[Show abstract][Hide abstract] ABSTRACT: A selection of cyclic and acyclic acetylenic scaffolds bearing two tetrathiafulvalene (TTF) units was prepared by different metal-catalyzed coupling reactions. The bridge separating the two TTF units was systematically changed from linearly conjugated ethyne, butadiyne and tetraethynylethene (trans-substituted) units to a cross-conjugated tetraethynylethene unit, placed in either acyclic or cyclic arrangements. The cyclic structures correspond to so-called radiaannulenes having both endo-and exocyclic double bonds. Interactions between two redox-active TTF units in these molecules were investigated by cyclic voltammetry, UV–vis–NIR and EPR absorption spectroscopical methods of the electrochemically generated oxidized species. The electron-accepting properties of the acetylenic cores were also investigated electrochemically.
[Show abstract][Hide abstract] ABSTRACT: The vinylheptafulvene (VHF) to dihydroazulene (DHA) electrocyclization is known to proceed from an s-cis conformation of VHF and cannot occur from the more stable s-trans conformation. Locking the VHF in the s-cis conformation by the introduction of a dihydronaphthalene (DHN) unit has been found to greatly enhance the speed of this reaction. Thus, the half-life was reduced by more than a factor of 150000 in cyclohexane and by a factor of approximately 950000 in ethanol. In addition, the characteristic absorption of the photoactive DHA isomer, now annulated to DHN, exhibited a desired redshift relative to the parent compound. Here, we present the synthesis and study of these DHN-DHA/VHFs, including a protocol for the incorporation of a pseudo-halide to enable the further functionalization of the molecule by metal-catalyzed cross-coupling reactions. For proof-of-concept, two different sulfur end-groups were incorporated as anchoring groups for potential molecular electronics applications.
European Journal of Organic Chemistry 05/2015; 2015(19). DOI:10.1002/ejoc.201500320 · 3.07 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 1,8a-Dihydroazulene-1,1-dicarbonitrile (DHA) is a photochromic molecule which upon irradiation undergoes ring-opening to a vinylheptafulvene (VHF). The system has many possible sites for functionalization and hence for tuning of its properties. In this account we summarize different synthetic protocols for attaching substituents at the ring carbon atoms of DHA as well as for replacing the cyano groups at position 1. In particular, positions 2 and 7 are most easily accessed, the latter from DHA by a regioselective bromination-elimination protocol.
ARKIVOC: archive for organic chemistry 12/2014; 2014(Part (i)):249. DOI:10.1002/chin.201451263
[Show abstract][Hide abstract] 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.
European Journal of Organic Chemistry 12/2014; 2014(35). DOI:10.1002/ejoc.201403117 · 3.07 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: Dihydroazulene (DHA)-vinylheptafulvene (VHF) photoswitches have attracted attention as potentially useful components in molecular electronics. The pi-conjugated dihydroazulene system is a rigid structure and can be strategically functionalised to place handles for further elaboration. Here we show that alkyne-functionalised dihydroazulenes can be subjected to copper and ruthenium catalysed azide-alkyne cycloadditions (CuAAC and RuAAC) with tolylazide, furnishing 1,4- and 1,5-disubstituted triazoles. The rates of ring-closure of the corresponding vinylheptafulvenes were compared with those of reference systems, which allowed determination of Hammett substituent constants (meta and para) for N-tolyl-substituted 1,2,3-triazoles.
Australian Journal of Chemistry 01/2014; 67(3):531. DOI:10.1071/CH13544 · 1.56 Impact Factor
[Show abstract][Hide abstract] 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; 79(1). DOI:10.1021/jo4020326 · 4.72 Impact Factor
[Show abstract][Hide abstract] 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 07/2013; 19(29). DOI:10.1002/chem.201300167 · 5.73 Impact Factor
[Show abstract][Hide abstract] 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; 78(9). DOI:10.1021/jo400362u · 4.72 Impact Factor
[Show abstract][Hide abstract] 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; 117(16). DOI:10.1021/jp400616c · 2.69 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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. DOI:10.1021/jo301306y · 4.72 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
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.
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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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). DOI:10.1021/jp205638b · 4.77 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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. DOI:10.1039/c1cc10804b · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An economical and effective protocol for large scale synthesis of the 2-phenyl-1,8adihydroazulene-1,1-dicarbonitrile (DHA) photoswitch has been developed. This compound is ring-opened by light to a vinylheptafulvene (VHF), which is thermally closed back to DHA. This compound serves as an important starting material for dihydroazulene photoswitches incorporating a substituent in the seven-membered ring and as a reference compound for comparison of properties. A detailed NMR spectroscopic characterization has allowed the assignment of all proton and carbon signals. In addition, the compound was characterized by Xray crystallography. A correlation between the rate constant for thermal ring-closure of VHF to DHA and empirical parameters of solvent polarity (ET (30)) was established.