Role of Voluminous Substituents in Controlling the Optical Properties of Disc/Planar-Like Small Organic Molecules: Toward Molecular Emission in Solid State

Abstract

Inspired by the role of coadsorbents in dye-sensitized solar cells, a pathway to disfavor aggregation in disclike luminophores was studied to enhance solid-state emission. By restricting the intense π-π stacking using a multicyclic aliphatic ring system, we brought the lithocholic ring system as bulky side substitution into the fluorophore design. Compared to the small-size cyclohexyl substitution in BC-CY6, which exhibited a bathochromic shift in solid-state emission owing to the intermolecular interactions, lithocholic-substituted BC-LTH had reduced intense intermolecular interactions. This very bulky/voluminous side substitution (lithocholic unit) helped us extract intermolecular interaction-free molecular emission in solid state. The cyclohexyl substitution provided solid-state emission, and the broad and high Stokes shift provided an insight into stacking interactions. Face-to-face stacking-originated dimerlike species was observed in the crystal packing, which was studied by theoretical geometry optimization. The dimer species exhibited an intermolecular distance of 3.5 Å. The molecular sizes of the developed chromophores were estimated by geometry optimization, and it was concluded that the dimeric interactions in BC-LTH may not be formed owing to the voluminous nature of the side substitution present. Hence, we have been able to successfully establish through molecular level understanding the role of lithocholic functionality in tuning the optoelectronic properties of various emissive materials for different applications.