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Sonication-Induced, Solvent-Selective Gelation of a 1,8-Napthalimide-Conjugated Amide: Structural Insights and Pollutant Removal Applications

Reported herein is the synthesis, characterization, and dye removal applications of a highly solvent-selective organogel-forming amide, compound 1, which contains a 1,8-naphthalmide moiety, flexible n-hexyl chain, and benzene ring. This compound displayed remarkable solvent selectivity, with gel formation occurring only in the presence of alkylated aromatic solvents. Detailed structural characterization of the gels, combined with notable theoretical insights, is invoked to explain the highly selective gelation properties of compound 1, as is a comparison to non-gel forming structural isomer 2, which contains the same structural elements in a different arrangement. Finally, the ability of the gel derived from compound 1 to act as a reusable material for the efficient removal of cationic organic dyes from contaminated aqueous environments is also reported, with up to 11 repeated uses of the gel still maintaining the ability to effectively remove Rhodamine B.


MATERIALS AND METHODS
All chemicals were purchased from commercial suppliers and used without further purification. All UV/Visible absorption spectra were recorded on a Varian Cary 50 Bio UV-Visible spectrophotometer. Fluorescence spectra were recorded on a Varian Cary Eclipse fluorescence spectrophotometer. 1 H NMR spectra were obtained using a Bruker Avance III spectrophotometer operating at 400 MHz, and were referenced to signals from the deuterated solvents. All fluorescence spectra were integrated vs. wavenumber on the X-axis using OriginPro 2020. All curve fitting was done using OriginPro curve fitting options for both linear and non-linear curve fitting. The morphologies of the reported amides were investigated using scanning electron microscopy (TESCAN MAIA3 Triglav). Polarized optical microscopic images were taken at 20-fold magnification (ZEISS, Axioscope 5 with polarizer and CCD camera). The rheological measurements were done on a MCR 102 rheometer (Anton Paar, Modular Compact Rheometer) with a steel parallel plate geometry having 40 mm diameter at 25 °C. The rheometer was attached to a Peltier circulator thermo cube in order to control the temperature accurately. All circular dichroism experiments were done on JASCO spectrophotometer.

DETAILED EXPERIMENTAL PROCEDURES EXPERIMENTAL PROCEDURES FOR AGGREGATION STUDIES
The molecular recognition and self-assembly of the reported naphthalic anhydride (NA)-based compounds 1 and 2 were studied by concentration dependent UV-visible and fluorescence spectroscopy. The concentration of the compounds ranged from 4.51 x 10 -2 mM to 11.88 x 10 -2 mM. UV-visible spectra were integrated from 250-400 nm using OriginPro software (with wavenumbers on the X-axis). Fluorescence spectra were measured from excitation at 330 nm, and integrated from 340-500 nm using OriginPro software (with wavenumbers reported on the X-axis).

EXPERIMENTAL PROCEDURES FOR SOLID -STATE FTIR STUDIES
Compounds 1 and 2, as well as the gel obtained from sonicating a solution of compound 1 in toluene for 90 seconds, were examined using solid-state FTIR spectroscopy. A small amount of all compounds was ground by hand with a mortar and pestle. The resulting solid-state mixture consisted of the compounds with KBr salt, and pellets were formed from this mixture using a hydraulic pressure apparatus. To measure the solidstate FTIR spectra of the samples, first a background correction scan was conducted in air. Then, the sample pellets were placed in the FTIR spectrometer and the spectra were acquired. All data were saved as ASCII files and fitted using Microsoft Excel software.

EXPERIMENTAL PROCEDURES FOR GELATION STUDIES
Gelation studies were conducted by sonicating a solution of compound 1 or 2 ([1] = [2] = 10 mg/mL) for 90 seconds (for sonication-induced gel formation) or by heating the solution of compound in a specified solvent to 90-100 o C for 2-3 minutes followed by slowly cooling the mixture to room temperature (for heatinduced gel formation). Minimum gelation concentrations were determined by varying the concentration of the compound and measuring the ability of the resulting solution to form gels. The existence of a gel was determined by inverting the vial containing the solution and observing if the material remained at the top of the vial (indicating formation of a gel) or if it fell to the bottom of the vial (indicating the absence of a gel).

EXPERIMENTAL PROCEDURES FOR DYE ABSORBANCE OVER TIME STUDIES
Dye absorbance studies were conducted by mixing a solution of compound 1 ([1] =10 mg/mL) with oxylene (0.75 mL) to create a gel, followed by adding a solution of the dye (compounds 10-14) ([dye] = 1 mg/mL in water). The UV-visible spectrum of the dye as a function of time was measured using a Varian Cary 50 Bio UV-Visible spectrophotometer, and recorded from 200-700 nm.

EXPERIMENTAL PROCEDURES FOR STUDIES OF GEL REUSABILITY
To check reusability, a gel formed from compound 1 in o-xylene (20 mg/mL) was treated with tap water containing rhodamine B (2 mg/mL). After effective absorption of the dye over 50 hours, the gel was treated with a dilute solution of sodium bicarbonate solution and the resulting solution was extracted with multiple portions of ethyl acetate. This led to partitioning of the compounds between the layers, with the aqueoussoluble dye remaining in water and the gelator compound dissolving in the organic solution. The combined organic layer was separated, washed with brine (6-7 times), and dried over sodium sulfate, followed by removal of the ethyl acetate under reduced pressure to obtain the solid product. The mass of the recovered compound was determined, followed by using the recovered compound to re-prepare the organogel. The resulting organogel was again treated with tap water containing the rhodamine B dye ([rhodamine B] = 2 mg/mL).

EXPERIMENTAL PROCEDURES FOR MICROSCOPIC IMAGING EXPERIMENTS
A small amount of gel (compound 1 in toluene), a solution of compound 1 in 1,2-dichlorobenzene, a solution of compound 2 in toluene, and a solution of compound 2 in 1,2-dichlorobenzene were placed on a previously cleaned silicon wafer. The samples were dried via slow evaporation of the solvent, and then kept S8 under vacuum at room temperature for two days. The materials were coated with gold, and the micrographs were taken using SEM instruments.

EXPERIMENTAL PROCEDURES FOR RHEOLOGY EXPERIMENTS
Small amounts of the gel were used to examine the mechanical strength of the gels. We prepared gels of compound 1 in toluene, o-xylene, and m-xylene, and performed oscillatory frequency sweep experiments to measure the storage modulus (G′) and loss modulus (G″) of the gel, using the experimental setup detailed in "Materials and Methods," above. The variation of G' and G'' were monitored as a function of angular frequency at a constant strain value of 0.01%.

EXPERIMENTAL PROCEDURES FOR CD EXPERIMENTS
To investigate supramolecular chirality and chirality changes in compounds 1 and 2, we conducted circular dichroism experiments. First solutions of compounds 1 and 2 were prepared in methanol ([1] = [2] = 4.51 x 10 -2 mM). After a baseline correction was run using methanol, the samples were run, and the CD spectra were recorded.  = 10 mg/mL; sonication occurred using a water sonicator for 1-2 minutes; heating-cooling occurred according to the following procedure: 10 mg of each compound in 1 mL of a selected solvent was measured into a closed vial. Then the vial was heated using a hot air gun until all components dissolved. After that, the vial was removed from heat and allowed to cool to room temperature undisturbed. b N/A = not applicable; no gel formation occurred c The term "sol-gel" refers to a state of neither a clear liquid nor the formation of a gel; rather a cloudy mixture forms from which a solid precipitates after 2-3 hours.    Integration was done from 250-500 nm, using wavenumbers on the X-axis and OriginPro software.

LACK OF GELATION OF COMPOUND 16
[10] = 1 mg/mL in water. All values represent an average of at least two trials. a The compound 1-derived gel was formed by mixing 7.5 mg of compound 1 with 0.75 mL of o-xylene. Integration was done from 250-500 nm, using wavenumbers on the X-axis and OriginPro software.
[11] = 1 mg/mL in water. All values represent an average of at least two trials. Integration was done from 250-500 nm, using wavenumbers on the X-axis and OriginPro software.
[12] = 1 mg/mL in water. All values represent an average of at least two trials. Integration was done from 250-500 nm, using wavenumbers on the X-axis and OriginPro software.
[13] = 1 mg/mL in water. All values represent an average of at least two trials.              Figure S59. Photograph of the self-healing experiment. A small portion of gel with Rhodamine B-absorbed dye (pink coloration in the above figure) merged after 15 minutes to another portion of gel without the dye.