at less than two repeat monomer units due to conformational disorder in the oligomer with increasing length. Positive solvatochromism of fluorescence with solvent shifts as large as ~70 nm was observed as a result of ICT from the arylamine donors to boryl-centered acceptor sites. The excited-state dynamics also show sensitivity to the solvent environment. Experimental findings suggest that these organoborane oligomers may have potential use as nonlinear material for optical power limiting (OPL) and two-photon sensing applications. The spectral properties of two bis-donor chromophores, (bis(diarylamino)biphenyl (TPD) and distyrylthiophene (DST), were investigated with and without the presence of AgNPs in order to better understand the local-field enhancement and subsequent effects on the photophysics and nonlinear behavior of 2PA dyes. While little changes were observed in the excited-state dynamics, measurements of nanoparticle aggregate-dye composite solutions with TPD revealed a 1.6-enhancement in the two-photon excited fluorescence signal. OPL measurements of nanosecond laser pulses at 532 nm revealed a reduction in threshold energy by a factor of 2 in solutions containing TPD and AgNP aggregates, relative to solutions of TPD alone. DST shows exceptional solubility (>1 M) in several organic solvents and exhibits a 2PA spectrum that overlaps well with its singlet-singlet and triplet-triplet ESA bands. Consequently, DST exhibits effective optical limiting of nanosecond laser pulses through two-photon induced excited-state absorption over a broad spectral range of approximately 200 nm in the red and near-IR.

There are two general ways to introduce functionalities into a polymeric structure: functionalization of the monomeric units before polymerization and postfunctionalization of the preformed polymer. Building libraries of polymers with different functionalities can be completed with significantly less effort by the second method, as each postfunctionalization of a single batch of polymeric backbone can involve as little as one synthetic step. One method of building a polymeric backbone for postfunctionalization involves the synthesis of hyperbranched conjugated polymers (HCPs) from AB2 monomeric units. A polymer formed from n AB2 monomeric units should contain n reactive B groups, which act as sites of functionalization. Utilizing this principle, two different hyperbranched poly(phenylene vinylene-phenylene ethynylene) scaffolds were synthesized and studied in both their inherent properties and functionalization. The first HCP synthesized was compared against a monomeric cruciform model and a linear polymer with a similar structure. The hyperbranched polymer has red-shifted absorption and emission in comparison to the cruciform model and linear polymer. The HCP quenches paraquat more efficiently than the linear polymer by a factor of about two, suggesting a greater rate of energy transfer. The functionalization of HCPs was studied; iodine groups decorating the HCPs were replaced with terminal alkynes by Pd-catalyzed coupling, providing a library of 24 differently functionalized HCPs. Elemental analyses of the postfunctionalized polymers show nearly complete substitution of the iodine groups. The postfunctionalized polymers show increased fluorescence compared to the original iodine decorated polymers, due to the loss of the heavy atom effect inducing iodine groups. The emissions of the postfunctionalized polymers in solution show a strong dependence on the groups attached to the conjugated structures, with emission maxima ranging from 505 nm to 602 nm; quantum yields range from 0.7% to 25%. Solid-state emission studies show stronger and more red-shifted spectra compared to emissions observed in solution.