Hydrogen bonded helices: Synthesis, crystal structure and self-assembled microtubes

Article abstract of DOI:10.1016/j.molstruc.2008.08.016

Dicarboxylic bola-shaped compounds 1-3, possessing phthalyl head groups and diol spacers are synthesized and characterized. Keeping phthalyl head group common for all three diester-dicarboxylic acids, the spacer moiety is systematically altered by two and four carbon atoms in 1 and 2, 3, respectively. The flexible spacer moiety ethane-1,2-diol in compound 1 is replaced by cis-but-2-ene-1,4-diol and 1,4-butane diol in 2 and 3, respectively, to study the effect on the morphology of the microcrystal grown on them. Thus compound 2 and 3 though posses four carbon atoms in their respective spacer moiety, they differ by their rigidity. The single crystal X-ray structure obtained for 1, 2 and 3 indicates the formation of self-assembled single stranded helical structure mediated through O-H...O interaction of the end carboxylic acids. Interestingly compound 1 self-assembled into microtubes in ethanol:water solvent mixture. The solvent and the O-H...O; C-H...O interaction combinedly play crucial role in molecular self-assembly process and defines the morphology for 1 into "microtube" whereas 2 and 3 forming "bar" fails to produce such tubular texture though their respective crystal structure shows single stranded helices. The role of weak C-H...O interaction, incorporation of rigid spacer and various other factors such as polarity of the solvents are discussed in detail to explore the difference in the morphology.

Full text of DOI:10.1016/j.molstruc.2008.08.016

Journal of Molecular Structure 919 (2009) 72–78
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Hydrogen bonded helices: Synthesis, crystal structure and
self-assembled microtubes
*
P. Mosae Selvakumar, E. Suresh, P.S. Subramanian
Analytical Science Division, Central Salt and Marine Chemicals Research Institute (Council of Scientific and Industrial Research), G.B. Marg, Bhavnagar, Gujarat 364002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 June 2008
Received in revised form 11 August 2008
Accepted 16 August 2008
Available online 28 August 2008
Dicarboxylic bola-shaped compounds 1–3, possessing phthalyl head groups and diol spacers are synthe-
sized and characterized. Keeping phthalyl head group common for all three diester-dicarboxylic acids, the
spacer moiety is systematically altered by two and four carbon atoms in 1 and 2, 3, respectively. The flex-
ible spacer moiety ethane-1,2-diol in compound 1 is replaced by cis-but-2-ene-1,4-diol and 1,4-butane
diol in 2 and 3, respectively, to study the effect on the morphology of the microcrystal grown on them.
Thus compound 2 and 3 though posses four carbon atoms in their respective spacer moiety, they differ
by their rigidity. The single crystal X-ray structure obtained for 1, 2 and 3 indicates the formation of
self-assembled single stranded helical structure mediated through O–H. . .O interaction of the end car-
boxylic acids. Interestingly compound 1 self-assembled into microtubes in ethanol:water solvent mix-
ture. The solvent and the O–H. . .O; C–H. . .O interaction combinedly play crucial role in molecular self-
assembly process and defines the morphology for 1 into ‘‘microtube” whereas 2 and 3 forming ‘‘bar” fails
to produce such tubular texture though their respective crystal structure shows single stranded helices.
The role of weak C–H...O interaction, incorporation of rigid spacer and various other factors such as polar-
ity of the solvents are discussed in detail to explore the difference in the morphology.
Keywords:
Hydrogen bond
Self-assembly
Helix
Microtubes
Supramolecular chemistry
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
Carboxylic bolaamphiphiles, upon treating with aliphatic amines,
or their Na, K salt [32,33] with controlled pH, were also demon-
Carboxylic amphiphiles form variety of self-assemblies such as
micelles, fibers and crystals [1–6]. Molecules with hollow tubular
structures [1–7], and pores as synthetic ion channels [8–9] offer
wide variety of applications in chemistry, biology, material science,
molecular transporters [10], molecular containers [11], drug deliv-
ery system in pharma/medicine [12–15], host–guest inclusion and
molecular separation, etc. Weak interaction [16–17], such as inter,
strated to form microtubes. All these compounds possessing NH
group in general, results into gel materials [20–33], caused by
the complex inter and intra molecular interaction. Up-to-date re-
port explores the formation of microtubes, mainly using carbox-
amide-based bolaamphiphiles, none of the above report, deals
amide-free dicarboxylic acid bolaamphiphiles. Further, Shimizu’s
odd-even effect [24,29,31] correlating the number of the CH2 group
in the alkyl linker and microtube formation, gains significant
importance. Carboxamide-based bolaamphiphiles with even num-
ber CH2 group in the spacer from 6 to 20 though already reported,
spacer moiety with less than six CH2 group, were not attempted till
today. With an interest to understand the nature of the supramo-
lecular network, we aimed to synthesis an amide-free dicarboxylic
system. Accordingly the compound 1–3 with phthalyl-based dicar-
boxylic head group, composed exclusively by terminal carboxylic
acids and spacer ester moieties are synthesized. The compound
1, possess (CH2)n spacer where n = 2, while the compound 2 and
3 consists four carbon atom with rigid (–C@C–) spacer and flexible
CH2 groups, respectively. Surprisingly no report has been found
exclusively on acid-based microtubes, though the understanding
on acid–acid [25] intermolecular interaction and the literature on
crystal structures of varieties of fatty acids are rich. To best of
our knowledge, the present paper in addition to reporting the com-
intra molecular H-bonding, Van der waals and
p-stacking are
known to generate superstructure covering nano to micro level fi-
bres, rods, ribbons, wires, sheets and tubular materials with attrac-
tive molecular architectures, such as helix, grids and knots, etc.
Among the different supramolecular architecture, self-assembled
helical structures are ubiquitous in nature and can be found in
many biologically important macromolecules like DNA which pos-
sess two intertwined helices, and many proteins have helical sub-
structures. Hence, chemists have made significant efforts to
introduce and control the helicity in many artificial systems [18–
19]. Extensive investigation on carboxamide-based bolaamphi-
philes, with varying head groups such as nucleotides [20–22], pep-
tides [23–27] and sugars [28–31] are reported to form tubes.
* Corresponding author. Tel.: +91 278 2567760; fax: +91 278 2567562.
E-mail addresses: siva140@yahoo.co.in, siva@csmcri.org (P.S. Subramanian).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.08.016

P. Mosae Selvakumar et al. / Journal of Molecular Structure 919 (2009) 72–78
73
pounds containing the lowest number of carbon atom in the spacer
segment (i.e. two in compound 1 and four in 2 and 3), illustrate the
formation of the first amide-free dicarboxylic-based microtube. All
these compounds possessing almost similar helical structure, the
SEM micrograph recorded for compound 1 indicates the formation
of ‘‘microtube”, while compound 2 and 3 does not form tube in the
chosen solvent medium is explored based on the supramolecular
self-assembly through vectorial propagation of the O–H. . .O and
C–H. . .O interactions in detail.
with SAINT [35] software. An empirical absorption correction
was applied to the collected reflections with SADABS [36]. The
structures were solved by direct methods using SHELXTL [37]
and were refined on F2 by the full-matrix least-squares method
using the SHELXL-97 [38] package. Graphics are generated using
PLATON [39] and MERCURY 1.3. [Mercury 1.3 Supplied with Cam-
bridge Structural Database; CCDC: Cambridge, U.K., 2003–2004]. In
both the compounds all non-hydrogen atoms were refined aniso-
tropically till convergence is reached. Most hydrogen atoms in both
the ligands were located from the difference Fourier map and re-
fined isotropically and the rest are stereochemically fixed at ideal-
ized positions. CCDC number 640778, 640779 and 686003 contain
the supplementary crystallographic data in CIF format for all three
compounds reported in this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
2. Experimental
All the chemicals are purchased from Aldrich & Co. and are used
without any further purification. Microanalysis of the compounds
was done using a Perkin-Elmer PE 2400 series II CHNS/O elemental
analyzer. Mass analysis was performed using electron spray ioniza-
tion (ESI+) technique on a waters Q Tof-micro mass spectrometer.
IR spectra were recorded using KBr pellets on a Perkin-Elmer Spec-
trum GX FT-IR spectrometer. 1H and 13C NMR spectra were re-
corded (200 and 50.3 MHz, respectively) on a BRUKER Avance
DPX 200 NMR spectrometer using methanol-d4 or CDCl3. 90°
2.2. Syntheses of compounds
2.2.1. 2-({2-[(2-Carboxybenzoyl)oxy]-ethoxy}carbonyl)benzoic acid
(1)
pulses for 1H (8.9 s) and 13C (5.9
l ls) nucleus was determined
Phthalic
anhydride
(0.04 mmol)
and
ethane-1,2-diol
(0.02 mmol) in dry dichloromethane were mixed homogeneously
under nitrogen atmosphere. The reaction mixture was thermo-
stated at 0 °C and added 0.06 mmol of triethylamine (TEA). The
whole reaction mass was allowed to continue with same condition
for three hours with constant stirring. The thermostat was re-
moved and continued with constant stirring for overnight at room
temperature. After all the anhydride was reacted, the solvent was
evaporated. The residue was cooled again at 0 °C and added
200 mL of saturated NaHCO3 solution in fractions and treated with
ether. The aqueous layer was cooled and acidified with dilute HCl
with constant stirring till the effervescence stops. A white precip-
itate obtained was filtered, washed by water thoroughly and dried.
Suitable single crystals were obtained in ethanol–water mixture in
a week time. Anal data: yield (85%). MS[ESI+] Calcd. for C18H14O8Na
(M+Na)+, 381.06, found: 381.12. Anal. Calcd. for C18H14O8: C, 60.33;
H, 3.93%, found: C, 60.65; H, 4.1%. 1H NMR (methanol-d4) 7.66 (m,
Ar, 2H), 7.62 (m, 2H), 7.58 (m, 4H) 4.60 (–CH2, 4H). 13C [methanol-
d4] NMR: 169.58 (C@O), 168.86 (C@O), 132.36, 132.01 (quat –C),
using Bruker XWIN-NMR software using standard ‘‘paropt” pulse
program. All 1H NMR spectra were calibrated with respect to
TMS and TMS was used as an internal reference for solvents such
as CDCl3 and CD3OD. SEM was performed on a LEO 1430VP. All sol-
vents were freshly purified by general distillation process [34] and
used as and when required.
2.1. Single crystal X-ray determination
In each case of compound 1, 2 and 3, a crystal of suitable size
was selected and mounted on the tip of a glass fiber and cemented
using epoxy resin. Summary of the crystallographic data for com-
pound 1, 2 and 3 are given in Table 1. Intensity data for both crys-
tals were collected using Mo-K (k = 0.71073 Å) radiation on a
a
Bruker SMART APEX diffractometer equipped with CCD area detec-
tor at 100 K. The data integration and reduction were processed
131.37, 131.05, 129.04, 128.53 (@CH), 63 (–CH2). IR spectra (m,
cmꢀ1). 3468, 2970, 1723, 1694, 1419, 1315, 1290, 1127.
Table 1
Summary of crystallographic data for compound 1, 2 and 3
2.2.2. cis-2-[({4-[(2-Carboxybenzoyl)oxy]but-2-en-1-yl}oxy)
(hydroxy)methyl] benzoic acid (2)
Identification Code
1
2
3
Chemical formula
Formula weight
Crystal color
Temperature (K)
Crystal system
Space group
a (Å)
C18H14O8
358.29
Colorless
293(2)
Monoclinic
P21/c
16.919(3)
4.9581(9)
20.580(4)
90
108.764(3)
90
4
1634.6(5)
1.456
0.116
C20H16O8
384.33
Colorless
293(2)
Monoclinic
P21/c
15.634(6)
7.292(2)
21.730(6)
90
133.220(17)
90
4
1805.3(10)
1.414
0.111
C20H18O8
386.34
Colorless
293(2)
Monoclinic
P21/c
7.4200(13)
10.2089(17)
11.947(2)
90
93.371(3)
90
2
903.4(3)
1.420
0.111
The above-mentioned procedure was repeated except the addi-
tion cis-but-2-ene-1,4-diol (0.02 mol, 1.645 ml) in place of ethane-
1,2-diol for the synthesis of 1. Suitable single crystals were ob-
tained in ethanol–water mixture in a week time. Yield (68%). MS[E-
SI+] Calcd. for C20H16O8Na (M+Na)+ 407, found: 407.02; Anal. Calcd.
for C20H16O8: C, 62.50; H, 4.19%, found: C, 62.80; H, 4.30%. 1H NMR
(methanol-d4): 7.65–7.64 (m, ArH, 2H), 7.60–7.56 (m, ArH, 6H)
5.94, 5.92, 5.90 (t, @CH, 2H, J = 4 Hz) 4.98, 4.96, 4.94 (d, –CH2).
13C NMR (methanol-d4): 170.35 (C@O), 169.68 (C@O), 134.03,
133.28 (quat –C), 132.48, 132.08, 130.26, 129.15 (@CH), 129.58
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Z
(quat –C), 62.31 (–OCH2). IR spectra (
1687, 1416, 1343, 1281, 1254, 1126.
m
, cmꢀ1). 3480, 2954, 1735,
V (3)
Density (mg/m3)
Absorption coeff. (mm-1
F(000)
)
744
7112
3230
0.0480
284
800
8741
3184
0.0695
309
404
4373
1576
0.0528
309
2.2.3. 2-({4-[(2-Carboxybenzoyl)oxy]butoxy}carbonyl)benzoic acid (3)
The above-mentioned procedure for the synthesis of 1, was re-
peated except the addition 1,4-butane-diol in place of ethane-1,2-
diol. Suitable single crystals were obtained in ethanol–water mix-
ture in a week time. Yield (69%). Anal. data. MS[ESI+]: Calcd. for
C20H18O8Na (M+Na+): 409, found: 409.28. Anal. Calcd. for
C20H18O8: C, 62.17; H, 4.70%, found: C, 62.21; H, 4.62%. 1H NMR
(CDCl3) d: 7.5 (b, 2H, OH), 7.536–7.822 (m, 8H, Ar-H), 1.879–
Reflections collected
Independent reflections
R(int)
Number of parameters
(Goodness of fit) on F2
Final R1
0.814
0.917
1.181
0.0575
0.1174
0.1182/0.1383
0.0539
0.1030
0.1311/0.1337
0.0661
0.1316
0.0897/0.1398
wR2(I > 2
r(I))
Weighted R1, wR2 (all data)

74
P. Mosae Selvakumar et al. / Journal of Molecular Structure 919 (2009) 72–78
1.896 (m, 4H, –CH2), 4.347–4.370 (m, 4H, –CH2); 13C NMR. (CDCl3)
d: 174.49 (C@O), 168.40 (C@O), 128.77 (quat –C), 129.33 (quat –C),
129.70 (@CH), 130.94 (@CH), 131.37 (@CH), 131.83 (@CH), 132.76
O2. . .O7 = 2.624(3) Å; hO2–H2. . .O7 = 168.11° and O8–H8. . .O1:
H8. . .O1 = 1.83 Å; O8. . .O1 = 2.639(3) Å; hO8–H8. . .O1 = 167°.]
The packing structure as shown in Fig. 2c, further indicates that
the alternate (P) and (M) helices are packed with their adjacent
helical strands in c-axis through a strong C–H. . .O interaction
[C15–H15. . .O3: H15. . .O3 = 2.48(3) Å, C15. . .O3 = 3.328(5) Å and
hC15–H15. . .O3 = 168(3)°] between the phenyl hydrogen H15 and
the ester carbonyl oxygen O3. Thus, the vectorial representation
of the C–H...O interaction shown in the packing diagram, and the
O–H. . .O mediated helices running down in b-axis combinedly cre-
ates a two-dimensional H-bonding network in ac-plane.
ORTEP diagram of the compound 2 with atom numbering
scheme is depicted in Fig. 3. Unlike compound 1, the double bond
introduced at the central carbon atom C10–C11, restricts the flex-
ibility at the spacer moiety in 2. The terminal phenyl rings are
tilted by 42.39° with respect to each other. The terminal carboxylic
groups C1–O1–O2 and C20–O7–O8 attached to the phenyl rings
C2–C7 and C14–C19 are twisted by 24.71° and 43.16°, respectively.
Similar to compound 1, the spacer segment O4–C9–C10–C11 and
O5–C12–C11–C10 with its respective torsion angles 120.94° and
ꢀ144.46° also indicates a twist. The H1 and H8 of the end carbox-
ylic acid is involved in O–H. . .O H-bond, with O7 and O2 of the
neighboring molecules and generates right and left handed helical
strands (Fig. 4a). [Details of this H-bonding interactions is O1–
H1. . .O7: H1. . .O7 = 1.80 Å; O1. . .O7 = 2.615(3) Å; and hO1–H1. . .
O7 = 170°; and O8–H8. . .O2: H8. . .O2 = 1.84 Å, O2. . .O8 =
2.636(4) Å and hO8–H8. . .O2 = 165°.] Thus the orientation and flex-
ibility of the spacer segment combinedly generates a helical
strands with pitch distance 7.29 Å extending down in b-axis via
intermolecular O–H. . .O hydrogen bond.
Packing diagram for compound 2 is depicted in Fig. 4c. The phe-
nyl hydrogen H6 and the middle carbonyl oxygen O3 of the adja-
cent helix (Fig. 4c) involves in H-bonding network along a-axis.
[Details of the C–H. . .O interaction: C6-H6. . .O3; H6. . .O3 =
2.52(4) Å; C6. . .O3 = 3.288(6) Å and hC6–H6. . .O3 = 138(4)°.] Simi-
larly, the weak short contact between the phenyl hydrogen H3
and O7 of the spacer C@O is running through C3–H3. . .O7;
H3. . .O7 = 2.71 Å; hC3–H3. . .O7 = 147.86(4)°, generates an another
H-bonded network, which connects the helices in c-axis. Thus
the O–H. . .O interaction promoting the formation of helical strand
in b-axis, the weak C–H. . .O contact, propagates a two-dimensional
supramolecular packing both in a-and c-axis (Fig. 4c), while helices
running down in b-axis as shown in Fig. 4b. The C–H. . .O bond an-
gle and distance are in the range reported by Desiraju [40] and Tay-
(@CH), 65.56 (–OCH2), 24.91 (–CH2). IR spectra (m
, cmꢀ1). 3438,
3236, 2644, 1957, 1757, 1732, 1598, 1489, 1451, 1392, 1247,
1130, 1074, 1039.
3. Results and discussion
Following the synthetic strategy shown in Scheme 1, compound
1–3 were synthesized upon desymmetrizing phthalic anhydride
with appropriate diols such as ethane-1,2-diol, cis-but-2-ene-1,4-
diol and 1,4-butane diol. The bola-shaped diester-dicarboxylic
compounds were characterized using various spectroscopic tech-
niques such as NMR, IR and MS. The single crystal X-ray structures
for 1, 2 and 3 were determined. All these compounds possess two
terminal phthalyl head groups and the diol spacers in common.
The two phthalyl moieties possessing dicarboxylic acid at its ter-
minal positions, they are bridged covalently through saturated
and unsaturated diol spacers, such as ethane-1,2-diol, cis-but-2-
ene-1,4-diol and 1,4-butane diol in compound 1–3, respectively.
ORTEP diagram with atom numbering scheme for compound 1
is depicted in Fig. 1. The presence of carboxylic acid group in the
ortho position to the phenyl ring and the flexibility at the central
glycolic chain, combinedly play significant role on the construction
of the hydrogen bonded helical chain. The terminal phenyl rings
(C2–C7 and C12–C17) show a slight twist by 8.64° from planarity.
While the carboxyl group (C1–O1–O2) with respective phenyl ring
(C2–C7), remains almost in the same plane (angle between the
phenyl ring and carboxyl plane is 4.83°), the other carboxyl group
(C18–O7–O8) is twisted by 23.11° with respect to the phenyl ring
C12–C17. The glycolic spacer moiety (O4–C9–C10–O5) in 1, bridg-
ing the ester carboxylic groups, also shows a similar twist with tor-
sion angle 65.28°. The opposite orientation of the end carboxylic
acid and the twist in the flexible aliphatic spacer segment, combin-
edly facilitates to interconnect the adjacent molecules and form
single stranded helical superstructure with pitch distance 4.958 Å
through its strong intermolecular carboxylic O–H...O hydrogen
bond (Fig. 2b). By virtue of the length and flexibility of the central
aliphatic spacer moiety, the terminal carboxylic acid constructs an
one-dimensional helical strand running along b-axis through its O–
H. . .O hydrogen bond and generates both left (M) and right handed
(P) helices as shown in Fig. 2a. [The pertinent O–H. . .O H-bond in-
volved in the helical assembly is O2–H2. . .O7: H2. . .O7 = 1.67 Å;
Scheme 1. Compound 1–3 with their functional role.

P. Mosae Selvakumar et al. / Journal of Molecular Structure 919 (2009) 72–78
75
Fig. 1. ORTEP diagram of 1 with atom numbering scheme.
Fig. 2. (a) Right (P) and left (M) handed helical motifs present in the unit cell of 1 via strong O–H. . .O interaction between the carboxyl groups (side view). (b) Single helical
strand of 1 running down b-axis through O–H. . .O interaction (red balls represents oxygen atoms and blue lines represents O–H. . .O H-bonding). (c) Packing diagram showing
the C–H. . .O interaction along c-axis, which bridge the (P) and (M) helices. (For interpretation of the references to color in this figure legend, the reader is referred to the web
version of this paper.)
Fig. 3. ORTEP diagram of 2 with atom numbering scheme.
lor and Kennard [41]. Similar to compound 1, the (M) and (P) heli-
ces in 2 are packed alternatively as shown in Fig. 4c. The propaga-
tion of helical assembly mediated through O–H. . .O interaction of
carboxylic acids being common for compound 1 and 2, the C–
H. . .O interaction mediated through C@O group of the ester unit,
involves in strong inter-helical interaction with the neighboring
helices and defines the dimensionality of the molecular packing
as shown in the Figs. 2c and 4c, respectively.
minal phenyl rings in the present compound projecting parallel,
they are oriented opposite to each other. The phenyl ring and the
ester spacer moiety possess a twist about ꢀ55.90°. The spacer moi-
ety C8O4C9C10 exists with torsion angle ꢀ157.55°. Similar to pre-
vious compounds, the terminal carboxylate oxygen O1 and O2
mediates a strong O–H. . .O H-bond, with O2 and O1 of the next
neighboring molecule from either end and forms COOH–COOH
mediated single stranded helices with pitch distance 11.947 Å.
[Details of this H-bonding interactions is O2–H2. . .O1: H2. . .O1 =
1.701 Å; O2–H2. . .O1 = 2.629(3) Å; and hO2–H2. . .O1 = 172°.] The
increase in the number of carbon and the flexibility on the spacer
ORTEP diagram with atom numbering scheme for compound 3
is given in Fig 5. Compound 2 and 3 possessing four carbons, they
differ by their rigid alkenyl and flexible alkyl spacer. Both the ter-

76
P. Mosae Selvakumar et al. / Journal of Molecular Structure 919 (2009) 72–78
Fig. 4. (a) Right (P) and left handed (M) helical motifs present in the unit cell of 2 via strong O–H. . .O interaction between the carboxyl groups (side view). (b) Top view of
helical strand via carboxylic O–H. . .O hydrogen bond viewed down b-axis (red balls represents oxygen atoms and Hydrogen bonding interactions by blue dotted lines). (c)
Packing diagram of the compound 2 showing the (P) and (M) helical strands bridged via C–H. . .O interactions both in c- and a-axis are indicated by dotted lines. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper.)
Fig. 5. ORTEP diagram of 3 with atom numbering scheme.
moiety, increases the pitch distance of the helical strands in the or-
der 1 < 2 < 3.
in few Angstrom. Keeping this huge difference of the hollowness
from molecule to material level, it is considered as essential to
understand the mechanism through which the supramolecular
self-assembly takes place.
The helical architecture obtained from single crystal X-ray
structure and the flower like crystal growth pattern (Supplemen-
tary Fig. S1) inspired us to investigate their morphology using
SEM. Accordingly an attempt to grow crystal in various solvents
was carried out. All these attempts have resulted the compound
1–3 in semisolid state except ethanol:water mixture. A simple
room temperature evaporation of compound 1 dissolved in 1:1
water ethanol mixture of volume ranging from 50 to 70 mL with
respective concentration 5–3.6 mM provided us microcrystalline
samples after 60 h. The SEM micrograph recorded for these sam-
ples collected from 50 to 70 mL batches are depicted in Fig. 6a
and b, respectively. Interestingly the SEM micrograph obtained
for compound 1, shown in Fig. 6a and b indicates the formation
of tubular texture. Thus while the SEM images of compound 1
showing tubular texture, a similar condition adopted to compound
2 and 3 failed to produce tube but forms bar type microcrystals
(Supplementary Fig. S2).
With an understanding from the crystal structure and its pack-
ing diagram influenced by the weak C–H. . .O and O–H. . .O interac-
tion, an attempt has been made to derive a plausible growth
mechanism to understand the morphological difference obtained
by SEM image. Accordingly, we have proposed two different mech-
anism for the formation of tube (Scheme 2a) and bar (Scheme 2b).
Both compound 1 and 2 possess (P) and (M) helices in adjacent po-
sition and constructs an almost similar packing pattern, as shown
in Fig. 2c and Fig. 4c. However they vary in their vectorial propaga-
tion of the C–H. . .O interaction as indicated in the dotted line. Thus
the alternate (P) and (M) helices assembles as layer in PMPM, etc.
manner through their inter-helical C–H. . .O interaction running
through c-axis. The compound 1 giving hollow tubes in nanometer
range suggest that the alternative layers PMPM, etc. rolls, one in-
side the other as indicated in the Scheme 2a, by making use of
the C–H. . .O interaction.
Interestingly, the diameter of the hollow tube for 1 measured
through SEM being nanometer range (200–700 nm), the corre-
sponding helices show the hollowness in the molecular structure
A similar C–H. . .O interaction in compound 2 spreading both in
c- and a-axis ties all the adjacent helices. Consequently, every (P)

P. Mosae Selvakumar et al. / Journal of Molecular Structure 919 (2009) 72–78
77
Fig. 6. SEM images of compound 1 in 1:1 ethanol:water mixture showing the tubes at (a) 50 mL. (b) 70 mL.
actions are combinedly plays significant role in bringing the
morphology.
a
b
M
P
P
P
P
MM
M
P
P
P
P
M
M
M
P
M
P
M
P
P
M
M
M
P
P
M
M
P
P
Acknowledgement
M
M
P
M
P
P
P
M
P
P
P
M
P
P
P
P
M
P
P
We are grateful to Department of Science and Technology New
Delhi (India), who supports this work, vide Project Number.SR/S1/
IC-19/2005. We acknowledge Mr. C.K. Chandrakanth for recording
the SEM.
M
P
M
M
M
P
P
P
P
P
M
M
M
M
P
P
P
M
M
M
P
M
P
P
P
M
P
P
M
P
P
M
P
P
M
Appendix A. Supplementary data
Scheme 2. Possible mechanism for the formation of tube and bar through H–bond.
The blue line represents the H-bonds and (P) and (M) indicates the right and left
handed helical strands. Growth mechanism for the formation of (a) tube and (b) bar.
(For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this paper.)
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2008.08.016.
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helix is tied with four (M) helix and vice versa through their C–
H. . .O interaction and forms two-dimensional packing network
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strict the molecules from folding and results into bars. Thus the
direction and strength of the C–H. . .O interaction observed from
the crystal structure dictates, the nucleation process during crys-
tallization, which ultimately defines the morphology as microtube
for compound 1. The increase in wall thickness from 400 nm to
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