1.2 EET mechanisms
1.2.3 Through-bond excitation energy transfer
(8) WhereLis the sum of van der Waals radii, JDA0 is the normalized spectral over-lap integral JDA0 = R∞
0 fD(λ)εA(λ)dλandK is a constant.[6]FRET cannot ex-plain TEET because that would violate the Wigner spin conservation law. Conse-quently, the Dexter exchange mechanism plays a crucial role to depict the TEET process.
In bichromophores of the type D-B-A, the bridge (B) may extend the range of the electron exchange due to interactions with bridging orbitals. Thisthrough-bond EET, will be described in more detail in the next section.
1.2.3 Through-bond excitation energy transfer
Many deviations from the idealized Förster and Dexter models have been re-ported for EET between bridged chromophores.[8,19–23] The deviations are at-tributed to the mediating influence of the bridge on the EET and therefor often termedthrough-bond EET.
MeO
MeO MeO
MeO O
MeO
MeO O
O
1 2 3
Figure 2:Bichromophores for studying through-bond SEET from 1,4-dimethoxynaphthalene (donor) via a rigid polynorbornyl system of varying length and geometry to a cyclic ketone (acceptor).[22,28]
In 1988 Verhoeven et al. reported fast singlet EET between 1,4-dimethoxynaphthalene as the energy donor and a cyclic ketone chro-mophores as acceptor which were connected by a rigid polynorbornyl systems of varying length (e.g. compounds1 and2, see figure 2). The EET rateskSEET decreased exponentially with the number of σ-bonds between donor and acceptor. But the distance dependence was too weak for Dexter exchange interaction and too strong for FRET.[28] Furthermore, it was shown later that
Chapter 1 EXCITATION ENERGY TRANSFER (EET)
energy transfer was more efficient in an all-s-trans than in a gauche or s-cis conformation (e.g. compounds1and3, see Figure 2).[8,22]
These findings underline the important role of the linker in a bichromophoric compound and demonstrate the limitations of the idealizedFörster andDexter models.
In order to explain the through-bond singlet EET, a superexchange mechanism was assumed, which was originally developed by McConnel in order to explain the electron transfer (ET) in anionic radicals mediated by a molecular bridge.[29]
The link between Dexter electron exchange and electron transfer may be seen in formal description of electron exchange as two-electron exchange event. Due to the mediation of the bridge, the electronic exchange coupling decreases much slower with increasing donor–acceptor distance than expected from the decrease in orbital overlap.
Empirically was found, that the electron transfer rate follows the expression[8]
kET =k0e−βrDA, (9) where k0 is the maximum rate constant at van der Waals contact, rDA is the edge-to-edge distance between the donor and acceptor and β is the so called attenuation factor which quantifies the mediating ability of a molecular frame-work.
Verhoeven et al. plottedkSEETversus the number ofσ-bonds between the donor and the acceptor and found a slope of –1.45 per bond. Assuming a carbon–carbon bond length of 1.54 Å this corresponds to a attenuation factor ofβ=0.94 Å-1.[28]
βdepends on the nature of the bridge, e.g. the bridge’s rigidity, and on the energy gap between bridge electronic energies and the energy levels of the excited donor and acceptor.[30]
McConnel suggested that low lying "virtual" orbitals localized on the bridge could mediate electronic coupling between an electron donor and acceptor (scheme 7). This coherent process, where no intermediate states (with excited bridge) are involved, can be seen as a tunneling process.[31]
EET mechanisms 1.2
Scheme 7:McConnell superexchange model for electron tunneling through molecular structures.[31–34]
The total electronic coupling between a donor and acceptorVDAmediated by a chain ofnidentical units, is given by:[31]
VDA = VDBVBA
The magnitude of the coupling is determined by the coupling of the donor and acceptor to the bridge (VDB and VAB), the nearest neighbor interactions, VBB, and the energy between the donor and bridge localized states∆EDB.[32]
If the edge-to-edge distance between the donor and acceptorrDA is equal to n·rB, where rB is the length of one repeating bridge unit, and the electronic couplingVBB is much smaller than∆EDB, than the attenuation factorβis given by:[8] The influence of the energy gap between the donor and the bridge on β was experimentally demonstrated by Albinsson et al. by using rod-like linkers with different lengths and energy levels (see section 2.3.2).[8,20]
Because the different EET mechanisms may be operative in parallel, the contri-butions of the individual processes needs to be considered in order to describe EET in bichromophores.[11,35,36] A variety of molecular bichromophore architec-tures was developed for the study of EET mechanisms as will be presented in the following chapter.
2 Bichromophoric compounds
A bichromophoric compound may be defined as a molecule which is built from two chromophores connected by a linker. The termslinker,bridgeandspacerare often used synonymously. However,bridgeis often used if the linker mediates electronic interactions between the chromophores, whereasspacer is commonly used if the linker does not mediate any electronic interaction.
Bichromophores have become a major tool for studying the mechanisms of ET and EET between a donor and an acceptor chromophore over the past decades.[36] The limited conformational freedom, as well as the possibility to explore the effect of linker length (donor-acceptor separation) and the choice of appropriate chromophores allow systematically investigation of EET mechanisms.[12] The well-defined separation, and in some cases also orienta-tion of chromophores, facilitates the development of models which explain the experimental data.
2.1 Molecular architecture of bichromophores
The properties of the linker determine the flexibility of the whole bichromophore.
The nature of linkers between chromophores is represented by a broad variety of chemical structures.[32,37] In the following, the selected bichromophores are classified according to linker properties (molecular rigidity and geometry).
The first group includes bichromophores with a flexible linker. A flexible linker can adopt many possible conformations which may result in various distances between chromophores and orientations. The second group comprises bichro-mophores with a rigid and semi-rigid linker, which to some extent fix the dis-tance between the chromophores. These include helical structures, linear and scaffold linkers. However, in most cases, the rotation around single bonds ex-cludes any fixed orientations of chromophores. Examples of bichromophores with fixed distances between them and also fixed orientation are very rare.