Stilbene ground state structure

It was suggested by Gegiou et al. that the photoisomerization of azobenzene probably proceeds via a mechanism different than the cis-trans isomerization of stilbene. Such an isomerization process in azobenzene could involve a pyramidal inversion of a nitrogen atom, in contrast to stilbene and its derivatives, where rotation about the central double bond is required. Based on the data, the AT->AC isomerization mechanism may be similar to that of the triphenylmethane dyes, whose ground state structure is known to resemble a three-dimensional, propeller-shaped D3 structure with the phenyl rings rotated 32 degrees from the central plane. 

Hammond, Saltiel and co-workers subsequently observed that the rate constant of energy transfer becomes slower than that predicted by Equation 2.47 when the structure of the relaxed excited triplet state of the acceptor differs substantially from that in ground state, as is the case for stilbene.112 These findings initiated extensive investigations on the twisted triplet state geometry of stilbene and related compounds (Section 5.5). The expression nonvertical energy transfer (NVET) was coined to describe this situation. 

The E Z photoisomerization of stilbene has been studied in considerable detail. We use the customary labels T (for trans) to designate structures near that of ( )-stilbene, c (for cis) for those near (Z)-stilbene and p (for perpendicular) for those near the transition-state geometry for E Zisomerization in the ground state. Figure 5.20 shows schematic potential energy surfaces (PESs) for S0, Si and Ti that are based largely on the experimental data discussed below. 

Stilbene 1, a hybrid of canonical structures (I) and (11), exhibits positive solvatochromism (Fig. 2). Its ground state resembles a neutral structure (I), with a calculated dipole moment of 9.85 D. Its more polar excited state, with the total electron density heavily concentrated on the acceptor fragment, has a calculated dipole moment of 23.5 D. 

INDO calculations on the potential curves of ethylene give very poor agreement with ab initio results.9 49>51 53 Reports have appeared on the geometries of excited states of small polyenes,54 the low-lying electronic states of the ethynyl free radical,55 the vibronic structure of the tt - tt transition in c/s-stilbene,56 the singlet-state geometries of diphenylacetylene,57 and the dissociation products of the HN2+ ion in its ground and excited states.58... 

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