A supramolecular approach for engineering functional solid-state chromophore arrays within metal-organic materials
Date of Award
Doctor of Philosophy (Ph.D.)
Jeremy Klosterman (Advisor)
Moira van Staaden (Other)
Ksenija Glusac (Committee Member)
H. Peter Lu (Committee Member)
The spacing and relative packing of aromatic chromophores highly affects the performance of molecular electronic devices and materials, such as organic light-emitting diodes (OLEDs) and conductive polymers. In this work, a novel supramolecular approach for the construction of ordered aromatic chromophore arrays is presented. Our approach consists of embedding aromatic chromophores within robust metal-organic materials through self-assembly of predesigned chromophore-functionalized organic ligands and transition metals. Utilizing our approach, we enforced representative aromatic chromophores (such as carbazole) to adopt unique packing motifs that are beneficial for their photophysical characteristics. By embedding carbazole in a rigid copper-based metal-organic framework (MOF), we constructed 1D infinite columnar stacks of carbazoles that have potential in anisotropic charge-transport. We achieved significant enhancement of solid-state fluorescence originating from carbazole via embedding the carbazole moiety within a rigid zinc-based MOF, which holds chromophores spaced out, thus preventing aggregation-caused fluorescence quenching. We discovered room-temperature phosphorescence in addition to fluorescence when the carbazole moiety was embedded within zinc-based metal-organic chains instead of zinc-based MOF. We expanded our approach towards mixed-chromophore MOFs and demonstrated that the mixed-chromophore (carbazole:anthracene 1:1) zinc MOF serves as a platform for carbazole-to-anthracene energy transfer.
Through the combination of thorough analysis of crystal structures and detailed photophysical studies, we established a correlation between chromophore packing motifs and bulk photophysical properties of metal-organic materials and their parent ligands. We concluded that extended cofacial aromatic interactions are detrimental for solid-state fluorescence, and disruption of cofacial stacks achieved via embedding of chromophores in zinc-based metal-organic materials leads to enhanced fluorescence. On the other hand, we discovered that although detrimental for fluorescence, carbazole-based cofacial stacks enhance triplet state population and thus are profitable for enhancing solid-state phosphorescence.
We investigated solution-state positive solvatochromism of the carbazolyl family of our ligands, N-carbazolyl-benzene carboxylate esters, and ascribed the origin of solvatochromic emission to the formation of twisted intramolecular charge-transfer state (TICT), which is not formed in solid state due to restricted torsion motion. We supported our conclusion that solution-state emission originates from TICT with a series of ultrafast transient absorption measurements.
In conclusion, we introduced a supramolecular approach for the construction of functional chromophore arrays that have potential in organic photovoltaic applications, such as solid-state lighting and anisotropic charge transport.
Lifshits, Liubov Mikhaylovna, "A supramolecular approach for engineering functional solid-state chromophore arrays within metal-organic materials" (2016). Photochemical Sciences Ph.D. Dissertations. 83.