Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Chromium 53
6.1
Self-Exchange Energy Transfer Between Identical Chromophores
As noted earlier (Chap. 1 of this volume, Sect. 4.4.2), electronic energy transfer
involving Cr(III) complexes is expected to proceed via an exchange
mechanism, and thus effective donor–acceptor orbital overlap is a necessity.
A large number of cross-exchange energy transfer studies utilizing Cr(III)
donors and/or acceptors have been undertaken with the objective of determining
the relative importance of thermodynamics, electronic factors (such
as orbital overlap), and nuclear factors associated with Franck–Condon restrictions
[87–92].
The study of self-exchange energy transfer is an attractive complementary
approach, since the effect of thermodynamics on the rate is eliminated.
However, monitoring self-exchange has proven a serious experimental challenge,
since the absorption and emission characteristics of the donor and
acceptor are identical. The only prior report of virtual self-exchange involving
transition metal systems is that of Balzani and coworkers on several Ru(II)
polypyridyl systems [93, 94]. In the paper to be discussed [81], advantage
was taken of the marked enhancements in emission lifetimes and steadystate
intensities in rt solution for the Cr(III) complexes listed in Table 1 upon
deuteration of the amine N – Hprotons.
For each complex, the solution absorption and emission maxima of the
deuterated and undeuterated compounds were essentially identical, indicating
the presence of effectively identical chromophores. Irradiation of acidified
mixtures of the isotopically labeled and unlabeled chromophores leads to the
Table 1 Emission lifetimes of Cr(III) complexes at 20 ◦ C
Complex Solvent τH τD Refs.
(µs) (µs)
trans-[Cr(cyclam)(CN)2] + H2O 335 1500 [55]
trans-[Cr(cyclam)(NH3)2] 3+ DMSO 135 1620 [57]
trans-[Cr(tet a)F2] + H2O 30 234 [59]
Scheme 1 Energy transfer between long-lived (CrL) andshort-lived(CrS) complexes