Photosensitive Dihydroazulenes with Chromogenic Properties

Abstract

Even daylight effects the rearrangement of dihydroazulene 1 into 8‐vinylheptafulvene 2; thermally, the reaction proceeds in the reverse direction. The behavior of 1 is largely determined by the residue R. The reaction cycle could find application in the storage of information.

Optimized synthesis and detailed NMR spectroscopic characterization of the 1,8a-dihydroazulene-1,1-dicarbonitrile photoswitch

“…The thermal ring-closure was performed by heating the sample (cuvette) by a Peltier unit in the UVVis spectrophotometer. (6). Acetophenone (18.5 g, 154 mmol) and malononitrile (28.4 g, 430 mmol) were dissolved in toluene (500 mL).…”

“…We have become interested in the dihydroazulene/vinylheptafulvene system (Scheme 1). 6 DHA undergoes a light-induced ring-opening reaction to VHF that in turn undergoes a thermal ringclosure back to DHA. A wide variety of derivatives has been synthesized, incorporating both electron-withdrawing and -donating groups in the five-membered ring 7 and very recently also in the seven-membered ring.…”

Synthetic protocols for the key functionalizations of the photochromic dihydroazulene scaffold

“…Functionalization of position 2 on the DHA skeleton has been most extensively explored. 5,6,[9][10][11][12][13][14][15][16][17][22][23][24][25][26][27][28] Aromatic groups are the substituents most widely incorporated, but few examples with methyl group on C-2 or cyclopentyl between C-2/C-3 have also been reported. 22 As described in the introduction (Scheme 3), the syntheses have followed either: (i) the [8+2] cycloaddition protocol, (ii) the route via nucleophilic addition of acetophenones to tropylium, followed by a Knoevenagel condensation with malononitrile, or (iii) an initial Knoevenagel condensation followed by reaction with tropylium species.…”

“…4 Thus, a molecule which upon excitation isomerizes to a more energetic metastable isomer, which in time will return to the more stable isomer by releasing energy as heat, may find use in lightharvesting energy storage devices. 3 In this context, the dihydroazulene/vinylheptafulvene (DHA/VHF) system 1/2 (Scheme 1), reported for the first time by Daub and co-workers in 1984, 5,6 is particularly interesting. DHA 1 is a yellow photochromic compound, which undergoes a light-induced 10-electron retro-electrocyclization to the red VHF 2.…”

Liquid crystalline dihydroazulene photoswitches

“…When irradiated with light, DHA 1 undergoes a ring-opening to the vinylheptafulvene (VHF) 2, which can then close back to the initial DHA via a thermal electrocyclization reaction. 7 The main advantage of the DHA-VHF photoswitch system is that only the DHA to VHF conversion is light-induced, which means that a broad spectrum of light can be used without triggering the VHF to DHA back-reaction. The DHA ringopening is associated with signicant changes in the physical properties of the corresponding VHF.…”