the increasing amounts of tetrabutylammonium iodide salt to the
protonated form of dyad 20 resulted in the gradual enhancement
of intensity of the 21-thiaporphyrin fluorescent band. This suggests
that the iodide ion was bound at the protonated 21,23-
dithiaphlorin site in dyad 20, which was reflected in the changes
in the intensity of the fluorescent band of the 21-thiaporphyrin
sub-unit, thus acting like a fluorescent sensor.{ More studies are
underway and will be reported in due course.
In conclusion, we report the first synthesis of 21,23-dithiaphlor-
ins and 21-monothiaphlorins containing one sp3 meso carbon
using easily available precursors. The method is simple, facile, and
allows the synthesis of any desired mono-functionalized thia
analogues of phlorin. The mono-fuctionalized thiaphlorins were
used further to synthesize the first examples of covalently linked
thiaphlorin–porphyrin dyads. The anion binding studies indicated
that the thiaphlorin and thiaphlorin–porphyrin dyads exhibit very
interesting anion binding properties.
Scheme 4 Synthesis of porphyrin–thiaphlorin dyads 19 and 21.
similar reaction conditions. The dyad 21 containing 21-mono-
thiaphlorin and ZnN4 porphyrin subunits was also synthesized in a
similar way by the coupling of 13 and 22 (Scheme 4).18 The dyads
19–21 are highly soluble in common organic solvents and
characterized by NMR, mass and absorption spectroscopic
techniques. The ES-MS mass spectra showed an M+ ion peak
confirming the dyads 19–21. The 1H NMR spectra of 19–21
showed peaks corresponding to both the monomeric thiaphlorin
and porphyrin sub-units in dyads 19–21 that interact very weakly.
The absorption spectra of 19–21 are essentially a linear combina-
tion of the spectra of the corresponding monomers with only
minor differences in wavelength maxima and band shapes,
indicating that the monomeric sub-units retain their individual
identities in the dyads 19–21.
This work was supported by a grant from the Department of
Science and Technology and Council of Scientific & Industrial
Research to MR. We thank Dr Neeraj Agarwal for his help during
the initial stages of this project.
Notes and references
{ Crystal data for C48H38N2S2, M = 706.92, monoclinic, space group P21/n
˚
(No. 14), a = 8.990(1), b = 16.034(1), c = 25.816(1) A, b = 98.78(1)u, V =
3677.7(5) A , Dc = 1.277 g cm23, m = 1.83 cm21, Z = 2, l = 0.71073 A, T =
3
˚
˚
198 K, 19039 reflections collected (¡h, ¡k, ¡l), [(sinh)/l] = 0.59 A21, 6457
˚
independent (Rint = 0.050) and 4623 observed reflections [I ¢ 2s(I)], 492
refined parameters, R = 0.064, wR2 = 0.169. CCDC 235993. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b607013b
The preliminary binding studies for halides as well as other
2
anions like HSO4 and SCN2 have been carried out19 with
protonated forms of the monomeric 21,23-dithiaphlorin 7 and
dyads 19 and 20. The protonated species of 7 was generated by
careful addition of a dilute solution of trifluoroacetic acid (2 6
1022 M) to 7 (1.5 6 1026 M) in dichloromethane. On addition of
trifluoroacetic acid to 7, the colour of the solution changes from
green to light yellow. While a detailed study is needed, the
preliminary investigation indicated that the protonated form of 7
binds halide anions as well as other anions as judged from optical
studies.19{ The protonated form of 7 showed a strong peak at
825 nm which decreased its intensity when it was titrated with
increasing amounts of anionic salts, confirming the binding of an
anion. The binding constants were evaluated using a 1 : 1 binding
model.20{
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The binding studies were also carried out with dyad 19, which
has the Zn(II) ion in the porphyrin sub-unit, by selectively
protonating the 21,23-dithiaporphotrimethene sub-unit with dilute
trifluoroacetic acid without inducing the demetalation of the
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intensity of the absorption band at 842 nm supported the binding
of anion at the 21,23-dithiaphlorin sub-unit site.{ Thus, the dyad
19 is the novel system which binds anion and cation simulta-
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thiaporphyrin and 21,23-dithiaphlorin sub-units as a fluorescent
sensor. Since the 21,23-dithiaphlorin sub-unit is more basic than
the 21-thiaporphyrin sub-unit in dyad 20, it is possible to
selectively protonate the 21,23-dithiaphlorin sub-unit and the
binding of anions at the protonated 21,23-dithiaphlorin site could
be then sensed by observing the changes in the fluorescence band
of the 21-thiaporphyrin sub-unit. It is observed that the addition of
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3728 | Chem. Commun., 2006, 3726–3728
This journal is ß The Royal Society of Chemistry 2006