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H nm

Fig. 12.14 (A) Illustration of the slip angle between porphyrin n and (n + 4) in polymer 68 and (B) the CD spectrum of 68 in CHCl3.

possess an opposite helicity, while the helicity of the polymer backbone remains the same [126].

Based on the same polyisocyanopeptide backbone, the groups of Nolte and Rowan prepared thiophene functionalized polyisocyanides 70-72 and perylene functionalized polymers 73 (Scheme 12.12). Thiophene polyisocyanide 70 with only one alanine unit appeared to be less soluble than polymer 71 which contains two alanine units in the backbone [127]. For 70, only short polymers were observed while longer ones, with lengths up to 300 nm were found for 71, as revealed by AFM measurements. IR, fluorescence and CD spectroscopic studies showed that the latter polymer contained a better defined backbone structure stabilized by hydrogen bonds, than the former of which the CD spectrum resembled that of the less well-defined polymer 27 derived from alanine-glycine [81]. The prepared polymers might be interesting compounds for the preparation of electron conducting nanowires, for instance, by applying a second topochemical polymerization of the thiophene side groups [128]. In a different approach the monomer from which 70 was prepared, was polymerized using the polystyrene nickel initiator complex 3a to yield block copolymer 72 [117]. Both in water and organic

73a: R=CoH5

b: R=C6H13

Scheme 12.12

solvents this block copolymer formed polymersomes, which are vesicles derived from amphiphilic polymers. In water, chemical oxidation resulted in cross-linking of the thiophene groups giving rise to electron conducting polymersomes. Cata-lytically active nanoreactors could be prepared in water by inclusion of enzymes in the interior of the polymersome. The polymersome bilayer proved to be permeable to substrate molecules and the building up of fluorescent product formed by the enzymes in the polymersome interior could be observed by fluorescence microscopy. Giant vesicles with sizes up to 100 mm were prepared from 72, namely by electroformation [129].

The perylene functionalized polyisocyanides 73 were synthesized as synthetic antenna systems, with possible applications as n-type materials in organic photo-voltaics [130, 131]. During polymerization a color change from yellow to red was observed due to the intramolecular stacking of the perylenes, which occurs as a result of the reaction. The polymer fibers were up to 1 mm in length, incorporating ca. 10 000 monomer units, as was concluded from AFM measurements (Fig. 12.15). A Cotton effect in the absorption region of the perylene chromophores revealed the helical organization of the perylenes around the helical polymer backbone.

Fluorescence and UV-vis spectroscopic studies on 73a proved the occurrence of excimer-like species in the close packed perylene arrays. Using a setup combining single-molecule confocal fluorescence and AFM, two species resulting from the polymerization reaction could be distinguished: (i) Ill-defined oligomer species displaying monomer-like fluorescence (green spots Fig. 12.16B) which were too small to be observed by AFM (Fig. 12.16A). The oligomeric character of the species was revealed by their step-wise blinking and bleaching (Fig. 12.16C) and the fact that their fluorescence spectrum was monomer-like (Fig. 12.16E). (ii)

Fig. 12.15 Color change during polymerization of 73b (left) and AFM picture of the polymer molecules on mica.

Fig. 12 16 (A) AFM and (B) confocal fluorescence (red: l > 590 nm, green l < 590 nm) images from the same 3.8 x 3.8 mm2 area of a diluted solution of polymer 73a spin-coated on glass (bar = 500 nm; polymers are encircled; (C) Fluorescence intensity trajectory for the green and (D) the red emissions in B; (E) Emission spectra integrated over the whole t = 0-25 s time window for the green emissions; (F) The same for the red emissions. (Adapted with permission from Ref. 130, Copyright 2004, Wiley-VCH.)

Fig. 12 16 (A) AFM and (B) confocal fluorescence (red: l > 590 nm, green l < 590 nm) images from the same 3.8 x 3.8 mm2 area of a diluted solution of polymer 73a spin-coated on glass (bar = 500 nm; polymers are encircled; (C) Fluorescence intensity trajectory for the green and (D) the red emissions in B; (E) Emission spectra integrated over the whole t = 0-25 s time window for the green emissions; (F) The same for the red emissions. (Adapted with permission from Ref. 130, Copyright 2004, Wiley-VCH.)

Well-defined polymers that could be observed by AFM, showing emission arising from multiple and independent excimer-like sites (red spots Fig. 12.16B). A continuous intensity decrease and an excimer-like emission spectrum, typical for a polymer, were observed for these species (Fig. 12.16D and F). Unfortunately, since polymer 73a turned out to be only poorly soluble in solvents like toluene and chloroform, application in a device was impossible.

Therefore, polymer 73b exhibiting similar photophysical properties, but with improved solubility in the aforementioned solvents was synthesized [132]. A photovoltaic cell with an active layer of 73b as electron acceptor and regioregular poly-thiophene (P3HT) as electron donor was prepared, displaying a 20-fold improved power output as compared to a cell with an active layer of a perylene monomer homolog and P3HT.

398 | 12 Polyisocyanides: Stiffened Foldamers 12.6

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