Efficient Negative Photochromism by the Photoinduced Migration of Photochromic Merocyanine/Spiropyran in the Solid State

Photoinduced fluorescence modulation through controllable intramolecular [2+2] photocycloaddition in single molecules and molecular aggregates

We report a general molecular design strategy of spatial proximity, which allows intramolecular [2+2] photocycloaddition reaction to take place in both single molecules and molecular aggregates. Sharply contrasting photoinduced fluorescence changes in solution and in the solid state were found and attributed to the aggregation-induced quenching property of the monomers and the aggregation-induced emission nature of the photodimers.

From Visible to Near-Infrared Light-Triggered Photochromism: Negative Photochromism

Photochromic compounds, whose key molecular properties can be effectively modulated by light irradiation, have attracted significant attention for their potential applications in various research fields. The restriction of photoisomerization coloration induced by ultraviolet light limits their applications in the biomedical field and some other fields. Negative photochromism, wherein a relatively stable colored isomer transforms to a colorless metastable isomer under low-energy light irradiation, offers advantages in applications within materials science and life science. This review provides a summary of negatively photochromic compounds based on different molecular skeletons. Their corresponding design strategies and photochromic properties are presented to provide practical guidelines for future investigations. Negatively photochromic compounds can effectively expand the range of photochromic switches for future applications, offering unique properties such as responsiveness to visible to near-infrared light.

Preparation of Photoswitchable Polyacrylic Nanocomposite Fibers Containing Au Nanorods and Spiropyran: Optical and Plasmonic Properties

Photoswitchable nanofibers and nanocomposite fibers containing plasmonic nanoparticles have attracted a great deal of interest in optical and plasmonic devices. Herein, photoswitchable poly(methyl methacrylate-co-vinylimidazole-co-spiropyran ethyl acrylate) (MVSP) and its copolymer with butyl acrylate (MBVSP) were prepared via emulsion polymerization, and the corresponding nanofibers (MVSP@NF and MBVSP@NF) and nanocomposite fibers (MVSP/Au@NF and MBVSP/Au@NF) containing AuNRs were fabricated through electrospinning. FTIR and ¹H NMR analyses confirmed the progress of the copolymerization reaction. The morphology of the prepared nanofibers containing AuNRs with an aspect ratio of 2.5 was identified by SEM and TEM techniques. The inclusion of vinylimidazole into the copolymer chains resulted in well-dispersed AuNRs. Photoisomerization studies revealed a higher photochromic efficiency for MBVSP@F (reflective intensity of 37.4%) with respect to MVSP@NF (reflective intensity of 62.5%) because of the greater flexibility of the chains. In addition, the presence of AuNRs in the nanocomposite fibers with high absorptivity intensified the photochromic properties for both samples. The polarization-dependent plasmonic band of AuNRs was switched between 650 and 634 nm through the photoisomerization of nonpolar SP to polar MC reversibly for MVSP/Au@NF. This displacement was just 4 nm for MBVSP/Au@NF, owing to the limited coupling between AuNRs and MC isomers. Besides, the capability of both nanocomposite fibers for reversible optical patterning was investigated by fast write-erase cycles. Enhanced photofatigue resistance in those fibers and the photomodulation of the plasmonic band of AuNRs using SP to MC isomerization revealed their promising potential for optical patterning and on-demand real-time plasmonic devices.

A Two-Step Baromechanical Cycle for Repeated Activation and Deactivation of Mechanophores

Mechanophores that are embedded in a polymer backbone respond to the application of mechanical stretching forces by geometric changes such as bond rupture. Typically, these structural changes are irreversible, which limits the applicability of functional materials incorporating mechanophores. Using computational methods, we, here, present a general method of restoring a force-activated mechanophore to its deactivated form by using hydrostatic pressure. We use the spiropyran-merocyanine (SP-MC) interconversion to show that repeated activation of the SP mechanophore and deactivation of MC can be achieved by alternating mechanical stretching and hydrostatic compression, respectively. In the baromechanical cycle, MC acts as a "barophore" that responds to hydrostatic pressure by bond formation. The activation and deactivation of SP/MC are understood in terms of strain and electronic effects. Beneficially, this two-step baromechanical cycle can be observed in real time by using UV/vis spectroscopy. Our calculations pave the way for improving the applicability and reusability of force-responsive materials.

Photoluminescent Nanoinks with Multilevel Security for Quick Authentication of Encoded Optical Tags by Sunlight: Effective Physicochemical Parameters on Responsivity, Printability, and Brightness

Investigation of developed photoactive security inks and anticounterfeiting technologies in recent years indicates significant challenges for future of this research area, such as increase of security, fast responsivity, and facile authentication. Here, amine-functionalized latex nanoparticles were synthesized by emulsion copolymerization of methyl methacrylate (MMA) and 2-(dimethylamino)ethyl methacrylate (DMAEMA). Size of the latex nanoparticles was increased as a function of poly(dimethylaminoethyl acrylate) (PDMAEMA) contents, and also a decrease of particle size was obtained in response to an increase of temperature from 25 to 70 °C, above the lower critical solution temperature (LCST) of PDMAEMA. Surface physical modification of the functional latex nanoparticle with spiropyran photoswitches led to the development of anticounterfeiting nanoinks that have multilevel security and photochromic/fluorescence properties with a higher intensity and also brightness. The photoluminescent nanoinks were made of spiropyran latex nanoparticles and used for printing of the encoded optical security tags on cellulosic papers and banknotes. The results displayed that an increase of the particle size above 100 nm and an increase of the PDMAEMA contents led to a remarkable decrease of printability, fluorescent emission, brightness, intensity of photochromism, and also resolution of the printed security tags. As a significant advantage of the developed security inks, the printed security tags could be authenticated easily and fast upon sunlight irradiation by means of photochromism. The responsivity of encoded tags from the invisible to visible state is immediate upon sunlight irradiation for some seconds, whose intensity of coloration is appropriate and detectable clearly by naked eyes. The security anticounterfeiting inks based on spiropyran with multilevel security have been reported for the first time for applying in printing of encoded security tags on cellulosic papers, banknotes, and other documents, where the printed marks are detectable on sunlight exposure.