A slippery molecular assembly allows water as a self-erasable security marker

Protection of currency and valuable documents from counterfeit continues to be a challenge. While there are many embedded security features available for document safety, they are not immune to forgery. Fluorescence is a sensitive property, which responds to external stimuli such as solvent polarity, temperature or mechanical stress, however practical use in security applications is hampered due to several reasons. Therefore, a simple and specific stimuli responsive security feature that is difficult to duplicate is of great demand. Herein we report the design of a fluorescent molecular assembly on which water behaves as a self-erasable security marker for checking the authenticity of documents at point of care. The underlying principle involves the disciplined self-assembly of a tailor-made fluorescent molecule, which initially form a weak blue fluorescence (λem = 425 nm, Φf = 0.13) and changes to cyan emission (λem = 488 nm,Φf = 0.18) in contact with water due to a reversible molecular slipping motion. This simple chemical tool, based on the principles of molecular self-assembly and fluorescence modulation, allows creation of security labels and optically masked barcodes for multiple documents authentication.

The crude product was subjected to column chromatography (2% methanol/chloroform) over silica gel that gave the pure product.

Synthesis of 3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benzoic acid (3a): Compound 2a
(3 g, 4.8 mmol) was taken in a 250 mL round bottom flask containing 50 mL ethanol and 10 ml 0.5 M KOH in ethanol was added to it. The reaction mixture was heated to 80 °C for 12 h and after cooling to room temperature the solvent was evaporated. The residue was extracted using chloroform and shake well with 10% HCl. The organic layer was washed with water, brine, dried over anhydrous sodium sulphate and then solvent was evaporated under reduced pressure to get the crude product. This was used for next step without further purification.

Synthesis of 3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benzoyl chloride (4a):
Compound 3a (1 g, 1.64 mmol) was taken in a two-neck round bottom flask containing 20 ml dry dichloromethane under nitrogen atmosphere. SOCl 2 (0.5 g, 5.0 mmol) was added dropwise through a syringe. The reaction mixture was allowed to stir at room temperature for 5 h and then purged with nitrogen to remove the solvent and unreacted SOCl 2 . The residue obtained was used for the next step without purification and characterization.

Synthesis of 3,4,5-tris(dodecyloxy)benzoyl chloride (4b):
Compound 3b (3 g, 4.4 mmol) was taken in a two neck round bottom flask containing 20 ml dry dichloromethane under nitrogen atmosphere. SOCl 2 (1.6 g, 13.3 mmol) was added dropwise through a syringe. The reaction mixture was allowed to stir at room temperature for 5 h and then purged with argon to remove the solvent and unreacted SOCl 2 . The residue obtained was used for the next step without purification and characterization.

Synthesis of N-(4-iodophenyl)-3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy) benzamide (6a):
Iodoaniline 5 (0.44 g, 2.0 mmol) was dissolved in 20 ml dry toluene in a two neck round bottom flask under nitrogen atmosphere. Dry triethylamine (2 ml) was added to the flask and allowed the reaction mixture to stir at room temperature for 15 min. Compound 4a was dissolved in 10 ml dry toluene separately and added to the reaction flask dropwise. The reaction mixture was then allowed to stir at room temperature for 12 h. After completion of reaction, the solvent toluene was evaporated and the residue was extracted using chloroform. The organic layer was washed with water, brine and then dried over anhydrous sodium sulphate. After the removal of solvent, residue was purified by silica gel column chromatography using 5% methanol/chloroform as an eluent.

Synthesis of 3,4,5-tris(dodecyloxy)-N-(4-iodophenyl)benzamide (6b):
Iodoaniline 5 (1.3 g, 5.2 mmol) was dissolved in 20 ml dry toluene in a two-neck round bottom flask under nitrogen atmosphere. Dry triethylamine (2 ml) was added to the flask and allowed the reaction mixture to stir at room temperature for 15 min. Compound 4b was dissolved in 10 ml dry toluene separately and added to the reaction flask dropwise. The reaction mixture was allowed to stir at room temperature for 12 h. After completion of reaction, solvent toluene was evaporated and the residue was extracted using chloroform. The organic layer was washed with water, brine and then dried over anhydrous sodium sulphate. After the removal of solvent, residue was purified by silica gel column chromatography using 5% ethylacetate/hexane as an eluent.

Synthesis of 4-iodophenyl 3,4,5-tris(2-(2-(2-methoxyethoxy)ethoxy)ethoxy) benzoate (8):
Iodophenol 7 (0.44 g, 2.0 mmol) was dissolved in 20 ml dry toluene in 250 ml two neck round bottom flask under nitrogen atmosphere. Dry triethylamine (2 ml) was added to the flask and allowed the reaction mixture to stir at room temperature for 15 min. Compound 4a was dissolved in 10 ml dry toluene separately and added to the reaction flask dropwise. The reaction mixture was then allowed to stir at room temperature for 12 h. After completion of reaction, solvent toluene was evaporated under reduced pressure and the residue was extracted using chloroform. The organic layer was washed with water, brine and then dried over anhydrous sodium sulphate. After the removal of solvent, residue was purified by silica gel column chromatography using 5% methanol/chloroform as an eluent. .

Synthesis of 1-ethynyl-4-(phenylethynyl)benzene (12):
To a solution of 11 (0.42 g, 1.71 mmol) in 5 ml dichloromethane, KF (1.0 g, 17.0 mmol) in 15 ml methanol was added to it and allowed to stir at room temperature for 6 h. After completion of the reaction, the reaction mixture was extracted using chloroform, washed with water, brine and then dried over anhydrous sodium sulphate. Solvent was evaporated under reduced pressure and the residue was used for the next step without further purification.

Synthesis and characterization of PE derivatives.
In a general synthetic procedure, the aryl halide (0.80 mmol), bis(triphenylphosphine)palladium (II) dichloride (10 mol%), and copper (I) iodide (10 mol%) were added to an oven-dried two neck round bottom flask equipped with a magnetic stirring bar. The round bottom flask was then sealed with a rubber septum, evacuated and backfilled with argon three times. Degassed triethylamine (10 ml) was added followed by degassed THF (10 ml) to serve as the co-solvent. After stirring for 5 minutes at room temperature, the terminal alkyne 1-ethynyl-4-(phenylethynyl)benzene (0.96 mmol) dissolved in 10 ml (1:1) mixture of degassed triethylamine and THF was added and the reaction mixture was stirred at room temperature until complete reaction was noted by the TLC. The reaction mixture was extracted using chloroform and washed with dilute hydrochloric acid. The organic layer was washed with brine and dried over anhydrous sodium sulphate and then evaporated under reduced pressure. The crude product was then purified by column chromatography using silica gel as adsorbent.

Measurements
Optical measurements. The solvents for the spectroscopic measurements are spectroscopic grade (99.8 %) and were used as received. The UV/Vis absorption spectra were recorded on a Shimadzu spectrophotometer UV-2100. The emission spectra were recorded on a SPEX- X-ray diffraction. Samples for the XRD studies were prepared by transferring chloroform solution of PE derivatives (5 mg/mL) onto glass plates (thickness: 0.05 mm), dried slowly to evaporate the solvent, and finally dried under vacuum. The X-ray diffraction pattern was recorded at ambient conditions using PANalytical 3 kW X'pert PRO diffractometer. The samples were irradiated using a monochromatic CuKa X-ray (wavelength, λ = 1.54 Å). In the case of PE1 derivative, the blue emitting film after recording XRD was exposed to moisture to get the cyan emitting film and then recorded the diffraction pattern. The cyan-emitting film was then exposed to hot air and diffraction pattern of the re-generated blue-emitting film was recorded.    Fluorescence decay profiles were recorded by excitation at 375 nm, emission monitored at the emission maximum.

Fluorescence Quantum Yield Measurements
Fluorescence quantum yield (Ф s ) of PE derivatives was determined using quinine sulfate (Ф r = 0.546 in 0.1 N H 2 SO 4 ) as the reference standard. The experiments were done using optically matching solutions and the quantum yield was calculated using equation 1 S1 , Ф s = Ф r (A r F s /A s F r ) (η s 2 /η r 2 ) -------- (1) where, A s and A r are the absorbance of the sample and reference solutions respectively at the same excitation wavelength, F s and F r are the corresponding relative integrated fluorescence intensities and η is the refractive index of the solvent.
Fluorescence quantum yield in the film state were measured using a calibrated integrating sphere attached to a SPEX Fluorolog spectrofluorimeter. A Xe arc lamp was used to excite the where E i (λ) and E 0 (λ) are the integrated luminescence as a result of the direct excitation of sample and secondary excitation, respectively; A is the absorbance of the sample calculated using Equation (3); L i (λ) is the integrated excitation when the sample is directly excited; L 0 (λ) is the integrated excitation when the excitation light first hits the sphere and reflects into the sample; and L e (λ) is the integrated excitation profile for an empty sphere.