Abstract
The combination of photochromic and nonlinear optical (NLO) properties of azobenzene-functionalized self-assembled monolayers (SAMs) constitutes an intriguing step towards novel photonic and optoelectronic devices. By utilizing the second-order NLO process of second harmonic generation (SHG), supported by density-functional theory and correlated wave function method calculations, we demonstrate that the photochromic interface provides the necessary prerequisites en route towards possible future technical applications: we find a high NLO contrast on the order of 16% between the switching states. These are furthermore accessible reversibly and with high efficiencies in terms of cross sections on the order of 10(-18) cm(2) for both photoisomerization reactions, i.e., drivable by means of low-power LED light sources. Finally, both photostationary states (PSSs) are thermally stable at ambient conditions.
Highlights
Molecular switches are molecules which can be switched reversibly between two or more stable states
Two clearly distinguishable second harmonic generation (SHG) signal levels are observed: a lower one subsequent to light exposure at 365 nm and a higher level measured after illumination at 445 nm
These are reproducible over 20 illumination steps without any sign of fatigue, i.e., no decrease in the nonlinear optical (NLO) contrast is observed
Summary
Molecular switches are molecules which can be switched reversibly between two or more (meta-) stable states. By means of X-ray photoelectron spectroscopy (XPS), conducted on simultaneously prepared twin-samples, are described elsewhere.[18] By following this preparation strategy the azobenzene chromophores can be sufficiently decoupled from their neighbors, allowing the photoinduced isomerization reactions between photostationary states (PSSs) enriched with azobenzene in its trans- and its cis-form, respectively (see Fig. 1). We access the switching state of the system via the interfacial nonlinear optical (NLO) response. This approach has already successfully applied for fulgimide-functionalized SAMs on Si(111). Thereby fulgimides undergo a reversible photoinduced ring-opening/closure reaction.[19] Second harmonic generation (SHG) probes the second-order nonlinear susceptibility w(2) of the sample. We determine the efficiency of the switching processes in terms of the cross sections These are on the order of 10À18 cm[2] for both isomerization reactions. We possess a two-dimensional NLO switching system with high potential regarding emerging photonic and optoelectronic technologies
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