Structural isomers, chromophores

Fig. 15 Structures of pyridine, picolinate, and three structural isomers of dipicolinate, overlaid with an electron density map of the highest occupied molecular orbital (HOMO) for each hgand. These chromophores were explored to better understand the binding properties of DPA. Electron density maps generated using Titan higher electron density is in black, lower in white.

Weigang95 has measured the solvent shifts for all the electronic transitions of phenanthrene, naphthalene and azulene in a series of paraffin hydrocarbon solvents. The effect of branching and cyclization was studied by the inclusion of various structural isomers. Normal hydrocarbons give remarkable linear shifts, and these shifts fit well into the McRae theory87. The corresponding non-cyclic branched and cyclic hydrocarbons produce lesser shifts. These small shifts have been attributed to the change in the physical conformation of the molecule and the resultant opportunity for closer interactions with the chromophore. 

For both cis- and Irans-dianthrylethylenes, the degree of deconjugation, which parallels the degree of deviation from coplanarity of the ethylene and anthracene ir-systems, is borne out in the shape of the electronic absorption spectra. Thus, the long-wavelength absorption of cis-dianthrylethylene 38a is characterized by the fine structure pattern which is typical of the anthracene chromophore, while the bathochromically shifted spectrum of the transisomer 39a is virtually structureless (see Figure 7). Substitution of the ethylene double bond is absorption spectroscopically noticeable for the cis-isomers 38b-f by distortion of the anthracene absorption. The absorption... 

Phytochrome is a chromoprotein , that is a molecule which consists of a relatively small chromophore unit linked to a large protein. The structure of the chromophore is now well established and the action of light leads to an isomerization the association of the isomer Pfr with the protein chain is different from that of the Pr isomer (Figure 5.16). 

The term photochromism can be defined as a light-driven reversible transformation between two isomers possessing different absorption spectra.111,21 The two isomers differ from one another not only in their absorption spectra, but also in their geometrical structures, oxidation/reduction potentials, refractive indices, and dielectric constants. When such photochromic chromophores are incorporated into functional molecules, such as polymers, host molecules, conductive molecular wires, or liquid crystals, the functions can be switched by photoirradiation.[3 61 Photostimulated reversible changes in refractive index can also be applied to optical waveguide switching.171 This chapter reviews applications of photochromic chromophores, especially diar-ylethene derivatives, in various photo switching molecular systems. 

As unique C2-symmetric transition metal complexes, the atropisomeric (Rp,Rp)-2,2 -bi([2]paracyclo[2]-5,8-quinolinophane) (52) and (Rp,Rp)-1,1 -bi([2]paracyclo [2]-5,8-isoquinolinophane) (53) were prepared from (/ p)-4-amino- and (Rp)-4-carboxy-[2.2]paracyclophanes, respectively. The CD spectra of 52 and 53 were significantly different from each other (Fig. 14) [60]. The bisquinoline moieties are in almost planar orientation in 2,2 -isomer 52 and the origin of chirality is ascrib-able primarily to the paracyclophane structure. In contrast, the main source of chirality in l,l -isomer 53 is the distorted bisquinoline chromophores (i.e., the axial chirality). 

RetinalS. The structure and photophysics of rhodopsins are intimately related to the spectroscopic properties of their retiny1-polyene chromophore in its protein-free forms, such as the aldehyde (retinal), the alcohol (retinol or vitamin A), and the corresponding Schiff bases. Since most of the available spectroscopic information refers to retinal isomers (48-55), we shall first center the discussion on the aldehyde derivatives. Three bands, a main one (I) around 380 nm and two weaker transitions at 280 nm and 250 nm (II and III), dominate the spectrum of retinals in the visible and near ultraviolet (Fig. 2). Assignments of these transitions are commonly made in terms of the lowest tt, tt excited states of linear polyenes, the spectroscopic theories of which have been extensively discussed in the past decade (56-60). In terms of the idealized C2h point group of, for example, all-trans butadiene, transitions are expected from the Ta ground state to B , A, and A" excited states... 

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