Chromophore polarisation

Litvinenko KL, Webber NM, Meech SR (2001) An ultrafast polarisation spectroscopy study of internal conversion and orientational relaxation of the chromophore of the green fluorescent protein. Chem Phys Lett 346 47-53... 

The values of the 15N CP MAS chemical shift of Lys296 nitrogen bonded to retinal via the —C=N bond ( Schiff base) was equal to 155.4 ppm for rhodopsin and 282.8 ppm for metarhodopsin (relative to 5.6 M aqueous NH4C1).70 The results proved the imine bond polarisation, which facilitates Schiff base hydrolysis. The comparison between chemical shifts for metarhodopsin and model compounds suggested that Schiff base linkage of the all-frans retinal chromophore in Metall is in a polar environment. 

Linear dichroism data with DNA oriented by an electric field [53, 54] or a linear flow [55, 56], under linearly polarised light, lead to determinations of the angle between the absorbing transition dipole moment of the chromophore in the molecule and the DNA helix axis conclusions concerning intercalation may thus be drawn from this technique. Finally, with chiral compounds, circular dichroism is also an attractive method to determine the enantioselectivity in the binding of the molecule [48, 57,58]. 

In some transitions, the polarity of the chromophore is weaker after absorption of radiation. One case of this is the n — jt absorption due to the carbonyl present in ketones in solution. Before absorption, the C+-0 polarisation stabilises in the presence of a polar solvent whose molecules will be clustered around the solute because of electrostatic effects. Thus, the n —> -rr electronic transition will require more energy and its maximum will be displaced towards a shorter wavelength, contrary to what would be observed in a nonpolar solvent. This is the hypsochromic effect. Because the excited state is readily formed, the solvent shell around the... 

The absorption of light by a chromophore is a directional process. Absorption is possible provided that the transition dipole of the chromophore and the plane of polarisation of the light are not at right angles to one another, and it is at a maximum when the two are parallel. Figure 7-12 shows the orientation of the transition dipole moment of adenine. Fluorescence emission is also a directional process, occurring in a plane parallel to the dipole moment. 

Many biologically interesting chromophores are chiral, or optically active, which means that they absorb right and left circularly polarised light to differing degrees. 

Fig. 7 a Temporal evolution of the PL spectrum of a single 3-phase chromophore recorded during the rotation of a polariser placed in the emission light path as sketched in b. c PL spectra of the 0-0 transition taken at the polarisation angle marked in a by the respective coloured lines. Note that the PL intensity of the ZPL centred at 2.806 eV does not show a full modulation, as expected for emission from a linear dipole... 

In the case of cytochrome c, no bands are inversely polarised, but some have anomalous polarisation (3/4 < pi < ). This may either indicate that the A2g modes are accidently degenerate with other depolarised modes, or that the effective fourfold symmetry of the haem chromophore has been reduced in cytochrome c (Fig. 20). 

X 10 ° esu) this can be in part explained by a deviation from coplanarity between the dicyanoethenyl fragment and the plane of the porphyrin, which decreases the electronic coupling across the donor-porphyrin-acceptor. Thus, direct comparison of the experimental hyper-polarisability for such a family of porphyrin chromophores is difficult because of differing experimental conditions at different incident wavelengths. Nevertheless, these studies show that metalloporphyrins behave as efficient 7t-linkers in push-pull systems. 

Liquid crystalline polyesters with 3-methylphthalimide and chromophore side chains, having good orientation stability and mechanical properties, were claimed for making nonlinear optical devices like filters, polarisers, waveguides and others [274]. 

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