Through-space conjugation

The potential for conjugation within polyene n ribbons has been examined for several other tt topologies [I]. 12.47 and 12.48 illustrate two bicyclic motifs. Interaction diagrams can easily be constructed and generalized electron counting rules have been established [66]. The through-space conjugation mode in 12.47 is... 

The through-space conjugation mode in 12.48 is called laticyclic. A number of systems have also been investigated here [66]. Definitive proof forthis type of overlap has been harder to come by. However, at least one class of bicyclic hydrocarbons has been shown to have large 7t—7i splittings [70] and more persuasively their radical cations have been shown to be delocalized by ESR spectrocopy [71]. 

From Table 1, it is apparent that beyond about 10 A, the use of a matrix cutoff has very little practical impact on the results. In fact, by the time 11 A is reached, errors in the energy and atomic charges are actually smaller than the default criteria used to determine SCF convergence. In the absence of significant through-space conjugation, it is clear that LCAO-MO chemical bonding is indeed localized. This localization is a primary motivation behind the D C method for MO calculations. 

Ultraviolet Spectra.—Interaction between the chromophores in phenylsulphonyl-guanidines through an empty c/-orbital on S, and between the sulphone w-system and the guanidine chromophore, is indicated by u.v. spectra, while related studies show the lack of homoconjugation (through-space conjugation) in aryl benzyl sulphides. U.v. fluorescence and phosphorescence data indicate exciton interaction between the phenyl chromophores in diphenyl sulphide. ... 

The optical properties of polymers 27 and 31, as well as compounds 28 and 32, were compared. UV-vis absorption specha and photoluminescence spectra are shown in Fig. 11, and the data are summarized in Table 1. The A , of 27 and 31 were observed at 380 and 436 nm, respectively, which were redshifted in comparison with those of 28 (X =310 nm) and 32 (2max=413 nm), as a result of the through-space conjugation among the stacked Jt-electron systems. 

Conjugated chains, 14, 46 Correlation diagrams, 44, 50 Cyclobutadiene, 171 Cyclobutane, 47, 222 orbital ordering, 26 through-space interactions, 26 Walsh orbitals, 27 Cyclobutene, 200 Cyclohexane, 278 Cyclohexene (half-boat), 274 Cyclopen tadiene, 225 Cvclopen tadienone, 269 Cyclopentadienyl anion, 237 Cyclopentane, 254 Cyclopen ten e, 241 Cyclopropane, 41, 47, 153 construction of orbitals, 19, 22 Walsh orbitals, 22, 36, 37 Cyclopropanone, 48, 197 bond lengths, 38 Cyclopropen e, 49, 132 reactivity, 40... 

An alternative approach is that adopted by Horrocks and co-workers, where the aromatic residues in metal-binding proteins are used as sensitizers. Since the distance between the metal and the donor is effectively fixed, this provides a rigid scaffold for the experiment, and the absence of a directly conjugated pathway between the metals means that Forster (through space) energy transfer can be assumed. The r-6 distance-dependence of this means that the extent of sensitized emission from the lanthanide ion provides information on the spatial relationship between the metal-ion binding site (lanthanide ions often bind at Ca2+ sites) and nearby aromatic residues. 58-60... 

The common feature of compounds [5]-[15] is that the electrophoric units are linked by saturated spacers, thus establishing only weak electronic (through-bond or through-space) interaction of the Tt-systems. In contrast, the binaphthyl [16], the biperylenyl [17] and the bianthryl [18] as well as the structurally related homologues [19], [20] and [21] allow for a direct 7r,7r-interaction of the subunits it will be shown, however, that for both steric and electronic reasons the inter-ring conjugation can be weak and thus lead to electronically independent redox groups in a similar fashion as in [5]-[15]. 

Although ab initio or semiempirical SCF calculations account rather well for this observation, it has proved extremely enlightening, especially from a qualitative point of view, to discuss the interactions between non-conjugated 7r-orbitals in terms of the concepts of through-space and through-bond interactions introduced by Roald Hoffmann200-203. With reference to the schematic diagram 43 of a non-planar diene, these interactions are defined as follows ... 

Through-space interaction. Although the two TT-orbitals 7ra and 7Tb are not in conjugation, there exists a small but finite cross term B between them which, to a first approximation, will be proportional to their overlap integral Sab = (7ra 7Tb). 

The conjugation in the molecular wire may be disrupted or modulated to create systems with different properties. For example, a porphyrin Ceo donor-acceptor system linked with a conjugated binaphthyl unit, has a preference for the atropi-somer where the fullerene unit is closer to the porphyrin system, thus increasing the through space interactions [82]. The charge transfer process on a dyad containing a crown ether in the linker structure can be modulated by complexation/ decomplexation of sodium cations [83] but even more interesting is the construction of supramolecular systems where the donor and acceptor moieties are... 

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