Stimuli-Responsive Room-Temperature N-Heteroacene Liquid: In Situ Observation of the Self-Assembling Process and Its Multiple Properties

Abstract

A novel stimuli-responsive room-temperature photoluminescent liquid 1 based on the N-heteroacene framework is developed and analyzed by several experiments such as differential scanning calorimetry, X-ray diffraction, dynamic viscoelasticity measurement, in situ observation by optical and polarized optical microscopes, UV–vis absorption and fluorescence spectroscopy, and by theoretical methods such as ab initio calculation and molecular dynamics (MD) computer simulation techniques. In contrast to stimuli-responsive solid materials reported previously, liquid 1 in response to HCl vapor as a single stimulus can involve dramatically multiple changes in physical properties such as rheological behavior, morphology, as well as photoluminescence. The present ab initio calculation and microsecond-timescale MD simulations reveal that the complexation of 1 and HCl molecules induces a large dipole moment, leading to the formation of stacking structures because of their dipole–dipole interaction. Upon exposure to HCl vapor, in situ microscopic observation of the stimuli-responsive liquid elucidates a self-assembling process involving the formation of the wrinkle structure in a micrometer scale, indicating disorder–order phase transition. Further exposure of 1 to HCl vapor from seconds to hours has an influence on the macroscopic physical properties such as viscosity, viscoelasticity, and photoluminescent colors. The synergy between the experimental and theoretical investigations opens a new strategy to develop a novel class of stimuli-responsive materials showing multiple changes in physical properties.

Stimuli-responsive Behaviors for Room-temperature Fluorescent Liquid Materials based on N-Heteroacenes and their Mixtures in Response to HCl Vapor and their Facile Synthesis

“…Also, the previous report has evaluated the protonated position by the single crystallographic X-ray analysis as well as 1 H NMR measurement of similar p-conjugated framework to Py and Phen. [25][26] These result suggests that the protonation of Bz and Phen occurs at imino-N atom of quinoxaline framework, whereas Py captures with proton at imino-N atom of pyridine ring. 26 Further, corresponding conjugated acids Figure 4a-c. 26,[49][50] The Commission Internationale de l'Eclairage (CIE) 1931 chromaticity diagram [53][54] in Figure 4d shows that each point for Bz, Py, and Phen before exposure to HCl vapor changes into each one after exposure, corresponding to the color change seen in the right photographs in Figure 3c.…”

“…In this study, we resolved this disadvantage by the redesigning of novel molecular structures for a series of SFLs, benzene-substituted (Bz), pyridine-substituted (Py), and phenanthrene-based quinoxaline (Phen), shown in Scheme 1. As a result, we successfully reduce the reaction step from 6 to 3 steps, compared to previous scheme, [25][26][27][28][29] increasing product yields up to 40%.…”

“…Phen as a solid state tends to form stronger p-p interaction through overlaps between p-conjugated frameworks rather than Bz and Py, making FL peak red-shifted as compared with those for Bz and Py. 25…”

“…fluorescent liquid materials (SFLs) based on N-heteroacene frameworks. [25][26][27][28][29] However, SFLs reported involve disadvantage on the product yield that previously molecular designs need at least 6 steps started from commercially available compound, to afford SFLs in very low yield (<10% in total steps). In this study, we resolved this disadvantage by the redesigning of novel molecular structures for a series of SFLs, benzene-substituted (Bz), pyridine-substituted (Py), and phenanthrene-based quinoxaline (Phen), shown in Scheme 1.…”

Circularly Polarized Luminescence from Solvent‐Free Chiral Organic π‐Liquids

“…It is a general method to introduce branched alkyl side‐chains to the π‐core for preventing the interaction between the π‐units and effectively lowering the glass transition temperature to facilitate the π‐liquid [6–20] . Although most of these branched alkyl chains contain the chiral carbon center, the racemic ones were often employed.…”