Tunable plastic films of a crystalline polymer by single-crystal-to-single- crystal photopolymerization of a diene: Self-templating and shock-absorbing two-dimensional hydrogen-bonding layers

Article abstract of DOI:10.1002/anie.201300690

Film review: Two amide-containing bisolefin monomers undergo solid-state polymerization (see example) through a [2+2] reaction in a single-crystal-to- single-crystal fashion. The transformation was favored by the self-templating and shock-absorbing nature of hydrogen-bonding layers. The pyridine-containing polymers were soluble and useful for making plastic films with considerable tensile strengths. Copyright

Full text of DOI:10.1002/anie.201300690

Angewandte
Chemie
DOI: 10.1002/anie.201300690
Solid-State Photopolymerization
Tunable Plastic Films of a Crystalline Polymer by Single-Crystal-to-
Single-Crystal Photopolymerization of a Diene: Self-Templating and
Shock-Absorbing Two-Dimensional Hydrogen-Bonding Layers**
Mousumi Garai, Ramkinkar Santra, and Kumar Biradha*
Polymerization reactions in the solid state have attracted the
interest of chemists because of their fundamental and
applicative aspects.[1,2] The advantage of solid-state polymer-
ization is that it results in the controlled formation of the
product in terms of geometry and stereochemistry and often
gives high yields.[3] In particular, single crystal to single crystal
(SCSC) reactions offer a detailed understanding of the
structure of the product polymer.[4] However, in most such
reactions the products obtained are either amorphous or
microcrystalline, thus making it difficult to characterize the
product structure. Recrystallization of these products is often
impossible because of the insolubility of the polymeric
products. Even in soluble cases, recrystallization of the
product is not guaranteed to result in the same structure as
was originally formed. Therefore, reactions in which the
single crystals maintain their integrity and crystalline nature
are particularly noteworthy. However, such reactions are
relatively rare and only a few molecules are known to
undergo such SCSC transformations to yield crystalline
polymers. The diacetylene, triacetylene, and muconic acid
derivatives of 7,7,8,8-tetrakis(alkoxycarbonyl)quinodime-
thane have been shown to undergo topochemical polymeri-
zation in an SCSC fashion.[5]
reaction. Even the crystal structure of the directly obtained
polymeric phase of DSP was not determined, as the original
crystal developed cracks, and only the cell parameters were
determined. However, Harris et al. recently used powder
diffraction to determine the structure.[8] Herein, we present
two examples of molecules containing amide functionalities
which undergo polymerization through a [2+2] reaction in
À
a SCSC fashion. These monomers form N H···O hydrogen-
bonded layers which act as a self-template to promote a [2+2]
reaction, and they also act as a shock absorber to enable the
reaction to occur in an SCSC fashion.
The molecules of 1 are of our interest for the study of
template-directed [2+2] reactions to produce tricylic mole-
cules by a double [2+2] reaction (Scheme 1). Indeed, recently
Scheme 1.
Although several bisolefins have been shown to form
polymers,[6] to date only 2,5-distyrylpyrazine (DSP) is known
to undergo [2+2] polymerization in an SCSC manner to
afford polymers containing cyclobutane rings. This reaction
was first reported in 1905 and named a four-center type
photopolymerization, but was only reported in detail in 1967
by Hasegawa.[7a] Following this discovery, several studies have
been conducted on various aspects of this reaction of DSP and
several other related molecules.[7]
The rarity of SCSC [2+2] polymerization reactions arises
from the difficulty in finding suitable monomers with the
required crystal packing to promote not only a reaction but
also a crystal packing that can withstand the changes that
occur at the molecular level over the entire course of the
we have shown that Ag···Ag interactions can template such
a reaction of 1 (R = 4-pyridyl; X = HN-NH) to yield a tricyclic
molecule with a 4-12-4 arrangement.[9a] Furthermore, our
systematic studies on bisamide derivatives indicated that this
class of materials have two possible hydrogen-bonding
modes: formation of a b sheet or a two-dimensional layer.[9b]
These studies hinted that the formation of two-dimensional
layers between molecules of 1 brings the possibility of
promoting a [2+2] polymerization reaction. Therefore, we
have prepared several derivatives of 1 by introducing 3-
pyridyl, 4-pyridyl, and phenyl groups as R and various
aromatic and aliphatic groups as linkers (X) to explore their
reactivity in crystalline solids with and without external
templates. During this process we found that 2 and 3 stand out
from all the other derivatives as they undergo SCSC photo-
reactions. We note here that 2, putrescine-1,4-dicinnamide, is
a natural product which can be isolated from fruiting bodies
of the gilled mushroom Pholiotaspumosa (Basidiomycetes,
Strophariaceae), and recently it was shown to inhibit the
growth of human prostate cancer cells.[10]
[*] M. Garai, Dr. R. Santra, Prof. K. Biradha
Department of Chemistry
Indian Institute of Technology
Kharagpur-721302 (India)
E-mail: kbiradha@chem.iitkgp.ernet.in
[**] We gratefully acknowledge financial support from the DST and the
DST-FIST for the single-crystal X-ray facility. M.G. thanks IIT-KGP for
a research fellowship.
Single crystals of compound 2 suitable for X-ray diffrac-
tion analysis were obtained from MeOH/DMF. In the crystal
structure,[11] the asymmetric unit contains only half of the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300690.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
molecule as there is inversion symmetry. Interestingly, the
molecule does not exhibit a linear geometry, as the -N-CH2-
CH2-CH2- fragment contains a gauche conformation with an
À
N-C-C-C torsion (t1) of 62.48. The molecules join through N
À
H···O hydrogen bonds (N···O, N H···O: 2.901(4) ꢀ, 1618) to
form a corrugated two-dimensional layer. Within this layer,
the double bonds are aligned for a photochemical reaction,
Figure 2. Crystal structure of polymer 3P.
=
with a nonbonding distance (d1) of 3.812 ꢀ between the C C
=
=
atoms of two layers and a C C···C C torsion of (t2) 08, thus
indicating ideal conditions for a topochemical [2+2] reaction
(Figure 1). Such an alignment of double bonds occurs through
The 3-pyridyl (3) and 4-pyridyl derivatives of these
compounds were prepared to analyze the generality of the
reaction. Furthermore, since polymer 2P was found to be
insoluble, which makes it difficult to study its properties, it
was also anticipated that the pyridyl derivatives of 2 would
result in better solubility in aqueous solutions. The single
crystals of 3 were not only isostructural with 2 but also
underwent a similar SCSC reaction to form crystals of the
corresponding polymer 3P. Some important differences were
observed in this polymer: the a-axis shrinks more than that of
2 (1.1 versus 0.731 ꢀ) and the c-axis expands (14.400 ꢀ in 3
versus 14.855 ꢀ in 3P) while it remains almost the same in the
case of 2 (15.496 ꢀ in 2 versus 15.305 ꢀ in 2P). In contrast,
the 4-pyridyl derivative was found not to be isostructural with
2 and 3, and was also photostable.
Full conversion of 2 into 2P or 3 into 3P was found to
require 22 h of irradiation in sunlight. In the case of 3, full
conversion (100%) into 3P was verified by 1H NMR
spectroscopic analysis in D2O containing a drop of HCl.
Furthermore, monitoring the conversion of 3 into 3P at
various time intervals by 1H NMR spectroscopy indicates that
1
the reaction occurs through an intermediate phase: the H
NMR spectra of a partially irradiated sample is not just
a mixture of 3 and 3P, it contains some new signals
corresponding to alkyl (n-butyl) and pyridine protons which
belong to neither 3 nor 3P. From this observation it may be
inferred that the reaction may progress through oligomer
formation. For example, it was shown earlier that controlled
irradiation of DSP at a wavelength greater than 400 nm
resulted in formation of an oligomer.[12] We are carrying out
further studies on 3 to obtain more details of this process.
We note here that the SCSC reactions of 2 and 3 do not
disrupt the hydrogen bonds involved in the layers, although
small changes in the hydrogen-bonding parameters are
Figure 1. Illustration of the hydrogen-bonding (large rectangle) layers:
a) before and b) after irradiation in 2 and 2P, respectively (views along
the a-axes). Please note the alignment of the double bonds (oval) and
amide–pyridyl stacking interactions (small rectangle).
one-dimensional stacks of molecules along the a-axis
arranged through p···p interactions between the amide and
phenyl groups: The distance between the amide C atom and
phenyl C atom (d2) is 3.36 ꢀ and the angle (t3) between the
planes of the phenyl and amide groups is 128.
À
Irradiation of crystals of 2 results in the [2+2] reaction
occurring in an SCSC manner to yield single crystals of the
one-dimensional polymer 2P. The volume of 2P was found to
be compressed by 6% compared to that of crystals of 2 (1765
versus 1881 ꢀ3). The major compression occurs through the
stacking axis (a-axis), which was reduced by 0.73 ꢀ (11.83
versus 12.56 ꢀ), while the other two axes remain almost the
same. As expected, the value of d1 decreased from 3.812 ꢀ to
1.588 ꢀ, while the values of t1 and t2 remain almost
unaffected (688 and 08, respectively) from those of the
parent structure. The cyclobutane moiety in the polymer
exhibits perfect planarity, with the groups attached to it
exhibiting torsions of 68 (cis groups) and 1258 (trans groups;
Figure 2). Furthermore, the d2 and t3 values in 2P are 3.2 ꢀ
and 378, respectively, which indicates a considerable change in
the stacking interactions.
observed (N···O, N H···O: 2.901(4) ꢀ and 1618 in 2,
3.035(2) ꢀ and 1628 in 2P; 2.891(3) ꢀ and 1728 in 3,
2.965(3) ꢀ and 1648 in 3P). The changes occurring at the
molecular level are absorbed by the 2D hydrogen-bond layer,
through adjustment of the interplanar angles between the
hydrogen-bonded amide groups (Figure 3): the interplanar
angle between two successive amide groups connected
through hydrogen bonds is 688 in 2 and 3, which decreases
to 498 upon photochemical reaction (in 2P and 3P).
As anticipated, the polymer 3P, unlike 2P, was found to be
soluble in formic acid, m-cresol, and dilute HCl or H2SO4.
Such solutions (formic acid and HCl) of 3P form films upon
drying in an oven at 70–808C. The films were found to have
a wrinkle-free nature: After twisting and turning they return
to their original shape without any evident residual marks
(Figure 4a, also see the video in the Supporting Information).
2
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
it was found that the irradiated crystals have smooth surfaces
(Figure 4d,e). The AFM images of 3 and 3P were also
recorded using the drop-casting technique. The powder
samples were dispersed in CH2Cl2 and drop-cast on the
glass slide with a micropipette. AFM analysis of 3 indicates
several peaks whereas that of 3P indicates very smooth
surfaces with several wedges parallel to each other (Fig-
ure 4 f–i).
In conclusion, we have demonstrated the facile [2+2]
SCSC transformation of two monomers—the first examples—
to produce crystalline polymers containing pyridine/phenyl,
amide, cyclobutane, and n-butyl moieties. The SCSC trans-
formation was favored by the self-templating and shock-
absorbing nature of the hydrogen-bonding layers. The pyri-
dine-containing polymers were shown to be soluble and useful
for making plastic films with considerable tensile strengths.
Figure 3. Illustration of the changes that occur in the hydrogen-
bonding layer: a side view of the layer in the crystal structures of a) 2
and b) 2P. Note the change in the angle between the amide planes
(circles).
Pyridine-containing
polymers
have been shown to be excellent
candidates for light-emitting
diodes because of their high elec-
tron affinity.[13] Currently, we are
investigating the electrolumines-
cent, photoluminescent, and ten-
sile strengths of 2P and 3P in the
presence of various counterions.
Received: January 26, 2013
Published
online:
&&
&&,
&&&&
Keywords: photopolymerization ·
.
polymers · self-templating ·
solid-state reactions
Figure 4. Film made from a) 3P·formic acid. SEM images of the b) 3P·formic acid film, c) 3P·HCl film,
d) crystal surface of 3 (inset: crystal image), and e) crystal surface of 3P (inset: crystal image). 3D AFM
height images of f) 3 and g) 3P. The bar graphs indicate the different degree of roughness for h) 3 and
i) 3P.
[1] a) K. Sada, M. Takeuchi, N.
Fujita, S. Shinkai, Chem. Soc.
However, the use of H2SO4 in place of HCl or HCOOH did
not result in films. Preliminary measurements of the tensile
Rev. 2007, 36, 415 – 435; b) D. J. Ahn, J. M. Kim, Acc. Chem. Res.
2008, 41, 805 – 816; c) G. Fernꢁndez, M. Stole, V. Stepanenko, F.
Wꢂrthner, Chem. Eur. J. 2013, 19, 206 – 217.
[2] a) S. Rondeau-Gagnꢃ, J. R. Nꢃabo, M. Desroches, J. Larouche,
J.-F. Morin, J. Am. Chem. Soc. 2013, 135, 110 – 113; b) T.-J. Hsu,
F. W. Fowler, J. W. Lauher, J. Am. Chem. Soc. 2012, 134, 142 –
145; c) S. Dawn, M. B. Dewal, J. A. Krause, P. J. Pellechia, L. S.
Shimizu, J. Am. Chem. Soc. 2011, 133, 7025 – 7032; d) A.
Matsumoto, T. Odani, K. Sada, M. Miyata, K. Tashiro, Nature
2000, 405, 328 – 330; e) X. M. Zhang, S. D. Xu, M. Li, Y. T. Shen,
Z. Q. Wei, S. Wang, Q. D. Zeng, C. Wang, J. Phys. Chem. C 2012,
116, 8950 – 8955.
strengths of the films of 3P·HCOOH and 3P·HCl indicate
values of 15.99 and 7.11 MPa, respectively, with an elongation
break at 20% strain. This implies that the nature of the film is
tunable by changing the counterions. Scanning electron
microscopy (SEM) images of both films were recorded on
a JEOL-JSM 5800 scanning electron microscope. These
images confirm the large structural differences between the
two films (Figure 4b,c).
[3] a) Y. Sonoda, Molecules 2011, 16, 119 – 148; b) A. Matsumoto,
Top. Curr. Chem. 2005, 254, 263 – 305; c) V. Rammurthy, K.
Venkatesan, Chem. Rev. 1987, 87, 433 – 481; d) I. G. Georgiev,
L. R. MacGillivray, Chem. Soc. Rev. 2007, 36, 1239 – 1248;
e) G. K. Kole, J. J. Vittal, Chem. Soc. Rev. 2013, DOI: 10.1039/
c2s35234f; f) K. Biradha, R. Santra, Chem. Soc. Rev. 2013, 42,
950 – 967.
Thermogravimetric analysis (TGA) of the films showed
that decomposition occurs at 4108C. It is interesting to note
here that the as-prepared crystalline polymer of 3P exhibits
a sharp melting point of 3438C, as indicated by differential
scanning calorimetry (DSC). Furthermore, polymer 2P was
found to have a higher melting point (3948C) than that of 3P.
The molecular weight of as-synthesized solids of 3P were
determined by MALDI-TOF analysis using a 2,5-dihydroxy-
benzoic acid matrix. The maximum molecular weight
observed is 4238, which indicates a 12-mer.
[4] a) G. Kaupp, J. Phys. Org. Chem. 2008, 21, 630 – 643; b) T.
ˇˇ ˇ
Friscic, L. R. Z. MacGillivray, Z. Kristallogr. 2005, 220, 351 – 363.
[5] a) J. W. Lauher, F. W. Fowler, N. S. Goroff, Acc. Chem. Res. 2008,
41, 1215 – 1229; b) L. Luo, C. Wilhem, A. Sun, C. P. Grey, J. W.
Lauher, N. S. Goroff, J. Am. Chem. Soc. 2008, 130, 7702 – 7709;
c) L. Addadi, J. V. Mil, M. Lahav, J. Am. Chem. Soc. 1982, 104,
3422 – 3429; d) G. Z. Wegner, Z. Naturforsch. B 1969, 24, 824 –
The SEM images for the as-synthesized and irradiated
crystals indicate significant differences. In both cases (2 and 3)
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
832; e) G. Wegner, Pure Appl. Chem. 1977, 49, 443 – 454; f) T.
Odani, S. Okada, C. Kabuto, T. Kimura, S. Shimada, H. Matsuda,
H. Oikawa, A. Matsumoto, H. Nakanishi, Cryst. Growth Des.
2009, 9, 3481 – 3487; g) D. Furukawa, A. Matsumoto, Macro-
molecules 2007, 40, 6048 – 6056; h) S. Nomura, T. Itoh, H.
Nakasho, T. Uno, M. Kubo, K. Sada, K. Inoue, M. Miyata, J. Am.
Chem. Soc. 2004, 126, 2035 – 2041.
Dillon, J. M. Kaiser, L. J. Mueller, G. Guirado, C. J. Bardeen,
Macromolecules 2007, 40, 9040 – 9044.
[8] F. Guo, J. Marti-Rujas, Z. Pan, C. E. Hughes, K. D. M. Harris, J.
Phys. Chem. C 2008, 112, 19793 – 19796.
[9] a) R. Santra, K. Banerjee, K. Biradha, Chem. Commun. 2011, 47,
10740 – 10742; b) M. Sarkar, K. Biradha, Cryst. Growth Des.
2006, 6, 202 – 208.
[10] A. Russo, M. Piovano, M. Clericuzio, L. Lombardo, S. Tabasso,
M. C. Chamy, G. Vidari, V. Cardile, P. Vita-Finzi, J. A. Garbar-
ino, Phytomedicine 2007, 14, 185 – 191.
[11] The crystal structure of 2 was reported earlier, but we re-
detrmined it for the sake of completeness: M. Clericuzio, M.
Piovano, M. C. Chamy, J. A. Garbarino, M. Milanesio, D.
Viterbo, G. Vidari, P. V. Finzi, Croat. Chem. Acta 2004, 76, 605.
[12] N. M. Peachey, C. J. Eckhardt, J. Am. Chem. Soc. 1993, 115,
3519 – 3526.
[13] a) Y. Z. Wang, D. D. Gebler, D. J. Spry, D. K. Fu, T. M. Swager,
A. G. MacDiarmid, A. J. Epstein, IEEE Trans. Electron Devices
1997, 44, 1263 – 1268; b) C. Wang, M. Kilitziraki, J. A. H.
MacBride, M. R. Bryce, L. E. Horsburgh, A. K. Sheridan, A. P.
Monkman, I. D. W. Samuel, Adv. Mater. 2000, 12, 217 – 222.
[6] a) P. Johnston, C. Braybrook, K. Saito, Chem. Sci. 2012, 3, 2301 –
2306; b) G. W. Coates, A. R. Dunn, L. M. Henling, J. W. Ziller,
E. B. Lobkovsky, R. H. Grubbs, J. Am. Chem. Soc. 1998, 120,
3641 – 3649.
[7] a) M. Hasegawa, Chem. Rev. 1983, 83, 507 – 518; b) M. Hase-
gawa, C. M. Chung, N. Muro, Y. Maekawa, J. Am. Chem. Soc.
1990, 112, 5676 – 5677; c) see Ref. [5c]; d) S. Takahashi, H.
Miura, H. Kasai, S. Okada, H. Oikawa, H. Nakanishi, J. Am.
Chem. Soc. 2002, 124, 10944 – 10945; e) G. M. Schmidt, Pure
Appl. Chem. 1971, 27, 647 – 678,; f) M. Hasegawa, H. Harashina,
S. Kato, K. Saigo, Macromolecules 1986, 19, 1276 – 1278; g) J. J.
Stezowski, N. M. Peachey, P. Goebel, C. J. Eckhardt, J. Am.
Chem. Soc. 1993, 115, 6499 – 6505; h) R. O. Al-Kaysi, R. J.
4
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Solid-State Photopolymerization
M. Garai, R. Santra,
K. Biradha*
&&&& — &&&&
Tunable Plastic Films of a Crystalline
Polymer by Single-Crystal-to-Single-
Crystal Photopolymerization of a Diene:
Self-Templating and Shock-Absorbing
Two-Dimensional Hydrogen-Bonding
Layers
Film review: Two amide-containing bis-
olefin monomers undergo solid-state
polymerization (see example) through
a [2+2] reaction in a single-crystal-to-
single-crystal fashion. The transformation
was favored by the self-templating and
shock-absorbing nature of hydrogen-
bonding layers. The pyridine-containing
polymers were soluble and useful for
making plastic films with considerable
tensile strengths.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!