Optical Molecular Switch Research Articles

Phenylazoimidazole as a possible optical molecular switch: An ab initio study

By applying nonequilibrium Green's function formalism combined first-principles density functional theory, we investigate the electronic transport properties of the phenylazoimidazole optical molecular switch. The molecule that comprises the switch can convert between the cis and the trans forms upon photoexcitation. The influence of HOMO–LUMO gaps and the spatial distributions of molecular orbitals on the electronic transport through the molecular device are discussed in detail. Theoretical results show that the current through the trans form is significantly larger than through the cis form, which suggests this system has attractive potential application in future molecular switch technology.

Electronic Transport Properties of a Naphthopyran-Based Optical Molecular Switch: an ab initio Study

The electronic transport properties of a naphthopyran-based molecular optical switch are investigated by using the nonequilibrium Green's function formalism combined with first-principles density functional theory. The molecule that comprises the switch can convert between its open and closed forms upon photoexcitation. Theoretical results show that the current through the open form is significantly larger than that through the closed form, which is different from other optical switches based on ring-opening reactions of the molecular bridge. The maximum on-off ratio (about 90) can be obtained at 1.4 V. The physical origin of the switching behavior is interpreted based on the spatial distributions of molecular orbitals and the HOMO-LUMO gap. Our result shows that the naphthopyran-based molecule is a good candidate for optical molecular switches and will be useful in the near future.

First-Principles Study of Triangle Terarylene as a Possible Optical Molecular Switch

Using non-equilibrium Green’s function formalism combined with first-principles density functional theory, we investigate the electronic transport properties of a triangle terarylene(open- and closed-ring forms) optical molecular switch. The influence of the HOMO-LUMO gaps and the spatial distributions of molecular orbitals on the quantum transport through the molecular device is discussed. Theoretical results show that the conductance of the closed-ring is 3-8 times larger than that of open-ring, which expect that this system can be one of good candidates for optical switches due to this unique advantage, and may have some potential applications in future molecular circuit.

Electronic Transport Properties of the Azobenzene-Based Optical Molecular Switch with Different Substituents

By applying nonequilibrium Green’s function formalism combined first-principles density functional theory, we investigate the electronic transport properties of the azobenzene -based optical molecular switch with different substituents. Theoretical results show that the donor/acceptor substituent plays an important role in the electronic transport of molecular devices. The switching performance can be improved to some extent through suitable donor and acceptor substituents.

Molecular Optical Switch Based on Polymer Nanosheet Assembly Operated at Visible Wavelength

Abstract We constructed a molecular optical switch, which controls the photocurrent direction using a polymer nanosheet assembly. Two polymer nanosheet photodiodes composed of ruthenium complex (sensitizer) and ferrocene (donor) polymer nanosheet pair and anthracene (sensitizer) and dinitrobenzene (acceptor) polymer nanosheet pair, respectively, were assembled on the ITO electrode. Selective excitation of ruthenium by 450 nm generated cathodic photocurrent, whereas excitation of anthracene at 389 nm generated anodic photocurrent. The results indicate that the current flow direction in the assembled film can be controlled by the irradiation wavelength.

A first-principles study of dihydroazulene as a possible optical molecular switch

By applying nonequilibrium Green’s function formalism combined with first-principles density functional theory, we investigate the electronic transport properties of the dihydroazulene optical molecular switch. Three kinds of adsorption sites including the hollow, bridge and top sites are studied. The two forms of this molecule, namely the open form and the closed form, can reversibly switch from each other upon photoexcitation. Their transmission spectra are remarkably distinctive. Theoretical results show that the current of the closed form is always significantly larger than that of the open form for all three adsorption sites, which promises this system as possibly one of the good candidates for optical switches due to its unique advantage, and which may have some potential applications in the future molecular circuit.

A possible salicylideneanilines-based optical molecular switch induced by a reversible hydrogen transfer: an ab initio study

The electronic transport properties of the salicylideneanilines-based molecular optical switch are investigated using a nonequilibrium Green's function formalism combined with first-principles density functional theory. The molecule that comprises the switch can convert between the enol and keto tautomeric forms upon photoinduced excited state hydrogen transfer in the molecular bridge. Theoretical results show that the current through the enol form is significantly larger than that through the keto form, which realize the on and off states of the molecular switch. The physical origin of the switching behaviour is interpreted based on the spatial distributions of molecular orbitals and the HOMO-LUMO gap. Furthermore the effect of the donor/acceptor substituent on the electronic transport through the molecular device is also discussed in detail. The switching performance can be improved to some extent through the acceptor substituent.

Effect of Chemical Substituents on the Energetical Landscape of a Molecular Photoswitch: An Ab Initio Study

The effect of chemical substitutions on the energetical landscape of an optical molecular switch (Phys. Chem. Chem. Phys. 2008, 10, 1243) was studied with the aid of ab initio electronic structure methods. Series of different chemical moieties were substituted into the molecular frame of 7-hydroxyquinoline as well as into the "molecular crane" at position 8 of the frame. It was shown that the π-electron-donating/withdrawing properties of substituents substantially modify the energetical landscape of the system in the ground as well as in the lowest excited ππ* and nπ* singlet states.

The I-V Characteristics of the 3,3',5',5-Tetra-Tert-Butyl-Azobenzene Optical Molecular Switch: A First-Principles Study

By applying nonequilibrium Green’s function formalism combined first-principles density functional theory, we investigate the electronic transport properties of 3,3′,5,5′-Tetra-tert-butyl-azobenzene(meta-TBA) optical molecular switch. This molecular switch comprises a meta-TBA molecule with the trans and cis forms, which can be reversed from one structure to another one upon photoexcitation. The influence of HOMO-LUMO gaps and the spatial distributions of molecular orbitals on the electronic transport through the molecular device are discussed in detail. Theoretical results show that there is a large current ratio in bias window, which suggests that this system can be one of good candidates for optical switches due to this unique advantage, and have real applications in the molecular circuit.

Squaraine‐Based [2]Rotaxanes that Function as Visibly Active Molecular Switches

In a flash! A squaraine-based optical molecular switch functions with striking fluorescence signal changes that are visible to the naked eye (see figure).

Spiropyrans as molecular optical switches

Optical microscopes use visible light and an arrangement of lenses to provide us with magnified images of small samples. Combined with efficient fluorescent probes and highly sensitive fluorescence detection techniques they allow the non-invasive 3D study of subcellular structures even in living cells or tissue. However, optical microscopes are subject to diffraction of light which limits optical resolution to approximately 200 nm in the imaging plane. In the recent past, powerful methods emerged that enable fluorescence microscopy with subdiffraction optical resolution. Since most of these methods are based on the temporal control of fluorescence emission of fluorophores, photochromic molecules that can be switched reversibly between a fluorescent on- and a non-fluorescent off-state are the key for super-resolution imaging methods. Here, we present our approach to use spiropyran-fluorophore conjugates as efficient molecular optical switches (photoswitches). In these photochromic conjugates fluorescence emission of the fluorophore is controlled by the state of the spiropyran, which can be switched reversibly between a colorless spiropyran and a colored merocyanine form upon irradiation with light. Thus, the efficiency of energy transfer from the fluorophore to the spiropyran can be modulated by the irradiation conditions. We present ensemble data of the switching process of various spiropyrans and spiropyran-fluorophore conjugates and demonstrate photoswitching at the single-molecule level. Our data suggest that spiropyrans have to be immobilized in polymers to stabilize the merocyanine form in order to be useful for super-resolution fluorescence imaging based on precise localization of individual emitters. Special emphasis is put on photobleaching of donor fluorophores due to UV irradiation, i.e. photoswitching of the photochromic acceptor. Furthermore, we present a water soluble switchable spiropyran derivative and demonstrate the first intermolecular single-molecule photoswitching experiments in polymers.

The I–V characteristics of the butadienimine-based optical molecular switch: An ab initio study

By applying nonequilibrium Green’s function formalism combined first-principles density functional theory, we investigate the electronic transport properties of the butadienimine-based optical molecular switch with and without substituents. The influence of HOMO–LUMO gaps and the spatial distributions of molecular orbitals on the electronic transport through the molecular device are discussed in detail. Theoretical results show that the donor/acceptor substituent plays an important role in the electronic transport of molecular devices. The switching performance can be improved to some extent through suitable donor and acceptor substituents.

Quantum chemical study of benzimidazole derivatives to tune the second-order nonlinear optical molecular switching by proton abstraction

A novel sequence for reversible second-order nonlinear optical (NLO) molecular switching with protonation/deprotonation has been achieved and tuned as well. The NLO switching with first hyperpolarizabilities (beta(0)) as low as 14 x 10(-30) esu (Off-phase) and as large as 1189 x 10(-30) esu (On-phase) have been computed by using density functional theory (DFT). Remarkably large differences between the beta(0) values of benzimidazole containing chromophores and their deprotonated anions have shown their significant potential for a new type of NLO molecular switching, as (1-(4-methoxyphenyl)-2-(2-thienyl)pyrrolyl)-1,3-benzimidazole anion (1(-)) has a beta(0) value computed to be 61 x 10(-30) esu, which is 4 times larger than its neutral molecule 1. This beta(0) value has been tuned up to 2028 x 10(-30) esu by effective substitutions in the derivatives of 1(-) (1a(-), 1b(-), 1c(-), and 1d(-)). Interestingly, the substituted compounds have illustrated robustly large off-on NLO switching with a difference in beta(0) values of 7, 63, 85 and 75 times larger than their neutral counterparts, respectively. TD-DFT calculations along with natural bond orbital (NBO), frontier molecular orbitals (FMOs) and molecular electrostatic potential (MEP) analyses show that the abstraction of an imido proton brings about a change in push-pull configurations resulting in a red shift for both absorption and emission spectra which subsequently leads to a high performance second-order NLO molecular switching. A similar trend of NLO switching in F(-) compounds of these chromophores has also been observed with significantly large beta(0) values having analogous electro-optical properties like deprotonated anions. Furthermore, gas-phase acidity (GPA) calculations for the neutral molecule 1 and its derivatives (1a, 1b, 1c, and 1d) have also revealed that these are rationally potent nitrogen acids and can easily be dissociated to produce stable deprotonated anions.

Transport properties of chrysazine-type molecules

Electronic transport properties are presented for the three lowest-energy conformers of the chrysazine molecule. The current–voltage characteristics of these isomers are calculated for two different connections of the molecule to gold leads. The results for the various structures differ so significantly from each other that the specific conformer and its connection to the leads can be identified by its transport characteristics. The calculated transport properties suggest that chrysazine may be used as a molecular optical switch.

Electronic transport properties of a molecular switch with carbon nanotube electrodes: A first-principles study

Abstract We have studied the electronic transport properties of a new kind of optical molecular switch with two single-walled carbon nanotube (SWCNT) electrodes using first-principles transport calculations. It is shown that the enol form shows an overall higher conductance than the keto form at low-bias voltage, which is independent of the SWCNTs’ chirality. Meantime, it is possible to tune the conductance of the molecular switch by changing the chirality of the SWCNTs.

1,3-diphenyltriazene as a possible optical molecular switch: A first-principles study

By applying nonequilibrium Green's function formalism combined with first-principles density functional theory, we investigate the electronic transport properties of a 1,3-diphenyltriazene-based optical molecular switch. The molecule that comprises the switch can convert between the cis and the trans forms upon photoexcitation. The transmission spectra of two forms are remarkably distinctive. Theoretical results show that the current through the trans form is significantly larger than that through the cis form, which suggests that this system has attractive potential application in future molecular switch technology.

Electronic transport properties of a diarylethene-based molecular switch with single-walled carbon nanotube electrodes: The effect of chirality

We have studied the electronic transport properties of an optical molecular switch based on the diarylethene molecule with two single-walled carbon nanotube (SWCNT) electrodes using first-principles transport calculations. It is shown that the closed form shows an overall higher conductance than the open form at low bias which is independent of the SWCNTs’ chirality. Meanwhile, the conductance of the molecular switch can be tuned by changing the chirality of the SWCNTs.

First-principles study of the switching characteristics of the 15,16-dinitrile DDP/CPD-based optical molecular switch with carbon nanotube electrodes

We have studied the switching characteristics of an optical molecular switch based on the 15,16-dinitrile dihydropyrene/cyclophanediene (DDP/CPD) molecule with two single-walled carbon nanotube (SWCNT) electrodes using first-principles transport calculations. It is shown that the DDP shows an overall higher conductance than the CPD at low bias which is independently of the SWCNT chirality. Meantime, the conductance of the molecular switch can be tuned by the chirality of the SWCNT.

Reversible molecular switching at a metal surface: A case study of tetra- tert-butyl-azobenzene on Au(1 1 1)

Molecular switches represent a fascinating class of functional molecules, whose properties can be reversibly changed between different molecular states by excitation with light or other external stimuli. Using surface science concepts like self assembly to align such molecules in a well-defined geometry at solid surfaces, new functional properties may arise, which are relevant for different fields like, e.g., molecular electronics, sensing or biocompatible interfaces. For a microscopic understanding of molecular switching at surfaces, it is essential to obtain detailed knowledge on the underlying elementary processes, for instance the excitation mechanism in photoinduced switching. Here we present a case study of a specifically designed azobenzene derivative on a metal surface, namely tetra- tert-butyl-azobenzene (TBA) adsorbed on Au(1 1 1), which is so far one of the best studied system for which reversible conformational changes have been demonstrated. TBA/Au(1 1 1) can thus be viewed as model system in order to gain deeper insights into molecular switching processes at metal surfaces. We have studied the photoinduced and thermally activated reversible switching of TBA in direct contact with a Au(1 1 1) surface using two-photon photoemission (2PPE) and high-resolution electron energy loss spectroscopy (HREELS). The trans/cis-isomerization of TBA is accompanied by reversible changes in the geometrical and electronic structure of the molecules, allowing to gain mechanistic and quantitative insight into the switching process. In particular, the cross sections for the photoisomerization, the ratio between the cis- and trans-TBA in the photostationary state, and the activation energy for the thermally induced cis → trans reaction have been determined and are found to be strongly reduced compared to the corresponding quantities in the liquid phase. Furthermore, the mechanism of optical excitation and molecular switching of TBA on Au(1 1 1) has been identified to arise from a substrate-mediated charge transfer process, whereby photogenerated hot holes in the Au d-band lead to transient positive ion formation, which may subsequently result in the conformational switching of adsorbed TBA. This substrate mediated excitation process for surface-bound TBA is thus completely different from the well-known direct (intramolecular) excitation mechanism operative in the photoisomerization of free molecules. These results demonstrate the feasibility of molecular switching at metal surfaces, but also indicate that the switching properties of the surface-bound species are strongly modified by the interaction with a metal substrate. Therefore, molecular switches at surfaces open the perspective to design and optimize novel properties of functional surfaces, which can be controlled by external stimuli.

New insights in the photochromic spiro-dihydroindolizine/betaine-system

We have revisited the photochromic spiro-dihydroindolizine/betaine (DHI/B) system applying state-of-the-art density functional theory (DFT) calculations in combination with stationary and time-resolved absorption measurements. DHI/B-systems are becoming increasingly important as potential molecular machines, molecular switches, and photoswitchable electron-acceptors. The knowledge of the exact mechanisms of ring opening and closure, as well as of the geometries of DHI and betaine can provide critical information that will enable the design of better molecular machines and optical switches. The first surprising result concerns the electronic structure of the betaines, which is quite different than commonly assumed. The photochemical ring opening of DHI's to betaines is a conrotatory 1,5 electrocyclic reaction, whereas the thermal ring-closing occurs in the disrotatory mode. According to our results, the electrocyclic back reaction of the betaines to the DHI is NOT rate determining, as previously thought, but instead the kinetics are dictated by the cis-trans-isomerization of the betaine.