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of theory suggest that the two limiting tautomeric forms have
a definite non-overlapping range of absorptions. Besides the
lack of isosbestic points in the VT-absorption spectra, the 15N
VT-NMR data and the computed spectra of each of the tauto-
meric forms suggest that the UV/Vis spectra might have contri-
butions from more than just the two limiting forms.
((11100) red triangles, (10101) aqua circles, the (00000) full
enol-imine blue triangles, and perhaps small contributions also
from the (11000) tautomer black rhombuses). The (00000)
enol-imine form gives the largest contribution to the M band
as described above and to the sides of the H band, leading to
widening of the band; the intensity does not decrease because
of the contribution of intermediate tautomeric forms. Further-
more, a plethora of low intensity transitions contribute to the
overall almost constant intensity of the spectrum.
The trend of the L band in the range 20000–17000 cmꢀ1
(ꢁ500–600 nm) hints that there is more than one transition
contributing to its shape, whereas only one intense peak is
predicted for the (11111) tautomer, the intensity of which arises
from two nearly degenerate transitions because the molecule
has close to C5h symmetry, and one far weaker transition. Fur-
thermore, the two most relevant Gaussian contributions
around 18500 (ꢁ540 nm) and 19900 cmꢀ1 (ꢁ500 nm) do not
have the same de/dT (Figure 23 inset and Tables S6–8 in the
Supporting Information), indicating that the two bands might
receive contributions from different phenomena. For this
reason, we focused our attention on the contribution to the
spectrum also by those tautomers whose computed free ener-
gies are within the range of energies of the two limiting keto-
enamine and enol-imine forms (Figure 15).
Conclusion
Campestarenes are a new family of Schiff-base macrocycles ex-
hibiting unique properties. They can be synthesized selectively
and easily through a simple procedure, which has been exem-
plified by using precursors containing different peripheral
groups, including bulky organosilyl substituents that prevent
aggregation of the molecules. The structures and tautomeric
equilibria of campestarenes have been analyzed by a combina-
tion of single-crystal X-ray diffraction, IR spectroscopy, variable-
temperature NMR (1H, 13C, 15N), and UV/Vis experiments, and
DFT and TD-DFT calculations on model compounds. Campes-
tarenes can exist as many possible tautomers, the predomi-
nance of one over the other being dictated by the solvent. In
the solid state, only the enol-imine tautomer is observed and it
also prevails in non-polar solvents such as benzene and tolu-
ene. When the solvent is changed to a more polar one, such
as chloroform or dichloromethane, the keto-enamine begins to
appear alongside other keto-enamine/enol-imine forms. The
relative quantities of each are temperature sensitive. In highly
polar solvents, such as DMF and DMSO, the largest contribu-
tion is given by the keto-enamine form of the campestarene
whereas intermediate tautomeric forms are responsible for the
temperature dependence of the UV/Vis spectra. Computational
models of all these species helped to rationalize this unique
behavior, and gave insight into the structure of campestarenes
in solution.
Observing that the energy of the most intense transition of
the enol-imine tautomer is accurately reproduced but that of
the keto-enamine tautomer is hyperchromically shifted by
1200 cmꢀ1 (ꢁ0.15 eV), the computed transition energies of
each of the 1g tautomers have been bathochromically shifted,
assuming that each NꢀH induces an error in the calculation of
transition energies of one fifth of this amount (less than
0.03 eV), Table S9 (in the Supporting Information).
At low temperature, the (11111) and the (11110) forms (green
triangles and yellow squares of Figure 24) prevail and their ab-
sorptions determine the shape of the L band and the shoulder
of the T band near 19900 cmꢀ1 (ꢁ500 nm). Upon increasing
the temperature, other tautomers start to be more populated
The rich tautomeric behavior of campestarenes combined
with their easy access make this new family of five-fold macro-
cycles suitable to form liquid crystalline phases, proton chan-
nels, and metallic complexes. Further investigations of the ap-
plications of campestarenes are underway.
Experimental Section
Computational methods
All of the reported computations were performed by using the
Gaussian 09 (rev. D.01) software.[80] All studies were performed by
applying the density functional theory (DFT).[81,82] SCF and structure
optimization convergence criteria are the default ones. Solvation
effects were included by the means of the IEFPCM method[83–85] im-
plemented in the standard software without any change with re-
spect to the default parameters. UV/Vis spectra were computed by
using the TD-DFT linear response theory[86,87] and using the 6–
311G(d,p) basis (standard basis sets in Gaussian 09) as in the case
of geometry optimization and M06 xc functional. Different xc func-
tionals, hybrid PBE1PBE,[88] double hybrid mPW2PLYP,[89] and meta-
Figure 24. Comparison of the experimental UV/Vis spectra of 1e in DMF at
213 K (green dashed), 258 K (red continuous), and 213 K (blue continuous).
Additionally, the TD-DFT computed transition energies (only transitions with
oscillator strength larger than 0.05 are reported) at the 6-311G(d,p)/M06/
DMF level of theory for the (11111) (green triangles), (00000) (blue triangles),
(11110) (yellow squares), (11100) (red triangles), (10101) (aqua circles), and
(11000) (black rhombuses) tautomeric forms of 1g.
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Chem. Eur. J. 2016, 22, 1 – 17
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
Chen, Zhengyu
