for extending the conjugation, or placing a dialkylamino
substituent on the boradiazaindacene system, both of which
lead to longer wavelength emitting dyes. Another possibility,
demonstrated elegantly by O’Shea10 and later by Carreira,11
is to synthesize an 8-aza-substituted boradiazaindacene,
which is, of course, better named as boratriazaindacene.
Tetraphenyl-substituted boratriazaindacenes show signifi-
cantly red-shifted absorption and emission spectra. This
property of boratriazaindacenes has been exploited10 within
the context of photodynamic therapy along with extended
conjugation distyryl-boradiazaindacenes.7d Boratriazain-
dacenes offer a tremendous potential as chemosensors
communicating in the NIR region of the spectrum, but until
now, only examples for acid switching of PET12 and ICT13
processes have been reported. As a part of our ongoing effort
in the design and synthesis of novel chemosensors, we
targeted a boratriazaindacene carrying two 2-pyridyl groups
at the 1 and 7 positions. Our modeling studies showed that
this would create a well-defined binding pocket for transition
metal sensing. The synthesis (Figure 1) starts with the known
compound 1. In analogy to the literature procedures, we
carried out a nitromethane conjugate addition to pyridine-
2-carboxaldehyde leading to the compound 2, which was then
converted into 2-phenyl-4-pyridylpyrrole (3). Partial nitro-
sylation, followed by a treatment with BF3-etherate, resulted
in the formation of the target fluorophore in the form of a
green powder following silica gel column chromatography.
Absorption spectrum of the compound 4 was acquired in
acetonitrile and shows a peak at 655 nm. The emission peak
in optically dilute solutions is at 682 nm in the same solvent.
In a survey of different metal ions, it was apparent that
mercuric ions produced the largest spectral changes in both
the absorption and emission spectra. The concentration
dependent changes in the absorption spectrum are shown in
Figure 2. On mercuric ion addition, a clean isosbestic point
of 672 nm was observed, and Hg(II)-4 complex has a red-
shifted absorbance peak at 690 nm. The extinction coefficient
at the absorption maximum is 77 000 M-1 cm-1. The
Figure 1. Synthesis of the near-IR emitting chemosensor 4.
emission spectrum (Figure 3) also shows a red-shift on Hg-
(II) binding. The free chemosensor has a λmax(emission) of
682 nm which moves to 719 nm on Hg(II) binding. The
(7) (a) Dost, Z.; Atilgan, S.; Akkaya, E. U. Tetrahedron 2006, 62, 8484-
8488. (b) Coskun, A.; Deniz, E.; Akkaya, E. U. Org. Lett. 2005, 7, 5187-
5189. (c) Saki, N.; Dinc, T.; Akkaya, E. U. Tetrahedron 2006, 62, 2721-
2725. (d) Atilgan, S.; Ekmekci, Z.; Dogan, A. L.; Guc, D.; Akkaya, E. U.
Chem. Commun. 2006, 4398-4400. (e) Coskun, A.; Akkaya, E. U. J. Am.
Chem. Soc. 2005, 127, 10464-10465. (f) Coskun, A.; Akkaya, E. U. J.
Am. Chem. Soc. 2006, 128, 14474-14475. (g) Coskun, A.; Turfan, B. T.;
Akkaya, E. U. Tetrahedron Lett. 2003, 44, 5649-5651.
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Dehaen, W.; Boens, N. Org. Lett. 2005, 7, 4377-4380. (b) Rohand, T.;
Baruah, M.; Qin, W.; Boens, N.; Dehaen, W. Chem. Commun. 2006, 266-
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2006, 4658-4663.
(9) Gabe, Y.; Ueno, T.; Urano, Y.; Kojima, H.; Nagano, T. Anal. Bioanal.
Chem. 2006, 386, 621-626.
(10) (a) Killoran, J.; Allen, L.; Gallagher, J. F.; Gallagher, W. M.; O’Shea,
D. F. Chem. Commun. 2002, 1862-1863. (b) Gorman, A.; Killoran, J.;
O’Shea, C.; Kenna, T.; Gallagher, W. M.; O’Shea, D. F. J. Am. Chem.
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Figure 2. Absorbance response of the boratriazaindacene 4 to
increasing concentrations of Hg(II) in the form of perchlorate salt.
Measurements were done in acetonitrile at 1.3 µM dye concentra-
tion. Metal ion concentrations were varied in the following order:
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20 µM.
dissociation constant was determined to be 5.4 × 10-6 M,
with a 1:1 binding stoichiometry as revealed in a Job plot
analysis. The quantum yields in acetonitrile are 0.19 for the
compound 4 and 0.17 for the 4-Hg(II) complex in reference
to tetra-tert-butylphthalocyanine.14 The boratriazaindacene
derivative 4 shows spectacular metal ion selectivity. This is
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Photochem. Photobiol. 1993, 57, 465-471. (b) Freyer, W.; Dahne, S.; Minh,
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