Tuning the dimensionality of functional thiazolo[5,4-d]thiazole based supramolecular polymers via competitive interactions

Abstract

The supramolecular organization of functional molecules at the mesoscopic level influences their material properties. Typically, planar π-conjugated (disc- or linear-shaped) molecules tend to undergo one-dimensional (1D) stacking, whereas two-dimensional (2D) organization from such building blocks is seldom observed in spite of their technological potential. Herein, we rationally achieve both 1D and 2D organizations from a single planar, π-conjugated molecular system via competitive interactions. We exemplify this concept through the supramolecular polymerization of amide functionalized thiazolo[5,4-d]thiazole chromophores into micron-sized, highly crystalline 2D sheets and 1D nanofibers in a range of different solvents. Solution-grown 2D sheets and 1D nanofibers with intermolecular hydrogen bonding were obtained for n-octyl and n-hexadecyl chain bearing derivatives, respectively. Molecular dynamics simulations reveal that for n-octyl derivatives weak C–H⋯S and C–H⋯N interactions between the π-conjugated cores of lateral monomers are dominant leading to 2D sheets, whereas with n-hexadecyl chains, enhanced van der Waals interactions of side-chains lead to 1D growth. For intermediate alkyl chain length (n-dodecyl), a competition between these two interactions leads to a combination of both 1D nanofibers and 2D sheets. The highly crystalline 2D sheets exhibit over two orders of magnitude higher electrical conductivity compared to their amorphous counterpart. We envisage that the strategy of competitive interaction could potentially be applicable to other heteroatom bearing chromophores to achieve tuneable soft functional materials.

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Introduction

The supramolecular assembly of functional monomeric units into high molecular weight polymeric materials via non-covalent interactions has been extensively pursued in the past few decades to create soft tuneable materials.^1–5 Various aspects of supramolecular polymerization such as the mechanisms of growth,^6–9 kinetically controlled assembly,^10–13 pathway complexity^11,14,15 and seed-mediated living supramolecular polymerization^16,17 have been studied. However, in most of these reports a disc-shaped π-conjugated core or bisfunctionalized linear chromophore is used as the building block, which often results in 1D polymers.^6,18

On the other hand, controlled synthesis of 2D architectures with potential applications in gas separation and optoelectronic devices is actively pursued.^19–24 Broadly, 2D architectures from organic precursors/polymers are achieved using the following well-established approaches: (i) crystallization-driven self-assembly of covalent polymeric systems pioneered by Manners and Winnik^25–29 and more recently adopted by others,^30–36 (ii) linking symmetrical, mostly planar building blocks, with complementary covalent bonds or dynamic-covalent chemistries to obtain covalent-organic frameworks,^37–39 (iii) construction of single molecular thick layers via non-planar building blocks with C₃-symmetric functionality for polymerization in two-dimensions,^40–42 and (iv) crystallization of small organic molecules via non-covalent interactions.^43–51 Although the non-covalent synthesis or supramolecular polymerization of small molecules provides an alternative, modular approach to obtain 2D architectures,⁵² often this approach has resulted in small-area (<1 μm) structures lacking in high crystallinity.^53–63 Thus rationally achieving crystalline, large-area 2D and 1D organization based on the same π-conjugated building block is a formidable challenge. Such a system/process could enable us to harness the advantages offered by both 1D and 2D organizations in a dynamic system.

Herein, we present the controlled supramolecular polymerization of thiazolo[5,4-d]thiazole (TzTz), a class of thermally stable, electronically active^64–66 and fluorescent molecular systems,^67–70 into both large-area, crystalline 2D hexagons and 1D nanofibers. Our molecular design comprises a central TzTz core appended with an amide motif on either side, which in turn is connected to a 3,4,5-trialkoxyphenyl (gallic) wedge (Fig. 1a). We envisaged that the amide groups and the π-surface would assist in 1D assembly, whereas the gallic wedge bearing n-octyl (TzTz-C8), n-decyl (TzTz-C10), n-dodecyl (TzTz-C12) or n-hexadecyl (TzTz-C16) will render the assemblies colloidally stable and also aid in tuning the dimensionality (2D to 1D) of the organization via van der Waals interactions of the alkyl chains. We shall first discuss the highly ordered 2D organization observed for TzTz-C8 and then present the systematic tuning of organization to 1D while moving from TzTz-C8 to TzTz-C16. Thereafter, the molecular level insights into the 1D and 2D organization from molecular dynamics simulation and the functional properties of TzTz-C8 based optoelectronic devices will be discussed.

Fig. 1 Self-assembly study of TzTz-C8. (a) Molecular structure of the studied TzTz derivatives. (b) UV-vis absorption spectra of TzTz-C8 in CHCl₃ : ACN solvent mixtures (v/v, l = 10 mm, c = 20 μM). The arrows indicate spectral changes with decreasing CHCl₃ percentage. (c) Partial ¹H-NMR spectra (400 MHz, 298 K) in CDCl₃ : ACN solvent mixtures (v/v, c = 3 mM). * Indicates the amide N–H proton. (d) Schematic representation of the assembly process leading to 2D sheets. (e) Representative FE-SEM image obtained by drop casting the assembled sample on a silicon wafer. The sample was sputtered with 5 nm of gold. (f) Representative polarized optical microscope (POM) image of the sample drop cast on a glass slide and viewed under a cross polarizer. For both (e) and (f), c = 20 μM and composition is 10% CHCl₃ in ACN. Insets of (e) and (f) show the histograms (analyzed using ImageJ) obtained on various FE-SEM and POM images, respectively.

Results and discussion

Synthesis and bulk properties

The designed TzTz molecules were synthesized by first coupling respective gallic amines and 4-formylbenzoic acid to obtain the aldehyde intermediate containing the amide functionality. This was further condensed with dithiooxamide to obtain the target TzTz molecules (Scheme S1). All the intermediates and final molecules were thoroughly characterized by ¹H and ¹³C NMR spectroscopy and mass spectrometry. First, the bulk properties of TzTz derivatives were characterized by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The TGA analysis showed <5% weight loss until 350 °C for all the TzTz derivatives, irrespective of the alkyl chain length, indicating their excellent thermal stability (Fig. S1). DSC thermograms showed endothermic melting transitions at 214 °C, 191 °C, 152 °C, and 119 °C for TzTz-C8, TzTz-C10, TzTz-C12, and TzTz-C16, respectively (Fig. S2). In addition to the melting and corresponding crystallization, a few other transitions were observed, indicating the potential to form liquid crystalline phases. POM images (viewed under cross-polarizers) of TzTz derivatives cooled from isotropic to room temperature showed birefringence with focal-conic or fan like texture for TzTz-C8/TzTz-C10/TzTz-C12 and spherulitic texture for TzTz-C16 (Fig. S3). The latter suggests the propensity of TzTz-C16 to form 1D organization.

Controlled solution-phase assembly of TzTz-C8

Next, we studied the solution-phase assembly of TzTz-C8 using UV-vis absorption and NMR spectroscopy. The UV-vis absorption spectrum of TzTz-C8 in chloroform (c = 20 μM) showed broad absorption from 300–420 nm with a maximum at 380 nm (ε_{380 nm} = 59 500 L mol⁻¹ cm⁻¹), attributed to the characteristic π–π* transition of the TzTz core (Fig. S4a). A similar spectral profile was observed in other non-polar solvents such as tetrahydrofuran (THF) and toluene, suggesting a molecularly dissolved state in these solvents, whereas, in non-polar methylcyclohexane (MCH), the absorption maximum was blue shifted to 370 nm (ε_{370 nm} = 47 190 L mol⁻¹ cm⁻¹) and a broad shoulder at around 415 nm was observed (Fig. S4a). To investigate whether these spectral changes are due to intermolecular aggregation in MCH, we carried out concentration-dependent studies. In the concentration range of 50–300 μM, the absorption spectra scaled linearly with the concentration (following Beer–Lambert's law), suggesting the lack of aggregation even in MCH (Fig. S4b and S4c). Although fluorescence spectra can provide insights into the aggregation of chromophores, TzTz-C8 was found to be non-emissive in the studied solvents (CHCl₃, THF, toluene and MCH). On the other hand, in polar solvents (methanol and acetonitrile) TzTz-C8 was found to be insoluble even at micromolar concentrations. Thus, we studied the assembly behaviour in a mixture of chloroform and acetonitrile (ACN). UV-vis absorption spectra (c = 20 μM) of TzTz-C8 showed no changes when changing the composition from pure CHCl₃ to 20% CHCl₃ in ACN. However, on further decreasing the volume fraction of CHCl₃ to 10%, a gradual drop in absorbance at 380 nm, tailing of the low energy transition until 450 nm and an isosbestic point at 407 nm were observed (Fig. 1b), suggesting intermolecular interaction. To confirm this, composition-dependent ¹H-NMR studies were performed. The ¹H-NMR chemical shift of amide N–H protons in pure CDCl₃ (c = 3 mM) was at 7.88 ppm. On increasing the fraction of polar ACN to 10%, the N–H proton downfield shifted to 8.32 ppm (Fig. 1c). This could be due to a combination of two factors: (i) a steep increase in the polarity of the composition and (ii) intermolecular hydrogen bonding. On further gradual increase of the ACN fraction to 50% (in steps of 10%), a gradual downfield shift of N–H protons to 8.53 ppm and broadening were observed. The gradual downfield shift of N–H protons beyond 10% ACN suggests that the sharp downfield shift (Δδ = 0.44 ppm) at 10% ACN composition was largely due to the sudden change in solvent polarity from pure CDCl₃ to 10% ACN in CDCl₃. Also, with the increase in the fraction of ACN, the aromatic protons of the core and gallic wedge undergo an upfield shift and broadening, clearly indicating the presence of intermolecular hydrogen bonding in a mixture of CHCl₃ and ACN.

Interestingly, a freshly prepared sample of TzTz-C8 in 10% CHCl₃ in ACN (c = 20 μM) was a clear solution. However, after 5 min of standing at room temperature (25–35 °C) it became turbid, indicating the potential assembly of the system. After 24 hours at room temperature (undisturbed), lustrous crystals visible to the naked eye were observed at the bottom of the vial (Fig. 1d). These crystals were transferred onto a suitable substrate and analyzed by field emission scanning electron microscopy (FE-SEM) and POM. FE-SEM micrographs showed micron-sized, hexagonal morphology with varied lateral dimensions (Fig. 1e and S5). Similar micron-sized structures with birefringence were observed using POM under cross-polarizers (Fig. 1f and S6), indicating the highly anisotropic arrangement of molecules in the hexagonal-shaped structures. The analysis of many such structures from FE-SEM and POM micrographs indicates that the typical long-axis of the hexagon (defined as length) is 40–50 μm (inset of Fig. 1e and f). The thickness of these hexagonal crystals as measured by atomic force microscopy (AFM) height analysis and a stylus profilometer was in the range of 100–500 nm (Fig. S7 and S8), indicating multiple layers of sheets stacked on top of each other. To understand if the sheets are in a metastable or kinetically trapped state, a freshly prepared sample was heated to the molecularly dissolved state and cooled to room temperature at a controlled rate (2 °C min⁻¹). The resultant sheets (after 12 hours) were highly birefringent and hexagonal (Fig. S9), suggesting the negligible role of sample preparation in obtaining such large-area sheets. Also, studies in dry 10% CHCl₃ in ACN (c = 20 μM) resulted in large-area sheets (Fig. S10), suggesting the negligible role of residual water in influencing the sheet formation. Furthermore, it is worth noting that the lateral dimensions observed in TzTz-C8 are significantly larger (>10 μm) than those observed for previously reported self-assembled small molecules (<1 μm).^53,60,61

To understand the formation of large-area sheets, time-dependent steady-state UV-vis spectroscopy and microscopic analysis were carried out on TzTz-C8 (c = 20 μM, 10% CHCl₃ in ACN). The time-dependent UV-vis spectral profile shows a minor decrease in absorbance up to 8 hours and thereafter a significant drop in absorbance was observed (Fig. S11). For microscopic analysis, aliquots of freshly prepared samples were drop-cast on a suitable substrate and analyzed by FE-SEM and POM at different intervals of time. It can be observed that until 8 hours slightly birefringent and ill-defined hexagons were formed (Fig. S12). However, the samples equilibrated for 12 hours and beyond at room temperature (25–30 °C) showed clear birefringent sheets in POM and well-defined hexagonal morphology in FE-SEM (Fig. S12). This suggests that at least 12 hours at room temperature (25–30 °C) are required to achieve well-defined sheets. However, as can be seen from the length histogram (inset of Fig. 1e and f), sheets of varied lengths (or high dispersity) are formed. In order to obtain sheets with controlled length distribution, the 10% CHCl₃ in ACN solution after 12 hours was carefully decanted to separate the apparently clear top layer and the bottom containing visibly large crystals. Imaging the supernatant showed sheets of 40–60 μm in length, whereas the crystals at the bottom were significantly large (hundreds of microns in length) (Fig. S13). This suggests that a non-covalent synthetic approach using the differential densities of the sheets could be employed to obtain sheets of markedly different lengths.

Next, we explored the assembly of TzTz-C8 in various solvent compositions and concentrations to explore the generality of large-area sheet formation. First, we varied the concentration (10–50 μM) and composition of CHCl₃ (10–50%) in ACN. The concentration-composition phase diagram shows that for 20–50 μM and 10–20% CHCl₃ in ACN, large-area sheets are observed (Fig. 2a). However, at a higher percentage of solvent (30–50% CHCl₃), as expected, no assembly was observed (Fig. 2a). To explore the versatility of sheet formation, various solvent mixtures were investigated. Notably, similar large-area, birefringent, hexagonal morphology was observed using different solvent combinations, such as methanol/CHCl₃, 1,2-dichloroethane/ACN, ethyl acetate/ACN, and toluene/ACN (c = 40 μM, Fig. 2b and S14). In these mixtures, CHCl₃, 1,2-dichloroethane, ethyl acetate, and toluene served as solvents, promoting molecular dissolution, whereas MeOH and ACN acted as anti-solvents, inducing assembly. This further reinforces that large-area sheets are observed when the right balance between solubility and aggregation is achieved, irrespective of the concentration and composition.

Fig. 2 (a) Concentration-composition phase diagram of TzTz-C8 showing the conditions under which sheets are observed. (b) Representative POM image (under cross-polarizers) of the large-area 2D sheets observed in different solvent combinations (c = 40 μM).

Influence of alkyl chain length on dimensionality of supramolecular polymers

To understand the molecular level origin of the high crystallinity in TzTz-C8, we studied other TzTz derivatives with variation in alkyl chain length (TzTz-C10, TzTz-C12, and TzTz-C16) on the gallic wedge. UV-vis absorption and ¹H-NMR spectroscopic studies suggest that TzTz-C10 and TzTz-C12 assemble in CHCl₃ : ACN solvent composition via intermolecular hydrogen bonding, analogous to that observed for TzTz-C8 (Fig. S15 and S16). The time-dependent UV-vis spectral profile of TzTz-C10 showed a gradual drop in absorbance with saturation around 6 hours (Fig. S17). Similarly, time evolution of morphology studied by FE-SEM (c = 10 μM, 10% CHCl₃ in ACN) showed no mesoscopic ordering until 2 hours; however at and beyond 4 hours elongated structures were observed (Fig. 3a and S18), suggesting the sluggish growth kinetics of TzTz-C10, analogous to TzTz-C8. On the other hand, the time-dependent UV-vis spectral profile (c = 10 μM, 30% CHCl₃ in ACN) of TzTz-C12 saturated within 10 minutes (Fig. S19). Also, time resolved FE-SEM studies showed that only 2D structures are formed until 5 minutes after sample preparation. At 10 minutes and beyond 1D nanofibers were observed, in addition to few 2D sheets (Fig. 3b and S20). Intriguingly, for a fully equilibrated (after 30 minutes) TzTz-C12 solution (c = 10 μM, 30% CHCl₃ in ACN), the supernatant and the material settled at the bottom of the vials showed exclusively 2D sheets and 1D nanofibers, respectively, thus providing access to both 1D and 2D morphologies from the same building block (Fig. 3c and S21).

Fig. 3 Morphological studies of TzTz-C10 and TzTz-C12. (a) and (b) Representative optical and FE-SEM micrographs of TzTz-C10 (c = 10 μM, 10% CHCl₃ in ACN) and TzTz-C12 (c = 10 μM, 30% CHCl₃ in ACN), respectively, obtained by drop casting assembled samples on a silicon wafer. The sample was sputtered with 5 nm of gold. (c) FE-SEM micrographs of TzTz-C12 (c = 10 μM, 30% CHCl₃ in ACN) obtained by drop casting of supernatant solution and precipitation on a silicon wafer.

Intrigued by the observation of 1D nanofibers in TzTz-C12, we further studied TzTz-C16 bearing longer alkyl chains. The UV-vis spectra (c = 10 μM) of TzTz-C16 showed a gradual decrease in absorbance and a hypsochromic shift of λ_max by 17 nm (to 364 nm) as the solvent composition was varied from pure CHCl₃ to 60% CHCl₃ in ACN (Fig. 4a). This is in contrast to the UV-vis spectral features observed for other TzTz derivatives in which only a decrease in absorbance was noted (Fig. 1b). The hypsochromic shift suggests a change in packing to H-type of aggregate for TzTz-C16. Concentration-dependent ¹H-NMR studies in CDCl₃ confirmed the presence of intermolecular hydrogen bonding for TzTz-C16 (Fig. S22). The time-dependent UV-vis spectral profile showed full equilibration within 30 min (Fig. S23), suggesting the rapid kinetics involved in the system. Furthermore, time-resolved FE-SEM studies showed exclusively high aspect ratio 1D nanofibers formed within 30 minutes (Fig. 4b and S24). Remarkably, the 1D nanofibers formed a weak gel (which becomes a partly flowing semi-solid on prolonged standing at room temperature) in CDCl₃ (Fig. 4c, d and S25), and also the 1D morphology was observed in solvents of different polarities (Fig. 4e), suggesting that the packing is independent of the solvent polarity.

Fig. 4 Self-assembly study of TzTz-C16. (a) UV-vis absorption spectra of TzTz-C16 in an ACN/CHCl₃ solvent mixture (v/v, c = 10 μM, l = 2 mm). The arrows indicate spectral changes with decreasing CHCl₃ percentage. (b) FE-SEM micrograph obtained by drop casting the assembled sample (c = 10 μM, 70% CHCl₃ in ACN) on a silicon wafer. The sample was sputtered with 5 nm of gold. (c) POM and (d) FE-SEM micrograph of a xerogel obtained from a CHCl₃ (c = 3.5 mM, CHCl₃) solution of TzTz-C16 drop cast on a glass slide (viewed under a cross polarizer) and silicon wafer, respectively. (e) FE-SEM micrographs of TzTz-C16 assembled in different solvents: toluene (c = 500 μM), THF (c = 500 μM), and EtOAc (c = 250 μM).

The marked differences in UV-vis spectral features and morphology between the different TzTz derivatives indicate that alkyl chain length on the gallic wedge plays a significant role in dictating the molecular packing. To study ordering in these morphologies, X-ray diffraction (XRD) patterns of the assembled structures were recorded. The XRD of TzTz-C8 sheets formed from CHCl₃ : ACN composition showed prominent and sharp diffraction peaks with the principal peak (q*) at 5.71°, followed by 2θ values of 11.17° (√4q*) and 22.42° (∼√9q*), corresponding to interplanar distances of 15.46 Å, 7.92 Å, and 3.96 Å, respectively (Fig. 5). Such an XRD pattern with q*, √4q* and √9q* relationship between the first three high intensity reflections suggests the presence of a highly ordered lamellar arrangement of molecules in the sheets.⁷¹ In addition to the sharp signals corresponding to the lamellar ordering, few low intensity peaks were also observed, which prohibited the full single crystal structural analysis to obtain the exact molecular arrangement. A similar XRD pattern was observed in other solvent mixtures such as CHCl₃/methanol (Fig. S26), indicating lamellar packing. On the other hand, XRD of drop-cast (from the as-synthesized) TzTz-C8 showed a broad peak around 20° (2θ), suggesting the amorphous nature of the sample (Fig. S27).

Fig. 5 X-ray diffraction pattern of the assembled TzTz derivatives from CHCl₃ : ACN composition.

The XRD pattern of TzTz-C10 exhibits multiple sharp reflections, with a prominent low angle peak at 3.65°. This hints towards a structure with at least one of the dimensions being elongated, which is in-line with the elongated 2D structures observed in FE-SEM. The XRD of as-synthesized sample TzTz-C10 showed a broad peak at 20° indicating the amorphous nature of the sample (Fig. S28), analogous to that observed in TzTz-C8. The XRD pattern of self-assembled TzTz-C12 was significantly broader and, interestingly, the reflections observed for TzTz-C8 were also present in TzTz-C12 (5.87°, 11.18° and 23.12°), in addition to other reflections. This again confirms the presence of 2D sheets in TzTz-C12 along with 1D nanofibers, as observed from FE-SEM micrographs (Fig. 3c). Finally, the XRD pattern of self-assembled TzTz-C16 was significantly broad with a prominent peak at 23.35° (3.80 Å), corresponding to π-stacking distance.^72,73 This is in agreement with the hypsochromic shift observed in UV-vis spectra due to strong interaction between the π-surfaces. Thus, the solution-phase assembly plays a paramount role in obtaining highly crystalline, large-area sheets and 1D nanofibers with the length of the alkyl chains playing a pivotal role.

Molecular dynamics simulations

Classical molecular dynamics (MD) simulations were conducted to investigate the molecular packing and the structural evolution of both TzTz-C8 and TzTz-C16 nanofibers. ¹H-NMR analysis confirmed the presence of intermolecular hydrogen bonds between amide groups in both TzTz derivative assemblies. Thus, for the MD simulations, 1D nanofibers comprising 30 monomeric units of TzTz-C8 and TzTz-C16 with intermolecular hydrogen bonding were constructed (Fig. S29 and S30). MD trajectory analysis reveals that both nanofiber assemblies show dynamic behaviour during the simulation timescale (100 ns). The structural evolution of assemblies was characterized using contact maps of all pairs of S, N, and H atoms of the TzTz core of each monomer pair for both nanofibers (Fig. 6a and b and S31). The contact maps revealed a distinct pattern of contacts between non-adjacent molecules. This pattern is more pronounced in TzTz-C8 nanofibers, where frequent interactions between the non-adjacent molecules are observed (off-diagonal elements in the contact map, Fig. 6a), whereas in TzTz-C16 interactions are largely with the neighbouring monomers (exhibiting a tridiagonal matrix pattern). This suggests that TzTz-C8 nanofiber displays more flexibility and bending compared to TzTz-C16, which, in turn, causes the non-adjacent molecules in TzTz-C8 to interact laterally, forming motifs similar to those found in 2D sheet-like structures. The hydrogen-bonding network between amide groups of neighbouring molecules remained stable throughout simulations, consistent with ¹H-NMR experimental observations. The alkyl side-chains show pronounced dynamic behaviour, characterized by intermolecular twinning and transient interactions among neighbouring side chains, resulting in a somewhat disordered arrangement. In TzTz-C16, the long side chains display stronger chain–chain van der Waals interactions. However, at the same time, steric congestions due to the longer alkyl chains prevent lateral interactions between peripheral atoms of the TzTz core. Furthermore, terminal monomers of the TzTz-C8 nanofibers also showed close contact with adjacent molecules during the simulations. This suggests that in the case of TzTz-C8, initially 1D nanofibers form; however, due to the increased lateral interaction between the monomers, they fold and adopt a brickwork-like arrangement within the contact regions. This is illustrated in Fig. 6c, which shows the arrangement of four TzTz-C8 monomers observed during the MD simulations. The lateral contacts mediated by weaker C–H⋯S and C–H⋯N hydrogen bonding interactions stabilize this arrangement, as shown in Fig. 6c, d and S32. This observation is consistent with the X-ray diffraction (XRD) pattern, which indicates a highly ordered lamellar arrangement for TzTz-C8. Moreover, the FE-SEM and POM micrographs reveal a hexagonal morphology that is characteristic of a brickwork-like arrangement of molecular units, as observed in the simulations.

Fig. 6 Molecular dynamics simulations. (a) and (b) Contact maps depicting short contacts between the peripheral atoms (S, N, and H) of the TzTz cores of 30 monomers of TzTz-C8 and TzTz-C16, respectively. (c) Representative arrangement of four TzTz-C8 monomers observed during the MD simulations. Lateral C–H⋯S and C–H⋯N hydrogen bonds are observed between non-adjacent molecules in TzTz-C8. (d) The side-view of the four TzTz-C8 monomers exhibiting a brickwork-like motif. Each pair of monomers (shown with the same color) is stabilized by intermolecular hydrogen bonding (dashed lines). (e) Representative extended 2D arrangement based on calculated approximate lattice parameters of the 2D unit cell being 14 Å and 9 Å (twice the lattice vector length shown in (d)). The alkyl side chains are not shown for clarity. Color codes used in (c): carbon in grey; hydrogen in white; nitrogen in blue; oxygen in red; sulphur in yellow.

Based on the observed arrangement of the four TzTz-C8 monomers in the MD simulations, the approximate lattice parameters of the 2D unit cell are calculated to be 14 Å and 9 Å (Fig. 6e), which is in close agreement with the experimental lamellar d-spacing of 15.6 Å. The core–core radial distribution functions (g(r)) corresponding to the TzTz-C8 and TzTz-C16 nanofibers (Fig. S33) indicate a higher degree of stacking order between monomer units in TzTz-C16 compared to TzTz-C8. This observation aligns with the experimentally observed hypsochromic shift of UV-vis spectra of TzTz-C16, suggesting stronger interactions between neighbouring TzTz-C16 molecules.

Electrical conductivity of TzTz-C8

The observation of solution-phase grown micron-sized 2D morphology in a system with an electronically active core (TzTz-C8) motivated us to further explore the electrical properties of this system. We utilized a custom-designed hollow chamber (2 mL) which was securely attached using silicone adhesive over a patterned Cr/Au electrode channel fabricated on glass substrates (Fig. 7a) for this purpose. A solution of TzTz-C8 (c = 20 μM) in CHCl₃ was first poured into the chamber and I–V characteristics were measured over a voltage range of −2 V to +2 V (Fig. 7b). The voltage range was chosen so that it is within the electrochemical window of the solvent used and artefacts from electrochemical modification of the solvents do not contribute to the observed channel current. Devices fabricated from the pristine CHCl₃ phase exhibited a rather low channel current of ∼10–20 nA. Upon incorporation of acetonitrile into the chamber we observed an increase in the conductivity (Fig. 7b and S34). The channel current increases and reaches a maximum value of ∼1 μA for 10% CHCl₃ in ACN composition, consistent with the microscopic studies in which birefringent hexagonal sheets were observed. Consequently, the conductivity reaches a maximum value of ∼63 μS cm⁻¹ (Fig. 7c). Typical fits for conductivity measurements are shown in Fig. S35. This behaviour of the increase in channel current and the overall conductivity with the increasing volume fraction of ACN can thus be attributed to the enhancement in crystallinity of the resultant assembled structure. From Fig. 7c we observe that the composition with maximum channel current (10% CHCl₃ in ACN) coincides with the composition at which formation of ordered 2D hexagonal structures is seen, thus suggesting that improved molecular ordering leads to higher channel currents. Interestingly, when the temporal evolution of the conductivity of the solution was monitored, we observed that the conductivity increases until 10 h (Fig. S34b). This is in-line with the 8–10 hours required to form equilibrated 2D hexagons of TzTz-C8, as observed from solution state kinetics. Our measurements provide important insights through sensitive conductometric measurement of the solvent induced crystallization/self-assembly processes in this class of semiconductors.

Fig. 7 Electrical characterization of TzTz-C8. (a) Device schematic for the conductivity measurements. (b) Comparison of I–V sweep between TzTz-C8 in pure CHCl₃ and in 10% CHCl₃ in ACN. (c) Comparison of conductivity on increasing the ratio of ACN in the mixture.

Conclusions

In summary, we have shown that in the family of TzTz based amides both micron-sized 2D sheets and 1D nanofibers can be obtained by tuning the van der Waals interactions of the peripheral alkyl chains. XRD studies on assembled structures and molecular dynamics simulations together indicate that in TzTz-C8, due to shorter alkyl chain length, side-on brickwork like packing mediated by the C–H⋯S and C–H⋯N interactions is observed. However, with the longer n-hexadecyl alkyl chain derivative (TzTz-C16), the van der Waals interactions in a 1D nanofiber are stronger leading to 1D morphology. Intriguingly, for an intermediate alkyl chain length (TzTz-C12), both 2D sheets and 1D nanofibers are observed. This represents a unique non-covalent synthetic methodology to achieve more than one kind of nanostructure from the same system. Furthermore, the temporal evolution of 2D sheets was observed by in situ electrical conductivity measurements, indicating the potential application of such systems. These findings highlight that even with a classical monomer design (hydrogen bonding units on either side of a linear π-conjugated core), it is non-trivial to foresee the morphology of the assembled structure. The subtle competition between the alkyl chain crystallization and the interactions between the π-conjugated core (via heteroatoms) dictates the mesoscopic morphology. We envisage that our findings can potentially be applied to a large number of heteroatom bearing π-conjugated systems known in the literature, resulting in exquisite control over the morphology and the resultant electronic properties.

Author contributions

C. K. and A. T. conceived the project. A. T. carried out all the experimental work (synthesis, spectroscopic and microscopic studies). U. P. and D. I. S. carried out the electrical measurements under the supervision of S. P. S. R. S. and S. K. R. carried out MD simulations. C. K. supervised the overall project. A. T. and C. K. wrote the initial draft of the manuscript. All authors discussed the results and contributed to the writing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. Supplementary information: Materials and methods, synthetic, computational and device fabrication details, supporting figures and tables, spectral copies, and references. See DOI: https://doi.org/10.1039/d5sc03874j.

Acknowledgements

C. K. acknowledges funding from the IIT Bombay Seed grant (RD/0520-IRCCSH0-001), DST-INSPIRE faculty fellowship (IFA19-CH316) and Science and Engineering Research Board, Government of India (SRG/2020/002485 and CRG/2023/004212). C. K. and A. T. thank the department of chemistry, IIT Bombay, and Institute of Eminence (IoE) for various facilities. A. T. and C. K. thank Mr Rajesh Gunjal for assistance with MALDI-TOF experiments. C. K. and A. T. thank Satyabrata Behera and Sourav Nandi for useful comments on the manuscript. S. K. R. acknowledges the Core Research grant (SERB-CRG) from the Science and Engineering Research Board (grant number CRG/2022/006457) for financial support. This work also used the computing resources of ‘PARAM Shakti’ at IIT Kharagpur which is a part of the National Supercomputing Mission (NSM), which is implemented by C-DAC and supported by the Ministry of Electronics and Information Technology (MeitY) and Department of Science and Technology (DST), Government of India. R. S. thanks UGC, India, for a Senior Research Fellowship. SPS acknowledges funding support from DAE, Government of India, for funding to NISER, Bhubaneswar, through project RIN-4001, and SERB for funding through IPA/2021/000096. UP and DIS acknowledge DAE, Government of India, for a fellowship.

Notes and references

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