Near IR absorbing planar aromatic [34]octaphyrins(1.1.0.1.1.0.0.0) containing a quaterthiophene subunit

Article abstract of DOI:10.1039/b614290g

The synthesis and structural characterization of the first examples of planar aromatic core modified [34]octaphyrins(1.1.0.1.1.0.0.0) with three different heteroatoms containing a quaterthiophene subunit are reported. The Royal Society of Chemistry.

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Near IR absorbing planar aromatic [34]octaphyrins(1.1.0.1.1.0.0.0)
containing a quaterthiophene subunit{{
Rajeev Kumar,a Rajneesh Misra,a Tavarekere K. Chandrashekar*ab and Eringathodi Sureshc
Received (in Cambridge, UK) 2nd October 2006, Accepted 24th October 2006
First published as an Advance Article on the web 14th November 2006
DOI: 10.1039/b614290g
The synthesis and structural characterization of the first
examples of planar aromatic core modified [34]octaphyr-
ins(1.1.0.1.1.0.0.0) with three different heteroatoms containing
a quaterthiophene subunit are reported.
Expanded porphyrins continue to attract the attention of chemists
and material scientists because of their diverse applications such as
PDT, MRI contrasting agents, neutral substrate binding, anion
recognition, and as NLO materials.1,2,9 Octaphyrins are a class of
expanded porphyrins in which eight pyrrole/heterocyclic rings are
connected to each other through meso carbon bridges as in 1–5,3–7
or through direct pyrrole–pyrrole link as in 6 (Fig. 1).8 Octaphyrins
exhibit different conformations such as figure-eight, twisted or
planar depending on the nature of linkage, number of meso carbon
bridges as well as the steric bulk of the meso aryl substituents.
Adaptation of twisted or figure eight conformations leads to loss
of aromaticity. Thus development of methods to synthesize planar,
aromatic octaphyrin is still a synthetic challenge. Recently we were
successful in synthesizing aromatic, planar octaphyrins6 by: (a)
replacing the few pyrrole rings by heavier heterocyclic rings such as
Fig. 1 Octaphyrins known in literature.
thiophene or selenophene in the core; and (b) by increasing steric
bulk on the meso aryl substituents to avoid the twisting of the
macrocycle. These core modified octaphyrins exhibit ring inver-
sions, where one or more pyrrole/heterocyclic rings are inverted
and this ring inversion depends on the nature of the meso
substituents and nature of heteroatom. In continuation of this
strategy of synthesizing aromatic, planar octaphyrins, in this
communication we wish to examine the effect of changes in the
steric bulk of the meso substituents and the increase in the number
of heteroatoms on the ring inversions and the conformation of the
octaphyrins.
reaction of 7 with 8–11, under acid-catalyzed condensation with
p-toluenesulfonic acid (PTSA) as catalyst, followed by chloranil
oxidation leads to formation of the desired octaphyrins 12–15
(Scheme 1). Reduction in the steric bulk in the precursor 8–11,
from mesityl to anisal and phenyl did not change the conformation
of the octaphyrins, suggesting the two meso mesityl substituents
are good enough to prevent twisting at the meso position, therefore
avoiding the formation of figure-eight conformation. The
advantage of this methodology is the formation of a single
product in good yields (18–20%), thus, avoiding tedious chroma-
tographic separations.
Our synthetic strategy involved preparation of
a novel
quaterthiophene fragment; 5,5--bis(mesitylhydroxymethyl)-
The proposed structure of the macrocycle comes from the
various spectroscopic analysis and the single-crystal X-ray
structure obtained for 12 (see ESI{). The FAB mass spectra show
peak at m/z = 1238.70 for 12, and m/z = 1144.25 for 13, confirming
the free base composition of these macrocycles. The UV/Vis
absorption spectrum of 12 in dry methylene chloride is
characterized by the presence of an intense Soret-band [578 nm
(log e = 4.98)] and Q-bands [766 nm (4.44) and 842 nm (4.70)] into
the near IR region and interesting results were obtained on careful
protonation with TFA. At less acid concentration (1 eq.), there
were three distinct bands at 650 nm (log e = 4.02), 757 nm (2.35),
and 1090 nm (2.13). This absorption spectrum represents the
diprotonated form of 12 (Fig. 2). Further addition of TFA results
in gradual increase in absorption at 650 and 1090 nm at the
expense of absorption at 757 nm. This changes are typical of
2,59 : 29,20 : 50,2--quaterthiophene, 7 as one of the precursors
and the modified tetrapyrranes (8–11) in which meso substituents
were varied from bulkier mesityl group to less bulkier anisal and
phenyl groups, as the other precursor. Further, X can be either S
or Se, leading to increase in the number of the heteroatoms. Thus,
aDepartment of Chemistry, Indian Institute of Technology, Kanpur,
208016, India. E-mail: tkc@iitk.ac.in
bDirector, Regional Research Laboratory, Trivandrum, 695019, Kerala,
India
cCentral Salt and Marine Chemicals Research Institute, Bhavnagar,
Gujarat -, 364 002, India
{ The HTML version of this article has been additionally enhanced with
colour images.
{ Electronic supplementary information (ESI) available: Details of
experimental procedures, characterization of compounds 7 and 12–15.
See DOI: 10.1039/b614290g
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Chem. Commun., 2007, 43–45 | 43

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Scheme 1 Synthesis of octaphyrins by [4 + 4] acid catalyzed condensation.
which suggests it has a C2 symmetric conformation in CDCl3.
Both the selenophene rings opposite to the tetrathiophene unit are
inverted in free base as well as in protonated forms. Very
interesting results were obtained when trifluoroacetic acid (TFA)
was added stepwise to a solution of free base octaphyrin 12 in
CDCl3. The degree of inversion of the selenophene rings was
found to be dependent on TFA concentration. In the freebase
form the b-CH protons of these inverted rings resonate at 5.57 and
4.05 ppm. On addition of 10 ml TFA solution, (10% TFA in
CDCl3) these b-CH protons resonate at 0.56 and 0.28 ppm and
this upfield shift increases further as more TFA was added. Finally
at excess TFA concentration (50 ml) these b-CH protons resonate
at 21.73 and 22.54 ppm and the NH protons resonate at
24.11 ppm (Fig. 3). These observations suggest that after adding
TFA, biselenophene unit comes into plane of the macrocycle and
macrocycle becomes more palnar. Dd value of 16.10 ppm supports
a strong diatropic ring current, inferring their aromatic nature.
Additional evidence for the aromatic character comes from the
cyclic voltammetric studies. Octaphyrins 12–15 exhibit two
reversible reductions and two reversible oxidations (see ESI{).
An estimated HOMO–LUMO gap of 1.09 V for 14 indicates a
significant reduction relative to meso aryl sapphyrin (1.88 V),
rubyrin (1.64 V), and heptaphyrin (1.38 V), thus explaining large
red shifts observed in the electronic spectrum. A linear correlation
obtained (Fig. 4) when the energy of the Soret maximum and
HOMO–LUMO gap (Dredox) were plotted versus increase in p
electrons in the delocalization pathway, gives a strong evidence of
the aromatic nature of 12–15.
Fig. 2 UV/Vis- near IR absorption spectra of 12 (- - - -) [7.73 6 1026
and the protonated derivative (—) in CH2Cl2.
]
binding of anion to the macrocycle. A similar behaviour was
observed for octaphyrins 12–15, suggesting that these macrocycles
bind trifluoroacetate anion in the diprotonated state.
Detailed 1H and 2D NMR studies provide important informa-
tion about the conformation and aromaticity of these octaphyrins.
Specifically, the 1H NMR spectrum of 12 (Fig. 3) taken in CDCl3
revealed eight sets of doublets between 8.75 and 4.05 ppm,
integrating to two protons each, observed for b-CH protons,
1
Fig. 3 TFA concentration dependent H NMR spectra of 12 in CDCl3. Amounts of TFA added, are shown in spectra.
44 | Chem. Commun., 2007, 43–45
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macrocycle. These subtle variations in the structure can play a
major role in the resulting conformation of the octaphyrin leading
to change in the electronic structure and aromaticity. Systematic
studies of these variations on the structure property corelations are
underway.
Notes and references
§ Crystallographic summary for 12: C72H60N2S4Se2, M = 1239.42. Crystals
were grown by slowly diffusing dry n-hexane over a chloroform solution of
12. Blue rectangular, triclinic, P-1, m = 1.134 mm21, Z = 4 in a cell of
˚
˚
˚
dimensions: a = 20.276(4) A, b = 21.240(4) A, c = 21.313(4) A; a =
,
F(000) = 2552. A total of 26051 independent reflections were measured on
81.06(3)u, b = 62.88(3)u, c = 66.62(3)u, V = 7495(3)A , rcal = 1.098 Mg m23
3
˚
a CCD area detector using graphite monochromatized Mo Ka radiation
˚
2
(l = 0.71073 A) at 2173 uC. The structure was refined on F to an Rw =
0.1411, with a conventional R = 0.0614, and a goodness of fit = 0.920 for
1465 parameters using the SHELXTL10 package. CCDC 622228. For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
b614290g.
Fig. 4 Variation of energy of the Soret band(&) and HOMO–LUMO
gap (.) with increasing number of p electrons.
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Fig. 5 Single crystal X-ray structure of 12, top view (a) and its side view
(b). Meso substituents in side view are removed for clarity and only one of
the two independent molecules in the asymmetric unit of 12 is shown.
4 M. Bro¨ring, J. Jendrny, L. Zander, H. Schmickler, J. Lex, Y.-D. Wu,
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The confirmation of planar structure for 12 came from the
single-crystal X-ray analysis10§ (CCDC-622228). The structure
shown in Fig. 5 confirms the inversion of both the selenophene
rings. There are two independent molecules in the asymmetric unit
and the inverted biselenophene unit, having different dihedral
angles (9.52 and 22.6u) with respect to the mean plane defined by
four meso carbon atoms. The molecules deviate slightly from
planarity as seen in side view (Fig. 5b). NMR studies also support
such an observation where the degree of inversion of the
biselenophene unit depends on the amount of TFA added and
at excess TFA concentration, the biselenophene unit comes into
the plane as defined by the four meso carbons. And the macrocycle
adopts a planar conformation.
In summary, it has been shown that: (a) use of rigid
quaterthiophene subunit as one of the precursors restricts the ring
inversion only on the tertapyrrane subunit unlike in 4, where both
the subunits are involved in ring inversions; (b) variation of X in
the tetrapyrrane moiety has no bearing on the ring inversions; (c)
reduction in the steric bulk of the meso substituents in the
tetrapyrrane moiety did not result in the twisting of the
8 D. Seidel, V. Lynch and J. L. Sessler, Angew. Chem., 2002, 114, 1480;
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Chem. Commun., 2007, 43–45 | 45