Photoionic switch

Since electrons are of rather low mass, PET rates can be extremely fast with transit times in the ps-ns range. Molecular switches where the only moving part is an electron are likely to be much faster than those involving nuclear motion. Since they require ion movement, photoionic devices based on PET will operate at slower rates. However these are fast enough compared to the human timescales. 

One of the earlier nonselective sensors of this type,1591 subsequently interpreted as a photoionic OR gate (Figure 8), was 17. Sufficient quantities of K+ or Rb+ elicited the correct fluorescence output, with K+ and Rb+ giving almost equal fluorescence enhancements. Only with neither input species present was the fluorescence seen to be switched off. However, the first intentionally designed OR logic gates1601 were 18 and 19, in which Ca2+ and Mg2+ produce essentially identical fluorescence enhancements. The receptor employed in this instance is one put forward by London 1611 compound 20. 

Topics of relevance to the content of this chapter which have been reviewed during the year include photoactive [2]rotaxanes and [2]catenanes, photochemical synthesis of macrocycles, phototransformations of phthalimido amino acids, photoaddition reactions of amines with aryl alkenes and arenes, photoreactions between arenenitriles and benzylic donors, photostability of drugs, polycyclic heterocycles from aryl- and heteroaryl-2-propenoic acids, photoreactions of pyrroles, photoamination reactions in heterocyclic synthesis, switching of chirality by light, photochromic diarylethenes for molecular photoionics and solid state bimolecular photoreactions. 

The validity of the statistical approximation in QET-RRKM-phase-space theories has been tested extensively both experimentally and theoretically. Some of the most incisive tests of statistical theory have been performed with energy-selected ions, prepared as described previously, using methods such as photoelectron-photoion coincidence (PEPICO). Particularly incisive tests of these statistical models have been performed on isomeric C4Hg systems by Baer and Bowers and their co-workers and on C4H by Bowers and collaborators. The latter study provided validation of an important variant of statistical theory called the transition state switching model. 

Alternatively, one may switch to another ionization technique, whereby the cation or anion formation takes place in the gas phase at atmospheric pressure. These techniques are chemical ionization (Atmospheric Presstne Chemical Ionization, APCI) and photoionization (Atmospheric Pressure Photo Ionization, APPI). 

The lower trace in Fig. 10A shows the number of ions Nign(2v2) produced with LI off. The peaks correspond to resonant transitions (4), as could be proved by observation of the corresponding fluorescence (lowest trace in Fig. 10B). When the pump laser LI is switched on, the upper trace in Fig. 10A is recorded. The difference between the two curves (note that the upper curve is vertically shifted by an amount indicated by the arrow in order to separate the two recordings) gives the number of ions produced by one photon hvg on a transition from a labelled level (v, J ) in the intermediate state. The auto-ionization lines can be seen superimposed on a continuum which increases with decreasing wavelength Aj. The continuum is attributed to the direct photoionization process (3) and from its onset at... 

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