Photo induced electron emission

Following the discovery of the hydrated electron in radiation chemistry, the reexamination of some fields of aqueous chemistry gave rise to a new concept of primary reduction processes. This paper surveys aspects of these investigations in which it appears that e aq, as opposed to its conjugate acid (H atom), is invariably the precursor to H2 when water is reduced. Evidence is reviewed for the production of e aq (a) photochemically, (b) by chemical reduction of water, (c) electrolytically, (d) by photo-induced electron emission from metals, (e) from stable solvated electrons, and (f) from H atoms. The basis of standard electrode potentials and various aspects of hydrated electron chemistry are discussed briefly. 

In all these experiments photo-induced electron emission is produced by photon energies substantially smaller than the work function of the metal. 

Excited state complexes are relevant in connection with photo-induced electron transfer, since their formation frequently competes with or precedes electron transfer. The simplest examples, excited state dimers (excimers), were discussed by Kautsky as early as 1939 [77], The first organic excimer, the dimer of pyrene, was identified by its characteristic, red-shifted, structureless emission spectrum by... 

Nine years later, Leonhardt and Weller detected an excimer type emission in solutions containing perylene and dimethylaniline [80]. This first heteroexcimer has become the prototype of an ever expanding area of research. Perhaps the impact of these observations are best illustrated by the monograph dealing with the new phenomenom published only 12 years after the first report [81]. The significance of this research for the proper understanding of photo-induced electron transfer is born out by the first positive identification of a radical anion resulting from the irradiation of a donor-acceptor system in polar solvents (vide infra) [82]. 

Tian and co-workers have prepared and investigated the properties of flexible dendrons and dendrimers that have NI or PDI cores with carbazole (CZ) or oxadiazole (OXZ) peripheral units (Scheme 38) [103,104]. Excitation of the peripheral chromophores result in emission of the NI of PDI cores. Interestingly, in the case of the NI core dendrons, excitation of the peripheral oxadiazole residues of 75c results in a 3.9 times enhancement of core liuni-nescence, while excitation of the peripheral carbazole residues of 76c results in only a 20% emission intensity, suggesting a second pathway for donor quenching in the CZ systems. This pathway is probably photo-induced electron transfer (PET) from the CZ units to the NI core [104], The OXZ units, however, have a relatively higher electron affinity and no PET can take place. 

Finally, both photo-induced electron transfer from the ligand to the metal ion, resulting in a reduction of Ln into Ln with a concomitant quenching of the metal-centered luminescence, and energy back transfer (see fig. 7) have to be avoided by an adequate ligand design positioning the LMCT and triplet states sufiiciently away from the emissive state. 

We have previously shown that when PPV is self-assembled with specific electronically active polyanions such as poly(thiophene acetic acid) (PTAA) or sulfonated fiillerenes (S-C60 )(7), the photoluminescence of the PPV is essentially completely quenched by the polyanion. The mechanism of this quenching is believed to be due to a photoinduc electron transfer process taking place between the excited PPV and the adjacent electroactive polyanion molecules. The quenching process, in this case, is not associated with a Forster type energy transfer since in both cases, the required spectral overlap of a donor emission band with an acceptor absorption band is not fulfilled. In addition, photo-induced electron transfer processes have previously been confirmed in PPV/C60 systems and can be exploited to fabricate thin film photovoltaic devices (77). In order to mediate this electron transfer process, we have constructed multilayer heterostructures in which the PPV donor and the polyanion electron acceptor are separated from each other with electronically inert spacer layers of known thickness. In addition to allowing studies of the electron transfer process, such structures provide important insights into the thermal stability of the multilayer structure. The "spacers" used in this study were bilayers of SPS/PAH with an experimentally determined bilayer thickness of 30 +/-5 A. 

When the counter-ion is TCNQ (tetracyanoquinodimethane anion), luminescence is no longer detected [41]. This likely results from a photo-induced electron transfer from the high-energy electron-rich MLCT triplet state (one can see that the unresolved emission band starts at about 400 nm at 77 K, designating the location of the 0-0 band), to the TCNQ" acceptor (a single TCNQ may exits under three forms neutral, mono- and di-anion). The reaction is shown in Eqs. 4.1 and 4.2 ... 

Oxazole 35 was studied for sensing water in organic solvents by photo-induced electron transfer since it exhibited a weak emission in organic solvents but demonstrated a drastic enhancement in the fluorescence intensity with increasing water content in organic solvents. Oxzizole 35 was formed in 32% yield by the reaction of diketone 33 and aldehydes 34 in the presence of NH4OAC in AcOH at 90 °C for 8 h. ... 

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