A Guide to Design Functional Molecular Liquids with Tailorable Properties using Pyrene-Fluorescence as a Probe

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.

Solvent‐Free Luminous Molecular Liquids

“…Moreover, such LMLs often exhibit low viscosities. For instance, the LMLs of alkylated tetrazines and alkylated naphthalenes exhibit complex viscosities (η*) in the ranges of 28–58 and 38–67 mPa • s, respectively, which are much lower than the η* of alkylated oligo( p ‐phenylenevinylene)s (0.64–34.6 Pa • s), alkylated 9,10‐diphenylanthracenes (0.28–84.0 Pa • s), alkylated pyrenes (2.3–58.0 Pa • s), and an alkylated bromonaphthalimide (0.79 Pa • s) . Such low viscous LMLs are promising for developing refreshable devices owing to their facile refilling.…”

“…For example, 9,10‐dibutylanthracene was a solid with a high T m (105 °C), whereas its isomer 9‐butyl‐10‐(1‐methylpropyl)anthracene ( Anth3 ) is a liquid at room termpature . The effect of molecular symmetry on the T m is probably caused by interrupted packing structures among neighboring molecules. ii) Chain Length : Essentially, T m decrease with increased chain length because longer side chains with a higher degree of bulkiness suppress interchromophore interactions more efficiently than shorter ones . However, the length of the side chains should be carefully controlled because the liquid phase was created by a delicate balance between the interchromophore π–π interactions and van der Waals interactions among side chains.…”

A Spin‐Active, Electrochromic, Solvent‐Free Molecular Liquid Based on Double‐Decker Lutetium Phthalocyanine Bearing Long Branched Alkyl Chains

“…Numerous studies of alkylated π‐molecules have proven that the substitution pattern, length, and level of branching of alkyl chains are very important factors for determining the bulk properties . The bulky alkyl chains can protect the functional core moiety from photodegradation .…”

Synthesis of Oligo(1,8‐pyrenylene)s: A Series of Functional Molecular Liquids

“…Unlike oligo(pyrenylene)s 1 – 5 , which are all solids, some recently reported pyrene‐containing oligoarylenes, e. g. 6 , were found to be liquids at room temperature . This property is very unusual for an aromatic molecule of its size.…”

“…Nakanishi's guidelines for the design of FMLs address certain key structural features that disfavor π–π interactions and thus suppress solidification . These include biaryl bonds, alkoxy substituents with branching and low symmetry in the π skeleton.…”