Cofacial

The polymers described so far have relatively flexible main chains which can result in complex confonnations. In some cases, tliey can double back and cross over tliemselves. There are also investigations on polymers which are constrained to remain in a confonnation corresponding, at least approximately, to a straight line, but which have amphiphilic properties tliat ensure tliat tliis line is parallel to tire water surface. Chiral molecules are one example and many polypeptides fall into tliis class [107]. Another example is cofacial phtlialocyanine polymers (figure C2.4.9). 

The selenium analogue [PhCNSeSeN] and cyano-functionalized diselenadiazolyl radicals adopt cofacial dimeric structures, e.g., 11.4 (E = Se), with unequal Se Se interactions of ca. 3.15 and 3.35 A. In the latter case the radical dimers are linked together by electrostatic CN Se contacts.Tellurium analogues of dithiadiazolyl radicals (or the corresponding cations) are unknown, but calculations predict that the radical dimers, e.g., 11.4 (E = Te), will be more strongly associated than the sulfur or selenium analogues. ... 

Firstly, we focus on cofacial dimers formed by stilbene molecules in such conformations, the amplitude of interchain interactions is expected to be maximized [57], Table 4-1 collects the INDO/SCl-calculated transition energies and intensities of the lowest two excited states of stilbene dimers for an interchain distance ranging from 30 to 3.5 A. 

Figure 4-4. Schematic rcprcsenlalion of ihc one-clcclron structure of a single siilbcnc molecule and that of a cofacial dimer formed by iwo chains separated by 4 A. lire INDO-ealculated energy splil ot the HOMO and LUMO levels when going from Ihc isolated molecule to the dimer are also given.

Figure 4-6. Evolution of the INDO/SCI-calculalcd. splitting between the lowest two oplieal transitions of cofacial dimers formed by two PPV chains as a function of the inverse number of bonds (1/u) along the conjugated backbone of the oligomer. The theoretical results are reported for interchain distances of 4 A (open circles) and C> A (tilled circles).

Table 4-2. Ground-slalc and lowcsl-cxcilcd slate AM l/CI-oplimized C-C bond lengths (in A) in a cofacial dimer formed by two stilbenc molecules separated by 3.5, 4.0, and 4.5 A, respectively. The geometrical parameters for single molecules arc reported in the right-hand-side column.

Figure 4-8. 1NDO/SCI simulation of the wavcfunclion y/(x,xi, = 16, chain I) of the lowest charge transfer-excited stale in a cofacial dimer formed by two five-ring PPV oligomers separated by 4A. Ili/(x,x/, - 16, chain 1) represents the probability amplitude in finding an electron on a given site xt. assuming the hole is centered on site 16 of chain I. The site labeling is the same as that reported on top of Figure 4-7.

Regarding the emission properties, AM I/Cl calculations, performed on a cluster containing three stilbene molecules separated by 4 A, show that the main lattice deformations take place on the central unit in the lowest excited state. It is therefore reasonable to assume that the wavefunction of the relaxed electron-hole pair extends at most over three interacting chains. The results further demonstrate that the weak coupling calculated between the ground state and the lowest excited state evolves in a way veiy similar to that reported for cofacial dimers. 

Figure 4-11. INDQ/SCI-caleulalcd evolution of the transition energies (upper pan) and related intensities (bottom pan) of the lowest two optical transitions of a cofacial dimer formed by two stilbenc molecules separated by 4 A as a function of the dihedral angle between the long molecular axes, when rotating one molecule around the stacking axis and keeping the molecular planes parallel (case IV of Figure 4-10). Open squares (dosed circles) correspond to the S(J - S2 (S0 — S, > transition.

Table 4-1. INDO/SCI-calculalcd iransilion energies, intensities, and Cl dcscriplions of llie lowest two excited stales (S and S2, respectively) of a cofacial dimer formed by Iwo stilbene molecules for various interchain distances.

Figure 4-5. lNDO/SCl-calculalcd transition energies of the lowest two optical transitions of a cofacial dimer formed by two slilhcnc molecules as a function of interchain distance. The horizontal line refers to the transition energy of the isolated molecule. Note that the upper value reported at 3.S A corresponds to the transition to the fifth excited stale, which provides the lowest intense absoiption feature. 

At this stage, we can draw several important conclusions regarding the absorption and emission properties of cofacial dimers formed by two identical PPV chains ... 

Figure 4-9. INDO/SCI-simulalcd absorption and emission spectra of two slilbene molecules with a huge interchain distance (solid lines) and those of a cofacial dimer formed by two slilbene chains separated by 4 A (dolled lines).

Figure 18.10 Three types of polarization curves typically manifested by simple Fe and Co porphyrins and cofacial metaUoporphyrins (simulated voltammograms).

Certain aspects of electrocatalysis by cofacial porph5Tins have been reviewed previously [Collman et al., 1994, 2003a]. 

Substantial effort has been invested in understanding how the linker(s) affect the metal-metal (M-M) distances, since the capacity of a cofacial porphyrin to accommodate a bridging diatomic molecule is often thought to be critical for achieving high catalytic activity. As of May 2007, 38 X-ray diffraction structures of cofacial porphyrins in Fig. 18.13 had been reported, with the DPA, DPB, DPD, and DPX... 

Figure 18.13 Chemical structures of selected cofacial strapped diporphyrins (a), pillared diporphyrins (h), and pillared porphyrin/corrole, dicorrole, and diphthalocyanine derivatives (c) whose metal complexes have heen studied as ORR catalysts. Conventional notations for the structures are also hsted (in bold). Other molecular architectures of cofacial porphyrins are known, hut the corresponding complexes have not yet been studied as ORR catalysts.

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