Hierarchical organization of a robust porphyrin cage self-assembled by hydrogen bonds

Article abstract of DOI:10.1039/c1cc11553g

Porphyrins appended with four rigid hydrogen bonding motifs on the meso positions were synthesized and self-assembled into a cofacial cage with four complementary bis(decyl)melamine units in dry solvents. The hydrocarbon chains on the melamine mediate the formation of nanofilms on surfaces as the solvent slowly evaporates.

Full text of DOI:10.1039/c1cc11553g

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COMMUNICATION
Hierarchical organization of a robust porphyrin cage self-assembled by
hydrogen bondsw
Sunaina Singh,a Amit Aggarwal,a Christopher Farley,a Brian A. Hageman,a
James D. Batteasb and Charles Michael Drain*a
Received 17th March 2011, Accepted 3rd May 2011
DOI: 10.1039/c1cc11553g
Porphyrins appended with four rigid hydrogen bonding motifs on
the meso positions were synthesized and self-assembled into a
cofacial cage with four complementary bis(decyl)melamine units
in dry solvents. The hydrocarbon chains on the melamine
mediate the formation of nanofilms on surfaces as the solvent
slowly evaporates.
of two different macrocycles are used to form the cages
(Scheme 2), where interconversion of the atropisomers of the
uracylporphyrins is required. NMR, light scattering, and
photophysical properties in solution indicate formation of
the cages, and atomic force microscopy (AFM) elucidates
the self-organization of the materials cast onto mica.
Uracylporphyrin 1b and N-alkyluracyl porphyrin 2b are
synthesized from 5-formyl-6-methyluracil8 and 1-ethoxy-
methyl-5-formyl-6-methyluracil6,7,9 using Adler conditions
with Zn(OAc)2 (Scheme 1).w N-alkylation of the uracil inhibits
tautomerization at this position, diminishes unproductive
H-bond formation, and improves solubility. Thus, we focus
on these porphyrins herein. The free bases, 1a and 2a, are
formed by demetalation reactions. The bis(decyl)melamine
was prepared similarly to previous reports.10
Self-assembly and self-organization of porphyrinoids1
continue to produce functional materials with applications
ranging from electronics2 to catalysis.3 Both coordination
bonds and H-bonds can mediate formation of supramolecular
porphyrinic materials. Porphyrinoids appended with H-bond
motifs can assemble into diverse arrangements such as
rosettes, squares, tapes, and nanoparticles for materials for
solar energy harvesting, photonic devices, sensors, catalysts,
and understanding biological electron transport.4–6 In addition
to directing supramolecular architectures, the functional
groups on the macrocycle and mode of assembly can modulate
photophysical and chemical properties of chromophoric arrays.
Responsive H-bond materials, whereby the supramolecular
structure can be modulated by environmental conditions such
as temperature or solvent, can find applications as catalytic
hosts for specific guests, or control of electronic communication
between subunits.7 While functional materials of porphyrins
self-assembled by hydrogen bonding have demonstrated utility,5
porphyrins bearing hydrogen bonding motifs rigidly attached
can be difficult to synthesize.4
For 2b the free energy barrier for atropisomer interconversion
was determined by a van’t Hoff plot (ln K versus 1/T) and the
DGz was found to be about 123 kJ molꢀ1 in CD3OD. This is
consistent with earlier results, which proposed the increased
rigidity of the macrocycle when complexed to a metal ion
raises tzhe rotational barrier by about 20 kJ/mol.11 Therefore,
the DG for 2a is estimated to be about 103 kJ molꢀ1. Similar
DGz values for atropisomerization of 1a and 1b are expected.
The porphyrins are poorly soluble in dry THF-d8 but slowly
become more soluble upon addition of the melamine and
heating between 40–50 1C for greater than four days for 1a
and 1b and two days for 2a and 2b. The formation of the cage
removes the a4 atropisomer from the equilibrating solution
Herein we report the synthesis of porphyrins possessing four
rigid uracylic hydrogen bonding units on the meso positions
(Scheme 1) and characterization of the self-assembly of these
subunits into robust cage structures mediated by complementary
2,4-di(n-decylamino)-6-amino-1,3,5-triazine, bis(decyl)melamine,
units. Two notable features are: the porphyrins are made in
one step, and the supramolecular dynamics are significantly
reduced because all four meso positions participate in the
assembly instead of two.4–6 The free base and zinc complexes
a Department of Chemistry & Biochemistry, Hunter College of
The City University of New York, 695 Park Avenue, New York,
NY 10065, USA. E-mail: cdrain@hunter.cuny.edu;
Scheme 1 (i) Reflux 10 h in 10% nitrobenzene in acetic acid with
0.05 M Zn(OAc)2. The porphyrin is prepared as a mixture of the four
atropisomers (a4, a3b, a2b2, abab) since the uracyl moieties are
nominally orthogonal to the macrocycle plane.
Fax: +1 212-772-5332; Tel: +1 212-650-3791
b Texas A & M University, College Station, TX 77842, USA
w Electronic supplementary information (ESI) available: Experimental,
DOSY NMR and AFM data. See DOI: 10.1039/c1cc11553g
c
7134 Chem. Commun., 2011, 47, 7134–7136
This journal is The Royal Society of Chemistry 2011

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Fig. 1 The five points on the curve are Dnorm of polystyrene
standards versus molecular weight, and the Dnorm of the two cages
to yield estimates of the molecular weights. (10 mM in THF-d8)
Scheme 2 Heating of two equivalents of one of the porphyrins in
the presence of four equivalents of the H-bond complimentary
bis(decyl)melamine (left) results in formation of the a4 atropisomer
and self-assembly of the cage in solution. The decyl groups mediate
formation of monolayer films of the cage on surfaces (right).
thereby increasing the yield of the cage.12 The self-assembled
cage is characterized by the NMR chemical shifts of the uracil
and melamine NH protons of the H-bonds (Table 1), and only
one resonance, instead of six, for the uracyl methyl group
indicating one atropisomer is present.1
Diffusion ordered spectroscopy (DOSY) allows the molecular
weights of the assemblies to be determined.6,7,13 For DOSY
experiments, the normalized diffusion coefficient (Dnorm) was
measured as the ratio of the observed value (Dobs) to Db-CD
,
where Db-CD is the diffusion coefficient for the internal
standard, heptakis(2,3,6-tri-O-methyl)-b-cyclodextrin (b-CD).
The molecular weight could be calibrated by Dnorm of a series
of polystyrene standards (Fig. 1). The value of Dnorm for
Por 1b + melamine was 0.64, which indicates a 1b cage with
a m of 3360 (calculated 3295), and Dnorm for Por 2b +
melamine was 0.60 which indicates the formation of a cage
with a m of 3820 (calculated 3717).
Fig. 2 After initially decreasing due to aggregation, the intensity of
the UV-visible and fluorescence spectra increase with time and heating
as the amount of the self-assembled 2b cage increases.
Dynamic light scattering data reveals the hydrodynamic
diameter of the initially formed aggregates of 2a, 2b, and for
2a–2b cages to be about 42 nm, and after heating the average
particle size is between 7–9 nm. There is a broad distribution
of sizes for the aggregates of 1a, 1b, and 1a–1b centered at
about 40 nm. The cages are found to be about 9–13 nm in
diameter. Assuming extended decyl groups, an estimation of
the cage dimensions from Chem 3Dr is about 5.2 nm from
terminal methyl to terminal methyl on opposite sides of the
cage and about 2 nm perpendicular to the porphyrin planes.
A 1 : 1 mixture of free base and Zn2+ macrocycles, cage
2a : 2b, was also examined. At equilibrium, if the intermole-
cular forces that mediate the assembly are equivalent, there
should be a 1 : 2 : 1 mixture of cages 2a : 2a, 2a : 2b, and 2b : 2b,
respectively. The UV-visible spectra for this mixture shows
similar trends as the above cages; the initial formation of
amorphous aggregates and the more soluble cage after heating
at 45–47 1C for a few days. The fluorescence spectra of cage
2a–2b evolve in a complex manner. Initially, in the aggregation
stage of the self-assembly process, some energy transfer from
the metalloporphyrins to the free bases is observed. The
solution may contain more aggregates of the Zn2+ complexes
because they are less soluble. After heating the solution, only a
small amount of energy transfer is observed, which is consistent
with separation of the porphyrins in the cage structure. Thus,
after about three days UV-visible, fluorescence, and NMR
indicate that the system has reached equilibrium.
The photophysical properties correlate with the NMR
results.14w UV-visible spectra of the 2a and 2b (Fig. 2) cages
(38 mM–42 mM in porphyrin) with bis(decyl)melamine in dry
THF were recorded over several days without heating. Heating
at 45–47 1C for B2 days causes an increase in the UV-visible
band intensities, indicating increased solubility and a shift in
the equilibrium towards formation of the a4 atropisomer and
the cages (Fig. 2). The emission spectra of the 2a and 2b cage
solutions, exciting in the Soret or Q bands, are in agreement
with the UV-visible results. An initial small decrease in the
fluorescence intensity due to porphyrin aggregation is followed
by an increase upon formation of the more soluble cages
(Fig. 2). For the 1a and 1b the porphyrin Soret bands near
420 nm red shift by about 4 nm with a small decrease in
intensity, indicating the formation of amorphous aggregates.
Table 1 NMR
Compound
Uracyl NH
Melamine NH
Por 1b
Por 2b
Melamine
Por 1b cage
Por 2b cage
11.5 ppm
12 ppm
na
14 ppm
13.8 ppm
na
na
4.5–5 ppm
5.5–6.7 ppm
5.5–6.1 ppm
10 mM in THF-d8.
c
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Table 2 Fluorescence lifetimes of porphyrins and cages
The synthesis of the porphyrins and aldehydes is straight
forward. The cooperative self-assembly of the porphyrin
cages mediated by four rigid meso uracil groups and the
bis(decyl)melamine form a stiff cage robust enough to allow
organization on surfaces driven by the long hydrocarbon
chains into nm thick films on mica. Assembly, in this case,
turns off electronic communication between chromophores.
Supported by the U.S. National Science Foundation
(NSF CHE-0554703, 0847997 to CMD, 0848786 to JDB);
Hunter College science infrastructure is supported by the
NSF, the National Institutes of Health, (including the RCMI
G12-RR-03037) and the City University of New York.
Compound
t1 (ns)
t2 (ns)
1a
1b
2a
2b
2.2 (19%)a
2.5
8.8 (81%)
1.8 (5%)a
3.8
8.6 (95%)
1a cage
1b cage
1a–1b cage mix.
2a cage
2b cage
2a–2b cage mix
2.8 (20%)
2.9
3.1(58%)
3.6 (11%)
2.4
10.2 (80%)
8.6 (42%)
9.6 (89%)
3.5 (60%)
9.7 (40%)
a
Some metalloporphyrin present, 408 nm excitation, 200 ps instru-
ment response time.
Notes and references
Time correlated single photon counting experiments on
these self-assembled cages were carried out in dry THF under
N2 at the same concentration used for the UV-visible studies,
and 408 nm excitation (Table 2). Incomplete demetalation of
the Zn2+ complexes to form the free bases results in some of
the metalloporphyrin present in the samples of 1a and 2a. The
lifetimes for 1a, 2a, 1b, 2b are somewhat shorter than standard
tetraphenylporphyrin (TPP, 11 ns) and ZnTPP (2.7 ns) under
similar conditions15 because of some aggregation. However, the
solutions with the cages, with reduced aggregation, generally
display lifetimes closer to those for other meso aryl porphyrins.
Since self-assembled materials must interact with surfaces
when incorporated into devices, we examined the self-
organization of the cages into films. The supramolecular cages
were drop cast from 30–40 mM THF solutions onto freshly
cleaved mica and imaged with AFM. For the 2a cage, a film
corresponding to a single layer of the cage structures is
observed (Fig. 3); estimated height = 2.8 nm.6 In this film,
the self-assembled cage is hierarchically organized by inter-
actions between the protruding hydrocarbon chains on the
four bis(decyl)melamine units.16 Friction images show no
indication of separation of the porphyrin and the melamine
components. When the 2b cage is cast onto mica, somewhat
thicker 12 nm films are observed, with root-mean-square
roughness of about 4 nm. The three different cages resulting
from the mixture of 2a and 2b, yield more complex patterns on
the surface.w This hierarchical self-organization into films
is analogous to those observed for squares of porphyrins
self-assembled by coordination chemistry.17
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Fig. 3 Contact mode AFM topography image of the 2a cage on mica
(left) and height profile (right).
c
7136 Chem. Commun., 2011, 47, 7134–7136
This journal is The Royal Society of Chemistry 2011