Abstract
Solvent-free, nonvolatile, room-temperature alkylated-π functional molecular liquids (FMLs) are rapidly emerging as a new generation of fluid matter. However, precision design to tune their physicochemical properties remains a serious challenge because the properties are governed by subtle π-π interactions among functional π-units, which are very hard to control and characterize. Herein, we address the issue by probing π-π interactions with highly sensitive pyrene-fluorescence. A series of alkylated pyrene FMLs were synthesized. The photophysical properties were artfully engineered with rational modulation of the number, length, and substituent motif of alkyl chains attached to the pyrene unit. The different emission from the excimer to uncommon intermediate to the monomer scaled the pyrene-pyrene interactions in a clear trend, from stronger to weaker to negligible. Synchronously, the physical nature of these FMLs was regulated from inhomogeneous to isotropic. The inhomogeneity, unexplored before, was thoroughly investigated by ultrafast time-resolved spectroscopy techniques. The result provides a clearer image of liquid matter. Our methodology demonstrates a potential to unambiguously determine local molecular organizations of amorphous materials, which cannot be achieved by conventional structural analysis. Therefore this study provides a guide to design alkylated-π FMLs with tailorable physicochemical properties.
Highlights
IntroductionA series of alkylated pyrenes, which have branched alkyl chains of different substitution patterns, is employed as a rational functional molecular liquids (FMLs) model
Precise tailoring of the bulk functions with rational molecular design is a great challenge because of the rapid but unguided development of alkylated-π functional molecular liquids (FMLs)
Pyrenes [1,2,3,4,5,6,7] (Fig. 1a) were synthesized by Suzuki coupling reactions (Supplementary Figs S1–S24). 1–5 were obtained as transparent viscous fluids at room temperature, while reference compounds 640 and 7 appeared as solid powders (Fig. 1c). 5 exhibits complex and broad signals in the 1H and 13C NMR spectra
Summary
A series of alkylated pyrenes, which have branched alkyl chains of different substitution patterns, is employed as a rational FML model All these compounds are solvent-free liquids without long-range ordering at room temperature. The inhomogeneity in locally stacked species is evidenced for most compounds by ultrafast time-resolved spectroscopy techniques, the mechanism for the formation of intermediate species remains an open question that needs further studies. The discovery of these inhomogeneous FMLs using unconventional techniques nearly dispels the unclear understanding regarding the homogeneity/inhomogeneity in amorphous soft-materials, i.e., liquid matter. We believe our new approach will provide a guide to design alkylated-π FMLs with tailorable π-unit-derived optoelectronic functions and a predictable liquid nature
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