Fast optical work-function tuning at an organic/metal interface

Abstract

In a two-color experiment, we demonstrate how light can be used as an external control to continuously tune the work function of a gold substrate functionalized with a dilute azobenzene-based self-assembled monolayer (SAM). The work function is measured by two-photon photoelectron spectroscopy. While in the ground state the azobenzene moiety adopts the trans configuration, illumination with pulsed laser light at a wavelength of 368 nm results in a photostationary state (PSS) mainly comprising cis isomers. An additional 450 nm continuous-wave laser with tunable intensity serves to shift the PSS back towards the ground state. This way the work function is freely adjustable in real time over a range of ∼240 meV between the two PSS extrema. We furthermore relate the change in work function to the average change in dipole moment per azobenzene chromophore. Quantum-chemical calculations that take into account available structural data of the molecules in the SAM must consider at least two different trans and four different cis orientations. The computed respective perpendicular trans-cis dipole-moment changes indicate that in experiment the cis molecules adopt different orientations along with a very high cis azobenzene yield in the UV PSS.

Azobenzene isomerization in condensed matter: lessons for the design of efficient light-responsive soft-matter systems

“…While SAMs represent systems where azobenzenecontaining molecules are bound strongly enough to be considered permanently in contact with the surface, surface interactions are also important to completely understand systems where transient adsorption is present. 58 Most commonly, the Au(111) surface has been used for formation of SAM structures, 12,[59][60][61][62][63][64][65][66] however, other metallic surfaces such as Ag(111) 67,68 have been used as well as silica, 69,70 mica, 71,72 and graphene. 73 In general, there are two common orientations in which azobenzene molecules associate with surfaces, dominated by forces of interaction with surface structures.…”

A novel naphthalene diimide-based conjugated polymer as an electron transport material for non-fullerene organic solar cells

“…PNDI-NN can form an interfacial dipole layer between the active layer and cathode, and reduce the WF of the air-stabilized Ag metal. [45][46][47] The secondary amine in the PNDI-NN side chain can form hydrogen bonds with photovoltaic materials in the active layer, resulting in good contact with the active layer. 22,48,49 Moreover, it promotes charge separation and helps in reducing charge recombination.…”

Influence of Light Polarization on Photoswitching of Fulgimide Monolayers on Surfaces

“…Immobilization of photochromic molecules on surfaces [19] [27], very often on gold, or self-assembled as Langmuir-Blodgett (LB) layers [28] is a further approach to control wettability of surfaces with external light [29], which is crucial in microfluidics, or to tailor surface conductance [30] [31] [32] [33] [34] and surface work function. [35] [36] [37] [38] Photochromic molecules can be also adsorbed on nanoparticles to control their properties [39].…”

“…For instance, photochromic molecules were incorporated into polymers , or thin films − to realize multifunctional layers, in particular to store information − or modulate charge transport. , Photochromic molecules can also be used in the context of biological systems, to structure soft matter with light, to control the function of biomaterials, or to promote catalytic activity . Immobilization of photochromic molecules on surfaces, − very often on gold, or self-assembled as Langmuir–Blodgett layers is a further approach to control wettability of surfaces with external light, which is crucial in microfluidics, or to tailor surface conductance − and surface work function. − Photochromic molecules can also be adsorbed on nanoparticles to control their properties …”