Photo-induced method

The radiation-induced method, in the y- or pulse regime [19,20] and, to a certain extent, the photo-induced method [21], provide a particularly powerful means to produce in condensed media, metal, and semiconductor clusters from monomers as precursors, to study their properties and to understand the exotic phenomena which occur whenever a new phase of oligomeric particles is formed in the bulk of a homogeneous mother phase [22-26], phenomena which are therefore rather frequent in physics and chemistry. Unlike a recent review concerning semiconductor clusters [19], the present chapter is specifically focused on metal clusters, induced by ionizing radiation or ultraviolet (UV)-visible photons. 

The excellent agreement between the TSC and P1A results has two implications. First, since the TSC method probes the product of mobility and carrier density, while the P1A probes only the carrier density, there seems to be no dominant influence of temperature on the carrier mobility. This was also found in other conjugated polymers like /ra/ry-polyacetylene [19, 36]. Second, photoconductivity (observed via the thermal release of photoexcited and trapped earners) and photo-induced absorption probe the same charged entity [36, 37J. 

As attractive as the transannular bridging of bis(thiolactones) to bicyclic bis(oxepane) frameworks is, our inability to convert the disulfide bridging product (see 25, Scheme 5) to a mmv-fused bre-vetoxin-type bis(oxepane) (see 28) necessitated the development of a modified, stepwise strategy. This new stepwise approach actually comprises two very effective methods for the construction of cyclic ethers the first of these is the intramolecular photo-induced coupling of dithioesters, and the second is the reductive cyclization of hydroxy ketones. We will first address the important features of both cyclization strategies, and then show how the combination of the two can provide an effective solution to the problem posed by trans-fused bis(oxepanes). 

Fig. 28 Raman spectra of polymeric sulfur (S ) prepared by various methods [109,173], of large disordered rings (S ) [182], and of photo-induced amorphous sulfur (a-S) [119], respectively. The spectrum of a-S has been smoothed for clarity. The position of the stretching vibration of a-S is pressure-shifted to higher wavenumbers. The very weak signals in the spectra of Sj, at ca. 150 and 220 cm are probably caused by the presence of Sg. In addition, the weak shoulder at ca. 470 cm observed in spectra of Sj, may originate from Sg, too...

Monte Carlo calculations have been carried out to simulate the spin transition behaviour in both mono- and dinuclear systems [197]. The stepwise transition in [Fe(2-pic)3]Cl2-EtOH as well as its modification by metal dilution and application of pressure have been similarly modelled by considering short- and long-range interactions [52, 198, 199]. An additional study of the effect of metal dilution was successfully simulated with the Monte Carlo treatment considering direct and indirect inter-molecular interactions [200]. A very recent report deals with the application of the Monte Carlo method to mimic short- and long-range interactions in cooperative photo-induced LS—>HS conversion phenomena in two- and three-dimensional systems [201],... 

But the method suffers from the difficulty of obtaining starting materials and photo induced rearrangement of silyl ketones... 

The third issue is most difficult to address theoretically since computational methods for treating radiationless and ET processes in large molecular systems are relatively primitive at this time [see the CT States and Photo-induced ET Processes"" section for further discussion]. Several interesting experimental approaches have, however, provided a number of useful insights into possible ways of reducing recombination, and thus enhancing the irreversibility of the photo-induced ET process. 

I propose to develop and apply such methods, based on ultrafast X-ray absorption spectroscopy, to study the ultrafast molecular motions of organometallics in solutions. In particular, initial studies will focus on photo-induced ligand dissociation and substitution reactions of transition metal carbonyls and related compounds in various solvent systems. 

In connection with the problem of oscillations discussed by previous speakers and other types of dynamical behavior of membranes, it would probably be timely to mention here in some more detail the experiments with vision rhodopsin that were performed in our institute by using the Mossbauer spectroscopy method [G. R. Kalamkarov et al., Doklady Biophys., 219, 126 (1974)]. These experiments manifested the existence of reversible photo-induced conformational changes in the photoreceptor membrane even at such low temperature as 77°K. We have labeled various samples of solubilized rhodopsin and of photoreceptor membranes by iron ascorbate enriched with Fe57 and looked for the change of Mossbauer spectra caused by the illumination of our samples. 

Exciplexes are defined as molecular complexes which are stable under electronic excitation30. The picosecond (or femtosecond) laser photolysis methods are suitable for investigating the very rapid photo-induced processes related to CT complexes. 

Cation radicals may be generated by direct or indirect electrochemical oxidation of the molecule of interest, and many such oxidations are synthetically useful. However, several other methods are also available, which fall into two broad categories thermally-induced electron transfer (TIET) and photo-induced electron transfer (PIET). 

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