Photoreaction process

Figure 11.15 The photoreactive compound psoralen-PEC>3-biotin can intercalate into double stranded DNA or RNA segments and covalently link to thymine bases via a photoreaction process.

Fig. 8.13. Schematic showing the photoreaction process of photlLOV2 detected by TG (a) light-induced association of two monomers and (b) light-induced dissociation of a dimeric form...

Scheme 2. Example for molecules possessing comi ting TICT and ESIPT photoreaction processes 286...

Absorption of a photon by the chromophore induces primary photoreaction, followed by conformational changes of protein, and eventually activates transducin. This is called the bleaching process because rhodopsin loses its color. To investigate the primary photoreaction processes in rhodopsin, two spectroscopic approaches have been applied low-temperature and time-resolved spectroscopies... 

The primary photoreactions in the PYP have been experimentally studied both in the protein and in solution environments. The initial structural change of PYP was directly observed by time-resolved x-ray crystallography. Ultrafast fluorescence spectroscopy was performed on the initial process of the photoreaction of PYP. It was shown that the photoisomerization reaction of the PYP chromophore is completed within 1 ps. The initial photoreaction processes were analyzed by using time-resolved spectroscopic data. Although the reaction of the PYP chromophore in solution environments has been studied by several groups, the characterization of the dynamics in these environments is not fully understood. 

Knowledge of photoiaduced electroa-transfer dyaamics is important to technological appUcations. The quantum efficiency, ( ), ie, the number of chemical events per number of photons absorbed of the desired electron-transfer photoreaction, reflects the competition between rate of the electron-transfer process, eg, from Z7, and the radiative and radiationless decay of the excited state, reflected ia the lifetime, T, of ZA ia abseace ofM. Thus,... 

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

Nitrile ylides derived from the photolysis of 1-azirines have also been found to undergo a novel intramolecular 1,1-cycloaddition reaction (75JA3862). Irradiation of (65) gave a 1 1 mixture of azabicyclohexenes (67) and (68). On further irradiation (67) was quantitatively isomerized to (68). Photolysis of (65) in the presence of excess dimethyl acetylenedicar-boxylate resulted in the 1,3-dipolar trapping of the normal nitrile ylide. Under these conditions, the formation of azabicyclohexenes (67) and (68) was entirely suppressed. The photoreaction of the closely related methyl-substituted azirine (65b) gave azabicyclohexene (68b) as the primary photoproduct. The formation of the thermodynamically less favored endo isomer, i.e. (68b), corresponds to a complete inversion of stereochemistry about the TT-system in the cycloaddition process. 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

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 acetoxy dienone (218) gives phenol (220). Here, an alternative primary photoreaction competes effectively with the dienone 1,5-bonding expulsion of the lOjS-acetoxy substituent and hydrogen uptake from the solvent (dioxane). In the case of the hydroxy analog (219) the two paths are balanced and products from both processes, phenol (220) and diketone (222), are isolated. In the formation of the spiro compound (222) rupture of the 1,10-bond in the dipolar intermediate (221) predominates over the normal electron transmission in aprotic solvents from the enolate moiety via the three-membered ring to the electron-deficient carbon. While in protic solvents and in 10-methyl compounds this process is inhibited by the protonation of the enolate system in the dipolar intermediate [cf. (202), (203)], proton elimination from the tertiary hydroxy group in (221) could reverse the efficiencies of the two oxygens as electron sources. 

It is evident from the nature of the products, especially those formed with toluene present, that the photoreaction in weakly acidic medium involves incursion of a radical species. The complete suppression of reactions leading to the above products, in the presence of oxygen, strongly suggests that it is an excited triplet trityl ion which undergoes reaction. It is postulated that the primary photochemical process is the abstraction of a hydrogen atom by the triplet trityl ion to form the radical cation 90, which was proposed as an intermediate in the dimerization reactions carried out in strong acid (Cole, 1970). 

The study of quantum yields. The quantum yield is the fraction of absorbed light that goes to produce a particular result. There are several types. A primary quantum yield for a particular process is the fraction of molecules absorbing light that undergo that particular process. Thus, if 10% of all the molecules that are excited to the state cross over to the T state, the primary quantum yield for that process is 0.10. However, primary quantum yields are often difficult to measure. A product quantum yield (usually designated ) for a product P that is formed from a photoreaction of an initially excited molecule A can be expressed as... 

On the other hand, the crystallization process of diolefin compounds often plays a significant role in determining their topochemical behaviour, by changing their crystal structure or by forming solvent inclusion complexes. Furthermore, topochemical photoreactions of crystals with )8-type packing are accompanied by thermal processes under moderate control by the reacting crystal lattice (see p. 140). These factors seriously complicate the whole reaction scheme. 

Assuming that the reaction probability of all the elementary processes is equal in the reaction of 1,4-DCB crystals, the calculated yields of unreacted 1,4-DCB, cyclophane, and oligomer by simulation, should be 1.8, 37.7, and 60.5% by weight, respectively. Furthermore, if all the photoexcited species of the monocyclic dimer are assumed to be converted into cyclophane, these yields should become 6.9, 65.6 and 27.5%. It is, therefore, rather surprising that in an extreme case of the experiment the yield of cyclophane is more than 90% while the amount of unreacted 1,4-DCB is less than 2%. One plausible mechanism to explain this result is that the first formation of cyclophane induces the successive formation of cyclophane so as to enhance its final yield. If such an induction mechanism plays an appreciable role, an optically active cyclophane zone may be formed, at least in a micro spot surrounding the first molecule of cyclophane, as illustrated in Scheme 13. The assumption of an induction mechanism was verified later in the photoreaction of 7 OMe crystals (see p. 151). 

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