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20
length alternation (BLA) values. The HOMA values of 0.772 Å from such NIR absorption in the smallest expanded porphyrin, which
the X-ray crystal structure and 0.824 Å from the theoretically paves the way towards efficient devices. The design and synthesis of
optimized geometry for 6 are well within the range of aromatic fused heteroannulenes that has become a competitive and mature
porphyrins. Maximum bond length alternations of 0.084 Å from the selectivity offering intriguing aromaticity with NIR absorption that
X-ray crystal structure and 0.086 Å from the theoretically optimized will be exploited in depth with much more to come from the author’s
geometry also support the aromatic character of the macrocycle 6 laboratory very soon, since the authors feel that this area of research
(
see ESI†).
is still in its infancy and there is much room for further development.
The work at IACS was supported by DST-SERB (SR/S1/IC-37/
Redox properties of the new macrocycles have been studied with
the help of cyclic voltammetry and differential pulse voltammetry. 6 2012), New Delhi, India, CSIR (02/(0120)/13/EMR-II), New Delhi,
exhibits one irreversible and two reversible reduction peaks at India and DST-SERB Ramanujan Fellowship (SR/S2/RJN-93/2011).
ꢀ
0.65 V, ꢀ1.15 V and ꢀ1.37 V with the HOMO–LUMO gap AM thanks CSIR, New Delhi for junior research fellowship, TG
estimated as 1.17 V. For 7, a HOMO–LUMO gap of 1.18 V was thanks IACS for a junior research fellowship. We thank Prof.
observed (see ESI†), which is comparatively much smaller than R. Murugavel for use of his single crystal X-ray diffraction facility
21
those of meso-tetraphenyl porphyrin (2.26 V) and [26] tetrathia established through DAE-SRC outstanding investigator award.
14a
rubyrin (1.64 V). Such a phenomenon may be due to the fusion of
heterocycles into the core of the macrocycle, leading to HOMO–
LUMO rearrangement, which is an offshoot of such core modifica-
tions that lead to a decrease in the Dredox value. For macrocycle 5,
a cyclic voltammogram has not been recorded due to very low
solubility in any organic solvent.
Further support for the proposition that the macrocycles are
aromatic came from solid state X-ray crystallographic analysis.
A single crystal of 6 suitable for X-ray analysis was obtained by slow
evaporation of a THF solution of the macrocycle. The steric demand
of the phenanthrene moiety causes the ring skeleton of the macro-
cycle to deviate from planarity, albeit slightly. The observed con-
formation (Fig. 3) is due to the intrinsic structural constraints that
arise from the requirement to form a cyclic structure and the steric
Notes and references
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1
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Fig. 3 X-ray crystal structures of 6; top view (left) and side view (right).
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9096 | Chem. Commun., 2014, 50, 9094--9096
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