Crystal Structures of Lignocellulosic Furfuryl Biobased Polydiacetylenes with Hydrogen-Bond Networks: Influencing the Direction of Solid-State Polymerization through Modification of the Spacer Length.

We present the topochemical polymerization of two lignocellulosic biobased diacetylenes (DAs) that only differ by an alkyl spacer length of 1 methylene (n = 1) or 3 methylene units (n = 3) between the diyne and carbamate functionalities. Their crystalline molecular organizations have the distinctive feature of being suitable for polymerization in two potential directions, either parallel or skewed to the hydrogen-bonded (HB) network. However, single-crystal structures of the final polydiacetylenes (PDAs) demonstrate that the resulting orientation of the conjugated backbones is different for these two derivatives, which lead to HB supramolecular polymer networks (2D nanosheets) for n = 1 and to independent linear PDA chains with intramolecular HBs for n = 3. Thus, spacer length modification can be considered a new strategy to influence the molecular orientation of conjugated polymer chains, which is crucial for developing the next generation of materials with optimal mechanical and optoelectronic properties. Calculations were performed on model oligodiacetylenes to evaluate the cooperativity effect of HBs in the different crystalline supramolecular packing motifs and the energy profile related to the torsion of the conjugated backbone of a PDA chain (i.e., its ability to adopt planar or helical conformations).

Polydiacetylenes (PDAs) are a class of semiconductor polymers that have attracted the attention of the scientific community due to their unique optoelectronic properties, in particular, their blue-red colorimetric transition when the leading conjugated backbone chains are disturbed by various external stimuli.[1,2] This feature has been exploited in a wide range of sensing applications,[3−5] going from virus detection,[6] to dosimeter system,[7] and so on.[8,9] The synthesis of PDA has been achieved in several forms, such as liposomes (also referred to as vesicles),[10−12] micelles,[13−15] Langmuir–Blodgett films,[16,17] strips,[18] nanotubes,[19,20] gels,[21,22] nanofibers,[23,24] and crystals.[25−29] In addition to applied research on this type of polymeric material, a fundamental understanding of the structure–property relationship of PDA can be obtained from crystallographic studies. Crystalline topochemical polymerization has been reported as an effective tool that offers control over polymers’ molecular and supramolecular structures.[30−32] A recent review has described several other advantages of topochemical polymerization in the crystalline state over conventional solution-phase polymerization.[33] On the other hand, suitable packing requirements for monomers must be satisfied to achieve the solid-state polymerization of diacetylenes[34,35] (Figure a): (1) The C1–C4′ contact distance (r) on adjacent diacetylene moieties must be less than or equal to 4 Å, (2) the stacking translational distance (d) of the monomers must be close to 4.9 Å, and (3) the orientation angle (θ) between the DA rod and the stacking axis must be close to 45°.

Figure 1

(a) Ideal geometrical conditions for the solid-state polymerization from DA to PDA. (b) Our previous work[27] on polychlorodiacetylene (PClDA) presenting a HB network close and parallel to the conjugated PDA backbone. (c) Our previous work[26] on biobased polydiacetylene having a HB network close but skewed to the conjugated PDA backbone.

However, these conditions are necessary but insufficient for the reaction to occur efficiently. Indeed, some examples in the literature suggest that if the hydrogen bond (HB) network is too close to the reactive DA, the topochemical reaction is partially inhibited and produces red PDA instead of the expected blue PDA with a long effective conjugation length.[36−38] This is probably one of the main reasons that flexible alkyl spacers with three or more methylenes between the DA reactive site and the group involved in the HB network have been widely used in PDA systems.[39−42] Our group focused on two exceptions that allowed us to determine the crystal structures of the resulting polymers even though HB networks were close to the DA functionalities:

The first exception has been using a highly reactive chlorodiacetylene (Figure b).[27] Using urethane (carbamate) functionality, we successfully obtained a single crystal structure determination of a polychlorodiacetylene with an HB network close and parallel to the conjugated PDA backbone. To the best of our knowledge, it is the first example of a complete structural determination of this kind of compact and parallel HB arrangement in PDA. Moreover, interactions through C–Cl···π halogen bonds (XB) are present, and this is another example of the incorporation of XBs into polymer science. This field has gained increasing importance recently.[27,43−48]

The second exception, also using the directional urethane functionality, consisted in the polymerization of DA in the direction skewed (forming an angle of 70°) to the HB network. This arrangement was first observed by the group of Collet in 1988.[49] However, there were no further investigations of this interesting molecular organization until our work with lignocellulosic biobased PDA in 2019 (Figure c).[26]

There have been numerous efforts to develop biobased polymers as a potential solution to reduce dependence on fossil resources.[50] In the case of PDA, we are the first group to report a furan-biobased PDA material.[26] Since furfural (FUR) has been identified as one of the most promising chemical platforms directly derived from biomass[51] and since furfurylamine (FAM) may either be obtained by reductive amination of furfural[52] or directly from biomass via a novel chemoenzymatic one-pot process,[53] our approach has focused on furfurylamine biobased diacetylene (FAM-DA) monomers as an interesting starting point in the development of environmentally friendly PDA materials (Figure a). More recently, Kim’s group developed furfuryl alcohol biobased diacetylene (FA-DA) and the corresponding PDA. They exploited the furan heterocycle in Diels–Alder and retro Diels–Alder reactions, proving that furan-based DA and PDA constitute a promising new family of functional materials.[54] We herein investigated the effect of the spacer length (n = 1 or 3) on the direction of polymerization of FAM-DAs (Figure b). We present the synthesis of the new FAM-DA-3CH2 derivative with a spacer of three methylenes (n = 3) and the comparative study between the single crystal topochemical polymerization of this new DA and our previous reported similar parent DA with a spacer of 1 methylene (n = 1).[26] Finally, based on atomic coordinates extracted from the X-ray single-crystal structures obtained from FAM-PDA-1CH2 and FAM-PDA-3CH2, we evaluated the cooperative behaviors of the HB networks through DFT calculations (Figure c). Using semiempirical tight-binding calculations, we also assessed the torsional energy profile around the planar conformation of individual PDA chains with intramolecular HBs (Figure d).

Figure 2

(a) Synthetic route from lignocellulosic biomass to furfurylamine (FAM)[51−53] and furfuryl alcohol (FA) to the corresponding furfurylamine-diacetylene (FAM-DA)[26] and furfuryl alcohol-diacetylene (FA-DA)[54] monomers. (b) Structures and direction of polymerization of the two studied diacetylenes which differ by an alkyl spacer length (n) of one methylene (n = 1) or 3 methylene units (n = 3) between the diyne and carbamate functionalities. (c) Schematic representation of HBs in the supramolecular motifs observed in the crystal structure of FAM-PDA-1CH2 and FAM-PDA-3CH2. (d) Illustration of the conformational change between the planar and helical PDA backbone having intramolecular HBs.

Synthesis

We synthesized derivative FAM-DA-3CH2 in two steps (Scheme ). At first, alkyne 1 was obtained from furfuryl isocyanate, which is commercially available or otherwise readily accessible from furfurylamine.[55] Second, using the Hay catalyst, we achieved the homocoupling reaction of the terminal alkyne 1 in 2 h 30 min to give the desired diacetylene.

Scheme 1

Synthetic Route of FAM-DA-3CH2

Recrystallization

The test tubes containing the desired product (spotted by TLC analysis) were left at room temperature for 1 week with minimum light exposure following the flash chromatography purification procedure. The slow evaporation of the eluent of the chromatography directly resulted in the formation of colorless crystals with blue parts (Figure S1a), which suggests some degree of polymerization. Addition of dichloromethane (DCM) to these crystals in test tubes formed partially insoluble blue fibers (Figure S1b, 0.6 mg of insoluble material over a total of 18 mg); the major part of the sample remained in the form of the monomer FAM-DA-3CH2 (97% if we consider the mass that could be dissolved and recovered with DCM). RAMAN spectroscopy also confirms that there are remaining monomers since the typical diacetylene peak at 2260.6 cm–1 is still present (Figure S7). Indeed, a plate-like specimen (Figure S1c) of FAM-DA-3CH2 was used for the X-ray crystallographic analysis and revealed the structure of the desired monomer.

X-ray Single Crystal Structure Analysis of the DA

The crystal structure of FAM-DA-3CH2 has an interesting feature similar to the parent crystal structure of FAM-DA-1CH2: the DA monomers stack in such a way that the polymerization may occur in two directions (Figure ). Indeed, the geometrical requirements (Figure a) are respected for the topochemical polymerization of DA in both the direction parallel and the direction skewed to the HBs. The main difference comes from the fact that in the case of FAM-DA-1CH2 (Figure a), all the parameters (d, r, and θ) have their values closest to the ideal conditions in the case of the polymerization skewed to the HBs. Unsurprisingly, polymerization proceeded in this direction.[26] In contrast, for FAM-DA-3CH2 (Figure b), the parameters r and θ seem to favor the polymerization in the direction skewed to HB, while the parameter d seems to favor the reaction that occurs in the direction parallel to HB. To the best of our knowledge, this is the first example of a DA system in which such a competition is present and raises the following questions: What will be the favored direction? Which distance parameter (r or d) is more critical for the process? A more in-depth examination of the origin of the reported optimal distances r and d allowed us to provide potential answers to these questions. First, the optimal value of r has often referred to a distance less than or equal to the van der Waals (vdW) contact distance between the two reactive unsaturated carbon atoms. Thus, various values have been reported in the literature (3.4 Å,[56] 3.5 Å,[57,58] or 3.8 Å[2]), since different methods can be used to determine the vdW distance.[59] Nevertheless, we did not find a clear explanation for the use of the vdW contact limit since the original r value reported by Enkelmann in 1984 was r ≤ 4 Å based on diacetylene reactivity and crystallographic data.[34] Second, since the average distance between equivalent atoms along the final PDA backbone is about 4.9 Å, the intermolecular spacing d between DA must be as near as possible to this value. An experience reported by the Laugher group has shown that it should not deviate more than about 0.2 Å from the ideal value of 4.9 Å.[35] So, the range for the d value often reported in the literature (d = 4.7 to 5.2 Å)[60−62] seems inconsistent with a logical ±0.2 Å deviation (it should be reported as d = 4.7 to 5.1 Å). Moreover, the Laugher group suggests that the repeat distance d is the most critical of the two distances. Therefore, taking into account that the r distance is less critical, the parallel direction of polymerization should be favored with FAM-DA-3CH2 (Figure b), since in the skewed direction, the d value (5.170 Å) is found to be outside the values recommended by Laugher (d = 4.7 to 5.1 Å) for topochemical reactivity (although it is within, but at the limit of, the traditional values reported in several articles of the literature (d = 4.7 to 5.2 Å)).

Figure 3

Geometrical parameters of the two potential directions of polymerization in DA single crystals of (a) FAM-DA-1CH2 and (b) FAM-DA-3CH2. A green box was used to highlight the parameters whose values are closest to the optimal values. Displacement ellipsoids are drawn at the 40% probability level.

Single Crystal Topochemical Polymerization

If a FAM-DA-3CH2 single crystal is heated at 60 °C for 6 days, the quality of X-ray diffraction (XRD) data is greatly diminished, but this is still sufficient to demonstrate that the resulting unit cell matches that of the initial monomer, even if the single crystal becomes darker (see Figure S2). Higher temperatures or longer heating times result in the loss of most of the diffraction of the single crystal, making structural analysis by XRD impossible under these conditions. We, therefore, opted for UV irradiation to solve this problem. Blue crystals (Figure a) of FAM-DA-3CH2 have been irradiated for five (5) minutes at 254 nm. After this period, a golden metallic luster is observed on the crystals.

Figure 4

Picture of the crystal chunks of FAM-DA-3CH2 used to study the color change after UV irradiation for (a) 0 min, (b) 5 min (picture shows the side not exposed to UV light), and (c) 20 min.

Nevertheless, some blue stains are still present on the side not exposed to UV (Figure b), which means that the process was done mainly on the surface and not in bulk. Fifteen (15) additional minutes of irradiation gave the crystals an almost completely golden color (Figure c). In Raman spectroscopy, the diacetylene peak of the monomer (2260.6 cm–1, Figure S7) completely vanished, while typical bands of polydiacetylene at 1454.5 and 2073.5 cm–1 became the strongest signals of the spectrum (Figure S8).

After a total of 1 h of irradiation, the conversion toward polymerization was estimated to be 58% based on the soluble mass of the monomer that was dissolved and recovered (Table , entry 4). The applicability of this gravimetric technique is relatively easy and general, but the accuracy in the measurements constitutes a significant drawback.[63] Also, depending on the size of the crystals, the penetration of UV rays to the bulk can vary. Nevertheless, we can follow approximately the progression of the polymerization. After 4 h of irradiation, the polymerization proportion reaches 63% (Table , entry 5). To confirm the determination of the crystallographic structure of polydiacetylene, we irradiated a small selected single crystal of FAM-DA-3CH2 for 4 h before analyzing it successfully by X-ray diffraction. We can assume that irradiation is an efficient method for thin crystals like this one, as the bulk is not very far from the surface.

Table 1

Determination of Polymer Concentration by the Residual Monomer Dissolution Method as a Function of UV Exposure Time of Crystalline Samples of FAM-DA-1CH2 and FAM-DA-3CH2

		FAM-DA-1CH2		FAM-DA-3CH2
Entry	Exposure Time (min)	A (%) of soluble organic mass recovered	% polym.	B (%) of soluble organic mass recovered	% polym.
1	0	92	8	97	3
2	5	68	32	78	22
3	20	67	33	62	38
4	60	41	59	42	58
5	240	17	83	37	63

X-ray Single-Crystal Structures Analysis of the Resulting FAM-PDA-3CH2

As discussed earlier, probably due to parameter d, the direction of chain propagation during the polymerization of FAM-DA-3CH2 is parallel to the HB network, as seen in the resulting crystal structure FAM-PDA-3CH2 (Figure a). As commonly observed during the polymerization of DAs, the unit-cell volume of the monomeric crystal is reduced (from 1034.6 Å3 for FAM-DA-3CH2 to 987.9 Å3 for FAM-PDA-3CH2, ca. 5% contraction), and consequently the density is increased (from 1.324 g/cm3 for FAM-DA-3CH2 to 1.386 g/cm3 for FAM-PDA-3CH2). The N–H···O HBs along the c axis shorten from 1.979 Å in the initial DA stack to 1.925 Å in the final polymer (Figure a). Meanwhile, along the ab plane, the C–H···π weak interactions between neighboring linear 1D supramolecular stacks of DA have shifted from the H7···C10 contact (2.826 Å, O–C2···C10) in the initial monomer to a new H8···C11 contact (2.779 Å, O–CH2–C2···C11) in the resulting polymer (Figure b).

Figure 5

(a) Main directional intermolecular interactions (N–H···O hydrogen bonds) of the carbamate group observed in the crystal structure of FAM-DA-3CH2 and the corresponding intramolecular HB interactions in FAM-PDA-3CH2. (b) Positional difference of the main C–H···π interactions present in FAM-DA-3CH2 and FAM-PDA-3CH2. Displacement ellipsoids are drawn at the 40% probability level.

Cooperative HBs in Polymers

Due to the different directions of polymerization of the initial DA monomers with various spacer lengths, the resulting supramolecular motifs observed in the crystalline state are very different for FAM-PDA-3CH2 than those observed for FAM-PDA-1CH2 (Figure ). The main difference comes from the presence of an intermolecular N–H···O HB network between the PDA chains for FAM-PDA-1CH2, whereas with FAM-PDA-3CH2, we find rather intramolecular HBs. Since HBs may have a considerable impact on the final properties of the overall polymeric material[64−71] or on individual macromolecules,[72] we were interested in modeling simplified oligodiacetylenes (ODAs) to get information on the cooperative effects of the HBs. Indeed, cooperative HBs play a crucial role in shaping macromolecular conformations and governing macroscopic properties of biomacromolecules and synthetic polymers.[72,73] For example, cooperative interchain HB network links polypeptide chains together to form a two-dimensional (2D) rigid β-sheet motif,[74−76] and on the other hand, the cooperative intramolecular HBs may stabilize the helical conformation of a polypeptide chain (such as an α-helix). Thus, using a computational approach, we were interested in

Figure 6

Illustration of the impact of the bond length modification on the direction of the polymerization of the furfurylamine biobased diacetylenes (FAM-DAs) and representation of the resulting HB network supramolecular architecture (left: 2D nanosheet for FAM-PDA-1CH2; right: 1D independent PDA chains for FAM-PDA-3CH2).

Studying the intermolecular cooperative effect of HBs in the 2D nanosheet of FAM-PDA-1CH2.

Evaluating the potential well around the planar conformation of the PDA with intramolecular cooperative HBs.

Computational Studies (Cooperative HB Effect in 2D Nanosheets)

We first compared the stabilization energy of the two types of 2D architectures, namely, 2D nanosheets built from PDA chains interlocked by N–H···O HBs in the case of FAM-PDA-1CH2 or by C–H···π weak interaction in the case of FAM-PDA-3CH2. To do so, we extracted 2D supramolecular stacks of 2 to 8 hexamer structures from the X-ray single-crystal data (Figure a). In order to reduce the size of the systems and keep them computationally tractable, we removed the furan cycles on the PDA side chains. We then performed DFT single-point calculations (B97D3/Def2SVP with density fitting using W06 as basis set) on these model systems and calculated the interaction energies depending on the number (m) of molecules and the position (k) of the desired interaction in the stack (Figure b), using a similar approach to the one reported by Dannenberg.[77]

Figure 7

Interaction energies (B97D3/Def2SVP/W06) between the organized hexamer chains as a function of the number (m) of model hexamers forming the stack and as a function of the position (k) of the interaction in the stack. Note that hexamers without the furan cycles were used as model oligomers for the calculations.

The first difference in behavior between the two studied PDA systems is highlighted by the variation of the interaction energies as the number of hexamer chains m in the stack increases. In the case of FAM-PDA-1CH2, where we have intermolecular hydrogen bonds, we go from an intermolecular interaction of −135.2 kcal/mol between two hexamers (m = 2, k = 1) to interaction energies ranging from −152.3 kcal/mol (k = 1) to −174.8 kcal/mol (k = 4) between two hexamers in a stacking of eight (m = 8) hexamers. This is evidence of a cooperativity effect that makes the HBs stronger as more units are added, in this case, up to 29% (39.6 kcal/mol) stronger in terms of interaction energy. By comparison, the FAM-PDA-3CH2 present an interaction energy of −189.9 kcal/mol (m = 2, k = 1) to an interaction energy ranging from −193.5 kcal/mol (k = 1) to −198.4 kcal/mol (k = 4), an increase of interaction energy of only 4% (8.5 kcal/mol). This small cooperative effect can potentially come from C–H···π interaction, since it has already been reported in the literature to be involved in a cooperativity effect.[78] Nevertheless, we saw that the stack with intermolecular N–H···O interactions (FAM-PDA-1CH2) has a higher cooperativity effect (29% vs 4%) than the one without these interactions (FAM-PDA-3CH2).

Another known characteristic of the HB network’s cooperativity is that the highest interaction value is located at the central position of the chain (for example, position k = 4 has higher energy interaction than position k = 1). We can extrapolate the curves using the equations proposed by Dannenberg,[77] resulting in maximal interaction energy of −176.2 kcal/mol at large m and k values in the FAM-PDA-1CH2 model. Thus, our model’s central interactions (k = 4) with eight hexamers have over 99% of the expected maximal interaction energy. Even the central interaction (k = 3) in the model of 6 hexamers achieves 97% of the maximal interaction energy and appears to be an adequate model of the infinite supramolecular stacking. The same analysis for the noncovalent interactions in FAM-PDA-3CH2 results in an extrapolated interaction energy of −198.5 kcal/mol, only 0.1 kcal/mol greater than the central HB of the eight hexamers model (k = 4, m = 8).

Finally, since the PDA chains of (FAM-PDA-1CH2) are interlocked by skewed directional and cooperatives HBs to form 2D nanosheets, it is conceivable that these kinds of material could eventually, following a tear, reassemble through the HBs, like a zipper, leading potentially to a new type of healable 2D conjugated polymer.[79]

Computational Studies (Planar Conformation: A Local Minimum of Potential Energy?)

To evaluate the ability of PDA chains to form planar or helical conformations with intramolecular HBs, we evaluated the torsional potential of the conjugated PDA backbone. Based on a similar procedure we used in our recent paper on PClDA,[27] we performed dihedral scans with the semiempirical tight-binding GFN2-xTB method (gas phase) on simplified oligodiacetylene (ODA) models (Model-ODA-1CH2 and Model-ODA-3CH2, Figure a). The purpose of these calculations was to verify if the helical conformation is preferred when the HBs are close to the backbone (i.e., for FAM-PDA-1CH2). In such a case, it could be another factor that supports the preference for FAM-PDA-1CH2 to not polymerize in the direction parallel to HBs. Indeed, if the helix conformation is lower in energy and an energy barrier must be overcome to reach the planar conformation, this could hinder polymerization in that direction. However, our results show that in both cases, the planar conformation with intramolecular HBs is a local minimum of potential energy, with deviations resulting in a roughly quadratic increase in energy. However, it is easier to access the helices with n = 1 than with n = 3. Indeed, the torsional energy profiles show that to form a helix with a 30° deviation from planarity, 29–33 kJ/mol is required for n = 3, while only 15–18 kJ/mol is required for n = 1 (Figure a). Based on the Boltzmann distribution, 99% of the population at 298 K is expected to deviate up to 27° from the minimum for n = 1, but only 19° for n = 3. A possible explanation for this observation is that the formation of HBs close to the conjugated backbone in the planar conformation (θ = 180°) is more difficult to achieve for n = 1 than for n = 3, as it is more difficult to align the carbamate HBs in a ring consisting of 14 atoms (when n = 1) rather than in a more flexible 18 atoms ring (when n = 3) (Figure b). The energy gap between the planar and helical conformation for n = 1 is thus reduced. These computational results highlight the influence of the spacer’s length on the polymer’s flexibility and may impact the design of PDA materials. Indeed, short spacer length (n = 1) may potentially increase the ability to induce planar–nonplanar conformation transitions of the conjugated backbone, which is related to the optoelectronic properties of the PDA chain (e.g., color changes).

Figure 8

(a) GFN2-xTB (gas phase) energy values for the torsional energy profiles of simplified ODA models with symmetrical carbamate side chains. (b) Illustration of the 14- or 18-membered ring formed depending on the spacer length.

Conclusion

We have reported the synthesis and single crystal structure of a novel furfurylamine biobased diacetylene (FAM-DA) monomer having three methylene units as a spacer between the carbamate functionality and the DA reactive group. The influence of the spacer length is present, since, with one methylene, the direction of solid-state polymerization is skewed with respect to HB orientation, and with three methylenes, the direction is parallel to it. Our results suggest that, when a choice arises, the polymerization occurs in the direction in which the repeat distance parameter d for polymerization of DA is within the range recommended by Laugher (d = 4.7 to 5.1 Å), even if the other direction have better parameters r and θ. To the best of our knowledge, this is the first clear single crystal structure demonstration on the influence of the spacer length on the direction of polymerization of DA relative to the HB network. Ordering polymers in different packing modes is a challenging task.[80] This study shows that modification of short spacer length allows different alignment of DAs and different packed forms of PDAs, resulting in a new method to tune the properties of polymers. Indeed, we have presented here the ability to obtain different orientations of conjugated polymers through subtle structural changes to the starting diyne derivative. Programming the orientation of π-conjugated polymers is a key facet in the development of organic optoelectronics, since orientation of the polymer chains has a strong influence on the final properties of the material.[81,82] Moreover, computational studies on model oligodiacetylenes built from the crystallographic data of the final PDAs reveal that

FAM-PDA-1CH2 forms a 2D nanosheet with a cooperative effect of the interchain HB network. This arrangement with such a strong structure stabilization could potentially be used to design healable 2D conjugated polymer, similar to a previously reported study on self-healable 2D supramolecular PDA material.[79]

Both PDA chains with intramolecular HBs present a local minimum of potential energy for the planar conformation. However, for FAM-PDA-1CH2, the helical conformations are more easily accessible. This preliminary result suggests that short spacer length (n = 1) can potentially increase the ability to make conformational transitions and indirectly affect the design of novel PDA material with specific optoelectronic properties.

Finally, this work on lignocellulosic furfuryl biobased PDA is a new example in the family of furan-containing polymers that have been reviewed in the literature recently.[83−86] Furfuryl groups can act as sites for post-polymerization modification, extending the range of applications for conjugated biobased polymers.[87]