Photochemical reactivity protons

A highly desirable property in information storage systems based on molecular switches is gated response.1491 Gated photochemical reactivity implies that no change occurs upon irradiation unless another external stimulus, either physical or chemical, is applied to the system. Scheme 18 shows a typical write-lock-unlock-erase cycle involving photoisomerization and protonation. 

A step forward along the route to the correct modelling of the spectroscopy and photochemical reactivity of photoreactive proteins is represented by the implementation of a Quantum Mechanics/Molecular Mechanics (QM/MM) computational strategy based on a suitable QM part coupled with a protein force field such as AMBER [34] (or CHARMM [35]). Very recently a CASPT2//CASSCF/AMBER method for rhodopsin has been implemented in our laboratory [36,37] within the QM/MM hnk-atom scheme [38]. Special care has been taken in the parametrization of the protonated Schiff base linkage region that describes the dehcate border region between the MM (the protein)... 

Lactams such as (258) can be synthesized from the phthalimides (259) by irradiation. Again the reactions are controlled by single electron transfer processes that are usually encountered in the photochemical reactions of phthalimides. The outcome of the reaction is a conventional proton transfer from the benzylic site within the zwitterionic biradical formed on irradiation. Cyclization within the resultant 1,5-biradical affords the final product. Griesbeck and his coworkers have studied the photochemical reactivity of the phthalimide derivatives (260). These compounds on irradiation under triplet sensitized conditions undergo decarboxylation and cyclization. The reaction involves SET and the key intermediates are shown as (261) and (262). The biradical anion (262) is the species that either cyclizes to afford (263) or abstracts hydrogen to yield (264). The reaction is controlled by a variety of factors that have been reported in some detail. Some photochemical reactions of phthaloylcysteine derivatives have been described. Typical of the processes are the decarboxylations of the derivative... 

Another significant and synthetically relevant observation was made in studies probing the photochemical reactivity of N-protonated pyridines. Mariano and co-workers have demonstrated that irradiation of a solution of pyridine in aqueous perchloric acid at 45 to 70°C results in clean generation of the cyclopentene aminodiol 39, isolated as its triacetyl derivative 40 (Scheme 14). Despite the fact that the quantum efficiency of this reaction is low, the process can be used to prepare 1-2 g quantities of 40 following 1-2 day irradiation periods. 

The experiments above indicate that the solvent dependence of < >d arises primarily from the solvent dependence of the triplet state reactivity. The absence of a primary deuterium isotope effect makes it unlikely that protic co-solvents affect < >d through a bond-breaking interaction (e.g., photochemical proton transfer). Also, since... 

---