Photoinduced Electron Transfer (PET) Reactions

Abstract

Charge transfer or electron transfer processes are of utmost importance in a variety of biochemical processes, with one of the most prominent examples being photosynthesis. While ensemble or bulk level charge transfer processes are fairly well understood [1][2][3][4], the study and characterization of these processes at the individual or single-molecule level is still in its infancy [5]. Electron transfer between one and another molecule occurs if one molecule can accept or donate electrons to the other molecule. Often electron transfer can be observed to or from electronically excited molecules, such as fluorophores. If excitation occurs through absorption of photons in a fluorophore (usually an organic molecule with a delocalized p-electron system), its redox properties change, which might enable so-called photoinduced electron transfer (PET) whereby the fluorescence of the fluorophore is quenched. Both fluorescence resonance energy transfer (FRET) [6-9] and PET [1][2][3][4][5][10][11][12][13][14] are two mechanisms that lead to variation of fluorescence emission by distance-dependent fluorescence quenching between a fluorophore and a quenching moiety. Whereas FRET from a donor (D) to an acceptor (A) chromophore scales with 1/[1 þ (R/R 0 ) 6 ], where R 0 is typically between 2 and 8 nm, PET can be designed in a way such that contact formation (van der Waals contact) is required for efficient quenching, with a separation between the fluorophore and quencher that can also be seen as an electron transfer donor and acceptor on the sub-nanometer length scale.To interpret fluorescence quenching caused by PET, the transfer mechanism and its distance dependence has to be well understood. In general, the electron transfer rate is proportional to the square of the electronic coupling between the donor and the acceptor, which in turn depends exponentially on the donor-acceptor distance [3]. Thus changes in the PET rate can also directly report on changes of the donoracceptor distance caused, for example, by conformational dynamics of a biopolymer (nucleic acid, peptide or protein). In standard electron transfer theory, quenching of fluorophores in the first excited singlet state by electron donors or acceptors results in charge separation with rate constant k cs and the formation of a radical ion pair D þ s A À s , which returns to the ground state via charge recombination with rate j189 constant k cr (Figure 7.1). The efficiency of charge separation and charge recombination is mainly controlled by the relationship between the free energy of the reactions, DG cs and DG cr , the reorganization energy l, and the distance between donor and acceptor [1][2][3][4][5]. Typically the total reorganization energy is written as the sum of an inner contribution, l in , and an outer contribution, l out , attributed to nuclear reorganization of the redox partners and their environment (solvent), respectively. Depending on the properties of the electron donor and acceptor, and the linker connecting both compounds, different charge trans...