Multi-Color Photoluminescence Based on Mechanically and Thermally Induced Liquid-Crystalline Phase Transitions of a Hydrogen-Bonded Benzodithiophene Derivative

Stimuli-fluorochromic smart organic materials

Smart materials based on stimuli-fluorochromic π-conjugated solids (SFCSs) have aroused significant interest due to their versatile and exciting properties, leading to advanced applications. In this review, we highlight the recent developments in SFCS-based smart materials, expanding beyond organometallic compounds and light-responsive organic luminescent materials, with a discussion on the design strategies, exciting properties and stimuli-fluorochromic mechanisms along with their potential applications in the exciting fields of encryption, sensors, data storage, display, green printing, etc. The review comprehensively covers single-component and multi-component SFCSs as well as their stimuli-fluorochromic behaviors under external stimuli. We also provide insights into current achievements, limitations, and major challenges as well as future opportunities, aiming to inspire further investigation in this field in the near future. We expect this review to inspire more innovative research on SFCSs and their advanced applications so as to promote further development of smart materials and devices.

Advanced Functional Liquid Crystals

Liquid crystals have been intensively studied as functional materials. Recently, integration of various disciplines has led to new directions in the design of functional liquid-crystalline materials in the fields of energy, water, photonics, actuation, sensing, and biotechnology. Here, recent advances in functional liquid crystals based on polymers, supramolecular complexes, gels, colloids, and inorganic-based hybrids are reviewed, from design strategies to functionalization of these materials and interfaces. New insights into liquid crystals provided by significant progress in advanced measurements and computational simulations, which enhance new design and functionalization of liquid-crystalline materials, are also discussed.

Unveiling the Role of Hydrogen Bonds in Luminescent N-Annulated Perylene Liquid Crystals

We report the liquid-crystalline (LC) and luminescent properties of a series of N-annulated perylenes (1-4) in whose molecular structures amide and ester groups alternate. We found that the LC properties of these compounds not only depend on the number of hydrogen-bonding units, but also on the relative position of the amide linkers in the molecule. The absence of amide groups in compound 1 leads to no LC properties, whereas four amide groups induce the formation of a wide temperature range columnar hexagonal phase in compound 4. Remarkably, compound 3, with two amide groups in the inner part of the structure, stabilizes the columnar LC phases better than its structural isomer 2, with the amide groups in the outer part of the molecule. Similarly, we found that only compounds 1 and 2, which have no hydrogen bonding units in the inner part of the molecule, exhibit luminescence vapochromism upon exposure to organic solvent vapors.

Amphiphilic Diazapyrenes with Multiple Stimuli-Responsive Properties

A series of amphiphilic diazapyrenes exhibiting switchable fluorescence under the stimulus of mechanical force or water vapor are reported for the first time. Comprehensive studies of their photophysical properties in different states based on UV-Vis absorption, FL emission, FT-IR spectroscopy, and XRD analysis have revealed a stimuli-induced excimer-based sensing mechanism. The relationship between molecular structures and optical responsive properties of these diazapyrene derivatives is illustrated. Moreover, the unique fluorescent, stimuli-responsive behaviors of these diazapyrene compounds in the solid state are used to fabricate sensory films for successively and orthogonally sensing mechanical force and water vapor. In contrast to the well-established knowledge on the transformation between the pyrene monomer and excimer, our study offers valuable information about the unknown diazapyrene excimers and demonstrates their potential applications in biocompatible force sensors, data storage, and humidity sensors.

Smart Supramolecular Self-Assembled Nanosystem: Stimulus-Responsive Hydrogen-Bonded Liquid Crystals

In a liquid crystal (LC) state, specific orientations and alignments of LC molecules produce outstanding anisotropy in structure and properties, followed by diverse optoelectronic functions. Besides organic LC molecules, other nonclassical components, including inorganic nanomaterials, are capable of self-assembling into oriented supramolecular LC mesophases by non-covalent interactions. Particularly, huge differences in size, shape, structure and properties within these components gives LC supramolecules higher anisotropy and feasibility. Therefore, hydrogen bonds have been viewed as the best and the most common option for supramolecular LCs, owing to their high selectivity and directionality. In this review, we summarize the newest advances in self-assembled structure, stimulus-responsive capability and application of supramolecular hydrogen-bonded LC nanosystems, to provide novel and immense potential for advancing LC technology.

Mechano-induced photoluminescence colour change in an alkyltolane-terminated cyanostilbene

A novel alkyltolane-terminated cyanostilbene is synthesized. The photoluminescence colour changed depending on the precipitation method, with either a red shift or blue shift in the photoluminescence spectra observed upon mechanical grinding.

Flexible π-Conjugated 2,5-Diarylamino-Terephthalates: A New Class of Mechanochromic Luminophores with Tunable Aggregation States

The generation of different thermodynamically (meta)stable states is crucial for the development of smart solid-state luminescent materials. However, the design of luminophores with tunable aggregation states is remaining a grand challenge. Herein, we present a family of mechanochromic luminophores with tunable metastable states, based on the dynamically controllable π-π stacking of the flexible π-conjugated structure of 2,5-diarylamino-terephthalates in the solid state. The experimental data revealed that both the kinetically controlled metastable state and thermodynamic controlled stable state can be generated via tuning the intermolecular interactions such as π-π stacking and hydrogen bonds. As a result, the highly sensitive mechano-stimuli response of these luminophores was successfully achieved.

Mechanochromic Delayed Fluorescence Switching in Propeller-Shaped Carbazole-Isophthalonitrile Luminogens with Stimuli-Responsive Intramolecular Charge-Transfer Excited States

Herein, the universal design of high-efficiency stimuli-responsive luminous materials endowed with mechanochromic luminescence (MCL) and thermally activated delayed fluorescence (TADF) functions is reported. The origin of the unique stimuli-triggered TADF switching for a series of carbazole-isophthalonitrile-based donor-acceptor (D-A) luminogens is demonstrated based on systematic photophysical and X-ray analysis, coupled with theoretical calculations. It was revealed that a tiny alteration of the intramolecular D-A twisting in the excited-state structures governed by the solid morphologies is responsible for this dynamic TADF switching behavior. This concept is applicable to the fabrication of bicolor emissive organic light-emitting diodes using a single TADF emitter.

Effect of electron-withdrawing moieties on mechanochromism of phenothiazine derivatives

Two phenothiazine derivatives with electron-withdrawing cyano and ester groups emitted longer wavelength fluorescence and exhibited different mechanochromic behaviors.