Polar-substituted stilbenes

These data can be explained in terms of the high stabilization energy resulting from solvation of the excited t state. High p values in these cases indicate that the polar solvent-solute intermolecular stabilization ofthe zwitterionic excited t state is very sensitive to intramolecular substituent effects. In contrast, there is no dependence of Stokes shifts on a-constants in cyclohexane, which is nonpolar aprotic solvent, where the vibrational relaxation ofthe Franck-Condon state plays a primary role in stabilizing the excited state. This implies that the vibrational relaxation is not sensitive to intramolecular donor-acceptor interactions. The observations showed that the ultrafast intra- and intermolecular electronic polarization plays a major role in determining the position of the Franck-Condon zwitterionic state and its sensitivity to the relaxation of polar-substituted stilbenes. 

One of the features of stilbene photochemistry is its essentially strong dependence on medium polarity and temperature the competition between fluorescence and trans-cis isomerization has been shown to be extremely sensitive to medium viscosity. Solvent polarity can affect both the dynamics and the pathway of the reaction. The dipolar character of asymmetrically substituted stilbenes and polarizability of the traws-stilbene transition state can explain the sensitivity of the photoisomerization rate to medium polarity [5, 6, 12, 31, 66-69]. 

The lifetimes of the first excited state of substituted stilbenes in solvents of different polarities were detected. An example of the time-resolved fluorescence decay profile is shown in Figure 11.6. 

The addition reactions of ring-substituted stilbenes with EtjN are nonregioselective. (Aminoalkyl) stilbenes in which a trialkylamine is appended to the stilbene ortho position with a mono-, di-, or trimethylene linker form fluorescent intramolecular exciplexes but fail to undergo intramolecular addition, even in polar solvents. It is possible that the exciplex or radical ion pair geometry is not suitable for a least-motion proton transfer process. 

An increase in the cA-stilbene concentration favors the chain propagation and decreases the probability of termination when the DCNA anion-radicals react with the stilbene cation-radicals. A decrease in the irradiation intensity has a similar effect The chain propagation is the first-order process, whereas termination of the chains is the second-order process. A temperature rise accelerates the accumulation of the stilbene cation-radicals. In this system, the free energy of electron transfer is -53- —44 kJ moD (the cation-radical generation is in fact an endothermal process). If a polar solvent is substituted for a nonpolar one, the conversion of the cii-stilbene cation-radical into the trani-stilbene cation-radical deepens. Polar solvents break ion pairs, releasing free ion-radicals. The cA-stilbene cation-radicals isomerize more easily on being released. The stilbene cation-radical not shielded with a counterion has a more positive charge, and therefore, becomes stabilized in the... 

In 1998, Yang and coworkers reported a series of (7 )-carvone derived ketones (63) containing a quaternary center at and various substituents at (Fig. 22) [119]. The ees of fran -stilbene oxide varied with different para and meta substituents when 63b was used as the catalyst. The major contribution for the observed ee difference is from the n-n electronic repulsion between the Cl atom of the catalyst and the phenyl group of the substrate. The substitution at also influences the epoxidation transition state via an electrostatic interaction between the polarized C -X bond and the phenyl ring on franx-stilbene (Table 6, entries 3-7, 10-14). In 2000, Solladie-Cavallo and coworkers reported a series of fluorinated carbocyclic ketones... 

Deuterium labelling studies have also been used to investigate the reaction of stilbenes and related compounds with amines (Lewis, 1979). It is known that tertiary amines form fluorescent exciplexes with stilbenes in nonpolar solvents and that polar solvents are necessary for chemical reaction to occur (Lewis and Ho, 1977). This suggests that radical ions are involved in product formation. When secondary amines are used, reaction occurs in solvents of widely differing polarity and this is presumably due to the acidity of the secondary N—H bond. N-deuteriated diethylamine reacts with 1,2-diphenyl-cyclobutene in benzene to give products [65], [66] and [67] incorporating deuterium (Scheme 6). For the reaction with unsymmetrically substituted... 

As discussed earlier, deprotonation of a-carbon forms a major reaction pathway for the disappearance of the amine radical cation. Studies of photoinduced electron-transfer reactions of tertiary amines by Lewis [7, 11] and by Mariano [5, 10] have contributed significantly towards our understanding of the factors that control this process. Lewis and coworkers used product-distribution ratios of stilbene-amine adducts to elucidate the stereoelectronic effects involved in the deprotonation process [5, 10, 121, 122]. In non-polar solvents, the singlet excited state of tran -stilbene forms non-reactive but fluorescent exciplexes with simple trialkylamines. Increasing solvent polarity brings about a decrease in the fluorescence intensity and an increase in adduct formation. For non-symmetrically substituted tertiary amines two types of stilbene-amine adduct can be formed, as is shown in Scheme 9, depending on whether the aminoalkyl radical adding to the stilbene radical is formed by de-... 

In the oxides of styrene, stilbene, and stilbazole, the chemical shifts of the cis protons of the oxirane ring are larger than those for the trans isomers. This can be explained by the polarization effect of the electric dipole moment of one of the CH protons on the other CH. The anisotropc shielding of the ring in the case of oxiranes substituted with an aromatic group can be utilized well in the determination of the configuration. ... 

The influence of solvent polarity and viscosity on the deactivation of the S., state of donor- acceptor in substituted trans-stilbenes has been determined by measuring the f. 1. u or e s cen c e... 

The prototype molecule for donor/acceptor substitution is 4-dimethyl-amino-4 -nitroazobenzene. Here, the n- n band is shifted far to the red due to the charge transfer character of the transition. The band has few vibrational features, and its energy is influenced by the polarity of the solvent. The weak n Tt band cannot be seen under the intense it —> it band (Figure 1.13). Most commercial azo dyes are pseudo-stilbenes rather than azobenzene-type molecules. 

Contrary to the Z-forms of protonated azobenzenes, the Z-forms of donor/acceptor di-substituted azobenzenes isomerize very quickly at room temperature. To investigate this reaction, the Z-form is produced by flash excitation. The isomerization of these pseudo-stilbenes is strongly dependent on the polarity of the solvent. " For instance, 4-diethyIamino-4 -nitroazobenzene isomerizes with k g = 0.007 s in hexane, but with kz, g = 600 s in N-methylformamide. 4-Anilino-4 -nitroazobenzene in cyclohexane obeys the relation In kz, E = 22( 3) - 72( 9)T0V(8.314 T). Schanze et al. " have established two linear relationships between the free-activation enthalpy of isomerization and the Kosower Z values of aprotic and protic solvents. Sanchez and de Rossi report a strong pH sensitivity of... 

For pseudo-stilbene-type molecules, the question of the mechanism of thermal isomerization was taken up again in the early eighties by Whitten et al. and later by Kobayashi et al., " who, on the basis of their isomerization experiments with donor/acceptor-substituted azobenzenes in polar solvents, postulate rotation. Asano and coworkers infer from the isomeriza-... 

Here, 3t is an upper excited triplet state with trans configuration which, depending on the further substitution and on external factors (e.g., solvent polarity) may be located above or below t. For 4-nitro- and 4,4 -dinitro-stilbene the energy of t is 335-375 kJ mol. For 4,4 -NMS (as well as for the more polar 4,4 -NDS) the energy of t is lowered by solvation. Because of the high dipole moment a large Stokes shift is observed in polar solvents. The energy of 3t (e.g., ET = 188 kJ mol"1 for 4,4 -NMS [400]) is probably... 

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