Effect of Tether Conductivity on the Efficiency of Photoisomerization of Azobenzene-Functionalized Molecules on Au{111}

Abstract

ABSTRACT:We establish the role of tether conductivity on the photoisomerization of azobenzene-functionalized molecules assembled as isolated single molecules in well-defined decanethiolate self-assembled monolayer matrices on Au{111}. We designed the molecules so as to tune the conductivity of the tethers that separate the functional moiety from the underlying Au substrate. By employing surface-enhanced Raman spectroscopy, time-course measurements of surfaces assembled with azobenzene functionalized with different tether conductivities were independently studied under constant UV light illumination. The decay constants from the analyses reveal that photoisomerization on the Au{111} surface is reduced when the conductivity of the tether is increased. Experimental results are compared with density functional theory calculations performed on single molecules attached to Au clusters. SECTION:Physical Processes in Nanomaterials and Nanostructures P recise control over the reversible isomerization of functional molecules when assembled on solid surfaces is of great importance to understand the rules of molecular-and supramolecular-scale action. 1−14 Various kinds of photochromic molecules such as azobenzene, 15−24 diarylethene, 9,25−27 spiropyran, 28−30 and more recently dihydroazulene 31−34 have been investigated as possible candidates for molecular switches. These families of functional molecules have relative advantages and disadvantages due to factors such as their ease of molecular assembly on substrates, quantum yield of photoisomerization, reversibility of isomerization, and quenching of isomerization by the underlying substrate. Of these families of photochromic molecules, azobenzenes have thus far attracted the greatest attention.Azobenzene (henceforth Azo) exists in a near-planar trans conformation in its thermodynamically stable state with nearly zero dipole moment. 35,36 When irradiated with UV light at ∼365 nm, the molecule isomerizes to a nonplanar cis conformation via rotation of a phenyl group out of the plane of the azobenzene moiety, carrying a dipole moment of 3 D. 36 Upon subsequent irradiation with ∼420 nm visible light, the cis conformation reverts back to its original trans conformation, although thermal relaxation from cis to trans has also been shown to be a common pathway for the reverse reaction where relaxation times typically depend on the substituents on the azobenzene moiety ranging from minutes (for alkyl-substituted molecules) to days (for unsubstituted azobenzenes). Because of their difference in planarity, it has been established that trans form is approximately 100 times more conductive than cis, 16 and hence azobenzenes are considered to be a type of molecular switches. 22 Various theoretical and experimental studies have been performed to understand the mechanisms of photoisomerization of azobenezenes in the gas phase, in various solvents, and on solid substrates when isolated as single molecules or in ensembles. For instance, it has been wellestablished that photoisomerization of...

Controlling Motion at the Nanoscale: Rise of the Molecular Machines

“…161,162 Physically and electronically decoupling these functional moieties from surfaces or from neighboring molecules is often necessary to avoid steric constraints or quenching of excited states. [163][164][165][166][167][168] For example, molecular rotors can be tethered such that the axis of rotation is aligned parallel or perpendicular to the surface, in either altitudinal or azimuthal orientations, respectively. Feringa and coworkers reported the tunable and reversible wettability of gold surfaces modified with SAMs of altitudinal rotors based on light-driven overcrowded alkenes bearing perfluorinated alkyl chains (Figure 12a).…”

Molecular switches from benzene derivatives adsorbed on metal surfaces

“…In particular, molecules that can be reversibly switched between two or more stable states show great promise for data storage and logical components 3 6 7 . Tremendous efforts were undertaken to trigger the function of materials at the molecular level, and some promising molecular switches have already been proposed in the literature 8 9 10 11 12 13 14 . Prominent examples of molecular switches include the cis-trans isomerization reaction of azobenzene and its derivatives on surfaces 8 9 10 11 12 , as well as the hydrogen tautomerization reaction of the naphthalocyanine molecule 6 14 .…”

Photoactive and Physical Properties of an Azobenzene-Containing Coordination Framework

“…Several studies of asymmetric azobenzenes have suggested that the decrease in relative intensity of peaks between 1400 and 1550 cm À1 is indicative of an isomerisation process. [51,52] Crystal Structure and Thermophysical Properties of [Zn 4 (tbazip) 3 (Fig. S3, Supplementary Material).…”