Photoinduced anisotropy

The photoinduced absorbance anisotropy in a TPD experiment relaxes according to the same correlation function as in Eq. (4.16).(29) Effects of spatial variations in the excitation and probe beams, and chromophore concentration, have been treated and shown not to alter the final result.(29) NMR dipolar relaxation rates are expressed in terms of Fourier transforms of the correlation functions, 4ji< T2m[fi(0)] T2m[i2(f)]>> where fl(f) denotes the orientation of a particular internuclear vector. In view of Eq. (4.7), these correlation functions are independent of the index m, hence formally the same as in Eq. (4.16). For the analysis of NMR relaxation data, it is necessary also to evaluate Fourier transforms of the correlation functions. Methods to accomplish this in the case of deformable DNAs have been developed and applied to analyze a variety of data.(81 83)... 

Not considered in this review are the removal of energetic equivalence by an applied field or stress where, as in a spinel, the energetically equivalent sites have differently oriented crystal-field axes. This lifting of site degeneracy coupled with charge transfer between mixed-valence states leads to such phenomena as magnetic after effect and photoinduced anisotropy ... 

Many spectroscopic methods have been employed for the investigation of such systems For example, wide-band, time-resolved, pulsed photoacoustic spectroscopy was employed to study the electron transfer reaction between a triplet magnesium porphyrin and various quinones in polar and nonpolar solvents. Likewise, ultrafast time-resolved anisotropy experiments with [5-(l,4-benzoquinonyl)-10,15,20-triphenylpor-phyrinato]magnesium 16 showed that the photoinduced electron transfer process involving the locally-excited MgP Q state is solvent-independent, while the thermal charge recombination reaction is solvent-dependent . Recently, several examples of quinone-phtha-locyanine systems have also been reported . 

Table 3.1 gives the ratio of the photoinduced anisotropy of the cis relative to that of trans isomers, i.e., For these calculations,... 

The effect of the structure of the polymer backbone on photo-orientation can be seen from the dynamic behavior as well as from the steady-state values of the photoinduced anisotropy in all azo-PURs. The photo-orientation dynamics of PUR-2 resemble but also contrast with those of PUR-1. In PUR-2, AbSi exceeds AbsQ, but not quite, as is the case for PUR-1, and the photostationary-state anisotropy is smaller than that of PUR-1, as can be seen in Figure 4.20. PUR-1 and PUR-2 exhibit exactly the same extinction coefficient at the analysis wavelength because they have the same azo chromophore furthermore, the rate of the cis- trans thermal isomerization is nearly the same in both polymers. The seemingly small difference into the... 

Figure 4.24 shows the influence of pressure on the photoinduced anisotropy in PMMA-DRI observed by the Kerr gate experiment for several applied hydrostatic pressures up to 150 MPa, as indicated. The moments when the irradiating light was turned on and off are indicated. After the thermal isomerization is completed after the end of the irradiation, circularly polarized irradiation randomized the in-plane orientation, and photo-orientation at the next higher pressure value followed. Figure 4.24 clearly shows the time... 

Blinov, L. M., Kozlovsky, M. V., Ozaki, M., Skarp, K., and Yoshino, K. Photoinduced diefaroism and optical anisotropy in a liquid crystalline azobenzene side-chain polymer caused by anisotropic angular distribution of trans and cis isomers./. Appl. Phys. 84, 3860 (1998). 

Besides azobenzene, spirobenzopyran, and salicylidene derivatives, other photochromic dyes have been used in LBK films. For example, anthocyanine dyes 5 (see Figure 6.5) have been used to obtain monolayers that change the area at constant pressure upon irradiation. Diphenyldiacetylerie chromophores 6 (Figure 6.5) have shown photoinduced anisotropy upon polar-... 

Another mechanism responsible for the optically induced anisotropy is angular redistribution (AR) of molecules. This mechanism has been widely developed to explain photoinduced birefringence and dichroism. In most experimental cases, there is evidence of some rotation of molecules during the photoisomerization cycle (see Reference 2, for example). This rotation results in AR, because the molecules remain longer in states with lower excitation probability, and so more molecules are accumulated perpendicular to the pump polarization. The AR process is initiated by the AHB, and these two processes should be studied simultaneously in the framework of general... 

Lefin, R, Fiorini, C., and Nunzi, J. M. Anisotropy of the photoinduced translation diffusion of azo-dyes. Optical Materials 1998, 9, pp. 323-328. 

Liquid-crystalline phases with two-dimensional order (Table 1) influence photochromic processes because their molecular orientation is reversibly altered by photochromic reactions owing to the marked structural changes of the photoactive molecules.3 Intensive interest has been focused on the photoinduced alteration of mesophases admixed with photochromic compounds, since photochromism can be transformed to the reversible modification of the optical anisotropy of liquid-crystalline states. This phenomenon is applicable to optical storage of information as nondestructive readout can be performed with light of wavelengths far from those causing the photochromic reactions. 

V. Matee, P. Markovsky, L. Nikolova, and T. Todorov, Temperature dependence of photoinduced anisotropy in rigid solutions of azo dyes, J. Phys. Chem. 96, 3055-3058 (1992). 

H. Akiyama, K. Kudo, and K. Ichimura, Command surfaces. 10. Novel polymethacrylates with laterally attached azobenzene groups displaying photoinduced optical anisotropy, Makromol... 

T. Fischer, L. Lasker, J. Stumpe, and S. G. Kostromin, Photoinduced optical anisotropy in films of photochromic liquid crystalline polymers, Photochem. Photobiol. A 80, 453 159... 

L. Lasker, T. Fischer, J. Stumpe, S. Kostromin, S. Ivanov, V. Shivaev, and R. Ruhmann, Photoinduced optical anisotropy in thin films of amorphous photochromic side chain polymers. Mol. Cryst. Liq. Cryst. 246, 347-350 (1994). 

In the first section, steady-state spectroscopy is used to determine the stoichiometry and association constants of molecular ensembles, emphasize the changes due to light irradiation and provide information on the existence of photoinduced processes. Investigation of the dynamics of photoinduced processes, i.e. the determination of the rate constants for these processes, is best done with time-resolved techniques aiming at determining the temporal evolution of absorbance or fluorescence intensity (or anisotropy). The principles of these techniques (pulse fluorometry, phase-modulation fluorometry, transient absorption spectroscopy) will be described, and in each case pertinent examples of applications in the flelds of supramolecular photophysics and photochemistry will be presented. 

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