Self-assembly and photoinduced electron transfer in a donor- $\beta$-cyclodextrin-acceptor supramolecular system${}^{\S }$

Equimolar amounts of native $\beta$-cyclodextrin ($\beta$-CD), pyrene-linked adamantane (PYAD) and tert-butylpyromellitic diimide (PMDI) when dissolved in water self-assembled to form the supramolecular donor-acceptor system $\mathrm{PYAD}⊐\beta$-$CD\succ PMDI$. The high affinity of adamantane derivatives for inclusion binding in the $\beta$-CD cavity and the propensity of PMDI to undergo rim-binding at the narrow rim of $\beta$-CD led to the formation of $\mathrm{PYAD}⊐\beta$-$CD\succ PMDI$. The ternary complex $\mathrm{PYAD}⊐\beta$-$CD\succ PMDI$ was thoroughly characterized using various spectroscopic techniques. $\beta$-CD performs three functions in the self-assembled complex: (1) encapsulate the adamantane unit and keep the pyrene (PY) moiety above the secondary rim, (2) rim-bind PMDI and keep it at the primary rim, and (3) act as a spacer between pyrene and PMDI. Thus, the ternary complex can function as a donor-spacer-acceptor system capable of undergoing photoinduced electron transfer (PET). Upon excitation of the pyrene moiety in $\mathrm{PYAD}⊐\beta$-$CD\succ PMDI$ an electron is transferred from the excited pyrene to the PMDI ground state. Steady state and time resolved fluorescence experiments were carried out to study the PET in $\mathrm{PYAD}⊐\beta$-$CD\succ PMDI$. Existence of the ternary system and PET processes taking place within it are further supported by laser flash photolysis experiments. Graphical abstract$\beta$-CD assembles donor pyrene through inclusion binding and acceptor pyromellitic diimide through rim-binding. Fluorescence intensity and lifetime quenching suggested photoinduced electron transfer from pyrene to pyromellitic diimide.: .