Stilbene potential energy curve

The third explanation is based on vertical excitations from the vibration functions outside the potential energy curves of the os-stilbene (Figure 9.5, mode d). 

Olson was the first to postulate that optical excitation of the ethylenic double bond involves rotation around a double bond in its excited state and that this rotation leads to an observable photoisomerization process [8-10]. Olson dealt with this aspect in terms of potential energy curves and mentioned the possibility of adiabatic photoisomerization process. Later, Lewis and co-workers [11] studied the photoisomerization process of tronj-stilbene with great interest but could not detect the cw-stilbene fluorescence. More recently, more detailed fluorescence studies carried out by Saltiel and co-workers [12-15] revealed that cw-stilbene fluoresces very weakly (Oa 0.(X)01) and shows an inefficient adiabatic isomerization process. The singlet mechanism currently proposed by Saltiel [16] is supported by quenching studies [17-20]. The extensive studies carried out on stil-bene and its analogs have already been reviewed [21-23]. Here the nature of the singlet excited state involved in the trans-cis isomerization process is dealt with. 

Figure 4 Potential energy curves depicting photoisomeiization of rra/u-stilbene.

Figure 2. Schematic potential energy curves for stilbenes, that is, energy as a function of the angle of twisting about the double bond.

Potential Energy Curves. The MO calculations for ethylene, as performed by Mulliken [384, 385], represent a basis for potential energy curves of various classes of ofefins. The potential surfaces of excited states of stilbene have been calculated by several researchers [38, 40, 42, 385-396], The question of whether tram - cis photoisomerization of stilbene proceeds via a singlet or a triplet mechanism has been discussed by several groups (see Section VI.A. 1). It seems to us that on the basis of theoretical calculations this question could not have been answered unequivocally. 

Figure 4. Potential-energy curves for central-bond twisting in So and T, of the stilbenes. From Ref. 36, reprinted with permission from 7. Phys. Chem. (1987)91,2755. Copyright 1987, American Chemical Society.

Figure4.5 Calculated potential energy curves for twisting about the central bond in So (O) and Ti (O) of stilbene. Except for the global minima at c-1 and t-1 in So and at in Ti for which geometries are fully optimized, 0i = 02 is fxed. Open and closed symbols distinguish points plotted in three dimensions from those that are...

Results of calculation of potential energy curves for twisting about the central bond in So and Ti of stilbene were reported (Figure 4.5) [22]. The origin of nonvertical triplet excitation transfer and the relative role of double-bond and phenyl-vinyl torsions in stilbenes were described. 

Figure 2.20 Potential energy diagram illustrating the room-temperature photoinduced isomerization of stilbene in solution wavy lines represent non-radiative transitions, curved arrows isomerizations and straight lines radiative transitions...

The ground state potential curve (state A, Fig. 7) shows a potential barrier in the vicinity of the excited state minimum. The dotted parts of branches b and c beyond point I correspond to the potential curves of the doubly excited configurations. Going from DHP to I requires an activation energy of 23 kcal/mole which is lowered to ca. 18 kcal/mole if the C.l. depression of state A is taken into account ) Going from cis-stilbene to 1 along the thermal path is very strongly endothermic... 

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