Synthesis of Fluorescent Gelators and Direct Observation of Gelation with a Fluorescence Microscope

Abstract

Fluorescein-, benzothiazole-, quinoline-, stilbene-, and carbazole-containing fluorescent gelators have been synthesized by connecting gelation-driving segments, including l-isoleucine, l-valine, l-phenylalanine, l-leucine residue, cyclo(l-asparaginyl-l-phenylalanyl), and trans-(1R,2R)-diaminocyclohexane. The emission behaviors of the gelators were investigated, and their gelation abilities studied against 15 solvents. The minimum gel concentration, variable-temperature spectroscopy, transmission electron microscopy, scanning electron microscopy, fluorescence microscopy (FM), and confocal laser scanning microscopy (CLSM) were used to characterize gelation. The intermolecular hydrogen bonding between the N-H and C=O of amide, van der Waals interactions and π-π stacking play important roles in gelation. The colors of emission are related to the fluorescence structures of gelators. Fibrous aggregates characterized by the color of their emission were observed by FM. 3D images are produced by the superposition of images captured by CLSM every 0.1 μm to a settled depth. The 3D images show that the large micrometer-sized aggregates spread out three dimensionally. FM observations of mixed gelators are studied. In the case of gelation, two structurally related gelators with the same gelation-driving segment lead to the gelators build up of the same aggregates through similar hydrogen-bonding patterns. When two gelators with structurally different gelation-driving segments induce gelation, the gelators build up each aggregate through individual hydrogen-bonding patterns. A fluorescent reagent that was incorporated into the aggregates of gels through van der Waals interactions was developed. The addition of this fluorescent reagent enables the successful observation of nonfluorescent gelators' aggregates by FM.

Fluorescent Gelators for Detection of Explosives

“…Carbz-1: The preparation of this compound was reported in our previous paper. 46 Carbz-2: A mixture of 7.03 g (20 mmol) of 11-(9H-Carbzol-9yl)undecanoic acid, 47 7.37 g (20 mmol) of L-valylaminooctadecane, 2.28 g (22 mmol) of DIPC (N,N'diisopropylcarbodiimide), 2.44 g (20 mmol) of DMAP (4dimethylaminopyridine), and 100 mL of dichloromethane was refluxed overnight. Chloroform (100 mL) was added to the reaction mixture and an insoluble matter was removed.…”

“…Calcd for C46H75N3O2: C 78.69, H 10.77, N 5.98%. Carbz-3: This compound was obtained from a mixture of 0.88 g (2.5 mmol) of 11-(9H-Carbzol-9-yl)undecanoic acid, 1.24 g (2.5 mmol) of L-isoleucyl-L-isoleucylaminooctadecane, 46 0.35 g (2.75 mmol) of DIPC, 0.31 g (2.5 mmol) of DMAP, and 20 mL of dichloromethane by the similar procedure described in Carbz-2. Recrystallization from methanol gave 0.92 g (44%) of Carbz-3.…”

“…Quino-1: The preparation of this compound was reported in our previous paper. 46 Quino-2: A mixture of 0.64 g (4.4 mmol) of 8-hydroxyquinoline, 0.88 g (6.6 mmol) of potassium carbonate, 0.66 g (4.0 mmol) of potassium iodide, 1.70 g (4 mmol) of 6-bromohexyloxycyclo(L-asparaginyl-L-phenylalanyl), 46 and 20 mL of dry DMF was stirred at 40°C overnight under an argon atmosphere. The mixture was poured to iced water and the precipitated matter was filtered off, washed with water, and dried.…”

“…Calcd for C33H41N3O5: C 70.82, H 7.38, N 7.51%. Quino-4: This compound was obtained from a mixture of 0.64 g (4.4 mmol) of 8-hydroxyquinoline, 0.88 g (6.6 mmol) of potassium carbonate, 0.66 g (4.0 mmol) of potassium iodide, 1.47 g (2.0 mmol) of trans-(1R,2R)-bis (11-bromoundecanoylamino)cyclohexane, 46 and 20 mL of dry DMF by the similar procedure described in Quino-2. A crude product obtained by recrystallization from 70 mL of acetone was purified by column chromatography on alumina (elution with chloroform; Rf = 0.7).…”

“…Benth-1, Benth-2, and Benth-3: These compounds were prepared according to our previous paper. 46 Stilb-1: A mixture of 2.16 g (10 mmol) of 4-nitrobenzyl bromide and 2.62 g (10 mmol) of triphenylphosphine in 50 mL of toluene was refluxed for 6 h. After evaporating toluene, the residue was dried in vacuo. 4-Dimethylaminobenzaldehyde (1.49 g, 10 mmol), 3.69 g (10 mmol) of tetrabutylammonium iodide, and 70 mL of CH2Cl2 were added to the residue.…”