Absorption ladder polymers

T. Pauck, R. Hennig, M. Pcnter, U. Lemmer, U. Siegncr, R. F. Malm, U. Scherf, K. Mullen, H. Bassler, E.O. Gobel, Femtosecond dynamics of stimulated emission and photoinduced absorption in a PPP-typc ladder polymer Client. Phys. Lett. 1995, 244, 171. 

IR and visible near-IR absorption, NMR, ESR and X-ray diffraction. It was emphasized that solid-state high-resolution 13C NMR spectroscopy is a powerful tool for the structure analysis of polydiacetylenes. THY is particularly attractive because of its potential for forming the polydiacetylene ladder polymer. 

Fig. 1. UV-VIS absorption spectrum of the poly(fluoreneacene) ladder polymer 71 and the open-chain polyalcoholic precursor 70 dashed line R -1.4-C6H4-C,oH2i solvent dichloromethane s in 1 -moL cm )...

The formation of the polymeric carbanions 81 of the fluorenyl-type is successful starting from the poly(para-phenylene) ladder polymer 71 with butyl-lithium as metallating agent. The degree of lithiation lies in the range of 90-95% (NMR). The UV/VIS absorption spectrum of these polymeric anions (81) is comparable with that of the 9-phenylfluorenyl anion and indicates the presence of mostly localized (anionic) sub-structures [101]. 

Moreover, semi-ladder polymers based on DIC units were obtained in much more classical ways. 2,11- and 3,10-dichloro DIC derivatives were polymerized through a nickel(0)-mediated Yamamoto polymerization reaction [95]. The resulting polymers are soluble in ODCB and partially soluble in CB. The absorption maxima of the BIC-based polymers clearly indicate a planar structure. The absorption maxima of the polymeric precursors show a primary maximum around 350 nm compared to the ladder-type polymers where the maximum is centered around 470 nm [50]. This significant red shift clearly indicates the higher degree of conjugation in the planar rigid polymers. The fluorescence spectra of those polymers also show the same trend. 

Poly butadiene rapidly becomes crosslinked when irradiated in vacuo at 253.7 nm [54]. A decrease amounting to 80% of the original unsaturation is observed by infrared spectroscopy. Since no new unsaturation has been detected this decrease has been accounted for by cyclization. However, the absence of absorption at 1020 cm-1 implies that formation of cyclopropyl groups does not occur. Formation of a ladder polymer is also unlikely since all attempts to accomplish the free radical post-polymerization of 1,2-poly butadiene have been unsuccessful namely,... 

Tour and Lambda [156,157] have prepared the aza-ladder polymers 81 from the alternating copolymers 82 (Scheme 36) by treatment with acid to remove the Boc protecting groups and induce imine formation. Unfortunately these materials are only soluble in strongly protic solvents, and there is no report of their luminescent properties. The absorption maxima in protic solvents are around 400 nm, with secondary bands between 510 and 550 nm, while in the solid state maxima between 460 and 490 nm were seen, indicating that protonation has a major effect on the conjugation length. 

Figure 6.7. UV-vis absorption spectrum of the poly(fluor-eacene) ladder polymer (58) and the open-chain polyalcohol precursor (57) (Reprinted with permission from ref 85).

As already briefly mentioned, easily the most obvious change regarding optical properties appears in the transition from the open-chain precursor polymers to the planarized PPP ladder polymers (LPPP). The primary single-stranded coupling product (a benzoyl-substituted polyphenylene) from a Suzuki-type polycondensation between an alkyl- or alkoxy-substituted 2,5-dibromo-1,4-dibenzoylbenzene derivative and 2,5-dihexyl-l,4-phenylenediboronic acid is colorless with an absorption maximum Amax of 264 nm [n-TT transition c = 39,000 L/(moI cm)l (19]. This absorption behavior supports the strongly twisted structure of the main PPP chain. The mutual distortion of the aromatic subunits reduces the conjugative interaction to a minimum, so that the typical electronic properties of a conjugated polymer are almost completely lost. 

In the cyclization to the ladder structure LPPP, the color of the reaction solution changes from colorless to an intense yellow, connected with the appearance of a considerably intense blue photoluminescence. The absorption spectrum of the resulting ladder polymer substantiates the drastic changes in the electronic structure. A very sharp, structured absorption band appears on the long-wave absorption maximum of about 450 nm (tt-tt transition) [19] (see Fig. 14.1). 

The polymer-analogous cyclization is accompanied by a remarkable change in the absorption properties. The colorless intermediate is converted to the deep yellow planar ladder polymer connected with this is a distinct bathochromic shift of the longest wavelength absorption maximum. The polymer possesses an absorption band with well-defined vibrational fine structure and a sharp absorption edge of the 0-0 transition (see Table 14.4 and Fig. 14.3). 

Table 14.4 Absorption and Emission Properties of the Angularly Annelated Polyacene Structures Derived from Monomers A and B as Fully Aromatic Ladder Polymers...

The absorption spectra of the synthesized ladder polymers (see Fig. 14.3) display two groups of signals in the visible part of the spectrum. One of these, with an absorption maximum Amax of 396-402 nm (with vibrational fine structure 356 and —375 nm) is very similar to the absorptions of the 7r-system of the angular polyacene structure of Scherf and Chmil described previously, although shifted somewhat (-30 nm) hypsochromically... 

In contrast to symmetrically substituted (tetra- or octa-substituted) phthalocyanines, reports on phthalocyanines with lower symmetry have rarely appeared, mainly because of the problems associated with their preparation [93,94]. Phthalocyanines with lower symmetry show interesting properties in NLO and are furthermore important materials for LB films [73] or ladder polymers [94]. They can also be useful in understanding the nature of phthalocyanines for example, fine-tuning of the absorption bands of phthalocyanines can be achieved by stepwise adjustment of the size of the 7r-conjugated macrocyclic system [95]. 

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