Dimerization of anthracene

We now address ourselves to the problem of accounting for the observations noted in Section 2.2a by building a mechanism for the dimerization of anthracene and related compounds. 

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... 

While it is in the triplet state a molecule may undergo typical diradical reactions. This provides a plausible mechanism for radical-like reactions of substances that are largely diamagnetic. They are partly converted to the triplet state by light, or in the case of low lying triplet states, by heat. Probable examples of this mechanism are the photooxidation of rubrene and the photooxidation and dimerization of anthracene and higher members of the acene series.76... 

It is known from literature that several reversible photochemical reactions, such as geometric isomerism of azobenzene [7], electrocyclic reaction of dihydroindolizines, fulgides and diarylethylenes with heterocyclic groups [8-10], dimerization of anthracene [11], and photochromic reaction of spirocompounds [12] have been also employed to provide photocontrol over metal-ion binding ability of crown ethers. 

Photoresponsive systems are seen ubiquitously in nature, and light is intimately associated with the subsequent life processes. In these systems, a photoantenna to capture a photon is neatly combined with a functional group to mediate some subsequent events. Important is the fact that these events are frequently linked with photoinduced structural changes in the photoantennae. This suggests that chemical substances that exhibit photoinduced structural changes may serve as potential candidates for the photoantennae. To date, such photochemical reactions as E/Z isomerism of azobenzenes, dimerization of anthracenes, spiropyran-merocyanine interconversion, and others have been exploited in practical photoantennae. It may be expected that if one of these photoantennae were adroitly combined with a crown ether, it would then be possible to control many crown ether family physical and chemical functions by means of an ON/OFF photoswitch. This is the basic concept underlying the designing of photoresponsive crown ethers. We believe that this is one of the earliest examples of molecular machines . 

J. O. Williams, J. M. Thomas, Photochemical reactions inside the electron microscope preferred dimerization of anthracene at dislocations, Mol. Cryst. Liq. Cryst., 1972, 16, 371-375. 

Figure 13. Calculated normalized refractive index change produced by the dimerization of anthracene, and of various other compounds, as functions of wavelength. Molecular structures, absorption spectra, and further details are given in ref. 28.

The reversible photodimerization of polycyclic aromatic hydrocarbons offers a number of advantages as a general technique for refractive-index imaging. We mentioned earlier the well-known reversible dimerization of anthracene and its relatives,... 

The dimerization of anthracene " has been studied extensively [243, 244]. With strongly electron-withdrawing substituents at position 9 the radical anions undergo reversible dimerization in aprotic solvents such as DMF, MeCN, propylene carbonate, DMSO etc. followed by rate determining a bond formation to furnish the stable dimer dianion [245]. In DMF k m were found to decrease in the... 

For as long as studies are carried out in which no more than the correspondence between the general level of reactivity and the total dislocation content is examined, little real advance will be accomplished in our understanding of the role of defects in chemical reactivity. An effort has to be made to characterize more fully the nature of the dislocations, and to exclude wherever possible effects which may arise from extraneous factors such as the presence of impurities. Quite clearly, model systems need to be investigated using techniques which readily reveal the presence and influence of the dislocations. In this subsection we shall concentrate, in detail, on three major model systems the oxidation of graphite, the thermal decomposition of calcium carbonate, and the solid-state dimerization of anthracene. Related systems will also be discussed where appropriate. 

Chart 1. Possible dimers of anthracene. (Reproduced with permission from reference 4. Copyright 1980 Pergamon Press.)... 

The dimerization of anthracene offers a convenient example. The reaction... 

One of the factors making carbonization so complex is the presence of so many possible polymerization sites in an aromatic molecule and the simple dimerization of anthracene [86] may form 11 different reaction products and the number of possible isomeric structures increases rapidly as the reaction progresses. 

The enhanced excimer fluorescence observed for naphthalene, pyrene and their derivatives in the presence of Y CD is evidence for the Type A complexation. Acceleration of dimerization of anthracene derivatives by Y"CD is obviously related to this type of complexation A phenomenon which suggests the Type B complex formation is the... 

The cyclization of stilbene occurs through an anti-Hiickel transition state analogous to anti-Hiickel phenanthrene. Photodimerizations of polycyclic aromatic systems are also very common. The dimerization of anthracene derivatives is a classical example ... 

Fiitsche discovers the dimerization of anthracene and other substances. 

Scheme 4.5 Formula of anthracene evidencing the particular valence status of meso carbons and dimer of anthracene where carbons have been restored to the normal valence, as reported in [21-23], The modem framula follows...

The Kobayashi group has also reported on two-substituted tetra-(dihydroxyboryl) cavitands and how they assemble in the presence of linkers such as l,2-h A-(3,4-dihydroxyphenyl)ethane to form dynamic boronic acid ester capsules (e.g., 35, Fig. 9.13) [90]. Guest binding has also been reported in these assemblies [91]. Additionally, these capsules have been shown to function as both photosensitizers and as a guard against photochemically-induced dimerization of anthracene guests [92]. Most recently, the scope of the h A-catechol linker that joins the cavitands together has been expanded [93]. 

Evidence that dimerization of anthracene involves addition of a molecule in its S state to one in its S state was provided by Bowen and Taimer (1955). They showed that the quantum yield of dimerization depends on the concentration of anthracene and oxygen in a manner which was related to the effect of these factors on the quantum yield of fluorescence. 

The dimerization of anthracene can also be sensitized by biacetyl (Back-strdm and Sandros, 1958). Despite the fact that the rate of triplet excitation transfer from biacetyl, Et = 54.9 kcal per mole, to anthracene, Et = 42 kcal per mole, should be diffusion-controlled, the rate of sensitized dimerization is extremely slow. Since singlet excitation transfer can be ruled out rigorously on energetic grounds, the results indicate that either T anthracene adds to S anthracene much less readily than does S anthracene, or in the sensitized experiments the dimerization occurs upon triplet-triplet... 

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