Helical supramolecular organization of a 1,2-diol appended naphthalene diimide organogelator via an extended intermolecular H-bonding network

Abstract

Self-assembly of a 1,2-diol appended naphthalene diimide derivative featuring chiral and J-type aggregation is reported. In MCH/CHCl₃, the compound exhibited intense yellow excimer and thermoreversible “sol–gel” behavior. Morphological and circular dichroism studies revealed long range M-helical nanofibre formation. A theoretical model of the cooperative hydrogen bonding was also proposed.

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Self-assembly of small molecular building blocks is one of the classical strategies for designing various functional nanoscale architectures.^1–3 Over the past two decades, incommensurable supramolecular assemblies of small organic molecules were explored utilizing noncovalent interactions.⁴ The hierarchical self-assembly of different chromophoric π-systems has unfolded diverse applications in chemistry, material science, and biology.^5–7 Among the class of rylene diimide dyes,⁸ 1,4,5,8-naphthalene diimides (NDIs) have been utilized extensively as critical units for constituting smart supramolecular nanostructures such as catenanes,^9,10 rotaxanes,^11–14 foldamers,^15–18 molecular knots,¹⁹ vesicles,²⁰ ion channels,²¹ nanotubes,²² hydrogels,^23,24 organogels,^25,26 etc. Longitudinally self-assembled NDIs from small molecules and polymers are also attractive n-type materials for organic electronic devices, photosystems,^27,28 and solar cells.^29,30 Other noncovalent interactions such as, hydrogen bonding (from carboxylic acids^31,32 and amides^20,33) van der Waals interactions,³⁴ etc. contribute synergistically to the ordered longitudinal orientation of these π-conjugated building blocks.

Recently, Ghosh and coworkers have reported a unique achiral extended self-assembly of NDIs based on simultaneous intra and inter-molecular hydrogen bonding motifs of tethered 1,3-dihydroxyl moieties.³⁵ The study paved the new ground in investigating the NDI aggregation based on hydrogen bonding of aliphatic hydroxyl groups. Solid state self-assembly of diketal protected mannitol derivatives with free 1,2-dihydroxyl groups was reported by us disclosing an extended network of alternate eight- and ten-membered rings through intermolecular hydrogen bond interactions (Fig. 1A).³⁶ Herein, we report a 1,2-dihydroxyl functionalized NDI amphiphile 1 (Fig. 1B) that provides a chiral and stable self-assembly through hydrogen bonding interactions palpably akin to those of the mannitol derivatives. The present work also features atypical red shifted NDI excimer emission and thermoresponsive gelation behavior of 1. A control NDI derivative 2 was also designed to address the importance of hydrogen bonding interactions in the self-assembly process.

Fig. 1 Network of alternate eight and ten-membered hydrogen bonded ring formation during the self-assembly of diketal protected mannitol (A). Structures of NDI-based organogelator 1 and control compound 2 (B).

In CHCl₃, compound 1 (see ESI† for the preparation of 1 and 2) exhibited well resolved π–π* absorption bands at 342, 361 and 381 nm indicating the monomeric state of the NDI chromophore (Fig. 2A). The change of solvent to nonpolar methyl cyclohexane (MCH) resulted in the considerable reduction of monomeric absorbance along with the concomitant 15 nm red-shifted absorption maxima indicating a J-type π–π aggregation state of NDI moieties.³⁷ The steady-state emission spectrum of 1 in CHCl₃ also provided characteristic monomeric signals at 387, 408 and 432 nm (Fig. 2B). The same compound when recorded in MCH, displayed red-shifted emission at 400–500 nm (a signature of vibrational progression),³⁸ and a strong excimer band centered at 578 nm. Such a red-shifted NDI excimer emission corroborates to strong inter-chromophoric interactions during the self-assembly process. The excitation spectrum collected at 578 nm displayed red-shifted excitation maxima as compared to monomeric absorption band confirming excimer emission is from J-aggregation of NDI chromophores (Fig. S5†). The ketal protected compound 2 displayed nearly identical UV-visible spectra in CHCl₃ and MCH. These results indicate that the observed spectral differences for 1 in these media are not caused by mere solvent effects. Fluorescence spectra of 2 in these two solvents were also comparable. This study confirms that the self-assembly of 1 in MCH is assisted predominately by hydrogen bonding interactions among hydroxyl groups leading to the aggregation induced enhanced emission (AIEE).³⁹

Fig. 2 UV-visible spectra of 1 (1 × 10⁻⁵ M) and 2 (1 × 10⁻⁵ M) in CHCl₃ and MCH (A). Fluorescence spectra of 1 (1 × 10⁻⁵ M) and 2 (1 × 10⁻⁵ M) in CHCl₃ and MCH (B). UV-visible (C) and fluorescence spectra (D) of 1 (1 × 10⁻⁵ M) in CHCl₃ with increasing percentage of MCH.

Further, to probe the effect of solvent composition on self-assembly, absorption spectra of 1 were recorded in variable solvent ratio of MCH and CHCl₃ (Fig. 2C). Up to 70 : 30 MCH/CHCl₃, the absorption spectra resembled that recorded in pure CHCl₃. A stepwise decrease of absorption bands at 338, 356 and 381 nm with the appearance of new absorption band at 402 nm wavelength was observed above 70 : 30 MCH/CHCl₃. However, the change of solvent from 95 : 5 MCH/CHCl₃ to pure MCH led to a 7 nm blue-shift of the absorption band suggesting a minor effect due to solvent polarity. The increase of MCH percentage in CHCl₃ also provided a considerable change in the emission spectrum of 1 (Fig. 2D). The monomeric behavior of 1 was evident till 70 : 30 MCH/CHCl₃. Further increase of MCH percentage resulted in the decrease of characteristic monomer signals and increase of aggregation bands. The appearance of the excimer around 560 nm was first observed in 75 : 25 MCH/CHCl₃. In 85 : 15 MCH/CHCl₃, the intensity of the excimer band increased and a shoulder band at 596 nm was observed. Further decrease in the solvent polarity to 95 : 5 MCH/CHCl₃ resulted in an averaged out excimer band at 578 nm. To understand the excited state dynamics of 1, time-correlated single-photon counting (TCSPC) experiments were performed with excitation at 340 nm (Fig. S8†). The emission monitored at 578 nm decayed bi-exponentially (τ₁ = 6.94 ns (30.54%), τ₂ = 18.33 ns (69.46%)). The emission monitored at 447 nm decayed mono-exponentially (τ = 1.02 ns). These longer lived decays are characteristics of J-aggregated strong intermolecular chromophoric interactions.^23,24

To prove the importance of hydrogen bonding in the aggregation of 1, absorption spectra of 1 were recorded with increasing percentage of MeOH in MCH. In these experiments, the signal at 396 nm displayed a 5 nm red-shift however, with a decrease in absorbance (Fig. S7A†). The addition of MeOH also resulted in the increase of signals corresponding to monomeric structure. A plot of maximum absorbance around 396–401 nm versus MeOH percentage indicated a complete disaggregation of around 2.6% of the protic solvent (Fig. S7B†). This study suggests that the longitudinal π-stacked self-assembly of 1 is sustained predominantly by intermolecular hydrogen bonding interactions among vicinal hydroxyl groups of 1. Based on the aforestated self-assembly, gelation behavior of the compound was explored in various solvent systems (Table S1†). In MCH/CHCl₃ solvent system at room temperature, 1 (1.5 mg mL⁻¹) formed white suspension which upon heating provided clear solution. However slow cooling of solution did not provide any gel formation below 60% of the less polar solvent. Interestingly, the stable gel formation was observed in 70 : 30 MCH/CHCl₃ (Fig. 3A). The gel also displayed thermoreversible “sol–gel” behavior up to five heating–cooling cycles. The preformed gel was also stable even after four months upon storing at room temperature. When placed under the hand-held UV-lamp (λ_ex = 364 nm), the solution of 1 in CHCl₃ provided weak blue fluorescence while that in 95 : 5 MCH/CHCl₃ exhibited strong yellow fluorescence (Fig. 3B). The gel formed in 70 : 30 MCH/CHCl₃ also provided a similar yellow fluorescence (Fig. 3C) indicating a comparable self-assembly in 95 : 5 MCH/CHCl₃ and in the gel. Table S1† also indicates the gel formation in other non-polar organic solvents. It is noteworthy that neither the gel formation in 95 : 5 MCH/CHCl₃ was affected, nor the preformed gel was disturbed upon addition of pyrene. These results suggest that the self-assembly of the 1 is rather strong and the aggregation does not allow interdigitation of electron rich pyrene within the π-stacked NDI layers. Additionally, UV-visible spectrum of 1 in the presence of pyrene did not indicate any charge transfer band formation and this result is also supportive of strong NDI self-assembly.

Fig. 3 Images of temperature dependent sol–gel transition of 1 (1.5 mg mL⁻¹) in 95 : 5 MCH/CHCl₃ (A). Fluorescence images of 1 (1 × 10⁻⁵ M) in CHCl₃ and 95 : 5 MCH/CHCl₃ under 364 nm UV-lamp illumination (B). Fluorescence image of the gel under 364 nm UV-lamp illumination (C).

To understand the morphology of self-assembly of 1, field-emission scanning electron microscopic (FESEM), transmission electron microscopic (TEM) and atomic force microscopic (AFM) data were recorded by drop casting a MCH suspension of the gel on solid surface. All these experiments confirmed the formation of left-handed (M) helical (helix pitch ∼126 nm) nanofibres of considerable length with variable entanglement (Fig. 4). FESEM and AFM data recorded by drop casting 95 : 5 MCH/CHCl₃ solution (Fig. S9†) of 1 also showed comparable left-handed helical nanofibres confirming the similarity of self-assembled morphology in the gel and 95 : 5 MCH/CHCl₃ solution. The long-range orientation of 1 within the self-assembled nanofiber was investigated using the circular dichroism (CD) spectroscopy. The CD spectrum of 1 displayed a flat profile in CHCl₃, whereas the same molecule when recorded in 95 : 5 MCH/CHCl₃ revealed a positive exciton couplet at 248 nm along with strong excitonic negative Cotton effect at 401 nm. The bisignated nature of CD curve indicates the strong excitonic coupling between NDI chromophores⁴⁰ and the long wavelength negative CD signal corresponds to the distinctive π–π* transition along the long axis of NDIs self-assembled in the M-helical arrangement.^22,41–44 (Fig. S10†). The observed left-handed chiral sense is induced by the synergistic effect of chirality transfer from the chiral diol linkage and steric congestion among alkyl chains in the fibrillar structure.^45,46

Fig. 4 FESEM (A), TEM (B) and AFM (D) images recorded after drop-casting the MCH suspension of the gel prepared from 1. Expanded regions of TEM (C) and AFM (E) images are also shown.

The X-ray diffraction study on the xerogel also provided the crucial insight about the self-assembled gel state. In the low angle region, sharp reflections at 2θ = 2.55° (d = 34.56 Å), 5.09° (d = 17.35 Å), and 7.50° (d = 11.55 Å) (i.e. at reciprocal spacing ratio of 1 : 1 : 1) demonstrating formation of strong lamellar type of packing in the gel state (Fig. S11A and B†).⁴⁷ The reflection at 5.09° (d = 17.35 Å) can also be correlated to the length of the monomer 1 (Fig. S15C†). In addition to this, the reflection at 2θ = 2.55° (d = 34.56 Å) can be correlated to a tail-to-tail distance of the 1-dimer preferably connected by hydrogen bonding interactions of hydroxyl groups. Reflections at 8.68 Å (D/4), 6.94 Å (D/5), 5.78 Å (D/6) and 4.33 Å (D/8) also support the lamellar structures in the self-assembled gel state.²⁶ Furthermore, a peak at d = 3.46 Å (2θ = 25.727°) was assigned as the π–π stacking distance between two adjacent NDI units.⁴⁸

Several attempts of NDI 1 crystallization from different solvent systems were unsuccessful. Thus, to obtain more structural insight of the observed self-assembled helical fibrillar packing, computational studies at semi-empirical as well as DFT level were performed. The calculations were done using Gaussian 09 (ref. 49) and MOPAC 2012⁵⁰ software. The initial geometry of the hydrogen bonded system M_2,2 was generated by placing two dimer M₂ units on each other at a stacking distance of 4.68 Å to avoid intermolecular steric clashes (Fig. S14†). Geometry optimization of the M_2,2 system at semi-empirical (Fig. S15A†) and DFT levels (Fig. S15B†) resulted near identical stacking distances of ∼3.4 Å. Subsequently, the octameric complex 1_2,8 was built by placing head-to-head hydrogen bonded dimer of 1 at a 4.68 Å distance above the original and rotating by 3° in the clockwise direction. The 1_2,8 complex was optimized using PM6-DH2 method available in MOPAC2012 software. The hydrogen bonds were not restrained during optimization of the octameric complex. The optimized structure obtained using PM6-DH2 method is well stabilized by hydrogen bond interactions of hydroxyl groups (i.e. via the network of alternate eight- and ten-membered hydrogen bonded rings),³⁶ offset π-stacking and dispersion interactions (Fig. 5A and S17†). Interestingly, the calculated stacking distance of 3.39 Å closely resembled that obtained from the PXRD data (3.46 Å). These results suggest that the strong chromophoric interactions may be responsible for red-shifted excimer emission. The theoretically calculated tail-to-tail distance (41.78 Å) was somewhat longer than that obtained from the PXRD study (34.56 Å). This observation along with SEM images (thickness of fibres = 10–100 nm) imply that the monomer units are involved in repetitive tail-to-tail self-assembly with interdigitation of alkyl chains,⁵¹ and this leads to further aggregation into thick fibres.

Therefore, a plausible mechanism of the self-assembly process is proposed based on the above studies. At first, the dimerization of NDI 1 through head-to-head hydrogen bonding gives 1₂ which upon supramolecular polymerization leads to the fibril 1_2,n with the M-helical arrangement (Fig. 5B). Further bundling of these fibrils forms thicker fibres (i.e. via tail-to-tail interactions of aliphatic chains).

Fig. 5 Top view of the geometry optimized (PM6-DH2) structure of 1_2,8 (A). Schematic representation of the self-assembly process (B).

The presence of extended π-stacked NDI self-assembly endowed us to examine conducting behavior of 1. The charge carrier mobility was evaluated by the current–voltage (I–V) measurement under inert gas atmosphere. The diode current–voltage (J–V) curve shows the deviation from the Ohm's law at the high voltage (Fig. S12†) indicating semiconducting nature of 1.

In conclusion, we have demonstrated M-helical J-type aggregation of naphthalene diimides upon appending a polar 1,2-dihydroxyl moiety and a long hydrophobic chain at two imide termini. Photoluminescence study showed the atypical red-shifted strong excimer emission in 95% MCH. Morphological and circular dichroism (CD) studies confirmed fibrous nature with the M-helical arrangement which is responsible for gelation from a non-polar organic solvent with very low critical gel concentration. An extended hydrogen bonded network of alternate eight- and ten-membered rings involving 1,2-dihydroxyl groups was proposed for the self-assembly. Hydrogen bonding, offset π-stacking, dispersion interactions and chirality of the vicinal diol moiety participate synergistically to ensure M-helical self-assembly with strong NDI chromophoric interactions.

Acknowledgements

This work was supported in part by grants from DST under SERB scheme (SR/S1/OC-65/2012 and SB/S1/PC-39/2012). S. V. S. thanks CSIR and M. K. thanks IISER Pune for research fellowships. We thank Dr K. Krishnamoorthy of NCL, Pune for the recording of conductivity data, Dr A. Mukherjee and Dr N. Ballav of IISER Pune for their suggestions and comments.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, supplemental data, and the ¹H-, ¹³C-NMR spectrum. See DOI: 10.1039/c6ra02729f

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