Excitation multistep

Regardless of the choice of method, excited-state modeling usually requires a multistep process. The typical sequence of steps is ... 

Electronically excited S02 has been found to be the principal emitter in the multistep reactions of a number of reduced sulfur compounds such as H2S, CH3SH, (CH3)2S, CS2, and thiophene with ozone [25, 27, 33, 34], Unfortunately, the mechanisms of these complex reactions are not understood. 

Rapid multistep Coulombic energy transfer takes place as the excitation energy is transferred between the antenna chromophores and the special pair of bacteriochlorophyll molecules (P) in the reaction centre. 

Nymphaea caerulea, for seven natural anthocyanins stabilizing a DNA triplex, etc. Sequential analysis of the oligosaccharide structures of the flavonol tamarixetin-7-O-rutino-side has been performed by ID multistep-relayed COSY-ROESY experiments. Selective excitation was performed by Gaussian-shaped soft pulses. 

Note that in the case of fluorescence, where the energies involved indicates that spontaneous emission in the form of a simple single transition should dominate. Nevertheless the typical pathways back to the ground state appear to involve multiple transitions where the excited state interchanges low energy photons with the environment. We thus have a case where the dynamics of the environment may facihtate a more efficient but complex multistep pathway back the ground state than spontaneous emission provides. 

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. 

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. 

Rather than attempting to cool warm molecules one can try to synthesize cold molecules by associating cold atoms. The molecules thus formed are expected to maintain the translational temperature of the recombining atoms because the center-of-mass motion remains unchanged in the association process (save for the little. momentum imparted by the photon). This idea was first proposed by Julienne and j co-workers [343, 344] who envisioned a multistep association, first involving the continuum-to-bound excitation of translational continuum states of cold trapped. atoms to an excited vibrational level in an excited electronic molecular state. This step was followed by bound-bound spontaneous emission to the ground electronic state. (I... 

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]. 

Extension from atriad (Fc—ZnP—Ceo) to atetrad (Fc—ZnP—H2P—C60) results in remarkable elongation of the final CS state (44). The multistep ET processes afford the final CS state, Fc —ZnP—H2P—Ceo which is detected as the transient absorption spectrum obtained by nanosecond laser flash photolysis [Fig. 8(rz)] (44). The Ceo fingerprint ( 1000 nm) NIR band is clearly seen, whereas the weak absorption features of the ferrocenium ion prevents its direct detection. The quantum yield of the CS state was determined to be 0.24 (44). The relatively low quantum yields results from the competition of ET from ZnP to H2P versus the BET from Ceo to H2P to give the triplet excited state ( H2P ... 

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