Double-bond twisting

The formation of a TICT state is often invoked even if no dual fluorescence is observed. For donor-acceptor stilbenes (PCT-2 and PCT-3), the proposed kinetic scheme contains three states the planar state E reached upon excitation can lead to state P (non-fluorescent) by double-bond twist, and to TICT state A by singlebond twist, the latter being responsible for most of the emission. 

Aromatic amines are not the only type of molecule to undergo a spontaneous twist in the excited state, but DMABN is the best-studied example. Other such compounds are stilbene-type molecules, where the double bond twists in the excited state. The electronic structure for both cases, twisting double bonds and twisting single or partly double bonds of 7r-donor linked to 7r-acceptor (TICT molecules), can be related to each other and characterized in the framework of quantum-chemical treatment as described in Section III. 

The data in Table 1 refer to the nonradiative decay rate k , in DCS and DCM and are indicative of the reaction to the photochemical funnel through double-bond twisting. They reveal that k, is highly polarity dependent, slowest in strongly and fastest in weakly polar solvents (negative solvatokinetic effect). In view of the above, we recognize this as signifying that the funnel state P is of less polar nature than the precursor state E. ... 

Figure 4.26. Examples of biradicaloid geometries expected to be favorable for spin-orbit coupling overlap leading to covalent interaction y between the localized nonbonding orbitals (full lines) and nonzero overlap after the action of the angular momentum operator /" on one of them (dotted lines) a) partial double-bond twist, b) disrotatory, and c) conrotutory ring closure, d) disproportionation, and e) fragmentation.

According to recent time-resolved triplet energy transfer experiments [224,225] with c/s-stilbene and 1,2-diphenylnorbornene (a model compound with inhibited double bond twisting), both compounds exhibit virtually the same nonvertical triplet excitation and relaxation behavior the respective relaxation energies are 63 and 42kJmol 1 [224], Single-bond (phenyl-... 

Ideally, this should have a (formal ) double bond twisted through 90° by the rigid scaffolding and exhibit Du symmetry. The molecule was conceived by Maier and Jeffrey, who examined it theoretically [1] and actually tried to synthesize it, a somewhat rare act of daring in this field [2]. Orthogonene has, in principle, the stereoisomers 1-6, which are shown in Fig. 7.1 using a schematic representation. In isomer 1 all the bridgehead C-H bonds point outward (away) from the molecule, which can be labeled (Fig. 7.1) oo,oo (or all-out). In 2 one of these bonds points inward (toward) the molecule, which we can label io,oo, and so on. In this notation the stereochemistry at the two ends of the double bond is separated by a comma, and i precedes o, alphabetically. To be systematic, the stmctures are drawn with any in C-H bonds preferentially at the top left and bottom front (above the plane of the paper), see e.g. io,io. The stereochemical situation is somewhat aMn to that in the ladderane 7 [3, 4] (an io,oo isomer is shown). 

Through double-bond twisting leading to trans-cis that consists of an initial emissive t state and subsequently a populated nonemissive p state (which is twisted by 90° about the C=C bond and undergoes internal conversion to give a mixture of trans- and ds-stUbene. 

Through Double-Bond Twisting (Sahiel) Mechanism... 

Double-Bond Twisting Mechanism in Linear Quinoid Structure... 

A theoretical study of the excited states of stilbene and stilbenoid donor-acceptor dye systems as potential laser was performed [4]. Semiempirical calculations within the CNDO/S framework were used to characterize the nature of the phantom-singlet excited state P (double-bond twisted geometry) of stilbene and stilbenoid donor-acceptor dye systems including 4-(dimethylamino)styrylpyridylmethylium and DCM laser dyes. It was shown that for stilbene, a slight geometric symmetry reduction is... 

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