Adsorption and switching properties of nitrospiropyran on Bi(1 1 4)

Abstract

Spiropyrans are prototype molecular switches, which undergo a reversible photoinduced ring-opening/-closure reaction between the closed three-dimensional spiropyran (SP) and the open, planar merocyanine (MC) form. In solution the SP isomer is the thermodynamically stable form. Using high resolution electron energy loss spectroscopy, we resolve a thermally-activated irreversible ring-opening reaction of nitrospiropyran resulting in the MC form for coverages above one monolayer. Thus, the situation found in solution is reversed for the adsorbed molecules, since the MC form is more stable due to the modified energetics by the presence of the substrate. In addition, illumination with blue light (445 nm) induced also the ring-opening, while the photostimulated back-reaction could not be observed. The photoisomerization is driven by a substrate-mediated process, i.e. a charge transfer from the substrate into molecular states. The situation changes completely in the monolayer regime. Neither a thermally-assisted nor a photoinduced ring-opening reaction has been identified. We ascribe the suppression to sterical effects stabilizing the SP form due to the surface structure of Bi(1 1 4), which consists of straight atomic rows separated by rough valleys.

Influence of N-introduction in pentacene on the electronic structure and excited electronic states

“…15–28 High-resolution electron energy loss spectroscopy (HREELS) provides the required insight into both adsorption and electronic properties of molecular compounds at interfaces to (semi)conducting surfaces and within thin films. 11,29–39 Depending on the primary energy of the incident electrons molecular vibrations or electronic transitions can be excited, including optically forbidden transitions (singlet to triplet states) as already successfully demonstrated for some organic compounds 40–42 including also the N-heteropolycycle 6,13-diazapentacene (6,13-DAP). 12 While the adsorption and electronic properties of pentacene (PEN) and its derivatives adsorbed on various metallic substrates have been investigated in great detail (see for example ref.…”

Influence of Core Halogenation on the Electronic Structure of Naphthothiadiazole Derivatives

“…Naphthothiadiazoles (NTDs) (see Figure ), NTD-containing polymers, and small molecule donor/acceptor (D/A) systems, in which the NTD moiety acts as the electron acceptor, are promising candidates for applications in organic semiconductor-based (opto)­electronic devices such as solar cells, light-emitting diodes, or field effect transistors. − For device performance the molecular orientation at the interface to an electrode as well as within a molecular film (film morphology) plays a central role because it strongly influences the electronic properties such as energy level alignment or charge transport properties. − High-resolution electron energy loss spectroscopy (HREELS) facilitates the analysis of both adsorption and electronic properties of molecules on (semi)­conducting surfaces and within thin films. − The determination of electronic properties include optical gaps and also optically forbidden transitions. − While the adsorption and electronic properties of S-heteropolycyclic aromatic molecules, in particular thiophene derivatives, on metal substrates have been studied in great detail (see e.g. refs − ) due to their relevance in vacuum-processed small molecule organic solar cells, − only a few examples on N-heteropolycycles are known in the literature. ,,,, S- and N-containing heteropolycycles, thus compounds such as NTDs, have not been investigated so far.…”

Light-Induced Quantitative and Electrical-Field-Induced Barrierless Switching of Spiropyran Derivative on Graphite Surface

“…We note that the proposed adsorption geometry of SP and MC states is similar to that of other SP derivatives adsorbed on Au(111) 9,13 and Bi(114). 11 This suggests that the microscopic changes associated to switching between SP and MC states is independent of the used substrate.…”

“…For solid-state applications, ordered thin films of the aforementioned molecules are essential. Self-assembly and on-surface switching of different types of molecular switches have been studied extensively during the past decades. − Different types of external triggers, such as photons, − tunneling electrons, ,− electrical field at molecular tunnel junction, temperature, and force-activation, are used to induce reversible switching of molecules on surfaces.…”

“…Spiropyran and its derivatives are known for photo- ,, and thermal-switching . The closed form of spiropyran (termed SP) upon photo irradiation undergoes a ring-opening reaction by cleavage of the C–O bond and converted into merocyanine (termed MC) through a cis–trans isomerization in solution .…”