-Substituted 2-Phenyldihydroazulene Photoswitches: Enhancing the Lifetime of the Photoisomer by -Aryl Interactions.

Photochromic molecules are systems that undergo a photoisomerization to high-energy isomers and are attractive for the storage of solar energy in a closed-energy cycle, for example, in molecular solar thermal energy storage systems. One challenge is to control the discharge time of the high-energy isomer. Here, we show that different substituents in the position of a phenyl ring at C-2 of dihydroazulene (DHA-Ph) significantly increase the half-life of the metastable vinylheptafulvene (VHF-Ph) photoisomer; thus, the energy-releasing VHF-to-DHA back-reaction rises from minutes to days in comparison to the corresponding - and -substituted systems. Systems with two photochromic DHA-Ph units connected by a diacetylene bridge either at the , and positions and corresponding to a linear or to a cross-conjugated pathway between the two photochromes are also presented. Here, the substitution was found to compromise the switching properties. Thus, irradiation of -bridged DHA-DHA resulted in degradation, probably due to the proximity of the different functional groups that can give rise to side-reactions.