Unimolecular photoreactions

The photochemistry of alkenes and dienes has already been mentioned in connection with the principles of orbital symmetry control in electrocyclic and cycloaddition processes in Section 13.2. Cycloadditions are considered, from a synthetic viewpoint, in Chapter 6 of Part B. This section will emphasize unimolecular photoreactions of alkenes and dienes. 

The photoisomerisation of stilbenes is found to be a simple and convenient model for the detailed study of factors affecting unimolecular photoreaction dynamics. Saltiel and coworkers first proposed a detailed mechanism for light- induced trans-cis photoisomerisation of franv-stilbene (Saltiel, and D Agostino, 1972 Saltiel et al.,1992, Waldeck,1996 Papper et.al., 1997, 1998 Papper and Likhtenshtein, 2001). 

This scheme represents two independent parallel reactions of the enantiomers. A general treatment for thermal kinetic resolution was given by Kagan and Fiaud [42]. For unimolecular photoreactions first-order equations seem to be appropriate [40]. Accordingly the rates are... 

The application of pressure to CH2CH2 solutions of either Cu(dmp)2 or Cu(dpp) led to systematic decreases in the emission lifetimes even in the absence of added Lewis bases. However, it is clear from the ln(T°/r) versus P plots of Figure 12 that the emission lifetimes of [Cu(dmp)2] are significantly more pressure sensitive. For both [Cu(dmp) ] and [Cu(dpp) ], the emission quantum yields at ambient temperature are small (< 10 3) and unimolecular photoreactions are not observed. Thus, the dominant deactivation pathways are nonradiative, and the pressure sensitivity of t reflects that of the various nonradiative deactivation pathways. 

The copper(I) complex Cu(dmp)2 (dmp = 2,9-dimethyl-l,10-phenanthroline) displays MLCT luminescence in ambient temperature CH2CI2 solutions [85]. This emission has been shown to be quenched by various Lewis bases (B), and the mechanism proposed is addition of B to the MLCT state at the metal center to give an exciplex which decays rapidly (Eq. 6.39). The validity of this mechanism was tested by comparing, in the presence and absence of Lewis base quenchers, the pressure effects on the emission lifetimes of Cu(dmp)2 vvith those on the emission lifetimes of the bulkier 2,9-diphenyl-phen analog Cu(dpp) [86]. The lattice ions should not be as susceptible to reaction of the copper center with B. For both ions, emission quantum yields are small (<10 ) at ambient T and unimolecular photoreactions are not observed, so the pressure sensitivity of x reflects non-radiative deactivation mechanisms. 

J. R. Scheffer, P. R. Pokkuluri, Unimolecular photoreactions of organic crystals the medium is the message, in Photochemistry in Organized and Constrained Media, ed. V. Ramamurthy, VCH Verlag GmbH, Weinheim, 1991, pp. 185-246. 

The largest family of luminescent metal compounds comprises those complexes which emit from MLCT states. This emission is facilitated by the fact that generally MLCT excitation has only a small effect on the metal-ligand bonding and does not initiate competing unimolecular photoreactions. Intense luminescence can thus be observed under ambient conditions. The most prominent emitters include [Ru (bipy)3]2+ [41,81] and Re(o-phen)(CO)3Cl [4,75] and their derivatives. Any discussion of these complexes is omitted here since they are treated elsewhere in this book. 

For a photoexcited molecule, the time allowed for a reaction to occur is of the order of the lifetime of the particular excited state, or less when the reaction step must compete with other photophysical processes. The photoreaction can be unimolecular such as photodissociation and photo isomerization or may need another molecule, usually unexcited, of the same or different kind and hence bimolectdar. If the primary processes generate free radicals, they may lead to secondary processes in the dark. 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

This chapter surveys unimolecular and bimolecular photochemical reactions in mixed molecular crystals and solid mixtures. Various photoreactions occurring in mixed crystals (solid solution), hydrogen-bonded cocrystals, donor-acceptor crystals, crystalline organic salts, and solid mixtures are described. In contrast to one-component crystals [2], the organic photochemistry of such multi-... 

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