Light-induced Photoswitching of 4-(Phenylazo)benzoic Acid on Au(111).

Photochromic molecules can undergo a reversible conversion between two isomeric forms upon exposure to external stimuli such as electromagnetic radiation. A significant physical transformation accompanying the photoisomerization process defines them as photoswitches, with potential applications in various molecular electronic devices. As such, a detailed understanding of the photoisomerization process on surfaces and the influence of the local chemical environment on switching efficiency is essential. Herein, we use scanning tunneling microscopy to observe the photoisomerization of 4-(phenylazo)benzoic acid (PABA) assembled on Au(111) in kinetically constrained metastable states guided by pulse deposition. Photoswitching is observed at low molecular density and is absent in tight-packed islands. Furthermore, switching events were noted in PABA molecules coadsorbed in a host octanethiol monolayer, suggesting an influence of the surrounding chemical environment on photoswitching efficiency.

A Scanning Tunneling Microscopy Study of the Photoisomerization of Diazocine.

Azobenzenes are fascinating molecular machines that can reversibly transform between two isomeric forms by an external stimulus. Diazocine, a type of bridged azobenzene, has been shown to possess enhanced photoexcitation properties. Due to the distortion caused by the ethyl bridge in the E-isomer, the Z-form becomes the thermodynamically stable configuration. Despite a comprehensive understanding of its photophysical properties, there is still much to learn about the behavior of diazocine on a metal surface. Here we show the operando photoswitching of diazocine molecules deposited directly on a Au(111) surface using scanning tunneling microscopy. Molecules were shown to aggregate into disordered islands with edge sites being susceptible to photon-induced movement. A few molecules were shown to undergo directional movement under UV irradiation with the motion reversed under blue light exposure. These findings contribute new insight into the activity of single and ensemble molecular systems toward purposefully guided motion.

Cluster Formation of Self-Assembled Triarylbismuthanes and Charge Transport Characterizations of Gold-Triarylbismuthane-Gold Junctions.

Organometallic molecules are promising for molecular electronic devices due to their potential to improve electrical conductance through access to complex orbital covalency that is not available to light-element organic molecules. However, studies of the formation of organometallic monolayers and their charge transport properties are scarce. Here, we report the cluster formation and charge transport properties of gold-triarylbismuthane-gold molecular junctions. We found that triarylbismuthane molecules with -CN anchoring groups form clusters during the creation of self-assembled submonolayers. This clustering is attributed to strong interactions between the bismuth (Bi) center and the nitrogen atom in the -CN group of adjacent molecules. Examination of the influence of -NH2 and -CN anchoring groups on junction conductance revealed that, despite a stronger binding energy between the -NH2 group and gold, the conductance per molecular unit (i.e., molecule for the -NH2 group and cluster for the -CN group) is higher with the -CN anchoring group. Further analysis showed that an increase in the number of -CN groups from one to three within the junctions leads to a decrease in conductance while increasing the size of the cluster. This demonstrates the significant effects of different anchoring groups and the impact of varying the number of -CN groups on both the charge transport and cluster formation. This study highlights the importance of selecting the appropriate anchoring group in the design of molecular junctions. Additionally, controlling the size and formation of clusters can be a strategic approach to engineering charge transport in molecular junctions.

Electronic energy levels of porphyrins are influenced by the local chemical environment.

Self-assembled islands of 5,10,15,20-tetrakis(pentafluoro-phenyl)porphyrin (2HTFPP) on Au(111) contain two bistable molecular species that differ by shifted electronic energy levels. Interactions with the underlying gold herringbone reconstruction and neighboring 2HTFPP molecules cause approximately 60% of molecules to have shifted electronic energy levels. We observed the packing density decrease from 0.64 ± 0.04 molecules per nm2 to 0.38 ± 0.03 molecules per nm2 after annealing to 200 °C. The molecules with shifted electronic energy levels show longer-range hexagonal packing or are adjacent to molecular vacancies, indicating that molecule-molecule and molecule-substrate interactions contribute to the shifted energies. Multilayers of porphyrins do not exhibit the same shifting of electronic energy levels which strongly suggests that molecule-substrate interactions play a critical role in stabilization of two electronic species of 2HTFPP on Au(111).