Reversible Switching of Spiropyran Molecules in Direct Contact With a Bi(111) Single Crystal Surface

Abstract

Photochromic molecular switches immobilized by direct contact with surfaces typically show only weak response to optical excitation, which often is not reversible. In contrast, here, it is shown that a complete and reversible ringopening and ring-closing reaction of submonolayers of spironaphthopyran on the Bi(111) surface is possible. The ring opening to the merocyanine isomer is initiated by ultraviolet light. Switching occurs in a two-step process, in which after optical excitation, an energy barrier needs to be overcome to convert to the merocyanine form. This leads to a strong temperature dependence of the conversion efficiency. Switching of the merocyanine isomer back to the closed form is achieved by a temperature increase. Thus, the process can be repeated in a fully reversible manner, in contrast to previously studied nitrospiropyran molecules on surfaces. This is attributed to the destabilization of the merocyanine isomer by the electron-donating nature of the naphtho group and the reduced van der Waals interaction of the Bi(111) surface. The result shows that molecules designed for switching in solutions need to be modified to function in direct contact with a surface.

Optical differential reflectance spectroscopy for photochromic molecules on solid surfaces

“…To account for this, NI 9239 uses independent AD converters, which are triggered simultaneously and convert with an accuracy of up to 24 bit and 50 kS/s. Automation is of high importance for the measurement of photochromic systems since typically cross sections for photoswitching on surfaces are reduced by several orders of magnitude 13,17 and result in extensive measurement times. To determine suitable combinations of molecules and substrates, experiments in a reproducible and reliable manner are required.…”

“…For the similar molecule spironaphthopyran on the same surface, this has been done before by x-ray absorption spectroscopy. 17 Figure 6 shows the thickness calibration for SNO on Bi(111). A quartz microbalance mounted in the evaporator provides a measure proportional to the amount of molecules arriving at the sample surface during evaporation.…”

“…15 Spiropyranes on Bi(110) were found in a photostationary state upon blue-light illumination, 16 and spironaphthopyran on a Bi(111) surface can be reversibly switched by using a UV LED and thermal energy. 17 From UV/Vis-spectroscopy in solution, the absorption bands of these molecules are already well known, but when in contact with a surface, the optical reflection represents a combination of the electronic and optical properties of the full system including the substrate. Therefore, DRS is a very suitable method to determine and quantify the ability to switch photochromic molecules on a surface.…”

Light-induced photoisomerization of a diarylethene molecular switch on solid surfaces

“…The x-ray absorption spectra were measured by means of the total electron yield method and normalized by the signal of a gold grid upstream to the measurement chamber as well as the corresponding background spectra of clean substrates. Thicknesses were defined by comparing to reference measurements of a spiropyran derivate [15] in a submonolayer on Bi(111), where the saturation of the first layer was monitored by XA (x-ray absorption) at the carbon K edge. This calibration is expected to give sufficient accuracy to confirm that the molecule coverage is below a saturated monolayer and the individual molecules have contact with the substrate.…”

Influence of Light Polarization on Photoswitching of Fulgimide Monolayers on Surfaces

“…Lateral steric effects, mechanical and/or electronic coupling with the substrate are frequently invoked to explain the different behaviour at surfaces. and this motivated various strategies to increase the free volume around photochromic compounds [56] [19] [57,58] or to reduce the electronic coupling with the substrate [59] [33] [31] using chemical spacers or using semimetals with a small density of states at Fermi level [26]. In spite of significant improvements, when the photo isomerization cross section ( ) is determined [60] [26] [61] [63], is generally smaller than the value measured for the same photochromic molecule dispersed in a solvent and the behaviour at the surface depends on the architecture of the assembly (for a review at metallic surfaces see Ref.…”