Photodimerization anthracenes

The discovery that ben/.o[6]quinolizinium salts could, like anthracene, photodimerize,154 or undergo cycloaddition reactions, has led to many interesting papers. An excellent review is available for cycloaddition reactions155 which occur with electron-rich rather than electron-deficient alkenes. For this reason, and because of the proposed mechanism, they... 

Anthracene photodimerization (see Fig. 6.21) is well known to occur in a variety of solvents and in the solid state [61]. The photodimers are stable at room temperature and can revert to their monomeric state by photoirradiation (l < 300 nm). 

More specifically, oxygenation reactions (involving addition of a molecule of oxygen) were also early individuated, although only in sparse instances. Thus, Fritzscke, the discoverer of anthracene photodimerization, also found that tetracene added oxygen to form what is now known as a cyclic peroxide and the reaction was thermally reversible [92]. 

Bouas-Laurent, H., Castellan, A. and Desvergne, J.-P. From anthracene photodimerization to jaw photochromic materials and photocrowns. Pure Appl. Chem. 52 2633-2648, 1980. 

Tamaki, T, Kawanishi, Y., Seki, T, and Sakuragi, M., Triplet sensitization of anthracene photodimerization in y-cyclodextrin,/. Photochem. Photobiol. A Chem., 65, 313, 1992. 

Figure 10.1 Photodimerization of anthracene (a) and trans-cinnamic acid (b).

Reversible Phase Separation Driven by Photodimerization of Anthracene A Novel Method for Processing and Recycling Polymer Blends... 

Figure 10.9 Reversible photodimerization of anthracene induced by light of two wavelengths 365 and 295 nm in a PSAF/PVME (20/80) mixture observed at 25 °C.

Derivatives of anthracene bearing substituents on the 1 or 2 position can be photodimerized with efficiencies comparable to that for the unsubstituted molecule. However, with substituents at the 9 meso) or 9, 10 dimeso) positions a very interesting photochemical problem results. Since dimerization occurs across the 9, 10 positions, substituents at these positions exert a first-order effect on the photochemical reaction. The mero-substituted anthracenes examined include the following as 19>... 

The photodimerization of anthracene has been utilized to produce a number of interesting synthetic derivatives which are essentially photo-isomers of the starting materials,[Pg.28]

By examining any correlation between excimer formation (as evidenced by characteristic excimer fluorescence) and dimerization quantum yield, one could perhaps determine whether dimerization is dependent upon prior excimer formation. Excimer fluorescence from anthracene solutions at room temperature is negligible although it has been observed in the solid state at low temperature.<75) Unfortunately, the data for substituted anthracenes allow no firm conclusions to be drawn. Some derivatives dimerize but do not exhibit excimer fluorescence. Others both dimerize and show excimer fluorescence. Still others show excimer fluorescence but do not dimerize and finally, some neither dimerize nor show excimer fluorescence. Hopefully, further work will determine what role excimer formation plays in this photodimerization. 

Up to this point we have concerned ourselves almost exclusively with the anthracene singlet state and its resulting photodimerization. However, we... 

We have already discussed one of the earliest photoreactions to be studied, that is, the (4w + 4w) photodimerization of anthracene. That the singlet state was involved in this reaction was conclusively shown in the period 1955-1957. The first reaction in which the triplet state of the molecule was shown to be involved was the photoreduction of benzophenone by Hammond and co-workersa) and Backstrom and co-workers<2) 1959-1961. This was the first in a series of many papers from Hammond s laboratory... 

Photochemical Techniques and the Photodimerization of Anthracene and Related Compounds... 

The photodimerization of anthracene, having been first studied by Fritzsche in 1867 (two years after Kekule proposed his revolutionary structure for benzene), was one of the first photochemical systems to be extensively investigated. Fritzsche found that upon exposure to sunlight, benzene solutions of anthracene yielded an insoluble substance which he called Para-photen. Observing that the photoproduct yielded anthracene upon melting, he concluded that he had obtained a polymer of anthracene/9 ... 

Clearly all the answers to the questions of the mechanisms of both the photodimerization and photooxidation of anthracene are not yet known. Hopefully, what we have seen in this chapter will serve to convince the reader that photochemistry is still an exciting and challenging field of study in which there is ample room for further research. 

The first photochemical reactions to be correlated with PMO theory were the dimerizations of anthracene, tetracene, pentacene, and acenaphthylene. 36> More detailed energy surfaces for the photodimerization reactions of butadiene have also been calculated. 30> In the category of simplified calculations lie studies of the regiospecificity of Diels-Alder reactions 37>, and reactivity in oxetane-forming reactions. 38,39) jn these... 

The [4+4]photodimerization of anthracene has already been reported more than 115 years ago (4.26)429). 

The literature of mechanistic aromatic photochemistry has produced a number of examples of [4 + 4]-photocycloadditions. The photodimerization of anthracene and its derivatives is one of the earliest known photochemical reactions of any type97. More recently, naphthalenes98, 2-pyridones" and 2-aminopyridinium salts100 have all been shown to undergo analogous head-to-tail [4 + 4]-photodimerization. Moreover, crossed [4+4]-photocycloaddition products can be obtained in some cases101. Acyclic 1,3-dienes, cyclohexadienes and furan can form [4 + 4]-cycloadducts 211-214 with a variety of aromatic partners (Scheme 48). 

We turn first to the (4 + 4) photodimerization of anthracenes, which has been most extensively studied in this context. In many anthracenes it has been possible to show that in the starting crystals defects are present at which the structure is appropriate for formation of the observed dimer in others it has been argued that the presence of such defects is very plausible. The weakness of this interpretation, at this stage, is that in no case has it yet proved possible to establish that the reaction indeed occurs at these defect sites. 

To give the reader a feel for these effects we refer to two examples. Anthracene itself crystallizes in a structure in which the extent of overlap between molecules in plane-to-plane close packing is negligibly small. We would therefore expect there to be no driving force for the reaction, and the crystal to be light stable. In fact, it undergoes photodimerization to yield 85a, albeit with low quantum... 

As in the former cases, k2 was calculated from the integrated extinction coefficients,149 k3 + kt was derived from fluorescence quantum yields,149 while k3 and k4 were separately estimated from the maximum quantum yields of photooxygenations at high oxygen concentrations.150 Flash spectroscopy techniques were used in order to determine k5 and k7, while kB was obtained from the Stern-Volmer quenching constant of oxygen.149 The ratio ke/kg was determined from the variation of AOz with the concentration of the anthracene.71 When photodimerization occurred, k13l(kia + k13) was calculated from the maximum yield of... 

Although the quinolizinium ion (1), like naphthalene, does not undergo photodimerization, its linear benzo derivative, the acridizinium ion, like anthracene, does so readily (Scheme 27) (57JOC1740). The photodimer dissociates when heated in ethanol. It has been reported that both the dimerization and dissociation in methanol are light-catalyzed and that the quantum yields for the two reactions are 0.23 and 0.49 respectively (78JPR739). 

In 1963, E. J. Bowen published his classic review The Photochemistry of Aromatic Hydrocarbon Solutions, in which he described two major reaction pathways for PAHs irradiated in organic solvents photodimerization and photooxidation mediated by the addition of singlet molecular oxygen, 02 ) (or simply 102), to a PAH (e.g., anthracene). For details on the spectroscopy and photochemistry of this lowest electronically excited singlet state of molecular oxygen, see Chapter 4.A, the monograph by Wayne (1988), and his review article (1994). For compilations of quantum yields of formation and of rate constants for the decay and reactions of 02( A), see Wilkinson et al., 1993 and 1995, respectively. 

A simultaneous increase in photodimerization yield and reduction in molecular fluorescence yield with increasing solute concentration provides convincing evidence that photodimerization involves the molecular singlet state,90,93 and in certain systems, notably derivatives of naphthalene48 and anthracene,41 excimer fluorescence is exhibited simultaneously. 

The photodimerization of anthracene derivatives has been advanced as a criterion of photoassociation in these systems it is therefore of interest to examine the extent to which similar behavior exhibited by the pyrimidine constituents of nucleic acids can be described in terms of the same reaction sequence. 

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