Synthesis of subphthalocyanines as probes for the accessibility of silica nanochannels

Article abstract of DOI:10.1021/ol2019983

The synthesis of a new subphthalocyanine is reported. Its structural and photophysical properties are ideal for probing the accessibility of arrays of silica nanochannels.

Full text of DOI:10.1021/ol2019983

ORGANIC
LETTERS
2
011
Vol. 13, No. 18
918–4921
Synthesis of Subphthalocyanines as
Probes for the Accessibility of Silica
Nanochannels
4
†
‡
†
,†,§
Mine Ince, Nando Gartmann, Christian G. Claessens, Tom ꢀa s Torres,* and
,‡
Dominik Br €u hwiler*
Departamento de Quı´mica Org aꢀ nica, Universidad Autonoma de Madrid, Cantoblanco,
8049 Madrid, Spain, Institute of Inorganic Chemistry, University of Zurich,
2
Winterthurerstrasse 190, 8057 Zurich, Switzerland, and IMDEA-Nanociencia,
Facultad de Ciencias, Cantoblanco, 28049 Madrid, Spain
tomas.torres@uam.es; bruehwi@aci.uzh.ch
Received July 25, 2011
ABSTRACT
The synthesis of a new subphthalocyanine is reported. Its structural and photophysical properties are ideal for probing the accessibility of arrays
of silica nanochannels.
1
Subphthalocyanines (SubPcs) are lower homologues of
4
applications in fields such as nonlinear optics, LEDs,
5
2
6
photovoltaics, photodynamic therapy, supramolecular
7
phthalocyanines, comprising a 14-π electron nonplanar
aromatic macrocycle made of three diiminoisoindole units
3
N-fused around a central boron atom. Unlike the related
8
chemistry, as well as in photosynthetic models for study-
9
ing energy- and electron-transfer processes. SubPcs have
planar phthalocyanines, SubPcs possess a peculiar conical
structure which provides them with relatively high solu-
bility and low tendency to aggregate. SubPcs exhibit a
number of unique properties that allow a wide range of
also been employed as intermediates in the synthesis of
unsymmetrically substituted phthalocyanines through a
ring expansion reaction.
1
0
(
6) Gommans, H.; Aernouts, T.; Verreet, B.; Heremans, P.; Medina,
A.; Claessens, C. G.; Torres, T. Adv. Funct. Mater. 2009, 19, 3435.
†
Universidad Autonoma de Madrid.
University of Zurich.
IMDEA-Nanociencia.
7) Rubio, N.; Jimenez-Banzo, A.; Torres, T.; Nonell, S. J. Photochem.
ꢀ
(
‡
Photobiol. A. 2007, 185, 214.
(8) Claessens, C. G.; Vicente-Arana, M. J.; Torres, T. Chem. Commun.
§
(
1) (a) Torres, T. Angew. Chem., Int. Ed. 2006, 45, 2834. (b) Claessens,
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Atienza Castellanos, C.; Guldi, D. M.; Torres, T. J. Am. Chem. Soc.
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(
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1
(
´
1
0.1021/ol2019983 r 2011 American Chemical Society
Published on Web 08/24/2011

1
6
of the particles. Pore sizes of mesoporous materials are
traditionally determined from nitrogen sorption data,
17
often by means of the BJH method. However, a pore
Scheme 1. Synthesis of SubPc 1
sizedistributiondoesnot necessarilyprovideunambiguous
1
information about the accessibility of the pores.
8
The adsorption of SubPcs in combination with CLSM
imaging allows us to draw a correlation between the pore
size distribution and the effective accessibility. We show
that the frequently used evaluation of pore sizes by the
BJH method fails to provide useful data on the pore
accessibility.
Table 1. Structural Properties of ASNCs
a
b
3
c
3
d
3
d
P
DFT (nm) Vtot (cm /g) V (cm /g) VP(DFT) (cm /g)
S-ASNCs
M-ASNCs
L-ASNCs
<2.0
2.6
0.24
0.57
0.75
0.23
0.52
0.70
0.23
0.54
0.71
3.2
a
b
Average pore diameter determined by NLDFT. Total pore vo-
c
lume. Primary mesopore volume determined by the R
Primary mesopore volume determined by NLDFT.
S
-plot method.
d
SubPc 1 (Scheme 1) was prepared in an overall yield of
1% by cyclotrimerization of 4,5-di(p-tert-butylphenoxy)-
2
phthalonitrile 3 in the presence of 1 equiv of BCl3,
1
9
followed by substitution of the axial chlorine atom by
20
0 0 00
1
,1 :4 ,1 -terphenyl-4-ol 2 in toluene.
00
0
0
1,1 :4 ,1 -Terphenyl-4-ol 2 was obtained in 55% yield by
a Suzuki cross-coupling reaction between 4-bromo(1,1 -
0
0
biphenyl)-4 -ol and phenylboronic acid in the presence of
tetrakis(triphenylphosphine) palladium(0) as catalyst in
DME by adapting a previously published procedure.
The synthesis of oxygenated subphthalocyanines such as
is still a challenge because BCl tends to break the ether
2
1
1
3
1
1
22
Mesoporous silica with ordered pores has become a
versatile host material in various fields, including drug
linkages. This could be overcome by slowly adding
substoichiometric amounts of BCl to 4,5-di(p-tert-butyl-
3
1
2
13
delivery and catalysis. These applications typically
require functionalization of the mesoporous silica, giving
rise to questions concerning the accessibility of the pores
phenoxy)phthalonitrile (see Supporting Information).
To test the ability of SubPcs to discern pore sizes, we
have synthesized ASNCs with different pore size distri-
butions. We found the well-defined morphology of the
ASNCs to be extremely sensitive to changes in the synth-
esis conditions and therefore decided to investigate possi-
bilities for a postsynthetic pore size adjustment. Indeed,
physisorption of dodecamethylpentasiloxane and sub-
sequent calcination gave ASNCs with reduced pore sizes
1
4
and the location of the functional groups. The use of
fluorescent probes and confocal laser scanning microscopy
CLSM) offers options for finding answers to these
questions. Arrays of silica nanochannels (ASNCs) have
proven to be an ideal material for this purpose. ASNCs are
hexagonally shaped fibers, each consisting of approxi-
mately 200 000 channels that run along the entire length
(
1
5
(
17) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc.
(
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007, 46, 7548. (b) Slowing, I. I.; Vivero-Escoto, J. L.; Wu, C.-W.; Lin,
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Vivero-Escoto, J. L.; Trewyn, B. G.; Lin, V. S.-Y. J. Mater. Chem. 2010,
´
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(b) Gartmann, N.; Sch u€ tze, C.; Ritter, H.; Br u€ hwiler, D. J. Phys. Chem.
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1993, 194.
(
2
(
(20) Claessens, C. G.; Gonz ꢀa lez-Rodrı
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T.; Mark, G.; Schuchmann, H.-P.; von Sonntag, C.; MacDonald, J. G.;
2
0, 7924.
(
15) (a) Gartmann, N.; Br u€ hwiler, D. Angew. Chem., Int. Ed. 2009,
8, 6354. (b) Blum, C.; Cesa, Y.; Escalante, M.; Subramaniam, V. J. R.
Nohr, R. S. Eur. J. Org. Chem. 2003, 14, 2547.
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(
Sokolov, I. J. Chem. Phys. 2008, 128, 151102.
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Org. Lett., Vol. 13, No. 18, 2011
4919

Figure 1. Top left: Estimation of the critical pore size for the inclusion of SubPc 1 in ASNCs. The SubPc macrocyclic core is highlighted
in black for clarity. Top right: Pore size distributions of L-ASNCs (ꢀ, green), M-ASNCs (O, blue), and S-ASNCs (b, black) calculated
from the nitrogen adsorption isotherms by a NLDFT model. The pore size distribution represented by the dashed green line has been
calculated by the classical BJH method using the adsorption isotherm of L-ASNCs. Bottom: CLSM images of ASNCs after deposition
of SubPc 1. The outermost right image of each group (shown in a white frame) was obtained after deposition of SubPc 4. Optical slices in
the center of the particles were selected. The length of the particles is approximately 5 μm.
(
(
Table1) but stillreasonablynarrowporesize distributions
Figure 1).
Physisorption of SubPc 1 on large pore ASNCs (L-
surface. We have also conducted reference experiments
2
4
with the smaller SubPc 4 (Figure 2). In this case, the
critical pore diameter for inclusion into the nanochannels
is 1.2 nm.
ASNCs) gave a uniform distribution throughout the
L-ASNCs (Figure 1). This is in agreement with an assess-
ment of the size of SubPc 1, resulting in a critical pore
diameter of 2.8 nm. The pore size distribution, calculated
Contrary to the pore size distribution calculated by
NLDFT, the pore size distribution of L-ASNCs deter-
mined by the classical BJH method indicates the presence
of pores exclusively smaller than 2.8 nm (Figure 1), which
would suggest a complete exclusion of SubPc 1. This is a
clear proof that the BJH method strongly underestimates
the pore size of ASNCs. The NLDFT model, on the other
hand, adequately describes the accessibility of the pores.
We have functionalizedthe externalsurface of L-ASNCs
by reaction with 3-aminopropyltris(methoxyethoxyeth-
oxy)silane (APTMEES) according to the method pro-
2
3
by a NLDFT (nonlocal density functional theory) model,
of L-ASNCs is positioned almost entirely at values larger
than 2.8 nm. The average pore diameter of medium
pore ASNCs (M-ASNCs) is 2.6 nm. Intuitively, one would
therefore expect that SubPc 1 cannot enter the pores.
CLSM images of the SubPc 1 distribution show that this
is only partially true. Luminescence can be observed
toward the center of the channels. According to the pore
size distribution of M-ASNCs, there is a considerable
fraction of pores with sizes larger than 2.8 nm, enabling
SubPc 1 to access the pore body. Complete exclusion of
SubPc 1 is only achieved if the entire pore size distribution
is located in a range below the critical diameter of 2.8 nm.
This is the case for small pore ASNCs (S-ASNCs). It is
interesting to note that there seems to be an accumulation
of SubPc 1 at the channel entrances of S-ASNCs as
opposed to a uniform coverage of the external particle
1
5
posed by Gartmann and Br u€ hwiler. Subsequent labeling
with fluorescein isothiocyanate (FITC) and CLSM ima-
ging reveals that the labeled amino groups are indeed
accumulated on the external particle surface. Figure 2A
shows that, despite the presence of the FITC-labeled
amino groups, the pores remain sufficiently accessible to
allow the inclusion of SubPc 1. An external surface area of
2
53 m /g was determined for L-ASNCs. Even in the event of
quantitative grafting onto the external surface, the density
of amino groups would remain in a reasonable range, that
(
23) (a) Ravikovitch, P. I.; Neimark, A. V. Colloids Surf. A 2001,
87ꢁ188, 11. (b) Ravikovitch, P. I.; Domhnaill, S. C. O.; Neimark,
A. V.; Sch u€ th, F.; Unger, K. K. Langmuir 1995, 11, 4765.
1
(24) Kasuga., K.; Idehara, T.; Handa, M.; Ueda, Y.; Fujiwara, T.;
Isa, K. Bull. Chem. Soc. Jpn. 1996, 69, 2559.
4
920
Org. Lett., Vol. 13, No. 18, 2011

In summary, a new SubPc was synthesized and used
in combination with CLSM to probe the accessibility of
ASNCs. Comparing the results of these studies to predic-
tions based on pore size distributions calculated from the
nitrogen adsorption isotherms revealed that an analysis by
a model based on NLDFT adequately describes the acces-
sibility of the pores. The classical BJH treatment was found
to draw a misleading picture of the accessibility by con-
siderably underestimating the pore size. The SubPc probe
was further employed to investigate the effect of surface-
grafted functional groups on the accessibility of the pores.
The photophysical properties of the SubPc are compatible
with those of the frequently used fluorescein labels, allow-
ing independent imaging of the distribution of fluorescein-
labeled functional groups and of the distribution of phy-
sisorbed SubPc.
Figure 2. CLSM images of ASNCs after external surface func-
tionalization with 100 μmol/g (A) or 500 μmol/g (B,C) of
APTMEES and labeling with FITC, followed by physisorption
of SubPc 1 (A,B) or SubPc 4 (C). The lower (green) images of
each panel show the luminescence of the coupled FITC labels,
whereas the upper (red) images were obtained upon excitation of
the SubPc molecules at 543.5 nm. Optical slices in the center of
the particles were selected.
Acknowledgment. Financial support by the Euro-
pean Commission through the Human Potential Program
(Marie-Curie RTN Nanomatch, MRTN-CT-2006-035884),
the Swiss National Science Foundation (200020-117591),
the MICINN and MEC (Spain) (CTQ2011-24187/BQU,
PLE2009-0070, and Consolider-Ingenio Nanociencia Mole-
cular CSD2007-00010), and Comunidad de Madrid (MAD-
RISOLAR-2, S2009/PPQ/1533) is gratefully acknowledged.
We thank Dr. Anais Medina (Universidad Aut oꢀ noma de
Madrid) for a sample of compound 4.
2
is, close to one group per nm , in the case of 100 μmol of
APTMEES per gram of L-ASNCs. Increasing the amount
of APTMEES by a factor of 5, on the other hand, would
produce a hypothetical density of more than 5 amino
2
groups per nm . It can be assumed that, in this case, a
considerable amount of amino groups is located on the
pore surface close to the pore entrances, leading to pore
blocking. CLSM images reveal that the distribution of
SubPc 1 on such highly loaded particles indeed tends to
be non-uniform (Figure 2B) with an accumulation of Sub-
Pc 1 at the pore entrances. However, the pores are appar-
ently still large enough to enable penetration of SubPc 4
Supporting Information Available. Synthesis and chara-
cterization of SubPc 1. Synthesis and functionalization of
ASNCs. This material is available free of charge via the
Internet at http://pubs.acs.org.
(
Figure 2C).
Org. Lett., Vol. 13, No. 18, 2011
4921