A new class of organogelators based on triphenylmethyl derivatives of primary alcohols: hydrophobic interactions alone can mediate gelation

In the present work, we have explored the use of the triphenylmethyl group, a commonly used protecting group for primary alcohols as a gelling structural component in the design of molecular gelators. We synthesized a small library of triphenylmethyl derivatives of simple primary alcohols and studied their gelation properties in different solvents. Gelation efficiency for some of the derivatives was moderate to excellent with a minimum gelation concentration ranging between 0.5–4.0% w/v and a gel–sol transition temperature range of 31–75 °C. 1,8-Bis(trityloxy)octane, the ditrityl derivative of 1,8-octanediol was the most efficient organogelator. Detailed characterizations of the gel were carried out using scanning electron microscopy, FTIR spectroscopy, rheology and powder XRD techniques. This gel also showed a good absorption profile for a water soluble dye. Given the non-polar nature of this molecule, gel formation is likely to be mediated by hydrophobic interactions between the triphenylmethyl moieties and alkyl chains. Possible self-assembled packing arrangements in the gel state for 1,8-bis(trityloxy)octane and (hexadecyloxymethanetriyl)tribenzene are presented. Results from this study strongly indicate that triphenylmethyl group is a promising gelling structural unit which may be further exploited in the design of small molecule based gelators.


General synthetic procedure for Series 1 compounds (TPM-G1 -TPM-G5)
Synthetic procedure for TPM-G1 is given below as a representative example. The same procedure was followed for the synthesis of the remaining compounds (TPM-G2 -TPM-G5) using similar molar scales. Trityl chloride (641 mg, 2.30 mmol) dissolved in 4 mL of dichloromethane (DCM) was added to a stirred solution of octan-1-ol (300 mg, 2.30 mmol) in DCM at room temperature in a 50 mL round bottom flask under nitrogen atmosphere. Triethylamine (232 mg, 2.30 mmol) diluted in 2 mL of DCM was added dropwise to the reaction mixture and stirring was continued for 24 h at room temperature. The reaction mixture was taken in ≈30 mL DCM and washed three times with water. The organic extract was dried over S3 anhydrous sodium sulfate and concentrated by rotary evaporation. The crude product was purified by column chromatography over silica gel 60-120 mesh using petroleum ether/ethyl acetate (99:1) as the eluent to give compound TPM-G1.

General synthetic procedure for Series 2 compounds (TPM-G6 -TPM-G10)
Synthetic procedure for TPM-G6 is described below as a representative example.
The same procedure was used for the synthesis of the remaining compounds (TPM-G7 -TPM-G10) at similar molar scales. Trityl chloride (697 mg, 2.50 mmol) S5 dissolved in 4 mL of DCM was added to a stirred solution of 1,6-hexanediol (300 mg, 2.50 mmol) in DCM at room temperature in a 50 mL round bottom flask under nitrogen atmosphere. Triethylamine (253 mg, 2.50 mmol) diluted in 2 mL of DCM was added dropwise to the reaction mixture and stirring continued for 24 h at room temperature. The reaction mixture was taken in ~30 mL DCM and washed three times with water. The organic extract was dried over anhydrous sodium sulfate and concentrated by rotary evaporation. The crude product was purified by column chromatography over silica gel 60-120 mesh using petroleum ether/ethyl acetate (9:1) as the eluent to give compound TPM-G6.    [M+Na] + ; found 415.420.

G15)
Synthetic procedure for TPM-G11 is described below as a representative example. The reaction mixture was taken in ~30 mL DCM and washed three times with water.
The organic extract was dried over anhydrous sodium sulfate and concentrated by rotary evaporation. The crude product was purified by column chromatography over silica gel 60-120 mesh using petroleum ether/ethyl acetate (98:2) as the eluent to give compound TPM-G11.

Organogel preparation
The required amount of compound was taken in 0.5 mL of solvent in a screw-capped glass vial. It was heated slowly on a hot plate till the solid dissolved completely and left to cool to room temperature and observation noted down after 0.5-2 h. Stable gel formation was confirmed by inversion of the glass vial.

Minimum gelation concentration (MGC) determination
Different concentrations of the compounds (% w/v) were taken in the required solvent and gelation test performed as described above. MGC is defined as the minimum concentration at which a stable gel is formed.

Gel-sol transition (T gel ) temperature measurement
T gel values were determined using the dropping ball method. A glass bead (weighing 742 mg, 3.96 mm in diameter) was placed at the top of the gel in a glass vial. This glass vial was placed in an oil bath and the temperature was raised slowly at a rate of 3 o C per minute. T gel is defined as the temperature at which the glass bead reached the bottom of the glass vial.

Differential scanning calorimetry
Gels prepared in different solvents (propan-1-ol and DMSO) were taken in DSC ampules and measurement taken in the range of 30-125 °C at a heating rate of 5 °C/min.

SEM analysis
A small volume of a hot solution of the gelator was placed on a glass coverslip and cooled to room temperature to allow gel formation. The gel was kept at ambient temperature overnight and then vacuum dried. The glass cover slip with the dried gel (xerogel) was transferred on a cylindrical brass stub and coated with Au. SEM images were obtained with an accelerating voltage of 20 kV.

Rheology
Rheology study was carried out on an Anton Parr MCR 302 instrument having cone and plate geometry. Gel (1.5 % w/v in propan-1-ol and DMSO) was carefully placed on the plate so that there was no air gap with the cone. Stress amplitude sweep experiments were carried out (in the range of 0.01-500 Pa) at a constant oscillation frequency of 1Hz at 25 o C. G' and G" were then measured as a function of oscillatory shear stress.

FTIR analysis
FTIR spectra of the gelator TPM-G12 in solution (CHCl 3 ) and dried gel (KBr pellet) were recorded with Perkin Elmer Spectrum II spectrometer.

XRD analysis
A few drops of the hot gelator solution was poured on a glass slide and cooled to room temperature till a gel was formed. It was vacuum dried to prepare the dried gel.
Powder XRD analysis of the dried gel was carried out with PANalytical XPERT Pro diffractometer. The X-ray source was Cu K radiation ( = 1.54056 Å) with a voltage of 40 kV, and current of 35 mA. The dried gel was scanned from 1-40° (2θ angle) with step size 0.02°.
Aqueous solution of Direct Red 80 (1.5 mL, 0.02 mM) was added on top of the organogel and this was left undisturbed at room temperature for time-dependent absorption studies ( Figure S2a). 1 mL of the dye solution was taken out and used to measure the residual dye concentration by UV-vis spectrophotometer at time points of 6, 12 and 24 h. A standard plot for absorbance (at  max = 529 nm) versus different dye concentrations was generated ( Figure S3a). Using this standard plot, the efficiency of dye absorption by TPM-G12 gels at different time points was calculated.
The same procedure was repeated for crystal violet dye solution ( Figure S2b and Figure S3b).  max of crystal violet dye is 583 nm. We have noted that during the dye absorption studies, the bulk of the gel remained fairly stable during the incubation period. However, the gel-liquid interface tend to be less stable, hence care should be taken during handling of the glass vial.