Michael Åxman Petersen

Arylethynyl derivatives of the dihydroazulene/vinylheptafulvene photo/thermoswitch: tuning the switching event.

A selection of dihydroazulene (DHA) photoswitches incorporating an arylethynyl-substituent in the seven-membered ring was prepared by palladium-catalyzed Sonogashira cross-coupling reactions employing a suitable bromo-functionalized DHA. Shielding of the alkyne bridge and separating the aryl and DHA units, by sterically demanding groups, was required to obtain stable compounds. The DHAs underwent a light-induced ring-opening to vinylheptafulvenes (VHFs) which were thermally converted to a mixture of two DHA regioisomers, one of which was the original one. The influence of the aryl groups on the DHA and VHF absorptions and on their interconversion was investigated in detail. The rates of the switching events were finely tuned by the donor or acceptor strength of the aryl group. The thermal ring closure was found to proceed most readily in the presence of an electron-donating group on the seven-membered ring. The rate constant was found to follow a Hammett linear free energy correlation, which signals that stabilization of a positive charge in the seven-membered ring plays a crucial role in the ring-closure reaction. In view of these findings, it was possible to control the switching event by protonation/deprotonation of an anilino-substituted DHA. Also, the light-induced ring opening reaction was strongly controlled by acid/base. In addition to the mesomeric effects exerted by an arylethynyl group, the inductive effects exerted by different groups on the thermal ring closure were elucidated. Although the alkyne bridge transmits the electronic character of the aryl group, the ring-closure is retarded for all the ethynylated compounds relative to the parent unsubstituted compound. Along with our synthesis of suitable arylalkynes, we discovered an interesting byproduct in a Sonogashira cross-coupling reaction involving a nitrophenyl group, namely a diaryl azoxy compound. Its structure was confirmed by X-ray crystallographic analysis.

Absorption Spectra of 4-Nitrophenolate Ions Measured in Vacuo and in Solution

From blue to red: While four pi-conjugated nitrophenolates absorb within a relatively narrow region in solution, they cover the entire visible spectrum when isolated in vacuo [picture: see text]. The work combines gas- and solution-phase spectroscopy and provides the first benchmark of theoretical excitation energies for nitrophenolates.

On the condensed phase ring-closure of vinylheptafulvalene and ring-opening of gaseous dihydroazulene.

Dihydroazulenes are interesting because of their photoswitching behavior. While the ring-opening to vinylheptafulvalene (VHF) is light induced, the back reaction is known to proceed thermally. In the present paper, we show the first gas phase study of the ring-opening reaction of 2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile (Ph-DHA) by means of time-resolved photoelectron spectroscopy which permits us to follow the ring-opening process. Moreover, we investigated s-trans-Ph-VHF in a series of transient absorption experiments, supported by ab initio computations, to understand the origin of the absence of light-induced ring-closure. The transient absorption results show a biexponential decay governed by a hitherto unknown state. This state is accessed within 1-2 ps and return to the ground state is probably driven through a cis-trans isomerization about the exocyclic C1═C2 double bond. The rapid decrease in potential energy disfavors internal rotation to s-cis-Ph-VHF, the structure that would precede the ring-closure reaction.

Photoreaction of Matrix-Isolated Dihydroazulene-Functionalized Molecules on Au{111}

Dihydroazulenes are photochromic molecules that reversibly switch between two distinct geometric and conductivity states. Molecular design, surface attachment, and precise control over the assembly of such molecular machines are critical in order to understand molecular function and motion at the nanoscale. Here, we use surface-enhanced Raman spectroscopy on special atomically flat, plasmonically enhanced substrates to measure the photoreaction kinetics of isolated dihydroazulene-functionalized molecules assembled on Au{111}, which undergo a ring-opening reaction upon illumination with UV light and switch back to the initial isomer via thermal relaxation. Photokinetic analyses reveal the high efficiency of the dihydroazulene photoreaction on solid substrates compared to other photoswitches. An order of magnitude decrease in the photoreaction cross section of surface-bound dihydroazulenes was observed when compared with the cross sections of these molecules in solution.

Substitution effects on the absorption spectra of nitrophenolate isomers

Charge-transfer excitations highly depend on the electronic coupling between the donor and acceptor groups. Nitrophenolates are simple examples of charge-transfer systems where the degree of coupling differs between ortho, meta and para isomers. Here we report the absorption spectra of the isolated anions in vacuo to avoid the complications of solvent effects. Gas-phase action spectroscopy was done with two different setups, an electrostatic ion storage ring and an accelerator mass spectrometer. The results are interpreted on the basis of CC2 quantum chemical calculations. We identified absorption maxima at 393, 532, and 399 nm for the para, meta, and ortho isomer, respectively, with the charge-transfer transition into the lowest excited singlet state. In the meta isomer, this π-π* transition is strongly redshifted and its oscillator strength reduced, which is related to the pronounced charge-transfer character, as a consequence of the topology of the conjugated π-system. Each isomers different charge distribution in the ground state leads to a very different solvent shift, which in acetonitrile is bathochromic for the para and ortho, but hypsochromic for the meta isomer.

New routes to functionalized dihydroazulene photoswitches

Molecular photoswiches are important for the development of advanced materials and molecular electronics devices. The dihydroazulene (DHA) is a particularly attractive molecule as it undergoes a light-induced ring opening to a vinylheptafulvene (VHF) isomer with altered optical properties. While functionalization of the five-membered ring of DHA has been possible for the last 25 years, this was not the case for the seven-membered ring. This article summarizes our synthetic efforts in achieving this goal. Incorporation of an alkyne at C-7 was accomplished by (i) regioselective bromination of DHA, followed by (ii) elimination of HBr, and (iii) Pd-catalyzed cross-coupling with triisopropylsilylacetylene. Light-induced ring opening of this DHA followed by thermal ring closure provided a mixture of 6- and 7-substituted DHAs with different absorption characteristics. The isomer ratio was controlled by the wavelength of irradiation and the solvent polarity. The dibromide formed in the initial step served as a precursor for a 3-bromo-functionalized azulene that was employed as a building block for acetylenic scaffolding. Incorporation of a dithiafulvene (DTF) unit at the five-membered ring of DHA resulted in a significant red-shift in the longest-wavelength absorption and consequently a lowering of the energy required for ring opening. Incorporation of a redox-active tetrathiafulvalene (TTF) unit allowed for redox-controlled photoswitching.

Double‐Bond versus Triple‐Bond Bridges: Does it Matter for the Charge‐Transfer Absorption by Donor–Acceptor Chromophores?

Double-Bond versus Triple-Bond Bridges : Does it Matter for the Charge-Transfer Absorption by Donor-Acceptor Chromophores?

Novel retinylidene iminium salts for defining opsin shifts: synthesis and intrinsic chromophoric properties

Retinal Schiff bases serve as chromophores in many photoactive proteins that carry out functions such as signalling and light-induced ion translocation. The retinal Schiff base can be found as neutral or protonated, as all-trans, 11-cis or 13-cis isomers and can adopt different conformations in the protein binding pocket. Here we present the synthesis and characterisation of isomeric retinylidene iminium salts as mimics blocked towards isomerisation at the C11 position and conformationally restrained. The intrinsic chromophoric properties are elucidated by gas phase absorption studies. These studies reveal a small blue-shift in the S0-->S1 absorption for the 11-locked derivative as compared to the unlocked one. The gas phase absorption spectra of all the cationic mimics so far investigated show almost no absorption in the blue region. This observation stresses the importance of protein interactions for colour tuning, which allows the human eye to perceive blue light.

New synthetic route to substituted dihydroazulene photoswitches

A new procedure for functionalization of the dihydroazulene photoswitch on its seven-membered ring was developed, which has allowed isolation of the first dihydroazulene with a phenyl substituent at position 5 from a mixture of regioisomers. Light-induced ring-opening to the corresponding vinylheptafulvene and the thermal back-reaction was studied in detail.

Molecular Switching in Confined Spaces: Effects of Encapsulating the DHA/VHF Photo-Switch in Cucurbiturils

Confinement of reactive chemical species uniquely affects chemical reactivity by restricting the physical space available and by restricting access to interactions with the solvent. In Nature, for example, confined protein binding pockets govern processes following photoisomerization reactions and the isomerizations themselves. Here we describe the first example of a dihydroazulene/vinylheptafulvene (DHA/VHF) photo-switch functioning in water, and we show how its switching behavior is strongly influenced by supramolecular interactions with a series of cucurbit[n]uril (CB) host molecules. In CB7 inclusion complexes, the kinetics of the thermal VHF-to-DHA back-reaction is accelerated, while in CB8 inclusion complexes, the kinetics is slowed down as compared to the free photo-switch. The effect of the CB encapsulation of the photo-switch can be effectively canceled by introducing a guest that binds the CB more strongly. According to DFT calculations, a stabilization of the reactive s-cis VHF conformer relative to the s-trans VHF appears to be a contributing factor responsible for the accelerated back-reaction when encapsulated in CB7.