Multi‐Photochromic Molecules Based on Dihydroazulene Units

Abstract

Multi-photochromic systemsi ncorporating individually addressables witching units are attractivef or development of advanced data storaged evices.H ere, we present the synthesis and properties of as election of such molecular systemsi ncorporating the dihydroazulene/vinylheptafulvene (DHA/VHF) photo-/thermoswitch. The influence of the linker (meta-phenylene vs. azulene-1,3-diyl vs. thiophene-2,5-diyl) separating two DHA units on the switching properties was investigated. An azulene-1,3-diyl spacer largely inhibited both the DHA-to-VHF photoisomerizations and the thermal VHF-to-DHA back-reactions;t he latter occurred ten times slower than for the related compound with a meta-phenyl-ene spacer.ADHA trimerc ontaining three DHA units aroundacentral benzene ring was found to undergo stepwise DHA-to-VHF photoisomerizations, whereas the thermal back-reactions occurred at similar rates for the three VHF entities.Ameta-phenylene-bridgedD HA dimerw as subjected to furthers tructural modifications at positionC-1 of each DHA,h avings trong implications for the switching events, and synthetic steps for further functionalizations at position C-7 of each DHA were investigated.F inally,t he molecular structure (from X-ray crystallographic analysis) between the meta-phenylene-bridgedDHA dimer and Cu I is presented.

Dihydroazulene‐Azobenzene‐Dihydroazulene Triad Photoswitches

“…Addition of a Cu I salt to VHF−Ph was previously found to shorten the VHF half‐life from 545 to approximately 20 min in CH 2 Cl 2 at 25 °C due to complexation of the CN groups [1k,13] . Addition of Cu I to a solution of meta‐ VHF‐ c AZB‐VHF did indeed accelerate the TBR and allowed detection of two different rate constants, but Cu I seemed to perturb also the AZB isomerization, and we could not access selectively meta ‐DHA‐ c AZB‐DHA in this way.…”

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

“…Molecules 2021, 26, x FOR PEER REVIEW 4 of 13 some degradation as well (Figure 1, bottom box) [29]. This "meta-rule" of photoactivity of phenylene-bridged photoswitch dimers was also established for azobenzenes [31,32], and is an important general design criterium.…”

“…The para-connected bridge separates the two DHA units by a linearly conjugated pathway, and the compound exhibits a redshifted absorption maximum in comparison to DHA-Ph and to the cross-conjugated meta-phenylene-bridged DHA dimer. Changes in the bridging unit from phenylene to thiophene-2,5-diyl ("para-like" connectivity, linear conjugation) or to azulene-1,3-diyl ("meta-like" connectivity, cross-conjugation) further confirmed that cross-conjugation allows sequential switchings, while linear conjugation reduces the photoactivity considerably, with full photoisomerization to VHF-VHF being accompanied by some degradation as well (Figure 1, bottom box) [29]. This "meta-rule" of photoactivity of phenylene-bridged photoswitch dimers was also established for azobenzenes [31,32], and is an important general design criterium.…”

“…In addition, we present a study on dimeric molecules with two DHA-Ph photochromic units connected by ortho, meta or para diacetylene spacers. It was previously demonstrated that with multimode photoswitches constituted by two DHA units separated by a phenylene bridge, communication between the two units depends on the relative positions of the single units on the central benzene ring (meta or para, Figure 1, bottom box) [29]. The photoactivity of one DHA unit depends on whether its neighboring unit has already been converted to VHF or not, and a sequential switching between the three possible states (i.e., from DHA-DHA to DHA-VHF, and then to VHF-VHF) is achieved only with a meta connectivity [29,30].…”

“…It was previously demonstrated that with multimode photoswitches constituted by two DHA units separated by a phenylene bridge, communication between the two units depends on the relative positions of the single units on the central benzene ring (meta or para, Figure 1, bottom box) [29]. The photoactivity of one DHA unit depends on whether its neighboring unit has already been converted to VHF or not, and a sequential switching between the three possible states (i.e., from DHA-DHA to DHA-VHF, and then to VHF-VHF) is achieved only with a meta connectivity [29,30]. The two sequential light-induced ring openings were explained by a significantly reduced photoactivity of DHA in the presence of a neighboring VHF electron acceptor unit (DHA-VHF).…”