A central theme in the research of organic nonlinear optical (NLO) materials is the modular synthesis and supramolecular engineering of highly efficient organic electro-optic (EO) materials for photonic applications. Besides the development of organic EO materials for optical modulation, these research endeavors can potentially simplify the synthesis of dipolar dyes and provide abundant access to the molecular complexity and diversity of new structures, self-assembled systems, and reaction mechanisms for advanced functionalities.

In this dissertation, chapter 2 focuses on the synthesis and development of thiophene-bridge-based push-pull chromophores by alternating the sequence of π-conjugation units, which furnishes a 5-(4-dialkylamino-phenyl)-thiophen-2-yl moiety, to boost the EO activities of poled polymers. Compared with traditional divinylenethienyl (FTC)-derived chromophores, our modifications lead to remarkable performance improvement of these chromophores in their molecular hyperpolarizabilities and effective translation to large r₃₃ values up to 106 pm V^-1 at 1304 nm in poled polymers. In chapter 3, we demonstrated a new strategy of hydrogen bonding interactions to self-assemble dendritic dipolar phenyltetraene chromophores, in which the co-dendrons with two different aromatic rings, including fluorinated benzene and benzene moieties, were attached to the donor end and the π-bridge center of push-pull chromophores. Single crystal structure revealed a new extended supramolecular system supported by hydrogen bonding different from the classical ArH-ArF interactions. It presents an exceptional example of hydrogen bonding consisting of C(sp²)-H hydrogen donor from the partially fluorinated aromatics, optically transparent in the near infrared, while optimizing the supramolecular assembly and NLO properties of dendritic chromophores for photonic application.

The dipolar chromophores in chapter 2 and 3 are obtained by sequential multistep synthesis. To further improve the synthetic efficiency, the stepwise condensation strategy of push-pull heptamethine chromophores based on the bisaldehydes are investigated. Chapter 4 focuses on the bond-length alternation (BLA) values from X-ray crystallographic structure to understand the relationship between ground-state polarization and β values of push-pull heptamethine chromophores with different electron donors, acceptors and isolation groups, as well as the molecular geometry. This chapter indicates that the nonlinear optical activities of chromophores can be optimized by tuning the strength of electron donors and acceptors and introducing suitable isolation groups in molecular design. Chapter 5 focuses on the synthesis of anionic cyanine dyes with proton sponge as the cationic counterpart based on a stepwise condensation strategy. The methodology possesses advantages, such as easy handling without chromatography purification and wide substrate scopes. The results suggest that the asymmetrical anionic cyanine dyes with different acceptors on the opposite ends of a conjugated π-system will adopt a dipolar form that shows a broad charge transfer (CT) transition and not the narrow cyanine transition. Based on this stepwise condensation strategy, in chapter 6, novel asymmetrical chromophores with two different acceptor (A and A’) substituents on the opposite ends of a conjugated π-system were prepared based on chloro-bisaldehyde and subsequent intramolecular substitution of chloride in a 1,6-relation cyclize to six-membered rings under basic conditions. The new asymmetrical A-pi-A’ chromophores with strong TCF and dicyanomethyleneindianone acceptors possess a low electrochemical band gap with lowest unoccupied molecular orbital (LUMO) energy levels from ~ -4.0 to -4.2 eV.

Chapter 7 discusses about an unexpected dehydrogenation of 8-alkoxyjulolidine as a strong electron donor to π-conjugated tricyanofuran (TCF) acceptors and tandem condensation for the synthesis of push-pull chromophores. Density-functional theory calculations identify the α-hydride transfer (HT) of julolidine derivatives, instead of nucleophilic addition, as the key initial step of reaction cascades, followed by efficient β-deprotonation to generate a nucleophilic enamine of isojuloline that condenses in situ with a second equivalent of π-conjugated TCF electrophiles. The scheme represents an exceptional example of intermolecular HT-enabled C(sp³)–H functionalization of both α and β hydrogens of 8-alkoxyjulolidine, notably under simple reaction conditions without using potent catalysts of metal complexes or Lewis acids. The resulting products of isojuloline-based merocyanines exhibit strong near-infrared (NIR) absorption and large first hyperpolarizabilities. The study suggests that HT-involved C(sp³)–H functionalization can be exploited to construct new NIR merocyanines by following rational, regioselective condensation pathways for photonic applications.