Wt. Yip et al., Energy transfer in bichromophoric molecules: The effect of symmetry and donor/acceptor energy gap, J PHYS CH A, 103(1), 1999, pp. 10-20
The dependence of the rate of singlet excitation transfer on the donor-acce
ptor energy gap was investigated in bichromophoric spiranes with symmetry-f
orbidden zero-order electronic coupling. The fluorescence measurements were
performed in a supersonic jet in order to avoid collisional and inhomogene
ous line broadening. Fluorescence excitation spectra and single-vibronic-le
vel emission spectra of the model chromophores cyclopentaphenanthrene and 1
,8-dimethylnaphthalene and the bichromophores spirofluorenephenanthrene and
spirofluorenenaphthalene are presented and analyzed. Although the transiti
on moments of the linked chromophores are rigorously perpendicular and the
exchange coupling between the v' = 0 states is computationally shown to be
zero, all spiranes with energy gaps larger than similar to 1000 cm(-1) exhi
bited complete electronic energy transfer from all vibrational states of th
e electronically excited donor, including the undistorted v' = 0 state. Thi
s behavior is explained in terms of vibronic coupling between the sparse st
ates of the donor and the dense manifold (pseudocontinuum) of the acceptor
states. The electronic energy transfer was sufficiently fast to result in m
easurable lifetime broadening of the donor absorption lines, from which the
k(EET) was estimated. The results demonstrate that the zero-order picture
overestimates the degree of the molecular orbital symmetry control over ele
ctronic energy transfer and charge-transfer rates and that at sufficiently
high driving forces the vibronically mediated "symmetry-forbiddcn" electron
ic energy transfer can be very rapid (similar to 1 x 10(12) s(-1)).