Exploration and exploitation of homologous series of bis(acrylamido)alkanes containing pyridyl and phenyl groups: β-sheet versus two-dimensional layers in solid-state photochemical [2 + 2] reactions

Sixteen derivatives of the homologous series of bis(acrylamido)alkanes have been synthesized with the aim of systematic analysis of their crystal structures and their solid-state [2 + 2] reactivities. Seven out of the 16 found to have reactive alignments for solid-state [2 + 2] reactions yield dimers and polymers.


Introduction
The systematic exploration of solid-state ensembles of organic molecules containing functional groups which possess similar interactive capabilities albeit minor variations laid the foundation for the gigantic rise of the field of crystal engineering (Schmidt, 1971;Desiraju, 1989;Lehn, 1995;Whitesides et al., 1995;Aakerö y et al., 2001;Moulton & Zaworotko, 2001;Zaworotko, 2001;Desiraju, 2002;Biradha, 2003;Vangala et al., 2005;Rajput et al., 2007a;Mukherjee & Biradha, 2011b). The comparison between various halogen derivatives, either by changing the halogen atoms or by changing their position on the aromatic ring is one of the finest examples of such systematic explorations (Bent, 1968;Legon, 1999;Metrangolo & Resnati, 2001;Metrangolo et al., 2005Metrangolo et al., , 2008Marras et al., 2006;Shirman et al., 2008;Samai & Biradha, 2009). On the other hand, the similarities or anomalies in the properties of homologues series of n-alkanes and ,!-substituted dithiols, diols, diamines and diacids are well understood from their crystal structures (Hicks & Nodwell, 2000;Xu et al., 2002;Vishweshwar et al., 2003;Gibson et al., 2003;Lee et al., 2007). Apart from these very few homologous series, in particular molecules containing more than one functional group that are capable of hydrogen bonding, are characterized crystallographically. The increase in functional groups and flexibility increases the complexity and decreases the probability of having isostructurality in the series. Further, the isostructurality of homologous series is not an obvious fact as the process of crystallization depends on several factors such as aggregation, size and solvation apart from the structure of a molecule.
In these lines we have previously reported our studies on homologous series of N,N 0 -bis(pyridinecarboxamido)alkanes (amides) (Sarkar & Biradha, 2006) and N,N 0 -bis(pyridyl)alkanediamides (reverse amides) (Rajput et al., 2007b) in which amide moieties are separated by alkyl (-(CH 2 ) n -) spacers ( Fig. 1). From these studies it was evident that both series exhibit an odd-even effect on the nature of the hydrogen bond and their network features (Mukherjee & Biradha, 2011a). As these classes of compounds contain two each of amide and pyridine moieties, the molecules were assembled through either N-HÁ Á ÁO or N-HÁ Á ÁN or both the interactions. The -sheets or (4,4)-networks, via N-HÁ Á ÁO or N-HÁ Á ÁN hydrogen bonds, are the two most common motifs displayed by the derivatives containing an even number of -CH 2groups. The amide molecules were found to assemble mostly via amide-to-amide recognitions in amides, while pyridine interferences to the amide-to-amide hydrogen bond was found to be more prominent in the case of the reverse amide series, i.e. the N-HÁ Á ÁN interaction is preferred over the N-HÁ Á ÁO hydrogen bond. In contrast to the formation of the -sheets or (4,4)-networks by the even ones, the formation of threedimensional structures were found to be more frequent in the case of odd ones.
From these studies it was understood that the hydrogen bonded (4,4)-networks can promote solid-state [2 + 2] reactions (Garai et al., 2013) to form polymers containing cyclobutanes and amides in the chain with the phenyl/pyridyl groups as dangling attachments, provided the bis-amide molecules contain double bonds at the terminals. In order to test this hypothesis several derivatives of 1-4 were prepared by changing the spacers between the amide functionalities (Fig. 2). The crystal structures were analyzed in comparison with those of homologues series of amides and reverse amides.
Further, the solid-state [2 + 2] reactions were explored wherever possible. It was shown by us earlier that the crystal structures of molecules 1c and 3c containing the butane spacer possess such unique features to form the hydrogen bonded (4,4)-layer which promotes the polymerization reaction as anticipated in a single-crystal-to-single-crystal (SCSC) manner to yield crystalline organic polymers (Garai et al., 2013). Such polymerization reactions are very rare and the only example in the literature is 2,5-distyrylpyrazine reported by Hasegawa et al. (Hasegawa, 1983;Hasegawa et al., 1986). In this manuscript the following points will be addressed by analyzing homologous series of crystal structures of 1-4: (1) competition between the O atom of the amide and the N atom of pyridine to form hydrogen bonds with the amide N-H group; (2) similarities or differences with previously published bis-amide analogues; (3) formation of the two-dimensional layer versus the -sheet hydrogen-bond networks (Fig. 3); (4) propensity for the formation of hydrates; (5) trends in their melting points; (6) alignment of double bonds for solid-state [2 + 2] reactions; (7) exploration of their reactivities and characterization of their products wherever possible.   Amides and reverse amides 1: R = phenyl; 2: R = 2-pyridyl; 3: R = 3-pyridyl; 4: R = 4-pyridyl, a: X = -HN-NH-, b: X = -HN-(CH 2 ) 2 -NH, c: X = -HN-(CH 2 ) 4 -NH-; d: X = -HN-(CH 2 ) 6 -NH-.

Figure 3
Representation of (a) -sheet and (b) two-dimensional layers.

Synthesis of 1a
In a round-bottom flask cinnamic acid (1.21 g, 0.0081 mol) and pyridine (15 ml) were taken and hydrazine hydrate (0.194 ml, 0.004 mol) was added to the reaction mixture and stirred for $ 15-20 min. After that triphenyl phosphite (2.25 ml, 0.0086 mol) was added to it and the reaction mixture was refluxed for 7-8 h.
After cooling to room temperature, the pyridine was distilled out to reduce the volume up to 5 ml. For the work up process, EtOH was added and then the solid product was filtered and washed with EtOH. The white solid product was recrystallized from methanol-DMF.

Synthesis of 4a
In a round-bottom flask (E)-3-(pyridine-4-yl) acrylic acid (0.596 g, 0.000399 mol), pentafluorophenol (0.8 g, 0.00434 mol) and DCC (0.908 g, 0.0044 mol) were taken in 20 ml of dry THF solvent and stirred over 24 h at room temperature, then the solvent was distilled out to collect the solid product. The solid product was recrystallized from petether. The recrystallized ester (1 g, 0.00313 mol) and hydrazine hydrate (0.077 ml, 0.00109 mol) were taken in dry DMF solvent and stirred at room temperature for 24 h. Then the solid product was filtered and recrystallized with MeOH.

Crystallographic data and refinement details
All the single-crystal data were collected on a Bruker APEX-II CCD X-ray diffractometer that uses graphite monochromated Mo K radiation ( = 0.71073 Å ) at room temperature (293 K) by the hemisphere method. The structures were solved by direct methods and refined by leastsquares methods on F 2 using SHELX97 (Sheldrick, 2015). Non-hydrogen atoms were refined anisotropically and hydrogen atoms were fixed at calculated positions and refined using a riding model.

Results and discussion
The compounds 1a-1d, 2a-2d and 3a-3d are synthesized by refluxing the corresponding diamines with cinnamic acid, (E)-3-(pyridin-2-yl)-acrylic acid and (E)-3-(pyridin-3-yl)-acrylic acid, respectively, in the presence of triphenyl phosphate in pyridine. The same procedure was found to be incompatible for the syntheses of derivatives of 4. Therefore, compounds 4a-4d are synthesized by reacting the pentafluoroester of (E)-3-(pyridin-4-yl)-acrylic acid with corresponding diamines. Single crystals suitable for X-ray diffraction were obtained by the slow evaporation technique from methanol, ethanol or methanol-DMF, methanol-THF solutions of the corresponding compounds. Despite several trails the single crystals suitable for X-ray diffraction analyses could not be obtained for 2c and 3d. The comparison of X-ray powder diffraction (XRPD) patterns with the related analogues hinted at their probable crystal structures. The crystal structures of all the derivatives have been analyzed in terms of hydrogen-bonding networks and comparisons were made with respect to the related derivatives of amides and reverse amides. Further, these studies revealed that the series of these 16 structures can be categorized as four types: (1) -sheet via N-HÁ Á ÁO hydrogen bonds; (2) -sheet via N-HÁ Á ÁN hydrogen bonds; (3) two-dimensional layer via N-HÁ Á ÁO hydrogen bonds; (4) two-dimensional layer via N-HÁ Á ÁN hydrogen bonds. In the following sections the results will be described based on the spacers as listed in Fig. 2. The photochemical [2 + 2] reactions were carried out on 1c, 2b, 2c, 3a, 3c, 4a and 4c as the structures indicated the possibility of such reactions. The pertinent crystallographic details are given in Table 1 and the hydrogenbonding parameters are given in Table 2.

b-sheet and two-dimensional layers with the -HN-NH-spacer
Molecules 1a, 2a, 3a and 4a all crystallize in monoclinic space groups C2/c, P2 1 /n, P2 1 /c and P2 1 /n, respectively. With the exception of 3a, which contains one molecule, all the other three contain half molecules in the asymmetric unit. Interestingly no solvent inclusion was found in all four lattices, making the comparison between these structures more realistic. With the exception of 1a, the hydrazine moieties (C-N-N-C: 164 in 2a and 3a and 180 in 4a) exhibited near co-planar geometry. In 1a the hydrazine moiety resembles that of a simple hydrazine or H 2 O 2 with C-N-N-C torsion of 115 . The molecules in 1a (along the c-axis with a repeat distance of 4.07 Å ) and 2a (along the a-axis with a repeat distance of 4.73 Å ) assemble via N-HÁ Á ÁO hydrogen bonds to form -sheets containing ten-membered rings.  Although both form -sheets the differences in both the structures are apparent given the differences in the geometry of the spacer and molecules: non-coplanar in 1a and nearly planar in 2a.  (Fig. 4). Given these differences the packing of -sheets also differ significantly: in 1a the sheets have a parallel packing (C-HÁ Á Á and Á Á Á interactions), while in 2a they have herringbone packing (C-HÁ Á ÁN, C-HÁ Á Á and Á Á Á interactions) (Fig. S33).
In 3a and 4a the pyridyl groups are found to exhibit interference and form N-HÁ Á ÁN hydrogen bonds. Although both are nearly planar molecules the hydrogenbonding patterns and packing are found to differ drastically. In 3a the molecules assemble to form a onedimensional chain that resembles a -sheet but via N-HÁ Á ÁN hydrogen bonds and also the molecules are off-set. These onedimensional chains pack in a parallel fashion which is somewhat similar to that of 1a (Fig. 5). Whereas in 4a, the molecules assemble to form a herringbone layer via N-HÁ Á ÁN hydrogen bonds containing rectangular cavities which are filled by the adjacent layers. The layers pack such that the double bonds are aligned for double [2 + 2] reaction (Fig. 6).
3.2. b-sheet, two-dimensional layers and water inclusion with -HN-(CH 2 ) 2 -NH-spacer The molecules 1b and 2b crystallized in space group P2 1 /c, while 3b and 4b crystallized in space groups C2/c and P2 1 /n, respectively. The asymmetric units of 1b and 4b are constituted by a half unit of the corresponding molecules, while that of 3b is constituted by two half units of 3b. The asymmetric unit of 2b contains one full molecule and four H 2 O molecules. The geometries of these molecules were found to be different, although all contain the HN-CH 2 -CH 2 -NH moiety with anti geometry (N-C-C-N: 180 in 1b, 3b and 4b and 176 in 2b). The interplanar angles between the amide planes are 0 in 1b, 3b and 4b, while it is 61 in 2b. Further, the plane of C C-C O creates the following angles with the central N-C-C-N plane: 19 in 1b, 80 and 19 in 2b, 77 and 12 in 3b and 72 in 4b.
In 1b and 3b, the molecules assemble via N-HÁ Á ÁO hydrogen bonds to form the usual -sheets along the b-axis.  parallel fashion in the case of 1b, while the sheets exhibit some angularity in the packing in 3b (Fig. 7).
The crystal structure of 2b is drastically different from all the structures presented here as the crystal lattice contains four water molecules which govern the overall hydrogenbonding interactions. No amide-to-amide or amide-to-pyridine hydrogen bonding was found in this structure. The water molecules form an octamer via O-HÁ Á ÁO hydrogen bonds. The octamer contains a flat six-membered ring with the other two water molecules linked at 1,4 positions. These octamers link the molecules of 2b into a three-dimensional network with a plethora of hydrogen-bonding interactions. In the octamer, in terms of hydrogen bonding three types of water molecules exists: (1) four water molecules involved in four hydrogen bonds each, two O-HÁ Á ÁO with H 2 O, one N-HÁ Á ÁOw with the amide and one O-HÁ Á ÁN with the pyridine N-atom; these O atoms exhibit near tetrahedral geometry in terms of hydrogen bonding; (2) two involved in exclusively three Ow-HÁ Á ÁOw hydrogen bonds each with water molecules; (3) two involved in one Ow-HÁ Á ÁOw and two Ow-HÁ Á ÁO C hydrogen bonds each. The second and third categories exhibit nearly planar geometry in terms of hydrogen bonding. In this three-dimensional network, it was found that the double bonds are aligned for a single [2 + 2] reaction (Fig. 8).
The crystal structure of 4b bears a close resemblance to that of 4a as it forms a similar herringbone layer via N-HÁ Á ÁN hydrogen bonds. However, here the double bonds are not aligned for the [2 + 2] reaction as the packing of the layers differ due to the presence of the -CH 2 -CH 2 -spacer. Further, this structure is found to be isostructural with that of the amide analogues with 4-pyridyl substitution and the ethyl spacer.

N-HÁ Á ÁO hydrogen-bonded two-dimensional layers and bsheet with -HN-(CH 2 ) 4 -NHspacer
In this series the single crystals suitable for X-ray diffraction were obtained for 1c, 3c and 4c.     The comparison of XRPD patterns of 2c with those of 1c indicates that the crystal structures of 2c could be similar to that of 1c (Fig. 9). Both 1c and 3c contain half molecules in the asymmetric units. The geometries of the molecules of 1c and 3c were found to be somewhat similar to those of the above structures as the planes of C C-C O create almost right angles (73.6 in 1c and 74.8 in 3c) with that of the spacer -C-C-C-C-plane, and the amide planes are parallel to each other. Further, the central butyl amine moiety is found to have non-planar geometry with the N-C-C-C torsion angles of 59 and 62 in 1c and 3c, respectively.
The crystal structure of 4c is found to be totally different from the above structures as it includes water in the crystal lattice. The crystals exhibit space group P2 1 /c and the asymmetric unit is constituted by half a unit of 4c and one water molecule. The geometry also differs from the above structures as the central plane of the alkyl spacer makes an angle of 64 with that of C C-C O. Unlike the above two structures the -N-(CH 2 ) 4 -N-fragment exhibits all anti geometry with N-C-C-C angles of 180 . The molecules assemble via amide-to-amide N-HÁ Á ÁO hydrogen bonds to form thesheet network along the b axis with a repeat distance of 4.68 Å . These sheets are further connected via Ow-HÁ Á ÁN pyridine hydrogen bonds to the chain of water molecules leading to the formation of a two-dimensional layer in which each water has three connectivity (Fig. 10). These layers stack on each other via C-HÁ Á Á and C-HÁ Á ÁO interactions.

Figure 10
Illustrations for the crystal structure of 4c: (a) linking of -sheets via H 2 O molecules to form a two-dimensional layer; (b) packing of the layers such that the double bonds are aligned for a possible [2 + 2] photopolymerization reaction.  despite several trails by varying solvents and their combinations. The asymmetric unit of 1d is constituted by one molecule while those of 2d and 4d are constituted by half a unit of the corresponding molecule and one H 2 O molecule. The molecular geometries are somewhat similar in all three cases as the amide planes are in-plane with the plane of hexyl spacer, which exhibits all anti geometry. In all three structures the molecules assemble in -sheets via amide-to-amide N-HÁ Á ÁO hydrogen bonds, along the b-axis in 1d and 4d, and along the c-axis in 2d with the same repeat distance of 4.9 Å . In 2d and 4d, neither the water molecules nor the pyridyl groups interfere in amide-to-amide. Rather water molecules form zigzag chains via O-HÁ Á ÁO hydrogen bonds and join the -sheets in two-dimensional layers via Ow-HÁ Á ÁN pyridine hydrogen bonds. Interestingly, given the positional differences of pyridines in 2d and 4d, the layers have different geometries although they have the same hydrogen-bonding connectivities. In 2d the layers are highly corrugated, while they are planar in 4d (Fig. 11). In both layers water molecules exhibit 3connectivity: two Ow-HÁ Á ÁOw with neighbouring water molecules and O-HÁ Á ÁN with pyridine moieties. We note here that the crystal structure of 4d was found to be isostructural with that of 4c. The layers pack on each other with an interlayer separation of 4 Å in 2d and 4d, whereas in 1d the -sheets pack in a parallel fashion. The XRPD of 3d was found to be similar to that of 1d, indicating it may also have similar -sheet formation (Fig. S30).

Molecular structure versus hydrogen-bonding networks
In previous studies on amides and reverse amides it was found that the geometry of the molecule and position of the pyridine groups play a key role in the competition of acceptors C O of amide and the N atom of pyridine to form hydrogen bonds with the N-H group of amides. In particular, the interplanar angle () between the amide group and the terminal aromatic ring tailors the resultant hydrogen bonds. Thevalue of less than 20 results in the formation of N-HÁ Á ÁN hydrogen bonds, while above 20 results in the formation of N-HÁ Á ÁO hydrogen bonds. This phenomenon was also explored using a larger set of compounds from the CSD. In the present series it was found that this hypothesis holds good with the exception of three compounds, namely 1a, 1c and 3c, which form amide-to-amide hydrogen bonds despite having a -value below 20 , i.e. 8.45 , 12.83 and 8.42 , respectively (Table S2).
The probable reasons could be due to their unusual molecular geometries which differ totally from the rest of the structures presented here as they deviate heavily from linearity. Further, it was found earlier that the phenyl and the 3pyridyl substituted derivatives do not form hydrates, while the 4-pyridyl derivatives do exhibit such a tendency. Similarly, out of four 4-pyridyl derivatives studied here, two form hydrates. Interestingly, the present studies show that the 2-pyridyl groups also have a similar tendency to form such hydrates as two out of three form hydrates.

Trends in the melting points of the homologues series
We are in agreement with the general conception that the decrease in melting points was observed with an increase in the number of CH 2 groups, with the caveat that the derivatives with a butyl spacer (1c, 2c, 3c and 4c) deviate from such trends (Hall & Reid, 1943;Thalladi, Boese & Weiss, 2000;Thalladi, Nü sse & Boese, 2000). The butyl derivatives 1c, 2c and 3c are found to exhibit higher melting points than the corresponding ethyl (1b, 2b, 3b) and hexyl derivatives (1d, 2d, 3d and 4d). A further decrease in the melting point was observed for hydrates compared with their respective analogues. Phenyl and 4-pyrydyl derivatives have been shown to have higher melting points than the 3-pyridyl and 2-pyridyl derivatives. A similar tendency was also observed in the melting points of amides and reverse amides (Mukherjee & Biradha, 2011a). No correlation of melting points was observed with either the dimensionality of the network or nature of the hydrogen bonds (N-HÁ Á ÁO versus N-HÁ Á ÁN) (Fig. 12). Further, in a given series a linear increase in the densities of the derivatives was observed with an increase in the number of -(CH 2 )groups with the exception of 1a. Interestingly, the densities of 4-pyridyl derivatives are found to be higher than those of the other three homologues. In contrast, the homologous series containing a phenyl substituent was found to exhibit lower densities than the other three series (Fig. S38). Illustrations for the crystal structure of 2d: (a) packing diagram, note the linking of -sheets into the three-dimensional network by water molecules; (b) linking of -sheets of 2d by the zigzag chain of water molecules.
3.7. Solid-state reactivities of 1c, 2b, 2c, 3a, 4a, 3c and 4c The crystal structure analysis of 2b reveals that the double bonds are aligned for a single [2 + 2] reaction with a CÁ Á ÁC distance (d 1 ) between the double-bonded C atoms of 3.84 Å and C CÁ Á ÁC C torsion of ( 2 ) 0 . The 1 H NMR spectra of irradiated 2b in DMSO-d 6 shows the appearance of cyclobutane protons at 4.44 and 4.03 p.p.m. with the presence of olefinic protons. From these observations it can be concluded that 2b was converted to a single dimer product in 59% yield after 72 h of irradiation in sunlight (Fig. 13).
As it was described earlier, the materials 1c, 2c, 3c and 4c have a required alignment for solid-state [2 + 2] polymerization reactions. The compounds 1c and 3c were shown by us earlier to undergo a polymerization reaction in a SCSC manner to yield crystalline covalent polymers. Although single crystals of 2c were not obtained, the comparison of XRPD patterns of 2c revealed that it also contains similar packing with two-dimensional (4,4)-layers as in 1c and 3c. Therefore, a polycrystalline material of 2c was irradiated in sunlight for 96 h. The irradiated product was found to be insoluble in common organic solvents similar to those of 1c and 3c. However, it was found to be soluble in DMSO or aqueous solution with a drop of HCl, HNO 3 or H 2 SO 4 .
The 1 H NMR spectra of an irradiated sample of 2c in DMSO-d 6 and one drop of H 2 SO 4 revealed that the reaction proceeds through the formation of oligomers as the resultant spectra contains some new peaks in addition to the peaks of the monomer and polymer. In the spectra the cyclobutane peaks of a polymer appeared at 4.27 and 4.64 p.p.m. and also the n-butyl protons of the polymer were found to exhibit an up-field shift from monomer to polymer, i.e. 3.22 to 2.56 p.p.m. and 1.51 to 0.77 p.p.m. For oligomers, the cyclobutane peaks appeared at the same p.p.m. as the polymer, however, the corresponding n-butyl peaks appeared at 3.05, 1.22 and 1.08 p.p.m.. From 1 H NMR, the yield of the reaction including oligomers was found to be 45%. The result of the polymerization reaction observed here is in line with that of 3c albeit the yield of the polymer is not 100% in the present case. The unreacted 2c and oligomers were removed by repeatedly washing the irradiated material of 2c with hot methanol. The 1 H NMR of the separated polymer in DMSO-d 6 with one drop of H 2 SO 4 reveals the presence of cyclobutane protons at 4.27 and 4.64 p.p.m. with n-butyl protons at 2.57 and 0.77 p.p.m. and the absence of olefin protons (Fig. 14). However, unlike the polymer of 3c, the polymeric material of 2c does not result in the formation of plastic films.
The molecular weight of the polymer (2c) was determined using MALDI-TOF (matrix-assisted laser desorption/ionization time-of-flight) with 2,5-dihydroxy benzoic acid (DHB) as a matrix in solution. The highest molecular ion peak was observed at 4415 (m/z), which corresponds to 12-mer of 2c (Fig. 15). Although 4c forms -sheet type N-HÁ Á ÁO hydrogen bonds, the double bonds of 4c are aligned with the other layer and fulfil [2 + 2] photopolymerization criteria with appropriate geometrical parameters: d 1 = 4.18 Å and 2 = 0 . However, 4c was found to be photostable even after prolonged irradiation. The probable reason could be loss of the lattice water which triggers the structural transformation to an unreactive form. Indeed, it was found that the XRPD pattern of 4c which is recorded at room temperature does not match the calculated XRPD pattern of 4c indicating the loss of water. The single-crystal data for   Trends observed in the melting points of the homologous series. this material was collected at low temperature.
Similarly, compounds 3a and 4a were found to be photostable despite the possibility of single and double [2 + 2] reactions, respectively. The d 1 and 2 values for 3a and 4a are 3.909 Å and 0.74 , and 3.779 Å and 0 , respectively. The probable reason could be that in 3a the double bonds exhibit the higher displacement value of 1.9 Å , whereas in 4a the existence of infinite stacks rather than discrete dimers might have prevented the reaction (Gnanaguru et al., 1985;Murthy et al., 1987;Nagarathinam et al., 2008;Yang et al., 2009).

Conclusions
The homologous series exhibit variation in their crystal structures depending upon the spacers and end groups such as phenyl, 2-pyridyl, 3-pyridyl or 4-pyridyl. Unlike in previously studied series, in the current one the hydrazine spacer and 2pyridyl derivatives were included for the first time. Some of the structural aspects observed here have direct correlations with those of amide/reverse amide homologues. For example, amide derivatives with a butyl spacer have previously been shown to form two-dimensional layers via N-HÁ Á ÁO hydrogen bonds for both 3-pyridyl and 4-pyridyl derivatives (Sarkar & Biradha, 2006). Similarly, in the current study the phenyl, 2-pyridyl and 3-pyridyl derivatives containing a butyl spacer form such a two-dimensional N-HÁ Á ÁO hydrogenbonded layer. However, the 4-pyridyl derivative deviates as it forms -sheets which are linked further by water molecules. Further, all the derivatives containing a hexyl spacer exhibited -sheets irrespective of the end attachments, which is in agreement with the amide derivatives containing a hexyl spacer and 3-pyridyl or 4-pyridyl attachments. MALDI-TOF mass data for 2c (irradiated).

Figure 14
1 H-NMR spectra recorded at various stages of irradiation of compound 2c: (a) before irradiation, (b) after irradiation (monomer, oligomers and polymer) and (c) after separation of polymer from monomer and oligomers.