Photoexcitation and subsequent

Photophysical processes Photoexcitation and subsequent events which lead from one to another state of a molecular entity through radiation and radiationless transitions. No chemical change results. 

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. 

Illumination of the sample by a light whose energy is greater than Eg and simultaneous recording of the infrared and/or electronic spectra allow probing of the structure and properties of the electronic excited state. The complex photophysics of photoexcitation and subsequent relaxation phenomena have been studied in detail by various groups, and excellent reviews are available [75,76]. 

Using a variety of transient and CW spectroscopies spanning the time domains from ps to ms, we have identified the dominant intrachain photoexcitations in C )-doped PPV films. These are spin-correlated polaron pairs, which are formed within picoseconds following exciton diffusion and subsequent dissociation at photoinduced PPV+/Cw> defect centers. We found that the higher-energy PA band of polaron pairs is blue-shifted by about 0.4 eV compared to that of isolated polarons in PPV. 

Visible light systems comprising a photoreducible dye molecule e.g. 87)293 or an a-diketone e.g. 85)2% and an amine have also been described. The mechanism of radical production is probably similar to that described for the ketone amine systems described above (i.e. electron transfer from the amine to the photoexcited dye molecule and subsequent proton transfer). Ideally, the dye molecule is reduced to a colorless byproduct. 

Photoexcitation of the electrode (of the electrons in its surface layer) and subsequent reaction of ordinary, nonactivated reactants at the electrode (Section 29.3)... 

As mentioned earlier, a great deal of literature has dealt with the properties of heterogeneous liquid systems such as microemulsions, micelles, vesicles, and lipid bilayers in photosynthetic processes [114,115,119]. At externally polarizable ITIES, the control on the Galvani potential difference offers an extra variable, which allows tuning reaction paths and rates. For instance, the rather high interfacial reactivity of photoexcited porphyrin species has proved to be able to promote processes such as the one shown in Fig. 3(b). The inhibition of back ET upon addition of hexacyanoferrate in the photoreaction of Fig. 17 is an example of a photosynthetic reaction at polarizable ITIES [87,166]. At Galvani potential differences close to 0 V, a direct redox reaction involving an equimolar ratio of the hexacyanoferrate couple and TCNQ features an uphill ET of approximately 0.10 eV (see Fig. 4). However, the excited state of the porphyrin heterodimer can readily inject an electron into TCNQ and subsequently receive an electron from ferrocyanide. For illumination at 543 nm (2.3 eV), the overall photoprocess corresponds to a 4% conversion efficiency. 

The formation of 5-hydroxy-2 -deoxycytidine (22) and 5-hydroxy-2 -de-oxyuridine (23) that arise from dehydration of dCyd glycols 20 and related dUrd derivatives 21, respectively, was assessed by HPLC-electrochemical detection within calf thymus DNA upon exposure to photoexcited menadione and subsequent enzymatic hydrolysis [57]. The latter two oxidized nucleo-... 

Ligand substitution reactions of NO leading to metal-nitrosyl bond formation were first quantitatively studied for metalloporphyrins, (M(Por)), and heme proteins a few decades ago (20), and have been the subject of a recent review (20d). Despite the large volume of work, systematic mechanistic studies have been limited. As with the Rum(salen) complexes discussed above, photoexcitation of met allop or phyr in nitrosyls results in labilization of NO. In such studies, laser flash photolysis is used to labilize NO from a M(Por)(NO) precursor, and subsequent relaxation of the non-steady state system back to equilibrium (Eq. (9)) is monitored spectroscopically. 

The following subsections are devoted to various mechanistic aspects of the ortho photocycloaddition. The possible role of ground-state complexes will be discussed and, subsequently, the intermediate species that are formed or may be formed upon photoexcitation will be treated the reactive excited state, exciplexes, and zwitterions, biradicals, and ion pairs. Empirical rules, aimed at predicting under what circumstances ortho photocycloaddition (or other modes of addition) may occur, will be discussed next and, finally, the results of theoretical considerations and calculations will be reviewed. 

In addition photoexcitation can also result in the transfer of an excited state electron to a distant acceptor group resulting in charge separation. This process can be understood within the framework of Marcus theory and subsequent more sophisticated theoretical treatments.2,5 The rate of electron transfer (ke]) drops with distance according to an attenuation factor / el ke °c exp(—/ el /yB) where /Xb is the distance between donor and acceptor components A and B. When the donor and acceptor components are separated by a vacuum J3el is estimated to be ca. 2-5 A-1. However when some kind of material substance is involved such as a bridge L the electron transfer process can be... 

Photoexcitation by photon absorption and subsequent events that lead from one to another state of a molecular entity through radiation and radiationless transitions without any chemical change are called photophysical processes. The processes are classified as radiative and radiationless ones, depending on the photon emission (or absorption) and energy loss without any photon emission according to the kinetic aspects the monomolecular (spontaneous) and bimolecular (quenched) processes are distinguished (see Figure 4.1). 

Rapid formation of a (hypothetical) bidentate, mononuclear surface complex is the important first step followed by photoexcitation of this surface complex and subsequent electron transfer to Fe(III) with formation of an oxalate radical and Fe(II). The oxalate radical undergoes a rapid decarboxyla-... 

We are now in a position to consider the lifetime of a RP. For simplicity, we will consider a RP born in a triplet state. Figure 8.5 shows the formation and subsequent reaction processes of a RP born from a photoexcited triplet state. 

In long-afterglow phosphors, optical excitation energy is stored in the lattice by trapping photoexcited charge carriers. The most prominent example is SrAl204 Eu,Dy after optical excitation of Eu, Eu is oxidized to Eu and Dy is reduced to Dy. Thermal excitation of Dy to Dy, followed by capture of the electron by Eu and subsequent Eu emission results in time-delayed Eu emission. The thermal excitation process of Dy determines the time delay. This particular material still generates visible emission after several hours in the dark. 

First, we would like to address the question how sample quality influences the observed results. Synthesis and sample treatment influence the electronic properties of conjugated materials in a defined way [23]. We have already shown [31] that the shape and intensity of photoinduced absorption spectra in different representatives of the LPPPs may vary (see Fig. 9-16), indicating at least different trap densities but also different electronic properties of these traps, depending on the synthesis and subsequent treatment of the polymers. However, the electronic properties for this class of polymers can be imderstood in terms of effective conjugation length [23-25] charge transfer by photoexcitation or redox reactions [31] and also photo-oxidation upon intense visible irradiation under the influence of oxygen [23]. Therefore, by optical spectroscopy (absorption, photoluminescence, or photoinduced absorption) we can assess the quality of a sample. 

Offermans et al. used several spectroscopic methods to study the formation and luminescence decay of exciplexes as a function of photoexcitation (of either polymer), and subsequent charge transfer at the heterojunction [229]. Exciplex formation was followed by a relaxation to the lower lying triplet state Ti of the MDMO-PPV. The energetic scheme is depicted in Fig. 48. 

Scheme 6.273 illustrates the photocatalytic addition of a tertiary amine (571) to the furanone 572,1474 which is initiated by electron transfer from the amine to a photoexcited benzophenone to form a radical ion pair.1475 The ketyl radical anion abstracts a proton from an amine radical cation and subsequently donates a hydrogen atom to the previously coupled radical intermediate 573 to regenerate the benzophenone molecule and to give the final addition product 574 (<94% chemical yield). 

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