Intermediate photoproducts

Further experiments for analyzing the electronic structure of intermediate states were not very successM, until Sixl et al. [23] and Bubeck et al. [24] published their first low-temperature spectroscopic experiments on partially photopolymerized TS crystals. Thereby, the monomer crystal is cooled to 4.2 K in the dark. Then it is irradiated with UV light (1 310 nm) for a short period. After this procedure a large number of different species show up in both, the optical absorption and the ESR spectrum. They persist at helium temperature. Subsequent annealing in the dark produces further intermediate reaction products which are also identified by their optical or ESR spectra. Finally, further irradiation with visible light, which is absorbed only by the intermediate reaction products, produces still more and different reaction products. In a comprehensive series of investigations [25-47] most of these intermediate products have been identified and classified. Moreover, the mechanisms of their production and their reaction kinetics have been analyzed. According to Sixl, there exist three different series of intermediate products  

The DR-DC series leads directly from the monomer to the polymer. It is initiated and processed in the following simple and clear way. The monomer crystal is irradiated with an UV flash and subsequently rests in the dark. The flash excites the monomers and produces dimers (DR2). These react by a thermally activated step by step addition of monomers, as illustrated in Fig. 9.8. In the following paragraphs we will show for a few selected examples how these results have been achieved and we will present details of both, the electronic structures and the dynamics. 

The AC and the SO series are produced by additional irradiation with light, i. e. these are photoproducts which do not necessarily arise during the solid state polymerization of TS6. Although they are an important part of the entire variety of structures in partially polymerized TS6 crystals, we will not treat them in this review. They have been described extensively by Sixl in his papers cited above and in Ref [48]. 

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We have examined two types of organized media that effectively control the charge separation and back reactions of the intermediate photoproducts. These include, (a) charged colloids i.e. SiC>2 and ZrC>2 colloids that introduce electrostatic interactions between the photoproducts and interface (7-10), and (b) water-in-oil microemulsions that provide aqueous-oil two phase systems capable of controlling the reactions by proper design of the hydrophobic-hydrophilic balance of the photoproducts ( 6). 

Intrinsic limitations of an artificial photosynthetic system include the thermodynamically favoured back electron transfer reactions of the intermediate photoproducts [47, 48]. For an oxidative ET quenching process the destructive back electron reactions are given by Eq. (9) and (10). The fraction of usable photo-... 

Photochromic transformations from the para- to ana-quinone structure of these compounds occur through intermediate photoproducts in the triplet state.51 The experimental evidence of the influence of oxygen in solution and the viscosity of solvents on the lifetime of these photoproducts supports this statement. It was found that both initial and photoinduced forms have lower triplet levels of the mi -type. 

For this purpose an electron transfer across the bilayer boundary must be accomplished (14). The schematic of our system is presented in Figure 3. In this system an amphiphilic Ru-complex is incorporated Into the membrane wall. An electron donor, EDTA, is entrapped in the inner compartment of the vesicle, and heptylviolo-gen (Hv2+) as electron acceptor is Introduced into the outer phase. Upon illumination an electron transfer process across the vesicle walls is initiated and the reduced acceptor (HVf) is produced. The different steps involved in this overall reaction are presented in Figure 3. The excited sensitizer transfers an electron to HV2+ in the primary event. The oxidized sensitizer thus produced oxidizes a Ru located at the inner surface of the vesicle and thereby the separation of the intermediate photoproducts is assisted (14). The further oxidation of EDTA regenerates the sensitizer and consequently the separation of the reduced species, HVi, from the oxidized product is achieved. In this system the basic principle of a vectorial electron transfer across a membrane is demonstrated. However, the quantum yield for the reaction is rather low (0 4 X 10 ). 

Grellmann and co-workers, who have contributed considerably to this area with their work on the photocyclization of diarylamines to carbazoles, now report that the only unknown isomer (157) of the five possible indolocar-bazoles can be conveniently synthesized from (158) in methylcyclohexane solution by using 300 nm radiation.Flash experiments and steady-state studies show that the formation of (157) occurs adiabatically in the triplet manifold by a two-photon process and a triplet energy transfer reaction prevents accumulation of large concentrations of the intermediate photoproduct (159). Photocyclization of the aryl-ethenyl compounds (160) has been... 

No less striking have been the advances made in our understanding of the substitution reactions peculiar to the much studied binuclear iron carbonyl, VIII (83-85). Here, too, a variety of experimental techniques has been brought to bear on the intermediate photoproducts— one a binuclear species IX and the other a mononuclear radical X— which differ hugely in their reactivity. 

The vectorial ET process and stabilization of the intermediate photoproducts against recombination as compared to the homogeneous phase significantly affects the quantum efficiency of ET products under continuous illumination. In the presence of a sacrificial electron donor (i.e., triethanolamine), irreversible reduction of the oxidized photoproduct occurs [Eq. (19)] and the reduced product, PVS, is accumulated in the system. The quantum efficiency for PVS formation in a homogeneous aqueous system corresponds to (j) = 0.04. In the presence of the Si02 colloid a substantially... 

The formation of the intermediate photoproduct was followed by in situ proton nmr spectroscopy. The most informative changes occured in the low-field region, where all signals of the photoproduct are shifted to higher fields. 

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