meso-Aryl sapphyrins with heteroatoms; synthesis, characterization, spectral and electrochemical properties

Article abstract of DOI:10.1039/a900137i

The synthesis, characterization and spectral properties of six new meso-aryl core modified sapphyrins are described. An efficient approach involving an acid catalyzed condensation of bithiophene diol 1 and modified tripyrranes 2a-2e allows preparation of the desired meso-aryl sapphyrins in 16-36% yield. The product distribution and the isolated yield were found to be dependent on the nature of the acid catalyst (Lewis acid or protic acid) and its concentration. Protic acid catalyst exclusively gave the expected sapphyrins while two additional products, an 18π tetraphenylporphyrin and a 26π modified rubyrin, were isolated under Lewis acid catalysis. An analysis of proton NMR and absorption spectral data suggests that in free base sapphyrins, the heterocyclic ring opposite to the bithiophene unit is inverted as in N-5 meso-aryl sapphyrin and the degree of inversion is dependent on the nature of the heterocyclic ring. The energy optimized structure calculated from the semi-empirical method substantiates such a conclusion. Protonation of sapphyrins generates respective mono- and dications and the heterocyclic ring retains an inverted structure in contrast to normal N-5 sapphyrins. The triplet excited lifetimes for free base and protonated derivatives are similar both under argon saturated and air equilibrated conditions, indicating that the triplet state quenching by oxygen is minimal. Cyclic voltammetric studies reveal easier reductions and harder oxidations relative to meso-aryl porphyrins and the Δ_redox observed for 3d suggests significant reduction of the HOMO-LUMO energy gap consistent with the large red shift observed for the Soret band.

Full text of DOI:10.1039/a900137i

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meso-Aryl sapphyrins with heteroatoms; synthesis,
characterization, spectral and electrochemical properties
Alagar Srinivasan,a Simi K. Pushpan,a Murugaeson Ravi Kumar,a Sumeet Mahajan,a
Tavarekere K. Chandrashekar,*a Raja Royb and P. Ramamurthyc
a Department of Chemistry, Indian Institute of Technology, Kanpur, U.P. 208 016, India
b NMR Division, Central Drug Research Institute, Lucknow-226 001, India
c Department of Inorganic Chemistry, University of Madras, Guindy Campus,
Chennai-600 025, India
Received (in Cambridge) 5th January 1999, Accepted 1st March 1999
The synthesis, characterization and spectral properties of six new meso-aryl core modified sapphyrins are described.
An efficient approach involving an acid catalyzed condensation of bithiophene diol 1 and modified tripyrranes
2a–2e allows preparation of the desired meso-aryl sapphyrins in 16–36% yield. The product distribution and the
isolated yield were found to be dependent on the nature of the acid catalyst (Lewis acid or protic acid) and its
concentration. Protic acid catalyst exclusively gave the expected sapphyrins while two additional products, an 18π
tetraphenylporphyrin and a 26π modified rubyrin, were isolated under Lewis acid catalysis. An analysis of proton
NMR and absorption spectral data suggests that in free base sapphyrins, the heterocyclic ring opposite to the
bithiophene unit is inverted as in N-5 meso-aryl sapphyrin and the degree of inversion is dependent on the nature
of the heterocyclic ring. The energy optimized structure calculated from the semi-empirical method substantiates
such a conclusion. Protonation of sapphyrins generates respective mono- and dications and the heterocyclic ring
retains an inverted structure in contrast to normal N-5 sapphyrins. The triplet excited lifetimes for free base and
protonated derivatives are similar both under argon saturated and air equilibrated conditions, indicating that the
triplet state quenching by oxygen is minimal. Cyclic voltammetric studies reveal easier reductions and harder
oxidations relative to meso-aryl porphyrins and the ∆redox observed for 3d suggests significant reduction of the
HOMO–LUMO energy gap consistent with the large red shift observed for the Soret band.
include ozaphyrins and bronzaphyrins,13 thiophene and furan
Introduction
containing annulenes,14 selenium and sulfur containing penta-
Research on expanded porphyrins1 in general and sapphyrins2
phyrins15 and furan containing 26π expanded macrocycles.16
in particular has received considerable attention in recent
Recent studies from this laboratory and of others describe the
years because of their potential biomedical applications as, for
synthesis of meso-aryl sapphyrins with heteroatoms.17,25
example, photosensitizers for PDT, MRI contrasting agents3
An understanding of the spectral and electrochemical prop-
erties of sapphyrins in the ground and excited states is an
important first step towards their potential biomedical appli-
and macrocyclic receptors for transport of neutral4 and anionic
substrates.4a,b,5 They are also of interest in terms of aromaticity
in large conjugated systems6 and the range of coordination
cation. In this direction, only β-substituted sapphyrin dication
environment available for transition metals.5d,7 Synthetic
has been probed for its redox behaviour,5a photoexcited
methods available in the literature for the synthesis of sapph-
singlet5a and triplet state properties18,19 and photodynamic
yrins include: a traditional [3 ϩ 2] acid catalysed MacDonald
activity.20 However, to the best of our knowledge there are no
condensation between the appropriate precursors,1 reaction
systematic studies on the meso-aryl sapphyrins in general and
core modified sapphyrins in particular. In this paper, we wish to
report not only the synthesis of a series of meso-aryl sapphyrins
of diformylbipyrrole with benzaldehyde and pyrrole,8 an acid
catalysed condensation of biladienes-ac with pyrrole-2-carb-
aldehyde9a and of terpyrranedicarboxylic acid with pyrrole-2,5-
bearing heteroatoms, but also their spectral and electrochemi-
dicarbaldehyde.9b The majority of the sapphyrins reported to
cal behaviour. A comparison of their properties with those
date are limited to β-pyrrole substituted macrocycles linked
observed for β-substituted sapphyrins reveals some important
through meso-carbon bridges. Only recently, an N-5 meso-aryl
differences attributable to the effect of meso-substitution and
sapphyrin was isolated as a byproduct from a Rothemund
the presence of heteroatoms. Unlike β-substituted sapphyrins,
the sapphyrins reported here show inversion of the heterocyclic
ring opposite to the bithiophene unit in their free base as well as
reaction10a and from an acid catalysed self coupling reaction of
dipyrromethane.10b
Core modification by replacement of one or more pyrrolic
in their protonated forms. However, a complete flip of the
units by other heterocycles such as furan, thiophene and seleno-
inverted heterocycle upon protonation as reported for N-5
phene leads to a new class of sapphyrins. These macrocycles by
virtue of their altered electronic structure are expected to have
meso-aryl sapphyrin is not observed in the core modified meso-
aryl sapphyrins reported here.10a
optical, electrochemical and excited state properties different
from all their pyrrolic counterparts.11 A perusal of the literature
reveals only limited reports on core modified expanded por-
phyrins. Sessler and coworkers have reported a series of core
modified β-substituted sapphyrins containing furan, thiophene
and selenophene rings and core modified β-substituted iso-
smaragdyrins containing a furan ring.12 The other reports
Results and discussion
(a) Synthesis and characterization
The synthetic method followed here is basically a modified
[3 ϩ 2] MacDonald condensation which has been traditionally
J. Chem. Soc., Perkin Trans. 2, 1999, 961–968
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Table 1 The chemical shifts of the protons of the inverted heterocyclic
ring opposite to the bithiophene unit before and after protonation
Chemical shift (ppm)
Compound Inverted ring Proton
Free base (∆δ)b
Dicationc
3a
3b
Pyrrole
NH
11.35
Ϫ0.84 (9.70)
—
2.71
Ϫ1.15 (10.03)
—
Ϫ0.73 (9.40)
—
0.61 (9.18)
—
Ϫ0.27 (9.10)
—
13.60
Ϫ1.06
Ϫ4.11
2.71
Ϫ1.09
Ϫ3.67
Ϫ1.20
Ϫ3.15
0.31
β-CH
NHa
N-CH3
β-CH
NHa
β-CH
NHa
β-CH
NHa
β-CH
NHa
N-methyl-
pyrrole
3c
3d
3e
Thiophene
Furan
Ϫ1.25
Ϫ1.17
Ϫ2.50
Selenophene
a Corresponds to NH protons of pyrrole after protonation. b Corre-
sponds to difference in the chemical shift of the ring inverted β-protons
and the adjacent pyrrole ring β-protons. c Dications were generated by
adding trifluoroacetic acid in CDCl3 solution.
(15%) and 5 (5%) at the expense of 3c (7%). Change of Lewis
acid to SnCl4 (4.35 × 10Ϫ4 M) also gave 5 (11%), 4 (3%) and 3c
(20%).
The cyclization has to occur through the formation of a carbo-
cation in 1 by the acid catalyst. The sole formation of the
expected sapphyrins with one equivalent of TFA indicates that
the generation of carbocation occurs through protonation
followed by elimination of water from 1. The formation of
additional products 4 and 5 in the presence of BF3ؒOEt2
catalysts can be explained by acidolysis of tripyrromethanes
on the time scale of sapphyrin formation in addition to carbo-
cation generation. It is possible that the metal on the Lewis acid
coordinates to the heteroatom on the tripyrrane, triggering its
acidolysis. The observed increase in the yields of 4 and 5 at the
expense of 3c on increasing the concentration of Lewis acid
supports such a possibility. There is spectroscopic evidence that
the tripyrromethanes undergo acidolysis. For example: (a) stir-
ring of 2c, 2d or 2e with benzaldehyde in CH2Cl2 under nitrogen
atmosphere for 15 min followed by addition of BF3ؒOEt2
(6.09 × 10Ϫ4 M) and subsequent oxidation with chloranil
under reflux for an hour resulted in the formation of ~2% meso-
tetraphenylporphyrin and monothia, monooxa or monoselena
porphyrin respectively. (b) The gradual decrease in the absorb-
ance of tripyrrane in CH2Cl2 in the UV–Visible spectrum (475
nm) upon addition of BF3ؒOEt2 with time and subsequent
TLC analysis (silica gel, ethyl acetate–petroleum ether (1:9))
of the mixture reveal a decrease in concentration of tripyrrane
and a new purple spot (Rf: 0.23) was noticed suggesting the
formation of uncyclized conjugated macrocycles. Further
support for the acidolysis of precursors dipyrromethane or
tripyrrane comes from recent work of Lindsey,22a Lash22c and
coworkers.
Scheme 1 Synthetic scheme for the preparation of core modified
meso-aryl sapphyrins.
used for the synthesis of sapphyrins1 (Scheme 1). Only recently,
Smith and coworkers9 have reported a new method which
avoids preparation of the bipyrrole precursor and is different
from the MacDonald approach. The present method is based
on Ulman’s reaction reported for the synthesis of core modified
mono- and dithia porphyrins.21 The key precursors required for
the synthesis were modified tripyrranes 2a–2f. The tripyrranes
2a, 2c, 2d and 2f were already known in the literature22 and
hitherto unknown 2b and 2e were synthesised by a similar
method by the reaction of 2,5-bis(phenylhydroxymethyl)-N-
methylpyrrole and 2,5-bis(phenylhydroxymethyl)selenophene
with pyrrole in the presence of 0.1 equivalent of TFA as the
catalyst in 33 and 72% yield respectively. Thus, the reaction
of 2a–2e with bithiophene diol synthesised from an earlier
reported method,23 on condensation in dichloromethane con-
taining one equivalent of TFA gave the desired core modified
sapphyrins 3a–3e in moderately good yields. The yields com-
pare well with those reported by Dolphin and coworkers24 for
the synthesis of meso-aryl N-5 sapphyrins (up to 39%) and are
better than those reported for 5,10-meso-diphenylsapphyrin8
(10%) and tetraphenylsapphyrin (1.1%), (7.5%),10 and 26,28-
dithia, dioxa and diselena substituted sapphyrins (15%).25
Change of catalyst from protic acid to Lewis acid changes the
product distribution and yield. For example, use of BF3ؒOEt2
(3.05 × 10Ϫ5 M) as the catalyst and 2c as the substrate under
similar conditions gave two additional products 4 and 5 in
addition to 3c in 4, 2 and 30% yield respectively. A higher con-
centration of BF3ؒOEt2 (6.05 × 10Ϫ5 M) produced more of 4
Grazynski and coworkers10a have recently reported that the
pyrrole ring opposite to the bipyrrolic unit in meso-aryl
1
sapphyrin is inverted in its free base form on the basis of H
NMR chemical shifts observed for the NH proton and the
β-pyrrole protons of the inverted ring. The chemical shifts
observed were 11.76 ppm for the NH proton and Ϫ1.28 ppm
for the inner β-CH protons. In the meso-aryl sapphyrins
reported here such a ring inversion is indeed observed and Table
1 lists the chemical shifts of the relevant protons of the inverted
heterocyclic ring opposite to the bithiophene unit. Thus, in 3a
the NH proton resonates at 11.35 ppm while β-CH protons
resonate at Ϫ0.84 ppm. A comparison of ∆δ values (calculated
from the difference between the chemical shift of the ring
inverted β-protons and the adjacent pyrrole ring β-protons)
reveals that those in 3a–3e (10.03–9.10 ppm) are slightly smaller
as compared to N-5 meso-aryl sapphyrin (10.25 ppm),10a sug-
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gesting a small decrease in the ring current. The non-planarity
of the sapphyrin on heteroatom substitution accounts for this
difference. The geometry optimized structure for 3d shown in
Fig. 1 reveals the ring inversion.
On protonation, N-5 meso-aryl sapphyrin undergoes a
dramatic 180Њ flip of the inverted pyrrole ring as evidenced by
a large upfield–downfield shift of the NH protons (11.75 to
Ϫ2.74 ppm) and β-pyrrole protons (Ϫ1.21 to 8.87 ppm). Both
monocationic and dicationic species have been identified and it
has been shown that the ring flipping occurs during formation
of dicationic species.25c,26 The NMR titration studies on 3d also
reveal formation of both mono- and dicationic species (Fig. 2).
It has been observed that the bithiophene, pyrrole and meso-
phenyl protons undergo gradual deshielding (0.28, 0.71 and
0.33 ppm) consistent with the observation of Grazynski.26 In
contrast, the inverted furan ring protons experience shielding
(0.31 ppm) suggesting that the dramatic ring flipping observed
for N-5 meso-aryl sapphyrin does not occur in the modified
sapphyrins reported here. This is also consistent with the
observation of Grazynski on 26,28-dioxa- and 26,28-dithia-
sapphyrins.25c It has been shown by detailed NMR studies that
in 26,28-dioxasapphyrin, the pyrrole ring opposite to the bi-
pyrrole is inverted while 26,28-dithiasapphyrin shows a planar
structure.25c It is also observed that on protonation no ring
flipping takes place in the dioxa case. It is pertinent to point out
here that the NH signals were not observed at room temper-
ature because of the possible rapid tautomerism. Formation of
mono- and dicationic species is shown in Scheme 2.
Fig. 1 Geometry optimized structure of 3d. (A) Top view, (B) side
view showing the ring inversion. The meso-aryl rings are deleted for
clarity.
Fig. 2 1H NMR spectrum of (a) 3d (1 × 10Ϫ4 M) in CDCl3, (b) 3d (1 × 10Ϫ4 M) containing 0.02 equiv. of TFA, (c) 3d (1 × 10Ϫ4 M) containing 0.12
equiv. of TFA, (d) 3d (1 × 10Ϫ4 M) containing 2 equiv. of TFA.
J. Chem. Soc., Perkin Trans. 2, 1999, 961–968
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Scheme 2 Protonation scheme for core modified meso-aryl sapphyrins.
Table 2 UV–Vis spectral data of meso-aryl core modified sapphyrins and their mono- and diprotonated derivatives in CH2Cl2
Q-band, λmax/nm (ε/103 dm3 molϪ1 cmϪ1
)
Soret, λmax/nm
Compound
(ε/104 dm3 molϪ1 cmϪ1
)
IV
III
II
I
3a
510 (4.7)
515 (3.9)
638 (7.0)
698 (10.0)
702 (7.1)
764 sh (3.7)
768 (7.75)
864 (4.2)
824 (5.0)
793 (16.75)
863 (3.74)
822 (6.0)
801 (20.40)
875 (4.5)
3aؒH؉
3aؒ2H؉
3b
519 (4.84)
510 (6.29)
515 (4.51)
520 (6.67)
507 (14.27)
512 (10.13)
520 (11.05)
511 (26.93)
520 (22.95)
525 (30)
650 (5.10)
705 (13.33)
709 (4.73)
770 (2.93)
772 (8.87)
658 (3.9)
777 (1.87)
778 (12.8)
778 (20.53)
780 (7.75)
773 (32.5)
766 (49)
3bؒH؉
3bؒ2H؉
3c
622 (9.73)
626 (5.2)
678 (20.13)
677 (10.13)
3cؒH؉
3cؒ2H؉
3d
838 (9.7)
838 (15.2)
883 (10.25)
843 (28.25)
832 (33.75)
871 (2.3)
846 (7.5)
842 (13.4)
624 (23.25)
680 (37.25)
681 (21.5)
3dؒH؉
3dؒ2H؉
3e
507 (10.25)
517 (6.94)
526 (11.20)
598 (9.7)
634 (5.1)
670 (8.90)
670 (5.90)
769 (1.5)
771 (8.3)
778 (16)
3eؒH؉
3eؒ2H؉
The UV–Visible absorption spectral data of the sapphyrins
are tabulated in Table 2. A comparison of this data with those
of β-substituted core modified sapphyrins7c,d,12 reveals inter-
esting substituent effects: (a) In general, the visible spectra of
the β-substituted core modified sapphyrins are not too much
different from those of the parent N-5 sapphyrins and only
small absorption band shifts are observed upon heteroatom
substitution. For example, the oxasapphyrins12 exhibit a small
blue shift of the Soret band (1–2 nm) and a red shift of the
Q-bands (10–35 nm) with only marginal changes in the ε-values.
On the other hand, the meso-aryl sapphyrins show a red shift of
both Soret and Q-bands upon heteroatom substitution and the
magnitude of the red shifts depends on the number and nature
of the heteroatoms with significant changes in ε-values. For
example, 3d shows an 18 nm red shift of the Soret band and an
88 nm shift for the Q-I band relative to parent N-5 meso-aryl
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Fig. 3 Absorption spectrum of 3d in CH2Cl2 and its mono- and dipro-
tonated derivative in (A) Soret and (B) Q-band region. (——) Free base
(1 × 10Ϫ4 M), (----) monocation (1 × 10Ϫ4 M containing 0.12 equiv. of
TFA) and (....) dication (1 × 10Ϫ4 M containing 2 equiv. of TFA).
sapphyrin10a and the ε-value for the Soret band is almost
doubled, suggesting that the electronic effect of the heteroatom
substitution is significant in meso-aryl sapphyrins.11 (b) Since all
the sapphyrins contain two pyridine type nitrogens, they can
be either monoprotonated or diprotonated like N-5 meso-aryl
sapphyrin. The monocations and dications of 3a–3e are identi-
fied through UV spectral titrations. A representative spectrum
of 3d is shown in Fig. 3. It is evident that the Soret and Q-I
bands of the monocation lie in between those of the free base
and the dication, while the other Q-bands almost remain at the
same position but with decreased intensity compared to the free
base. On further increasing the acid concentration, the Soret
band is shifted further towards the red and the number of
Q-bands decreases to two from four. Change of solvent from
dichloromethane to methanol did not result in any appreciable
change, further confirming that the ring flipping is not taking
place upon protonation.26 It is well known that the meso-aryl
porphyrins27 undergo a structural change upon protonation by
releasing the repulsive interaction between the ortho-hydrogens
of the meso-phenyl rings and the adjacent pyrrole protons. This
results in phenyl rings becoming more coplanar with the
porphyrin plane, facilitating the delocalization of π-electrons
into the phenyl rings by resonance interaction and this is
spectroscopically manifested as a red shift of the absorption
bands. Such an interaction is not possible in the β-substituted
derivatives due to the lack of a meso-phenyl substituent.
However, it is pertinent to point out here that the N-5 meso-aryl
sapphyrin shows a blue shift of the Soret band (9 nm) upon
protonation10a in contrast to the red shift observed for the
meso-aryl modified sapphyrins. Thus, the meso-aryl core
modified sapphyrins behave like meso-aryl porphyrins while
β-substituted sapphyrins resemble octaethyl porphyrins in their
spectral behaviour.27
Fig. 4 T–T transient absorption spectrum of (A) 3d (3.5 × 10Ϫ5 M in
CHCl3) and (B) its protonated derivative (3.5 × 10Ϫ5 M containing
~10Ϫ3 M of TFA in CHCl3). The inset shows the triplet excited state
decay.
Q-bands of the ground state and a positive absorbance change
in the region 380–600 nm. The triplet lifetimes calculated
from the decay profiles (Fig. 4 inset) are tabulated in Table 3.
Comparison of triplet lifetimes of meso-aryl sapphyrins with
those of texaphyrins19 reveals that the lifetimes observed here
are about two orders of magnitude lower. The β-substituted
sapphyrin dication18 has a triplet lifetime of 60 5 µs which is
again much higher than those of meso-aryl sapphyrins. The
short lived triplet suggests that the rate of internal conversion
from the singlet excited state is much more efficient for meso-
aryl sapphyrins relative to texaphyrins and sapphyrin dication.
Another reason for the short lifetime could be because of the
interaction with the solvent CHCl3. It is known that the halo-
gen atoms can affect the non-radiative deactivation process
through a spin-orbit coupling effect.28 Thus, the triplet lifetime
data suggest that the β-substituted sapphyrin dication5a,20 and
texaphyrins19 are better suited for the PDT application in view
of their longer triplet lifetimes and better quantum efficiency of
singlet oxygen generation19 than the meso-aryl core modified
sapphyrins. No attempts were made to measure the quantum
yield of singlet oxygen generation.
(c) Electrochemical studies
The cyclic voltammograms for 3c and 3e in CH2Cl2 at 0.1 M in
TBAP recorded in the potential region 1.5 to Ϫ1.5 V vs. SCE
are shown in Fig. 5. All the sapphyrins exhibit two quasi revers-
ible reductions (∆Ep = 90–150 mV) and two irreversible oxid-
ations.29 However, for 3a and 3b, scanning the voltage in the
positive potential also showed two irreversible oxidations
but on repetitive scans, the peak potentials kept on shifting
indicating some decomposition. The E1/2 values listed in Table 4
refer to the average of the two peak potentials at slow scan
(b) Triplet excited state properties
The triplet–triplet transient spectra of 3d and its protonated
derivative, shown in Fig. 4, show bleaching in the Soret and
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Table 3 The triplet excited state parameters of core modified meso-aryl sapphyrins and their protonated derivatives in CHCl3
Triplet lifetime/µs
Excited T–T absorption/nm
Argon saturated
Free base
Air equilibrated
Free base
Compound
Free base
Dication
Dication
Dication
3c
3d
3e
435, 570
430, 565
445, 540
430, 595
430, 585
440, 570
0.88
1.26
0.03
0.81
1.44
0.05
0.75
1.85
—
0.70
1.90
—
Table 4 Redox potentials of core modified meso-aryl sapphyrins
ox1
1/2
ox2
1/2
red1
1/2
red2
1/2
Compound
E
/V
E
/V
E
/V
E
/V
∆
a/V
redox
3a
3b
3c
3d
3e
—
—
0.80
0.76
0.82
—
—
1.19
1.14
1.16
Ϫ0.93
Ϫ1.09
Ϫ0.91
Ϫ0.94
Ϫ0.92
Ϫ1.39
Ϫ1.30
Ϫ1.12
Ϫ1.15
Ϫ1.12
—
—
1.71
1.70
1.74
ox1
a Calculated from difference in E and E1r/e2d1
.
1/2
Fig. 5 The cyclic voltammogram of (A) 3c and (B) 3e in CH2Cl2
containing TBAP (0.01 M) recorded at 50 mV sϪ1 vs. SCE. The concen-
trations of sapphyrins were ~10Ϫ3 M.
rates (50 mv sϪ1). In general, there are only minor changes in
the reduction potentials among the meso-aryl sapphyrins.
Comparison of these with the protonated derivatives of
β-substituted sapphyrins5a (containing BrϪ and ClϪ counter
anions)30 suggests easier reductions for the meso-aryl core
modified sapphyrins by about 120–200 mV. This is not surpris-
ing since the substitution of a heteroatom into the porphyrin
core leads to easier reductions and harder oxidations relative
to the normal porphyrins suggesting changes in the energies of
the HOMO and LUMO. Indeed, Ulman and coworkers31 have
shown that in heteroatom substituted porphyrins, both the
HOMO and LUMO are stabilized by different mechanisms. In
the present study, the ∆redox calculated from the difference of
first oxidation potential and first reduction potential indicates
significant decreases in the ∆redox values relative to meso-aryl
porphyrins (2.26 V for H2TPP).27b,31 This suggests a decrease
in the HOMO–LUMO gap in meso-aryl sapphyrins relative to
porphyrins. Thus, the observed red shifts of the Soret and
Q-bands in the UV–Visible spectra are consistent with this.
Among the meso-aryl sapphyrins reported here, 3d has the
least ∆redox value and indeed the Soret band of 3d shows the
maximum red shift relative to the others. The redox processes of
the meso-aryl sapphyrins are summarized in Scheme 3.
Scheme 3 The redox process of core modified meso-aryl sapphyrins.
the synthesis of core modified expanded porphyrins, we hope to
explore their rich and fascinating chemistry in the coming years.
Experimental
All the chemicals used for the synthesis were reagent grade
unless otherwise specified. Solvents for spectroscopic measure-
ments were purified and dried according to the standard
methods. The instrumentation used for UV–Visible, 1H NMR,
FAB mass and elemental analysis was the same as that
described previously.32 Chemical shifts are given in ppm and
J values in Hz. For the triplet excited state studies, a Quantel
481 Nd:Yag laser operated in the Q-switched mode was used as
excitation source. All the samples were excited at 520 nm. A
high intensity Xe-arc lamp was used as monitoring beam. The
absorbance changes were detected with a red sensitive photo-
multiplier tube digitized with a Biomation 8100 recorder before
being analysed with a PDP/1170 computer. ILOAR argon
degassed 10Ϫ5 M solutions of sapphyrins were used for all the
measurements.
Conclusion
In this paper, we have described the synthesis of five meso-aryl
sapphyrins bearing heteroatoms. The spectral, electrochemical
and excited state studies reveal many interesting differences and
similarities relative to β-substituted core modified sapphyrins. It
has been shown that the meso-aryl hetero sapphyrins behave
more like tetraphenyl porphyrins in their spectral and electro-
chemical behaviour. The hetero sapphyrins have a very short
lived triplet excited state relative to β-substituted sapphyrin
dication. Preliminary studies on the protonated derivatives of
meso-aryl hetero sapphyrins suggest binding of the anions simi-
lar to that observed for the dicationic β-substituted sapphyrins.5
Finally, with the availability of good synthetic methodology for
Acid titration
For UV–Visible titration, a constant volume of a dry dichloro-
966
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methane solution of sapphyrin (1 × 10Ϫ4 M) was transferred
to 10 ml standard volumetric flasks. Then, different equivalents
(0.02–1.5) of 1.0 × 10Ϫ6 M TFA solution in CH2Cl2 were added
and the volume was made up to the mark with dry dichloro-
methane and the absorption spectra in the desired region
were recorded in overlay mode. In the case of NMR titration,
standard solutions were prepared by using CDCl3.
5,10,15,20-Tetraphenyl-27-N-methyl-25,29-dithiasapphyrin (3b)
5,5Ј-Bis(phenylhydroxymethyl)-2,2Ј-bithiophene (200 mg,
5.29 × 10Ϫ4 mol), 5,10-diphenyl-16-N-methyltripyrrane (210
mg, 5.37 × 10Ϫ4 mol), trifluoroacetic acid (0.04 ml, 5.29 × 10Ϫ4
mol) and chloranil (390 mg, 1.58 × 10Ϫ3 mol) under similar
reaction conditions as mentioned above gave a green lustrous
solid identified as 3b in 26% yield. 1H NMR (300 MHz, CDCl3):
δ 10.38–10.36 (d, 2H, J 6), 9.94–9.93 (d, 2H, J 3), 8.89–8.87 (d,
2H, J 3), 8.81–8.80 (d, 2H, J 3), 8.40 (m, 4H), 7.92–7.80 (m,
10H), 7.72–7.53 (m, 6H), 2.71 (s, 3H), Ϫ1.15 (s, 2H). 1H NMR
(300 MHz, CDCl3–TFA): δ 10.60–10.58 (d, J 6, 2H), 10.30–
10.28 (d, J 6, 2H), 9.53–9.51 (d, J 6, 2H), 9.47–9.45 (d, J 6, 2H),
8.72–8.60 (m, 4H), 8.20–8.13 (m, 10H), 8.10–8.02 (m, 6H), 2.71
(s, 3H), Ϫ1.09 (s, 2H), Ϫ3.67 (br, s, 2H). Anal. calcd. for
C49H33N3S2: C, 80.85; H, 4.57; N, 5.77. Found: C, 80.92; H,
4.73; N, 5.62%.
Molecular mechanics calculations
The geometry-optimized structure was calculated on an HCL-
HP Pentium 120 MHz desktop computer using Hyperchem
version 5.0. The semiempirical AM1 method and the Polak–
Ribiere algorithm with the gradient set at 0.1 were used for the
calculation.
5,10-Diphenyl-16-N-methyltripyrrane (2b)
A mixture of 2,5-bis(phenylhydroxymethyl)-N-methylpyrrole
(500 mg, 1.7 × 10Ϫ3 mol)21 and pyrrole (4.73 ml, 6.8 × 10Ϫ2 mol)
was degassed by bubbling with argon for 10 min. Trifluoro-
acetic acid (0.01 ml, 1.7 × 10Ϫ4 mol) was added and the mixture
was stirred for 30 min at room temperature. It was diluted with
CH2Cl2 (100 ml), then washed with 0.1 M NaOH, followed by
water washing. The organic layer was dried over anhydrous
Na2SO4. The solvent was removed under reduced pressure and
the unreacted pyrrole was removed by vacuum distillation at
room temperature. The resulting viscous dark yellow liquid was
purified by column chromatography (silica gel 100–200 mesh,
ethyl acetate–petroleum ether (10:90)). After the initial tailing
material, a pale orange band eluted which gave orange oil
identified as 2b in 33% yield. 1H NMR (300 MHz, CDCl3): δ 7.9
(br, s, 2H), 7.12–7.34 (m, 10H), 6.685 (m, 2H), 6.14 (m, 2H),
5.83 (m, 2H), 5.58 (m, 2H), 5.38 (m, 2H), 3.10 (s, 3H). EI mass:
m/z calcd. for C27H25N3 392, found 389 (37%) [(M Ϫ 3)ϩ].
5,10,15,20-Tetraphenyl-25,27,29-trithiasapphyrin (3c)
5,5Ј-Bis(phenylhydroxymethyl)-2,2Ј-bithiophene (150 mg,
3.97 × 10Ϫ4 mol), 5,10-diphenyl-16-thiatripyrrane (156 mg,
3.97 × 10Ϫ4 mol), trifluoroacetic acid (0.03 ml, 3.97 × 10Ϫ4 mol)
and chloranil (293 mg, 1.191 × 10Ϫ3 mol) under similar reaction
conditions as mentioned above gave a green lustrous solid
1
identified as 3c in 36% yield. H NMR (300 MHz, CDCl3):
δ 10.27–10.26 (d, 2H, J 3), 9.82–9.80 (d, 2H, J 6), 8.67–8.66 (d,
2H, J 3), 8.57–8.56 (d, 2H, J 3), 8.39–8.32 (m, 8H), 7.91–7.65
1
(m, 12H), Ϫ0.73 (s, 2H). H NMR (300 MHz, CDCl3–TFA):
δ 10.61–10.59 (d, J 6, 2H), 10.20–10.18 (d, J 6, 2H), 9.39–9.38
(d, J 3, 2H), 9.33–9.31 (d, J 6, 2H), 8.77–8.67 (m, 8H),
8.16–7.95 (m, 12H), Ϫ1.24 (s, 2H), Ϫ3.10 (br, s, 2H). FAB MS:
m/z calcd. for C48H30N2S3 731, found 732 (100%) [(M ϩ 1)ϩ].
Anal. calcd. for C48H30N2S3: C, 78.87; H, 4.14; N, 3.83. Found:
C, 78.52; H, 4.36; N, 3.91%.
5,10-Diphenyl-16-selenatripyrrane (2e)
5,10,15,20-Tetraphenyl-27-oxa-25,29-dithiasapphyrin (3d)
2,5-Bis(phenylhydroxymethyl)selenophene (500 mg, 1.46 × 10Ϫ3
mol), pyrrole (4 ml, 5.83 × 10Ϫ2 mol) and trifluoroacetic acid
(0.01 ml, 1.46 × 10Ϫ4 mol) under similar reaction conditions
as mentioned above gave pale green oil identified as 2e in 72%
yield. 1H NMR (300 MHz, CDCl3): δ 7.91 (br, s, 2H), 7.35–7.24
(m, 10H), 6.80 (s, 2H), 6.68 (m, 2H), 6.16–6.13 (m, 2H), 5.97
(s, 2H), 5.59 (s, 2H). EI mass: m/z calcd. for C26H22N2Se 441,
found 443 (25%) [(M ϩ 2)ϩ].
5,5Ј-Bis(phenylhydroxymethyl)-2,2Ј-bithiophene (150 mg,
3.97 × 10Ϫ4 mol), 5,10-diphenyl-16-oxatripyrrane (150 mg,
3.97 × 10Ϫ4 mol), trifluoroacetic acid (0.03 ml, 3.97 × 10Ϫ4 mol)
and chloranil (293 mg, 1.191 × 10Ϫ3 mol) under similar reaction
conditions as mentioned above gave a green lustrous solid
1
identified as 3d in 36% yield. H NMR (300 MHz, CDCl3):
δ 9.75–9.74 (d, 2H, J 3), 9.41–9.40 (d, 2H, J 3), 8.57–8.56 (d,
2H, J 3), 8.49–8.48 (d, 2H, J 3), 8.28–8.23 (m, 8H), 7.86–7.49
1
(m, 12H), 0.61 (s, 2H). H NMR (300 MHz, CDCl3–TFA):
5,10,15,20-Tetraphenyl-25,29-dithiasapphyrin (3a)
δ 10.06–10.05 (d, J 3, 2H), 9.63–9.61 (d, J 6, 2H), 9.33–9.31 (d,
J 6, 2H), 9.23–9.21 (d, J 6, 2H), 8.60–8.57 (m, 8H), 8.04–7.74
(m, 12H), 0.51 (s, 2H), Ϫ1.30 (br, s, 2H). FAB MS: m/z calcd.
for C48H30N2S2O 715, found 715 (25%) [Mϩ]. Anal. calcd. for
C48H30N2S2O: C, 80.64; H, 4.23; N, 3.92. Found: C, 80.87; H,
4.10; N, 3.69%.
5,5Ј-Bis(phenylhydroxymethyl)-2,2Ј-bithiophene (200 mg,
5.29 × 10Ϫ4 mol) and 5,10-diphenyltripyrrane (200 mg, 5.29 ×
Ϫ4
10 mol) in dry dichloromethane (200 ml) were stirred under
nitrogen atmosphere for 15 min at room temperature. Trifluoro-
acetic acid (0.04 ml, 5.29 × 10Ϫ4 mol) was added to the above
mixture. The solution was stirred for a further 1 h under dark
conditions. The resulting solution was opened to air and chlor-
anil (390 mg, 1.58 × 10Ϫ3 mol) was added and the mixture
was heated to reflux in a preheated oil bath at 50 ЊC for 1 h.
After removal of the solvent, the crude product was purified by
column chromatography (basic alumina). An orange band
eluted with CH2Cl2–ethyl acetate (95:5) gave green lustrous
solid identified as 3a in 16% yield. 1H NMR (300 MHz, CDCl3):
δ 11.35 (br, s, 1H), 10.31–10.29 (d, 2H, J 6), 9.89–9.87 (d, 2H,
J 6), 8.86–8.85 (d, 2H, J 3), 8.78–8.80 (d, 2H, J 6), 8.37 (m, 4H),
7.80–8.05 (m, 10H), 7.61–7.67 (m, 6H), Ϫ0.84 (s, 2H). 1H NMR
(300 MHz, CDCl3–TFA): δ 13.60 (br, s, 1H), 10.61–10.59 (d,
J 6, 2H), 10.20–10.18 (d, J 6, 2H), 9.65–9.63 (d, J 6, 2H), 9.55–
9.53 (d, J 6, 2H), 8.65 (m, 4H), 8.13–8.07 (m, 10H), 7.99–7.93
(m, 6H), Ϫ1.06 (s, 2H), Ϫ4.11 (br, s, 2H). MS (electrospray):
m/z calcd. for C48H31N3S2 714, found 714 (100%) [Mϩ]. Anal.
calcd. for C48H31N3S2: C, 80.75; H, 4.38; N, 5.89. Found: C,
80.92; H, 4.15; N, 6.02%.
5,10,15,20-Tetraphenyl-27-selena-25,29-dithiasapphyrin (3e)
5,5Ј-Bis(phenylhydroxymethyl)-2,2Ј-bithiophene (150 mg,
3.97 × 10Ϫ4 mol), 5,10-diphenyl-16-selenatripyrrane (176 mg,
3.97 × 10Ϫ4 mol), trifluoroacetic acid (0.03 ml, 3.97 × 10Ϫ4 mol)
and chloranil (293 mg, 1.191 × 10Ϫ3 mol) under similar reaction
conditions as mentioned above gave a green lustrous solid iden-
tified as 3e in 36% yield. 1H NMR (300 MHz, CDCl3): δ 10.65–
10.63 (d, 2H, J 6), 10.12–10.10 (d, 2H, J 6), 8.84–8.82 (d, 2H,
J 6), 8.55–8.53 (d, 2H, J 6), 8.41–8.29 (m, 8H), 7.91–7.63
1
(m, 12H), Ϫ0.27 (s, 2H). H NMR (300 MHz, CDCl3–TFA):
δ 10.53–10.52 (d, J 3, 2H), 10.14–10.13 (d, J 3, 2H), 9.29–9.28
(d, J 3, 2H), 9.13–9.11 (d, J 6, 2H), 8.72–8.65 (m, 8H), 8.15–
7.92 (m, 12H), Ϫ1.17 (s, 2H), Ϫ2.50 (br, s, 2H). FAB MS calcd.
for C48H30N2S2Se 778, found 778 (50%) [Mϩ]. Anal. calcd. for
C48H30N2S2Se: C, 74.12; H, 3.89; N, 3.60. Found: C, 74.31; H,
3.56; N, 3.92%.
J. Chem. Soc., Perkin Trans. 2, 1999, 961–968
967

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Acknowledgements
This work was supported by a grant from the Department of
Science and Technology, New Delhi, India to T. K. C.
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J. Chem. Soc., Perkin Trans. 2, 1999, 961–968