Model compound photoreaction

The greater intensity of the band of the metabolite at 220 mis probably due to the presence of a second, superimposed chromophore which could also account for the shift of the minimum. On the other hand, the band near 300 m/u. has the expected intensity. Its broadness and displacement towards longer wavelength are probably due to the presence of a substituent on the double bond or benzenoid ring. That the assignment to a coumaroyl chromophore is essentially correct is evidenced by the fact that both M and the model compound underwent the same type of reaction on irradiation in the near-ultraviolet (Figure 4). The observed isosbestic points imply that the photoreaction is a simple one, such as A -> B or A = B, and is obviously the well-known light-induced trans- to c/r-isomerization (7) of cinnamic acid derivatives. 

Another model compound, the tris(2,2 -bipyridine)ruthenium(II) complex, has prompted considerable interest because its water-splitting photoreactivity has been demonstrated in various types of photochemical systems (77,99,100,101). Memming and Schroppel (102) have attempted to deposit a monolayer of a surfactant Ru(II) complex on a Sn02 OTE. In aqueous solution, an anodic photocurrent attributable to water oxidation by the excited triplet Ru complex was observed. A maximum quantum efficiency of 15% was obtained in alkaline solution. 

In the present studies this difficulty has been eliminated by synthesizing polymers with well-defined chromophoric groups situated at known locations with respect to the polymer chains. By studying the photoreactions of these polymers in comparison with those of suitable model compounds, it was possible to infer much about the photochemistry of macromolecules. 

The photochemistry of chlorophenols has been studied by many other authors, and depending on the conditions chosen, numerous product types have been identified. Crosby and co-workers examined the photoreactions of 2,4-dichlorophenol (Crosby and Tutass, 1966) and 2,4,5-trichlorophenol (Crosby and Wong, 1973), model compounds for the herbicides 2,4-D and 2,4,5-T. The products isolated appeared to have been formed by photonucleophilic aromatic substitution of OH groups for Cl atoms (Figure 6.16). Photolysis of 3-chlorophenol as well as other 3-halogenated phenols afforded resorcinols, presumably by similar mechanisms... 

The model compound results revealed by Hasegawa et al. [178] emphasizes that the presence of alkyl substituents at the ortho position contributes to an increased photoreactivity owing to the N-Ar molecular plane distortion, which reduces the intramolecular CT character. This is the reason why BTDA-PIs derived from the ortho- ky substituted diamine are highly reactive but that from the mcta-substituted one is less reactive, as stated by Pfeifer et al. [155]. 

Computer modelling of the photoreactions of two other lignin model compounds, i.e. tert-butylguaiacylcarbinol (4.125) and p-methoxypropio-phenone (4.126) shows the existence of 29 elementary reactions from 91 elementary steps with 58 different species formed [2164] ... 

Table I summarizes the quantum yields for the photoreaction of diazo-containing model compounds and those for the benzophenone-oontaining models,...

Table I. Quantum yields for photoreaction of model compounds...

A model proposed for photochemical conversion of solar energy 11,14) is shown in Fig. 3. The system is made of a photoreaction couple, two kinds of electron mediator, and reduction as well as oxidation catalysts. It is designed to share the necessary functions among the various compounds because it would be difficult for one single compound to bear all the functions. A single component carrying out the total conversion would of course be the best system. 

In regard to the mechanism for these solid-state photoreactions, there are two possible pathways from the starting thioesters to phthalides (Scheme 12). In the first model (Path A), the reaction is initiated by homolytic dissociation between C-S bonding to form a radical pair intermediate. Such pathway is well recognized as the excitation reaction of thioester compounds. The other model (Path B) consists of direct... 

The photoreaction investigated to test the above model is the well-known electron-transfer-initiated intermolecular hydrogen abstraction reaction of carbonyl compounds [277]. Under the conditions employed, one of the guest mole-... 

Experimental work in model systems supplemented with theoretical investigations proved that the main photoreaction mode of Roussin s black salt is a photoredox reaction leading to generation of NO and a Fe(II) compound. The n NO-d transitions were assumed to be responsible for the photochemical reactivity of these compounds, which result in photodissociation of the NO group (33,36,50-52). It has been found, however, that irradiation of [Fe4(p3-S)3(NO)7] in the presence of S , thiolate anions or other S-nucleophiles produces Fe(III) species and N2O, instead of NO (53). 

Examples of photoreactions may be found among nearly all classes of organic compounds. From a synthetic point of view a classification by chromo-phore into the photochemistry of carbonyl compounds, enones, alkenes, aromatic compounds, etc., or by reaction type into photochemical oxidations and reductions, eliminations, additions, substitutions, etc., might be useful. However, photoreactions of quite different compounds can be based on a common reaction mechanism, and often the same theoretical model can be used to describe different reactions. Thus, theoretical arguments may imply a rather different classification, based, for instance, on the type of excited-state minimum responsible for the reaction, on the number and arrangement of centers in the reaction complex, or on the number of active orbitals per center. (Cf. Michl and BonaCid-Kouteck, 1990.)... 

---