Synthesis of Nonaromatic and Aromatic Dithia Benzisapphyrins

Article abstract of DOI:10.1021/acs.joc.8b01737

A series of [24?€] dithia meta-benzisapphyrins and [22?€] dithia para-benzisapphyrins were synthesized by 3 + 2 condensation of appropriate benzitripyrrane with bithiophene diol under mild acid catalyzed conditions. The dithia m-benzisapphyrins and dithia p-benzisapphyrins were thoroughly characterized by HR-MS, 1D and 2D NMR, absorption, and electrochemical techniques. Our studies showed that dithia m-benzisapphyrins are nonaromatic, whereas the dithia p-benzisapphyrins are aromatic in nature. Thus, we demonstrated here that the dithiabenzisapphyrins can be made aromatic by replacing the m-phenylene moiety of the benzisapphyrin macrocycle with a p-phenylene unit. Furthermore, the studies also indicated that the aromaticity of the dithia p-benzisapphyrins was relatively more compared to the reported heterosapphyrins. The structural and spectral characteristics including aromaticity of the m-benzisapphyrins and p-benzisapphyrins were also discussed with the help of DFT, NICS, and TD-DFT studies.

Full text of DOI:10.1021/acs.joc.8b01737

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Article
Synthesis of Nonaromatic and Aromatic Dithia Benzisapphyrins
Rima Sengupta, Kishor Gulab Thorat, and Mangalampalli Ravikanth
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01737 • Publication Date (Web): 07 Sep 2018
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Synthesis of Nonaromatic and Aromatic Dithia Benzisapphyrins
Rima Sengupta,† Kishor G. Thorat† and Mangalampalli Ravikanth*
Indian Institute of Technology, Powai, Mumbai, 400076, India, Fax:91-22-5723480;
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Tel: 91-22-5767176; E-mail: ravikanth@chem.iitb.ac.in
Abstract
A series of [24] dithia meta-benzisapphyrins and [22] dithia para-benzisapphyrins
were synthesized by 3+2 condensation of appropriate benzitripyrrane with bithiophene diol
under mild acid catalyzed conditions. The dithia m-benzisapphyrins and dithia p-
benzisapphyrins were thoroughly characterized by HR-MS, 1D, 2D NMR, absorption and
electrochemical techniques. Our studies showed that dithia m-benzisapphyrins are
nonaromatic whereas the dithia p-benzisapphyrins are aromatic in nature. Thus, we
demonstrated here that the dithiabenzisapphyrins can be made aromatic by replacing m-
phenylene moiety of the benzisapphyrin macrocycle with p-phenylene unit. Furthermore, the
studies also indicated that the aromaticity of the dithia p-benzisapphyrins was relatively more
compared to the reported heterosapphyrins. The structural and spectral characteristics
including aromaticity of the m-benzisapphyrins and p-benzisapphyrins were also discussed
with the help of DFT, NICS and TD-DFT studies.
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Introduction
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Sapphyrins 1 (Chart 1) are one class of expanded porphyrins with five pyrrole rings
joined with four meso-carbon bridges and one direct pyrrole bond and these macrocycles are
oldest and simplest member of the expanded porphyrin family.1–3 Sapphyrins are 22π
electron aromatic macrocycles and were discovered serendipitously by Woodward and co-
workers3 during their pioneering work on synthesis of Vitamin B12. The pentapyrrolic
macrocycles were called sapphyrins because of their intense blue-colored crystals.3
Although some work on sapphyrins have been reported in 1970’s and 1980’s, an
intensification of interest in these macrocycles was generated when Sessler and co-workers4
accidentally discovered in 1990’s that these macrocycles can bind/sense anions, a feature that
had not previously been recognized for porphyrinoid macrocycles. Sessler and co-workers5
extensively investigated sapphyrin chemistry and developed facile methods for their
synthesis, studied their aromatic character and their ability to bind metals and their specific
versatile sensing ability of anions. Sapphyrins are also singlet-oxygen-producing
photosensitizers and have been explored for potential applications in photodynamic therapy
and viral photoeradication.6 In addition, sapphyrins were also found to be potential
compounds for treating leishmaniasis disease.7 Because of their widespread applications,
many structural variants of sapphyrins have been reported including meso-substituted
sapphyrins,8–11 carbasapphyrins,6,12–16 heterosapphyrins, N- or heteroatom confused
sapphyrins,17 doubly N-confused sapphyrin derivatives,13 sapphyrins incorporated with
polycyclic aromatic hydrocarbons and so on.13 Among many structural variants of
sapphyrins, benzisapphyrins containing benzene and four pyrrole/heterocycle rings within a
conjugated pathway is scarcely explored.18 On the other hand, benzene-containing
porphyrins have been extensively investigated by Lash et. al.19,20 and other researchers and
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have shown that the benziporphyrins readily forms organometallic derivatives and have
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potential applications in the development of chemical sensors and in molecular recognition
studies.8,21 Benziporphyrinoid macrocycles have been used to understand aromaticity and
conjugation since the characteristics of benziporphyrinoids vary from nonaromatic to highly
aromatic and in few cases antiaromatic structures are formed.22–26 Thus, benzisapphyrins are
highly desirable macrocycles to understand their diverse properties and applications. A
perusal of literature revealed that there is only one report available on benzisapphyrins.27
Sessler, Lee and co-workers27 reported one example of dithia benzisapphyrin 2 (Chart 1)
bearing exocyclic double bonds at the meso-positions which was obtained in 6% yield. The
dithia analogue of meso-alkylidenyl sapphyrin 2 was nonaromatic since the conjugation
pathway was interrupted by the m-phenylene unit.27 Interestingly, this is the only report
available on benzene-containing sapphyrin systems so far. However, the other reported dithia
analogues of sapphyrins such as heterosapphyrins 3-5 (Chart 1) were found to be aromatic
and showed interesting physico-chemical and anion sensing properties.28 Therefore, it is
interesting to design and synthesize benzene-containing sapphyrins so that they can be
compared to previously reported aromatic heterosapphyrins 3-5.28 Furthermore, to the best of
our knowledge, there is no report on benzene containing sapphyrins that are aromatic in
nature. Thus, we made an attempt to design and synthesize aromatic dithia benzisapphyrins
using readily available precursors. During our synthetic attempts, we realized that it is
possible to synthesize aromatic dithia benzisapphyrins by changing the m-phenylene moiety
to a p-phenylene unit. Herein, we report synthesis and properties of series of nonaromatic
[24] dithia m-benzisapphyrins 6-7 and aromatic [22] dithia p-benzisapphyrins 8-10 which
were prepared under simple acid catalyzed reaction conditions (Chart 2). The dithia m-
benzisapphyrins and dithia p-benzisapphyrins exhibit quite different spectral and
electrochemical properties from each other because of their differences in conjugation and
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aromaticity.
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Chart 1: Structures of the sapphyrins and ditha meta-benzisapphyrin reported in the literature
Chart 2. Structures of the dithia meta-benzisapphyrins (6 and 7) and dithia para-
benzisapphyrins (8-10).
Results and Discussion
Synthesis and characterization of dithia m-benzisapphyrins 6-7
Initially, we made an attempt to synthesize dithia m-benzisapphyrins by adopting 3+2
condensation approach. The desired diols 11a and 11b were prepared by treating
isophthaldehyde with freshly prepared ArMgBr in THF followed by standard work up and
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column chromatographic purification and afforded diols 11a and 11b as white solids in 65-
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70% yields.
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Scheme 1: Synthesis of ditha m-benzisapphyrins 6 and 7.
The appropriate diols 11a/11b were treated with pyrrole in 1,2-dichloroethane at reflux in
the
presence of catalytic amount of BF3.(OEt)2 followed
by
column
chromatographic purification to afford the corresponding benzitripyrranes 12a/12b in 60-65%
yields. The other desired bithiophene diol 13a was prepared by following the literature
procedure.29,30 All precursors were characterized by HR-MS, H & 13C NMR techniques.
1
The dithia m-benzisapphyrins 6 and 7 were synthesized by condensing one equivalent of the
appropriate benzitripyrrane 12a/12b with one equivalent of bithiophene diol 13a in CH2Cl2 at
room temperature in the presence of one equivalent of TFA under inert atmosphere for about
30-45 min, followed by oxidation with DDQ in open air at room temperature for 30 min
(Scheme 1). TLC analysis showed one major dark red spot corresponding to the desired
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dithia m-benzisapphyrin macrocycle 6-7. The crude compounds were subjected to basic
alumina column chromatography and collected the dithia m-benzisapphyrin macrocycle 6-7
in 5-7% yields. The dithia m-benzisapphyrins 6-7 were freely soluble in common organic
solvents and their identities were confirmed by molecular ion peak in HRMS and detailed 1D
and 2D NMR spectroscopy.
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The H, H-1H COSY and NOESY NMR spectra of dithia m-benzisapphyrin 6 is
presented in Figure 1. The proton assignments were made on the basis of their location,
integration, coupling constants and cross-peak correlations observed in COSY and NOESY
spectra. The macrocycle 6 is symmetrical and showed resonances for half of the macrocycle
which were easily identified using 1D and 2D NMR spectroscopy. The selected chemical
shifts (1H) were tabulated in supporting Information (Table S1). The inner type g proton of
m-phenylene unit appeared as broad singlet at 8.03 ppm, whereas type e and type f protons of
m-phenylene unit appeared as multiplet in the region of 7.38-7.44 ppm and a triplet at 6.19
ppm, respectively. The meso-phenyl protons (type j, k and l) also appeared in the region of
7.38-7.62 ppm and merged with other proton resonances. The NMR spectrum of macrocycle
6 showed that the inner meta-phenylene –CH proton (type g) appeared at downfield region,
whereas the outer meta-phenylene protons (type e and type f) appeared at upfield region
supporting that the macrocycle 6 is nonaromatic in nature. The other dithia meta-
benzisapphyrin 7 also showed similar nonaromatic NMR features (Figure S14).
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Figure 1: (A) Partial H NMR spectra of compound 6; Resonances were assigined on the
basis of correlations observed in H-1H COSY (B) and H-1H NOESY (C) spectrum as
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shown in this figure.
Synthesis and characterization of dithia p-benzisapphyrins 8-10:
Since dithia m-benzisapphyrins 6-7 were found to be nonaromatic 24π electron
systems, we thought that if we replace m-phenylene moiety of the macrocycle with p-
phenylene moiety, the resulting p-benzisapphyrin macrocycle would be a 22π aromatic
system like 1 and core-modified sapphyrins 3-5.28 With this idea in mind, we attempted the
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synthesis of dithia p-benzisapphyrins 8-10 as shown in Scheme 2. The desired p-phenylene
based diols 14a-14b were synthesized by treating terephthaldehyde with freshly prepared
ArMgBr in THF followed by work-up and column chromatographic purification yielded diols
14a-14b in 65-70% yields. The corresponding p-benzitripyrranes 15a-15b were prepared by
treating appropriate diol 14a/14b with excess pyrrole in 1,2-dichloroethane at reflux for
overnight followed by column chromatographic purification to afford p-benzitripyrranes 15a-
15b in 60-65% yields.
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Scheme 2: Synthesis of ditha p-benzisapphyrins 8-10.
The dithia p-benzisapphyrins 8-10 were synthesized by condensing one equivalent of
p-benzitripyrrane 15a/15b with one equivalent of bithiophene diol 13a/13b in CH2Cl2 in the
presence of one equivalent of TFA at room temperature for 1 h followed by oxidation with
DDQ in open air for additional 30 min. TLC analysis showed one major spot corresponding
to the required macrocycle along with trace amounts of one or two additional spots
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corresponding to the unidentified products. The crude compound was subjected to basic
alumina column chromatographic purification and afforded the pure dithia p-benzisapphyrins
8-10 in 4-6% yields.
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Figure 2: (A) Partial 1H NMR spectra of the compound 8; Resonances were assigined on the
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basis of correlations observed in H-1H COSY (B) and H-1H NOESY (C) spectrum as
shown in this figure.
The molecular ion peak in HR-MS confirmed the identities of dithia p-
benzisapphyrins 8-10. Macrocycles 8-10 were thoroughly characterized by 1D and 2D NMR
spectroscopy and 1H, 1H-1H COSY and NOESY NMR spectra of macrocycle 8 are presented
in Figure 2. The 1H NMR spectra of macrocycle 8 was very well resolved and all resonances
were identified easily using cross peak correlations observed in 2D NMR. The selected peak
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resonances for the compound 8 for comparion with compounds 6 and 4 were included in
supporting information (Table S1). Interestingly, the four p-phenylene protons in macrocycle
8 were completely absent in the NMR spectrum recorded at room temperature due to its rapid
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rotation in NMR time scale. Hence, we recorded H NMR and H-1H COSY spectra at -40
⁰C to slow down the rotation of the p-phenylene ring in macrocycle 8. In macrocycle 8, the
four protons of p-phenylene ring appeared as two sets of slightly broad singlets at -1.91 ppm
(type f) and 9.24 ppm (type e). The other protons of macrocyclic ring showed almost
negligible shifts at low temperature NMR compared to the room temperature NMR.
Inspection of the dithia p-benzisapphyrin NMR clearly showed that the downfield shifts of
outer ring protons and upfield shift of inner ring protons (Figure 2 and Table S1) supporting
the operation of diatropic ring current effect and the aromatic nature of p-benziporphyrin
macrocycle 8. Furthermore, we compared the NMR features of p-benzisapphyrin macrocycle
8 with structurally analogous aromatic heterosapphyrin 4. For example, in compound 4, the
type a and type b protons of bithiophene unit appeared as doublets at 9.40 and 9.75 ppm,
respectively were downfield shifted in p-benzisapphyrin macrocycle 8 and appeared at 10.51
and 10.01 ppm, respectively. Similarly, the β-pyrrole protons (type c and type d) were more
downfield shifted in macrocycle 8 compared to macrocycle 4. In macrocycle 4, the furan
ring which was opposite to bithiophene unit was inverted and the β-protons of inverted furan
ring which was under diatropic ring current effect appeared at 0.61 ppm whereas in p-
benzisapphyrin macrocycle, the inner p-phenylene-protons appeared at -1.91 ppm. All these
upfield and downfield shifts of various protons clearly indicated that the macrocycle 8
possesses more diatropic ring current than the structurally analogous heteroatom substituted
sapphyrin 4. The other two aromatic p-benzisapphyrin macrocycles 9 and 10 also showed
similar NMR features like that of macrocycle 8 (Figures S21 and S24).
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Absorption and electrochemical properties of m-benzisapphyrins and p-
benzisapphyrins:
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The absorption and electrochemical properties of dithia m-benzisapphyrins 6-7 and dithia p-
benzisapphyrins 8-10 were investigated and the data are presented in Table 1. The
comparison of absorption spectra of dithia m-benzisapphyrin 6 and dithia p-benzisapphyrin 8
recorded in toluene are shown in Figure 3A. The m-benzisapphyrin 6 showed an intense
broad band at 397 nm along with a shoulder band at 493 nm and a broad less intense band at
823 nm supporting the nonaromatic nature of the macrocycle. Whereas, the p-benzisapphyrin
8 showed one intense sharp Soret like band at 484 nm along with three Q-type bands at 668,
762 and 854 nm which are in agreement with the aromatic nature of the macrocycle. We also
carried out protonation titration of m-benzisapphyrin 6 and p-benzisapphyrin 8 by systematic
addition of increasing amounts of TFA to the macrocycles 6 and 8 in toluene as shown in
Figures 3B and 3C, respectively. Addition of increasing amounts of TFA to the solution of 6,
the colour of the solution was changed from yellow to dark brown due to formation of
diprotonated species 6.2H2+ and the broad band at 397 nm was shifted bathochromically to
relatively sharp intense band at 431 nm and the band at 823 nm was shifted to 1071 nm with
two clear isosbestic points at 444 and 895 nm (Figure 3B). The compound 8 upon sequential
addition of increasing amounts of TFA showed distinct colour change from red to pink and
the resulting 8.2H2+ showed two bands at 507 and 806 nm and two clear isosbestic points at
493 and 703 nm were noted.
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The redox properties of dithia m-benzisapphyrins 6-7 and dithia p-benzisapphyrins 8-
10 were studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using
0.1 M tetrabutyl ammonium perchlorate (TBAP) as the supporting electrolyte in CH2Cl2.
The comparison of reduction waves of compounds 6 and 8 are presented in Figure 3D and the
relevant data for all compounds were included in Table 1.
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(B)
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Wavelength (nm)
Wavelength (nm)
(C)
(D)
2x10-5
1x10-5
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TFA
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-1.5
Wavelength (nm)
Potential (V)
Figure 3: (A) Comparison of absorption spectra of the compounds 6 (brown line) and 8 (pink
line) (5 µM) recorded in toluene, (B) Change in the absorption spectra of 6 upon the
systematic addition of trifluoroacetic acid (0-150 eq.) in toluene, (C) Change in the
absorption spectra of 8 upon the systematic addition of trifluoroacetic acid (0-90 eq.) in
toluene, and (D) Comparison of cyclic voltamograms of compounds 6 (brown line) and 8
(pink line), recorded in CH2Cl2 containing 0.1 M TBAP as the supporting electrolyte and a
saturated calomel electrode (SCE) as the reference electrode at scan rates of 50 mV s-1.
Comparison of redox waves of all the compounds is included in supporting information
(Figure S27). In general dithia m-benzisapphyrins 6-7 and dithia p-benzisapphyrins 8-10
showed two irreversible oxidations and two to three quasi-reversible/reversible reductions.
For example, compound 6 showed two oxidations at 0.68 V, 1.14 V and two reductions at -
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0.66 V and -0.88 V, whereas compound 8 showed two oxidations at 0.89 V, 1.35 V and three
reductions at -0.60 V, -0.95 V and -1.46 V. A close inspection of redox data indicates that
dithia p-benzisapphyrins 8-10 were difficult to oxidize but relatively easier to reduce
compared to dithia m-benzisapphyrins 6-7. The reversible nature of reduction waves noted
for dithia m-benzisapphyrins 6-7 and dithia p-benzisapphyrins 8-10 indicates that these
macrocycles are stable under reduction conditions.
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Table 1: Absorption and electrochemical data of the compounds 6-10.
Absorption data
Redox data
Reduction
Compound
λ1 nm
λ2 nm
λ3 nm
λ 4 nm
Oxidation
(V)
(log ɛ) (log ɛ) (log ɛ) (log ɛ)
(V)
397
(4.77)
493
(sh)
823
(4.29)
-
-
-
-
1.14
0.68
-0.66
-0.88
-
6
6.2H2+
7
431
503
1071
-
1.00
-
-
0.62
-
-
-
-
(4.55) (4.69) (4.26)
406
(4.58)
500
(sh)
837
(4.04)
-0.70
-0.92
-
451
567
1113
7.2H2+
8
-
-
-
(4.41) (4.85) (3.98)
484 668 762
(5.14) (4.75) (3.37)
854
(3.47)
1.35
-
0.89
-
-0.60
-0.95
-1.46
507
(5.19) (4.76)
487 676
806
8.2H2+
9
-
-
-
-0.63
-
-
-0.95
-
-
-1.46
-
767
858
(2.86)
1.31
-
0.82
-
(4.61) (4.16) (2.75)
519
(4.59) (4.23)
484 671
(4.71) (4.32) (2.95)
504 803
(4.79) (4.34)
840
9.2H2+
10
-
-
757
849
(2.49)
1.60
-
0.99
-
-0.50
-
-0.69
-
-0.97
-
10.2H2+
-
-
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DFT and TD-DFT studies:
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8
DFT and TD-DFT studies were used to understand the structural and spectral characteristics
of dithia m-benzisapphyrins 6-7 and dithia p-benzisapphyrins 8-10. The geometry in S0 state
optimization, NICS (Nucleus-Independent Chemical Shifts)31,32 and AICD (Anisotropy
Induced Current Density)33 calculations for the representative compounds 6 and 8 were
performed using with B3LYP/6-31g(d,p) level of theory. The optimized structures of the
compounds 6 and 8 are presented in Figure 4. The DFT studies revealed distorted structure
for the dithia m-benzisapphyrin 6 with m-phenylene moiety deviated from the mean plane
(defined by four meso carbon atoms) to great extent as shown in Figure 4 (a, b). Whereas, the
macrocyclic core of the dithia p-benzisapphyrin 8 showed an almost planar arrangement,
except the p-phenylene ring which deviated from the mean plane (defined by four meso
carbons) by 21.9 ° (Figures 4c and 4d).
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a)
b)
S
S
N
N
S
S
N
N
d)
c)
S
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Figure 4. DFT (B3LYP/6-31g(d,p)) optimized structures of the compound 6 ((a)-side view
and (b) top view) and compound 8 ((c)-side view and (d) top view).
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Compound 8
Compound 6
Figure 5. AICD plots of the compounds 6 and 8.
The AICD plot (Figure 5) of the compound 6 displayed the clockwise ring current which was
interrupted at m-phenylene moiety of the macrocyclic core favoring nonaromatic character of
the compound 6. Whereas the AICD plot of the compound 8 displayed continuous clockwise
ring current favouring aromatic character of the macrocycle 8 which was also supported by
the NICS(0) value of -13.48 ppm.
The analysis of frontier molecular orbitals revealed that the HOMO of the compound 6 was
much more destabilized compared to that of the compound 8, whereas the LUMO of the
compound 6 was stabilized to some extent compared to that of compound 8 (Figure 6). This
leads to the decrease in the band gap (ΔE = ELUMO-EHOMO) for the compound 6 (ΔE = 0.587
eV) compared to that of compound 8 (ΔE = 0.948 eV), supporting the observed red shift in
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-6.0
-6.2
-6.4
-6.6
-6.8
-7.0
-7.2
-7.4
-7.6
-7.8
-8.0
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EH-1
EH
EL
ΔE=0.587 eV
ΔE=0.948 eV
EL+1
ΔE=1.172 eV
ΔE=0.971 eV
Compound 6
Compound 8
Figure 6. Energy level diagram (selected frontier molecular orbitals) of the compounds 6 and
8 calculated by DFT (B3LYP/6-31g(d,p)) method; (EH-1= Energy of HOMO-1; EH= Energy
of HOMO; EL= Energy of LUMO; and EL+1= Energy of LUMO+1 orbitals).
the absorption of dithia m-benzisapphyrins 6-7 compared to that of dithia p-benzisapphyrins
8-10. The analysis of the FMOs of the compound 6 suggest the transfer of charge from one
part of the molecule to another.
Further, the TD-DFT studies were carried out to evaluate the excitation energies and
oscillator strengths for first 50 S0-Sn transitions and comparison of calculated excitation
energies (black vertical lines) and experimental UV-Vis spectra (colored lines) is presented in
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Figure S28. The oscillator strengths (right axes) for the compounds 6 and 8 represents the
probability of the occurrence of corresponding transitions and are proportional to the molar
extinction coefficients (left axes) found for the compounds 6 and 8. From figure S28 it is seen
that, the TD-DFT studies suggested very close match between the observed and calculated
absorption spectra for the compounds 6 and 8. The HOMO→LUMO transitions were
responsible for the observed low energy charge transfer band and Q bands in compounds 6
and 8, respectively. It is clear from the figure 6 that the HOMO-1 orbital of the compound 6
was destabilized compared to that of compound 8. This resulted in the decrease of the EL–
EH-1 gap for compound 6 which reflected in its red shifted low energy absorption band
compared to that of compound 8.
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Conclusions:
In conclusion, we prepared a series of benzisapphyrins by 3+2 condensation of
appropriate benzitripyrrane with bithiophene diol under acid catalysed conditions. We
showed that the dithia benzisapphyrins can be made aromatic by replacing m-phenylene
moiety of the macrocycle with p-phenylene moiety using NMR, absorption, redox and DFT
studies. The studies also indicated that the dithia p-benzisapphyrins are relatively more
aromatic in nature compared to the reported hetero sapphyrins. Metalation and sensing
studies of these new aromatic meso-aryl dithia p-benzisapphyrins are currently under
investigation.
Experimental Section
General Experimental: The chemicals such as BF3·OEt2, 2, 3-dichloro-5, 6-dicyano-1, 4-
benzoquinone (DDQ) were used as obtained from Aldrich. All other chemicals used for the
synthesis were reagent grade unless otherwise specified. Column chromatography was
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performed on silica gel and basic alumina. Compounds 11a-11b34, 12a-12b35, 13a-13b29,30
,
5
6
7
8
1
13
14a-14b23,36,37, 15a-15b23 were synthesized by the reported methods. The H, and C NMR
spectra were recorded in CDCl3 on Bruker 400 and 500 MHz instruments. The frequencies
for 13C nucleus are 100.06 and 125.77 MHz for 400 MHz and 500 MHz instruments
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1
13
respectively. Tetramethylsilane [Si (CH3)4] was used as an internal standard for H and C
NMR. Absorption spectra were obtained with Shimadzu UV-Vis-NIR Spectrophotometer.
Cyclic voltammetry (CV) studies were carried out with BAS electrochemical system utilizing
the three electrode configuration consisting of a glassy carbon (working electrode), platinum
wire (auxiliary electrode) and saturated calomel (reference electrode) electrodes. The
experiments were done in dry dichloromethane using 0.1 M tetrabutylammonium perchlorate
as supporting electrolyte. The HR mass spectra were recorded with a Q-Tof micro mass
spectrometer.
Computational details: All the calculations were done using of the Gaussian 09 program
package.38 The density functional theory (DFT)39 method; hybrid functional B3LYP in
conjunction with basis set 6-31G(d,p),40 was used to optimizes the structures of the
compounds 6 and 8 in ground (S0) states. Using same hybrid functional and basis set the
vertical excitation energies and oscillator strengths were obtained for the 50 lowest S0→Sn
transitions at the optimized S0 state equilibrium geometries using time dependant density
functional theory (TD-DFT) method.41–49 All the computations in the toluene media were
carried out using the Self-Consistent Reaction Field (SCRF) under the Polarisable Continuum
Model (PCM).50,51 The electronic absorption spectra, including oscillator strengths were
systematically investigated using TD-DFT with PCM model on the basis of the S0 optimized
structures.
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Synthesis of 1,3-benzene-bis((4-methoxyphenyl)methanol (11b).
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8
The solution of isophthalaldehyde (1.00 g, 7.46 mmol) in dry THF (100 mL) was
cooled to 0 ° C and freshly prepared anisyl magnesium bromide (22 mL in THF, 44.74
mmol) was added to it by maintaining reaction temperature at 0 °C under N2 atmosphere. The
resulting mixture was stirred at room temperature for 2 h and the mixture was slowly poured
on to the ice cold aqueous ammonium chloride solution. The resulting mixture was extracted
with diethyl ether (70 mL x 3) and the combined organic layers were dried over anhydrous
sodium sulphate. The crude compound obtained after removal of solvent under vacuum was
subjected to silica gel (100-200) column chromatography using pet ether and ethyl acetate
(40:60 v/v) and the desired product 11b was obtained as white semi-solid in 65% yield (1.69
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g). H NMR (500 MHz, CDCl3) δ 7.43-7.46 (m, J = 9.9 Hz, 1H), 7.23 – 7.31 (m, 7H), 6.84
13
(d, J = 8.6 Hz, 4H), 5.76 (s, 2H), 3.80 (s, 6H), 2.53 (s, 2H). C NMR (126 MHz, CDCl3) δ
159.1, 144.4, 144.3, 136.2, 129.3, 128.6, 128.1, 128.0, 125.7, 125.6, 124.5, 114.0, 113.3,
75.8, 55.4. HRMS (ESI): calcd. for C22H21O3 [M -OH]+ m/z 333.1485; found m/z 333.1494.
Synthesis of 1,3-bis((4-methoxyphenyl)(1H-pyrrol-2-yl)methyl)benzene (12b).
To the solution of 11b (1.00 g, 2.86 mmol) and pyrrole (20 mL) in 1, 2-
dichloroethane (40 mL), BF3·Et2O (1.5 mL) was added and the resulting mixture was stirred
under N2 atmosphere at reflux for 16 h. The reaction mixture was then cooled to room
temperature and quenched by addition 2N sodium hydroxide solution (100 mL). The mixture
was extracted using dichloromethane (50 mL x 3) and combined organic layers were dried on
anhydrous sodium sulphate. The solvent and excess pyrrole were removed using high
vacuum. The black semisolid obtained was purified by column chromatography on silica gel,
eluting with pet ether and dichloromethane (60:40) mixture to afford pure compound 12b as
pale yellow semi-solid. The NMR data were consistent with the presence of two
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diastereomers. Yield- 0.77 g, (60%); 1H NMR (400 MHz, CDCl3) δ 7.77 (s, 2H), 7.22 (t, J =
7.6 Hz, 1H), 7.07 – 6.96 (m, 7H), 6.82 (d, J = 8.6 Hz, 4H), 6.67 (s, 2H), 6.13 (dd, 2H), 5.75
(s, 2H), 5.34 (s, 2H), 3.79 (s, 6H). 13C NMR (126 MHz, CDCl3) δ 158.3, 143.7, 135.3, 134.1,
131.3, 129.9, 129.5, 128.7, 127.1, 127.0, 117.2, 113.9, 112.9, 108.2, 107.8, 55.3, 49.8, 49.7.
HRMS (ESI): calcd. for C30H28KN2O2 [M + K]+ m/z 487.1782; found m/z 487.1781.
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Synthesis of [2,2'-bithiophene]-5,5'-diylbis((4-nitrophenyl)methanol (13b).
Dry and distilled n-hexane (30 mL) was added to a 250 mL three-necked round-
bottom flask containing 2,2’-bithiophene (1.00 g, 6.02 mmol) which was equipped with a
glass inlet tube, a reflux condenser and a rubber septum. N,N,N’,N’-Tetramethyl ethylene
diamine (2.25 mL, 15.06 mmol) and n-BuLi (9.00 mL of 1.6 M solution in n-hexane) were
injected into the stirred solution and refluxed for 1h. The reaction mixture was then allowed
to attain room temperature and cooled to 0 °C. The ice-cold solution of p-nitrobenzaldehyde
(1.52 mL, 15.06 mmol) in dry THF (30 mL) was added drop wise to the reaction mixture.
After the addition was complete, the reaction mixture was allowed to attain room
temperature. After 15-20 min, saturated ammonium chloride solution was added and the
resulting mixture was extracted with diethyl ether (50 mL x 3). The combined organic layers
were washed with brine water and dried over anhydrous Na2SO4. The crude compound
obtained after removal of solvent was subjected to silica gel column chromatography using
petroleum ether/ethyl acetate (65:35) and the desired diol 13b was collected as white solid.
1
Yield- 70% (1.97 g); mp. 160-162° C; H NMR (500 MHz, CDCl3) δ 8.22 (d, J = 8.8 Hz,
4H), 7.63 (d, J = 8.4 Hz, 4H), 6.96 (d, 2H), 6.83 (d, J = 3.6 Hz, 2H), 6.11 (s, 2H), 2.70 (s,
13
2H). C NMR (126 MHz, CDCl3) δ 149.6, 147.7, 145.9, 138.0, 127.1, 126.4, 124.0, 123.6,
71.5. HRMS (ESI): calcd. for C22H16NaN2O6S2 [M + Na]+ m/z 491.0342; found m/z
491.0349.
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General procedure for synthesis of dithia benzisapphyrins (6-10).
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To the solution of appropriate tripyrrane (0.50 mmol) in dichloromethane under N2
atmosphere, appropriate bis-thiophene dicarbinol 13a/13b (0.50 mmol) was added and
resulting mixture was stirred at room temperature until dicarbinol was completely dissolved.
Trifluoroacetic acid (TFA) (0.50 mmol) was added to initiate the condensation and the
reaction mixture was stirred at room temperature under inert atmosphere for 30-45 min.
DDQ (1.25 mmol) was then added and the reaction mixture was stirred at room temperature
in open air for additional 30 min. After completion of reaction as judged by TLC and UV-
Vis spectroscopy, the solvent was removed on a rotary evaporator under vacuum. The
resulting crude compounds were purified by basic alumina column chromatography using pet
ether-dichloromethane (80:20) as eluent and the desired dithia m-benzisapphyrins (6-7) and
dithia p-benzisapphyrins (8-10) were obtained as dark brown solids in 4-7% yields.
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Synthesis of compound 6
The compound 6 was synthesized from 12a (194 mg, 0.50 mmol) and 13a (203 mg, 0.50
mmol) by following general procedure reported above for compounds 6-10; Yield- 7% (26
1
mg); mp. >300° C; H NMR (400 MHz, CDCl3, in ppm) δ 8.03 (s, 1H), 7.78 (d, J = 5.5 Hz,
2H), 7.59-7.62 (m, 6H), 7.56 (d, J = 5.5 Hz, 2H), 7.51 (d, J = 7.5 Hz, 4H), 7.38-7.44 (m,
8H), 7.35 (d, J = 7.9 Hz, 4H), 7.08 (d, J = 4.6 Hz, 2H), 6.19 (t, J = 7.8 Hz, 1H), 2.49 (s, 6H).
13C NMR (126 MHz, CDCl3, in ppm)) δ 145.5, 139.6, 138.6, 137.4, 134.9, 133.3, 131.4,
129.9, 129.3, 129.1, 128.4, 125.6, 21.5. HRMS (ESI): calcd. for C52H37N2S2 [M + H]+ m/z
753.2393; found m/z 753.2452.
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Synthesis of compound 7
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The compound 7 was synthesized from 12b (224 mg, 0.50 mmol)) and 13a (203 mg, 0.50
mmol) by following general procedure given above for compounds 6-10; Yield- 5% (20 mg);
1
mp. >300° C; H NMR (400 MHz, CDCl3) δ 8.12 (s, 1H), 7.74 (d, J = 5.5 Hz, 2H), 7.57 –
7.59 (m, 6H), 7.50-7.53 (m, J = 7.0 Hz, 6H), 7.42 (dd, J = 7.8, 1.7 Hz, 2H), 7.34 (d, J = 7.8
Hz, 4H), 7.07 (d, J = 4.9 Hz, 2H), 6.97 (d, J = 7.1 Hz, 4H), 6.21 (t, J = 7.8 Hz, 1H), 3.88 (s,
13
6H), 2.49 (s, 6H). C NMR (101 MHz, CDCl3) δ 161.0, 138.6, 137.3, 134.8, 134.6, 131.4,
129.3, 129.1, 125.6, 114.0, 55.6, 29.8. HRMS (ESI): calcd. for C54H41N2O2S2 [M + H]+ m/z
813.2604; found m/z 813.2603.
Synthesis of compound 8
The compound 8 was synthesized from 15a (194 mg, 0.50 mmol) and 13a (203 mg, 0.50
mmol) by following general procedure reported above for compounds 6-10; Yield- 6% (22
mg); mp. >300° C; 1H NMR (600 MHz, CDCl3, -40° C) δ 10.51 (d, J = 4.8 Hz, 2H), 10.01 (d,
J = 4.8 Hz, 2H), 9.24 (s, 2H), 8.79 (d, J = 4.3 Hz, 2H), 8.71 (d, J = 4.3 Hz, 2H), 8.43 (d, J =
6.8 Hz, 4H), 8.27 (d, J = 7.6 Hz, 4H), 7.84–7.91 (m, 6H), 7.74 (d, J = 7.5 Hz, 4H), 2.76 (s,
13
6H), -1.91 (s, 2H). C NMR (101 MHz, CDCl3) δ 159.4, 155.8, 144.0, 143.6, 141.1, 140.1,
138.2, 138.0, 137.7, 135.8, 134.5, 131.6, 130.1, 128.8, 128.2, 128.1, 127.4, 21.7. HRMS
(ESI): calcd. for C52H37N2S2 [M + H]+ m/z 753.2393; found m/z 753.2389.
Synthesis of compound 9
The compound 9 was synthesized from 15b (224 mg, 0.50 mmol) and 13a (203 mg, 0.50
mmol) by following general procedure reported above for compounds 6-10; Yield- 4% (16
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mg); mp. >300° C; 1H NMR (600 MHz, CDCl3, -40° C) δ 10.44 (d, J = 4.9 Hz, 2H), 9.94 (d, J
= 4.8 Hz, 2H), 9.17 (s, 2H), 8.77 (d, J = 4.3 Hz, 2H), 8.66 (d, J = 4.3 Hz, 2H), 8.40 (d, J = 8.3
Hz, 4H), 8.26 (d, J = 7.6 Hz, 4H), 7.72 (d, J = 7.6 Hz, 4H), 7.45 (d, J = 8.4 Hz, 4H), 4.15 (s,
9
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6H), 2.76 (s, 6H), -1.64 (s, 2H). C NMR (126 MHz, CDCl3) δ 160.1, 159.6, 156.0, 143.5,
141.4, 140.1, 139.1, 138.3, 137.9, 137.6, 137.1, 135.5, 134.4, 131.2, 129.7, 128.8, 128.0,
113.1, 55.8, 29.8. HRMS (ESI): calcd. for C54H41N2O2S2 [M + H]+ m/z 813.2604; found m/z
813.2596.
Synthesis of compound 10
The compound 10 was synthesized from 15a (194 mg, 0.50 mmol) and 13b (234 mg, 0.50
mmol) by following general procedure reported above for compounds 9-10; Yield- 4% (17
mg); mp. >300° C; 1H NMR (600 MHz, CDCl3, -40° C) δ 10.43 (d, J = 4.4 Hz, 2H), 9.76 (d, J
= 4.6 Hz, 2H), 9.10 (s, 2H), 8.77 (m, 6H), 8.47-8.49 (m, J = 6.7 Hz, 6H), 8.38 (d, J = 6.6 Hz,
13
4H), 7.95 – 7.86 (m, 6H), -1.32 (s, 2H). C NMR (101 MHz, CDCl3) δ 159.1, 155.7, 148.0,
147.6, 143.8, 143.5, 143.2, 140.3, 138.8, 138.6, 135.8, 134.9, 130.5, 129.2, 128.6, 127.6,
126.4, 123.2. HRMS (ESI): calcd. for C50H31N4O4S2 [M + H]+ m/z 815.1781; found m/z
815.1792.
Associated Content
Supporting Information
HRMS spectra, spectral, electrochemical and computational data is included in supporting
information. This material is available free of charge via the Internet at http:// pubs.acs.org.
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Corresponding Author
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*E-mail: ravikanth@chem.iitb.ac.in.
Author Contributions
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† R.S. and K.G.T. contributed equally.
Acknowledgements
M.R thanks the Department of Science and Technology, Govt. of India (File No.
EMR/2015/002196 to M.R.) R.S. thanks the CSIR India for research fellowship and K.G.T
thanks to IIT Bombay for a post-doctoral fellowship.
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