Bis(amino acid) oxalyl amides as ambidextrous gelators of water and organic solvents: Supramolecular gels with temperature dependent assembly/dissolution equilibrium

Bis(LeuOH) (1a), bis(ValOH) (2a) and bis(PhgOH) (5a) (Phg denotes (R)-phenv iglycine) oxalyl amides are efficient low molecular weight organic gelators of various organic solvents and their mixtures as well as water, water/DMS O, and water/DMF mixtures. The organisational motifs in aqueous gels are do minated primarily by lipophilic interactions while those in organic solvent s are formed by intermolecular hydrogen bonding. Most of the gels are therm oreversible and stable for many months. However, 2a forms unstable gels wit h organic solvents which upon ageing transform into variety of crystalline shapes. For some 1a/alcohol gets, a linear correlation between alcohol diel ectric constants (epsilon) and gel melting temperatures (T-g) was found. Th e H-1 NMR and FTIR spectroscopic investigations of selected gels reveal the existence of temperature dependent network assembly/dissolution equilibriu m. In the H-1 NMR spectra of gels only the molecules dissolved in entrapped solvent could be observed. By using an internal standard. the concentratio n of dissolved gelator molecules could be determined. In FTIR spectra, the bands corresponding to network assembled and dissolved gelator molecules ar e simultaneously present. This enabled determination of the K-gel values by using both methods. From the plots of ln K-gel versus 1/T, the DeltaH(gel) values of selected gels have been determined (-DeltaH(gel) 10-36 kJ mol(-1 ) range) and found to be strongly solvent dependent. The DeltaH(gel) values determined by H-1 NMR and FTIR spectroscopy are in excellent agreement. Cr ystal structures of 2a and rac-5a show the presence of organisational motif s and intermolecular interactions in agreement with those in get fibres elu cidated by spectroscopic methods.