Photoinduced Electron Transfer in α‐Cyclodextrin‐Based Supramolecular Dyads: A Free‐Energy‐Dependence Study

Abstract

Photoinduced electron transfer (PET) between alpha-cyclodextrin-appended pyrene (PYCD) and a few acceptor molecules was studied in aqueous solutions. The pyrene moiety in PYCD is located above the narrower rim of the alpha-CD and is fully exposed to water. The acceptors are monocyclic organic molecules and, upon dissolution in water in the presence of PYCD, a fraction of the donor-acceptor systems is present as supramolecular dyads and the remaining fraction as free molecules. Free-energy-dependence studies showed that electron transfer in the supramolecular dyads follows the Marcus equation. The donor-acceptor coupling and the reorganization energy were determined from fits of the data to the Marcus equation. The electronic coupling was found to be similar to those reported for hydrogen-bonded systems. It appears that the actual lambdaout values are somewhat lower than values calculated with the continuum model. The experimental design has also allowed, for the first time, a visual demonstration of the inverted region on the basis of the raw fluorescence lifetime data.

An Anthracene‐Appended β‐Cyclodextrin‐Based Dyad: Study of Self‐Assembly and Photoinduced Electron‐Transfer Processes

“…Photoexcitation of the system would then lead to unimolecular and bimolecular electron-transfer processes, as demonstrated previously for a-CD-appended systems [4] and hydrogenbonded systems. [18] An important difference in the present study is that the triplet excited state of the AN chromophore is also involved in the electron-transfer process.…”

“…[4] The acceptors were monocyclic aromatic molecules capable of encapsulation in the a-CD cavity. PET processes in these dyads were probed by fluorescence lifetime quenching experiments, which showed that PET reactions in the CD-based dyad systems obey the Marcus equation.…”

“…In our earlier study [4] with a-CD-appended pyrene and encapsulated acceptors, we obtained H el = 3 cm…”

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

“…As per the scheme both static and dynamic quenching pathways exist and this leads to the observed bi-exponential decay profiles. 8,30,31,35,61 The bi-exponential profiles were fitted using equation 3 to obtain lifetimes τ 1 and τ 2 and their respective contributions χ 1 and χ 2 .…”

“…Cyclodextrins (CDs), which are cyclic oligosaccharides with hydrophobic cavities, are ideal molecular receptors for building water-soluble supramolecular functional assemblies. [15][16][17][18][19] Recently others [20][21][22][23][24][25][26][27][28][29] and we 30,31 have employed chromophore-appended CDs to assemble supramolecular D-A systems for PET reactions in water. The hydrophobic nature of the CD cavity acts as the assembler of the D-A system here.…”