Photochemistry and Photophysics of Coordination Compounds

20 V. Balzani et al.
where H el and FC el are the electronic coupling and the Franck–Condon density
of states, respectively.
In the absence of any intervening medium (through-space mechanism),
the electronic factor decreases exponentially with increasing distance:
H el = H el �
(0) exp – βel
2
� �
rAB – r0
�
, (26)
where rAB is the donor–acceptor distance, H el (0) is the interaction at the “contact”
distance r0,andβ el is an appropriate attenuation parameter.
For donor–acceptor components separated by vacuum, β el is estimated to
be in the range 2–5 ˚A –1 . When donor and acceptor are separated by “matter”
(e.g., a bridge L), the electronic coupling can be mediated by mixing of
the initial and final states of the system with virtual, high-energy electrontransfer
states involving the intervening medium (superexchange mechanism)
[60, 61].
The FC el term of Eq. 25 is a thermally averaged Franck–Condon factor connecting
the initial and final states. In the high temperature limit (hν