The mechanically induced color change from UV to visible region

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.

Mechanochromic Luminescence (MCL) of Purely Organic Two-Component Dyes: Wide-Range MCL over 300 nm and Two-Step MCL by Charge-Transfer Complexation

Despite recent extensive studies on mechanochromic luminescence (MCL), rational control over the magnitude of the emission-wavelength shift in response to mechanical stimuli remains challenging. In the present study, a two-component donor-acceptor approach has been applied to create a variety of organic MCL composites that exhibit remarkable emission-wavelength switching. Dibenzofuran-based bis(1-pyrenylmethyl)diamine and typical organic fluorophores have been employed as donor and acceptor dyes, respectively. Outstanding wide-range MCL with an emission-wavelength shift of over 300 nm has been achieved by mixing the diamine with 3,4,9,10-perylenetetracarboxylic diimide. Unprecedented two-step MCL in response to mechanical stimuli of different intensity has also been realized for a two-component mixture with 9,10-anthraquinone. Fluorescence microscopy observations at the single-particle level revealed that the segregation and mixing of the two-component dyes contribute to the stimuli-responsive emission-color switching of the MCL composites.

A Mechanochromic and Photochromic Dual-Responsive Co-assembly with Multicolored Switch: A Peptide-Based Dendron Strategy

In this contribution, we report a unique co-assembly composed of pyrene and spiropyran that were linked separately at the focal point of the same peptide-based dendron (Phe-Glu), in which the dendrons offer driving forces for the coaggregation. A series of co-assemblies with different weight ratios (Py-Phe-Glu/SP-Phe-Glu) were prepared and the morphology could be tuned. It is found that the resulting stable co-assembled organogel is double switchable triggered by light and heat. TEM revealed that, in the xerogel, Py-Phe-Glu formed rigid rod nanofibers with large diameters and acted as a rigid sketelon where the gracile interwoven fibrous structure of SP-Phe-Glu grew. More interestingly, the original powder of the co-assembled xerogel (1.0 mg/0.1 mg) not only displayed a sequential high-contrast tricolored switch from dark blue to bright cyan and to red under external force but also presented multistate accessible photochromic properties. Such mechanochromic and photochromic behaviors of the xerogel are mainly due to the transition of different excimers of pyrene and the force/photoinduced ring-opening reaction of spiropyran. It is rarely reported that self-assembled soft materials achieve mechanochromic and photochromic dual-responsive behaviors with a high-contrast multicolored switch. We believe the co-assembly strategy based on polypeptide dendrons can be extended to other systems for establishing novel intelligent fluorescent materials.

How side-chain substituents and substrates influence mechanochromic luminescence: case study with pyrene

Many organic molecules exhibit reversible, force-induced emission change known as mechanochromic luminescence (ML) and can potentially be used as mechanosensors.

A Novel Mechanochromic and Photochromic Polymer Film: When Rhodamine Joins Polyurethane

A rhodamine-based molecule, Rh-OH, is synthesized. Rh-OH exhibits a reversible mechanochromic luminescent character but a passivating response to UV light. An elastomeric polymer film based on polyurethane with embedded Rh-OH is prepared via a polycondensation reaction. The film shows mechanochromic and photochromic properties with reversible color change, which originates from the isomerization of the Rh-OH molecule from a twisted spirolactam in the ring-closed form to a planarized zwitterionic structure in the ring-open state.