Spontaneous Formation of Fluorescent Nanofibers from Self-Assembly of Low-Molecular-Weight Coumarin Derivatives in Water

Abstract

A solvent-exchange process was applied to three fluorescent dyes belonging to the 2-benzimidazolyl-7-diethylaminocoumarin series (namely Coumarin 7 (1), Coumarin 30 (2), and one of their derivatives bearing a butyl chain (3)). The three compounds only differ by the substitution of the nitrogen atom of the benzimidazolyl group. They were first dissolved in acetone and then suddenly placed in an aqueous environment where they generated molecular assemblies. The size and shape of the latter were studied by fluorescence microscopy and transmission and scanning electron microscopy. It appeared that 1 gave aggregates and flat microcrystals that evolved toward elongated structures. 2 formed straight nanofibers that are 10-20 microm long and exhibit a crystal structure. 3 gave short fibers (1 micromx25 nm), which finally arrange into entangled solid nanofibers. The formation of fibers arising from low-molecular-weight molecules was particularly interesting. The optical properties of the free-standing particles in suspension were analyzed and compared to those of highly dilute dyes, with the aim to get additional information about the dye arrangement in the nanostructures.

Organic Fluorescent Nanofibers and Sub-Micrometer Rods

“…The first study Mille et al, 2008) deals with four dyes of the coumarin series. This type of dyes is currently used for applications in fields as varied as the coloring of textile fibers, medical imaging, lasers, photonic devices and organic lightemitting diodes and their spectroscopic behavior in solution is well known.…”

Cucurbit[8]uril-templated H and J dimers of bichromophoric coumarin dyes: origin of contrasting emission

“…Strikingly, the NMR spectrum also shows 20 clear shifts in the ethyl protons of 7-N,N′-diethylamino group as well as the coumarin protons (see Fig.2(a/b)), which is likely when both the coumarin ends of the 1:2 complex are further encapsulated by another two CB8 moieties as shown in the inset of Fig.2. This 3:2 stoichiometry, where the H d protons of C30 are 25 placed in the deshielding region of the carbonyl portals of the CB8s, appears with ~ 0.54 ppm downfield shift (Fig.2). Similar changes found with C7 dye are discussed in Note S3, Fig.S4, ESI.…”

“…2, 3 Therefore, gaining control over these noncovalent interactions 20 and the molecular orientation/stacking parameters are crucial for management of optical properties of the fluorophores or its aggregates 4 and can be accomplished by introducing, more specifically, macrocyclic hosts. 5 Among various synthetic cavitand receptors, the cyclic oligomers obtained in the acid 25 catalysed condensation of glycoluril with formaldehyde, known as Cucurbit[n]uril (CBn, n = 5-8, 10,14) family of macrocyclic hosts, attracted a great deal of attention due to their hydrophobic cavity with carbonyl laced portals at either ends, which offer strong ion dipole interaction for cationic guests, such as organic 30 or organometallic cationic moieties, metal ions/complexes, etc. [6][7][8] Among the CBn homologues, by virtue of its larger cavity, the cucurbit[8]uril (CB8) often accommodates more than one small guest molecules in its cavity, which by and large, leads to dramatic makeover in the guest properties.…”

“…However, due to the lack of 20 convincing ITC titration data, the association constant for CB8-C6 system was evaluated as K = (8.3±1.0)×10 11 M -2 , based on fluorescence measurements (Note S8,C, ESI). Note that the thermodynamic parameters shown with Figs.S8/9 could be associated with a larger error due to the higher order 25 complexation pattern and low absolute heats, limited by the solubility of CB8. 17 In this study, we exclusively established hitherto unknown dimer emission from CB8-templated bichromophoric coumarin fluorophores, presenting > 150 fold increase in the excited state 30 lifetime.…”

Fluorescent Organic Nanoparticles

“…Very few papers reported fluorescence quantum yields of nanoparticles; fluorescence quantum yields of microparticles of coumarin 6, 7, and 30 are reported to be 0.010 0.004, 0.080±0.020, and 0.046±0.020, respectively [18,19]. No one has reported fluorescence of doped organic nanoparticles yet to the authors' knowledge.…”