Fused core-modified meso-aryl expanded porphyrins

Article abstract of DOI:10.1039/c000387e

The synthesis and characterization of the first examples of singly and doubly fused expanded porphyrins containing dithienothiophene (DTT) cores are reported.

Full text of DOI:10.1039/c000387e

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Fused core-modified meso-aryl expanded porphyrinsw
T. K. Chandrashekar,*ab V. Prabhuraja,a S. Gokulnath,a R. Sabarinathana and A. Srinivasanb
Received 7th January 2010, Accepted 7th June 2010
DOI: 10.1039/c000387e
The synthesis and characterization of the first examples of
singly and doubly fused expanded porphyrins containing
dithienothiophene (DTT) cores are reported.
Expanded porphyrins receive considerable attention from
chemists and material scientists due to their potential applications
in opto-electronic devices and sensors.1 Various modifications
have been carried out on them such as core-modification,2a
confusion2b and fusion3 which lead to modulation in electronic
[4+2] MacDonald type condensation of DTT-diols 4 with
modified tetrapyrranes 5 in the presence of 1 equivalent of
trifluoroacetic acid (TFA) as a catalyst followed by oxidation
with p-chloranil in air gave mono-fused rubyrins 6a–c in
B20% yields as a single product (Scheme 1). The mass spectra
peaks at m/z = 1010.66 [M]+ and 1107.31 [M+1]+ for 6a
and 6c indicated formation of the mono-fused rubyrins. The
1H NMR spectrum of 6a in CDCl3 shows a sharp singlet at
d = 10.76 ppm corresponding to the b-CH of the DTT moiety
(Fig. 1). The four bithiophene protons resonate as two doublets
at 11.82 and 10.52 ppm. Two sets of pyrrolic protons resonate
closely at 9.04 and 9.02 ppm. The mesityl phenyl protons are
observed at 7.46 ppm. These assignments are based on the
correlations found in the 1H–1H COSY spectrum. The
aromaticity of fused rubyrins estimated by 1H NMR chemical
shifts are in comparison to rubyrin without fusion. For
example on protonation with TFA the bithiophene and
pyrrole protons were shifted by 0.8 and 1.1 ppm compared
to 0.29 and 0.35 ppm for the same protons for a similar
rubyrin without fusion.6 Furthermore the NH protons are
also more shielded and appear (at 233 K) at ꢀ7.15 ppm
compared to ꢀ5.15 ppm observed for rubyrin without fusion
(ESIw). Overall, the chemical shift difference (d) between most
shielded (pyrrolic NH) and deshielded (b-CH) protons of 6a is
20 ppm compared to 17 ppm for rubyrin without fusion
suggesting an increase in the aromaticity upon fusion.
structure, conjugation and electrochemical properties. Osuka
and co-workers first reported fusion in expanded porphyrins
in N-fused pentaphyrin3a 1 followed by many reports mainly
involving pyrrolic nitrogens.3 However, there are only few
reports where fusion does not involve pyrrolic nitrogen(s).
Recently sapphyrins derived from benzodipyrrole (2a) and
benzodifuran (2b) have been reported by using 3+2 approach.4
Very recently, Wu and co-workers reported rubyrins with
phenanthrene-fused pyrrole rings which serve as a Hg2+
sensor.1e To the best of our knowledge, there are no other
reports on expanded porphyrins where fusion does not involve
pyrrolic nitrogen(s). Thus, it is important to synthesize fused
expanded porphyrins by easy and efficient methods using a
rational approach so that the design principles used can be
applied to generate a variety of fused skeletons. In this
communication we report that we have succeeded in designing
efficient methodology to synthesize a range of expanded
porphyrins using a preformed fused precursor. The fused
precursor is dithienothiophene (DTT) 3 which has an electron
rich rigid core and has been used as a building block for many
electronic and opto-electronic materials.5 We speculated that
the incorporation of a planar DTT core in expanded porphyrins
can lead to conformational rigidity resulting in planar fused
macrocycles with increased aromaticity, which is a basic
requirement for any macrocycle for use in optical materials.
The versatility of the method was demonstrated by synthesizing
singly and doubly fused rubyrins and fused heptaphyrins using
appropriate conditions. It has been shown that these macro-
cycles are almost planar and show aromatic characteristics.
The key precursor required for the synthesis was fused
bithiophene diol 4. This was synthesized by dilithiation
of 3 followed by the addition of aldehydes in THF to give
fused bithiophene diols (DTT diols) 4 in 60–70% yield. Thus the
Similarly, doubly fused rubyrins were synthesized by simply
condensing DTT-diols 4 with pyrrole under modified Lindsey’s
conditions with 1 eq. of TFA. The rubyrins were obtained in
B15% yield as a single product. The proposed structure was
initially confirmed by the mass spectrum peak at m/z = 1040
[M]+ for 7a. The 1H NMR spectrum confirms the symmetrical
nature of the molecule. For example in the aromatic region the
1H NMR spectrum of 7a contains three singlets at 10.92,
9.24 and 7.56 ppm, corresponding to DTT, pyrrole and phenyl
protons which are slightly deshielded compared to normal and
mono fused rubyrins. On protonation, similar to 6a, the CH
protons were more deshielded and the NH proton appears at
ꢀ7 ppm (ESIw).
a Department of Chemistry, Indian Institute of Technology,
Kanpur 208016, India. E-mail: tkc@iitk.ac.in
b School of Chemical Sciences, National Institute of Science Education
and Research (NISER), Bhubaneswar, 751005, India.
E-mail: tkc@niser.ac.in
w Electronic supplementary information (ESI) available: Detailed
experimental procedures, characterization data and preliminary
crystal structures of 6c and 7a. CCDC 694219–694221. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c000387e
The final proof of the proposed structures came from single
crystal X-ray crystallography.7 The crystal structure of 6a is
shown in Fig. 2.z The unit cell contains two molecules. Due to
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Scheme 1 Syntheses of mono and doubly fused rubyrins.
Fig. 1 1H NMR spectrum of 6a in the downfield region in CDCl3 (for
labeling (a–e) see Scheme 1).
the pseudo-symmetrical nature of the macrocycle the S3 atom
is in positional disorder and refined with 0.5 occupancy factor.
Crystal structure analysis reveals the complete planarity of the
molecule as envisaged. As expected, the fused bithiophene unit
is planar and deviation from the molecular plane defined by
the four meso carbons is very small (1.256(30)1). Similarly,
the pyrrole plane is also coplanar with the molecular plane
making the molecule completely flat. This is the first time such
a flat structure has been observed for rubyrins. In all other
rubyrins (all aza and core-modified) reported earlier, two or
more heterocyclic units are canted.6,8 The meso phenyl
groups are almost perpendicular due to the bulkiness of the
mesityl group, typical of meso-aryl expanded porphyrins. The
preliminary crystal structures of 6c and 7a also show planar
structure (ESIw). In 6c, S and Se atoms are in positional
disorder due to pseudo-symmetry. However, compared to all
thia analogue 6a the biselenophene unit is slightly tilted with
respect to the mean plane, probably due to the bigger size of Se
compared to S.
Fig. 2 Crystal structure of 6a. (a) Top view with 30% probability
thermal ellipsoids. (b) Side view (meso aryl groups and hydrogens are
omitted for clarity).z
peak at m/z = 1093.28 [M+1]+ confirms the formation of the
fused heptaphyrin. The effect of the fusion is clearly visible
from the analysis of the 1H NMR spectrum (Fig. 3). The
reduction in the number of peaks in the aromatic region
compared to similar non-fused heptaphyrin clearly reveals
the symmetry of the molecule. The normal heptaphyrin is
not symmetric, non-planar and shows fourteen doublets for
the seven heterocycles.9 In contrast, the aromatic region in the
1H NMR spectrum of 10 in CDCl3 contains four singlets and
four doublets. The singlets at 10.59 and 11.42 ppm have
been assigned to the b-CH protons of DTT (a) and central
thiophene (f) (see Scheme 2). The doublets at 11.23 and 10.16 ppm
were assigned to b-CH of the other two thiophenes (d, e). The
pyrrole protons (b, c) resonate closely at 8.88 and 8.76 ppm
respectively as doublets. The two singlets at 7.54 and 7.48 ppm
were assigned to phenyl protons of the mesityl group.
It is clear from the NMR spectrum that no ring inversion
takes place in sharp contrast to non-fused heptaphyrin (11)
where one of the thiophene rings of the bithiophene unit is
inverted.9b
This methodology was extended to obtain a higher order
macrocycle such as fused heptaphyrin. For this the precursor
required was fused tetrapyrrane 8 which was obtained under
Lindsey’s conditions by treating diols 4 with excess pyrrole in
the presence of 0.1 eq. of TFA as catalyst in 60% yields.
The versatility of our synthetic strategy was demonstrated
by the availability of an alternate route for the syntheses of
doubly fused rubyrins. Thus, MacDonald condensation of
tetrapyrranes 8 with diols 4 also yielded doubly fused rubyrins
7 in 15–20% yields. The fused heptaphyrin was synthesized by
a condensation reaction of tetrapyrrane 8a with terthiophene
diol 9 in the presence of 1 equivalent of TFA, which gave fused
heptaphyrin 10 in 23% yield (Scheme 2). The mass spectrum
The effect of conformational restriction on the spectral
properties of macrocycles (Fig. 4) upon fusion was inferred
from the following observations: (i) the blue shift of the Soret
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Notes and references
z Crystal data for 6a C64H52N2S5:9 crystals were grown by slowly
diffusing dry n-hexane over a chloroform solution of 6a. Green
rectangular, monoclinic, space group P21/n, a = 14.6226(29) A,
b
= 15.0918(30) A, c = 15.9295(32) A, a = 90.001, b =
114.978(30)1, g = 90.001, V = 3186.6(13) A3, T = 293(2) K,
Z = 2, m(Mo-Ka) = 0.218 mmꢀ1, F(000) = 1060. 45977 reflections
were measured, of which 5924 were unique (Rint) = 0.0430, R1 =
0.0948, wR2 = 0.2006, final R1 (I > 2s(I))=0.0843, wR2 = 0.1933,
GOF on F2 = 1.047. CCDC 694219. The S3 atom is in positional
disorder and refined with 0.5 occupancy factor, where two independent
molecules are overlapped with opposite orientation.
Scheme 2 Synthesis of fused heptaphyrin.
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Fig. 3 1H NMR spectrum of 10 in the downfield region in CDCl3 (for
labeling (a–f) see Scheme 2).
band on fusion (6 nm for mono-fused (6a) and 10 nm for
doubly fused (7a)), (ii) the increase in the molar extinction
coefficient by 4 times upon fusion in rubyrins (6, 7),10 and
the 1.5 times increase in fused heptaphyrins (10),10 (iii) the
increase in planarity of 10 relative to 11, (as reflected by
1H NMR) and (iv) the dramatic downfield shift of a, a0
protons of 119a from ꢀ0.6 ppm to 10.6 ppm in 10.
In conclusion we have synthesized a new range of mono and
doubly fused core-modified expanded porphyrins containing
DTT cores. We also demonstrated that fused expanded
porphyrins can be designed by choosing appropriate precursors.
In rubyrins fusion resulted in increased aromaticity, probably
due to their flat structure. By introducing fusion, we were also
able to restrict the conformation of the heptaphyrin. Possibly,
fusion can be used as a tool for attaining planarity/rigidity in
larger expanded porphyrins. Research in this direction is
underway.
6 A. Srinivasan, V. G. Anand, S. J. Narayanan, B. Sridevi,
S. K. Pushpan, M. Ravikumar and T. K. Chandrashekar, Angew.
Chem., Int. Ed. Engl., 1997, 36, 2598–2601.
7 SHELXTL-PC Package, Bruker Analytical X-ray Systems,
Madison, WI, 1998.
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T. K. Chandrashekar, A. Vij and R. Roy, J. Am. Chem. Soc.,
1999, 121, 9053–9068.
The authors thank Prof. Singh U P, IIT Roorke for single
crystal X-ray data collection. T K C thanks the Department
of Science and Technology (DST), India for J C Bose
fellowship. We greatly acknowledge Dr E. Suresh, CSMCRI–
CSIR, Bhavnagar, for useful discussion on crystal structure
analysis.
9 (a) V. G. Anand, S. K. Pushpan, A. Srinivasan, S. J. Narayanan,
B. Sridevi, T. K. Chandrashekar, R. Roy and B. S. Joshi, Org.
Lett., 2000, 2, 3829–3832; (b) B. S. Joshi, V. G. Anand,
S. K. Pushpan, A. Srinivasan, T. K. Chandrashekar and R. Roy,
J. Porphyrins Phthalocyanines, 2002, 6, 410–422; (c) V. G. Anand,
S. K. Pushpan, S. Venkatraman, S. J. Narayanan, A. Dey,
T. K. Chandrashekar, R. Roy, B. S. Joshi, S. Deepa and
G. N. Sastry, J. Org. Chem., 2002, 67, 6309–6319.
10 The electronic spectral data for 6a, 7a and 10 are as follows: 6a.
UV-Vis (CH2Cl2) [lmax (nm) (e ꢂ 10ꢀ5)]: 517 (6.10), 536 (3.10), 606
(0.09), 659 (0.30), 711 (1.30), 822 (0.05); (CH2Cl2/TFA): 538 (4.70),
559 (3.40), 783(0.30) and 831(1.00). 7a. UV-Vis (CH2Cl2)
[lmax (nm) (e ꢂ 10ꢀ5)]: 513 (3.80), 540 (2.10), 606 (0.09), 665
(0.17), 710 (0.82), 800 (0.06); (CH2Cl2/TFA): 535 (2.40), 566 (1.64),
766 (0.17), 843 (0.77). 10. UV-Vis (CH2Cl2) [lmax (nm) (e ꢂ 10ꢀ5)]:
553 (2.00), 579 (1.38), 797 (0.70); (CH2Cl2/TFA): 613 (1.60), 860
(0.12), 933 (0.70).
Fig. 4 Effect of fusion: conformational restriction.
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 5915–5917 | 5917