Susceptibility photoinduced

Characterization of the Photoinduced Susceptibility I 1.2.4 Optimization of the Poling Efficiency... 

FIG. 11.13 Dependence of the initial growth of the photoinduced susceptibility in function of the writing beams intensities (in arbitrary units). The sample is a film of the DRI -MMA 35/65 copolymer, 0,1 pm-thick. 

The photoinduced susceptibility shown in Equation 11.14a is the sum of two terms one with exp(-2Dt) (relaxation of the first-order parameter A ) decay and the second with exp(-12D ) (relaxation of the third-order pammeter A3) decay. Hence, the first very rapid decay may contain the fast exp(-12D ) contribution. However, as can be seen from Figure 11.14, the relative magnitude of this initial very fast decay does not depend on the optimization of the intensity ratio between the writing beams. So, this first rapid decay may not be due to the decay of the third-order parameter A3. In addition, because the hyperpolarizability P of DRl is different in the ds and in the trans state, the first very rapid decay also contains a contribution connected with the hferime of the metastable ds form, which is due to molecules coming back to the trans form without any net orientation. A better model would have to account for a distribution of diffusion constants for molecules embedded with various free volumes, which may explain the multiexponential behavior of the decay. 

To determine the magnitude of the photoinduced a quartz plate with known refractive index (W2a) 1-5) and susceptibility di, (du = 0.5 pmA ) is used as a reference. The magnitude of the photoinduced susceptibility d ff in the DRl-MMA 35/65 is estimated from the ratio between the SH intensity generated in the reference quartz plate and in the polymer sample, for equal (O-reading beam intensities ... 

HG. 11.18 Erasure of the photoinduced polar orientation with monochromatic irradiation at (O frequency. Negative times correspond to the seeding process. Circles and squares correspond, respectively, to the dark and to the photostimulated decays. The insert shows the spatial profile of the photoinduced susceptibility, obtained by SH generation inside the sample with a beam at ftmdamental frequency strongly attenuated before focusing. The spatial profile was measured before (triangles) and after irradiation (squares) of the central prepared area with an intense and focused beam at CO frequency (about 10 more intense than the beam used for the reading). 

The S-S linkage of disulfides and the C-S linkage of certain sulfides can undergo photoinduced homolysis. The low reactivity of the sulfur-centered radicals in addition or abstraction processes means that primary radical termination can be a complication. The disulfides may also be extremely susceptible to transfer to initiator (Ci for 88 is ca 0.5, Sections 6.2.2.2 and 9.3.2). However, these features are used to advantage when the disulfides are used as initiators in the synthesis of tel ec he lies295 or in living radical polymerizations. 96 The most common initiators in this context are the dithiuram disulfides (88) which are both thermal and photochemical initiators. The corresponding monosulfides [e.g. (89)J are thermally stable but can be used as photoinitiators. The chemistry of these initiators is discussed in more detail in Section 9.3.2. 

Carbonyl insertion is preferentially observed in the photoinduced reaction of 22 to give the cyclohexenones 25 and 26 as shown in Scheme 9 [17]. The acyl complex 24 is involved as an intermediate. The cyclohexenone formation appears to be susceptible to conformational effect, as observed in the facile rearrangement of 27 to 28. 

A typical cyclic voltammetric trace for the anodic oxidation of the fluorenyl anion 2 at platinum is shown in Figure 1. The oxidation potential for this and several other resonance stabilized carbanions lies conveniently within the band gap of n-type Ti02 in the non-aqueous solvents, and hence in a range susceptible to photoinduced charge transfer. Furthermore, dimeric products (e. g., bifluorenyl) can be isolated in good yield (55-80%) after a one Faraday/mole controlled potential (+1.0 eV vs Ag quasireference) oxidation at platinum. 

Complexes of the form [Rh(NH3)5X]"+ have been studied and restudied for many years, and the work continues. Firmly bonded to the Rh111, the five ammines are thermally quite inert, leaving the sixth site for ligand substitution studies. Numerous thermodynamic and kinetic studies of ligand substitutions have been reported and the reactivity at the sixth site is generally, but not always, between Co and Ir. Ligands at the sixth site are often susceptible to photoinduced substitution, but not to redox processes, making [R NH X] 14 complexes popular candidates for photochemical studies. 

The heptanuclear complex [Cu°(tren)]6[Mo (CN)g] (C104)g (204) is characterized by the presence of six noninteracting Cu(ll) ions that surround the central diamagnetic [Mo (CN)g]" fragment [Fig. 45(Z))]. Upon irradiation with blue light (406 15 nm) at lOK, the compound exhibits a gradual increase in magnetic susceptibility that saturates after 10 h of continuous irradiation [Fig. 70(a)]. This effect was explained as a photoinduced electron transfer from Mo(lV) to Cu(ll) ions, which is promoted by the irradiation into the intervalence MMCT band... 

FIG. 11.16 Spatial profile (in the plane of the film) of the photoinduced second-order susceptibility X The sample vwis a 0.3 tm thick spin-coated film of the DRl-MMA 35/65 copofynoer.The polarization direction of the probe beam is identical to the polarization direction of the writing beams. The probe beam diameter was about 100 gm. This profile yields a full width at half height of about I mm, which is consistent with the diameters of the writing beams. 

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