Triplet excitation transfer through the walls of hemicarcerands: Dependence of the electronic coupling on the size of the molecular cage

Triplet excitation transfer from biacetyl trapped inside three hemicarceran ds of different size (1, 2, and 3) to acceptors in the surrounding medium w as investigated. The largest hemiracerand I employs four butyl linkers and the intermediate hemicarcerand 2 four o-xylyl linkers. The smallest hemicar cerand 3 contains only three methylene linkers. Both neat liquid triplet ac ceptors and acceptors dispersed in solvents were used. The primary objectiv e of this work was to determine the dependence of the energy transfer rate on the size and the electronic structure of the molecular cages. There is a pronounced, more than 10-fold, increase of triplet energy transfer rates w ith decreasing size of the cage. The corresponding electronic coupling, \V\ , increases approximately by a factor of similar to3.5 from the largest hem icarceplex I to the smallest hemicarceplex 3. This increase of the electron ic interaction is similar to that observed in covalently bound systems when the distance between the triplet donor and the acceptor is reduced by one carbon-carbon sigma -bond. The electronic structure of the hemicarcerand ap pears to be of secondary importance, at least when T-1 states of the donor and the acceptor are far from a resonance with the T-1 state of the cage. A very good agreement between the results obtained in neat acceptors and in solution was found, indicating that the association between the acceptors a nd the molecular cages is negligible, if at all present. An unexpectedly la rge interaction between the guest and the polarizable walls of the hemicarc erands manifested by emission red-shifts was observed in all cases. This su ggests that the entrapment within the molecular cage gives rise to an envir onment considerably different from that of a single molecule in the gas pha se. An interesting correlation between the magnitude of the phosphorescence spectral shift, Delta nu (0-0), and the guest-to-external acceptor electro nic coupling, \V\, was found.