Vinylheptafulvene

We demonstrate by using ultrafast time resolved spectroscopy that the photoconversion from dihydroazulene (DHA) to vinylheptafulvene (VHF) is governed by two mechanisms The ring opening proceeds on the excited energy surface on the picosecond time scale. It is followed by an internal conversion to the VHF ground state that is accelerated by the presence of a conical intersection in the case of cyclopenta-DHA. This conical intersection hinders the photoinduced back reaction from the final VHF products. However, we successfully photo-converted the cyanophenyl-VHF-cis back to the DHA in an experiment with two delayed pulses. This opens the route to the development of bistable dihydroazulene switches. 

Optoelectronic Molecular Switches Based on Dihydroazulene-Vinylheptafulvene (DHA-VHF)... 

Scheme lb Synthesis of dihydroazulenes via corresponding vinylheptafulvenes AT = reflux. 

Molecular switches based on the dihydroazulene-vinylheptafulvene system are discussed in Chapter 3. Chiroptical switching was achieved with a carbohydrate-modified chiral polyazuleneJ721... 

The dihydroazulene vinylheptafulvene couple (9) [85, 98], dihydroindolizine (10) [95], and fulgides are further examples of photochromic open-closed ring systems... 

Several previous reports have described various features of the reversible photoisomerization of l,8a-dihydroazulene dicarbonitriles to vinylheptafulvenes. The first time-resolved investigation of this ring-opening reaction has now been reported and, using sub-30 fs transient absorption spectroscopy, prominent workers in this area have determined that the ring opening of (13) to (14) occurs within 1.2 ps, which is followed by internal conversion of Si (14) to Sq (14) in 13... 

Spreitzer, H. Daub, J. Functional dyes for molecular switching- dihydroazu-lene/vinylheptafulvene photochromism-effect of pi-arylenes on the switching behavior. Liebigs Ann. 1995, (9), 1637-1641. 

Hammett correlations have been shown for the ring closure of vinylheptafulvenes to their corresponding dihydroazulenes. ... 

The dihydroazulene/vinylheptafulvene (DHA/VHF) system, first reported by Daub in 1984, represents such a photochromic system. DHAs such as 448, a ring-closed system, are prepared by [8+2] cycloaddition of 8-methoxyheptafulvene 449 and dicyanoethylenes 450. The double bond between C-2 and C-3 is introduced by elimination of methanol with phosphorus pentoxide (Scheme 6.117a). The yellow solutions of 448 turn dark red to 451 (VHP) on exposure to daylight. The thermolabile 451 rearranges quantitatively within 70 h at 25 °C into 448 in the dark (Scheme 6.117b) [272]. 

In a study of multimode switching based on dihydroazulene/vinylheptafulvene photochromism, Spreitzer and Daub [72] reported a [8+2]cycloaddition reaction of 8-methoxyheptafulvene 284 and dicyanovinylene compound 285 in chloroform to give the tetrahydroazulene 286 (Scheme 7.62). 

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