Chromophoric backbones

Occasionally, the UV response factors can be reasonably assumed to be quite similar for example, the parent and degradation product(s) share the same chromophoric backbone, with structural differences only in regions unassociated with the chromophore. Favorable comparison of UV spectra (e.g., from a PDA array detector) can help confirm such a situation (15). 

Alternatively, the dendrimer backbone itself can concurrently be used as the energy donor or acceptor. Several types of chromophoric dendrimer backbones such as poly(phenylacetylene) [3], poly(phenylene) [4], and poly(benzylether) [5] have been used as light absorbers, and the energy was efficiently transferred to the core acceptor. While most of these systems have high energy transfer efficiencies, they still suffer from a weak fluorescence or a low fluorescence quantum yield. However, polyphenylene dendrimers composed of tens or hundreds of out-of-plane twisted phenyl units can be used as chromophoric backbones [6] carrying highly luminescent dyes at the periphery. 

In light of tire tlieory presented above one can understand tliat tire rate of energy delivery to an acceptor site will be modified tlirough tire influence of nuclear motions on tire mutual orientations and distances between donors and acceptors. One aspect is tire fact tliat ultrafast excitation of tire donor pool can lead to collective motion in tire excited donor wavepacket on tire potential surface of tire excited electronic state. Anotlier type of collective nuclear motion, which can also contribute to such observations, relates to tire low-frequency vibrations of tire matrix stmcture in which tire chromophores are embedded, as for example a protein backbone. In tire latter case tire matrix vibration effectively causes a collective motion of tire chromophores togetlier, witliout direct involvement on tire wavepacket motions of individual cliromophores. For all such reasons, nuclear motions cannot in general be neglected. In tliis connection it is notable tliat observations in protein complexes of low-frequency modes in tlie... 

Although the n-n and tz-tz electronic transitions of the urea chromophore have not been studied as extensively as amides, the contribution of the backbone is expected to dominate the far UV spectra of oligoureas in a fashion similar to that which is observed for peptides. The CD spectra recorded in MeOH of oligoureas 177 and 178 show an intense maximum near 204 nm (Figure 2.48). This is in contrast to helical y" -peptides that do not exhibit any characteristic CD signature. 

Figure 1. Chromophore (A) and polymeric backbone structures (B) used for synthesis of the polymeric dye...

However, the significant changes in the electronic spectrum of PDHS at the transition temperature suggest that the nature of the backbone chromophore is also being altered during the process. 

Owing to their chemical structure, carotenes as polyterpenoids are hydrophobic in nature (Britton et al., 2004). Therefore, as it might be expected, the carotenes are bound within the hydrophobic core of the lipid membranes. Polar carotenoids, with the molecules terminated on one or two sides with the oxygen-bearing substitutes, also bind to the lipid bilayer in such a way that the chromophore, constituted by the polyene backbone is embedded in the hydrophobic core of the membrane. There are several lines of evidence for such a localization of carotenoids with respect to the lipid bilayers. 

The CP MAS NMR spectroscopy has been also extensively used for studies of proteins containing retinylidene chromophore like proteorhodopsin or bacteriorhodopsin. Bacteriorhodopsin is a protein component of purple membrane of Halobacterium salinarium.71 7 This protein contains 248 amino acids residues, forming a 7-helix bundle and a retinal chromophore covalently bound to Lys-216 via a Schiff base linkage. It is a light-driven proton pump that translocates protons from the inside to the outside of the cell. After photoisomerization of retinal, the reaction cycle is described by several intermediate states (J, K, L, M, N, O). Between L and M intermediate states, a proton transfer takes place from the protonated Schiff base to the anionic Asp85 at the central part of the protein. In the M and/or N intermediate states, the global conformational changes of the protein backbone take place. 

Barondeau, D. P., Kassmann, C. J., Tainer, J. A. and Getzofif, E. D. (2005). Understanding GFP chromophore biosynthesis Controlling backbone cyclization and modifying post-translational chemistry. Biochemistry 44, 1960-70. 

The material system is a Langmuir-Blodgett film of the S enantiomer of a chiral polymer deposited on a glass substrate. The polymer is a poly(isocyanide)30 functionalized with a nonlinear optical chromophore (see Figure 9.14). In this particular system the optical nonlinearity and chirality are present on two different levels of the molecular structure. The chirality of the polymer is located in the helical backbone whereas the nonlinearity is present in the attached chromophores. Hence, this opens the possibility to optimize both properties independently. 

Porphyrine chromophore units have also been introduced to the PPV backbone but the PLQY of such materials decreased rapidly with increasing ratio of the porphyrine units and no EL devices have been reported [177,178]. 

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