Studies of polycyclic aromatic hydrocarbon-based organic materials for optoelectronic applications

Faghih Abdollahi, Maryam (2023) Studies of polycyclic aromatic hydrocarbon-based organic materials for optoelectronic applications. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

π-Conjugated polycyclic aromatic hydrocarbons (PAHs) have found increasing applications in the fields of organic photonics and optoelectronics owing to their highly tunable photophysical properties, redox activity, and supramolecular self-assembling behavior. This PhD thesis mainly focuses on the studies of four classes of PAH-based organic materials, namely diphenyl dibenzofulvenes (DP-DBFs), pentacenetetraone-derived π-extended tetrathiafulvalene analogues (TTFAQ-AQ),donor/acceptor(D/A)-functionalized phenanthroimidazoles(PIs), and bis(diarylmethylene)dihydroanthracenes (Ar₄-DHAs). DP-DBFs have been known to show intriguing crystallization-induced emission enhancement(CIEE) and aggregation-induced emission (AIE) properties; however, the molecular origins for them to exhibit such unique photophysical properties have not yet been clearly reported in the literature. In the first project of this thesis work, we designed and prepared a series of DP-DBFs with various substituents. With the aid of solution and solid-state fluorescence spectroscopic analysis, the AIE and CIEE properties of DP-DBFs we reinvestigated. Our studies confirmed that the twist angle around the C=C bond in the DP-DBF is a key factor to control its fluorescence behavior. In the second study, the structure-property relationship of TTFAQ-AQs were systematically examined. Analyses of their redox properties and electronic band gaps suggested that they could be promising organic semiconducting materials. Moreover, the synthesized TTFAQ-AQs featured double curved π-surfaces that contain both electron-deficient and electron-rich segments. In this way, they can serve as versatile supramolecular partners to interact with 3-dimensional π-conjugated host molecules such as fullerenes. A benzo-fused TTFAQ-AQ derivative was observed to form organized organic co-crystals with electron deficient C₇₀ fullerene through concave- convex shape complementarity and strong donor-acceptor interactions. The results point to promising application in redox-active organic electronic materials. The third project embarked on the design of novel PI-derived organic chromophores and fluorophores. PI derivatives containing electron-donating dithiafulvenyl (DTF) groups and electron-accepting aldehyde groups were synthesized and characterized in terms of their fluorescence and electrochemical properties. The DTF-functionalized PI derivatives were found to show fluorescence sensitivity to singlet oxygen through a chemiluminescent mechanism. Moreover, the chemical oxidative dimerization of DTF end groups led to the construction of a new type of TTFV-PI macrocycles, which showed interesting fluorescence turn-on/off behavior when they interacted with C₆₀ and C₇₀ fullerenes at different ratios. These features make the synthesized PI materials interesting candidates in the fabrication of efficient fluorescence sensors for singlet oxygen and fullerenes. The last project focuses on the synthesis, characterization, and mechanistic analyses on the photocyclization of a series of Ar₄-DHAs. We have made three different derivatives functionalized with methoxy groups at various positions of the phenyl ring, a tethered Ar₄-DHA derivative as a rigid model compound, and their photocyclized products. Although the cyclodehydrogenation reaction of this class of PAHs through the Scholl reaction has been well studied, photocyclization reaction still lacks sufficient understanding and evidence to support detailed mechanisms. In our work, it has been observed that the photocyclization only happens on one side of Ar₄-DHA through a two-fold Mallory reaction. To better understand the mechanisms involved and the molecular reasons behind the two-fold Mallory reaction, we have utilized experimental and theoretical approaches including X-ray crystallographic analyses and density functional theory (DFT) calculations. The frontier molecular orbital (FMO) analyses offered convincing explanation for the regioselectivity of the two-fold Mallory reaction on a Ar₄-DHA. In addition, the importance of the position of the methoxy substituent on the mechanism of photocyclization has been revealed; in particular, when the methoxy groups are at the ortho positions, cyclodemethoxylation occurred instead of cyclodehydrogenation. Collectively, the work in Chapter 5 offers in-depth understanding of the photochemical properties of Ar₄-DHAs as well as valuable guidance for future work on the synthesis of π-extended PAHs and nanographenes from readilly accessbile Ar₄-DHA precursors.

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