Excitation, electronic multistep transfer

The sodium D-line radiation dominates the system because the Nad is long-lived, the vibrational-electronic energy transfer is efficient and the excited atom radiates in 10-B seconds. The multistep process bleeds off the excitation energy. This behavior probably is common in systems containing atoms with low-lying energetically accessible electronic states29,55. 

In Sections 7.3 and 7.4, the temperature dependence of radiationless transitions and the effect of deuteration on the lifetimes of excited electronic states are examined. In Section 7.5, a contribution to time-resolved spectroscopy is presented. In that section, we will discuss a problem dealing with transport phenomena of electronic excitations in doped molecular crystals. The theory of singlet excitation energy transfer uses an effective Hamiltonian to account for intramolecular excited-state depopulation and energy transfer by multistep migration among guest molecules. 

An electronic excited state of a metal complex is both a stronger reductant and oxidant than the ground state. Therefore, complexes with relatively long-lived excited states can participate in inter-molecular electron transfer reactions that are uphill for the corresponding ground state species. Such excited state electron transfer reactions often play key roles in multistep schemes for the conversion of light to chemical energy ( 1). 

As discussed above, the photosynthetic reaction center solves the problem of rapid charge recombination by spatially separating the electron and hole across the lipid bilayer. In order to achieve photoinitiated electron transfer across this large distance, the reaction center uses a multistep sequence of electron transfers through an ensemble of donor and acceptor moieties. The same strategy may be successfully employed in photosynthesis models, and has been since 1983 [42-45]. The basic idea may be illustrated by reference to a triad Dj-D2-A, where D2 represents a pigment whose excited state will act as an electron donor, Di is a secondary donor, and A is an electron acceptor. Excitation of D2 will lead to the following potential electron transfer events. 

The details of the multistep electron transfers undergone by 40 may best be appreciated by reference to the results for two model compounds 41 and 42. Triad 41 is similar to the tetrad, except that it lacks the final benzoquinone moiety. Excitation of the porphyrin leads to the production of C-P+-QA with a quantum yield of essentially 1, as was observed for 40. In common with other C-P-Q triads, this state goes on to produce a final C+-P-Qx species. However, the quantum yield of this state is only 0.04, and its lifetime is about 70 ns (Fig. 7). The low quantum yield is due to the fact that only a single, relatively inefficient electron transfer step (analogous to step 4 in Fig. 6) competes with charge recombination of C-P+-Qx. With the tetrad 40, a similar pathway is still available, but in addition there is a second, relatively efficient pathway which also competes with charge recombination and is responsible for most of the quantum yield of the final state. 

Importantly, all photoinduced processes share some common features. A photochemical reaction starts with the ground state structure, proceeds to an excited state structure and ends in the ground state structure. Thus, photochemical mechanisms are inherently multistep and involve intermediates between reactants and products. In the course of a photoinduced charge transfer reaction the molecule passes through several energy states with different activation barriers. This renders the electron transfer pathway quite complex. 

Solvated electrons do not inevitably interfere in photoinduced electron transfer. Their observations are often made under laser irradiations in order to detect these transients efficiently. Under these conditions processes may occur in a multistep and biphotonic way [68], the triplet state being one of the possible intermedites [69], The two photon process of electron ejection may dominate under pulsed laser conditions of high excitation energy while a monophotonic process prevails under continuous laser intensity conditions. These differences may explain the quantum yields observed for instance for the electron photoejection from excited phenolate in water under different irradiation conditions (0.23 [70], 0.17 [71], 0.37 [72]). When using conventional light sources, a relatively low yield of solvated electron is to be expected [69, 72]. 

Zinc chlorins are more easily oxidized than their free base counterparts, and this fact has been used in the design of some chlorin-porphyrin-imide triads that demonstrate multistep electron transfer [16]. For example, triad 47 consists of a zinc chlorin linked to a free base porphyrin that also bears a pyromellitimide acceptor moiety. Phenyl linkers join all active constituents. Excitation of the zinc chlorin moiety of 47 in tetrahydrofuran gives CZ-P-Im, which decays with a 20-ps time constant to yield CZ +-P -Im. This initial charge-separated state evolves into CZ +-P-Im with a lifetime of 120 ps. The CZ -P-Im " state is formed with an overall quantum yield of 0.90 and decays biphasically with time constants of 110 ns and 400 ns. 

Figure IS Two paths for energy flow from an electronically excited ethylene molecule to the water solvent, (a) Multistep energy transfer where the ethylene rotation acts as a mediator between the ethylene vibration and a single water solvent molecule, (b) Direct energy transfer from the ethylene vibration to a single water molecule. Adapted from ref. 119.

PEDI fluorescence. The intramolecular CT excited state formed in the PI chains is preferably non-fluorescent as described in the previous section. Similar multistep electron transfer was observed in a porphyrin connected to two quinones and in several porphyrin-quinone-carotenoid systems [105-108]. The probability of exciplex formation (electron transfer fluorescence quenching) in polar solvents can be estimated from the free energy changes, AGet, given by the Rehm-Weller equation [109,110] ... 

The long lifetime of the CS state was also achieved by the introduction of the multistep-electron-transfer system. Liddell et al. reported that carotenoid-porphyrin-fullerene (C-H2P-C60) triad molecule (Fig. 29f) generates the final charge-separated state (C +-H2P-C6o ) via the C-P +-C6o upon excitation of porphyrine moiety [127]. In 2-methyltetrahydrofuran, the lifetime and the quantum yield for the generation of the final CS state were 170 ns and 0.14, respectively. 

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