Modification of symmetrically substituted phthalocyanines using click chemistry: Phthalocyanine nanostructures by nanoimprint lithography

Article abstract of DOI:10.1021/ja905683g

Phthalocyanines (Pcs) are commonly applied to advanced technologies such as optical limiting, photodynamic therapy (PDT), organic field-effect transistors (OFETs), and organic photovoltaic (OPV) devices, where they are used as the p-type layer. An approac

Full text of DOI:10.1021/ja905683g

Published on Web 08/28/2009
Modification of Symmetrically Substituted Phthalocyanines
Using Click Chemistry: Phthalocyanine Nanostructures by
Nanoimprint Lithography
Xiaochun Chen,† Jayan Thomas,‡ Palash Gangopadhyay,‡ Robert A. Norwood,‡
N. Peyghambarian,‡ and Dominic V. McGrath*,†
Department of Chemistry and College of Optical Sciences, The UniVersity of Arizona,
Tucson, Arizona 85721
Received July 9, 2009; E-mail: mcgrath@u.arizona.edu
Abstract: Phthalocyanines (Pcs) are commonly applied to advanced technologies such as optical limiting,
photodynamic therapy (PDT), organic field-effect transistors (OFETs), and organic photovoltaic (OPV)
devices, where they are used as the p-type layer. An approach to Pc structural diversity and the incorporation
of a functional group that allows fabrication of solvent resistant Pc nanostructures formed by using a newly
developed nanoimprint by melt processing (NIMP) technique, a variant of standard nanoimprint lithography
(NIL), is reported. Copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), a click chemistry reaction,
serves as an approach to structural diversity in Pc macrocycles. We have prepared octaalkynyl Pc 1b and
have modified this Pc using the CuAAC reaction to yield four Pc derivatives 5a-5d with different peripheral
substituents on the macrocycle. One of these derivatives, 5c, has photo-cross-linkable cinnamate residues,
and we have demonstrated the fabrication of robust cross-linked photopatterned and imprinted nanostruc-
tures from this material.
Introduction
solid state properties including self-association and interfacial
behavior are dependent on substituents on the periphery of the
Phthalocyanines (Pcs) are commonly applied to advanced
technologies such as optical limiting,1 photodynamic therapy
(PDT),2 organic field-effect transistors (OFETs),3 and organic
photovoltaic (OPV) devices,4-6 where they are used as the
p-type layer. Critical to the performance of Pc materials in these
applications is the condensed phase morphology that they adopt
and the nanostructures into which they can be fabricated. Pc
chromophore.7 Extremely subtle structural variations in disk-
like chromophores can lead to dramatic and effective alteration
of their condensed phase organization, with a resultant effect
on the electronic interactions between adjacent molecules.8 To
study the influence of Pc structure on condensed phase
morphology, a library of different Pcs must be created to survey
the effects of structural alterations. However, while the extent
of structural variation of Pc materials to date is quite impres-
sive,9 there is a need for general methods of producing Pc
materials with structural diversity, especially with substituents
that will not readily survive the relatively harsh conditions of
Pc chromophore synthesis.10
† Department of Chemistry.
‡ College of Optical Sciences.
(1) (a) Dini, D.; Barthel, M.; Hanack, M. Eur. J. Org. Chem. 2001, 3759–
3769. (b) O’Flaherty, S. M.; Hold, S. V.; Cook, M. J.; Torres, T.;
Chen, Y.; Hanack, M.; Blau, W. J. AdV. Mater. 2003, 15, 19–32. (c)
Chen, Y.; Hanack, M.; Araki, Y.; Ito, O. Chem. Soc. ReV. 2005, 34,
517. (d) de la Torre, G.; Vazquez, P.; Agullo-Lopez, F.; Torres, T.
Chem. ReV. 2004, 104, 3723.
Here we disclose an approach to Pc structural diversity based
on “click chemistry”.11 We have chosen the copper(I)-catalyzed
azide-alkyne cycloaddition (CuAAC)12 as an approach to
structural diversity in Pc macrocycles. A click chemistry
reaction, CuAAC has proven useful for the synthesis of novel
polymers and materials in many laboratories13 and is an ideal
(2) (a) Bonnett, R. Chem. Soc. ReV. 1995, 24, 19–33. (b) Pandey, R. K.;
Zheng, G. Porphyrin Handbook 2000, 6, 157–230. (c) Moreira, L. M.;
dos Santos, F. V.; Lyon, J. P.; Maftoum-Costa, M.; Pacheco-Soares,
C.; da Silva, N. S. Aust. J. Chem. 2008, 61, 741–754.
(3) Bouvet, M. Anal. Bioanal. Chem. 2006, 384, 366–373.
(4) Shaheen, S. E.; Ginley, D. S.; Jabbour, G. E. MRS Bull. 2005, 30,
10–19.
(5) (a) Yoo, S.-Y.; Domercq, B.; Donley, C. L.; Carter, C.; Xia, W.; Minch,
B. A.; O’Brien, D. F.; Armstrong, N. R.; Kippelen, B. SPIE: Organic
PhotoVoltaics IV 2004, 5215, 71–78. (b) Brumbach, M.; Placencia,
D.; Armstrong, N. R. J. Phys. Chem. C 2008, 112, 3142.
(6) (a) Xue, J.; Rand, B. P.; Uchida, S.; Forrest, S. R. J. Appl. Phys. 2005,
98, 124903. (b) Xue, J.; Rand, B. P.; Uchida, S.; Forrest, S. R. AdV.
Mater. 2005, 17, 66–71. (c) Thompson, B. C.; Fre´chet, J. M. J. Angew.
Chem., Int. Ed. 2008, 47, 58–77. (d) Ma, W.; Yang, C.; Gong, X.;
Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617–1622. (e)
Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.;
Yang, Y. Nat. Mater. 2005, 4, 864–868. (f) Reyes-Reyes, M.; Kim,
K.; Carroll, D. L. Appl. Phys. Lett. 2005, 87, 083506. (g) Xue, J. G.;
Uchida, S.; Rand, B. P.; Forrest, S. R. Appl. Phys. Lett. 2004, 84,
3013–3015.
(7) Elemans, J. A. A. W.; van Hameren, R.; Nolte, R. J. M.; Rowan, A. E.
AdV. Mater. 2006, 18, 1251–1266.
(8) Huijser, A.; Savenije, T. J.; Kotlewski, A.; Picken, S. J.; Siebbeles,
L. D. A. AdV. Mater. 2006, 18, 2234+.
(9) de la Torre, G.; Claessens, C. G.; Torres, T. Chem. Commun. 2007,
2000–2015.
(10) de la Torre, G.; Claessens, C. G.; Torres, T. Eur. J. Org. Chem. 2000,
2821–2830.
(11) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed.
2001, 40, 2004–2021.
(12) (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B.
Angew. Chem., Int. Ed. 2002, 41, 2596–2599. (b) Tornoe, C. W.;
Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057–3064.
(13) Meldal, M.; Tornøe, C. W. Chem. ReV. 2008, 108, 2952.
9
13840 J. AM. CHEM. SOC. 2009, 131, 13840–13843
10.1021/ja905683g CCC: $40.75  2009 American Chemical Society

Click Chemistry Modification and NIMP of Pcs
A R T I C L E S
Scheme 1
reaction for Pc modification due to its extremely high yields
and fidelity.14,15 Incorporation of alkyne functionalities on the
periphery of a Pc will facilitate alkyne-azide click chemistry,
allowing for a variety of high-functioning substituents to be used.
With this approach, a library of Pcs can be prepared from a
single Pc core. Modifying the Pc after the macrocycle has been
formed eliminates repeating the difficult, low-yielding cycliza-
tion step with every desired change in structure. We also
demonstrate herein the incorporation of a functional group that
allows fabrication of solvent resistant Pc nanostructures, formed
by using a newly developed nanoimprint by melt processing
(NIMP) technique, a variant of standard nanoimprint lithography
(NIL).16
Results and Discussion
Pc Modification by Click Chemistry. In designing a target
for a “clickable” Pc, it was necessary to incorporate multiple
terminal alkyne moieties on the Pc periphery while maintaining
good solubility in the typical organic solvents used for click
chemistry (e.g., DCM, THF, DMSO). We also sought to
minimize the synthetic steps needed to prepare the phthalonitrile
precursor to the Pc. The ready availability of dihydroxyphtha-
lonitrile 217 and the facile access to alkylated derivatives using
Williamson ether or Mitsunobu chemistry18 led us to posit
octaalkynyl 1b for our clickable Pc. In addition to the requisite
alkynyl moieties, the four methylene groups between each
alkyne moiety and the Pc ring would lend degrees of freedom
to the structure that may translate to good solubility.
We prepared octaalkynyl Pc 1b via phthalonitrile 4a or 4b,
both of which were prepared from dihydroxy phthalonitrile 217
by Mitsunobu coupling with either 3a19 or commercially
available 3b, respectively (Scheme 1). Linstead cyclization of
phthalonitrile 4a yielded TIPS-protected Pc 1a which was
subsequently fluoride-deprotected to provide target 1b (26%
from 4a). The TIPS groups were initially used to provide a
soluble precursor to the potentially poorly soluble 1b. However,
we discovered to our delight that 1b is readily soluble in a wide
range of organic solVents (e.g., CH2Cl2, CHCl3, THF, acetone,
EtOAc). Hence, 1b is also readily obtained by direct cyclization
of phthalonitrile 4b in 68% yield (Scheme 1). Both 1a and 1b
were successfully characterized by NMR, UV-vis spectroscopy,
mass spectrometry, and combustion analysis. The UV absorption
spectrum of both 1a and 1b (ca. 10 µM in CH2Cl2) were
consistent with reported spectra of nonaggregated Pcs20 with a
split Q-band at 702 and 668 nm, vibrational bands at 640 and
607 nm, and B-band at ca. 340 nm (Figure 1a). The GPC traces
of both 1a and 1b were indicative of monodisperse materials,
and the hydrodynamic size of TIPs-protected 1a was larger than
that of its deprotected analogue 1b (Figure 1b).
Octaalkynyl 1b is an effective platform for Pc chromophore
modification through click chemistry (Scheme 2). Among click
conditions surveyed, a combination of CuBr(PPh3)3 and Hunig’s
base in CH2Cl2 proved most effective. Using a ratio of 2 azide
to 1 alkyne equivalents under these conditions, a reaction time
at ambient temperature of 7 days was sufficient to provide
octatriazenyl Pcs 5a-d in high yield from 1b and azides 6a-d.
A greater than stoichiometric amount of CuBr(PPh3)3 was
necessary as the metalated CuPcs were obtained under these
conditions. We expect that metalation of 1b prior to click
chemistry will allow the use of catalytic amounts of Cu in the
preparation of Pc with other metal centers.14 Pcs 5a-d were
characterized by a combination of NMR, UV-vis spectroscopy,
(14) During the preparation of this manuscript, an analogous approach to
Pc modification was reported: Jur´ıcek, M.; Kouwer, P. H. J.; Reha´k,
J.; Sly, J.; Rowan, A. E. J. Org. Chem. 2009, 74, 21–25.
(15) (a) Campidelli, S.; Ballesteros, B.; Filoramo, A.; Diaz, D.; de la Torre,
G.; Torres, T.; Rahman, G. M. A.; Ehli, C.; Kiessling, D.; Werner,
F.; Sgobba, V.; Guldi, D. M.; Cioffi, C.; Prato, M.; Bourgoin, J.-P.
J. Am. Chem. Soc. 2008, 130, 11503–11509. (b) Yoshiyama, H.;
Shibata, N.; Sato, T.; Nakamura, S.; Toru, T. Org. Biomol. Chem.
2008, 6, 4498–4501.
(16) Rogers, J. A.; Lee, H. H. UnconVentional Nanopatterning Techniques
and Applications; Wiley: NJ, 2009.
(17) Wo¨hrle, D.; Eskes, M.; Shigehara, K.; Yamada, A. Synthesis 1993,
194–196.
(18) (a) Kernag, C. A.; McGrath, D. V. Chem. Commun. 2003, 1048–1049.
(b) Kernag, C. A.; McGrath, D. V. Israel J. Chem. 2009, 49, 9–21.
(19) Crimmins, M. T.; Zhang, Y.; Diaz, F. A. Org. Lett. 2006, 8, 2369–
2372.
Figure 1. (Left) UV absorption spectra of Pcs 1b (4.85 × 10-6 M) and 5c
(5.16 × 10-6 M) in DCM. (Right) GPC chromatograms of Pcs 1a, 1b, and
5a-5d in THF.
(20) Brewis, M.; Clarkson, G. J.; Humberstone, P.; Makhseed, S.; McKe-
own, N. B. Chem.sEur. J. 1998, 4, 1633–1640.
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A R T I C L E S
Chen et al.
Scheme 2
Figure 2. (a) UV absorption spectra of a spin-cast film of 5c before and
after irradiation and wet development. (b) Fluorescence microscopy image
of patterned, cross-linked thin films of Pc 5c (495 ( 10 nm excitation and
620 ( 90 nm emission filter). (c and d) AFM image (full images in
Supporting Information) of patterned Pc 5c showing 30 µm square features
in a 50 nm thin-film.
mass spectrometry, and GPC. The absorption spectra of 5a-5d
confirmed the presence of the Cu in the macrocycles, evidenced
by the collapsed Q-band resonance at 682 nm (Figure 1a,
Supporting Information). The GPC traces of 5a-5b were
indicative of monodisperse materials, and the hydrodynamic
sizes of all derivatives were larger than those of the correspond-
ing starting material 1b (Figure 1b).
state [2 + 2] photodimerization of the cinnamate residues was
easily monitored by changes in the UV absorbance.23 Continu-
ous irradiation resulted in a decrease in the cinnamate peak in
the UV for 5c (37% overall decrease at 295 nm, λmax) after 95
min (Figure 2a). The decrease in cinnamate absorbance is most
likely due to both EfZ isomerization and [2 + 2] photodimer-
ization. Subsequent wet development (CH2Cl2 for 3 min)
resulted in negligible change in absorbance for the film of 5c,
indicating the formation of an insoluble cross-linked network
(Figure 2a).
Using the conditions for effective cross-linking determined
above, we next produced features from these thin films with
pattern sizes in the 30 µm range and characterized them by
fluorescence microscopy and AFM. Films with ca. 30 µm feature
sizes were produced by irradiation (310 nm for 150 min) of a
film masked with a transmission electron microscopy (TEM)
copper grid (500 mesh) and subsequent wet development in
CH2Cl2. Patterns of 5c visualized under a fluorescence micro-
scope (495 ( 10 nm excitation; 620 ( 90 nm emission filter)
exhibited resolved features with sharp edges (see Supporting
Information). These positive tone images have dark areas with
no emissive material and light areas of patterned 5c with the
dimensions of the pattern of the copper grid pattern used as a
mask. AFM images (Figure 2b) of the patterned thin film
confirmed the feature dimensions (30 µm × 30 µm) as well as
the thickness (ca. 50 nm). The similar thickness of the films
before and after cross-linking and wet development is evidence
of complete polymerization of the pendant cinnamate residues.
Significantly smaller feature sizes were effected in the surface
of a thin film of 5c using a simple but highly efficient technique
(NIMP) developed to create nanoarchitectures (Figure 3a). The
fundamental difference between NIMP and conventional nan-
imprinting lithography (NIL) processes is that NIMP starts from
a melted material, whereas NIL starts with a spin coated film.
Micropatterning and Nanofabrication. This method of chro-
mophore modification allows the installation of functional
moieties for further manipulation. We demonstrate here both
the photopatterning of thin films and imprinting of robust
nanostructures using the photo-cross-linking of the cinnamate
residues installed onto Pc 5c by click chemistry. Nanostructures
of Pcs are of importance for advanced technology applications
such as OFETs and OPVs. The control of nanostructure
dimension is critical to surmounting the exciton bottleneck
problem in heterojunction-based OPVs.21 To date, OPVs have
only reached a maximum power conversion efficiency of ∼5%.6
With a target efficiency of 15-20%,4 both materials and device
design need to be dramatically improved. An increase in
efficiency is expected as feature sizes decrease to approach the
exciton diffusion length. In addition, exciton mobility is
dramatically affected by chromophore organization.8 The rapid
method of chromophore modification described above will allow
us to explore both of these aspects of OPV device design. The
feature sizes of the nanostructures presented herein approach
the exciton diffusion length for OPVs (ca. 20 nm22).
To first demonstrate the effectiveness of cross-linking the
cinnamate residues on 5c for producing robust condensed
structures, we prepared thin films (ca. 50 nm by ellipsometry)
of 5c by spin-casting from ca. 10-6 M CH2Cl2 solutions onto
quartz. The conditions necessary for successful cross-linking
of these films were investigated by subjecting them to irradiation
(λ ) 310 nm, 6 nm slits) using a 75 W Xe arc lamp. The solid
(21) Forrest, S. R. MRS Bull. 2005, 30, 28–32.
(22) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107,
(23) Bernstein, H. I.; Quimby, W. C. J. Am. Chem. Soc. 1943, 65, 1845–
1324.
1846.
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Click Chemistry Modification and NIMP of Pcs
A R T I C L E S
90, and 100 nm, respectively. The size of the mold was ∼1 ×
1 in2, and the size of the patterned area was 3 × 3 mm2. In the
imprinting step, ∼5 mg of 5c were heated just above its melting
temperature (∼70 °C) on a glass substrate (1 × 1 in2). The mold
was then pressed against the sample at a pressure less than 1
bar and held there until the sample cooled to ambient temper-
ature. High mobility and low magnitude of wall slip at the
melted sample/silicon interface enable us to directly imprint the
nanostructures with high fidelity. After cooling, the glass
substrate was released carefully from the mold without damaging
the printed structure. A high resolution scanning electron
microscopy image of nanopatterned 5c (Figure 3b) shows
nanostructures with an average diameter of 40 ( 5 nm and a
height of 80-100 nm in a periodic array of cubic structures
(with several squares of 0.5 mm × 0.5 mm area each). Cross-
linking of the cinnamate residues by irradiating the surface with
300 nm light results in patterned nanostructures that are resistant
to dissolution in chlorinated solvents (CH2Cl2, CHCl3), which
should allow subsequent wet processing steps to be performed.
Figure 3c shows the SEM image of the nanostructures after
cross-linking. (The image location on the nanostructure and
imaging angle are not the same in Figure 3b and 3c.) The printed
structures show high fidelity with the nanostructures of the
silicon mold both before and after cross-linking. These nano-
patterns can be printed in a few minutes and are highly
reproducible.
Figure 3. (a) Schematic of the NIMP process. (b) SEM image of a 0.6
µm × 0.4 µm area of pillars (diameter 40 nm and height 80-100 nm) of
5c created by NIMP. (c) SEM image of a similar area of pillars after photo-
cross-linking.
In NIL, a spin-coated film of a high molecular weight polymer
is used as the printing medium. This film is heated to in general
60-100 °C above the glass transition temperature (Tg) of the
polymer, and the highly viscous polymer is shaped by pressing
the hard mold into it under high pressure. The mold is removed
once the polymer is cooled below its glass transition temper-
ature. In NIMP, by contrast, a relatively low molecular weight
material is melted on the substrate with no prefabrication of a
thin film. Hence, the required pressure for the imprinting process
is much lower (about an order of magnitude) in NIMP compared
to NIL since the viscosity of the melt is much lower. Due to
the low viscosity and small wetting angle of a completely melted
material on a silicon (mold) surface, highly efficient nanopatterns
can be readily imprinted by the NIMP process. An added
advantage of NIMP over NIL is that material loss can be
minimized if the required amount of the sample and the
thickness are optimized. We have found NIMP to be extremely
reliable in printing versatile nanostructures not only for solar
cells but also for nanoelectronic and nanophotonic devices, as
the technique enables nanoimprinting of the active material
without breaking vacuum conditions during the device fabrica-
tion process.
Summary
In summary, we have demonstrated the modification of Pc
chromophores with click chemistry and the subsequent use of
installed functional groups to create robust cross-linked pho-
topatterned and imprinted nanostructures. The investigation of
these nanostructures as components of bulk heterojunction solar
cells is currently underway in our laboratories.
Acknowledgment. This work was supported in part by the
National Science Foundation (CHE-0719437) and by AZRise. Mold
fabrication was done in the UCSB nanofabrication facility, part of
the NSF funded NNIN network. We thank Neal Armstrong for
helpful discussions, Mindy Horst for preliminary work, and Mayank
Mayukh for synthesis of azides.
Supporting Information Available: Experimental details and
additional characterization data for all new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
High resolution e-beam lithography and inductively coupled
plasma reactive ion etching were used to make a silicon mold
having a square lattice pattern of circular holes. The diameter,
depth, and pitch of the patterned holes were approximately 40,
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