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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  ortho  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  para - and  meta -substituted systems. Systems with two photochromic DHA-Ph units connected by a diacetylene bridge either at the  para ,  meta  and  ortho  positions and corresponding to a linear or to a cross-conjugated pathway between the two photochromes are also presented. Here, the  ortho  substitution was found to compromise the switching properties. Thus, irradiation of  ortho -bridged DHA-DHA resulted in degradation, probably due to the proximity of the different functional groups that can give rise to side-reactions.

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