Photoisomerization potential energy curves

A kinetic scheme and a potential energy curve picture ia the ground state and the first excited state have been developed to explain photochemical trans—cis isomerization (80). Further iavestigations have concluded that the activation energy of photoisomerization amounts to about 20 kj / mol (4.8 kcal/mol) or less, and the potential barrier of the reaction back to the most stable trans-isomer is about 50—60 kJ/mol (3). 

Although the phase space of the nonadiabatic photoisomerization system is largely irregular, Fig. 36A demonstrates that the time evolution of a long trajectory can be characterized by a sequence of a few types of quasi-periodic orbits. The term quasi-periodic refers here to orbits that are close to an unstable periodic orbit and are, over a certain timescale, exactly periodic in the slow torsional mode and approximately periodic in the high-frequency vibrational and electronic degrees of freedom. In Fig. 36B, these orbits are schematically drawn as lines in the adiabatic potential-energy curves Wo and Wi. The first class of quasi-periodic orbits we wish to consider are orbits that predominantly... 

Triplet state and Z/E isomerization of p-styrylstilbene induced by various sensitizers and the T, potential energy curves and one way photoisomerization (c -> t only) of styryl aromatics " are the subject of two papers on the behaviour of triplets of this class of compound. Time resolved resonance Raman spectra of the triplet state and radical cation of 5-dibenzosuberenol has been used to study and examine the mechanism of photoisomerization of this compound. ... 

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. 

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. 

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