Caffeine as a Gelator

Caffeine (a stimulant) and ethanol (a depressant) may have opposite effects in our body, but under in vitro conditions they can “gel” together. Caffeine, being one of the widely used stimulants, continued to surprise the scientific community with its unprecedented biological, medicinal and physicochemical properties. Here, we disclose the supramolecular self-assembly of anhydrous caffeine in a series of alcoholic and aromatic solvents, rendering a highly entangled microcrystalline network facilitating the encapsulation of the solvents as illustrated using direct imaging, microscopy analysis and NMR studies.


Recrystallization of Caffeine
In a 250 mL round-bottom flask a mixture of commercial caffeine (200 mg) and ethanol (25 mL) was heated over an oil bath until the mixture turned clear solution. The hot solution was quickly filtered using Whatman No. 4 filter paper and allowed to crystallize. The crystalline solid was dried under vacuum and used for further studies.

Gelation Tests
A typical gelation test was performed dissolving a known amount of sample in a test tube (l = 10 cm, d = 1.0 cm) in a solvent (0.5 mL) under investigation. The mixture was heated slowly until it turns into a clear solution (Note: We avoid rapid heating due to sublimation properties of caffeine, which may lead to erroneous results). The solution was either allowed to attain room temperature (~22 °C) or subjected for sonication for 1 min. The formation of the gel was tested using the "resistance to flow upon inversion of the test tube). Depending on the appearance the gels are denoted as, P = precipitate after attaining a hot solution to room temperature. G = gel, Gs = required sonication, S = solution (Table S1).

Scanning Electron Microscopy (SEM) Studies
The sample preparation for SEM measurements was carried out by placing the hot sol (10 µL) on a carbon tape fixed over a sample stub. The sample was allowed to dry under ambient conditions for 24 h. The sample was sputter coated with gold in a JEOL Fine Coat Ion Sputter JFC-1100 and the images were collected using Bruker Quantax400 EDS microscope equipped with a digital camera. The scanning electron microscopy images are shown in Figures S1 and S2.

Gel Melting Temperature (Tgel) Measurement
The gel melting temepratures were measured for different solvents ( Figure S3) as well as for 1-octanol gel at different concentrations ( Figure S4) using inverted test tube method.

Solid-State NMR Studies
The 13 C{ 1 H}CP/MAS and 15 N{ 1 H}CP/MAS NMR spectra were recorded on a Bruker AV400 spectrometer equipped with a 4 mm standard bore CPMAS probe head whose X channel was tuned to 100.62 MHz for 13 C and 40.55 MHz for 15 N, respectively. The other channel was tuned to 400.13 MHz for broad band 1 H decoupling. Approximately, 100 mg of dried and finely powdered samples were packed in the ZrO2 rotor closed with Kel-F cap and spun at 10 KHz rate. The 13 C{ 1 H}CPMAS NMR was carried out for all samples under Hartmann-Hahn conditions with TPPM (tppm15) decoupling. The /2 pulse for proton and carbons were found to be 4.0 s and 5 s at power levels of −5.0 dB and −4.0 dB, respectively. The experiments were conducted at contact time of 2 ms. A total of 20,000 scans were recorded with 5 s recycle delay for each sample. All FIDs were processed by exponential apodization function with line broadening of 20 Hz prior to FT ( Figure S5).

Gelation Tests in Alcoholic and Aromatic Solvents
The results of gelation tests carried out using alcoholic and aromatic solvents are summarized below in Table S1. Table S1. Gelation studies of caffeine. P= precipitate after attaining a hot solution to room temperature. G = gel, Gs = required sonication, S = solution.

Variable Temperature NMR of Toulene-d8 Gel
Twelve milligrams of recrystallized caffeine was taken in NMR tube (d = 5 mm) and 600 µL of toluene-d8 were added. The sample was slowly heated until it turned into a clear solution. The solution was subjected for sonication and stabilized for 2 h. VTNMR was recorded from 30 °C to 90 °C with 10 °C increment at a time with 5 min of stabilizing time at each temperature ( Figures S6-S8).

Variable Temperature NMR of 1-octanol Gel and Tgel Measurement
Twelve milligrams of recrystallized caffeine was taken in NMR tube (d = 5 mm) and 600 µL of 1-octanol were added. The sample was slowly heated until it turned into a clear solution. The solution was subjected for sonication and stabilized for 2 h. VTNMR was recorded without lock from 30°C to 90 °C with 10 °C increment at a time with 5 min of stabilizing time at each temperature (see Figures S9-S11).    The 1 H NMR of caffeine in CDCl3 (clear solution) shows the vinylic porton (C-8 protons) having 4 bond coupling with C-14 protons (CH3) and appears as a broad quartet ( Figure S6). The proton signals from N-CH3´s are well separated from each other.
The 1 H NMR of the toulene-d8 gel ( Figure S7) showed broad signals, apart form that the signals from 10-CH3 and 12-CH3 now moved downfield (Shielded) compared to that in CDCl3 solution. Upon  heating both 10-CH3 and 12-CH3 signals showed a significant shift and while 10-CH3 signal moved upfield upon heating, the 12-CH3 signal moved in the opposite direction. Meanwhile 8-H also shifted upfield in the nmr spectrum ( Figures S8-S11). This observation is attributed to involvement of carbonyl oxygens O11 and O13 and N-9 in hydrogen bonding.