Reverse-twist state

Fig. 4.7.9 LC molecular configuration of splayed-twist state (a) and reverse-twist state (b).

Bourn (107) as 10.8 kcal mole-1 (AH4 temperature range 156—246 K solution of cyclohexane in carbon disulfide). By applying an interesting trapping technique Anet and coworkers (110) have recently measured a value of 5.3 kcal mole-1 for the barrier of the reverse twist/chair process (AH4 temperature range 72.5—74.0 K solid state). Our calculated (for the gas phase) activation enthalpies AH4 for the former process are at 73,... 

The fluorescence lifetime of trans-4-[4-(dimethylamino-styryl]-l-methylpyridinium iodide trapped in water-containing AOT-reversed micelles has been found to be markedly influenced by R, implying a significant effect on its excited-state twisting motion [122],... 

Olsen S, Lamothe K, MartiAAnez TJ (2010) Protonic gating of excited-state twisting and charge localization in GFP chromophores a mechanistic hypothesis for reversible photoswitching. J Am Chem Soc 132 1192-1193... 

V,/V-Dimethylamino)benzonitrile (DMABN) and its derivatives, as a class of organic donor-acceptor compounds, exhibit dual fluorescence, one related to the local excited state ( B band) and the other ascribed to the twisted intramolecular charge transfer (TICT) state ( A band).17 As expected, compound 818 exhibits dual fluorescence, showing two fluorescence bands centered at 350 and 432 nm, which can be ascribed to the corresponding band (from the local excited state) and A band (from the TICT state), respectively. After oxidation of TTF unit in 8, the fluorescence intensity of A band decreases while that of band increases slightly. As expected, further reduction of TTF" + into neutral TTF unit leads to the restoration of the fluorescence spectrum of 8. Therefore, the dual fluorescence spectrum of 8 can be reversibly modulated by redox reactions of TTF unit in 8. 

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

The light-induced reversible trans-cis photoisomerisation of franv-stilbene molecule in condensed media includes at least four macroscopic stages excitation of the stilbene chromophore, radiative deactivation of the excited state with the rate constant kr, medium relaxation around the excited stilbene molecule with the rate constant km (to provide space for torsional distortion during the photoisomerisation process) and eventually, twisting transition with the rate constant kt c. 

Upon introduction of substituents which produce a sufficient electronic asymmetry d, the situation of the right hand side of Fig. 8 is expected, with a state reversal, i.e. weakly polar (dot-dot) and strongly polar Sq (hole-pair) for the twisted double-bond. 

For trienes like (5)-<7) with dienophile activating groups located at C(2) ( internally activated systems), however, the situation is reversed. The LUMO coefficient at C(l) is greater than at C(2) and so bonding between C(l) and C(9) should be more advanced early in the transition state. This form of twist asynchronicity, however, is probably less significant than that involving the C(2)—C(6) bond for terminally activated nonatrienes U,2) since the selectivity of the IMDA reactions of (5)-<7) is comparable with that obtained with the unsubstituted nonatriene (3) (also compare results with (4L Figure 2). 

---