Hydrogen Bonded non-covalent synthesis by reaction of porphyrin appended calix[4]arene with 5,5-diethylbarbituric acid in solution

Article abstract of DOI:10.1007/s10847-012-0276-8

Calix[4]arene, diametrically substituted at the upper rim, with two porphyrin appended melamine units spontaneously form well-defined double rosette in the presence of 5,5-diethylbarbituric acid. This assembly consists of nine components which utilize 36 hydrogen bonds and are quite stable in apolar solvents of up to 10^-4 M concentration. The formation of assembly between porphyrin appended calix[4]arene and diethylbarbiturate has been broadly studied by UV-visible, fluorescence and ¹H NMR spectroscopic techniques. The stoichiometries of proposed aggregates have been analyzed by facile titration of calixarene and barbital. Furthermore, the MALDI-TOF mass measurement is found fully compatible with the observed stoichiometries. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10847-012-0276-8

J Incl Phenom Macrocycl Chem
DOI 10.1007/s10847-012-0276-8
ORIGINAL ARTICLE
Hydrogen Bonded non-covalent synthesis by reaction of porphyrin
appended calix[4]arene with 5,5-diethylbarbituric acid in solution
•
Tanuja Bisht Bhaskar Garg
•
Shive Murat Singh Chauhan
Received: 25 August 2012 / Accepted: 11 December 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract Calix[4]arene, diametrically substituted at the
upper rim, with two porphyrin appended melamine units
spontaneously form well-defined double rosette in the
presence of 5,5-diethylbarbituric acid. This assembly con-
sists of nine components which utilize 36 hydrogen bonds
Introduction
The non-covalent synthesis is a dominant synthetic method
for the preparation of complex molecular structures to
understand the molecular mechanism of multifarious biolog-
ical reactions in nature and development of newer materials
[1, 2]. Multi-hydrogen bond mediated synthesis has been used
to understand the photosynthesis, secondary and tertiary
structures of biomolecules and formation of newer materials
in physical sciences. In particular, hydrogen bonds are con-
sidered to be useful for controlling molecular self-assembly
due to the reversibility, specificity, directionality, and coop-
erative strength of this class of interactions [3, 4]. For the last
two decades, the supramolecular chemistry has utilized the
advantages of self-assembly to control the synthesis of non-
covalent structures in solutions [5–7].
and are quite stable in apolar solvents of up to 10-4
M
concentration. The formation of assembly between por-
phyrin appended calix[4]arene and diethylbarbiturate has
1
been broadly studied by UV–visible, fluorescence and H
NMR spectroscopic techniques. The stoichiometries of
proposed aggregates have been analyzed by facile titration
of calixarene and barbital. Furthermore, the MALDI-TOF
mass measurement is found fully compatible with the
observed stoichiometries.
Keywords Non-covalent interaction ꢀ Self-assembly ꢀ
Porphyrin appended calix[4]arene ꢀ
5,5-diethylbarbituric acid
The supramolecular organization of functional porphy-
rin is involved in many important biological systems. The
self-assembly of porphyrin derivatives has garnered con-
siderable attention because of their photoelectric proper-
ties, as components of nano scale photonic devices and as
novel functional materials [8]. Much effort has been
applied to the design and synthesis of covalent porphyrinic
arrays, analogous to those found in nature for charge sep-
aration, electron transport [9, 10], and signal transduction
[11–13]. Multi-hydrogen bonds between complementary
molecular components are widely used in the fabrication of
supramolecular assemblies such as, a linear strand (ribbon
or tape) [14–16] and crinkled tape or rosette [17]. The
functional porphyrin arrays have been primarily formed by
hydrogen bonding [18–21], axial metallo-porphyrin coor-
dination [22–24] and coordination of exocyclic ligands
[25–27]. Nevertheless, the other non-covalent interactions,
in particular, p–p interaction contribute significantly in the
formation of supramolecular porphyrinic structures.
T. Bisht ꢀ B. Garg ꢀ S. M. S. Chauhan (&)
Bioorganic Research Laboratory, Department of Chemistry,
University of Delhi, Delhi 110007, India
e-mail: smschauhan@chemistry.du.ac.in
Present Address:
T. Bisht
Department of Chemistry, National Taiwan University,
Taiwan, Republic of China
Present Address:
B. Garg
Department of Chemistry, National Tsing Hua University,
Taiwan, Republic of China
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J Incl Phenom Macrocycl Chem
Over the years, calixarenes have been widely used as
efficient building blocks in supramolecular chemistry to
construct various three-dimensional assembly structures
because of their bowl shaped unique constitution and easy
functionalization at lower and upper rims [28–30]. A
variety of box- or capsules like arrangements with calixa-
renes were constructed using neutral hydrogen bonds in
polar solvent systems [31–33]. In this milieu, it should be
pointed out here that the structural diversity at supramo-
lecular level was generated just simply by mixing the
various components under thermodynamically controlled
conditions.
[4-amino-6-[5-(40-aminophenyl)porphinatozinc(II)]-1,3,5-
triazin-2-yl]diamino-25,26,27,28-tetrakis(propyloxy)calix
[4]arene. Further, the formation of hetero-composite self-
assembly (HCSA) of synthesized derivative with barbital
1
has been studied using UV–visible, fluorescence, H NMR
and MALDI-MS techniques in solution. The supramolec-
ular assembly is fashioned due to the development of
complementary hydrogen bonds between the donor-
acceptor-donor (DAD) arrays of calix[4]arene dimelamine
derivative and the acceptor-donor-acceptor (ADA) array of
barbituric acid building block (Scheme 1).
The rosette formation by three-point intermolecular
hydrogen-bonding interactions between substituted tria-
zines and barbituric acid derivatives has been reported
previously by us [34]. Inspired by the earlier report by
Reindhoudt et al. [35] together with our high interest in
self-assembly [36, 37] and non-covalent interactions [38–
41], herein we report the synthesis of 5,17-N,N’-Bis
Experimental
Materials and methods
All the reagents and chemicals for synthesis were obtained
commercially and suitably purified when required. Solvents
Scheme 1 Schematic
presentation of hetero-
composite self-assembly
(HCSA)
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J Incl Phenom Macrocycl Chem
used were dried and distilled before use. The solid compounds
were dried under vacuum in the presence of P2O5. Column
chromatography was carried out using silica gel (60–120 mesh
and 230–400 mesh) of Spectrochem. The 1H (300 MHz) and
13C NMR (75 MHz) were recorded on Bruker Avance-300
spectrometer using tetramethylsilane (TMS) as internal stan-
dard. The chemical shifts (d ppm) are referenced to the
respective solvents and splitting patterns are designed as
s(singlet), d(doublet), m(multiplet), br(broad) and bs(broad
singlet). The UV–visible spectra were recorded on a Perkin-
Elmer (Lambda 35) spectrophotometer with a quartz cuvette
(path length =1 cm) at 298.2 0.1 K and the absorption
maxima (kmax) are expressed in nanometers. The FTIR spectra
were recorded on Perkin-Elmer spectrum FT-2000 spectrom-
eter and mmax are expressed in cm-1. Matrix assisted laser
desorption ionization time of flight mass spectrometry
(MALDI-TOF-MS) was performed on a Bruker Ultraflex TOF
mass spectrometer equipped with a 337 nm UV nitrogen laser.
The matrix used in MALDI-TOF-MS was 2,5-dihydroxyben-
zoic acid (DHB). The ESI-MS were recorded on LC-TOF (KC-
455) mass spectrometer of Waters. Fluorescence spectra were
recorded on Shimadzu RF-5301 spectrophotometer.
Later, 1 lL of the resulting mixture was spotted on the
MALDI-TOF plate and spectrum was recorded.
Synthesis
The 5,5-diethylbarbituric acid, 2 was synthesized as described
previously [42]. The reaction of cyanuric chloride with
ammonia gave 2-amino-4,6-dichloro-1,3,5-triazine which on
reaction with 5-(40-aminophenyl)-10,15,20-triphenylpor-
phyrinatozinc(II) in the presence of DBU in 1,4-dioxane
resulted in the formation of 2-amino-4-chloro-6-[5-(40-ami-
nophenyl)-10,15,20-triphenylporphyrinatozinc(II)]-1,3,5-tri-
azine, 3 along with a dimer in appreciable quantity. Further,
the reaction of 3 with 5,17-Diamino-25,26,27,28-tetra-
propoxycalix[4]arene6 inthepresenceofDBUin1,4-dioxane
resulted in 1 in appreciable yield (Scheme 2).
2-amino-4,-Chloro-6-[5-(40-
aminophenyl)porphyrinatozinc(II)]-1,3,5-triazine (3)
In a 100 mL round bottomed flask, equipped with an efficient
condenser were taken 2-amino-4,6-dichloro-1,3,5-triazine, 4
(13 mg, 0.08 mmol) and 5-(40-aminophenyl)porphyrinato-
zinc(II), 5 (80 mg, 0.17 mmol) in dry 1,4-dioxane (30 mL) and
thoroughly mixed. To this solution, DBU (30 lL, 0.20 mmol)
was added with vigorous stirring and reaction mixture was
allowed to reflux at 110 °C for 6 h under nitrogen. After
cooling, distilled water (150 mL) was poured into the reaction
mixture. The precipitate was filtered on suction pump, washed
with water (3 9 30 mL) and air dried. The crude was column
chromatographed on silica gel (60–120 mesh). The elution of
column with chloroform: petroleum ether (1:4, v/v) afforded
monomer 3 and dimer in appreciable yields. 32 mg, 42 %;
UV–Visible (kmax, CHCl3, log e): 416 (2.9), 545 (1.56), 582
(0.084); IR mmax, (Nujol)/cm-1: 3342, 3076, 2854, 1598, 1512,
1487, 1440, 1344, 1296, 1200, 1172, 1104, 1073, 989, 862, 843,
792, 747, 715, 702, 654; 1H NMR (300 MHz, CDCl3, 298 K):
d = 8.96 (s, 4H, b-pyrrole), 8.80 (d, J = 5.2 Hz, 2H, b-pyr-
role), 8.67 (d, J = 5.3 Hz, 2H, b-pyrrole), 8.50 (d, J = 8.1 Hz,
2H, phenyl), 8.22 (d, J = 8.1 Hz, 2H, phenyl), 8.10 (d,
J = 8.1 Hz, 6H, phenyl), 7.87–7.95 (m, 9H, phenyl), 7.78 (br,
1H, NH), 4.36 (s, 2H, NH2); ESI–MS (m/z, positive ion):
842.2009 [M (Cl35)?Na]? (100 %), 844.5614 [M(Cl37)?Na]?
(33 %).
General methods to study the rosette formation
between compounds 1 and 2
All experiments were carried out at 298.2 0.1 K, unless
otherwise mentioned. The stock solutions of 5,17-N,N’-
Bis[4-amino-6-[5-(40-aminophenyl)
porphinatozinc(II)]-
1,3,5-triazin-2-yl]diamino-25,26,27,28-tetrakis(propyloxy)
calix[4] arene, 1 (2.2 mg, 1.0 lmol) and 5,5-diethylbarbi-
turic acid, 2 (1.84 mg, 10 lmol) were prepared by dis-
solving in CHCl3 (5 mL and 50 mL) in different
volumetric flasks. The 500 lL solution of 1 in chloroform
(2.0 mL) was placed in a quartz cell and titrated with
increasing amount of 2. After each addition of 100 lL
solution of 2, UV–visible spectra was recorded and the
changes in absorbance and shift in wavelength were mea-
sured. A similar procedure was employed for the fluores-
cent spectroscopic titrations.
1
In the H NMR experiment, the stock solutions were
prepared by dissolving 1 (8.8 mg, 4 lmol) and 2 (3.68 mg,
20 lmol) in 1 and 5 mL of CDCl3 respectively. The
0.5 mL solutions of 1 and 2 were transferred to the separate
NMR tubes and the spectra were recorded. Further, the
CDCl3 solution of 1 (200 lL) and 2 (400 lL) were mixed
Dimer: 48 mg, 50 %; UV-Visible (kmax, CHCl3, log e):
419 (4.42), 548 (3.31), 587 (2.32); H NMR (300 MHz,
1
1
together and H NMR spectra were recorded. The changes
in each resonance were carefully measured.
CDCl3, 298 K): d = 8.98 (d, J = 5 Hz, 4H, b-pyrrole), 8.96
(s, 8H, b-pyrrole), 8.84 (d, J = 5 Hz, 4H, b-pyrrole), 8.64
(d, J = 8 Hz, 4H, phenyl), 8.41 (d, J = 8 Hz, 4H, phenyl),
8.22 (m, 12H, o-phenyl), 7.77 (m, 18H, m- and p-phenyl),
7.75 (s, broad 2H, -NH-), 4.21 (s, 2H, amino).; ESI-MS (m/z,
In MALDI-TOF-MS experiment, the stock solutions
were prepared by dissolving 1 (2.2 mg, 1 lmol) and 2
(1.84 mg, 10 lmol) in CHCl3 (1 and 10 mL respectively).
Solutions of 1 (100 lL) and 2 (200 lL) were taken from
stock solutions and mixed thoroughly with CH3CN (1 mL).
?
positive ion):1501.3534 [M Na]?
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J Incl Phenom Macrocycl Chem
Scheme 2 Synthesis of
bisporphyrin-melamine
appended calix[4]arene 1
N
N
N
Cl
N
Cl
Zn
H2N
N
+
N
N
NH2
4
5
1,4-dioxane, DBU
Reflux
NH2
H2N
N
n
Z
N
N
+
O
O
O
O
N
Cl
N
NH
N
N
3
6
NH2
1,4-dioxane, DBU
Reflux
N
N
Zn
N
N
N
N
Zn
N
N
HN
NH2
N
NH
N
H2N
N
N
N
N
HN
NH
O
O
O
O
1
123

J Incl Phenom Macrocycl Chem
Synthesis of 5,17-N,N’-Bis[4-amino-6-[5-(40-aminophenyl)
porphinatozinc(II)]-1,3,5-triazin-2-yl]diamino-25,26,27,
28-tetrakis(propyloxy)calix[4]arene (1)
distereotropic protons of methylene bridge appeared as two
doublets resonating at d 4.5 and 3.1 ppm, indicative of the
coneconformation ofcalix[4]arene. Theremaining signalsfor
sixteen b-pyrrolic protons and the propyl chain appeared as
usual and were assigned with ease. The ESI mass spectrum of
1 gave singly charged, mono-sodiated peak at 2230.7651 Da
for [M?Na]?.
A solution of 2-amino-4,-Chloro-6-[5-(40-aminophenyl)por-
phinatozinc(II)]-1,3,5-triazine, 3 (163 mg, 0.2 mmol) and
5,17-diamino-25,26,27,28-tetrakis(propyloxy)calix[4]arene,
6 (63 mg, 0.1 mmol) in THF (50 mL) was refluxedfor 16 h in
the presence of DBU (2 equiv). After completion of reaction,
the mixture was evaporated to dryness. The residue so
obtained was dissolved in CH2Cl2 (50 mL), washed with H2O
(3 9 25 mL) followed by brine (25 mL) and dried over
anhydrous Na2SO4. The compound was purified by column
chromatography on silica gel (60-120 mesh). Elution of col-
umn with chloroform: methanol (95:5, v/v) gave porphyrin
appended calix[4]arene, 1 as purple solid. Yield: 136 mg,
62 %; UV–Visible (kmax, CHCl3, log e): 421 (1.24), 542
UV–visible and Fluorescence spectroscopy
The assembly pattern of porphyrin appended calix[4]arene, 1
with barbital, 2 was initially studied by UV–Visible spec-
troscopy, the most appropriate technique to study the por-
phyrin-based systems. The UV–visible spectrum of 1 exhibits
a characteristic Soret band, corresponding to the porphyrin B
transition, at 421 nm. The gradual addition of 2 (2 9 10-5 to
2 9 10-4 M) to the solution of 1 (*2 9 10-4 M) resulted in
pronounced UV–visible spectral changes. Specifically, upon
successive addition of aliquots of 2 to the solution of 1, the
Soret band at 421 nm progressively decreased and experi-
enced a bathochromic shift by 2–4 nm together with consis-
tent broadening. The analogous bathochromic shift
(*3–4 nm) and broadening was also recognized in visible
bands, corresponding to the Q transitions, of the porphyrins.
At the end point of titration, the soret band get splitted and
1
(0.86), 596 (0.62); H NMR (300 MHz, CDCl3, 298 K):
d = 9.38 (s, 8H, b-pyrrole), 8.81 (d, J = 4.8 Hz, 2H, b-pyr-
role), 8.69 (d, J = 4.8 Hz, 4H, b-pyrrole), 8.51 (d,
J = 7.8 Hz, 4H, phenyl), 8.23 (d, J = 7.8 Hz, 4H, phenyl),
8.16(d, J = 8.1 Hz, 12H,phenyl), 7.92(m, 18H, phenyl),7.80
(s, 2H, NH), 7.75(s, 2H, NH), 7.62(s, 4H, phenylcalix), 7.10(d,
J = 8.0 Hz, 4H, phenylcalix), 6.89 (t, 2H, phenylcalix), 4.40 (s,
2H, NH2), 4.42(d, 4H, ArCH2Ar), 3.81(t, 8H, OCH2), 3.11(d,
4H, ArCH2Ar), 2.15(m, 8H, CH2), 0.98 (t, 12H, CH3); ESI-
MS (m/z, positive ion): 2230.7658 [M?Na]?.
Results and discussion
Spectral characterization
The structures of all synthesized derivatives (Scheme 2)
were unambiguously confirmed by different spectroscopic
techniques. The UV–visible spectrum of 3 exhibited a
Soret band at 416 nm and two Q bands at 545 and 582 nm.
1
The H NMR spectra of 3 showed three signals resonating
at d 8.80 (d, 2H), 8.68 (d, 2H) and 8.95 ppm (s, 4H) which
were assigned to the eight b-pyrrolic protons. Two doublets
at d 8.53 and 8.22 ppm, integrated to two protons each,
were assigned to the protons on the phenyl ring attached to
the triazine unit. In addition, two broad resonances at
d 7.78 and 4.23 ppm were assigned to the -NH and -NH2
protons respectively. The ESI-MS spectra of 3 showed a
sodiated peak at 842.2009 Da [10Cl35?Na]? along with an
isotopic peak at 844.5614 Da [10Cl37?Na]? in 3:1 ratio
which indicated the presence of the halogen atom.
The 1H NMR spectrum of 1 showed two broad resonances
at d 7.70 and 7.80 ppm for different NHs while a signal
at d 4.40 ppm was assigned due to free NH2 protons. Inter-
estingly, the phenyl protons of calixarene scaffold appeared
upfield relative to other phenyl protons. Furthermore, the
Fig. 1 Study of rosette formation by electronic transition spectra;
A1 = 1 (2 9 10-4 M); A2 = 1 (2 9 10-4 M) and 2 (0.2 9 10-4 M);
A3 = 1 (1.98 9 10-4 M) and 2 (0.39 9 10-4 M); A4 = 1 (1.97 9
10-4 M) and 2 (0.59 9 10-4 M); A5 = 1 (1.97 9 10-4 M) and 2
(0.78 9 10-4 M); A6 = 1 (1.96 9 10-4 M) and 2 (0.98 9 10-4 M);
A7 = 1 (1.96 9 10-4 M) and 2 (1.17 9 10-4 M); A8 = 1 (1.95 9
10-4 M) and 2 (1.38 9 10-4 M); A9 = 1 (1.95 9 10-4 M) and 2
(1.57 9 10-4 M); A10 = 1 (1.94 9 10-4 M) and 2 (1.77 9 10-4 M);
A11 = 1 (1.94 9 10-4 M) and 2 (1.96 9 10-4 M) A12 = 1 (1.93 9
10-4 M) and 2 (2.16 9 10-4 M); A13 = 1 (1.93 9 10-4 M) and 2
(2.36 9 10-4 M)
123

J Incl Phenom Macrocycl Chem
broadened into two minor but observable peaks at 417 and
423 nm (Fig. 1), a ‘hallmark’ for self-assembly between two
components [43]. It is reasonable because the rotational con-
formations of 1 are restricted to an eclipsed state such that the
effective cross-section per porphyrin is significantly reduced.
The binding stoichiometry between two components was
determined by Job’s method and the assembly showed a 1:2
binding pattern.
Spectrofluorometry is another important tool for the
studies of aggregated porphyrins in solutions and accord-
ingly used in our studies as well. In particular, the fluo-
rescence spectrum of 1 (2 9 10-4 M) in CHCl3 showed a
characteristic fluorescent peak at 665 nm. Upon addition of
aliquots of 2 (2 9 10-5 M) to the solution of 1, the fluo-
rescence peak decreased with the concomitant formation of
new peak at 700 nm (Fig. 2). The significant bathocromic
Fig. 2 Fluorescence spectroscopic titrations of 1 (2 9 10-4 M) with
5,5-diethylbarbituric acid 2 (2 9 10-5 M) at 298 K
Fig. 3 1H NMR spectra of
a 5,5-diethylbarbturic acid 2,
b bisporphyrinmelamine
appended calix[4]arene 1 and
c after mixing 1:2 molar
solution of 1 and 2
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J Incl Phenom Macrocycl Chem
shifting (* 35 nm) as well as a 20 fold increase in the
intensity of new peak at the end point of the titration
clearly indicated the formation of super structure (the
proposed cyclic motifs) between porphyrin appended
calix[4]arene and barbiturates.
diagnostic for box-like assembly (double rosette) forma-
tion [45, 46]. Reasonably, it is due to unsymmetrical
substitution of the melamine units. In line with this, the
significant downfield shifting of NHs of 2 further indi-
cates the involvement of theses protons in intermolecular
hydrogen bonding (Fig. 3). A similar participation in
intermolecular hydrogen bonding was also recognized for
two NHs of 1 which experienced a downfield shifting
from d 7.62 and 7.75 ppm to 7.82 (f) and 8.72 (e) ppm
respectively.
1H NMR spectroscopy
1
The H NMR spectroscopy can provide a means of moni-
toring the homo and hetero-composite hydrogen bonded
self-assembly of variety of supramolecular architectures. In
this context, the doubly rosette formation with calixarene
The careful examination of spectrum of assembly 13ꢀ26
in CDCl3 clearly reveals that the assembly exists exclu-
sively as one single conformational isomer (the D3 isomer)
[47]. It is worth mentioning here that in their earlier
research work, Whitesides et al. presented a model rosette
motif which is formed by combining melamine, having
small substituents, and alkylated cyanuric acid. However,
this rosette motif was found quite unstable at the concen-
tration of less than 4.0 mM in chloroform solution [46].
Nevertheless, the our current studies clearly revealed that
the new rosettes, assembled from 1 and 2 are more stable
relative to the model system and dictate that the large
1
scaffold has been conveniently characterized by H NMR
spectroscopy [44]. Accordingly, CDCl3 solution of 2
(4 mM) was titrated against CDCl3 solution of 1 (2 mM) at
room temperature.
Upon the addition of two equivalents of 2 into the
CDCl3 solution of 1, two new signals originated at d 12.8
(a) and 13.2 (b) ppm which were easily assigned to the
hydrogen bonded NH protons of diethylbarbituric acid, 2,
in the proposed assembly. The appearance of these signals
1
at different chemical shifts in the H NMR spectra is the
Fig. 4 MALDI-MS spectrum of self-assembled architecture (HCSA)
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J Incl Phenom Macrocycl Chem
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matrix assisted laser desorption ionization–time of flight–mass
spectrometry (MALDI–TOF–MS) was used. In particular, the
MALDI–TOF mass spectra of 5,17-N,N’-Bis[4-amino-6-[5-
(40-aminophenyl)porphinatozinc(II)]-1,3,5-triazin-2-yl] dia-
mino-25,26, 27,28-tetrakis(propyloxy)calix[4]arene, 1 and
barbital, 2 (1:2) (3.0 9 10-3 mol L-1 in CHCl3 and diluted by
acetonitrile) gave doubly charged complexation peak. The
mass spectrum of self-assembled architecture exhibited a peak
at 7776.818 Da which was assigned to [13ꢀ26?2Na]2?. A
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Conclusions
In conclusion, doubly rosette formation (13ꢀ26) between three
moles of 5,17-N,N’-Bis[4-amino-6-[5-(40-aminophenyl)por-
phinatozinc(II)]-1,3,5-triazin-2-yl]diamino-25,26,27,28-tetra-
kis(propyloxy)calix[4]arene 1 and six moles of diethylbarbi-
turic acid 2 has been recognized in solution and adequately
characterized by UV–visible, fluorescence and MALDI-MS
spectroscopic techniques. The exact binding mode between
well-defined aggregates in solution could be determined by 1H
NMR spectroscopy with ease. The fact that no higher or lower
order oligomers were obtained in mass spectrum clearly sug-
gests that the predominant form under the experimental con-
ditions is indeed (3?6) species.
We have shown that just by simple switching of smaller
substituents from melamines or alkylated cyanuric acids to
larger and hindered one, for instance porphyrins, the for-
mation of super structures could also changed from tape to
cyclic one.
17. Zerkowski, J.A., Mathias, J.P., Whitesides, G.M.: New Varieties
of crystalline architecture produced by small changes in molec-
ular structure in tape complexes of melamines and barbiturates.
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Acknowledgments The authors are thankful to the Department of
Science and Technology (DST) for project number SR/S1/OC-54/
2003, Council of Scientific and Industrial Research (CSIR) and
University of Delhi, New Delhi for financial assistance.
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Self-assembly of monopyrazolylporphyrins by hydrogen bonding
in solution. Chem. Commun. 1759–1760 (1999)
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