Zs. Romanova et al., Triplet excitation transfer through the walls of hemicarcerands: Dependence of the electronic coupling on the size of the molecular cage, J AM CHEM S, 123(44), 2001, pp. 11029-11036
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.