Reorientation within the trans-cis Photoisomerization

FIHSURE 3.6 (A) Structure formula and (B) UV-vis spectra of a poly-(5-(2-(4-(4-decylox) henyla2o) pheHo ) ediy -L-glutaiTiate), denoted by P2,io solution in CHCI3 in the dark (tnms) imd at the photostationary state (cis) of a 360 nm UV light irradiation. After reference 21, redrawn by permission. 

Irradiation with unpolarized blue light does not erase the induced dichroism, because only the cis isomer has significant absorption in the blue region around 450 nm consequently, the trans molecules cannot be excited and reoriented. The in-plane isotropy in both the trans and the cis molecular distributions can be restored only after successive unpolarized UV and blue light irradiations. The initial spectra of a freshly prepared sample prior to irradiation is not restored by this procedure, however, because a net out-of-plane orientation of the azomolecules remains. It is noteworthy that spectra recorded with different analysis light polarizations do not intersect at isosbestic points because of the anisotropy that exists in the sample. Heating the azo-polyglutamate sample at 80°C for 30 minutes and 14.5 hours failed 

FIGURE 3.7 UV-vis absorption spectra of the azo-polyglutamate film before (I) and after 35 min (2. 3) of linearly polarized UV (360 nm) irradiation. The probe light was also linearly polarized and spectra were obtained for both parallel (3) and perpendicular (2) orientations.The inset is an expanded view of the cis absorption. After reference 22, redrawn by permission of ACS. 

between Pump and Probe Beam Polarization 

FIGURE 3.8 Dependence of the absorbance of linearly polarized probe light at 360 nm (squares) and 450 nm (circles) on the angle. between the probe and the UV light polarization.TMs behavior is fitted by a cos with an amplitude that decays with the cis trans thermal isomerization rate. After reference 22, redrawn by permission of ACS. 

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