A supramolecular approach for engineering functional solid-state chromophore arrays within metal-organic materials

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.