Promotion of charge‐transfer complex formation by β‐ or γ‐cyclodextrin

The formation of charge‐transfer (CT) complexes between eleven naphthalene derivatives and eight di‐ or trinitrobenzene derivatives in water and in water‐organic solvent systems has been studied in the presence and absence of β‐ or γ‐cyclodextrin (β‐ or γ‐CDx), respectively. Data of UV absorption and circular dichroism (CD) clearly revealed that γ‐CDx, in several cases, assists CT complex formation by accommodating both electron donor (D) and acceptor (A) molecules in its cavity simultaneously. Thus the weak CT band was intensified and even some systems in which CT complex formation was usually not observed produced CT complexes by the addition of γ‐CDx. In contrast, β‐CDx did not promote markedly the CT complex formation except in one case. It did promote the CT complex formation between α‐naphthylacetic acid (α‐NAA) and picric acid (PA), manifesting the capability of β‐CDx to include two aromatic molecules. Magnetic circular dichroism (MCD) associated with the CT bands was detected for a π ‐ π* type CT complex for the first time, but its intensity was quite weak as anticipated from theory.