Indolino-Oxazolidine Acido- and Photochromic System Investigated by NMR and Density Functional Theory Calculations

Rewritable acidochromic papers based on oxazolidine for anticounterfeiting and photosensing of polarity and pH of aqueous media

Oxazolidine is a new category of stimuli-chromic organic compounds with unique characteristics in response to polarity, pH changes, water, light, and metal ions that were well-known as solvatochromism, acidochromism, hydrochromism, photochromism, and ionochromism, respectively. Therefore, oxazolidine derivatives have been developed for their potential applications in chemosensors, anticounterfeiting, and rewritable hydrochromic papers. In this study, various oxazolidine derivatives containing hydroxyl and naphthalene substituted groups were synthesized by using two different indolenine compounds. The synthesized oxazolidine derivatives were used for investigation of solvatochromism in different solvents, and also acidochromism in various pHs by using UV–Vis and fluorescence spectroscopies. In addition, the oxazolidine derivatives were coated on cellulosic papers using a layer-by-layer strategy to develop rewritable acidochromic papers for printing of security tags on cellulosic papers by using acidic and alkaline solutions as water-based inks. Therefore, the developed rewritable acidochromic papers could be used as security papers.

Inclusion-Activated Reversible E/Z Isomerization of a Cyanostilbene Derivative Based on Cucurbit[8]uril under 365 nm Ultraviolet Irradiation

The photoisomerization behavior of cyanostilbene molecules is a hotspot in supramolecular configuration transformation research. Here, we reported a cyanostilbene derivative that converted from the Z,Z-isomer to the E,E-isomer under UV light irradiation at 365 nm. This process can be reversibly converted only in the presence of cucurbit[8]uril under the same light source, accompanied by the reversible conversion of fluorescence from green to yellow. No effective configuration transformation occurred with guest molecules only or upon the addition of cucurbit[7]uril. The photoisomerization was fully characterized by UV-vis and fluorescence spectroscopy, NMR, high-resolution mass spectrometry, and transmission electron microscopy. This work provides a new method for the supramolecular macrocyclic-activated configuration transformation.

Development of highly sensitive metal-ion chemosensor and key-lock anticounterfeiting technology based on oxazolidine

Optical chemosensors and ionochromic cellulosic papers based on oxazolidine chromophores were developed for selective photosensing of metal ions and information encryption as security tags, respectively. The oxazolidine molecules have been displayed highly intense fluorescent emission and coloration characteristics that are usable in sensing and anticounterfeiting applications. Obtained results indicated that oxazolidine molecules can be used for selective detection of pb²⁺ (0.01 M), and photosensing of Fe³⁺, Co²⁺ and Ag⁺ metal ion solutions by colorimetric and fluorometric mechanisms with higher intensity and sensitivity. Also, oxazolidine derivatives were coated on cellulosic papers via layer-by-layer method to prepare ionochromic papers. Prepared ionochromic papers were used for printing and handwriting of optical security tags by using of metal ion solutions as a new class of anticounterfeiting inks with dual-mode fluorometric and colorimetric securities. The ionochromic cellulosic papers can be used for photodetection of metal ions in a fast and facile manner that presence of metal ions is detectable by naked eyes. Also, key-lock anticounterfeiting technology based on ionochromic papers and metal ion solution as ink is the most significant strategy for encryption of information to optical tags with higher security.

Photoluminescent and Chromic Nanomaterials for Anticounterfeiting Technologies: Recent Advances and Future Challenges

Counterfeiting and inverse engineering of security and confidential documents, such as banknotes, passports, national cards, certificates, and valuable products, has significantly been increased, which is a major challenge for governments, companies, and customers. From recent global reports published in 2017, the counterfeiting market was evaluated to be $107.26 billion in 2016 and forecasted to reach $206.57 billion by 2021 at a compound annual growth rate of 14.0%. Development of anticounterfeiting and authentication technologies with multilevel securities is a powerful solution to overcome this challenge. Stimuli-chromic (photochromic, hydrochromic, and thermochromic) and photoluminescent (fluorescent and phosphorescent) compounds are the most significant and applicable materials for development of complex anticounterfeiting inks with a high-security level and fast authentication. Highly efficient anticounterfeiting and authentication technologies have been developed to reach high security and efficiency. Applicable materials for anticounterfeiting applications are generally based on photochromic and photoluminescent compounds, for which hydrochromic and thermochromic materials have extensively been used in recent decades. A wide range of materials, such as organic and inorganic metal complexes, polymer nanoparticles, quantum dots, polymer dots, carbon dots, upconverting nanoparticles, and supramolecular structures, could display all of these phenomena depending on their physical and chemical characteristics. The polymeric anticounterfeiting inks have recently received significant attention because of their high stability for printing on confidential documents. In addition, the printing technologies including hand-writing, stamping, inkjet printing, screen printing, and anticounterfeiting labels are discussed for introduction of the most efficient methods for application of different anticounterfeiting inks. This review would help scientists to design and develop the most applicable encryption, authentication, and anticounterfeiting technologies with high security, fast detection, and potential applications in security marking and information encryption on various substrates.

Encryption and authentication of security patterns by ecofriendly multi-color photoluminescent inks containing oxazolidine-functionalized nanoparticles

Counterfeiting of confidential documents has been a costly challenge for banks, companies, and customers. Encryption of invisible security marks, such as barcodes, quick response codes, and logos, in national or international confidential documents by high-security anticounterfeiting inks is the most significant solution for counterfeiting problems. Ecofriendly multi-color photoluminescent anticounterfeiting inks based on highly-fluorescent polymer nanoparticles functionalized with new oxazolidine derivatives were developed for the fast and facile encryption of security labels on cellulosic documents, such as paper currency, passport, and certificate. Depending on the polarity of functionalized polymer nanoparticles, a wide range of colors and fluorescence emissions were observed as a result of polar-polar interactions between the oxazolidine molecules and surface functional groups of the nanoparticles. The fluorescent polymer nanoparticles showed spherical, vesicular, and cauliflower-like morphologies resulted from different surface functional groups. Functional polymer nanoparticles displayed high stability and printability on cellulosic substrates due to hydrogen bonding interactions. The highly-fluorescent polymer nanoparticles were also used to prepare anticounterfeiting inks with different colors and fluorescence emissions. All the ecofriendly polymeric anticounterfeiting inks were loaded to stamps with specific marks, and then applied to different confidential documents. Printed labels displayed highly intense fluorescence emission in different colors (green, orange, pink, and purple depending on the matrix polarity) under UV irradiation (365 nm). These water-based multi-color fluorescent anticounterfeiting inks with highly intense, bright, and sensitive fluorescence emission have potential applications in encryption and authentication of security patterns.

Photo-/Baso-Chromisms and the Application of a Dual-Addressable Molecular Switch

Two typical molecular switches of spiropyran (SP) and benzoxazine (OX) were fused by sharing an indole to achieve a new dual-addressable molecular switch (SP-OX-NO₂ ). Through proper molecular modification with NO₂ , the transformation from merocyanine (MC) to ring-closed SP or ring-closed OX can be controlled separately with visible light or base stimuli in solution, respectively, and these processes are verified by UV-vis and NMR spectroscopy as well as control experiments. The cis-merocyanine (cis-MC) form is involved in the basochromic process in solution. DFT calculation suggests that the bidirectional switching property of the fused SP-OX molecular switch can be controlled separately, when the OX isomer is more stable than the deprotonated SP isomer. Because of the significant color variations in solution, the simple dual-addressable switch has been further successfully applied to construct a multicolor reversible display on paper.