Charge-transfer cyclophanes

Stoddart representation blue box with included p-dimethoxy benzene in red 

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The efficient formation of the cyclophane suggests that the [DDD, TCNQ] charge-transfer complex is converted to the ion-radical pair via a thermal electron transfer. Subsequent fragmentation of the resulting DDD+ and a homolytic coupling with TCNQ leads then to a zwitterionic intermediate which collapses to the cyclophane within the ion pair, as summarized in Scheme 20. 

These structural data demonstrate that 12 is a rather less distorted molecule than [2.2]paracyclophane. However, a dramatic effect of the strong cr(Si—Si)—w interaction was observed in UV spectra as shown in Fig. 5. In the UV spectrum of phenylpentamethyldisilane, an intramolecular crfSi—Si)—7T charge-transfer band appears around 231 nm (11a, 12). Octamethyltetrasila[2.2]ortho- (15) and metacyclophane (16) show similar absorptions, but the band splits into two bands at 223 nm (e = 19,100) and 263 nm (e = 22,500) in 12. This type of red shift in the UV spectra occurs only in 12 among other polysilapara-cyclophanes such as 13 and 14. 

The desulphurization of disulphides by tervalent phosphorus compounds has been the subject of a review.70 The light-induced desulphurization of benzylic sulphides by phosphites has found further use in the synthesis of cyclophanes which exhibit the formation of intramolecular charge-transfer complexes, e.g. (61) and (62).71... 

In addition to chemical reactions which result in direct valency coupling, other reactions, for example, those that lead to the formation of charge-transfer complexes are rather generally characteristic of the [2.2]para-cyclophane system. 

Figure 13.22 The circumrotation of the tetracationic cyclophane component of catenane 254+ can be controlled reversibly by adding-protonating -hexylamine that forms a charge transfer adduct with the diazapyrenium unit of the catenane.

Donald Cram attempted synthesis of cyclophane charge transfer complexes with (NC)2C=C(CN)2 1961 - N.F. Curtis first Schiff s base macrocycle from acetone and ethylene diamine 1964 - Busch and Jager Schiff s base macrocycles... 

Molecular shuttle 154+ consists of a tetracationic cyclophane macrocycle, a linear thread containing two hydroquinol stations and a polyether spacer. The macrocycle binds the stations via n - n and charge-transfer interactions between the electron-poor cyclophane and the electron-rich hydroquinols. As explained above, because both stations are energetically degenerate (they are chemically identical) the macrocyclic unit has no preference for either of them and randomly shuttles between them, in this case at a rate of k = 2360 s 1 in (CDs CO at 34 °C, measured by JH NMR spectroscopy. It was already noted in Stoddart s seminal 1991 paper that including two stations of different binding affinity in the thread could allow a stimuli-induced change of position of the macrocycle in a molecular shuttle. 

Donor-acceptor cyclophanes have been extensively investigated as models for intermolecular charge transfer (CT) complexes. The [m.n]cyclophane structure provides the adjustable well-defined distance and orientation for the donor and acceptor units incorporated in its framework. These systems have been systematically investigated by Staab et al. using absorption spectroscopy [34]. 

Fig. 4. Assumed charge transfer-stabilization in [m.n)para-cyclophane/TCNE-oomplexes...

The absorption spectrum of a carbazole-acceptor cyclophane shows transannular 7t-7t electronic interaction and little charge transfer interaction, while the fluorescence spectrum exhibits intramolecular exciplex emission around 525 nm <2003CL910>. 6,13-Bis(9-ethyl-9//-carbazol-3-yl)pentacene (a pentacene derivative bearing two carbazole moieties) displays an absorption spectrum that consists of the bands due to the carbazole (352 nm) and the pentacene moieties (483, 523, 561, 605 nm), indicating that the electronic transition level of pentacene is not affected by the carbazole chromophore <2006MI185>. 

For investigations of intramolecular charge-transfer interactions, donor- and acceptor-substituted cyclophanes are particularly well suited, since they allow for wide variation of substitutents. Interplanar-distance and orientation of aromatic planes can be tuned by choosing appropriate bridge lengths. Synjanti conformational preferences have an additional influence on charge-transfer interactions. Severals studies of this kind were undertaken by Staab et al. 17,34-36]. In usually good yields they synthesized methoxy-sutetituted [2.2]meta- and metaparacyclo-phanes with bromo-, cyano-, nitro-, or ester-substituents. In the case of methoxy-substituted substrates, often a few percent of quinoid by-products do form, e.g., 40, 41, and 44, which have been isolated in some cases. 

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