Photochemistry four-electron systems

The gas-phase photochemistry of haiogenated ethenes has been studied in the UV and VUV [60, 61], as well as in the infrared, using multiple-photon-absorption excitation with powerful CO2 laser sources [62-66]. Also, sensitized decompositions, for example using electronically excited Hg( P) atoms, have also been reported [67-69]. The net gas-phase photochemistry of these systems usually involves hydrogen halide elimination via three-and/or four-center transition states, with some evidence for simple bond fission producing halogen atoms in the case of Hgf Pj) photosensitization [70]. 

There are totally 11 chromophores in the bacterial photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps.) virids. Since the excitation process of the reaction center is the primary event of the photo-induced electron transfer in the reaction center, the detailed analysis of the absorption spectrum is one of the key steps for the understanding of photochemistry of the system. The chromophores included in the PSRC are bacteriochlorophyll b dimer (special pair, P), bacteriochlorophyll in L- and M-branches (Bl and B ), bacteriopheophytin in L- and M-branches (Hl and Hm), menaquinone (MQ), ubiquinone (UQ) and four different hemes, c-552, c-554, c-556, and c-559 in c-type cytochrome subunit. 

We illustrate the method for the relatively complex photochemistry of 1,4-cyclohexadiene (CHDN), a molecule that has been extensively studied [60-64]. There are four it electrons in this system. They may be paired in three different ways, leading to the anchors shown in Figure 17. The loop is phase inverting (type i ), as every reaction is phase inverting), and therefore contains a conical intersection Since the products are highly strained, the energy of this conical intersection is expected to be high. Indeed, neither of the two expected products was observed experimentally so far. 

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