Energy transfer, after photoexcitation

Figure 3.50 Energy or electron transfer after photoexcitation of a chromophore.

A transfer of the excitation energy from the donor to the acceptor will occur when an energy acceptor molecule is placed at the proximity of an excited energy donor molecule. After energy transfer, the donor relaxes to its ground state and the acceptor is promoted to one of its excited states. A photo-induced electron transfer can be initiated after photoexcitation when an excited single electron in the LUMO of the electron donor is transferred to a vacant molecular orbital (LUMO) of the acceptor. 

The electronic coupling between an initial (reactant) and a final (product) state plays a key role in many interesting chemical and biochemical photoinduced energy and electron transfer reactions. In excitation (or resonance) energy transfers (EET or RET) [1,2], the excitation energy from a donor system in an electronic excited state (D ) is transferred to a sensitizer (or acceptor) system (A). Alternatively, in photoinduced electron transfers (ET) [3,4], a donor (D) transfers an electron to an acceptor (A) after photoexcitation of one of the components (see Figure 3.50). 

Both energy transfer and electron transfer from a photoexcited compound to a given substrate are distance dependent. This property allows one to delineate—at least on paper—a catalytic cycle for a sensitised process with an appropriately modified template (Scheme 14). If the passive tetrahydronaphthalene shield in 12 is replaced by a photoactive moiety, this part of the compound can, after excitation, facilitate an energy or electron transfer significantly faster at a bound than at an unbound substrate. 

Morteani et al. demonstrated that after photoexcitation and subsequent dissociation of an exciton at the polymer-polymer heterojunction, an intermediate bound geminate polaron pair is formed across the interface [56,57]. These geminate pairs may either dissociate into free charge carriers or collapse into an exciplex state, and either contribute to red-shifted photoliuni-nescence or may be endothermically back-transferred to form a bulk exciton again [57]. In photovoltaic operation the first route is desired, whereas the second route is an imwanted loss channel. Figure 54 displays the potential energy ciu ves for the different states. 

Time-resolved Raman spectroscopy has proved to be a very useful tool to elucidate fast processes in biological molecules, for instance, to follow the fast structural changes during the visual process where, after photoexcitation of rhodopsin molecules, a sequence of energy transfer processes involving isomerization and proton transfer takes place. This subject is treated in more detail in Chap. 6 in comparison with other time-resolved techniques. 

When 7AI-water clusters are excited into the Si band origin, ESPT occurs slow [16, 17, 32]. For 7AI(H20)3, for instance, while the excited-state Ufetime (an upper bound to the ESPT time) is more than 10 ns at Oq, it is reduced to only 15 ps upon an energy excess of 300 cm [43]. Ab initio molecular dynamics (AIMD) simulations on 7AI(H20)i,2 reported by Kina et al. [44] showed that the excited-state transfer occurs about 50 fs after photoexcitation for 12,000 cm energy excess. 

The linear relationship between k and k can be explained as follows. Just after photoexcitation, hot trans-stilbene is formed, and it quickly shares the excess energy with the surrounding solvent molecules in the first solvation shell. In the case of decane in Figure 8.10. the temperature of this first solvation shell is approximately 80°C at 0 ps. The excess energy is then transferred from the solvent molecules in the first solvent shell to those in the outer shells and eventually to the bulk solvent molecules. These energy transfers occur on the... 

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