applying the principles of supramolecular chemistry to stabilize mixed D–A stacks
with noncovalent bonding interactions greatly enhances their ferroelectric properties, allowing them to maintain their polarization at and even above room temperature. The crystalline mechanically interlaced D–A foldamers we describe herein
possess similarly stabilized D–A stacks. The investigation of this new class of
compounds is therefore driven, not only by curiosity and fundamental inquiry, but
also by the prospect of uncovering new applications based on crystalline organic
materials.
2.1 Early Signs of Extended D–A Stacking
Donor–acceptor rotaxanes and catenanes and their interlaced precursors are, more
often than not, constructed (Fig. 1a) around 4,4
0 -bipyridinium (BIPY
2+ ) and
1,5-dioxynaphthalene (DNP) recognition units and their macrocyclic counterparts,
cyclobis(paraquat-p-phenylene)
(CBPQT
4+ )
and
dinaphtho[38]crown-10
(DN38C10). The charge and planarity present in these compounds and complexes
lend themselves to facile crystal growth, and dozens of mechanically entwined
crystal structures based on these recognition units have accumulated over the years.
Evidence for extended D–A π–π stacking in these molecules was apparent from the
very beginning. Indeed, we noted the continuous sequence of alternating donors and
acceptors (Fig. 1b) back in 1989 when we crystallized [97] BIPY
2+ and DN38C10
in a 2:1 ratio to obtain the complex [BIPY
2+ ] 2 & DN38C10. Likewise, the reverse
recognition system [DNP] 2 & CBPQT
4+ crystallizes [98] in a 2:1 stoichiometry,
accommodating an extended D–A stack (Fig. 1c). This 2:1 binding motif is only
observed in the solid state; the same pairs of components form stable 1:1 inclusion
complexes in solution [99, 100]. The [2]catenane based on DN38C10 and CBPQT
4+
,
which was initially synthesized [101] in 1991, contains within itself two D–A pairs
and also packs with its neighbors into an infinite D–A stack (Fig. 1d).
The most significant noncovalent bonding interactions that stabilize the sorts of
D–A systems illustrated in Fig. 1 are [C–H Á Á Á O] hydrogen bonding interactions
between the acidic α-BIPY
2+ protons and the polyether oxygen atoms, and π–π and
charge transfer interactions between aromatic donors and acceptors in van der
Waals contact. When DNP is a guest inside of CBPQT
4+ (as in Fig. 1c, d),
additional [C–H Á Á Á O] interactions between the 4/8 DNP protons and the
phenylene units of CBPQT
4+ further stabilize the complex. The [C–H Á Á Á O]
hydrogen bonds [102, 103] are especially vital to these systems and have been
estimated [104] to contribute fourfold more stabilization energy to the BIPY
2+ &
CDN38C10 complex than π–π interactions. The [C–H Á Á Á O] interactions dominate
DNPC & CBPQT
4+ complexes likewise, since appending polyether chains to the
DNP unit increases the binding constant by almost two orders of magnitude [105].
The natural progression of our research dealing with structures of the kind
depicted in Fig. 1 led to the development of oligomeric DNP and BIPY
2+ threads.
Crystallization of tetraethylene glycol-linked DNP trimers encircled by CBPQT
4+
274
C.J. Bruns and J.F. Stoddart
with noncovalent bonding interactions greatly enhances their ferroelectric properties, allowing them to maintain their polarization at and even above room temperature. The crystalline mechanically interlaced D–A foldamers we describe herein
possess similarly stabilized D–A stacks. The investigation of this new class of
compounds is therefore driven, not only by curiosity and fundamental inquiry, but
also by the prospect of uncovering new applications based on crystalline organic
materials.
2.1 Early Signs of Extended D–A Stacking
Donor–acceptor rotaxanes and catenanes and their interlaced precursors are, more
often than not, constructed (Fig. 1a) around 4,4
0 -bipyridinium (BIPY
2+ ) and
1,5-dioxynaphthalene (DNP) recognition units and their macrocyclic counterparts,
cyclobis(paraquat-p-phenylene)
(CBPQT
4+ )
and
dinaphtho[38]crown-10
(DN38C10). The charge and planarity present in these compounds and complexes
lend themselves to facile crystal growth, and dozens of mechanically entwined
crystal structures based on these recognition units have accumulated over the years.
Evidence for extended D–A π–π stacking in these molecules was apparent from the
very beginning. Indeed, we noted the continuous sequence of alternating donors and
acceptors (Fig. 1b) back in 1989 when we crystallized [97] BIPY
2+ and DN38C10
in a 2:1 ratio to obtain the complex [BIPY
2+ ] 2 & DN38C10. Likewise, the reverse
recognition system [DNP] 2 & CBPQT
4+ crystallizes [98] in a 2:1 stoichiometry,
accommodating an extended D–A stack (Fig. 1c). This 2:1 binding motif is only
observed in the solid state; the same pairs of components form stable 1:1 inclusion
complexes in solution [99, 100]. The [2]catenane based on DN38C10 and CBPQT
4+
,
which was initially synthesized [101] in 1991, contains within itself two D–A pairs
and also packs with its neighbors into an infinite D–A stack (Fig. 1d).
The most significant noncovalent bonding interactions that stabilize the sorts of
D–A systems illustrated in Fig. 1 are [C–H Á Á Á O] hydrogen bonding interactions
between the acidic α-BIPY
2+ protons and the polyether oxygen atoms, and π–π and
charge transfer interactions between aromatic donors and acceptors in van der
Waals contact. When DNP is a guest inside of CBPQT
4+ (as in Fig. 1c, d),
additional [C–H Á Á Á O] interactions between the 4/8 DNP protons and the
phenylene units of CBPQT
4+ further stabilize the complex. The [C–H Á Á Á O]
hydrogen bonds [102, 103] are especially vital to these systems and have been
estimated [104] to contribute fourfold more stabilization energy to the BIPY
2+ &
CDN38C10 complex than π–π interactions. The [C–H Á Á Á O] interactions dominate
DNPC & CBPQT
4+ complexes likewise, since appending polyether chains to the
DNP unit increases the binding constant by almost two orders of magnitude [105].
The natural progression of our research dealing with structures of the kind
depicted in Fig. 1 led to the development of oligomeric DNP and BIPY
2+ threads.
Crystallization of tetraethylene glycol-linked DNP trimers encircled by CBPQT
4+
274
C.J. Bruns and J.F. Stoddart