Polarizability, photoisomerization

J. Troe I would like to comment on the role of the solvent in the photoisomerization of frans-stilbene, as discussed by Prof. Marcus. From our extensive studies in series of nonpolar and polar solvents in compressed gases and in the liquid phase, a very detailed picture arises we now can distinguish specifically different types of solvation first, there is strong interaction with polar solvents second, we can distinguish two kinds of interaction with nonpolar solvents, one site specific of only mildly polarizable small alkanes that possibly can squeeze in between the two phenyl groups in stilbene and one non-site specific of more polarizable large alkanes that can only solvate around the outer periphery of stilbene. These different types of solvation result in characteristically different solvent effects on the kinetics. 

One concludes from these facts that pseudo-stilbenes are not suitable for persistent switching of the molecular form. Any information based on E-Z isomerization is quickly lost. If, however, fast interconversion of E- and Z-forms is the aim, as it is in the alignment of the higher-order polarizability tensor of donor/acceptor azobenzenes, then thermal isomerization supports the photoisomerization process. 

PHOTOISOMERIZATION EFFECTS IN ORGANIC NONLINEAR OPTICS PHOTO-ASSISTED POLING AND DEPOLINGAND POLARIZABILITY SWITCHING... 

FIG. 8.8 Nonlinear polarizability switching by reversible photoisomerization of the NLO azo chromophore in PI-2 observed by EFISH, a feature that leads to all-optlcaHight modulation of the SH signal of the Pf-2 polymer. The irradiation and sample conditions are those explained in dte caption to Figure 8,6, and the moments of turning on and off the irradiation are indicated. Adapted from reference 9, and redrawn by permission of ACS. 

Change in the dipole moment and polarizability upon electronic excitation was determined for various linear polyenes by adopting electric field-induced changes in the optical absorption spectrum [70]. Polyenes studied (Exhibit 2) were diphenylbutadiene (DPB 7), diphenylhexatriene (DPH 8), diphenyloctate-traene (DPO 9), diphenyldecapentaene (DPD 10), and all-tranj-retinal (3). Exhibit 2 describes the experimental values determined. It is proposed that (based on the results obtained) the excited state dipole moments determined on these polyene systems have a role to play in the mechanism of trans-cis photoisomerization. 

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