Søren Lindbæk Broman

IT University of Copenhagen, København, Capital Region, Denmark

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Publications (20)80.84 Total impact

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    ABSTRACT: The concept of cross-conjugation has in recent years attracted increasing interest in materials science as well as in the field of molecular electronics as a means to tune the optical properties and molecular conductance of nanoscale devices. In this paper we show how the rate of an electrocyclic reaction, the conversion of vinylheptafulvene (VHF) into dihydroazulene (DHA), is influenced by an electron-withdrawing p-nitrophenyl group separated from the VHF by either cross-conjugated or linearly π-conjugated bridging units. A selection of DHA/VHF molecules with ethyne, 1,1- and 1,2-diethynylethene, and 1,3-butadiyne bridges was prepared by metal-catalyzed coupling reactions. The effect of the different conjugated spacers on the kinetics of the conversion of VHF into DHA was examined by UV/Vis absorption spectroscopy by monitoring the decay of the VHF absorption. The cross-conjugated diethynylethene spacer was found to increase the half-life of the VHF relative to the linearly conjugated spacers.
    Annalen der Chemie und Pharmacie 10/2014; · 3.10 Impact Factor
  • Søren Lindbæk Broman, Mogens Brondsted Nielsen
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    ABSTRACT: Recent synthetic advances allowing large-scale preparation and systematic functionalization of the dihydroazulene (DHA) / vinylheptafulvene (VHF) photo- / thermoswitch have enabled detailed studies on how to tune optical and switching properties and have paved the way for using this system as a functional unit in molecular electronics and materials chemistry. Since discovery of its photochromism in the 1980’ies, numerous examples of DHA/VHF systems have been developed, allowing multimode switching, fluorescence-control and fine tuning of absorbance and VHF half-lives, giving insights into the mechanism of the switching event. Here, we present an overview of the properties of the DHA/VHF system, together with some selected synthetic procedures which have paved the way for its development.
    Physical Chemistry Chemical Physics 08/2014; · 4.20 Impact Factor
  • ARKIVOC: archive for organic chemistry 03/2014; 2014(Part (i)):249. · 1.25 Impact Factor
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    ABSTRACT: The dihydroazulene (DHA) / vinylheptafulvene (VHF) photo- / thermoswitch has found interest for molecular electronics and advanced materials. The switching between the two isomers has previously been found to depend strongly on the presence of donor and acceptor groups. The fine-tuning of optical and switching properties relies on ready access to new derivatives via efficient synthetic protocols. The central DHA core is conveniently prepared in a four-step synthesis starting from acetophenone and tropylium substrates. The outcome of this reaction as a function of the nature of a substituent group on the phenyl unit of acetophenone is here investigated in detail. A wide variety of functional groups (nitro, cyano, halo, alkyl, amido, and thioether) was tolerated, and the route thus provided access to a large selection of substituted DHA derivatives (position 2 of DHA). These compounds were investigated for their ability to undergo subsequent functionalization in the seven-membered ring by a regioselective bromination-elimination protocol, introducing a bromo-substituent at position 7. Halo-substituted DHAs were subjected to palladium-catalyzed cyanation, Sonogashira, Cadiot-Chodkiewicz, and Suzuki couplings and for the latter reaction, optimized conditions were developed by varying the palladium catalyst. In general, focus has been on reducing formation of fully unsaturated azulene by-products.
    The Journal of Organic Chemistry 12/2013; · 4.56 Impact Factor
  • Søren Lindbaek Broman, Martyn Jevric, Mogens Brøndsted Nielsen
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    ABSTRACT: Linear free-energy relationships, like Hammett correlations, are fundamental in physical organic chemistry for the elucidation of reaction mechanisms. In this work, we show that Hammett correlations exist for the ring closure of six different model systems of vinylheptafulvenes (VHFs) to their corresponding dihydroazulenes (DHAs). These first-order reactions were easily followed by UV/Vis absorption spectroscopy on account of the significantly different absorption characteristics between VHFs and DHAs. Opposing effects displayed by substituent groups at two different positions are conveniently accounted for by simply subtracting the two Hammett σ values of each group. The linear correlations readily allow us to obtain unknown and approximate Hammett σ values for previously uninvestigated substituents. We also show that they can provide alternative values to the standard ones. We present values for a variety of substituent groups ranging from alkynes, sulfones, sulfoxides, and different heteroaromatics. The electronic effects exerted by substituent groups on VHFs are also reflected in their absorption maxima. Thus, we have established an empirical relationship between the absorption maximum of the VHF and the Hammett σ values of its substituents. This fine-tuning of electronic properties is particularly important for the ongoing efforts of using the DHA/VHF molecular switch in molecular electronics devices.
    Chemistry - A European Journal 06/2013; · 5.93 Impact Factor
  • Martyn Jevric, Søren Lindbæk Broman, Mogens Brøndsted Nielsen
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    ABSTRACT: The dihydroazulene (DHA)/vinylheptafulvene (VHF) photo/thermoswitch has attracted interest as a molecular switch for advanced materials and molecular electronics. We report here two synthetic approaches using palladium catalysis for synthesizing dihydroazulene (DHA) photoswitches with thioacetate anchoring groups intended for molecular electronics applications. The first methodology involves a Suzuki coupling using tert-butyl thioether protecting groups. Conversion to the thioacetate using boron tribromide/acetyl chloride results in the formation of the product as a mixture of regioisomers mediated by a ring-opening reaction. The second approach circumvents isomerization by the synthesis of stannanes as intermediates and their use in a Stille coupling. Although fully unsaturated azulenes are formed as byproducts during the synthesis of the DHA stannanes, this approach allowed the regioselective incorporation of the thioacetate anchoring group in either one of the two ends (positions 2 or 7) or at both.
    The Journal of Organic Chemistry 04/2013; · 4.56 Impact Factor
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    ABSTRACT: 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 1,1-dicyano-2-phenyl-1,8a-dihydroazulene (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 which would precede the ring closure reaction.
    The Journal of Physical Chemistry A 04/2013; · 2.77 Impact Factor
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    ABSTRACT: Molecular switches play a central role for the development of molecular electronics. In this work it is demonstrated that the reproducibility and robustness of a single-molecule dihydroazulene (DHA)/vinylheptafulvene (VHF) switch can be remarkably enhanced if the switching kernel is weakly coupled to electrodes so that the electron transport goes by sequential tunneling. To assure weak coupling, the DHA switching kernel is modified by incorporating p-MeSC6H4 end-groups. Molecules are prepared by Suzuki cross-couplings on suitable halogenated derivatives of DHA. The synthesis presents an expansion of our previously reported bromination–elimination–cross-coupling protocol for functionalization of the DHA core. For all new derivatives the kinetics of DHA/VHF transition has been thoroughly studied in solution. The kinetics reveals the effect of sulfur end-groups on the thermal ring-closure of VHF. One derivative, incorporating a p-MeSC6H4 anchoring group in one end, has been placed in a silver nanogap. Conductance measurements justify that transport through both DHA (high resistivity) and VHF (low resistivity) forms goes by sequential tunneling. The switching is fairly reversible and reenterable; after more than 20 “ON-OFF” switchings, both DHA and VHF forms are still recognizable, albeit noticeably different from the original states.
    Advanced Functional Materials 10/2012; 22(20). · 10.44 Impact Factor
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    ABSTRACT: The dihydroazulene (DHA)/vinylheptafulvene (VHF) photo/thermoswitch has recently attracted interest as a molecular switch for molecular electronics. In this field, Buckminsterfullerene, C(60), has been shown to be a useful anchoring group for adhering a molecular wire to an electrode. Here we have combined the two units with the overall aim to elucidate how C(60) influences the DHA-VHF switching events. Efficient synthetic protocols for making covalently linked DHA-C(60) conjugates were developed, using Prato, Sonogashira, Hay, and Cadiot-Chodkiewicz reactions. These syntheses provide as well a variety of potentially useful DHA and C(60) building blocks for acetylenic scaffolding. The two units were separated by bridges of various lengths, such as oligo(phenyleneethynylene) (OPE2 and OPE3) wires. The distance of separation was found to influence strongly the light-induced ring-opening reaction of DHA to its corresponding VHF. Thus, C(60) was found to significantly quench this conversion when situated closely to the DHA unit.
    The Journal of Organic Chemistry 09/2012; 77(20):8922-32. · 4.56 Impact Factor
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    ABSTRACT: Intramolecular cyclization reactions of Green Fluorescent Protein chromophores (GFPc) containing an arylethynyl ortho-substituent at the phenol ring provide new aryl-substituted benzofuran derivatives of the GFPc. Some of these heteroaromatic compounds exhibit significantly enhanced fluorescence at red-shifted emission wavelengths relative to the parent GFPc structure.
    RSC Advances 08/2012; 2(22):8243-8249. · 3.71 Impact Factor
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    ABSTRACT: Background: The dihydroazulene (DHA)/vinylheptafulvene (VHF) system (with two cyano groups at C1) functions as a photo-/thermoswitch. Direct ionic bromination of DHA has previously furnished a regioselective route to a 7,8-dibromide, which by elimination was converted to a 7-bromo-substituted DHA. This compound has served as a central building block for functionalization of the DHA by palladium-catalyzed cross-coupling reactions. The current work explores another bromination protocol for achieving the isomeric 3-bromo-DHA and also explores the outcome of additional bromination of this compound as well as of the known 7-bromo-DHA.Results: Radical bromination on two different VHFs by using N-bromosuccinimide/benzoyl peroxide and light, followed by a ring-closure reaction generated the corresponding 3-bromo-DHAs, as confirmed in one case by X-ray crystallography. According to a (1)H NMR spectroscopic study, the ring closure of the brominated VHF seemed to occur readily under the reaction conditions. A subsequent bromination-elimination protocol provided a 3,7-dibromo-DHA. In contrast, treating the known 7-bromo-DHA with bromine generated a very labile species that was converted to a new 3,7-dibromoazulene, i.e., the fully unsaturated species. Azulenes were also found to form from brominated compounds when left standing for a long time in the solid state. Kinetics measurements reveal that the 3-bromo substituent enhances the rate of the thermal conversion of the VHF to DHA, which is opposite to the effect exerted by a bromo substituent in the seven-membered ring.Conclusion: Two general procedures for functionalizing the DHA core with a bromo substituent (at positions 3 and 7, respectively) are now available with the DHA as starting material.
    Beilstein Journal of Organic Chemistry 01/2012; 8:958-66. · 2.80 Impact Factor
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    ABSTRACT: 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.
    Organic Letters 12/2011; 14(1):318-21. · 6.14 Impact Factor
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    ABSTRACT: Derivatives of 1,1-dicyano-1,8a-dihydroazulene (DHA) undergo light-induced ring-opening to a corresponding vinylheptafulvene (VHF), which in turn is thermally reverted to DHA. Here we have fabricated single-molecule DHA/VHF junctions and measured light-triggered conductance switching of these junctions. The DHA/VHF system studied includes a substituent group at the seven-membered ring. Light-induced conversion of this DHA to VHF in the junction is supported by a reduced tunnelling gap in tunnelling density of states. In fact, the reduced tunnelling gap corresponds to the reduced HOMO–LUMO gap of VHF relative to that of DHA as measured by absorption spectroscopy and electrochemistry. For this comparison, electrochemical measurements were performed on both the parent DHA/VHF system and the functionalized system that was subject to transport measurements. In one junction, it was possible to switch back and forth between DHA and VHF three times, the forward reaction being induced by light and the back-reaction occurring after waiting a period of time.
    The Journal of Physical Chemistry C. 08/2011; 115(37).
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    ABSTRACT: 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.
    Organic & Biomolecular Chemistry 08/2011; 9(19):6498-501. · 3.57 Impact Factor
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    ABSTRACT: Mild Lewis acids enhance the rate of the thermal conversion of vinylheptafulvene (VHF) to dihydroazulene (DHA). In the absence of light, stronger Lewis acids promote the otherwise photoinduced DHA to VHF conversion.
    Chemical Communications 06/2011; 47(21):6102-4. · 6.38 Impact Factor
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    ABSTRACT: A protocol for functionalizing the seven-membered ring of the DHA/VHF photochromic system via a Suzuki cross-coupling was developed. Kinetic studies reveal that direct attachment of an aromatic moiety on the seven-membered ring strongly affects the thermal back reaction. By the introduction of an anilino group, pH control of both the photochromic and thermochromic properties was obtained. Furthermore, it was possible to switch between three different states: two photochromic and one thermochromic.
    Annalen der Chemie und Pharmacie 02/2011; 2011(6). · 3.10 Impact Factor
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    ABSTRACT: 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.
    Journal of the American Chemical Society 07/2010; 132(26):9165-74. · 10.68 Impact Factor
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    Pure and Applied Chemistry - PURE APPL CHEM. 01/2010; 82(4):843-852.
  • Synfacts 09/2009; 2009(09):0978-0978.
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    ABSTRACT: The first synthetic protocol for functionalizing the dihydroazulene (DHA) photoswitch in its seven-membered ring has been developed. This protocol is based on regioselective bromination, followed by regioselective elimination of HBr, and finally a palladium-catalyzed cross-coupling reaction with a terminal alkyne. The position of functionalization (C-7) was confirmed by X-ray crystal structure analysis. Light-induced ring opening of this compound to its vinylheptafulvene (VHF) isomer followed by thermal ring closure provides a mixture of two DHA regioisomers in a ratio that depends on the wavelength of irradiation and solvent polarity.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
    Annalen der Chemie und Pharmacie 06/2009; 2009(17):2733-2736. · 3.10 Impact Factor