Electric and Magneto-Electric Nonlinear Optical Response of Twisted Intramolecular Charge Transfer Chromophores

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Nonlinear optical (NLO) effects are used universally in modern day telecommunication and form the foundation of emergent photonic technologies. Twisted intramolecular charge transfer (TICT) chromophores, composed of a donor and acceptor fragment connected by a twisted bi-aryl bridge, combine large molecular NLO response with properties such as transparency in the IR region, thermal stability, and compact chemical structure, which makes them promising candidates for use in electro-optic devices. In this work we elucidate the impact of structural and environmental changes on 2nd order (), 3rd order (), 5th order ((3):(1)), and magneto-electric (My and Pz) NLO response of TICT chromophores. Chapter 1 reports the synthesis and characterization of a series of high- TICT chromophores that utilize a new type of acceptor group and exhibit intermediate bi-aryl torsional angles which were previously inaccessible. Whereas earlier TICT chromophores exhibited poor performance in polar solvents, this series maintains a high  value in DMF. The use of polar solvents has the added benefit of mitigating aggregate formation, thereby enhancing  in concentrated solutions. Chapter 2 explores the interactions of the highest performing chromophore of the series, B2TMC-2, with organic salt additives, which lead to large spectroscopic changes and improved  values. The origin of these effects is determined to be coulombic attraction between the salt and the chromophore acceptor group, which competes with dipole-driven aggregate formation. The increases in  arise entirely from the disaggregating property of the salt, and we demonstrate that the same interactions in solution persist in host/guest films. In Chapter 3, we explore the impact of hybridizing a stilbene and TICT chromophore on  and (3):(1). The observed two-photon absorption is attributed to the stilbene fragment, while the excess polarizablity is related to the twisted fragment, which causes localization of ground and excited states. The (3):(1) response contributes significantly to nonlinear refraction, and highlights the drastic change in NLO response resulting from a subtle structural modification. In Chapter 4, we outline the theoretical underpinnings of the first connections between magneto-electric response and molecular structure, showing that the twist motif can be productively applied to 2-D molecules. The sum of this work provides insight into the impact of bi-aryl twist angle, acceptor group identity, and environmental factors on several NLO effects, and in many cases yields productive guidelines based on deep understanding of specific cause/effect relationships.

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  • 10/08/2019
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