Chromophoric main chain

Helical polysilanes whose optical activity is induced by chiral side chains are particularly suitable chiroptical polymers for elucidating the inherent nature of the polymer helix since they embody a fluorophoric and chromophoric main chain, exhibiting intense UV, CD, and FL bands due to the Sia-Sia ... 

Illustrations of coincident CD and UV absorptions for chromophoric main chain screw sense and (b) equal proportion of P and M screw senses. 

The conformational mobility of a chromophoric main-chain polymer is often connected to its electronic structure. Therefore, changes in the UV-visible absorption spectra and/or chiroptical properties are spectroscopically observable as thermo-, solvato-, piezo-, or electrochromisms. It is widely reported that o-conjugating polysilanes exhibit these phenomena remarkably clearly.34 However, their structural origins were controversial until recently, since limited information was available on the correlation between the conformational properties of the main chain, electronic state, and (chir)optical characteristics. In 1996, we reported that in various polysilanes in tetrahydrofuran (THF) at 30°C, the main-chain peak intensity per silicon repeat unit, e (Si repeat unit)-1 dm3 cm-1, increases exponentially as the viscosity index, a, increases.41 Although conventional viscometric measurements often requires a wide range of low-dispersity molecular-weight polymer samples, a size exclusion chromatography (SEC) machine equipped with a viscometric detector can afford... 

In the early stage of helical polymer stereochemistry, a few polymers were known to retain a helical main chain with a predominantly single screw sense in solution at room temperature. For example, in cases of poly( f-bulyl isocyanides) [22], poly(triphenylmethyl methacrylate) [23], polyisocyanate [24], and poly-a-olefins [19], helical structures are kept through side group interactions. Since these pioneering works, many synthetic optically active polymers with a chromophoric main chain bearing chiral and/or bulky side... 

Scheme 1 Typical optically active polymers with chromophoric main chain and/or side chain...

The conformational mobility of a chromophoric main chain polymer is often connected to its electronic structure. Therefore, changes in the UV-visible absorption spectra and/or chiroptical properties are spectroscopically observable as thermo-, solvato-, piezo-, or/and electrochromisms. It is widely reported that a-conjugating polysilanes exhibit these phenomena remarkably clearly [54], However, their structural origins were controversial until recently, since limited information was available on the correlation between the conformational prop-... 

Waveguides of the chromophore main chain polymer PSTF66 have been measured, disclosing also a too large two-photon absorption for applications. 

R S. Nalwa. T. IMtanabe. A. Kakuta. and S. Miyata. (1993) Aromatic polyureas a new daas of chromophore main-chain polymers for second-order nonlinear optics. Nonlinear Optics. 8 137(1994)... 

In contrast, if photoactive chromophores are covalently tethered to the polyelectrolyte main chain, the chromophore reactivity in the photoinduced ET is much more greatly changed because the reaction sites are totally constrained to the polyelectrolyte molecular surface. 

The logical continuation of the stepladder strategy outlined above for minimizing the mutual distorsion of adjacent main chain phenylene units was the incorporation of the complete PPP-parent chromophore into the network of a completely planar ladder polymer. The complete flattening of the conjugated 7T-system by bridging of all the subunits should then lead to maximum conjuga-tive interaction. As with the PTHP 11 systems, alkyl or alkoxy side chains should lead to solubilization of the polymers. 

Fig. 3 Protonation states, isomerism and mesomerism of the HBI chromophore (p-hydroxybenzi-lidene-imidazolinone). The chromophore is shown in its most stable Z ( cw ) conformation, conventionally associated to a 0° value of the dihedral angle t, while the E ( trans ) conformation corresponds to t = 180°. For model compound HBDI (4 -hydroxy-benzylidene-2,3-dimethyl-imidazolinone), Ri = R2 = CH3, for chromophore in GFP, Ri, and R2 stand for the peptidic main chains toward N-terminus and C-terminus, respectively, (a) Possible protonation states of HBI (a) neutral, (b) anionic, (c) enolic, (d) cationic, and (e) zwitterionic. (b) Two resonance structures of the anionic form of HBI...

Since for the diarylpolysilanes, as for all polysilanes, the main chain is chromophoric due to the electronic transition between the delocalized silicon a and a orbitals, the electronic spectroscopies of CD, UV-Vis, and FL are particularly powerful probes of the structures of these materials. The magnitude of dimensionless quantity, gabs, is perhaps the most useful in comparing the CD spectra of optically active poly(diarylsilane)s. 

Figure 4.35 shows the CD and UV spectra for 48 in isooctane. The observation of the Cotton effect indicates that the chromophore in the polymer, which is a main-chain segment over which the silicon a and o orbitals are electronically delocalized, exists in a chiral structure a helix. 

The ORD and CD curves of optically active polymers containing chromo-phoric groups show that the chromophores can be asymmetrically perturbed by the chirality of the substituents and of the main chain conformation. This is the case with poly( ec-butyl vinyl ketone) (377), which presents a Cotton effect at 292 nm, its intensity being greater in the prevalently isotactic polymer than in the atactic polymer. 

The secondary structure of poly(iV-alkynylamides) is influenced by the position of the chiral center and amide group.The position of the chiral center mainly affects the helical pitch, which becomes short when the chiral center is positioned away from the main chain. The stability of the helical structure is also influenced by the position of the amide group. Based on molecular orbital study, it is concluded that poly(iV-propargylamides) with right-handed helical structure display a plus Cotton effect around 390 nm. This is also confirmed by the exciton chirality method using porphyrin as a chromophore. ... 

The photoisomerization of several copolymers was studied, in order to determine the effects of the structure and switching of the chiral side chain on the helicity of the main chain. A delicate balance of parameters was found, including separation and nature of the stereocenters, solvent, and concentration of azobenzene moieties J77 Stereoselectivity was often greatly enhanced if the chiral moieties were closer to each other. Accordingly, it was found that the incorporation of the stereocenter into a short, two-carbon spacer resulted in much more pronounced helical preference, as well as CD effects at lower chiral chromophore concentrations. The greater helical twist and improved thermal stability of the cis form (half-life 40h at RT) are notable features. 771 It was also found that the relationship between the trans-cis... 

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