Conspectus
In order to fabricate efficient molecular photonic
devices, it
has been a long-held aspiration for chemists to understand and mimic
natural light-harvesting complexes where a rapid and efficient transfer
of excitation energy between chlorophyll pigments is observed. Synthetic
porphyrins are attractive building blocks in this regard because of
their rigid and planar geometry, high thermal and electronic stability,
high molar extinction, small and tunable band gap, and tweakable optical
as well as redox behavior. Owing to these fascinating properties,
various types of porphyrin-based architectures have been reported
utilizing both covalent and noncovalent approaches. However, it still
remains a challenge to construct chemically robust, well-defined three-dimensional
porphyrin cages which can be easily synthesized and yet suitable for
useful applications both in solution as well as in solid state.
Working on this idea, we recently synthesized box-shaped organic
cages, which we called porphyrin boxes, by making use of dynamic covalent
chemistry of imine condensation reaction between 4-connecting, square-shaped,
tetraformylporphyrin and 3-connecting, triangular-shaped, triamine
molecules. Various presynthetic, as well as postsynthetic modifications,
can be carried out on porphyrin boxes including a variation of the
alkyl chain length in their 3-connecting subunit, chemical functionalization,
and metalation of the porphyrin core. This can remarkably tune their
inherent properties, e.g., solubility, window size, volume, and polarity
of the internal void. The porphyrin boxes can therefore be considered
as a significant addition to the family of multiporphyrin-based architectures,
and because of their chemical stability and shape persistency, the
applications of porphyrin boxes expand beyond the photophysical properties
of an artificial light-harvesting complex. Consequently, they have
been exploited as porous organic cages, where their gas adsorption
properties have been investigated. By incorporating them in a lipid
bilayer membrane, an iodide selective synthetic ion channel has also
been demonstrated. Further, we have explored electrocatalytic reduction
of carbon dioxide using Fe(III) metalated porphyrin boxes. Additionally,
the precise size and ease of metalation of porphyrin boxes allowed
us to utilize them as premade building blocks for creating coordination-based
hierarchical superstructures. Considering these developments, it may
be worth combining the photophysical properties of porphyrin with
the shape-persistent porous nature of porphyrin boxes to explore other
novel applications. This Account summarizes our recent work on porphyrin
boxes, starting with their design, structural features, and applications
in different fields. We also try to provide scientific insight into
the future opportunities that these amazing boxes have in store for
exploring the still uncharted challenging domains in the field of
supramolecular chemistry in a confined space.
