pH triggered self-assembly induced enhanced emission of phosphonic acid appended naphthalenediimide amphiphile

Abstract

pH triggered self-assembly induced enhanced emission of water soluble phosphonic acid–NDI amphiphiles 1 is described. At pH 4.5, the amphiphile self-assembled into interwoven fibres, whilst a ladder-type network was observed at pH 7. At pH 9.5, NDI amphiphile assembled into more complex fractal nanostructures. Interestingly, enhancement of emission was observed under both acidic and basic conditions.

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Self-assembly of naphthalene diimides (NDIs) is rapidly emerging as an attractive strategy to create a new class of n-type conducting functional materials.^1,2 NDI systems and nanostructures thereof have wide applicability towards biomolecule signalling,³ chirality induction,⁴ charge transfer interactions,⁵ semiconductors,⁶ organic light emitting diodes or organic field effect transistors,⁷ artificial supramolecular photosystems,⁸ and sensors.⁹ Various stimuli are known to be responsible for inducing specific shapes and sizes of NDI nanomaterials.¹⁰ There are also a few reports of water soluble NDI with supramolecular architectures.¹¹

Literature survey shows that NDI derivatives exhibit low fluorescence quantum yields when compared to perylene diimide derivatives.¹² There is a considerable focus on rational molecular design to derive fluorescent NDI molecules with controlled emission properties, and this has led to enormous changes in their optical and emission properties.¹³ Nevertheless, despite the huge benefits offered by morphology tuning, the enhancement of emission of hierarchical supramolecular NDI self-assemblies has largely been unexplored to-date.¹⁴ George et.al. investigated the self-assembly of NDI bolaamphiphile with aggregation induced green emission enhancement.¹⁵ Moreover, there are also few reports of supramolecular gel materials with aggregation-induced enhanced emission (AIEE).¹⁶ However, to the best of our knowledge, there is no protocol on pH triggered supramolecular assembly of NDIs that exhibits aggregation induced fluorescence enhancement.

In our previous work, we reported a pH dependent self-assembled material of a phosphonato derived porphyrin.¹⁷ As a follow-up, we have constructed phosphonic acid based NDI bolaamphiphile and fabricated a family of soft materials using L- and D-arginine.¹⁸ In this work, we report the first example of a phosphonic acid appended NDI amphiphile motif 1 (Fig. 1fig1) and pH-dependent aggregation induced fluorescence enhancement in aqueous media. Spectroscopic and microscopic techniques were employed to demonstrate the growth mechanism of the observed self-assembly.

Fig. 1 Schematic illustration of a proposed mechanism for self-assembly of NDI (1) triggered by pH tuning and representative SEM images.

The NDI used in this study, 1, bears three important features, allowing molecules to self-assemble into a variety of supramolecular nanostructures with pH control (Fig. 1): (i) a hydrophilic bidentate phosphonic acid head group that is sensitive to pH tuning, (ii) an NDI core exhibiting π–π-stacking of aromatic systems, and (iii) a hydrophobic long n-octyl tail to increase hydrophobic interactions, and to minimize the repulsive electrostatic forces between the negatively charged moieties of stacked molecules, especially at highly basic pHs.

NDI amphiphile 1 was synthesized (Scheme 1sch1) by reacting N-octyl-naphthalene-1,8-dicarboxyanhydride-4,5-dicarboximide (3)¹⁹ with phosphonated anilines 4 in dry DMA at 120 °C. 2 was obtained in 73% yield. Deprotection of the phosphonate esters of 2, was achieved by TMSBr in acetonitrile at 50 °C, followed by protonation using methanol, resulting in a 66% yield (for details see ESI†).

Scheme 1 Synthesis of NDI amphiphile 1.

NDI 1 can be dissolved easily in water, resulting in a stable, transparent yellow solution. At pH 7, NDI 1 (1 × 10⁻⁵ M) in water showed two well-resolved strong absorption bands at 363 nm, and 383 nm, assigned to the S₀ → S₂, and the S₀ → S₁ transitions respectively, and two shoulders at 363 nm and 342 nm, which is typical of the NDI chromophore.²⁰ The UV-Vis absorption spectra of 1 and its aggregates were greatly influenced by the pH of the aqueous media (Fig. 2fig2).

Fig. 2 UV-Vis of 1 ([1] = 8 × 10⁻⁵ M) in water with (a) decreasing pH and (b) increasing pH.

Lowering the pH (adjusted by addition of 0.1 M HCl) leads to a sharp hypochromic shift and decrease in band intensities in the absorption spectra (Fig. 2a), indicative of the aggregating nature of 1. In basic pH (7 to 11.5, adjusted by 0.1 M NaOH addition), the two characteristic NDI absorbance bands at 383 and 363 nm, decrease in their intensity, along with broadening. This provides evidence for the stacked assembly as a result of protonation and deprotonation of the phosphonate group in the molecular structure, both in acidic and basic conditions (Fig. 2b). The plot of absorbance vs. pH clearly shows that molecular self-assembly into aggregates occurs in both cases, acidic as well as basic conditions (Fig. S1†). The mode of deprotonation/protonation of phosphonic acid was investigated by UV-vis spectrophotometric titrations (Fig. S2†). The analysis of absorption intensity changes as a function of pH yielded the pK_a = 4.58 and 8.25 for first and second deprotonation of phosphonic acid protons respectively (Fig. S3†).

The emission spectra of aqueous NDI 1 at various pHs using S₀ → S₁ excitation band at 380 nm (Fig. 3fig3), where at pH 7 shows an emission maximum at 392 nm. In acidic pH (7.0 → 2.0), emission intensity increased, with about a 30 nm red shift of the emission to 422 nm. This was attributed to the π–π* transition of the stacked NDI molecules. The decreasing pH increases aggregation, and the resultant enhancement of emission is ascribed to J-stacking of the neighbouring molecules (Fig. 1a).²¹ In basic conditions (pH 7.0 → 12), an increase in fluorescence intensity, with 7 nm red shift, was observed (Fig. 3b). To induce molecular aggregation at neutral pH, concentration depended emission was investigated (Fig. S4†). This shows decrease in emission intensity with increasing concentration of 1 from 8 × 10⁻⁵ M to 6 × 10⁻² M. These spectral features suggest the formation of larger aggregates, as similar behaviour was observed in the case of tryptophan based NDI bolaamphiphiles.¹² This prompted us to further investigate the morphology of aggregates of 1, after solvent evaporation on a glass slide, using field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM).

Fig. 3 Fluorescence emission spectra of 1 (8 × 10⁻⁵ M, λ_ex = 380 nm) in water with (a) decreasing acidic pH and (b) increasing basic pH.

FE-SEM showed that at acidic pH = 4.5, NDI 1 form a tuned entangled network of micrometre long interwoven fibres, with fiber width of 600 to 800 nm (Fig. 4afig4). Whereas at neutral pH, interconnected ladder type networks of micrometre long fibres (∼1 μm in width) formed (Fig. 4b). We presume that ladder type aggregation of 1 is due to strong aromatic π–π stacking of the NDI core, and van-der Waals interactions of the hydrophobic alkyl chain, along with strong hydrogen bonding interactions between the peripheral phosphonic acid groups at the surface with water. At pH = 9.5, the NDI amphiphile 1 undergoes deprotonation of the phosphonic acid groups, resulting in repulsive electrostatic forces and diversion from the parallel arrangement of the molecules at neutral pH. Which further introducing a curvature in the supramolecular sheets to form larger fibrile structures, which further grow into well-defined fractals (Fig. 4c, see ESI Fig. S8b & c†). The angle resulting from electrostatic forces, may occur in-plane resulting in a twisted sheet instead of a tube, to minimize the repulsive electrostatic forces. Increasing the pH further to 10.3 leads to larger growth of the fractals into a leaf-like morphology (see ESI Fig. S7†). pH dependent self-assembly of 1, also studied in buffer solution upon filtration and obtained results are similar to self-assembly in water (see ESI Fig. S8†). Atomic Force Microscopy (AFM) support the fractals assembly at pH 9.5 (see ESI Fig. S9†).

Fig. 4 NDI amphiphle 1 aggregate SEM micrographs produced by solvent evaporation at pH (a) 4.5, (b) 7.0, and (c) 9.5 and (d) the normal particle size distribution of these aggregates in colloid at various pHs starting from a filtered solution at pH 9.0 then reducing the pH and aggregate growth between pH 8.02 and 3.32 as measured by DLS.

The particle size distribution of the aggregates were studied at various pH between 9 and 3 solution using dynamic light scattering (DLS) as shown in Fig. 4d. Solution at pH 9 was filtered using cellulose acetate 0.21 μm syringe filter, the DLS measurement showed no a clear solution and no particle was detected, the pH was adjusted using 0.1 M HCl to lower pHs down to pH 3. The NDI 1 solution at 1 × 10⁻⁴ M at pH 9 produced no detectable particles. When pH was reduced to 8.02 NDI aggregates start becoming detectable at 74 nm mean diameter. The particles continue to grow with further reducing pH, these particles cluster to produce precipitate over few hours that by filtration can be easily removed (for detail DLS see ESI Fig. S10 and Table S1†).

A schematic illustration of the morphologic tuning of 1, triggered by pH conditions, is shown in Fig. 1. This clearly suggests that the π–π stacking, hydrophobic interactions, van-der Waals interactions, hydrogen bonding and electrostatic interactions play a crucial role in the aggregation of naphthalene diimide amphiphile 1, and was studied using UV-vis, fluorescence, FE-SEM and AFM techniques. Thus, at neutral pH, the peripheral phosphonic acid head groups undergo strong hydrogen bonding with water, whilst under acidic conditions these groups undergo molecular dimerization. Upon increasing the pH to basic conditions (pH > 7), a solution of 1 formed fractals, with the deprotonated head group at the outer surface, which subsequently organised into higher level molecular aggregates with increasing pH.²²

Herein, we have demonstrated that a novel water soluble amphiphile, consisting of a NDI bearing phosphonic acid head group self-assembled into well-defined architectures consisting of: systematically-arranged or entangled networks of fibres; or leaf like, or bone skeletal type arranged fractals, fine-tuned by pH control. This is the first report of pH induced fluorescence enhancement of phosphonic acid amphiphiles. The morphology of these self-assembled materials has been characterised using FE-SEM and AFM techniques. These results are promising, due to aggregation induced fluorescence enhancement, for application in the construction of optoelectronic devices.

Acknowledgements

S.V.B. (IICT) is grateful for financial support from the DAE-BRNS (Project Code: 37(2)/14/08/2014-BRNS), Mumbai, and intelcoat project CSC0114, S.V.B. acknowledges the Australian Research Council for financial support under a Future Fellowship Scheme (FT110100152) and the RMIT Microscopy and Microanalysis Facility (RMMF).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis of 1, and details of UV/vis and AFM data for all assemblies and aggregates. See DOI: 10.1039/c4ra07925f

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