One-dimensional porphyrin H-aggregates induced by solvent polarity

Article abstract of DOI:10.1016/j.tetlet.2008.09.140

A series of new porphyrin derivatives possessing the side arms of alkyl-substituted thiophene oligomer were synthesized. The effects of solvent polarity on the formation of supramolecular assembly have been studied by UV-vis absorption, fluorescence emiss

Full text of DOI:10.1016/j.tetlet.2008.09.140

Tetrahedron Letters 49 (2008) 7050–7053
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
One-dimensional porphyrin H-aggregates induced by solvent polarity
*
Myung-Seok Choi
Department of Materials Chemistry and Engineering, Konkuk University, Seoul 143-70, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new porphyrin derivatives possessing the side arms of alkyl-substituted thiophene oligomer
were synthesized. The effects of solvent polarity on the formation of supramolecular assembly have been
studied by UV–vis absorption, fluorescence emission, and TEM measurement. The linear-shape porphyrin
derivatives bearing two thiophene pentamers at meso-position showed the significant spectral changes in
both blue-shift and band broadening of Soret bands which indicate the formation of a relatively larger
porphyrin H-aggregate that occurred in nonpolar solvent such as n-hexane.
Received 7 August 2008
Revised 24 September 2008
Accepted 25 September 2008
Available online 30 September 2008
Keywords:
Porphyrin
Ó 2008 Elsevier Ltd. All rights reserved.
H-Aggregates
p–p Stacking
Supramolecular assembly
Supramolecular assemblies of
p-conjugated systems are of
alkyl-substituted thiophene oligomer. In this synthetic strategy,
increasing interest in recent years due to its potential applications
for optoelectronic and photovoltaic devices.1 Recently, much
efforts have been made on porphyrin assembly which is one of
the promising candidates for light-driven energy transduction sys-
tems, mimicking photophysical processes of photosynthetic organ-
isms.2–6 Two-dimensional character of porphyrin molecule with 22
thiophene oligomers play an important role in enhancing
interactions to form porphyrin supramolecular aggregates.
p–p
Porphyrin derivatives 9, 10, and 11 were prepared by using a
fundamental building block 5, which was synthesized from the
coupling of hexanoyl chloride to 2,20:60,200-terthiophene (1) to af-
ford 5-hexanoyl-2,20:50,200-terthiophene (2), followed by reduction
by LiAlH4/AlCl3 to afford 5-hexyl-500-2,20:50,200-terthiophene (3).
Compound 3 is converted into 5-hexyl-tributylstannyl-2,20:50,200-
terthiophene (4), and 500-hexyl-2,20:50,200-terthiophene-5-carboxal-
dehyde (5) using BuLi/Bu3SnCl and POCl3/DMF, respectively
(Scheme 1). Compound 8 was synthesized by condensation of
5-bromo-2,20-bithiophene-50-carboxaldehyde together with meso-
pentyldipyrrolmethane (7). Finally, conventional synthetic pro-
cesses of porphyrin have been performed to afford 9 and 10 via
acid-catalyzed condensation and oxidation reactions.18 Stille-type
palladium(0)-catalyzed reaction19 was carried out to give 11 by
coupling of 4 with porphyrin derivative 8. The porphyrin deriva-
tives and other molecules were purified by recycling preparative
size-exclusion chromatography and were fully characterized by
means of 1H NMR and UV–vis spectroscopy, together with
MALDI-TOF-MS.20
p
-electrons enables strong p–p
interaction,7 facilitating the forma-
tion of one-dimensional nanostructures such as J- and H-aggre-
gates via side-by-side and face-to-face stacking, respectively.
Recent reports clearly show that self-assembly via strong
p–p
stacking is a useful method to form one-dimensional supramolec-
ular systems, consisting of hexabenzocoronene,8 perylenediimide
derivative,9 and regioregular polythiophenes.10 To date, a variety
of supramolecular aggregates of porphyrins have been studied by
employing the auxiliary intermolecular forces or functionality,
reinforcing the usual
tors,11,12 metal–ligand coordination,13 LB film,14 surfactant,15 bi-
cyclic guanidine,16 and ( )-prolinium moiety.17 However, there
are few one-dimensional porphyrin H-aggregates in organic sol-
vents using purely interactions.
p–p interactions, which are H-bonding gela-
L
p–p
Porphyrin H-aggregates have potential advantages for unidirec-
tional transport of excitation energy and electric charge along the
axis of molecular stacking, which play crucial roles in electronic
devices such as organic solar cell and field-effect transistor.
Herein, we report on the solvent-induced porphyrin H-aggre-
gates where porphyrin derivatives possess the side arms of
To investigate the effect of solvent polarity on absorption spec-
tral characteristic, 10 ll of high concentrated CHCl3 solution of 11
is added to 5 ml of CHCl3, toluene, and n-hexane, respectively,21
where the concentration of each solution is adjusted to 3 ꢀ
10ꢁ6 M. The UV–vis spectrum of 11 in CHCl3 showed an intense
Soret band with an absorption maximum at 431.5 nm, together
with relatively weak Q-band around 560.5 and 615.0 nm (Fig. 1).
We found that a stable colloidal dispersion of 11 could be obtained
upon decreasing polarity of solvent, from CHCl3 (kSoret = 431.5 nm,
full width at half-maximum (FWHM) = 1916 cmꢁ1) to n-hexane
* Tel.: +82 2 450 4270; fax: +82 2 444 3490.
E-mail address: mchoi@konkuk.ac.kr
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.140

M.-S. Choi / Tetrahedron Letters 49 (2008) 7050–7053
7051
BuLi
CH3(CH2)3SnCl
CH3(CH2)4COCl
SnCl2
LiAlH4, AlCl3
O
S
S
S
S
S
S
R2
R2
S
Bu3Sn
S
S
S
S
S
CH2Cl2
70%
toluene
73%
THF
100%
1
2
3
4
POCl3, DMF
S
S
R2
OHC
R1
S
C2H5Cl2
72%
5
S
Br
CHO
p-chloranil Zn(O2CCH3)2
S
R1
C5H11CHO, TFA
30%
N
N
N
TFA,
S
S
Br
Zn
R1
Br
S
N
N
H
N
H
S
CHCl3
46%
H
N
6
7
8
TFA, -chloranil
p
TFA,
p
-chloranil
CHCl3
5
Pd(PPh3)4
toluene
14%
5
CHCl3
9%
30%
4
Zn(O2CCH3)2
Zn(O2CCH3)2
3
R2
R1
R1
Zn
R1
S
N
N
N
S
N
N
N
N
S
S
S
R2
R2
Zn
R1
R2
R2
5
5
N
3
3
N
N
S
R1 = C5H11
R2 = C6H13
Zn
R2
R2
S
10
11
3
N
N
3
9
S
3
R2
Scheme 1. Synthetic routes for porphyrin derivatives 9, 10, and 11.
but was also slightly broadened (D
FWHM = 421 cmꢁ1) compared
to a CHCl3 solution (kSoret = 447.5 nm, FWHM = 2189 cmꢁ1) (S4).
According to exciton theory,22 these observations, including blue-
shift and band broadening of Soret band, indicate a characteristic
of the formation of H-aggregates of porphyrin derivatives 9, 10,
and 11. A blue-shift of Soret band is consistent with a parallel
stacking of the porphyrin molecules denoted as H-aggregate, due
to excitonic coupling of the electronic transitions in the porphyrin
p
-systems,23 whereas the red-shift is a sign of J-aggregation forma-
tion.24 To be highlighted, significant spectral changes of both kSoret
and FWHM for 11 indicate that the formation of a relatively larger
H-aggregate occurred in nonpolar solvent such as n-hexane than 9
and 10.
For a further insight into the solvent-induced H-aggregation of
11, Soret absorption bands were monitored at different CHCl3/n-
hexane ratios, where when the content of n-hexane increased from
0% to 80%, the blue-shift gradually increased (431.5 nm for 0%,
430.5 nm for 20%, 429.5 nm for 40%, 427.5 nm for 60%, and
427 nm for 80%), and when it reached 100% (424 nm) a dramatic
increase was observed (inset A in Fig. 2). A similar trend of broad-
ening of Soret band was also observed, for which FWHM increase
with the content of n-hexane to be 2211 cmꢁ1 for 0%, 2304 cmꢁ1
for 20%, 2374 cmꢁ1 for 40%, 2501 cmꢁ1 for 60%, 3038 cmꢁ1 for
80%, and 6817 cmꢁ1 for 100% (inset B in Fig. 2).
Figure 1. UV–vis spectra of 11 (3.0 ꢀ 10ꢁ6 M) in CHCl3, toluene, and n-hexane. Inset
shows blue-shift of Soret absorption peaks in each solvent.
(kSoret = 424.0 nm, FWHM = 6817 cmꢁ1
430.5 nm, FWHM = 2051 cmꢁ1), where the Soret band in n-hexane
was significantly both blue-shifted ( kSoret = 7.5 nm) and broad-
ened (
FWHM = 4766 cmꢁ1) relative to that of CHCl3 (inset in
)
via toluene (kSoret =
D
D
Fig. 1). The Q-bands in the visible region for 11 were red-shifted
to 575.5 and 620.5 nm in n-hexane. The n-hexane solution of 10
(kSoret = 427.0 nm, FWHM = 1781 cmꢁ1) also showed similar blue-
Upon excitation at k = 400.0 nm in CHCl3, fluorescence of 11
showed the emission at 634 nm predominantly from the zinc por-
phyrin (Fig. 3A). However, the n-hexane solution showed a rather
weak and blue-shifted emission at 609 nm by porphyrin, where
the relatively large emissions at 506 and 537 nm were newly pro-
duced which may be attributed to the excitation wavelength scat-
tered by hundreds of rod-like architecture of H-aggregates. It is
noted that the emission at 506 and 537 nm does not result from
shift (DkSoret = 7.0 nm) as large as 11 but was hardly broadened
(D a CHCl3 solution (kSoret =
FWHM = 37 cmꢁ1), compared to
434.0 nm, FWHM = 1744 cmꢁ1) (S4). On the other hand, the Soret
band of the n-hexane solution of 9 (kSoret = 444.5 nm, FWHM =
2610 cmꢁ1) was not only slightly blue-shifted (
DkSoret = 3 nm)

7052
M.-S. Choi / Tetrahedron Letters 49 (2008) 7050–7053
porphyrin molecules, conformational transition of 11 undergo
from random phase in polar solvent to a more ordered H-aggregate
in order to decrease the unfavorable interaction between the sol-
vent and aromatic main moiety.
The most directed evidence for the formation of large one-
dimensional H-aggregates was given by transmission electron
Figure 2. UV–vis spectra of 11 (7.5 ꢀ 10ꢁ6 M) in a mixture of CHCl3/n-hexane. Inset
A and B show changes of Soret absorption peaks and FWHM, respectively, upon the
content of n-hexane in the solvent mixtures.
Figure 3. Fluorescence spectra of 10 (B) and 11 (A) (3.0 ꢀ 10ꢁ6 M) in CHCl3 (broken
line) and n-hexane (solid line), upon excitation at Soret absorption peak.
side arms of oligothiophene because a CHCl3 solution shows no
fluorescence emission at same wavelengths (Fig. 3B). In sharp con-
trast, 10 emitted mainly red color of fluorescence both in CHCl3
(k = 634 nm) and n-hexane (k = 620 nm) without emission at short-
er wavelength, indicating the formation of smaller H-aggregates
than that of 11.
From these results, it was revealed that the formation of supra-
molecular H-aggregates depends on the morphology of porphyrin
derivatives for 9 (star-type) and 11 (linear-type), and on the length
of oligothiophene units, attached to a meso-position of zinc por-
phyrin, for 10 (thiophene trimer) and 11 (thiophene pentamer).
In nonpolar solvent such as n-hexane, which is a good solvent for
alkyl side chains and a poor solvent for oligothiophene-appended
Figure 4. TEM images of 9 (C), 10 (B), and 11 (A), deposited onto the carbon-coated
copper grid from n-hexane solution containing a small amount (0.02%) of CHCl3.

M.-S. Choi / Tetrahedron Letters 49 (2008) 7050–7053
7053
microscope (TEM). Samples for the TEM experiment were prepared
by drop-casting from the n-hexane solution (1.0 ꢀ 10ꢁ7 M) of por-
phyrin derivatives onto a carbon-coated copper grid. TEM images
taken without a staining reagent are shown in Figure 4 for 9, 10,
and 11. We clearly find rod-type supramolecular architectures
for 11 with 100–500 nm in length and 40–60 nm in diameter in
Figure 4A. We suggest that the large dimension of 1D aggregates
may result from weakly intercalated porphyrin H-aggregates via
van der Waals forces between alkyl chains (Fig. 4A), which is well
consistent with a significant absorption spectral change in n-hex-
ane. For sharp comparison, the TEM images of 9 and 10 were
recorded where spherical particles of 15–30 nm in diameter for
10 were entangled continuously (Fig. 4B). However, it is hard to
see a stable ordered assembly structure for 9 (Fig. 4C). From mor-
phology and size of structures in TEM images, significant changes
in absorption spectra together with fluorescence emission profiles
were reflected in the formation of large H-aggregates of 11 in
n-hexane.
data associated with this article can be found, in the online version,
at doi:10.1016/j.tetlet.2008.09.140.
References and notes
1. Schenning, A. P. H. J.; Meijer, E. W. Chem. Commun. 2005, 3245.
2. Choi, M.-S.; Aida, T.; Yamazaki, I.; Yamazaki, T. Angew. Chem., Int. Ed. 2004, 43,
150.
3. Choi, M.-S.; Aida, T.; Luo, H.; Araki, Y.; Ito, O. Angew. Chem., Int. Ed. 2003, 42,
4060.
4. Choi, M.-S.; Aida, T.; Yamazaki, I.; Yamazaki, T. Chem. Eur. J. 2002, 8, 2667.
5. Choi, M.-S.; Aida, T.; Yamazaki, I.; Yamazaki, T. Angew. Chem., Int. Ed. 2001, 40,
194.
6. Luo, H.; Choi, M.-S.; Araki, Y.; Ito, O.; Aida, T. Bull. Chem. Soc. Jpn. 2005, 78, 405.
7. van de Craats, A. M.; Warman, J. M. Adv. Mater. 2001, 12, 130.
8. Nguyen, T.-Q.; Martel, R.; Avourious, P.; Bushey, M. L.; Brus, L.; Nuckolls, C. J.
Am. Chem. Soc. 2004, 126, 5234.
9. Ahrens, M. J.; Sinks, L. E.; Rybtchinski, B.; Liu, W.; Jones, B. A.; Giaimo, J. M.;
Gusev, A. V.; Goshe, A. J.; Tiedo, D. M.; Wasielewski, M. R. J. Am. Chem. Soc. 2004,
126, 8284.
10. Kiriy, N.; Jahne, E.; Adler, H.-J.; Schneider, M.; Kiriy, A.; Gorodyska, G.; minko,
S.; Jehnichen, D.; Simon, P.; Fokin, A. A.; Stamm, M. Nano Lett. 2003, 3, 707.
11. Shirakawa, M.; Kawano, S.-I.; Fujita, N.; Sada, K.; Shinkai, S. J. Org. Chem. 2003,
68, 5037.
12. Shirakawa, M.; Fujita, N.; Sada, K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 4164.
13. Terech, P.; Scherer, C.; Demé, B.; Ramasseul, R. Langmuir 2003, 19, 10641–
10647.
In summary, porphyrin derivatives bearing thiophene oligomer
were newly synthesized and the formation of 1D supramolecular
assembly has been studied by means of UV–vis absorption, fluores-
cence spectroscopy, and TEM measurement. Porphyrin derivative
11, bearing two pentameric thiophenes at meso-position of
porphyrin molecule, has been driven to form a relatively large
14. Li, C.; Imae, T. Langmuir 2003, 19, 779.
15. Maiti, N. C.; Mazumdar, S.; Periasamy, N. J. Phys. Chem. B 1998, 102, 1528.
16. Král, V.; Schmidtchen, F. P.; Lang, K.; Berger, M. Org. Lett. 2002, 4, 51.
17. Monti, D.; Venanzi, M.; Mancini, G.; Di Natale, C.; Paolesse, R. Chem. Commun.
2005, 2471.
18. Lee, C.-H.; Lindsey, J. S. Tetrahedron 1994, 50, 11427.
19. Labadie, J. W.; Stille, J. K. J. Am. Chem. Soc. 1983, 105, 6129.
20. Full experimental details are provided as Supplementary data.
21. Solvent polarity index obtained from Phenomenex catalog is known to be 0 for
n-hexane, 2.4 for toluene, and 4.1 for CHCl3.
22. (a) Kasha, M.; Rawls, H. R.; El Bayoumi, A. Pure Appl. Chem. 1965, 11, 371; (b)
Valdes-Aguilera, O.; Neckers, D. C. Acc. Chem. Res. 1989, 22, 171; (c) Ojeda, P. R.;
Amashta, I. A. K.; Ochoa, J. R.; Arbeloa, I. L. J. Chem. Soc., Faraday Trans. 2 1988,
84, 1; (d) Hessemann, J. J. Am. Chem. Soc. 1980, 102, 2167; (e) Mooney, W.;
Brown, P. E.; Russel, J. C.; Costa, S. B.; Pederson, L. G.; Whitten, D. G. J. Am. Chem.
Soc. 1984, 106, 5659.
H-aggregate by solvophobic interactions. Such
a stable 1D
H-aggregate of porphyrins is a promising building block for future
nanoscale and for molecular electronic devices such as single-
electron transistors.
Acknowledgment
This work was supported by the faculty research fund of
Konkuk University in 2007.
23. Adachi, M.; Yoneyama, M.; Nakamura, S. Langmuir 1992, 8, 2240.
24. Kano, K.; Fukada, K.; Wakami, H.; Nishiyabu, R.; Pasternack, R. F. J. Am. Chem.
Soc. 2000, 122, 7494.
Supplementary data
Synthetic methods and characterization data (1H NMR, MALDI-
TOF-MS, and UV–vis) for compounds are available. Supplementary