Mvs2

Fig. 12.12 Schematic representation of the three univalent and the two mixed valence redox states of polymer 56 and the colors of the corresponding polymer solutions. (Adapted with permission from Ref. 114 Copyright 2005, Wiley-VCH.)

distinguishable chiroptical properties as was shown by CD spectroscopy measurements and therefore can act as a multistate redox-switchable organic system.

One of the early examples of polyisocyanides bearing electron acceptor and donor groups was reported by Oostveen and Drenth [115, 116]. They synthesized polyisocyanides bearing pyridinium iodide in their side chains. In these polymers the pyridinium functions act as electron acceptors and the iodide ions as electron donors (Fig. 12.13). A large bathochromic shift was observed in the charge transfer spectra of copolymer 57, which was attributed to intercalation of the iodide ions in the cavities created by the co-monomers leading to strong CT interactions. Similarly, copolymer 58 with short and long side-chains could effectively bind donor molecule 59 leading to electron transfer from the donor to acceptor as was confirmed by EPR measurements. The electron transfer, however, was not ob-

Fig. 12.13 Schematic representation of polyisocyanides bearing acceptor and donor groups, showing the intercalation of the iodide ion and compound 59 in the cavities of the copolymers 57 and 58. In polymer 60 the acceptor and donor functions are covalently attached to each other.

served between the monomeric, that is, nonpolymerized species. In an attempt to create a polymeric photoconductor, polymer 60 having side groups capable of internal charge transfer, was synthesized [117]. Unfortunately, the polymer was barely soluble and no conductivity measurements could be performed.

A polyisocyanide-based light harvesting system has been constructed by Hong and Fox [118, 119]. Using the nickel catalyst 1, developed by Deming and Novak, a living polymerization reaction allowed the formation of homo, di- and triblock copolymers incorporating acceptor and donor blocks (Scheme 12.11). From fluorescence spectroscopic studies it was concluded that the stiff polyisocyanide backbone was able to spatially define the chromophores, thereby preventing excimer formation as often observed in more flexible polymers functionalized with chro-mophores. This is remarkable since the chromophores are connected to the backbone by a relatively flexible ethyl spacer.

Directional singlet energy migration to the acceptor-donor interface was observed in the block copolymers 61 and 62, resulting in electron transfer at the block interface in the case of 62 and exciplex formation in the case of 61. Energy

Scheme 12.11

wasting exciplex formation was suppressed in triblock copolymer 63, which contains an intervening pentamethylphenyl block between the two blocks present in 61. For block copolymer 62, transient absorption spectroscopy revealed the formation of a radical ion pair with a lifetime of 1.1 ms.

Extending the field of chromophore functionalized polyisocyanides, the group of Takahashi reported on porphyrin functionalized polyisocyanides 64-67 and related compounds, prepared by using Pd-Pt catalyst 5a [120-123]. The degree of polymerization was varied between 2 and 200 and block copolymers of type 66 with various block length and low polydispersities, mostly below 1.1, were prepared. Photophysical studies revealed that the porphyrins in the stacks are exci-tonically coupled in a face-to-face manner. It was found that exciton-exciton annihilation rate constants were independent of the length of the polymer indicating a fast exciton migration through the stacks. In the di- and tri-block copoly-mers 66 and 67, energy transfer from the zinc to the free-base porphyrins was observed. The rate constants for the excitation energy transfer process appeared to be the same for different block lengths of the free base and zinc porphyrins, again pointing to a fast exciton migration [122].

The achiral porphyrin moieties were also incorporated in different ratios as the middle block in a triblock copolymer, namely between two blocks of chiral iso-cyanide 11, functionalized with a (l)-menthyl group. The included porphyrins were used as spectator functions to determine the helical sense of the poly(aryl isocyanide)s [124].

Using the well-defined polyisocyanopeptide, as a scaffold, De Witte et al. synthesized helical porphyrin functionalized polyisocyanides 68 and 69 [125]. AFM-measurements revealed that the polymers had an average length of 87 nm, corresponding to an average degree of polymerization of 830. Resonance light scattering measurements demonstrated that at least 25 porphyrins in a stack were interacting with each other and that the slip angle between the porphyrins in a stack was 30°. The presence of a chiral interaction between monomer n and (n + 4) in this slipped conformation and between the neighboring porphyrins in the helix (n and (n + 1)) could be observed by CD spectroscopy (Fig. 12.14). Upon addition of the bifunctional ligand dabco, polymer 69, which has zinc porphyrin side groups, could be switched to a conformation in which the porphyrin stacks

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