Cross-linking of discotic tetraazaporphyrin dyes in 2 and 3 dimensions by "click" chemistry

Article abstract of DOI:10.1039/c3tc31588f

An intrinsic shortcoming of self-organizing materials is their susceptibility to structural changes by mechanical forces and exposure to chemicals and radiation. Cross-linking of the molecules in the desired supramolecular structure is a generally applicable pathway to structurally more stable materials but is difficult to apply to self-organizing materials because the introduction of cross-linkable groups affects their self-organization and the process of cross-linking may alter the supramolecular structure of the preceding phase. Reported here are the synthesis, thermal properties, and cross-linking of octaalkylthio substituted tetraazaporphyrins that contain either eight terminal azide groups or eight terminal acetylene groups. Synthesis of these compounds requires the preparation of the azide and alkyne containing side-chains and their reaction with 1,2-dicyanoethylene-1,2-dithiolate to the corresponding maleodinitrile intermediates. All maleodinitriles are successfully converted to tetraazaporphyrins by the established Mg templated cyclotetramerization in typical yields of 60-70%. Thermal properties of the metal-free and copper(ii) metallated tetraazaporphyrins were studied by thermal gravimetric analysis, polarized optical microscopy, differential scanning calorimetry, and variable temperature powder X-ray diffraction measurements. Azide substituted tetraazaporphyrins with trimethylene and hexamethylene spacers as well as acetylene derivatives with trimethylene spacers unexpectedly display columnar mesophases over ranges of temperature from 30 °C to 110 °C. A thermally activated cross-linking of a hexagonal columnar mesophase by cycloaddition at 65 °C is demonstrated for a 1:::1 mixture of azide and acetylene derivatives. At this temperature the reaction progresses for up to 48 hours but renders the mesophase insoluble and stable to above 200 °C. The structure of the mesophase is surprisingly little affected by the cross-coupling process that reaches a conversion of 60% of all azide and acetylene groups based on IR measurements. Conversion of up to 80% of azide and acetylene groups is reached by copper catalysed cross-linking of a 1:::1 mixture in solution to generate insoluble polymers. A similar degree of conversion is achieved by copper catalysed cross-linking of a 1:::1 mixture as Langmuir film after 3 hours. However, transfer of intact cross-linked Langmuir films onto substrates was not successful.

Full text of DOI:10.1039/c3tc31588f

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Cross-linking of discotic tetraazaporphyrin dyes in 2 and 3 dimensions
by “click” chemistry
Himadri Kayal,a Mohamed M. Ahmida,a Scott Dufour,a Hi Taing,a and S. Holger Eichhorn*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
An intrinsic shortcoming of self-organizing materials is their susceptibility to structural changes by
mechanical forces and exposure to chemicals and radiation. Cross-linking of the molecules in the desired
supramolecular structure is a generally applicable pathway to structurally more stable materials but is
difficult to apply to self-organizing materials because the introduction of cross-linkable groups affects
10 their self-organization and the process of cross-linking may alter the supramolecular structure of the
preceding phase. Reported here are the synthesis, thermal properties, and cross-linking of octaalkylthio
substituted tetraazaporphyrins that contain either eight terminal azide groups or eight terminal acetylene
groups. Synthesis of these compounds requires the preparation of the azide and alkyne containing side-
chains and their reaction with 1,2-dicyanoethylene-1,2-dithiolate to the corresponding maleodinitrile
15 intermediates. All maleodinitriles are successfully converted to tetraazaporphyrins by the established Mg
templated cyclotetramerization in typical yields of 60-70%. Thermal properties of the metal-free and
copper(II) metallated tetraazaporphyrins were studied by thermal gravimetric analysis, polarized optical
microscopy, differential scanning calorimetry, and variable temperature powder X-ray diffraction
measurements. Azide substituted tetraazaporphyrins with trimethylene and hexamethylene spacers as well
20 as acetylene derivatives with with trimethylene spacers unexpectedly display columnar mesophases over
ranges of temperature from 30 ºC to 110 ºC. A thermally activated cross-linking of a hexagonal columnar
mesophase by cycloaddition at 65 ºC is demonstrated for a 1:1 mixture of azide and acetylene derivatives.
At this temperatures the reaction progresses for up to 48 hours but renders the mesophase insoluble and
stable to above 200 ºC. The structure of the mesophase is surprisingly little affected by the cross-coupling
25 process that reaches a conversion of 60% of all azide and acetylene groups based on IR measurements.
Conversion of up to 80% of azide and acetylene groups is reached by copper catalysed cross-linking of a
1:1 mixture in solution to generate insoluble polymers. A similar degree of conversion is achieved by
copper catalysed cross-linking of a 1:1 mixture as Langmuir film after 3 hours. However, transfer of
intact cross-linked Langmuir films onto substrates was not successful.
and electronic properties of the generated adducts due to their
30 Introduction
strong dipole and aromatic character. Both, calamitic,11-13
polymeric,14 and discotic15-17 liquid crystals containing the 1,2,3-
50 triazole motive have recently been reported.
First reported in 2002, “click” reactions based on the copper
catalyzed azide-alkyne [3+2] cycloaddition (CuAAC) have
quickly become a versatile tool in organic, biological, and
materials chemistry. CuAAC reactions are tolerant to most
35 functional groups, proceed in high yields with few possible side-
reactions, and form stable 1,2,3-triazole rings as linker.1-3 Their
application in the synthesis and processing of self-organizing and
self-assembling materials, however, has been limited to the
synthesis of molecular building blocks and the modification of
40 self-assembled monolayers4 and Langmuir-Blodgett films.5
CuAAC is a particularly powerful reaction for the construction
of complex structures through covalent bond formation such as
the attachment of large dendrons to a rotaxane core,6 the
synthesis of oligomeric phthalocyanines,7-9 and the attachment of
45 dyes to polymers and carbon nanotubes.10 The 1,2,3-triazole rings
that result from the CuAAC may also benefit self-organization
Surprisingly, neither CuAAC nor thermally activated AAC
have been employed for the cross-linking of Langmuir films and
mesophases. Cross-linking of Langmuir and Langmuir-Blodgett
films is important to stabilize these monolayers and thin films for
55 potential applications but have been given relatively little
attention. Reported approaches include the photochemical cross-
19
linking of cinnamic acid and acrylate derivatives,18,
the
polymerization of amphiphilic diacetylene derivatives,20 the
oxidative polymerization of aniline derivatives,21 the ionic
60 interaction between ionic amphiphiles and ionic polyelectrolytes
23
of opposite charge,22,
the polycondenzation at the air-water
interface,24 and the linking by sol-gel chemistry of siloxanes.25, 26
Cross-linking of discotic columnar mesophases has been
mainly achieved by the polymerization of vinyl groups that are
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DOI: 10.1039/C3TC31588F
attached via alkyl spacers. Early work by Ringsdorf and
Schumacher was based on liquid crystalline triphenylene
Mass spectrometry measurements were performed by Kirk Green
at the Regional Center for Mass Spectrometry (McMaster
University) and Jiaxi Wang at the Mass Spectrometry and
derivatives that were photopolymerized in the presence of a
28
radical initiator.27,
A limitation of this approach is the low 60 Proteomics Unit (Queen’s University). UV/VIS absorption
5
discogenic potency of the triphenylene core that limits the
number of vinyl groups that can be attached without
compromising their mesomorphism. In contrast, the columnar
mesomorphism of macrocycles like phthalocyanines can tolerate
the attachment of 8 terminal acrylate groups and their columnar
spectra were run on a Varian Cary 50 Conc UV-visible
spectrophotometer. Elemental analysis was performed on a
Perkin Elmer 2400 combustion CHNS analyser at the Centre for
Catalysis and Materials Research at the University of Windsor.
Described below are the synthesis and characterization of all
TAP derivatives. Details on the synthesis of all starting materials
(Scheme 1) and spectroscopic data for all TAPs are provided as
ESI†.
65
10 mesophases were successfully polymerized by thermally
activated radical polymerization.29 Photopolymerization was not
successful, most likely because the light was preferentially
absorbed by the phthalocyanine core. However, this work has not
been pursued much further because the generated materials were
15 contaminated by the required radical initiator, typically AIBN,
and the fast progress of the cross-linking reaction generates
defects and cracks in the material. Müllen et al. recently
succeeded in the photochemical and thermal polymerization of a
70
Synthesis of octa-azide and octa-acetylene TAPs (6a-c, 7a-c,
6a-cCu, and 7a-cCu)
Mg powder (24.6 mg, 1 mmol) and 5 mg of iodine crystals were
refluxed overnight in 50 mL of dry propan-1-ol under nitrogen
until all Mg is reacted to Mg(II) propanolate. Maleodinitrile 4a
columnar
mesophases
of
an
acrylate
substituted 75 (1.54 g, 5 mmol) (or 4b (1.96 g, 5 mmol), 4c (2.38 g, 5 mmol), 5a
20 hexabenzocoronene derivative without the aid of
a
(1.372 g, 5 mmol), 5b (1.793 g, 5 mmol), 5c (2.21 g, 5 mmol))
was added and the suspension was heated at reflux for 24 hrs.
The resulting greenish-blue suspension was cooled to room
temperature, diluted with 50 mL of water, filtered, and the filter
80 residue was washed with methanol/water 1:1 until the filtrate
remains colourless. The filter residue was dissolved in a mixture
of 50 mL acetic acid and 10 mL THF and stirred at 80 C for
about 8 hours until demetallation was completed. Progress of the
demetallation was monitored by UV-Vis spectroscopy. Water
photoinitiator.30 The same group, together with Thünemann, also
developed an alternative approach to the cross-linking of discotic
mesophases by combining ionic hexabenzocoronene derivatives
with oppositely charged polyelectrolites.31, 32
25
Reported here is the synthesis of octaalkylthio substituted
tetraazaporphyrins (TAPs) that contain eight terminal azide or
acetylene groups and the cross-linking of their 1:1 mixtures via
CuAAC in solution and as Langmuir monolayers. Also presented
o
is the characterization of their discotic columnar mesophases and 85 (100 mL) was added to the now purple solution and the product
30 the cross-linking of a hexagonal columnar mesophase of a 1:1
mixture by thermally activated [3+2] cycloaddition.
was extracted with H2CCl2 (3 x 100 mL). The combined H2CCl2
layers were dried over anhydrous MgSO4 and the solvent was
removed under vacuum to give the crude TAP. Precipitation of
the crude TAP from acetone solution by slow addition of a 2:1
90 mixture of methanol and water purifies the TAPs to about 90%
purity and these samples were further purified by flash
chromatography on silica gel with H2CCl2/hexane 1:2 as eluent.
The purified metal-free TAPs were obtained as dark purple
solids.
Experimental
Synthesis
Chemicals: All reagents and solvents were purchased from
35 Sigma-Aldrich and Fluka Chemical Companies and used as
received unless otherwise stated. 1-Propanol and methanol were
dried over 4 Å and 3 Å molecular sieves, respectively, whereas 95 2,3,7,8,12,13,17,18-octakis(azidopropylthio)-5,10,15,20-
dry and air-free dichloromethane, tetrahydrofuran, and diethyl
ether were obtained from solvent purification system
tetraazaporphyrin (6a) (839 mg, 68%): 1H NMR (300 MHz,
CDCl3, δ): -1.45 (2H, s, NH), 2.18 (16H, tt, J = 6.5 Hz, J = 7.0
Hz), 3.67 (16H, t, J = 6.3 Hz), 4.19 (16H, t, J = 6.6 Hz); 13C NMR
(130 MHz, HMBC, CDCl3, ): 29.21, 34.42, 49.21, 139.82, 151
a
40 (Innovative Technology Inc. MA, USA, Pure-Solv 400). Silica
gel 60 (35-70 mesh ASTM, from EM Science, Germany) was
used for column chromatography and Silica Gel 60 aluminum 100 (not observed); IR (cm-1): 3290 (m,ν(N-H)), 2930 (m, ν(C-H)),
backed sheets (EM Science, Germany) for thin layer
chromatography.
2098 (s, ν(N3)); UV/VIS in THF (max in nm,  in dm3 mol-1 cm-
1): 344 (44300), 513 (20600), 651 (26100), 712 (34200); EA
calcd for C40H50N32S8: 38.88 %C, 36.28 %N, 4.08 %H; Found:
39.12 %C, 36.00 %N, 4.15 %H.
1
45 Instrumentation: H-NMR and 13C-NMR spectra were obtained
on Bruker NMR spectrometers (DRX 500 MHz and DPX 300
MHz). The residual proton signal of the deuterated solvent 105 2,3,7,8,12,13,17,18-octakis(azidohexylthio)-5,10,15,20-
(usually chloroform (CDCl3)) was used as a reference signal and
multiplicities of the peaks are given as s = singlet, d = doublet, t =
50 triplet, and m = multiplet. Coupling constants are given in Hz and
only calculated for 1st-order systems. Data are presented in the
tetraazaporphyrin (6b) (1.098 g, 70%): 1H NMR (300 MHz,
CDCl3, δ): -1.12 (2H, s, NH), 1.31-1.45 (16H, m), 1.47-1.71
(32H, m), 1.88 (16H, t, J = 6.9 Hz), 3.19 (16H, t, J = 6.6 Hz),
4.09 (16H, t, J = 6.9 Hz); 13C NMR (75 MHz, CDCl3, δ): 26.12,
following order: integration, multiplicity, and coupling constant. 110 28.63, 28.81, 29.72, 34.89, 49.86, 138.01, 149.93; IR (cm-1):
Fourier Transform Infrared spectra (FT-IR) were obtained on a
Bruker Vector 22 as KBr pellets or thin films on KBr windows.
55 Relative peak intensities and shapes in IR are abbreviated as vs =
very strong, s = strong, m= medium, w = weak, and br = broad.
3285 (m, ν(N-H), inner core NH), 2932 (m, ν(C-H), CH2), 2095
(s, ν(N3)); UV/VIS in THF (max in nm, ε in dm3 mol-1 cm-1): 342
(44100), 515 (20500), 655 (26200),714 (34200); EA calcd for
C64H98N32S8: 48.89 %C, 28.51 %N, 6.28 %H; Found: 48.80 %C,
2
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28.30 %N, 6.44 %H.
concentrated, and filtered through a small amount of silica gel
60 with ethyl acetate/hexane 1:3 to generate the purified copper
TAPs as dark blue solids.
2,3,7,8,12,13,17,18-octakis(azidopropylthio)-5,10,15,20-
tetraazaporphyrinato-copper (6aCu) (571 mg, 88%): IR (cm-1):
2917, 2849 (m, ν(C-H), CH2), 2093 (s, ν(N3), azide); UV/VIS in
2,3,7,8,12,13,17,18-octakis(azidononylthio)-5,10,15,20-
tetraazaporphyrin (6c) (1.259 g, 66%): 1H NMR (300 MHz,
CDCl3, δ): -1.06 (2H, s, NH), 1.28 (64H, m), 1.59 (32H, m),
1.97(16H, m), 3.20 (16H, broad), 4.12 (16H, broad); 13C NMR
(75 MHz, CDCl3, δ): 26.71, 28.84, 28.96, 29.15, 29.26, 29.44,
5
30.54, 35.32, 51.45, 140.63, 153.43; IR (cm-1): 3283 (m,ν(N-H)), 65 THF (max in nm, ε in dm3 mol-1 cm-1): 362 (21820), 500 (7576),
2920 (m, ν(C-H)), 2100 (s, ν(N3)); UV/VIS in THF (max in nm, 
667 (30606); EA calcd for C40H48CuN32S8: 37.04 %C, 34.56 %N,
3.73 %H; Found: 36.79 %C, 34.35 %N, 3.91 %H.
in dm3 mol-1 cm-1): 344 (44 300), 510 (21 000), 651 (25 900), 713
10 (34115); HRMS-MALDI (m/z): calcd for C88H146N32S8+H:
1908.0252; Found: 1908.0242 [M+H]+; EA calcd for
2,3,7,8,12,13,17,18-octakis(azidohexylthio)-5,10,15,20-
tetraazaporphyrinato-copper (6bCu) (653 mg, 81%): , MW: IR
C88H146N32S8: 55.37 %C, 23.48 %N, 7.71 %H; Found: 55.25 %C, 70 (cm-1): 2929, 2853 (m, ν(C-H), CH2), 2094 (s, ν(N3), azide);
23.31 %N, 7.84 %H.
UV/VIS in THF (max in nm, ε in dm3 mol-1 cm-1): 365 (21970),
505 (7758), 670 (30667); HRMS-MALDI (m/z): calcd for
C64H96CuN32S8+H: 1632.5635; Found: 1632.5630 [M+H]+.
2,3,7,8,12,13,17,18-octakis(azidononylthio)-5,10,15,20-
2,3,7,8,12,13,17,18-octakis(pent-4-yn-1-ylthio)-5,10,15,20-
15 tetraazaporphyrin (7a) (770 mg, 70%): 1H NMR (300 MHz,
CDCl3, δ):-1.30 (2H, s, NH), 1.90 (8H, s), 2.08 (16H, tt appears
as p, J = 6.9 Hz), 2.52 (16H, t, J = 2.1 Hz), 4.18 (16H, t, J = 7.2 75 tetraazaporphyrinato-copper(6cCu) (788 mg, 80%); IR (cm-1):
Hz); 13C NMR (75 MHz, CDCl3, δ): 17.72, 29.28, 34.00, 69.33,
83.43, 140.59, 153.35; IR (cm-1): 3290 (s, ν(C-H) alkyne and w,
20 ν(N-H)), 2917, 2850 (m, ν(C-H), CH2), 2115 (w, ν(C≡C));
UV/VIS in THF (max in nm, ε in dm3 mol-1 cm-1): 355 (43090),
2915, 2851 (m, ν(C-H), CH2), 2090 (s, ν(N3), azide); UV/VIS in
THF (max in nm, ε in dm3 mol-1 cm-1): 365 (24857), 504 (8354),
670
(30616);
HRMS-MALDI
(m/z):
calcd
for
C88H144CuN32S8+H: 1968.9391; Found: 1968.9329 [M+H]+.
565 (18940), 645 (26470), 710 (34200); HRMS-MALDI (m/z): 80 2,3,7,8,12,13,17,18-octakis(pent-4-yn-1-ylthio)-5,10,15,20-
calcd for C56H58N8S8+H: 1099.2625; Found: 1099.2646 [M+H]+.
2,3,7,8,12,13,17,18-octakis(oct-7-yn-1-ylthio)-5,10,15,20-
tetraazaporphyrinato-copper (7aCu) (493 mg, 85%): IR (cm-1):
3353, 3295 (s, ν(C-H) alkyne), 2930, 2853 (s, ν(C-H), CH2), 2116
(w, ν(C≡C)); UV/VIS in THF (max in nm, ε in dm3 mol-1 cm-1):
360 (25151), 500 (8667), 665 (29090); HRMS-MALDI (m/z):
25 tetraazaporphyrin (7b) (976 mg, 68%): 1H NMR (300 MHz,
CDCl3, δ):-1.13 (2H, s, NH), 1.38-1.72 (48H, m), 1.79-1.96 (24H,
m), 2.04-2.19 (16H, m), 4.08 (16H, t, J = 7.2 Hz); 13C NMR (75 85 calcd for C56H56CuN8S8+H: 1160.1767; Found: 1160.1754
MHz, CDCl3, δ): 18.37, 28.39, 28.45, 29.75, 30.41, 35.24, 68.26,
84.49, 140.63, 153.2; IR (cm-1): 3290 (s, ν(C-H) alkyne and w,
30 ν(N-H)), 2930, 2854 (m, ν(C-H), CH2), 2115 (w, ν(C≡C));
UV/VIS in THF (max in nm, ε in dm3 mol-1 cm-1): 355 (43030),
[M+H]+.
2,3,7,8,12,13,17,18-octakis(oct-7-yn-1-ylthio)-5,10,15,20-
tetraazaporphyrinato-copper (7bCu) (600 mg, 80%): IR (cm-1):
3352, 3300 (s, ν(C-H) alkyne), 2930, 2853 (s, ν(C-H), CH2), 2117
562 (19091), 642 (26060), 710 (33333); HRMS-MALDI (m/z): 90 (w, ν(C≡C)); UV/VIS in THF (max in nm, ε in dm3 mol-1 cm-1):
calcd for C80H106N8S8+H: 1435.6384; Found: 1435.6380 [M+H]+.
2,3,7,8,12,13,17,18-octakis(undeca-10-yn-1-ylthio)-5,10,15,20-
35 tetraazaporphyrin (7c) (1.241 g, 70%): 1H NMR (300 MHz,
CDCl3, δ): -1.09 (2H, s, NH), 1.2-1.3 (64H, m), 1.42 (16H, m),
365 (24850), 503 (8364), 670 (30606); HRMS-MALDI (m/z):
calcd for C80H104CuN8S8+H: 1496.5523; Found: 1496.5552
[M+H]+.
2,3,7,8,12,13,17,18-octakis(undeca-10-yn-1-ylthio)-5,10,15,20-
1.60 (16H, m), 1.89 (24H, broad), 2.10 (16H, broad), 4.10 (16H, 95 tetraazaporphyrinato-copper(17cCu) (726 mg, 82%); IR (cm-1):
broad); 13C NMR (75 MHz, CDCl3, δ): 18.33, 28.40, 28.68,
28.78, 28.90, 29.04, 29.26, 30.49, 35.28, 68.05, 84.65, 140.42
40 (HMBC at 130 MHz), 151 (not observed); IR (cm-1): 3300 (s,
ν(C-H) alkyne and w, ν(N-H)), 2933, 2845 (m, ν(C-H), CH2),
3300, 3350 (s, ν(C-H) alkyne), 2928, 2850 (s, ν(C-H), CH2), 2110
(w, ν(C≡C)); UV/VIS in THF (max in nm, ε in dm3 mol-1 cm-1):
365 (21975), 505 (7760), 670 (30655); EA calcd for
C104H152CuN8S8: 68.09 %C, 6.11 %N, 8.35 %H; Found: 67.83
2113 (w, ν(C≡C)); UV/VIS in THF (max in nm, ε in dm3 mol-1 100 %C, 5.99 %N, 8.42 %H.
cm-1): 360 (43100), 565 (18965), 645 (26500), 712 (34200); EA
calcd for C104H154N8S8: 70.46 %C, 6.32 %N, 8.76 %H; Found:
45 70.19 %C, 6.15 %N, 8.91 %H.
Results and Discussion
Synthesis
Synthesis of octa-azide and octa-acetylene copper TAPs (6a-
cCu and 7a-cCu)
Synthesis of maleonitriles 4-5 required the preparation of azide
and acetylene terminated alkanes that contain a leaving group at
Metal-free TAP 6a (617 mg, 0.5 mmol) (or 6b (785 mg, 0.5 105 their other terminal position (Scheme 1). Azide terminated
50 mmol), 6c (954 mg, 0.5 mmol), 7a (550 mg, 0.5 mmol), 7b (718
mg, 0.5 mmol), 7c (886 mg, 0.5 mmol)) was stirred in a solution
of copper(II) acetyl acetonate (262 mg, 1.0 mmol) in THF (50
mL) at 60 ºC for 6-8 hrs. Progress and completion of the
bromoalkanes 1 have previously been prepared in 46–65% yield
by a statistical mono-substitution of dibromoalkanes with sodium
azide in DMF at 80 ºC.33-35 Reported here is an alternative
approach based on the selective substitution of OH by N3 in
metallation with copper(II) was monitored by UV-VIS 110 bromoalkanols via
a
Mitsunobu-type reaction with
55 spectroscopy. Ethyl acetate (100 mL) was added and the mixture
was then extracted with an aqueous solution of ammonium
chloride (5% w/w, 3 x 100 mL) and finally with water (2 x 100
mL). The organic layer was dried over anhydrous MgSO4,
diisopropylazodicarboxylate (DIAD) and
diphenylphosphorylazide (DPPA).36 This methodology avoids the
tedious chromatographic separation of mono- and di-azide
products but the removal of DIAD, DPPA, and their side-
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products still required chromatographic separations to generate
the products in only slightly higher yields of 60%-70%.
Alkyne 2a was also obtained in one step by converting the
commercially available 4-pentyne-1-ol to its methylsulfonate.37-40
A different approach is required for alkynes 2b-c, which were
35
TAPs
6-7
were
prepared
by
the
established
cyclotetramerization of 2,3-bis(alkylthio)maleonitrile drivatives
4-5 in a suspension of magnesium propanolate in propanol at
reflux temperature. The initially obtained Mg containing TAPs
were demetallated in situ under acidic conditions to give the
5
prepared by statistical nucleophilic substitution of 1,6- 40 metal-free macrocycles 6-7. Overall yields of 60-70% for
dibromohexane and 1,9-dibromononane with 1.5 eq. of TMS-
acetylene.41 The obtained product mixtures of about 80% 2b-c
and 20% disubstituted compounds were used for the following
cyclization and demetallation are equal to yields previously
reported
for
cyclotetramerizations
of
other
2,3-
bis(alkylthio)maleonitrile derivatives.44-46 Clearly, the presence of
terminal alkyne and azide groups does not affect the cyclization
10 reactions with 3 without prior separation. Compound 3 only
reacts with the mono-acetylides 2b-c and the chromatographic 45 and both functional groups are surprisingly stable to the reaction
removal of the di-acetylides from maleonitriles 5b-c was
straightforward. Previously reported preparations of unprotected
2b-c are lower yielding and some require fractionated
15 distillation.42
conditions. Perhaps this is not an unexpected result since both,
azide and acetylene groups, have been previously shown to be
compatible with the lithium alcoholate mediated synthesis of
phthalocyanines.47-49 Metallation of TAPs 6-7 with Cu(II) was
50 achieved with CuAcAc in THF solution in >80% yield.
Mesomorphism
The thermal stability of TAPs 6 and 7 is limited by the stability of
their alkynyl and alkylazide chains and was studied by thermal
gravimetric analysis under He (Tab. S1 in ESI†). Least stable are
55 the TAPs with azide groups and propyl spacers (6a and 6aCu)
that start to decompose between 110-120 ºC. All other TAPs
decompose between 140-160 ºC, which is still significantly lower
than the reported 240 ºC for analogous octa-alkenyl substituted
TAPs.45 Differences in thermal stability between metal-free and
60 copper containing TAPs are small and non-systematic.
Scheme 1 Reaction conditions and yields: (a) THF, PPh3, DIAD, DPPA,
0-20 ºC, 24 hrs; 60%-70%. (b) MsCl, Et3N, H2CCl2, 0 ºC, 5 hrs; 90%. (c)
1.5 eqiv. trimethylsilylacetylene, BuLi, HMPA, THF, −78-20 ºC, 15 hrs;
20
25
about 56% 2b,c and 14% disubstituted product. (d) For 4a-c and 5a:
NaIcat, MeOH, 20 ºC, 5-24 hrs; 65-70%. For 5b-c: first Na2CO3, MeOH,
20 ºC, 5 hrs; then addition of 3 and NaIcat, 20 ºC, 24 hrs; 60%. (e)
Mg(OC3H7)2, HOC3H7, refl., 24 hrs; not purified. (f) AcOH/MeOH 5:1,
refl., 8 hrs.; 60-70% for both steps. (g) CuAcAc, THF, 60 ºC, 8 hrs.;
>80%.
Scheme 2 Phase behaviour of TAPs as determined by POM, DSC, and
XRD. Transition temperatures are given in ºC (peak temperatures of 2nd
65
heating and 1st cooling runs) and enthalpies (in brackets) in kJ/mol.
Transition temperatures not observed by DSC were determined by POM
(POM). Colh and Colr are columnar mesophases of hexagonal and
rectangular symmetry, respectively. aTwo overlapping transitions.
Compounds
1
and 2a readily reacted with 1,2-
dicyanoethylene-1,2-dithiolate 3 to give maleodinitriles 4 and 5a
but, unexpectedly, reactions of 2b-c with 3 generated complex
product mixtures. The observed side-reactions were apparently
All twelve TAPs were studied by polarized optical microscopy
70 (POM), differential scanning calorimetry (DSC), and X-ray
diffraction (XRD) to probe their mesomorphism (Scheme 2).
POM images, DSC curves, and XRD data are provided as ESI†.
All TAPs with hexamethylene and nonamethylene spacers are not
liquid crystalline except for 6bCu that displays a Col mesophase
75 of probably rectangular symmetry over a temperature range of 30
30 caused by the TMS group since the problem was resolved when
the TMS groups were removed (in situ) prior to the addition of 3.
Compound 3 was prepared by the method of Davison and Holm
and can be stored for weeks in a freezer under argon if
sufficiently pure.43
4
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ºC on heating. TAPs with nonamethylene spacers either melt just
of a freshly prepared 1:1 mixture of TAPs 6aCu and 7aCu to 100
above room temperature or are isotropic liquids at room 60 ºC showed an increase in slope of the baseline when the
temperature (7cCu). In fact, compound 7cCu does not show any
transition in DSC down to -50 ºC and is an isotropic liquid down
to 0 ºC based on POM, which is rather unusual for TAPs46 and
porphyrins in general.50
temperature surpassed 70 ºC (Fig. S29 in ESI†). Measurements to
200 ºC revealed that this increase in slope is actually the onset of
an irreversible exothermic transition of 293 J/g (approximately
360 kJ/molTAP) (Fig. S30 in ESI†). This peak is interpreted as the
5
All Cu metallated and one metal-free TAP (7a) with 65 heat generated by the AAC reaction, although the measured
trimethylene spaces display liquid crystal phases. Particularly
surprising is the Colr mesophase displayed by 7a over a
10 temperature range of 80 ºC. Only few metal-free octaalkylthio-
substituted TAPs have been reported to display columnar
enthalpy may represent only a part of the total reaction enthalpy
as the reported exothermic standard reaction enthalpy is 188-230
kJ/mol per pair of azide and acetylene.1 Consequently, a total
reaction enthalpy of about 800 kJ per mole of TAP is expected if
mesophases, in contrast to their metallated analogues, and the 70 four groups per TAP react.
temperature ranges of their mesophases are below 20 ºC.51
Another evidence of the high stability of the mesophase of 7a is
15 the large enthalpy of its clearing transition when compared to the
clearing enthalpy of the Colh mesophases of 7aCu, although a
A slow cross-linking of the columnar mesophase by AAC at
65 ºC was chosen to minimize structural changes to the film and
columnar mesophase. The progress of the AAC reaction was
studied by isothermal DSC measurements at 65 ºC and IR
larger enthalpy can be reasoned with the higher order of a Colr 75 spectroscopy. Isothermal DSC measurements show a constant
mesophase. Nematic discotic mesophases that have been reported
for metal-free TAPs with alkene terminated aliphatic chains have
20 not been observed for TAPs 6 and 7.51
A comparison of the Colh mesophases of TAPs 6aCu and
7aCu reveals similar melting points but a higher clearing point of
TAP 6aCu. This and the mesomorphism of 6bCu suggest that the
attachment of terminal azide groups is less detrimental to the
25 mesomorphism of octaalkylthio substituted TAPs than terminal
acetylene groups. However, a comparison of the mesomorphism
exothermic slope of the baseline for the first 700 minutes that is
followed by a slow decrease in slope (Fig. S31 in ESI†). The end
point of the reaction is difficult to determine by this method but
the slope of the baseline approaches 0 after about 22 hours.
More sensitive to the progress of the reaction is the decrease of
the azide/alkyne absorptions at 2100 cm-1 (decrease in area)
observed by IR spectroscopy. A thin film of the 1:1 mixture of
TAPs 6aCu and 7aCu was coated onto a KBr window and kept
at 65 ºC for 3 days and IR spectra were taken after 4, 12, 24, 48,
80
of TAPs 6 and 7 with the mesomorphism of analogous octa- 85 and 72 hours (Fig. 1). No further decrease of the combined
alkenyl substituted TAPs45 shows that terminal alkenes are better
accommodated by their columnar mesophases than terminal
30 azides and alkynes. All three terminal functional groups
destabilize mesophases in comparison to TAPs with eight
alkylthio chains of comparable length.51
azide/alkyne absorption peak was observed after 48 hours and the
majority of the cross-linking had occurred after 24 hours, which
overall agrees with the DSC data.
1.00
0.95
Thermally activated AAC of columnar mesophase
1:1 Mixtures of any of the azide TAPs 6 and alkyne TAPs 7 can
35 be cross-linked in solution by standard CuAAC to give
amorphous insoluble polymers of the dyes. Conversions of azide
and acetylene groups based on the decrease of the overlapping IR
stretching absorptions of the acetylene and azide groups reach
values of 60 to 80%, similar to the IR measurements shown in
40 Figure 1. However, detailed studies of cross-linking in solution
have not been pursued because the formation of insoluble
amorphous polymers was not our objective and they are difficult
to characterize.

(C-H)alkyne
0.90
0.85
0.80
0.75
alkyne

(C-H)aliphatic
0h
4h
12h
24h
48h
N3
3400 3200 3000 2800
2200
2100
2000

(cm-1)
More intriguing to us was the cross-linking of TAPs in their
45 columnar mesophases, especially when conducted under thermal
activation without the use of a catalyst. A 1:1 mixture of TAPs
6aCu and 7aCu was chosen for this experiment because it does
not crystallize and its Colh mesophase is stable from below -50 to
above 100 ºC. The absence of any transitions in this temperature
50 range was confirmed by DSC (Fig. S29 in ESI†) and the presence
of a Colh mesophase was verified by POM and XRD studies. In
fact, the defect texture observed by POM shows only minor
changes even at 200 ºC (Fig. 2), although partial decomposition
cannot be excluded at these high temperatures. This high thermal
55 stability of the Colh mesophase is reasoned with a fast cross-
linking process at temperatures above 100 ºC.
90 Fig. 1 IR spectra of the Colh mesophase of a 1:1 mixture of TAPs 6aCu
and 7aCu as thin film on a KBr disk after heat treatment at 65 ºC for
specific periods of time. No further change was observed for heat
treatments longer than 48 hrs.
After 48 hours the area of the azide/alkyne absorption peak
95 was decreased to 40% of its original area. This is equivalent to a
conversion of 60% of the azide and acetylene groups or 4.8
azide/alkyne groups per TAP. A conversion of only a fraction of
all azide and alkyne groups is reasonable because some azide and
alkyne groups will not be able to reach each other once the
100 molecules are locked into place after the first groups have
reacted. Sufficient for the formation of insoluble polymers is
A more quantitative investigation of the cross-linking process
was conducted by DSC and IR measurements. DSC heating runs
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likely the conversion of only 2 azide/alkyne groups per TAP
(25%) and, indeed, the thin films are completely insoluble in
organic solvents after 12-16 hours at 65 ºC.
pure 6aCu and 7aCu have d-spacings of the (10) reflections at
14.6 Å and 15.8 Å, respectively.
CuAAC in Langmuir Film
Observation of a thin film of the 1:1 mixture of TAPs 6aCu
and 7aCu on glass by POM at 65 ºC for 3 days revealed only
minor changes to the texture (Fig. 2). The absence of structural
changes to the Colh mesophase during the cross-linking by AAC
was independently confirmed by powder XRD measurements
(Fig. 2). In fact, the cross-linking process hardly affects the
65 The overall low amphiphilicity of TAPs 6 and 7 is a disadvantage
for the formation of Langmuir films. Monolayers of reasonable
quality were obtained for TAPs 6b and 7a and their 1:1 mixture
served as model systems for testing their cross-linking by
CuAAC at the air-water interface.
5
140
Both TAPs 6b and 7a are crystalline at room temperature but
their surface pressure versus area (π–A) isotherms are rather
different (Fig. S32 in ESI†). The isotherm of 6b is featureless and
the pressure exponentially increases with decreasing surface area
until the film collapses at a pressure of 45 mN m-1 and a limiting
145 area per molecule of 138 Å2. In contrast, the L-film of 7a shows a
distinct step in its π–A isotherm. A first steep increase in surface
pressure occurs at a limiting area per molecule of 152 Å2 and
briefly falls off to 23.5 mN m-1 after reaching a surface pressure
of 26 mN m-1. Upon further compression the surface pressure
150 again increases exponentially until the final collapse pressure of
44.5 mN m-1 at a limiting area per molecule of 113 Å2 is reached.
The in-between decrease in surface pressure to 23.5 mN m-1 may
indicate the intermediate formation of 3-dimensional structures
but was not studied in more detail. Also uncertain remains the
155 exact orientation of the TAPs at the air-water interface but the
observed surface areas per molecule agree with a more face-on
orientation of the macrocycle while an edge-on orientation would
result in surface areas well below 100 Å2.46, 52
10 columnar mesophase despite the formation of relatively bulky
and polar triazole rings. What is observed by XRD is a small
decrease in intensity of the (10) reflection and its gradual shift to
smaller lattice spacings by 0.2 Å over 44 hours. The former
change indicates a small decrease in columnar packing order
15 while the latter change is explained with the expected shrinkage
of the intercolumnar packing distance during cross-linking.27
Fig. 2 POM photomicrographs (100 x 100 m, crossed polarizer) and
20 powder XRD patterns (staggered for clarity) of a 1:1 mixture of 6aCu and
7aCu kept at 65 ºC for up to 48 hours. The top left image shows the
freshly prepared sample (less than 30 minutes after mixing) at 65 ºC, the
top centre image shows the same area after 48 hours at 65 ºC for, and the
top right image shows the same area (after 48 hours at 65 ºC) at 200 ºC.
90
Fig. 3 π–A isotherm of a 1:1 mixture of TAPs 6b and 7a spread from
chloroform solution onto pure water as subphase (solid line). Change of
surface pressure over time of the same mixture at constant surface area
(135 Å2) on pure water (triangles) and on water containing copper (II)
acetate (0.638 x 10-3 M) and sodium ascorbate (1.92 x 10-3 M) (circles).
25
Cross-linking of the mesophase probably occurs both within
columnar stacks to generate 1,5-disubstitiuted 1,2,3-triazole
linkers and between columnar stacks to generate 1,4-
disubstitiuted 1,2,3-triazole linkers. Both types of cycloadditions
are thermally allowed, in contrast to CuAAC that exclusively
110
The π–A isotherm of the 1:1 mixture of TAPs 6b and 7a used
for the cross-linking by CuAAC has a shape similar to the
isotherm of 7a but the surface areas per molecule shifted to larger
molecular areas (Fig. 3). This increase in surface area indicates an
overall looser and less compressible packing of the TAP mixture,
30 generates 1,4-disubstitiuted 1,2,3-triazoles. Also, each columnar
stack likely consists of a 1:1 mixture of TAPs 6aCu and 7aCu
since all data are consistent with the formation of one phase. The
unlikely possibility of a mixture of columnar stacks that contain
either just 6aCu or just 7aCu is excluded as it should generate a
35 more complex XRD pattern and the average d-spacing of the (10)
reflection should be at about 15.0 Å. Columnar mesophases of
115 which agrees with the observed overall flattening of the π–A
isotherm and the decreases in transition and collapse pressures by
12 mN m-1. However, the observed limiting area per molecule of
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151 Å2 at a surface pressure of 16.5 mN m-1 used for the cross-
linking experiment suggests a predominantly face-on orientation
of the macrocycles, which is expected to bring most azide and
alkyne groups in contact with the catalyst in the aqueous
subphase.
5
L-films of the 1:1 mixture on an aqueous solution of copper
(II) acetate and sodium ascorbate become insoluble in organic
solvents after about 1 hour for the concentrations given in Fig. 3.
Progress of the cross-linking of the TAPs by CuAAC was
10 monitored by the decrease in surface pressure at a constant area
and the point at which the surface pressure remains constant is
interpreted as the completion of the CuAAC reaction (after about
190 min for the example given in Fig. 3).
The occurrence of CuAAC and its completion was
15 independently confirmed by ATR-FTIR measurements on parts
of the Langmuir film removed from the water surface (Fig. 4).
Integrations of the overlapping azide/alkyne absorptions at 2100
cm-1 reveal an astonishing 78% conversion of all azide/alkyne
groups, which is 18% more than in the Colh mesophase and equal
20 to the maximum conversions obtained by CuAAC in solution.
Fig. 5 UV-Vis spectra of a 1:1 mixture of TAPs 6b and 7a as LB films
on quartz transferred right after spreading and after cross-linking by
CuAAC (0.638 x 10-3 M copper (II) acetate and 1.92 x 10-3 M sodium
ascorbate) for 200 minutes. A solution spectrum is shown for comparison.
60
Conclusions
Octaalkylthio substituted TAPs that contain either eight terminal
85 azide or acetylene groups can be prepared based on the
established Mg templated cyclization of maleodinitriles. Several
TAPs with shorter alkyl spacers display columnar mesophases
over surprisingly wide ranges of temperature. Overall, terminal
azide groups appear to be more compatible with self-organization
90 into columnar mesophases than terminal acetylene groups but
both groups are less compatible than terminal vinyl groups.
Demonstrated for the first time is the cross-linking of
Langmuir films by CuAAC and of columnar mesophases by
thermally activated azide-alkyne cycloaddition (AAC). A 1:1
135 mixture of azide and alkyne TAPs was successfully cross-linked
via CuAAC as Langmuir film to reach surprisingly high
conversions of almost 80% of all alkyne and azide groups within
3 hours. Thermally activated AAC in a columnar mesophase of a
1:1 mixture of azide and alkyne TAPs reached about 60%
140 conversion of all azide and acetylene groups after 48 hours. The
structure of the columnar mesophase is surprisingly little affected
by the slow cross-linking process while detailed structural studies
on the Langmuir films were not possible because cross-linked
films fractured and buckled when removed from the water
145 surface. This shortcoming may be overcome by the design of
more amphiphilic TAPs that contain fewer azide and acetylene
groups.
Fig. 4 IR spectra of LB films of a 1:1 mixture of TAPs 6b and 7a
transferred onto a KBr window before (dashed line) and after cross-
linking by CuAAC (0.638 x 10-3 M copper (II) acetate and 1.92 x 10-3
M
25 sodium ascorbate) for 200 minutes (solid line). The decrease of the N3
and C≡C stretching absorptions at 2100 cm-1 (arrow) indicates the
progress of the CuAAC reaction and was completed within 200 minutes.
UV-Vis spectra of cross-linked films transferred onto quartz
slides verify that the TAP macrocycle is not affected by the cross-
30 linking and remains metal-free (Fig. 5). The spectra also reveal a
red-shift of the Q-bands in the Langmuir and transferred films
that indicate a J-type aggregate structure before and after the
cross-linking process. However, the transfer of intact cross-linked
mono-layers has not yet been achieved; instead the films rupture
35 and buckle during transfer to generate 3-dimensional structures as
confirmed by AFM measurements on the transferred films (Fig.
S33 in ESI†).
Acknowledgements
MMA thanks the Libyan Ministry of Education for scholarship
90 awards and SHE acknowledges financial support by NSERC,
CFI, and OIT. HK thanks the University of Windsor for a partial
tuition scholarship.
Notes and references
a Department of Chemistry & Biochemistry, University of Windsor,
90 Windsor, Ontario, Canada. Fax: 519-973-7098; Tel: 519-253-3000
x3990; E-mail: eichhorn@uwindsor.ca
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Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C3TC31588F
Graphical contents entry
N3
N3
Cross-linking of discotic tetraazaporphyrins in
Langmuir films by “click” chemistry and in
columnar mesophases by thermally activated
azide-alkyne cycloaddition.
S
S
S
S
S
click
N
N
N
N
N
M
N
N
S
S
S
S
N
N
M
N
N3
S
S
S
N3
N3
N
N
N
N
N
N
S
N
N
N
S
S
S
N3
N3