Hydrogen-bonding driven luminescent assembly and efficient Förster Resonance Energy Transfer (FRET) in a dialkoxynaphthalene-based organogel

Abstract

This paper reports self-assembly, photophysical studies and energy transfer in a bis-amide functionalized dialkoxynaphlane (DAN)-based organogel. Due to the synergistic effect of hydrogen bonding, π–π stacking and hydrophobic interaction, DAN showed long-range ordering in a non-polar solvent, methylcyclohexane (MCH), as evidenced by the fibrous morphology observed in TEM. It showed gelation in various organic solvents with very low critical gelation concentration (in some solvents as low as <0.1 wt%, suggesting very strong gelating propensity). UV/vis and photoluminescence studies of the gel indicated offset π-stacking between the DAN chromophores. Gel to sol transformation was noticed in presence of trace amount of H-bond competing solvent MeOH, indicating strong influence of H-bonds on gelation. Direct evidence for hydrogen bonding was further probed by a variable temperature NMR study in which the NH peak corresponding to the amide functionality was significantly upfield shifted at elevated temperature, signifying breakage of the assembly. Gelation induced enhanced emission was noticed which was attributed to offset π-stacking. As DAN and pyrene are known to form an efficient D–A pair for FRET, photoluminescence properties of the gel were tested in presence of pyrene. It was found that the fluorescence from the donor was completely quenched. In turn strong emission was observed from pyrene suggesting a very efficient FRET process possibly due to close proximity of the pyrene acceptor and the DAN chromophore in the gel state.

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Introduction

π-conjugated chromophores are promising candidates for next-generation low-cost and flexible electronic devices.¹ For such applications, however, the major challenge lies in achieving the desired supramolecular ordering by tuning molecular-level interactions which can control the resulting photophysical properties.² In this regard, low molecular weight organic gelators³ based on pi-conjugated building blocks⁴ are particularly relevant since they are known to form elongated one-dimensional (1-D) fibers on gelation. One of the major advantages of these ordered supramolecular chromophoric arrangements is their ability to facilitate fast and efficient transport of charges (electrons and holes) over a long range,⁵ which is highly desirable in many device applications such as bulk-heterojunction solar cell, organic thin-film-transistors etc. Furthermore, pi-conjugated organic gelators also exhibit rich photophysical properties in the gel state such as aggregation induced enhanced emission.⁶ Organogels can also act as a media for co-assembly of donor and acceptor dyes for efficient energy transfer process.⁷ For example, organogel obtained from oligo(phenylenevinylene) (OPV) and doped with suitable energy acceptor organic dye, was proved to be an excellent candidate for artificial light harvesting material.⁸ Since supramolecular organization is very critical in all of these photophysical processes, it is foremost important to control the inter-chromophoric interaction at the molecular level and possibly correlate this interaction further with the macroscopic properties such as gelation, morphology and energy or electron transfer processes. With this aim, we have studied the self-assembly of a dialkoxynaphthalene gelator,⁹ which is an interesting and well-studied electron donor chromophore in the context of donor–acceptor interaction.⁹ Herein we report H-bonding mediated gelation of DAN-1 (Scheme 1)¹⁰ and elaborate on its photophysical properties in the gel state. Further we show highly efficient FRET from DAN-1 chromophore to a non-covalently co-assembled pyrene guest in the gel state.

Scheme 1 Structure of DAN-1.

Results and discussion

Gelation study

Gelation property of DAN-1 (C = 5.0 mM) was examined in aliphatic hydrocarbon (cyclohexane, methylcyclohexane, n-octane), aromatic (benzene, toluene), and chlorinated solvents (carbon-tetrachloride, tetrachloroethylene). Interestingly, DAN-1 was able to gelate all investigated hydrocarbon- as well as chlorinated-solvents giving transparent or opaque gels (Fig. 1). In contrast, no gelation was observed for aromatic solvents even at relatively higher gelator concentration (10.0 mM). Critical gelation concentration (CGC) was found to be <0.2 wt% for all the cases. In aliphatic hydrocarbon medium, the values were even lower (∼0.1 wt%) and the gel-to-sol melting temperature (T_gel) values were rather high indicating very strong gelation property (Table 1).

Fig. 1 Gelation test (c = 5 mM) in different organic solvents.

Table 1 Gelation data (c = 5 mM) for DAN-1

Solvent	Observation	Tgel^c (°C)	CGC (wt%)
MCH	Gel	75	0.09
CH	Gel	77	0.07
TCE	Gel	51	0.19
CCl4	Gel	46	0.3
^nOctane	Gel	>95	0.13
Benzene	Sol	N/A	N/A
Toluene	Sol	N/A	N/A

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The enthalpy of melting for gel-to-sol transition was determined by analyzing the concentration-dependent T_gel using Schroeder–van Laar equation (eqn (1)).¹¹

where c is the concentration of the gelator, ΔH_m is the enthalpy of melting, R is the universal gas constant (value of R taken as 8.314 J mol⁻¹ K⁻¹) and T_gel is the temperature (in K) at which the gel transforms into sol. Firstly, T_gel was determined as a function of gelator concentration in MCH. As expected, a sharp decrease in T_gel was observed as the gelator concentration decreased (Fig. 2a). From the slope of the Arrhenius plot of ln c vs. 1/T_gel (Fig. 2b), the enthalpy of melting was estimated as 41 kJ mol⁻¹, which indicates a moderately strong gel in comparison to other hydrogen bonding driven organo-gelators reported in the literature.¹²

Fig. 2 (a) Variation in T_gel as a function of gelator concentration; (b) plot of ln[gelator] vs. 1/T_gel.

The morphology of the gel sample examined by TEM (Fig. 3) appeared as a fibrillar network. The elongated fibers were found to be micrometer long, which is typical for a good gelator and the width of the fibrils was found to be in the range of 30–50 nm.

Fig. 3 TEM images of DAN-1 gel; width of the fibril shown by arrow (∼28 nm).

The rigidity and the flow behavior of the gel were investigated in MCH (5.0 mM concentration) by rheological studies. Fig. 4 shows that initially G′ (storage modulus, 112 Pa) was higher than G′′ (loss modulus, 29.6 Pa), indicating presence of a gel phase.¹³ With a gradual increase in the applied stress, G′ and G′′ remained almost invariant up to a certain extent until they crossed each other at the yield stress of 1.9 Pa, suggesting moderate stability of the gel.

Fig. 4 Rheological data for DAN-1 gel in MCH (5.0 mM concentration at 25 °C).

Mode of chromophore assembly in gel state

UV/vis studies. To understand the chromophore packing in the gel state, absorption spectra of the MCH gel (c = 2.0 mM) was compared with that of the sol in CHCl₃. Going from sol to the gel-state, distinct differences were noticed in the absorption spectrum (Fig. 5a). For example, the major absorption peaks at 327 and 314 nm exhibited hyperchromic shift with concomitant red shift of ∼1 nm in the gel compared to the monomeric unit. Furthermore, the absorption bands became sharper in the aggregated state as indicated by the reduction of the full width at half maxima by 0.5 nm. These observations indicate an offset π-stacking,¹⁰ which was predicted as one of the favorable mode of assembly for electron-rich systems.¹⁴ Noteworthy that self-assembly of DAN-1 under a different experimental condition (solution in 90 : 10 MCH/THF) did not show¹⁰ such red-shifted absorption indicating difference in nature of inter-chromophoric interaction in solution and gel state. To determine the thermal stability of the assembly, variable temperature UV-vis spectroscopy studies was carried out (Fig. 5a). The intensity of the absorption bands at 327 nm and 314 nm gradually decreased as the temperature was raised from 25 °C to 75 °C. The absorption spectrum at 75 °C almost overlapped with the monomeric spectrum in CHCl₃, suggesting reversible self-assembly and also the melting temperature obtained from UV/vis studies corroborates very well with the observed T_gel (Table 1).

Fig. 5 (a) Temperature variable UV-vis spectra of DAN-1 gel in MCH (c = 2.0 mM) and in CHCl₃ (for comparison); (b) variation of mole fraction of aggregates with temperature.

From the variable temperature absorption spectra, we further calculated the mole fraction of the aggregates (α_agg) as a function of temperature (Fig. 5b) by using eqn (2).⁴ⁿ

where α_agg(T) is the mole fraction of the aggregate at temperature T (in °C), A(T), A_mon and A_agg are the absorbance at temperature T (at 327 nm), absorbance in the monomeric form (taken from the spectrum in CHCl₃), and the absorbance at the fully aggregated state at 25 °C, respectively. From the above plot α₅₀ (T) (the temperature at which α_agg = 0.5) was estimated as 62 °C.

¹H NMR studies. In order to probe that hydrogen bonding is indeed responsible for the self-assembly, ¹H-NMR was recorded at variable temperatures (VT-NMR) in the gel state and was compared with that in CDCl₃ in which the chromophore remains in the monomeric form. For comparison, only the aromatic region of the spectra is shown in Fig. 6. In CDCl₃, sharp and distinct signals corresponding to the aromatic peaks were observed. The triplet at δ = 6.5 ppm can be attributed to the N–H proton with J_N–H = 4.4 Hz, typically consistent with the ¹⁴N–H coupling. To elucidate the nature of the aggregate, we attempted to record VT-NMR of the gel in MCH (10% benzene-d₆ was added for locking the signal). However, no peak corresponding to either aromatic protons or amide NH were observed at low temperature, signifying very high relaxation time due to strong gelation.¹² Then VT-NMR was recorded in TCE (5.0 mM concentration with 10% benzene-d₆ for locking) in which the self-assembly was found to be weaker compared to that in MCH (see Table 1 for CGC and T_gel of DAN-1 in these two solvents). While going from CDCl₃ to TCE at 30 °C, significant upfield shifts (H_b: 6.86 to 6.52 ppm; H_c: 7.28 to 7.05 ppm; and H_d: 7.77 to 7.64 ppm) were observed for all the signals corresponding to the DAN-aromatic protons, supporting self-assembly in TCE. Moreover, the peak corresponding to the NH proton appeared as a broad singlet at δ = 6.61 ppm, in contrast to the clear triplet peak pattern in CDCl₃, indicating hydrogen bonding in TCE. To probe this point further, we recorded NMR spectra of the gel at elevated temperatures. The peak corresponding to the amide proton (NH) experienced significant upfield shift as the temperature was raised gradually from 30 °C (δ = 6.60 ppm) to 75 °C (δ = 6.12 ppm).

Fig. 6 Variable temperature ¹H NMR spectra of DAN-1 gel in TCE (5.0 mM) (10% benzene-d₆ was used for locking). For comparison ¹H NMR spectrum in CDCl₃ was recorded at room temperature.

Also the peak pattern changed from a broad singlet (at 30 °C) to a clear triplet (at 75 °C), indicating disruption of the hydrogen bonding at elevated temperature. Concurrently, with gradual increase in temperature, the signals corresponding to the DAN-1 aromatic protons (H_b, H_c and H_d) moved downfield, clearly illustrating the dissociation of the self-assembled structure.

Additional evidence of hydrogen bonding in the gel network was established by FT-IR spectroscopy. Going from CHCl₃ to MCH, the peak corresponding to N–H stretching shifted from 3454 cm⁻¹ to 3284 cm⁻¹ (Fig. 7). Also clear shifts of the peaks corresponding to amide-I (1653 cm⁻¹ in CHCl₃ and 1628 cm⁻¹ in MCH) and amide-II (1600 cm⁻¹ in CHCl₃ and 1591 cm⁻¹ in MCH) were observed. These spectroscopic data are unambiguous proof of hydrogen bonding arising from the amide functionalities and in line with the reported literature.¹⁵

Fig. 7 Selected region of FT-IR spectra of DAN-1 in CHCl₃ (black) and in MCH (red) (DAN-1 c = 5.0 mM).

Proposed model. Based on UV-vis experiments and the variable temperature NMR and FT-IR studies, we propose that the chromophore assembly prefers an arrangement that would synergize both pi–pi stacking and hydrogen-bonding (Fig. 8). This results in self-assembled fiber which bundles up to produce thicker fiber (from TEM the width was noticed 30–50 nm). They further entangle to form network structure which entraps solvent molecules to produce organogel. Further to elucidate the fact that hydrogen bonding is indeed major cause for such facile assembly and gelation, the effect of a hydrogen bond disrupting solvent methanol (MeOH) was examined. It was found that even in the presence of only 2% (v/v) MeOH, the gel was completely destroyed (Fig. 8) confirming H-bonding as the key force for gelation. It is assumed that the possibility of such extended hydrogen bonding among the amide groups in case of off-set stacking of the DAN chromophores does not allow the system from attaining edge-to-face arrangement, which is another preferable mode of arrangement for electron rich chromophores.¹⁴ In order to support this model, we carried out an X-ray diffraction study of the xerogel of DAN-1 prepared from MCH. The diffraction pattern showed a very sharp primary peak at 2θ = 2.74° (Fig. 9), corresponding to a length of d = 32.2 Å, matching very well with the molecular length (32.0 Å) of a fully stretched DAN-1 molecule (Fig. 8) as obtained from the energy minimized structure using Chem3D MM2 level molecular modeling (Fig. S1†). The presence of second- and third-order peaks at periodic distance of d/2 and d/3 suggests a lamellar structure with a repeat unit of 32.2 Å.

Fig. 8 Proposed model for self-assembly and gelation of DAN-1 showing synergistic effect of π–π interaction and H-bonding and gel-to-sol transition in presence of 2% MeOH in MCH suggesting strong influence of H-bonding on gelation.

Fig. 9 Powder X-ray diffraction of DAN-1 xerogel.

Photoluminescence properties

Further we carried out the photoluminescence study of the DAN-1 gel in MCH (c = 2.0 mM) and compared the emission property with that of the monomeric form in CHCl₃ or THF (Fig. 10a). In sharp contrast to solution state, where the emission is almost quenched (peaks are not visible in Fig. 10a), the emission intensity in the gel state increased significantly (Fig. 10a). Further, in the gel state the emission spectrum appeared with small Stoke's shift (18 nm), showed mirror-image relationship with the absorption spectrum. These features in addition to the observations made in UV-vis studies show similar trend that was observed for J-aggregating perylene-derivatives.¹⁶ To gain further insight into the emission property of the self-assembly, time-correlated single photon counting (TCSPC) experiments were carried out with the gel in MCH (Fig. 10b) and compared with the emission in the sol state (in THF, 2.0 mM concentration) (Fig. 10c). For both sol and gel, the emission profiles were monitored at 346 nm after exciting at 295 nm. Fluorescence decay time in the gel state was found to be 5.1 ns (Table S1†), which was faster compared to the same in THF (7.2 ns). The fast decay profile in gel can be attributed to fast delocalization of the exciton in the aggregated state, matching well with the previous observation on self-assembled perylene¹⁶ and naphthalene-diimide¹⁷ chromophores. Further we checked the emission properties of the gel as a function of temperature (Fig. 11a).

Fig. 10 (a) Absorption (dotted lines) and normalized (absorption and emission intensity in the gel state were normalized to 1 and intensity of rest of the spectra are adjusted proportionately) emission spectra (solid line) of DAN-1 in gel (MCH, blue line) and sol (THF, black line; CHCl₃, wine color) state. c = 2.0 mM; λ_ex = 295 nm; time resolved fluorescence decay of DAN-1 in (b) gel in MCH and (c) sol in THF. λ_ex = 295 nm, λ_em = 346 nm.

Fig. 11 (a) Temperature dependent emission spectra of DAN-1 gel (c = 2 mM, MCH); (b) absorption (solid line) and emission (dotted line) of DAN-1 gel (black, MCH) and sol (red line) in presence of 2% MeOH, λ_ex = 295 nm.

It was found that the emission intensity remained almost invariant up to the temperature 70 °C. Beyond 75 °C (where the gel melting was observed before) a sharp decrease in the emission intensity was noted, confirming the enhanced emission was indeed due to gelation. We have shown (Fig. 8) in presence of a protic solvent MeOH gelation could be destroyed. Thus we checked effect of MeOH on emission of the gel and noticed (Fig. 11b) in presence of 2% MeOH the emission intensity reduced by more than 60% reconfirming gelation induced enhancement in the emission.

Co-assembly with pyrene and FRET study

In contrary to common observation that aggregation quenches emission, the present system shows enhanced emission in gel state. This prompted us to explore utilizing this luminescent gel scaffold as a light harvesting system⁷ by FRET from the DAN donor to a pyrene (py) acceptor,¹⁸ which was non-covalently assembled in the gel matrix. Prior to energy transfer studies, the heterogel of DAN-1 + py (1 : 1.1, DAN c = 5.0 mM) was characterized in MCH and was compared with the DAN-1 gel alone. The CGC of the mixed gel was found to be 0.06 wt% and T_gel = 79 °C, indicating improved gelating ability of DAN-1 in presence of py. Further rheological studies (Fig. 12a) revealed a yield stress value of 4.1 Pa which is significantly more than that of DAN-1 gel confirming more robust gelation in presence of py. In broader sense, enhanced gelation propensity of DAN-1 in presence of py is similar to well known phenomenon of salt induced lowering of critical gelation concentration of a surfactant molecule. The morphology of the hetero-gel sample was further examined by TEM. TEM image of the mixed gel shows (Fig. 12b) entangled fibrillar network (width ∼30–50 nm) similar to that observed for DAN-1 and thus suggests that the gel-network remains unperturbed in presence of pyrene. In order to gain further insight about the nature of the hetero-gel, its ¹H NMR was recorded in MCH (DAN c = 5.0 mM, 10% benzene-d₆ was added for locking the signal) (Fig. 12d) and compared with the ¹H NMR of DAN-gel alone in MCH at similar concentration (Fig. 12c). At room temperature, peaks were observed only at δ = 7.99 to 7.77 ppm, corresponding to pyrene protons while all the signals corresponding to the DAN-aromatic protons were absent, signifying different relaxation time for pyrene and DAN-protons in the gel matrix. At elevated temperature, peaks corresponding to DAN-aromatic protons and amide proton (NH) appeared along with the pyrene peaks and all the peaks were assigned according to the NMR spectra in Fig. 6. It is noteworthy that no upfield (amide proton) or downfield (DAN-aromatic and pyrene protons) shifts of the peaks were observed even at 70 °C, suggesting stronger gelation in MCH than TCE. Increasing the temperature further resulted in minor downfield shift of the amide proton at 75 °C indicating onset of disassembly. Then UV/vis absorption spectrum of the hetero-gel DAN-1 + py (1 : 1.1, DAN c = 0.5 mM) was recorded and compared with the individual spectrum of DAN-1 and py at similar concentrations (Fig. 12e). The spectrum of the mixture exhibits peaks in the region of 250–340 nm, corresponding to both DAN-1 and py chromophores. The spectrum of DAN-1 + py matches well to the spectrum obtained from mathematical summation of the individual spectrum of DAN-1 and py chromophores (Fig. 12e), suggesting no ground state interaction between DAN and pyrene chromophores as also indicated by their ¹H-NMR spectrum in the mixed gel which showed sharp pyrene peaks but no DAN peak at rt. Further FT-IR of DAN-1 + py mixed-gel shows identical spectral features compared to the FT-IR spectrum of DAN-1 gel alone in MCH (Fig. 12f), which unambiguously proves the hydrogen bonding network among amide functionalities remains unperturbed even after inclusion of py. Based on these observations it is conceivable that py chromophores do not intercalate between the stacked DAN-chromophores, rather are located in the peripheral region (within Förster distance) of the 1D stack.

Fig. 12 (a) Rheological study for DAN-1 + py gel (DAN c = 5.0 mM); (b) TEM image of DAN-1 + py gel; (c) and (d) variable temperature ¹H NMR of DAN-1 and DAN-1 + py gel, respectively (DAN c = 5.0 mM), 10% C₆D₆ was used for locking, X denotes peak from C₆D₆; (e) UV-vis spectrum of DAN-1 (red), py (black), DAN-1 + py (blue) and mathematical summation of DAN-1 and py (dotted line) (DAN-1:py = 1 : 1.1, DAN c = 0.5 mM, path-length = 1 mm); (f) comparative FT-IR spectra of DAN-1 (black) and DAN-1 + py (red) (DAN c = 5.0 mM) in MCH.

After successful characterization of the hetero-gel, FRET from the DAN donor and pyrene acceptor was tested. For FRET to happen, it is prerequisite that the absorption band of the acceptor should have significant overlapping with the emission spectrum of the donor, which is satisfied for the DAN–pyrene pair (Fig. S2†). Moreover, the supramolecular alignment and the distance between the donor- and acceptor-chromophores also play very crucial role in dictating the efficacy of FRET process. To check whether all these criteria are fulfilled in the present gel scaffold we studied emission properties of a gel made from DAN-1 + py (1 : 1.1) in MCH (Fig. 13).

Fig. 13 Emission spectra (λ_ex = 295 nm) of DAN-1 gel (MCH, c = 2 mM) in the absence (red) and presence (black) of pyrene (2.2 mM); the blue line (inset shows same data in zoomed Y-axis scale) represents the emission of pyrene (2 mM) in MCH solution (λ_ex 295 nm).

When the mixture was excited at DAN absorption (λ_ex = 295 nm) a complete quenching of the emission bands ∼350 nm corresponding to the DAN-1 chromophore was noticed with a concomitant appearance of the intense emission bands in the range of 375–425 nm corresponding to py chromophore suggesting extremely efficient energy transfer with maximum possible FRET ratio. When py chromophore alone was excited at λ_ex = 295 nm where its absorption is very negligible (black line, Fig. 12e), it showed negligible fluorescence (blue line, Fig. 13), indicating intense emission band that was observed in the hetero-gel is solely due to energy transfer from DAN to py. The broad band present at λ_em = 472 nm possibly corresponds to the excimer formation of py in the gel state. Such a high FRET efficiency can possibly indicate that the pyrene acceptor is located near the peripheral hydrophobic alkyl chains; which brings them well within the Förster radius facilitating efficient energy transfer.

Experimental

Materials and characterization

All the chemicals were purchased from Sigma Aldrich Chemical Corporation and used without further purification. Solvents were purchased from commercial sources and purified by reported protocol.¹⁹ Spectroscopic grade solvents were used for physical studies. All ¹H and ¹³C NMR spectra were recorded on Bruker DPX 500 MHz machine and calibrated against TMS peak. Chemical shift (δ) and coupling constant (J) are reported in parts per million and Hertz, respectively. The abbreviations for splitting patterns are s, singlet; bs, broad singlet; d, doublet; t, triplet; q, quartet; dd, doublet of doublets, and m, multiplet. Variable temperature NMR was recorded on Bruker DPX 300 MHz machine. UV-visible absorption spectra were recorded on a PerkinElmer Lamda 25 spectrometer and photoluminescence studies were performed on a Horiba Fluoromax-3 spectrometer. Rheological studies were performed on an AR 2000 advanced Rheometer (TA instrument) by cone and plate method. FT-IR data were recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer. XRD data were recorded on a Seifert XRD3000P diffractometer having a Cu Kα radiation source (wavelength λ = 0.1541 nm) operating at a voltage and current of 40 kV and 300 mA, respectively.

Synthesis

DAN-1 was synthesized according to the literature-reported procedure.¹⁰

Gelation test

The gelation property of DAN-1 was tested in a series of solvents such as MCH, CH, CCl₄, TCE, n-octane, and toluene. In a typical experiment, a stock solution of the gelator was made from a good solvent like CHCl₃ at a fixed concentration and measured amount of the stock solution was further taken in a screw-capped sample vial and the solvent was evaporated with a hot air gun. To the solid film, measured amount of gelating solvent was added and the contents were heated in closed condition to obtain a homogeneous solution. After cooling to room temperature and standing for 30 minutes, gelation property was tested by standard ‘stable-to-inversion of a vial’ method.

To determine the critical gelation concentration of DAN-1 in MCH, the gel was gradually diluted with known amount of MCH and each time the vial was heated to obtain a homogeneous solution and further cooled down to room temperature before the gelation was tested. The concentration below which no gelation was observed even after waiting for hours is reported as the critical gelation concentration.

By using above-mentioned procedure, T_gel of DAN-1 in MCH was obtained in five different gelator concentrations and ΔH_m was determined using Schroeder–van Larr equation.

Similar procedure was used to check the gelation data of DAN-1 + py.

UV-visible and photoluminescence studies

DAN-1 gel (2.0 mM) was prepared and equilibrated for 2 h before performing the experiments. For variable temperature UV-vis experiment, the gel (2.0 mM in MCH) was heated from the lowest temperature (25 °C) to the highest temperature (75 °C) with an increment of 5 °C and allowed to equilibrate for 10 min at each step before carrying out the measurement. Similar protocol was used for photoluminescence studies.

UV-vis spectrum of DAN-1 + py was recorded according to the procedure as describe above.

Variable temperature NMR

DAN-1 gel was prepared in TCE at 5.0 mM concentration. 10% C₆D₆ was added for locking NMR signal. NMR spectra at variable temperatures were recorded on a 300 MHz NMR instrument from 30 °C to 70 °C with an increment of 10 °C. The sample was allowed to equilibrate for 10 min at each temperature before recording the spectrum.

Variable temperature NMR experiments of DAN-1 in MCH and DAN-1 + py in MCH were carried out according to the procedure as describe above.

Rheological measurement

DAN-1 gel (5.0 mM) was prepared in MCH and equilibrated for 2 h before performing the experiment. Stress-amplitude sweep measurement was carried out using an Advanced Rheometer AR 2000 (TA instrument) with the cone diameter, cone angle and truncation of 40 mm, 4°0′22′′ and 121 μM, respectively. The runs were conducted at 25 °C with a constant oscillation frequency of 1 Hz.

Rheological measurement of DAN-1 + py was conducted using similar protocol.

FT-IR measurement

FT-IR spectra were recorded on a PerkinElmar Frontier FT-IR spectrometer (DAN-1 c = 2.0 mM).

TEM study

One drop of the gel sample (0.5 mM in MCH) was put on a copper grid coated with carbon and the grid was air-dried for 1 h before the experiment was performed.

TEM study of DAN-1 + py (1 : 1.1, DAN c = 0.5 mM) was carried out by similar method.

Powder X-ray diffraction studies

DAN-1 gel in MCH (2.0 mM) was repeatedly drop-casted on a glass slide to prepare a thick film, which was air dried for 12 h at room temperature for powder X-ray diffraction study. Data was recorded from 1°–30° with sampling interval of 0.02 Å per state.

Conclusion

In conclusion, we described the self-assembly and gelation of DAN-1 chromophore. DAN-1 was found to gelate a variety of organic solvents ranging from aliphatic hydrocarbon (cyclohexane, methylcyclohexane, n-octane), aromatic (benzene, toluene), to non-polar chlorinated solvents (carbon-tetrachloride, tetrachloroethylene) with very low critical gelation concentration. TEM study of gel showed fibrous morphology, indicating long-range ordering due to synergistic effect of π–π stacking and hydrogen bonding. DAN chromophores were found to form offset π-stacking in the gel state leading to enhanced emission. A highly efficient light harvesting system could be constructed by gelation in presence of py as an acceptor which did not perturb the self-assembly of DAN-1. Emission spectra of the mixed gel showed very efficient energy transfer suggesting the present gel is a highly promising soft scaffold for co-assembly of functional chromophores. Luminescent gelation and highly effective energy transfer process is encouraging to explore this building block and structurally related systems for co-assembly of more than one chromophore to realize cascade energy transfer producing tunable emission color.

Acknowledgements

SG thanks the Alexander von Humboldt Foundation for donating a spectro-fluorimeter. DB and AD thank CSIR, India for a research fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Energy minimized structure and fluorescence lifetime data. See DOI: 10.1039/c4ra08920k

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