Synthesis of alkyl-substituted, strapped porphyrin to prepare stable alkyl-chain-assisted self-assembled monolayers of porphyrin conjugates

Article abstract of DOI:10.1246/cl.2004.1418

An alkyl-substituted, strapped porphyrin was synthesized aiming at the preparation of stable alkyl-chain-assisted self-assembled monolayers (SAMs) of porphyrin conjugates. We confirmed that the synthesized strapped porphyrin could form stable SAMs by scanning tunneling microscopy (STM). This approach will lead to the functionalization of alkyl-chain-assisted SAMs by introducing a variety of functional groups in the strapped moiety.

Full text of DOI:10.1246/cl.2004.1418

1418
Chemistry Letters Vol.33, No.11 (2004)
Synthesis of Alkyl-substituted, Strapped Porphyrin to Prepare Stable Alkyl-chain-assisted
Self-assembled Monolayers of Porphyrin Conjugates
Taichi Ikeda,ꢀ Masumi Asakawa,ꢀ Koji Miyake,y and Toshimi Shimizuꢀ
Nanoarchitectonics Research Center (NARC), National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565
yAdvanced Manufacturing Research Institute (AMRI), AIST, 1-2-1 Namiki, Tsukuba, Ibaraki, 305-8564
(Received July 29, 2004; CL-040894)
An alkyl-substituted, strapped porphyrin was synthesized
aiming at the preparation of stable alkyl-chain-assisted self-as-
sembled monolayers (SAMs) of porphyrin conjugates. We con-
firmed that the synthesized strapped porphyrin could form stable
SAMs by scanning tunneling microscopy (STM). This approach
will lead to the functionalization of alkyl-chain-assisted SAMs
by introducing a variety of functional groups in the strapped
moiety.
The strapped porphyrin 10 was synthesized via condensa-
tion of the alkyl-substituted dialdehyde (6) and alkyl-substituted
dipyrromethane derivative (9) (Figure 1). The alkylation of
2,4-dihydrobenzaldehyde (1) with stearylbromide (C18H37Br)
(molar ratio: 1/C18H37Br = 2/1) gave the product 2. 1 N NaOH
aqueous solution containing 3 and 2-chloroethanol (molar ratio:
3/2-chloroethanol = 1/5) was refluxed for 24 h under dark to
obtain the diol (4). After the tosylation of 4, the dialdehyde 6
was obtained from the reaction of 2 and 5 in DMF (molar ratio:
2/5 = 2/1) in the presence of CsCO3 at 110 ꢁC. The aldehyde 8
was treated with pyrrole in the presence of catalytic amount
of TFA for 1 h under dark and nitrogen atmosphere to get the
product 9.
Two-dimensional (2-D) self-assembly on a substrate has at-
tracted much attention to arrange functional molecular devices
with a regular pattern.1 To date, SAMs of organothiol com-
pounds on a noble metal (organothiol SAMs) are the most exten-
sively studied SAMs to immobilize functional molecules.2 In the
organothiol SAMs, however, the control of the distance between
the molecules or the symmetry of the molecular pattern is now
unattainable. On the other hand, SAMs of alkyl-substituted com-
pounds (alkyl-chain-assisted SAMs) have boomed to create pe-
riodic molecular patterns on a substrate (graphite3 or gold4) and
these molecular patterns were analyzed by means of STM.
Recently, some groups reported 2-D array of 5,10,15,20-tetra-
(4-alkyloxyphenyl) porphyrin (CnOPP, n: number of the carbon
atoms in an alkyl chain).5 Since the porphyrins or metallopor-
phyrins are attractive materials for photosynthesis,6 optoelec-
tronic devices,7 and catalysts,8 the well-ordered molecular array
of functionalized porphyrins on the substrate would be of great
importance in the fields of nanoscience and nanotechnology.
On the basis of this perspective, we have studied molecular ar-
rangement of the porphyrin conjugates composed of CnOPP
and a functional molecule. In the previous study, we confirmed
SAMs of the pyridine-coordinated C30OPP rhodium chlorides,
in which the pyridine molecule bound to the metalloporphyrin
through axial-coordination bond (axial-coordination approach).9
In this letter, we describe the covalently linked approach as an
alternative to the axial-coordination approach. In the covalently
linked approach, the functional moiety was covalently linked to
CnOPP. Therefore, we synthesized an alkyl-substituted, strapped
porphyrin and confirmed the formation of stable SAMs of the
synthesized strapped porphyrins.
The condensation reaction between the dialdehyde 6 and
dipyrrolylmethane derivative 9 was conducted in CH2Cl2 with
For the simple organic synthesis, hydroquinone was cova-
lently linked to C18OPP. We considered that the strapped por-
phyrin 10 (Figure 1) would be superior to the non-strapped
C18OPP derivatives, because the strapped part hinders free rota-
tion of the phenyl group of C18OPP not only in the solution but
also on the surface. Thus, the strapped porphyrin 10 has no un-
favorable atropisomer for molecular adsorption, which means
one porphyrin face is always free from the strapped moiety.
Figure 1. Reaction scheme for the synthesis of strapped por-
phyrin 10. Reagents and conditions: (i) C18H37Br, KI, K2CO3,
butanone, reflux, 57%; (ii) 2-chloroethanol, NaOH, water, under
dark, reflux, 60%; (iii) Ts-Cl, pyridine, DMAP, THF, N2, rt,
72%; (iv) CsCO3, DMF, 110 ꢁC, 70%; (v) C18H37Br, K2CO3,
DMF, reflux, 85%; (vi) pyrrole, TFA, under dark, rt, 55%;
.
(vii–i) BF3 OEt, TFA, CH2Cl2, N2, under dark, rt; (vii–ii)
DDQ, reflux, 8%.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.11 (2004)
1419
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BF3 OEt2 and TFA catalysis for 3 h under dark and nitrogen at-
one. We believe that the porphyrin free face, not hydroquinone
face, should attach to the graphite surface. The entropic contri-
bution to the adsorption free energy is calculated to be ca.
1.7 kJ molꢄ1 (¼ RT ln 2, T ¼ 293 K) from the difference in sym-
metry number.13 In the case of pyridine-coordinated rhodium
chloride with the same alkyl chain length (C18), we had no mo-
lecular image.9 Therefore, the covalently linked approach using
the strapped porphyrin 10 is more attractive than the axial-coor-
dination approach from the viewpoint of the formation of stable
alkyl-chain-assisted SAMs.
This protocol will lead to the functionalization of alkyl-
chain-assisted SAMs by introducing the functional molecule in
the strapped moiety. The molecular design of the strapped moie-
ty is considered to be free since the structure of the opposite side
(porphyrin free face) is always the same. 2-D array of the func-
tional porphyrin conjugates is expected to exhibit the novel at-
tractive characteristics. The detailed STM characterization of
the strapped porphyrin 10, characterization of SAMs as a func-
tional device, and further functionalization of the strapped por-
phyrin are now in progress.
mosphere. After the addition of DDQ, the solution was refluxed
for 1 h under dark. After cooling to room temperature, the solu-
tion was directly adsorbed to dry silica gel and eluted the crude
product by CHCl3. The recovered product was purified by silica
gel chromatography twice [First: CHCl3/hexane = 7/3 (v/v),
Second: hexane/AcOEt = 10/1 (v/v)]. The product 10 was
confirmed by H, 13C NMR, MS, and elemental analysis.10
1
SAMs of the strapped porphyrin 10 were confirmed by
STM. STM observations were conducted in a droplet of the sam-
ple solution (solvent: dichlorobenzene) on a fleshly cleaved
graphite surface. Figure 2a is a representative STM image of
SAMs of the strapped porphyrin 10. We were able to confirm
the characteristic two-fold symmetry structure.5 The bright spots
aligned in lines and the dark stripes correspond to the strapped
porphyrin moiety and the closely packed alkyl chains on the
graphite surface, respectively. The lattice parameters of the unit
cell a ꢂ b and ꢀ (Figure 2c) were 3:94 ꢃ 0:1 ꢂ 2:01 ꢃ 0:1 nm
and 101 ꢃ 5ꢁ. As for the proposed structure of SAMs shown
in Figure 2c, the lattice parameters were 3:96 ꢂ 2:02 nm and
104.8ꢁ (a ꢂ b and ꢀ). These values are well consistent with
the observed ones.
This study was supported by the Industrial Technology
Research Grant Program of the New Energy and Industrial
Technology Development Organization (NEDO) of Japan.
References and Notes
1
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Figure 2. (a) STM image of self-assembled monolayers con-
sisting of strapped porphyrin 10 on a graphite surface (10 ꢂ
10 nm2). Solvent: dichlorobenzene. Tunneling conditions: I ¼
200 pA, V ¼ ꢄ1000 mV. (b) Partial molecular structure of the
strapped porphyrin 10 (MM2 calculation). (c) Proposed structure
of SAMs consisting of the strapped porphyrin 10. Each molecu-
lar structure was optimized using MM2 calculation.
8
9
10 1H NMR (400 MHz, CDCl3): ꢁ ꢄ2:63 (s, NH), 0.88 (t, CH3), 1.20–1.55
(m, CH2), 1.62 (m, CH2), 1.97 (m, CH2), 2.77 (br, CbH2), 3.62 (s,
CaH), 3.81 (br, CcH2), 4.24 (m, OCH2), 6.79 (s, CdH), 7.03 (d,
J ¼ 8:4 Hz, CeH), 7.20, 7.25 (br, CgH), 7.87, 8.22 (br, ChH), 8.37
(d, J ¼ 8:4 Hz, CfH), 8.83 (m, CiH); 13C NMR (100 MHz, CDCl3): ꢁ
14.6, 23.2, 26.7, 29.9, 30.0, 30.1, 30.2, 32.4, 68.8, 69.0, 69.1, 69.6,
101.6, 105.6, 113.1, 115.4, 115.8, 119.6, 125.3, 131 (br), 134.9,
135.8, 136.2, 151.0 (br), 152.8, 159.3, 160.8, 161.2; MS (MALDI):
m=z: 1881.8 ½Mꢅþ; C126H184N4O8 (1882.8): calcd C 80.38, H 9.85,
N 2.98; found C 80.74, H 9.80, N 2.71%.
11 This value is consistent with that for the crystal structure of the capped
porphyrin. See: M. R. Johnson, W. K. Seok, and J. A. Ibers, J. Am.
Chem. Soc., 113, 3398 (1991).
12 ‘‘Scanning Probe Microscopy and Spectroscopy: Method and
Application,’’ ed. by R. Wiesendanger, Camblidge University Press,
New York (1994).
From the section analysis of STM image, the average appa-
rent height of the strapped porphyrin 10 was 0:57 ꢃ 0:05 nm,
which is clearly more than that of C18OPP (0:20 ꢃ 0:03 nm).
In the molecular structure calculated by MM2 (Figure 2b), the
average distance between the porphyrin (pyrrole and meso car-
bons) and strapped benzene planes was 0.38 nm.11 However,
we must consider the fluttering of the strapped moiety and the
interaction between the strapped benzene and porphyrin. In prin-
ciple, the apparent STM height is not comparable to the real
height of the molecule.12
In compared to C18OPP, the molecular adsorption of the
strapped porphyrin 10 is entropically unfavorable owing to less
molecular symmetry. Namely, C18OPP has two equivalent faces
to adsorb to the surface while the strapped porphyrin 10 has only
13 W. F. Bailey and A. S. Monahan, J. Chem. Educ., 55, 489 (1978).
Published on the web (Advance View) October 2, 2004; DOI 10.1246/cl.2004.1418