Study of β‑Cyclodextrin–Pyromellitic
Diimide Complexation. Conformational Analysis of Binary and Ternary
Complex Structures by Induced Circular Dichroism and 2D NMR Spectroscopies
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Abstract
Complexation of <i>N</i>-alkyl derivatives
of PMDI with
β-CD is probed using a variety of techniques. Although MALDI-TOF
and CV experiments suggested complex formation, it is very evident
from UV–vis and NMR experiments that these complexes are different
from regular inclusion complexes. A clear understanding of the structure
of the binary complex PMDI@β-CD could be obtained using ICD
and NMR ROESY experiments. ICD signals were negative which suggest
that the PMDI moiety is placed outside of the cavity. ROESY experiments
provide support for this contention. When the alkyl group is <i>t</i>-butyl or 2-propyl, the CH<sub>3</sub> protons exist very
close to the inner protons of β-CD, but the aromatic proton
of PMDI is clearly outside the β-CD cavity. Based on these results
we proposed a structure for PMDI@β-CD with the PMDI moiety placed
at the narrow rim of β-CD and the <i>N</i>-alkyl group
projecting into the cavity and designated these as “rim-binding”
complexes. Additional experiments showed that β-CD can accommodate
a PMDI moiety at the narrow rim and an adamantane moiety in its cavity
simultaneously, resulting in the formation of ternary complexes PMDI⊃β-CD⊂ADA.
Structure of the ternary complex was also probed by ROESY. The ternary
complex formation can be utilized for the design of higher order functional
materials such as CD-based hydrogels