by a mathematical expression. Like the oligomers in Sect. 2.2, any thread
containing an odd number of DNP units cannot accommodate rings on exactly
50% of its recognition sites. For these compounds, rotaxanes with [0.5(n–1) + 1]
components have slightly less than 50% of their DNP units occupied by rings and
belong to the “Confused” family of oligorotaxanes. Those with [0.5(n–1) + 2]
components are “Happy” oligorotaxanes that express slightly more than 50%
occupancy, as do the “Frustrated” [0.5(n–1) + 3] oligorotaxanes. These nicknames,
which reflect the “moods” of each oligorotaxane family, stem from their observed
solution-state behavior as probed by
1 H NMR spectroscopy.
Because the oligorotaxanes possess more DNP units than CBPQT
4+ rings to
occupy the DNP units, one can envision a variety of isomers involving the occupation of rings at different sites along the oligomer backbones, a phenomenon
known [115] as translational isomerism. Our
1 H NMR spectroscopic investigations
led us to uncover the following empirical selection rule: the most stable translational isomers of any oligorotaxane will have no two CBPQT
4+ rings occupying
adjacent DNP sites. This selection rule is consistent with the notion that
unencircled adjacent DNP sites can participate in the stabilization of the rotaxanes
by extending the D–A stack, and also elegantly explains the trends observed in the
1 H NMR spectra of the oligorotaxanes in CD 3 CN at 233 K. The moods we
designate to each family of oligorotaxanes are rooted in this selection rule. The
Confused oligorotaxanes are so-named because they adopt multiple stable translational isomers that are slow to equilibrate on the NMR timescale, leading to highly
complicated
1 H NMR spectra. The Frustrated oligorotaxanes are likewise complicated by translational isomerism, but the signals are more broad and featureless
because they cannot adopt a co-conformation that fully obeys the selection rule.
Only the Happy oligorotaxanes express singular rule-compliant translational isomers in solution. These moods are demonstrated by the isomers illustrated in Fig. 8
using the oligorotaxanes derived from the heptameric DNP thread, the smallest
oligomer from which compounds in all three families were isolated. Note that,
although an uninterrupted D–A stack can be achieved in both the Confused and
Happy rotaxanes, the product distribution is typically biased towards the Happy
compounds. Although this situation is convenient for NMR interpretation, it most
likely results from the use of a relative excess of CBPQT
4+ in the reaction, rather
than from major differences in their stabilization energies.
Structural diagrams of the solution-state co-conformations of every isolated
Happy oligorotaxane are presented in Fig. 9. Our detailed
1 H NMR spectroscopic
analysis of each member in the Happy family confirmed that the highly folded
co-conformation with extended D-stacking is indeed a major contributor to the
overall solution-state secondary structure of this class of molecules. To summarize
our findings, the protons of all the DNP and BIPY
2+ recognition units resonate at
lower and lower frequencies as the lengths of the oligomers are extended. DNP and
BIPY
2+ protons at the center of the molecules always resonate at the lowest
frequencies, whereas protons in the most peripheral recognition sites always resonate at the highest frequencies. Comparison of the chemical shifts of the resonances
for the α- and β-BIPY
2+ protons in the homologous series of Happy oligorotaxanes
Mechanically Interlaced and Interlocked Donor–Acceptor Foldamers
285
containing an odd number of DNP units cannot accommodate rings on exactly
50% of its recognition sites. For these compounds, rotaxanes with [0.5(n–1) + 1]
components have slightly less than 50% of their DNP units occupied by rings and
belong to the “Confused” family of oligorotaxanes. Those with [0.5(n–1) + 2]
components are “Happy” oligorotaxanes that express slightly more than 50%
occupancy, as do the “Frustrated” [0.5(n–1) + 3] oligorotaxanes. These nicknames,
which reflect the “moods” of each oligorotaxane family, stem from their observed
solution-state behavior as probed by
1 H NMR spectroscopy.
Because the oligorotaxanes possess more DNP units than CBPQT
4+ rings to
occupy the DNP units, one can envision a variety of isomers involving the occupation of rings at different sites along the oligomer backbones, a phenomenon
known [115] as translational isomerism. Our
1 H NMR spectroscopic investigations
led us to uncover the following empirical selection rule: the most stable translational isomers of any oligorotaxane will have no two CBPQT
4+ rings occupying
adjacent DNP sites. This selection rule is consistent with the notion that
unencircled adjacent DNP sites can participate in the stabilization of the rotaxanes
by extending the D–A stack, and also elegantly explains the trends observed in the
1 H NMR spectra of the oligorotaxanes in CD 3 CN at 233 K. The moods we
designate to each family of oligorotaxanes are rooted in this selection rule. The
Confused oligorotaxanes are so-named because they adopt multiple stable translational isomers that are slow to equilibrate on the NMR timescale, leading to highly
complicated
1 H NMR spectra. The Frustrated oligorotaxanes are likewise complicated by translational isomerism, but the signals are more broad and featureless
because they cannot adopt a co-conformation that fully obeys the selection rule.
Only the Happy oligorotaxanes express singular rule-compliant translational isomers in solution. These moods are demonstrated by the isomers illustrated in Fig. 8
using the oligorotaxanes derived from the heptameric DNP thread, the smallest
oligomer from which compounds in all three families were isolated. Note that,
although an uninterrupted D–A stack can be achieved in both the Confused and
Happy rotaxanes, the product distribution is typically biased towards the Happy
compounds. Although this situation is convenient for NMR interpretation, it most
likely results from the use of a relative excess of CBPQT
4+ in the reaction, rather
than from major differences in their stabilization energies.
Structural diagrams of the solution-state co-conformations of every isolated
Happy oligorotaxane are presented in Fig. 9. Our detailed
1 H NMR spectroscopic
analysis of each member in the Happy family confirmed that the highly folded
co-conformation with extended D-stacking is indeed a major contributor to the
overall solution-state secondary structure of this class of molecules. To summarize
our findings, the protons of all the DNP and BIPY
2+ recognition units resonate at
lower and lower frequencies as the lengths of the oligomers are extended. DNP and
BIPY
2+ protons at the center of the molecules always resonate at the lowest
frequencies, whereas protons in the most peripheral recognition sites always resonate at the highest frequencies. Comparison of the chemical shifts of the resonances
for the α- and β-BIPY
2+ protons in the homologous series of Happy oligorotaxanes
Mechanically Interlaced and Interlocked Donor–Acceptor Foldamers
285