Effect of Substituents and Polarity

The photochemistry of the substituted stUbenes opens up a unique possibility to follow the different timescale processes (in the femto-, pico-, and nanosecond regions) occurring in the molecules after irradiation. The investigated processes are the electronic polarization, vibrational and polar relaxation, radiative and non-radiative decay of the excited state, and twisting transition in the excited state. All these processes take place in the elementary act of a chemical reaction, but these are overlapped by each other and thus are undetectable by direct experimental measurements. It means that it is practically impossible to elucidate and differentiate the contribution of such factors as substituent or solvent effects to the above-mentioned processes. However, a Hammett-like correlation approach in photochemistry allowed one to elucidate and differentiate the contribution of these factors. 

The various processes occurring in the stilbene molecule after its excitation exhibit different sensitivity to intramolecular donor-acceptor effects of substituents. This sensitivity was quantitatively characterized by p-constant of the linear Hammett-like relationships [54]. It has been shown that the Stokes shift in nonpolar cyclohexane was not dependent on the structure of the stilbene molecule (Section 3.4.1) [12, 31). Therefore, the substituent effects on vibrational relaxation in the nonpolar solvent can be neglected. Nevertheless, these intramolecular electronic effects on the excitation energy of the substituted stilbenes were found to be very essential even in the nonpolar media (the excitation energy difference between stilbenes substituted with weak and strong donor-acceptor groups can reach 20kcal/mol). 

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 polarity effect strongly depends on the nature of substituents in a stilbene molecule. The data obtained in Refs [12,31, 69] may be served as an example of such 

The photoisomerization rate of the 4-(CH3)2N-suhstituted stilbenes from the second stilbene group decreases considerably compared to the weak donor-acceptor-substituted stilbenes from the first group, hut usually follows the same trends of 

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