Anne Ugleholdt Petersen

Controlling Two-Step Multimode Switching of Dihydroazulene Photoswitches

We present the synthesis and switching studies of systems with two photochromic dihydroazulene (DHA) units connected by a phenylene bridge at either para or meta positions, which correspond to a linear or cross-conjugated pathway between the photochromes. According to UV/Vis absorption and NMR spectroscopic measurements, the meta-phenylene-bridged DHA-DHA exhibited sequential light-induced ring openings of the two DHA units to their corresponding vinylheptafulvenes (VHFs). Initially, the VHF-DHA species was generated, and, ultimately, after continued irradiation, the VHF-VHF species. Studies in different solvents and quantum chemical calculations indicate that the excitation of DHA-VHF is no longer a local DHA excitation but a charge-transfer transition that involves the neighboring VHF unit. For the linearly conjugated para-phenylene-bridged dimer, electronic communication between the two units is so efficient that the photoactivity is reduced for both the DHA-DHA and DHA-VHF species, and DHA-DHA, DHA-VHF, and VHF-VHF were all present during irradiation. In all, by changing the bridging unit, we can control the degree of stepwise photoswitching.

A Bis(heptafulvenyl)-dicyanoethylene Thermoswitch with Two Sites for Ring Closure

Suitably functionalized vinylheptafulvenes (VHFs) act as thermoswitches undergoing ring closure to the corresponding dihydroazulenes (DHAs). Here we present the synthesis of a new such thermoswitch incorporating two heptafulvene rings on a dicyanoethylene unit. The synthetic protocol explores both the tropylium species as an electrophile and as a leaving group in the generation of the heptafulvene units. The thermally induced ring closure was enhanced as a result of two accessible sites for the reaction to occur.

Single-molecule detection of dihydroazulene photo-thermal reaction using break junction technique

Charge transport by tunnelling is one of the most ubiquitous elementary processes in nature. Small structural changes in a molecular junction can lead to significant difference in the single-molecule electronic properties, offering a tremendous opportunity to examine a reaction on the single-molecule scale by monitoring the conductance changes. Here, we explore the potential of the single-molecule break junction technique in the detection of photo-thermal reaction processes of a photochromic dihydroazulene/vinylheptafulvene system. Statistical analysis of the break junction experiments provides a quantitative approach for probing the reaction kinetics and reversibility, including the occurrence of isomerization during the reaction. The product ratios observed when switching the system in the junction does not follow those observed in solution studies (both experiment and theory), suggesting that the junction environment was perturbing the process significantly. This study opens the possibility of using nano-structured environments like molecular junctions to tailor product ratios in chemical reactions.

Aluminum Chloride Mediated Alkynylation of Boron Subphthalocyanine Chloride Using Trimethylsilyl-Capped Acetylenes

A mild and versatile procedure is presented for functionalization of boron chloride subphthalocyanine at the axial boron position with trimethylsilyl-protected alkyne nucleophiles in the presence of aluminum chloride. The method allows a large variety of substituents on the alkyne units, including electron-donating/withdrawing aryl groups, silyl-protected alkynyl groups, as well as ferrocenyl and azulenyl groups. In addition, ferrocene itself reacts smoothly under these conditions allowing for directly anchoring it to the boron of the subphthalocyanine.

Liquid crystalline dihydroazulene photoswitches

A large selection of photochromic dihydroazulene (DHA) molecules incorporating various substituents at position 2 of the DHA core was prepared and investigated for their ability to form liquid crystalline phases. Incorporation of an octyloxy-substituted biphenyl substituent resulted in nematic phase behavior and it was possible to convert one such compound partly into its vinylheptafulvene (VHF) isomer upon irradiation with light when in the liquid crystalline phase. This conversion resulted in an increase in the molecular alignment of the phase. In time, the meta-stable VHF returns to the DHA where the alignment is maintained. The systematic structural variation has revealed that a biaryl spacer between the DHA and the alkyl chain is needed for liquid crystallinity and that the one aromatic ring in the spacer cannot be substituted by a triazole. This work presents an important step towards employing the dihydroazulene-vinylheptafulvene photo/thermoswitch in photoactive liquid crystalline materials.

Photoswitching of Dihydroazulene Derivatives in Liquid-Crystalline Host Systems

Photoswitches and dyes in the liquid-crystalline nematic phase have the potential for use in a wide range of applications. A large order parameter is desirable to maximize the change in properties induced by an external stimulus. A set of photochromic and nonphotochromic dyes were investigated for these applications. It was found that a bent-shaped 7-substituted dihydroazulene (DHA) photoswitch exhibited liquid-crystalline properties. Further investigation demonstrated that this material actually followed two distinct reaction pathways on heating, to a deactivated form by a 1,5-sigmatropic shift and to a linear 6-substituted DHA. In addition, elimination of hydrogen cyanide from the photoactive DHA gave both bent and linear azulene dyes. In a nematic host that has no absorbance around 350 nm, it was found that only the linear DHA derivative has nematic properties; however, both 6- and 7-substituted DHAs were found to have large order parameters. In the nematic host, ring opening of either DHA to the corresponding vinylheptafulvene resulted in a decrease in dichroic order parameter and an unusually fast back-reaction to a mixture of both DHAs. Likewise, only the linear azulene derivative showed mesomorphic properties. In the same nematic host, large order parameters were also observed for these dyes.

Inverting the Selectivity of the Newman–Kwart Rearrangement via One Electron Oxidation at Room Temperature

The discovery that the Newman-Kwart rearrangement can be performed at room temperature by action of a simple and readily available oxidant, cerium ammonium nitrate, is described. The conditions give clean conversion when using electron-rich aromatic substrates, and the reactions are often quantitative. Computational studies support a reaction mechanism where the O-thiocarbamate is first oxidized to the radical cation, followed by nucleophilic attack by the ipso carbon of the aromatic system.