Other photoisomerizations

On irradiation in solution 9-t-butylanthracene, unlike other 9-substituted anthracenes, gives a Dewar isomer (3), which reverts thermally to the parent anthracene (ii at 20 °C = 6.5 h). Although the quantum yield is low (f) 0.01), the absorption properties of this system make it more suited to further study for possible use in solar energy storage than many other photoisomerizations. 

None of these papers referred to each other or to their apparent deviation from NEER. However, since they were the only exceptions to NEER (not counting the large aryl systems mentioned above), we suspected that there must be a shared common feature. After much deliberation, it suddenly dawned on us that these four examples share common reaction conditions in that aU four photoisomerization studies were conducted in rigid media. This feature differs from numerous other photoisomerizations that obeyed NEER and were carried out either in fluid solutions or under other unconstrained conditions. 

Also, using dyes as laser media or passive mode-locked compounds requires numerous special parameters, the most important of which ate the band position and bandwidth of absorption and fluorescence, the luminiscence quantum efficiency, the Stokes shift, the possibiHty of photoisomerization, chemical stabiHty, and photostabiHty. AppHcations of PMDs in other technical or scientific areas have additional special requirements. 

P -Bonding is obviously also the initiating step in the complex photoisomerization sequence of the stereoisomeric 1,5-dien-3-ones (162) and (163) in ethanol. After low conversions of the starting dienones, an isomer containing an analogous chromophoric system [(164) and (165), respectively] was found to build up temporarily in each case. On longer photolysis times, both compound pairs (162)/(164) and (163)/(165), are consumed, and the mixtures of the four diastereomers (166)-(169) were isolated from both runs. According to separate irradiation experiments with each of these products, (166) and (167) on one hand, and (168) and (169) on the other, are... 

Nonadiabatic transitions definitely play crucial roles for molecules to manifest various functions. The theory of nonadiabatic transition is very helpful not only to comprehend the mechanisms, but also to design new molecular functions and enhance their efficiencies. The photochromism that is expected to be applicable to molecular switches and memories is a good example [130]. Photoisomerization of retinal is well known to be a basic mechanism of vision. In these processes, the NT type of nonadiabatic transitions play essential roles. There must be many other similar examples. Utilization of the complete reflection phenomenon can also be another candidate, as discussed in Section V.C. In this section, the following two examples are cosidered (1) photochromism due to photoisomerization between cyclohexadiene (CHD) and hexatriene (HT) as an example of photoswitching molecular functions, and (2) hydrogen transmission through a five-membered carbon ring. 

The photoisomerization reaction gives a valuable tool to investigate the molecular friction at the interface. Depending on the exact location of the reaction path, whether predominantly in one or the other phase, the time scale of the rate of the reaction is severely modified. The photoisomerization reaction of the dyes malachite green and 3,3 -dieth-yloxadicarbocyanine iodide (DODCI) has been reported at several interfaces, the air-... 

A comparison of the rate constant for photoisomerization of the unsubstituted 3-phenyl derivative (kT = 3 x 1010 sec-1) to that of the 3-(p-methoxy phenyl) derivative (kr = 1.5 x 1010 sec-1) indicates that the presence of the p-methoxy groups imparts no special stability to the intermediate responsible for isomerization even though cleavage of a cyclopropane bond is predominant. Clearly these results are inconsistent with an intermediate possessing electron-poor or electron-rich species such as would be obtained from heterolytic cleavage of the cyclopropane. On the other hand, the results are consistent with a biradical species as intermediate. Further evidence consistent with this conclusion was obtained in a study of trans-3-p-cyanophenyl-/ra w-2-phenyl-1 -benzoylcyclopropane,<82)... 

A quick survey of the photochemistry of the different complexes described above shows that the mechanism of photoactivation and the subsequent nature of the observed photoproducts varies from complex to complex and from one geometric isomer to another. Photochemical pathways often involve a combination of photosubstitution, photoisomerization, and photoreduction steps. In general, photolysis is rather slow in water and many different products are obtained if the complex is irradiated alone. The presence of nucleophilic biomolecules, on the other hand, can have a major influence, as photoreduction is usually rapid and accompanied by simpler reaction pathways. NMR methods... 

The first CEM system described by Griffing and West (84) consisted of an organic dye dispersed in an inert polymer film that is spin coated onto the surface of a resist and subsequently removed following exposure but prior to resist development. The chemistry of this system is based on the photoisomerization of an aromatic dye to an oxaziridine (87) (Figure 10). Other workers have evaluated polysilanes (88) and diazonium salt chemistry (89,90) for CEM applications. 

Photochemical response of these liposomes is different from each other. With progression of trans - cis photoisomerization of azobenzene, ICD at the absorption band of the trans isomer decreases. As shown in Figure 4, depression in ICD is almost proportional to the amount of photoisomerization for the phase separated system. Photoisomerization in the domain of azobenzene aggregate proceeds independently from the rest of DPPC aggregate so that the depression in ICD corresponds to the concentration of remaining transazobenzene. When the two components are molecularly mixed, change of... 

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

There has been considerable debate over whether the ,Z-photoisomerization (as well as other photoprocesses) of conjugated polyenes proceeds via neutral (2XA) or zwitterionic... 

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. 

In calcium chelators Indo-1 (17) and Fura-3 (18b) (Figure 2.9),(18) the fluoropho-res have donor-acceptor stilbene-like structures rigidified so as to avoid photoisomerization. Based on the same principle, Fura-2 (18a)a8) is one of the most popular calcium indicator for microscopy of individual cells because, in contrast to Quin-2 (see Section 2.2.5.), the excitation spectrum is blue shifted on cation binding, thus allowing intensity-ratio measurements. On the other hand, there is almost no shift of the emission spectrum, which can be interpreted along the same line as DCM-crown (see earlier in this section). 

When an aqueous solution containing 1,2,3,4-tetrachlorobenzene and a nonionic surfactant micelle (BriJ 58, a polyoxyethylene cetyl ether) was illuminated by a photoreactor equipped with 253.7-nm monochromatic UV lamps, no photoisomerization was observed. However, based on photodechlorination of other polychlorobenzenes under similar conditions, it was suggested that tri- and dichlorobenzenes, chlorobenzene, benzene, phenol, hydrogen, and chloride ions are likely to form. The half-life for this reaction, based on the first-order photodecomposition rate of 5.24 x lO Vsec, is 2.2 min (Chu and Jafvert, 1994). 

---