Effects of Molecular Interactions on the Photophysical Properties of Luminescent Materials

Molecular luminescent materials have been widely studied and used in different applications, including optoelectronics, sensors, and bioimaging. The photophysical properties of these materials are controlled by several factors, such as their aggregation behaviors, molecular electronic structures, and modes of assembly. Molecular interactions, either intra- or intermolecular, play a significant role in regulating these factors. Therefore, understanding these interactions, which are unique to each material system, improves our ability to rationally design molecular luminescent materials with desired electronic and optical properties. In this dissertation, we explore the effects of molecular interactions on the photophysical properties, such as emission colors and photoluminescence quantum efficiencies (PLQEs), for two different classes of materials: cyclometalated Pt(II) complexes and organic metal halide hybrids. Cyclometalated Pt(II) complexes adopt a square-planar geometry that favors their assembly through Pt-Pt interactions. These complexes are known to display very distinguished photophysical properties depending on the level of interactions between the Pt centers. They can either exhibit emissions from the high energy ligand centered/metal-to-ligand charge transfer (LC/MLCT) state or the lower energy metal-to-metal-ligand charge transfer (MMLCT) state. In binuclear Pt(II) complexes, the Pt centers are bridged by an appropriate bridging ligand (e.g., pyrazole) which dictates their molecular geometries. Chapter 2 and 3 of this dissertation discuss binuclear Pt(II) complexes with H-frame and A-frame geometries, respectively. In Chapter 2, the synthesis and characterization of a series of binuclear 2,4-difluorophenylpyridine Pt(II) complexes bridged by thiazol-2-thiolate ligands with different bulkiness are reported. The three complexes adopt H-frame geometries or half-lantern structures with short Pt-Pt distances ranging from 2.916 to 2.945 Å, favoring their intramolecular Pt-Pt interactions. All these complexes are found to exhibit strong and stable 3MMLCT emissions in the orange/red region, with PLQEs reaching near-unity. Chapter 3 covers another series of binuclear Pt(II) complexes that have no intramolecular Pt-Pt interactions due to their A-frame geometries and long Pt-Pt distances. These complexes are bridged by pyrazole and have different cyclometallating ligands (N,N-dimethyl-3-(pyridin-2-yl)aniline and 2-(naphthalen-1-yl)pyridine). The complex with N,N-dimethyl-3-(pyridin-2-yl)aniline as its cyclometallating ligand exhibit a positive solvatochromic behavior introduced by the electron-donating dimethylamino group. This behavior is studied in several solvents with different polarities and evaluated using the Lippert-Mataga correlation and the empirical solvent polarity parameter ET(30). The positive solvatochromism recorded for this complex is assigned to the intramolecular charge transfer character of its emission. In addition to binuclear Pt(II) complexes, mononuclear cationic Pt(II) complexes are studied in Chapter 4. Intermolecular Pt-Pt interactions can exist between these complexes, which are regulated in a series of four mononuclear cationic Pt(II) complexes by modifying their counter-anions and molecular structures. These complexes exhibit strong 3MMLCT emissions in solid state with colors ranging from green to deep red. The structural and photophysical characterizations of these complexes reveal a clear correlation between their stacking modes and photophysical properties. The second class of materials studied in this dissertation is the ionically bonded organic metal halide hybrids. The remarkable structure diversity of this class of materials, originating from the infinite combinations of organic cations and metal halides, unlocks many new properties of both components. In Chapter 5, the synthesis and characterization of a series of benzoquinolinium (BZQ+) metal halides with a unique two-dimensional (2D) structure are reported. The structures are composed of layers of face-sharing metal halides (Pb2X5-, X= Cl, Br) separated and balanced by cationic BZQ+ arrays. The BZQ+ cations are found to be responsible for the emission of these hybrid materials, as revealed by their optical characterizations and theoretical calculations. The inorganic layers in the 2D structure scaffold the organic cations and provide a more rigid architecture due to stronger molecular interactions, enhancing the PLQEs and stability of the BZQ+ cations in the 2D metal halide hybrids compared to the pristine BZQ halides. In addition to the scaffolding effect, the composition of metal halide also affects the photophysical properties of BZQ+ cations. Replacing Cl by the heavy atom Br results in room temperature phosphorescence (RTP) from cationic BZQ+ due to heavy atom effects.